Causes and Effects of a Late 20th Century Fault Slip Event in Coastal Louisiana

Wetlands loss in the delta plain of coastal Louisiana is best understood as an episode that peaked in the late 20th century, and has been generally declining for the past three decades.  Losses were concentrated in the Barataria and Terrebonne hydrologic basins, as well as some intense hot spots in the Mississippi River Delta.  This land loss episode had a strong temporal correlation with an episode of higher subsidence rate estimates derived from tide gauge records (Penland et al, 1988) and elevation benchmark leveling surveys (Shinkle and Dokka, 2004).  Many of the major hot spots of wetlands loss associated with this episode are located along the surface traces of major faults, and it appears that a late 20th century fault slip event was a principal cause of increased rates of subsidence.  Wetlands loss was primarily a function of the submergence of marshes on the hanging walls of the faults.

The implications for coastal sustainability are striking.  The hot spots of wetlands loss are for the most part hot spots of higher subsidence rates associated with faults.  The objective of most coastal sustainability projects is to build and maintain the elevation of the marsh surface.  There is a much lower probability of maintaining elevation in areas with higher subsidence rates.  Conversely, stable areas outside of the subsidence hot spots have been effectively maintaining elevation for decades through natural sediment accretion.  Sustainability projects that seek to work with nature by supplementing natural accretion on stable marsh platforms will be the most effective.

THE TERREBONNE LINKED TECTONIC SYSTEM

The fault slip event occurred mainly on the major faults in the “Terrebonne Linked Tectonic System”.  The faults and salt domes in this interconnected system form the subsurface structural framework of the delta plain.  There appears to be a strong genetic relationship among the individual elements of the system.  Salt and faults appear to have moved interactively throughout the geological history of the area. 

Most of the salt domes in the system have a rounded conical shape in the subsurface above depths of about 10,000 feet.  For much of the 20th century geologists assumed that most domes rose vertically from a sedimentary layer called the Louann Salt.  The advent of 3-D seismic made it clear that most domes have a distinctly tilted orientation at greater depths.  Salt appears to have been squeezed out laterally from the source layer under the weight of an advancing sedimentary load during much of the Cenozoic Era.  The inset profile across the Lake Washington Dome shows this configuration, and the relationship between the dome and the Magnolia and Barataria faults.  It is likely that there was a feedback loop between sediment accumulation, movement of the salt, and movement on the faults throughout their geologic history.  The combined effects of fault and salt movement have created accommodation capacity for deltas throughout much of the Cenozoic Era, and the weight of the accumulated sediment has induced further movement.

The faults are arranged as connected linear segments within the system.  Faults along the north side of the system tend to be down to the south.  Their surface traces, which are coincident with the 0-depth contour of the fault plane map, are shown in light blue.  Faults along the south side of the system tend to be down to the north, and their surface traces are shown in red.  This arrangement tends to group the faults into conjugate pairs of opposing faults that create large structural grabens between them.  Sediments of the Late Miocene Epoch within the major grabens are noticeably thicker than they are outside of the system.  This network of grabens that runs along the coastline is commonly called the Terrebonne Trough.

The regional profile across southern Louisiana from Nelson et al, 2000 shows the dramatic thickening of the Late Miocene sediments into the Terrebonne Trough.  It also indicates that faults of the Terrebonne Linked Tectonic System merge into a larger detachment surface in which sediments of the late Mesozoic and Cenozoic Eras have moved laterally to the south above deeper sediments.  A horizontal vector of motion toward the south is measured at the surface today, and it probably is an expression of this deeply-rooted lateral movement (Karegar et al, 2017).  Salt also appears to have moved laterally along this same detachment surface before following tilted upward paths to form diapirs that rise toward the surface.

The grabens of the Terrebonne Linked Tectonic System provided accommodation capacity for deltas throughout the Miocene Epoch.  The black and purple contours of the faults planes and salt domes show the configuration of the linked tectonic system.  The outlines of the Late Miocene Regressive Phase deltas numbered for cycles XI – XIII (from Curtis, 1970) are overlain on the contours and a colored grid representing Late Miocene sediment thickness (from Wu & Galloway, 2006).  It appears that Late Miocene deltas “took up residence” in the grabens of the structural system where maximum sediment accumulation occurred.  More detailed studies of faults within this system (Bullock et al, 2018, Levesh et al, 2019, McLindon, 2017) have shown a strong relationship between sediment loading and inferred rates of fault slip.  Large-scale, long-term fault slip events are indicated by the expansion indices measured across these faults.

The accumulation of thick deltaic sands within the grabens of the linked tectonic system and the continual movement of the faults in response to sedimentary loading created a very efficient system for removing water from the sedimentary column.  This allowed the sand and shales to dewater and compact.  Faults within the system have been shown to be good conduits for fluid movement, and the thick clean sand layers formed aquifers that were also very efficient at moving water.  On either side of the central grabens the movement of water in the subsurface was not as efficient.  Fluids became trapped in the pore spaces, which prevented the mineral grains of the sediment from compacting with increasing overburden.  The onset of geopressure in these sediments occured when the mineral grains of the sediment matrix could not increase grain to grain contact to support the increased overburden, and so the pore fluids took on a portion of the overburden stress resulting in increased fluid pressure. 

Within the major grabens pore fluid escape and compaction continued to greater depths than on either side of the graben.  This has resulted in a thick column of normally-pressured sediments with the grabens that is more compacted and denser than the sedimentary columns on either side of the graben.  This density differential between sediments downthrown to the graben faults and the sediments upthrown to those faults has been a continual impetus for fault movement since the Late Miocene.  The major delta systems of the Pliocene and Pleistocene Epochs continued to build out basinward and prograding the continental shelf.  The major deltas from these epochs primarily interacted with faults on the modern shelf, but Fisk, 1960 showed that Pliocene and Pleistocene sediments were also thicker within the area of the Terrebonne Linked Tectonic System.  This indicates that fault movement continued into the Pleistocene.  There is evidence of Holocene movement on the Bastian Bay, Barataria, and Ironton faults in the Pleistocene.

The Holocene Epoch represents the end of last glacial cycle of the Pleistocene.  It is a reasonable expectation that episodic movement, which had been continual on the faults and domes of the Terrebonne Linked Tectonic System throughout the Miocene, Pliocene and Pleistocene Epochs would have continued into the Holocene.  One of the best indications of Holocene fault movement is the relationship among the configuration of the deltas, the thickness of the sediments and the linked tectonic system. The dashed colored outlines of the the most recent delta lobes (from Frazier, 1967) shows that each lobe appears to have had a distinct relationship with one of the conjugate pairs of faults that define the Terrebonne Trough.  The thickness of the “topstratum” of the Holocene (from Kulp, 2000) represents the entire sedimentary accumulation of the Holocene deltas.  Sediment thickness is greater within the grabens of the linked tectonic system than it is outside of them. These grabens provided accommodation capacity to the deltas, just as they had done with the Miocene deltas.  Each of the most recent delta lobes is underlain by the sediments of at least one previous Holocene delta lobes.  Frazier defined sixteen historical delta lobes of the late Holocene.  Most of them have submerged below the surface, which further evidences subsidence throughout the Holocene.

The dramatic increase in Holocene thickness in the far eastern graben associated with the Balize delta may be evidence of a sedimentation anomaly associated with the onset of agriculture in the Mississippi Basin the the 18th and 19th centuries.  This will be discussed in more detail in a later section on possible causes for the late 20th century fault slip event.

THE BASTIAN BAY AND LAKE BOUDREAUX FAULTS

Research on the surface traces of the faults in the Terrbonne Linked Tectonic System has expanded significantly in the past decade.  Most of the traces shown here are documented by academic research.  Two of the major faults will be considered in more detail to examine characteristics that are shared all of the faults. 

The fault plane contours of the Bastian Bay fault have been mapped in the subsurface by the interpretation of the petrophysical logs from oil and gas wells and seismic data.  The trace of the fault where it crosses a Late Miocene sand layer at a depth of about 10,000 feet is parallel to the surface trace.  The fault creates a “rollover” anticlinal structure on the sand layer, which allows for the trapping of natural gas in the sand at the top of the anticline. Gas accumulations in several Late Miocene sands form Bastian Bay field.  Most of the major faults in the Terrebonne Linked Tectonic System have a similar relationship with oil and gas fields.  There is a causal relationship between the location of the surface traces of the faults and the locations of large oil and gas fields. 

These block diagrams show the Bastian Bay fault in context with other elements of the Terrebonne Linked Tectonic System, and in isolation to reconstruct its history.  The fault probably formed as a low-relief slump feature on the continental slope some time before the start of the Miocene Epoch.  Sediment loading across the fault from the Miocene deltas caused episodic fault movement.  Fluids migrating along the fault plane during episodes of movement included hydrocarbons which had been cooked out of deeper sources rock.  The migrating hydrocarbons accumulated in Late Miocene sand layers forming the gas reservoirs of Bastian Bay field.  Episodic fault movement continued throughout the Pliocene and Pleistocene Epochs and into the Holocene and recent periods. 

Studies have shown that oil and gas reservoirs which have experienced significant depletion in reservoir pressure during extraction may have induced compaction of the reservoir sand resulting in subsidence at the surface (Chan and Zoback, 2007).  None of the reservoirs in the Bastian Bay field experienced adequate depletion in reservoir pressure to have induced compaction and subsidence.  Hydrocarbon extraction is not considered to be a viable cause for the subsidence measured at the surface in this area.  It is likely that the subsidence estimates made with data from the Grand Isle tide gauge reflect some contribution of recent movement on the Bastian Bay fault.

In the period between 1973 and 1975 a large area across the hanging wall of the Bastian Bay fault submerged and converted from mostly saline marsh to open water.  Gagliano et al (2003) estimated that the maximum total displacement on the fault at the surface was 3.0 to 3.5 feet based on his boring profiles and anecdotal evidence from oyster fishermen and camp owners.  Martin (2006) also quoted anecdotal evidence from Greg Linscombe of the Louisiana Department of Wildlife and Fisheries that the marsh break associated with the fault escarpment of the Adams Bay fault just to the north formed between 1976 and 1977.  The magnitude and pattern of these two fault slip events are similar, but they appear to be separated in time by 3 or 4 years.  The relationship between the Bastian Bay and Adams Bay fault slip events may provide insights into the nature of the connection between elements in the linked tectonic system.  It may be possible that the Bastian Bay fault slip event changed the stress fields in the near-surface in a way that affected and contributed to the cause of the Adams Bay fault slip event.

The dramatic submergence of marshes on the hanging wall of the Bastian Bay fault can be seen in a comparison between 1973 and 1975 reconstructions of the marsh surface (from ProPublica, 2015).  This appears to have been the peak of the fault slip event, which is estimate of have extend across a period of about 18 years between 1970 and 1988 based on the records of the Grand Isle tide gauge.

The Lake Boudreaux fault has a distinct northwest-southeast orientation within the linked tectonic system but its relationship with a rollover structure oil and gas field and a surface trace are very similar to the Bastian Bay fault.  The Lapeyrouse field was studied by Chan and Zoback, 2007 to investigate the relationships between oil and gas extraction and subsidence at the surface.  They determined that reservoirs in the Lapeyrouse field had experienced reductions in reservoir pressure due to extraction that were adequate to have induced compaction in the subsurface. They found that this could have caused subsidence at the surface.  Their models showed that subsidence associated with these processes would have been expressed as bowl-shaped depressions at the surface immediately above the outline of the depleted reservoirs.  The Madison Bay hot spot of wetlands loss lies along the surface trace of the Lake Boudreaux, and not above the depleted reservoirs.  There was no oil and gas production beneath the Madison Bay hot spot, and it is unlikely that hydrocarbon extraction contributed to subsidence in this area.  Subsidence in this area is more likely to have been caused by a slip event on the Lake Boudreaux fault.  Patterns of wetlands loss suggest that fault movement may have been happening nearly simultaneously on the Lake Boudreaux, Montegut and Isle de Jean Charles faults. 

Morton et al, 2005 used boring profiles to study patterns of subsidence at the Madison Bay hot spot.  Their investigation shows that a layer of muddy sediment was deposited on top of the marsh surface beginning about 150 years ago.  The original thickness of the mud layer can be seen in the un-subsided portions of the profile.  Below this layer there is a layer of peat that indicates that the marshes maintained elevation through natural accretion and organic growth of marsh plants for a period from about 700 years ago until 150 years ago.  The magnitude of subsidence in the Madison Bay hot spot can be seen in the current elevation of boundary between the mud layer and the peat layer relative to the elevation of the original marsh surface.  The approximately 80 centimeters of subsidence inferred by this method is equivalent to about 2.6 feet.  This is roughly comparable to the value estimated by Gagliano at the Bastian Bay and Adams Bay faults, and it appears to have happened within the same time frame.  As will discussed in the section on possible causes of the fault slip event, the deposition of layer of muddy sediment on the marsh surface over the past 150 years may be associated with the same sedimentation anomaly responsible for the thick Holocene sediments at the eastern end of the Terrebonne Trough.  Sediment loading across the fault may have contributed to the cause of the fault slip event.

LATE 20TH CENTURY SUBSIDENCE EPISODE

Evidence for the patterns of recent movement on the Lake Boudreaux and Bastian Bay faults can be seen on faults throughout the Terrebonne Linked Tectonic System.  There is good evidence that the elements of system have moved interactively throughout the Cenozoic Era into the Holocene and recent.  It appears that as the fault slip event propagated throughout the system it created an episode of increased subsidence rates across the delta plain that was recorded in historical tide gauge records (Penland et al, 1988) and the re-surveying of elevation benchmark (Shinkle & Dokka, 2004).

Estimates of current subsidence rates were made using static GPS measurements by Byrnes et al, 2021.  These are direct measurements of the vertical velocity of the surface of the earth at the sites of the benchmarks.  The total value of subsidence measured at the surface is the cumulative result of the contribution of multiple processes.  Byrnes suggested that the very strong relationship between the magnitude of the subsidence values and the thickness of the Holocene topstratum at the site of the benchmark indicate that the compaction of the Holocene sediment is the primary driver of current subsidence rates.  It is likely that other processes including deep lithospheric flexure of the mantle, continuing compaction of Pleistocene through Miocene sediments and Glacial Isostatic Adjustment are contributing to the total value of subsidence being measured.  It is also likely that the magnitude of subsidence attributable to these processes is small, and that it would not vary significantly across the area.

None of the processes listed above provides a viable mechanism to account for the episode of increased subsidence rates measured in the late 20th century.  A sampling of the values of subsidence rates estimated from Penland et al, 1988 and Shinkle & Dokka, 2004 are shown in blue and green, respectively.  In some cases the elevated rates of subsidence are two to three times the value of current subsidence rates in the same area.  The most viable explanation for the episode of increased subsidence rates in the late 20th century is a fault slip event in the Terrebonne Linked Tectonic System.

The temporal relationship between rates of subsidence and rates of wetlands loss over the past century is striking.  It appears likely that natural subsidence has been the primary driver of wetlands loss over that time period.  This would be consistent with the inferred relationships between subsidence and wetlands loss in the delta lobes delineated by Frazier, 1967 that are now submerged 20 to 30 feet below the surface.  The strength of the relationship between subsidence and wetlands loss also implies that patterns of wetlands loss may be used to add detail to the values of subsidence that have been measured at discrete locations over the past few centuries.

THE NATURE OF FAULT SLIP EVENTS

Very little is known about the nature of fault slip events in the Louisiana delta plain.  The measurement of accurate current subsidence rates has only been taking place for about two decades.  The density of measurement stations is much too low to have detected variations on subsidence rate that may be attributable to fault slip.  Earthquakes are periodically recorded in the state, and they very likely indicate fault slip events of some kind.  The only direct observation of a fault slip event that ruptured the surface occurred at Vacherie between April 12th and 15th in 1943.  The magnitude of that slip event was measured at 8 inches.  Shen et al, 2017 did precise dating of sedimentary layers and accurate elevation measurements of those layers on either side of faults in the Baton Rouge fault system.  They were able accurately estimate the average rate of fault slip over several millennia.  The total cumulative offset of one Pleistocene layer due to fault slip was estimated to be about 1100 millimeters over 32,000 years or an average rate of fault slip of 0.034 mm/year.  The offset of a Holocene layer on a different fault was measured by Shen et al at about 600 mm over 2780 years or an average slip rate of 0.216 mm/year.  The difference between these two values can be attributed to an inverse relationship between the magnitude of average rate of fault slip and the time span over which it was measured (Sadler, 1981).

A semi-quantitative distribution of fault slip events may be constructed by combining the data from Shen et al, 2017 with Gagliano’s observation of 3 feet of displacement at the Bastian Bay fault, which is spread over a span of 18 years based on the span of increased subsidence rates measured by tide gauge and benchmark data, and the displacement of the Vacherie fault over 3 days in 1943.  What is more important than the derivation of any actual data from this very sparse population is the characterization of the nature of fault slip events.  The values estimated by Shen et al, are averages of a total population of fault slip events that happened on those faults over periods of 32,000 and 2,780 years respectively.  The average values of fault slip rate estimated by Shen et al are also the mean value of the population. The total population of fault slip events responsible for the total offset probably has a size distribution similar to the semi-quantitative power-law distribution in the figure.  In other words the total displacement of the Pleistocene layer across the fault measured by Shen et al, was the result of many individual fault slip events over the span of 32,000 years.  If slip events on this fault happened once a year there would be 32,000 individual events in this population.  If slip events happened once a century, there would be 320 in the population.  It is likely that rather than being a population of events of equal magnitude, that there would be a distribution of different magnitudes.  If this distribution followed a power law, it would consist of many smaller events and very few larger events.  Large slip events would be very rare, and small slip events would be more common.  The magnitude and frequency of the small slip events may be so low as to be manifest as a slow creeping motion of the fault rather than a distinct slip event.  If the pattern of accelerated subsidence in the late 20th century as seen in tide gauge data in the delta plain is a reflection of the nature of fault slip that caused it, then it is likely that rate of fault slip increased somewhat gradually over time rather than there being distinct slip events.  In other words the movement on faults in the Terrebonne Linked Tectonic System may have been a gradual creeping motion, and the rate of creep increased for a period of several years.

POSSIBLE CAUSE OF THE FAULT SLIP EVENT

A possible cause of the late 20th century fault slip event in the Louisiana delta plain is suggested by the sites of the subsidence hot spots relative to the distributary channels of the last few historical deltas and the surface traces of the major faults.  The timing of the fault slip event suggests a slow build-of surface stresses due to abnormal sedimentary loading in the 18th and 19th centuries and a release of that stress caused by a triggering event in the last three decades of the 20th century. 

The map of the 1874 flood of the Mississippi River (Hardee, 1874) shows that without artificial levees major floods covered the entire delta plain.  Sediment-laden flood waters were carried throughout the entire distributary channel network.  It also shows that the modern Balize delta lobe has been built over the last two centuries.  Tweel and Turner, 2012 showed that agricultural expansion across the Mississippi River basin caused a dramatic increase in the suspended sediment lobe of the river at the beginning of the 19th century.  The anomalously thick accumulation of sediment in the Balize delta lobe seen on the Holocene topstratum thickness map is probably a direct result of the increased sediment load of the river.  The Hardee map suggests that sedimentary loading would have been spread across the delta plain.  It is likely that the layer of muddy sediment seen in the boring profile at the Madison Bay hot spot is a function of increased rates of sedimentation over the last 150 years.

The most logical sites for increased sediment accumulation in the delta plain during the period of higher suspended sediment load of the Mississippi River would have been off the flanks of the natural levees of the distributary channel networks where those networks branch out.  Because of the genetic relationship between the Holocene delta lobes and the faults of the Terrebonne Linked Tectonic System, the distributary channel networks of each delta lobe tend to branch out where they cross the major down to the south faults.  Sediment-laden flood waters would have fanned out across these branching channel networks and deposited their load on the flanks of the natural levees and across the hanging walls of the faults.  There is a very strong correlation between the sites where branching distributary channel networks cross the major faults and the hot spots of wetlands loss in the late 20th century.  It is likely that the accumulated sediment in these areas, such as the muddy layer seen in the Madison Bay borings, would have caused stress differentials on either side of the major faults.  It appears that these stress differentials were released in the fault slip event brought on by a trigger mechanism.

Historical tide gauge data has been used around the world to measure “relative sea level rise”, which is the combination of global “eustatic” sea level rise and subsidence.  By knowing the rate of global sea level rise it is possible to estimate a history of the rate of subsidence at the site of the tide gauge (Boon et al, 2010).  Blum and Roberts, 2012 compared water levels from the Pensacola and Grand Isle tide gauges to the established global mean sea level.

The Pensacola gauge tracks very closely to the global mean sea level, but the Grand Isle gauge appears to indicate that sea level was rising nearly four times faster than it was at Pensacola.  This is because the rate of subsidence a Pensacola is near zero, and so the gauge is recording actual global sea level.  Conversely, the Grand Isle gauge began subsiding at an accelerated rate in the latter part of the 20th century, and the apparent higher water levels are due to subsidence. 

The level of the Grand Isle gauge relative to the station datum is shown in the figure below.  Increasing water level anomalies indicate increasing rates of subsidence.  These anomalies peak in the 1970s and 1980s, then begin to decline.  The relationship between the subsidence anomaly and the episode of land loss in the Terrebonne and Barataria basins is obvious, but there also appears to be a relationship between the discharge of the Mississippi River and the rate of subsidence measured by the tide gauge.

This relationship may suggest a trigger mechanism for the fault slip event.  Increases in river discharge coincide with the elevation of the river stage within the channel.  These are major flood events on the Mississippi River.  Elevated river stages are also accompanied by the exchange of groundwater in near surface aquifers and the river channel.  Kolker et al, 2013 showed that groundwater fluxes during high river stages extended across the delta plain in the near-surface aquifers.  Because near-surface aquifers are the natural levee deposits of the last few historical deltas, the flux of increased groundwater flow during periods of high river discharge would have extended throughout the distributary channel network.  Increased flow of groundwater in the natural levee sands would have tended to dilate the aquifers and affect the stress fields in the area.  Changes in stress would have been concentrated in the distributary channel networks at the exact sites where sediment loading has caused stress differentials across the major faults.  In this manner the periods of high river discharge could have served as a trigger mechanism for a fault slip event.  In simplest terms the dilation of near surface aquifers may have served to “lubricate” the faults allowing for increases in the rate of slip movement. The increased rates of fault slip across the Terrebonne Linked Tectonic System would have been measured as increased rates of subsidence in the tide gauge and benchmark leveling surveys.

The geological history of the Terrebonne Linked Tectonic System is one of continual episodic movement in response to sediment loading from the deltas of the Mississippi River.  Throughout the Miocene Epoch and into the late Holocene there is evidence of fault movement in response to sediment loading.  It is logical that an increased suspended sediment load of the river during the 18th and 19th century would have resulted in loading at the sites where the distributary channels crossed the major faults.  A possible trigger mechanism for a fault slip event may have been caused by changes in the stress fields around the sites of sediment loading caused by the dilation of near-surface aquifers during periods of high river discharge.  This set of processes would accurately account for the location of the hot spots of a late 20th century episode of subsidence.  Recognizing the relationship between this episode of subsidence and wetlands loss in the delta plain has significant implications for coastal sustainability planning.

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Penland, S.,, Ramsey, K.E., McBride, R.A., Mestayer, J.T. and Westphal, K.A., 1988, Relative Sea Level Rise and Delta-Plain Development in the Terrebonne Parish Region, LA Geological Survey Coastal Geology Technical Report No. 4, 140 p.

ProPublica, 2015, Losing Ground

Shinkle, K. D., and R. K. Dokka, 2004, Rates of vertical displacement at benchmarks in the Lower Mississippi Valley and the northern Gulf Coast, NOAA Tech. Rep. 50

Stadler, P.M., 1981, Sediment Accumulation Rates and the Completeness of Stratigraphic Sections, The Journal of Geology, Vol. 89 p. 569-584

Thorne, C., Harmar, O., Watson, C., Clifford, N., Biedenharn, D., and Measures, R., 2008, Current and historical sediment loads in the Mississippi River, Report to the European Research Office of the U.S. Army, 170 p.

Tweel, A.W. and Turner, R.E., 2012, Watershed land use and river engineering drive wetland formation and loss in the Mississippi River birdfoot delta, Limnology and Oceanography, v. 57, p. 18‐28.

Zou, L., Kent, J., Lam, N. S.-N., Cai, H., Qjang, Y., and Li, K., 2016, Evaluating Land Subsidence Rates and Their Implications for Land Loss in the Lower Mississippi River Basin, Water, v. 8, 15 p.

A Geological Evaluation of the vicinity of the Mid-Barataria Sediment Diversion

This evaluation was submitted as comments on the Draft Environmental Impact Statement for the Mid-Barataria Sediment Diversion. They propose that a thorough geological evaluation should be conducted at the site to determine the potential for sediment loading by the project to cause a major fault slip event.

These comments are being submitted on the Draft Restoration Plan and Environmental Impact Statement: Mid-Barataria Sediment Diversion (the “Draft EIS”).  Comments submitted prior to drafting this Statement (McLindon et al, 2017) recommended: ”that a thorough subsurface geological evaluation of the vicinity of MBSD [Mid-Barataria Sediment Diversion] be conducted to attempt to determine the location of geological faults, the recent history of fault movement and the effects of active faults on subsidence rates and variations in the thickness of highly compactible soils.”  This recommendation included evaluation including the use of oil and gas industry seismic data, the acquisition of high resolution seismic data, the acquisition of sediment cores, and the development of subsidence measuring capabilities.  The Draft EIS concluded in response to this recommendation that “there is insufficient information on which to evaluate the impact of faulting on the proposed Project or the impact of the proposed Project on the future fault movement.”  It was the intention of the McLindon et al (2017) recommendation to collect the information necessary to make such an evaluation.

The comments submitted here are in the form of a geological evaluation of the vicinity of the MBSD.  There are two principal objectives to this evaluation:

1.       To provide a framework within which to develop the ability to forecast a probabilistic distribution for the frequency and magnitude of fault slip events in the vicinity of MBSD and other large infrastructure projects on the coastal plain, and

2.       To provide insights into the historical relationships between sediment loading, fault slip events, and fault-induced subsidence that may be incorporated into predictive models for land elevation and land area gain in the MBSD project area.

This evaluation has been constructed with the best available data, and is in no way intended to replace or substitute for the thorough geological evaluation recommended in McLindon et al (2017).  The subsurface geological maps used here represent a compilation of geological interpretations over many decades.  Inputs to the interpretation have included well logs, biostratigraphic data from micropaleontology, seismic data, gravity data, published subsurface geological interpretations from peer-reviewed technical literature and atlases constructed by the New Orleans and Lafayette Geological Societies, and interpretations submitted to the Louisiana Office of Conservation in support of oil and gas unitization

The Ironton fault

The Ironton fault is the most likely geological feature to have a direct impact on the MBSD Project.The fault plane has been mapped in the subsurface, and it is expressed on subsurface structural contour maps as a fault trace delineating the intersection of the fault plane and the mapped stratigraphic horizon.The fault can also be seen in cross sections constructed with oil and gas well logs, as well as 2-D and 3-D seismic data.Gagliano et al (2003) described this fault and other faults and salt domes in the area as being part of a “linked tectonic system”.This implies that individual elements within the system may be impacted by activity on other elements such as episodic fault slip events, diapiric salt movement (halokenesis), or salt dissolution.It also implies that faults within the system are likely to share similar characteristics.There is not adequate data available on the Ironton fault to derive the objective framework for modeling fault slip activity and its impacts, so data from other faults in the system and within the basin will be used here to construct the framework.

This evaluation will consider aspects of the ten faults labeled here to derive a set of characteristics that may be applied to the Ironton fault.  Taken together this set of characteristics can be used to make reasonable estimates of a probabilistic distribution of the frequency and magnitude of episodic slip events on the Ironton fault. The map of surface fault traces shown in Fig.1 is a compilation of numerous individual interpretations including university theses and dissertations and peer-reviewed publications (Akintomide & Dawers (2019), Armstrong et al (2014), Bullock et al (2018), Culpepper et al (2019), Frank (2017), Johnston et al (2017), Levesh et al (2019), McLindon (2017)).  A portion of this map is available in a GIS application on the Louisiana Department of Transportation and Development website. 

A subsurface contour map of the Ironton fault plane is shown in Fig. 2 along with subsurface contours on the top of salt at the Lafitte and Lake Hermitage salt domes.A fault plane map is constructed by integrating the interpretation of biostratigraphy, well log correlation and seismic data.The values in blue in Fig. 2 are the depths at which the fault can be interpreted on a well log by the “missing section” in the log relative to surrounding well logs.A cross section of six well logs across the fault plane is shown in Fig. 3.  Wells 2, 3

and 4 exhibit missing stratigraphic section due to the fault at depths of -2175, -9,195', and -10,512’ respectively.  These values are integrated with those from other wells that intersect the fault plane along with seismic data interpretation to construct the fault plane map.  The tops of the biostratigraphic intervals are shown as colored lines on the cross section.  Each of these interval tops is correlated between the well logs and could be a potential mapping horizon.  Mapped horizons at the top of the Pliocene and top of the Mid-Miocene Epoch are indicated on the cross section.  The subsurface structure maps for these horizons are shown in Figs. 6 and 7, respectively.  The vertical change in elevation across the fault, or fault throw, is shown for 5 horizons.  The throw of the fault is 35’ and 840’ for the top of Pliocene and top of Mid-Miocene horizons respectively.  These values could be measured from the cross section or by comparing the values of the subsurface elevation contours on either side of the fault on the subsurface structure maps.  The throw of the fault continually increases with depth in the classic form of a Gulf Coast “growth fault”.  Increasing throw with depth indicates that the fault has been continually, if episodically, moving throughout the geological timespan measured by biostratigraphic control.

The interval thickness between each horizon is shown for a well on the upthrown, or foot wall side of the fault, and one on the downthrown or hanging wall side.  The ratio of the hanging wall thickness to the footwall thickness for any interval is called the “expansion index”, and it is used gain insight into historical fault movement.  The deepest correlated interval is 485’ thick on the foot wall side and 704’ thick on the hanging wall side.  The expansion index across the fault at this interval is 1.45, or the hanging wall side is 145% thicker than the foot wall side.  The cross section also shows that the Ironton fault has a classic listric shape, which flattens with depth.  The magnitude of the horizontal component of fault displacement increases with depth relative to the vertical component as the fault plane flattens.  This illustrates the significant lateral movement that is associated with fault slip over time.  It is likely that the horizontal vectors of movement measured at CORS stations in Fig. 33 are due to fault slip.

Fig. 4 is a portion of an oil and gas industry 2-D seismic profile across the Ironton fault.  Seismic data can be integrated with subsurface well log interpretations in the construction of the fault plane map and subsurface structure maps within the interval of biostratigraphic control.  It can also be used to map the fault plane and subsurface structure below this interval, allowing for the projection of fault plane contours to depths of up to 20,000 feet.  Oil and gas industry seismic data is acquired and processed to optimize imaging of the subsurface between depths of about 2,000 and 15,000 feet below the surface, where most oil and gas is found.  Seismic profiles such as that shown in Fig. 4 are less than optimal for imaging the near-surface extent of a fault, and interpretations generally have to be extrapolated to the surface.  The acquisition of new high resolution seismic data across the Ironton fault recommended in McLindon et al (2017), and reiterated here, is intended provide the necessary imaging to evaluate the near-surface extent of the fault.

Fig. 5 is a graph of the variation of the expansion index across the Ironton fault through the span of geological time between the Mid-Miocene and Pliocene mapping horizons.  The expansion index is generally above the value of 1 throughout most the time span indicating continual movement of the fault.  As will be discussed in more detail in a subsequent section, the higher values of expansion index on the fault can be related to periods of increased sedimentary loading associated with an active delta system.  Graphs like this can be used to assign a generalized relative quantitative value to the distribution of fault slip events over geologic time.  It is likely that the history of fault movement has consisted of a distribution of individual events over time whose variation in magnitude is similar to other natural phenomenon such as earthquakes, floods and hurricanes.  Unlike tectonic fault movement associated with earthquakes, the movement of listric faults in a passive margin setting is generally aseismic.  The relative distribution of the magnitude of slip events over time may, however, be similar.  In other words, fault slip on these faults may consist primarily of low magnitude slip events that manifest as a creeping motion on the fault, but larger events may occur sporadically over time.  The primary intention of a thorough geological evaluation is to attempt to put ranges on the magnitude and frequency of slip events on the Ironton fault.

The Ironton fault plane is represented as a colored grid overlain on the subsurface structure map for the top of Mid-Miocene mapped horizon in Fig. 6.  The subsurface trace of the fault is the brown polygon on the structure map based on the lateral component of displacement.  Black squares indicate the down-dropped side of the fault.  Well 3 from the cross section falls within the fault trace polygon for the Ironton fault on this horizon meaning that the horizon is “faulted out” of the well.  It can be seen on the cross

section that the Mid-Miocene mapped horizon is not present in the well, as it has been displaced vertically and laterally by the fault.  A comparison of the Mid-Miocene map in Fig. 6 and the Pliocene map in Fig. 7 shows how the trace of the fault on the map reflects the decrease in throw with depth.  Subsurface geological structures generally become simpler and flatter at shallower depths.  At the depth of the Pliocene mapped horizon only the major faults are still active.  These are the same faults that appear to reach the surface.  The 0’-depth contour of the Ironton fault plane is coincident with the surface trace of the fault shown in Fig. 1.  The most significant impacts of a potential future fault slip event should be expected along this trace, and future geological evaluation of the fault should be focused on more fully delineating the near-surface fault plane with high resolution seismic and sediment cores.  Fig.8 also shows the connection of the Ironton fault plane to the Lafitte and Lake Hermitage salt domes.  These are elements of the linked tectonic system described by Gagliano et al (2003).  There is likely to be a genetic relationship between the structural evolution of the fault and the salt domes.  It is probable that halokentic salt movement on the domes has affected the history of movement on the fault.  It is also possible that the dissolution of salt at the domes may have contributed to triggering fault slip events.  Halokenesis and salt dissolution should be considered as potential causes for future slip events on the Ironton fault.

A near-surface expression of the Ironton fault can be seen in a profile of cone penetrometer tests (CPT) taken along the proposed conveyance structure for the MBSD.  Interpreted images of a profile including the same tests were published in the Geotechnical Baseline Report for 30% Design (CPRA, 2014).  These interpreted images indicated the correlation of the top of the Pleistocene interval with an overlying deposit of “Near Shore Gulf” sand shown in orange in Fig. 9.  This sand layer is likely to represent a basal Holocene transgressive unit associated with the first submergence of the Pleistocene surface by rising sea level.

Frazier (1967) interpreted borings taken along the Barataria Waterway, and defined the top of deposits associated with the Bayou Families Delta at between 30 and 40 feet below the surface in this area.  The sandy, silty layer in this depth range on Fig. 9 is likely to be associated with this delta lobe.  These approximate ages may be used to generally approximate rates of fault slip on the Ironton fault, however much more accurate age determinations from sediment cores used in association with high resolution seismic profiles would be necessary to provide estimates accurate enough to be used in predictive modeling. 

Fig. 9 indicates 6 feet of throw on the Ironton fault at the top of the Pleistocene surface.  If the more detailed geological evaluation recommended here supports this interpretation, it may concluded that the 6 feet of throw is consistent with the pattern of throw versus depth seen on the cross section in Fig. 3.  It is likely that the magnitude of this throw is the result of the cumulative effect of multiple fault slip events that have occurred since the deposition of the basal transgressive sand.  Fig. 10 shows the fault plane contours of the Ironton fault crossing the CPT profile at the interpreted location of the fault on the profile.  Oil and gas wells in the area are identified by their Louisiana DNR serial number, and the depth of the Ironton fault is indicated in blue.

Characteristics of other faults in the area

Each of the nine other faults shown in Fig. 1 will be considered here to provide a collective characterization that may be used as a framework within which to develop the ability to forecast a probabilistic distribution for the frequency and magnitude of fault slip events on the Ironton fault.  Each of these faults exhibits some indication of recent fault movement.  Some reveal the potential impacts of faults on infrastructure, and some document differential rates of Holocene sediment accumulation across the fault.

The Vacherie fault (Fig. 11) provides the highest quality data point for the magnitude of a recent individual fault slip event.  The event occurred between April 12th and 15th, 1943, and the magnitude of the vertical displacement was reported by Fisk (1944) to be 8 inches.  There was some reported local ground shaking, but there was no detection of seismic activity at the Loyola University seismometer only 50 miles away.  This appears to have been an aseismic fault slip event.  Fisk used a boring profile across the fault escarpment to measure 3 feet of vertical displacement across the fault at the top of the Pleistocene.  He interpreted this to indicate recurrent movement on the fault.  Fisk also noted “Of particular interest is the fact that faulting occurred at a place along the river where repeated crevassing took place.”

It is probable that the 3 feet of displacement at the top of the Pleistocene is representative of the cumulative total of all of the recurrent fault slip events throughout the Holocene.  If each recurring event produced a displacement of 8 inches, then there could have been three previous events over the 7,000 year span that is likely to be represented by the Holocene sediments.  Some of the displacement could have been taken up by compaction of the poorly consolidated sediments, which may allow for more events of the same magnitude, but it may also be true that larger magnitude events in the past would have been necessary to cause a crevasse of the river.  If the history of movement on the Vacherie fault could be accurately reconstructed, it would likely show a log-normal distribution in the magnitude of the fault slip events.  Subsurface mapping indicates that the fault has been active since at least the early Miocene.  Movement since the deposition of the first Holocene sediments would probably be represented by many small slip events that would have propagated a creeping movement on the fault, and a few large events, perhaps with magnitudes of a foot or more vertical displacement adequate to cause a crevasse of the river. 

The correlation of the fault and the site of the historical crevasses of the river is significant.  The 1943 fault slip event occurred during flood stage on the river.  It is likely that the dilation of near-surface aquifers near the river channel during flood stages may alter the stress fields within the near surface sedimentary layers, and provide a potential trigger mechanism for fault slip events.  The 6 feet of vertical displacement at the top of the Pleistocene seen on the CPT profile in Fig. 9 is twice the value seen on the Vacherie fault.  This may suggest that the Ironton fault has had more and higher magnitude fault slip events during the Holocene.  There is no evidence of historical crevasses of the river at the Ironton fault, but some effort should be made to evaluate the magnitude of an event that would be necessary to cause a crevasse of the river, and the probability that such an event could occur using predictive modeling based on a likely distribution of the magnitude of historical events.  This is particularly true given that differential sedimentary loading across the fault, as intended by the MBSD project, may provide a trigger mechanism for a fault slip event.

The Highway 11 fault (Fig. 12) provides the most tangible evidence for the impact of a fault on infrastructure in the area.  The fault also provides valuable insights because it is the single place in south Louisiana where a fault is crossed by a high resolution seismic line capable of measuring near-surface fault throw on multiple horizons (Lopez et al, 1997).  Vertical displacement on the fault at the surface has also been measured by elevation surveys across the Highway 11 Bridge (Hopkins et al, 2016).

The seismic profile across the fault exhibits high-quality imaging of Pleistocene sedimentary layers to a depth of about 150 feet with a vertical resolution of less than a few feet.  This type of imaging capability is necessary to evaluate faults in the near-surface, and is recommended for a geological evaluation of the MBSD project area.  Lopez et al (1997) provided interpreted correlation of seismic horizons associated with sedimentary layers across the fault.  The fault exhibits increasing throw with depth and thickening of stratigraphic intervals on the hanging wall side up to the surface.  It is likely that this pattern extends to all faults delineated on Fig. 1, and it suggests continual episodic movement on the faults throughout the Quaternary.  If this seismic profile had been combined with high resolution dating of sediment cores, it would have been possible to reconstruct a detailed history of fault movement.  A reconstruction of fault movement from this type of data is likely to reveal averaged movement over intervals of time rather than individual episodic fault slip events, but it would be valuable for putting reasonable ranges on the magnitudes of individual events within a given interval. 

The elevation profiles from Hopkins et al (2016) indicate the magnitude and span of displacement across the fault, and valuable estimates of average rates of fault slip.  The lower of the two elevation profiles in Fig. 12 shows the entire span of the Highway 11 Bridge.  It is important to note that while there is a localized increase in offset immediately adjacent to the fault, the entire hanging wall side of the fault has a lower elevation than the footwall side.  This indicates that subsidence due to cumulative fault slip has affected an area of up to 100 square miles or more.  The average rate of subsidence associated with fault slip is between 1.3 and 3.0 mm year over the time span measured.  This is likely to be the result of cumulative slow slip movement, as there have been no reported episodic events on the fault over the past few decades.  Lopez et al (1997) did however document the occurrence of two small earthquakes associated with the South Point fault near the end of the bridge in 1987.

The St. Rose fault (Fig. 13) also appears to exhibit the impact of faults on infrastructure.  The surface trace of the fault is clearly delineated by a sharp tree line in the cypress swamp.  Trees on the hanging wall side of the fault have mostly died.  The plane of the fault can be mapped in the subsurface with well log correlation and 3-D seismic data interpretation.  One possible explanation for the death of the trees is the migration of saline fluids to the surface along the fault plane.  A similar configuration exists at the Montegut fault (Fig. 16) where Kuecher et al (2001) measured a distinct anomaly in total dissolved solids in soils adjacent to the fault.  Saline fluid migration has also been documented on faults in the Baton Rouge fault system. 

The clearly defined surface trace of the St. Rose fault very closely coincides with two apparent impacts on infrastructure.  An elevation survey on Highway 626 shows a vertical offset on the road bed.  The trace of the fault also coincides with a crack and vertical offset in a T-wall in the St. Rose drainage structure. 

The Gentilly fault (Fig. 14) was identified in a boring profile by Fisk (1944).  The vertical offset at the top of the Pleistocene (blue on the profile) is nearly 20 feet.  As a result, the Holocene sediments, and in particular the organic clay and peat deposits near the surface are thicker on the hanging wall side of the fault than they are on the foot wall side.  A map of peat thickness across the area by Gould and Morgan (1962) shows that

the thickest accumulation of peat in the area is on the hanging wall side of the fault.

Zou et al (2015) used US Coast and Geodetic Survey data from repeat surveys of elevation benchmarks in the area to delineate a high subsidence anomaly that coincides the thick accumulation of peat on the hanging wall side of the Gentilly fault.  This is likely to be a result of a feedback loop between fault slip and sediment accumulation.  Subsidence rates are logically higher across an area of thick peat accumulation because sedimentary layers composed primarily of poorly consolidated organic material are likely to exhibit higher rates of compaction resulting in subsidence expressed at the surface.  A consideration of why the peat layer is thick in the first place would lead to the recognition that peat accumulation is itself a response to subsidence.  The accumulation of peat has both a cause and effect relationship with subsidence in the area.  Marsh and swamp ecosystems on a delta plain maintain elevation through organic growth with some input of mineral sediment from river flooding or tidal flux.  As plants in the ecosystem die and are submerged new plants grow on top of them.  The rate of the accumulation of organic material is controlled by the rate of subsidence.  Areas with the high subsidence rates, such as the hanging walls of faults, tend to have thicker peat accumulations.

The total vertical displacement of 20 feet at the top of the Pleistocene suggests that the Gentilly fault has had higher magnitude and possibly more frequent individual slip events than either the Vacherie or Ironton faults.  Possible ranges for values of a distribution of magnitude and/or frequency of fault slip events can be considered.  If a distinct fault slip event occurred an average of once a century over the 7,000 year span representing the accumulated Holocene sediments, then there would have been 70 individual slip events and the average magnitude of each event would have been about 3.4 inches of vertical displacement.  If distinct slip events occurred once a decade, then there would have been 700 events with an average magnitude of about 1/3 inch.  Given the size distribution patterns of other natural phenomenon like earthquakes, floods and hurricanes, it is more likely that fault slip events have not occurred at evenly spaced intervals of time with equal magnitudes.  It is more likely that the historical distribution the magnitude of slip events on the Gentilly fault exhibits a log-normal pattern, and that high-end magnitudes with less than a 1% chance of occurrence may have values of 2 feet or greater, while low-end magnitudes may be fractions of an inch.  The cumulative effect of all of the individual fault slip events since the beginning of Holocene deposition accounts for the 20 feet of vertical offset at the top of the Pleistocene.

The Central Wetlands Unit, which overlies the thick peat accumulation on the hanging wall side of the Gentilly fault is recognized as a hot spot of wetlands loss.  It is likely that subsidence associated with fault slip and differential compaction of the peat layer across the fault has contributed significantly to the submergence of the wetlands.  The magnitude of the rate of subsidence is evidenced by the submergence of Old Paris Road, which as constructed in the 1930s, and is now 2 feet below the water’s surface.  This is an obvious impact on infrastructure.  It also appears likely that the surface trace of the Gentilly fault can be extrapolated to connect with a fault segment documented by Dokka (2011).  He used a LIDAR digital elevation model to reveal the escarpment of a fault crossing Gentilly Blvd. in eastern New Orleans.  Dokka also documented shear and extension fractures in the streets that coincided with the location of the fault.

The Barataria fault (Fig 15.) is a critical component of the linked tectonic system that includes the Ironton fault.  The fault plane connects the Bay de Chene and Lake Washington salt domes at the western and eastern ends of the fault plane respectively.  It is probable that the domes and the fault have interacted throughout the Cenozoic Era, and that halokenesis and salt dissolution have played a causal role in fault slip events.  Expansion index evaluation across the fault indicates continual episodic movement of the fault throughout the Pleistocene, but there is not adequate data to determine the vertical offset at the top of the Pleistocene. A boring profile from Frazier (1967) crosses the fault near its western edge.  Frazier interpreted the sedimentary layers associated with the Bayou des Families (#7), Bayou Blue (#10) and early Mississippi (#13) deltas.  The vertical trace of the fault has been superimposed on the profile to highlight the offset of the sedimentary layers across the fault.  It is likely that each of the Holocene deltas defined by Frazier interacted with the components of the linked tectonic system, and that the architecture of each delta lobe reflects its interaction with the structural system.  As will be discussed in a subsequent section, there is good evidence that sedimentary loading associated with the Miocene deltas that were active across this area caused accelerated slip on certain faults.  It is probable that the deposition of deltaic sediments in the Holocene also contributed to fault slip events, and fault slip probably played an important role in the submergence of the delta lobes.  The same depth interval associated with the Bayou des Families delta on the boring profile in Fig. 15 can be seen in the CPT profile in Fig. 9, and it appears that this interval has been offset by the Ironton fault. 

The Montegut fault (Fig 16.) is along a trend of faults that extend westward from the Barataria fault that Gagliano et al (2003) called the Golden Meadow fault trend.  The fault is one of several faults that were mapped in the subsurface with 3-D seismic data and extrapolated to surface traces that coincide with lineations in the marsh (Culpepper et al, 2019).  Gagliano et al (2003) used a boring profile to document thicker peat accumulations on the hanging wall side of the fault, and related the fault to a lineation in the marsh surface and an area of dead cypress trees.  Keucher et al (2001) identified a distinct anomaly in total dissolved solids in the soils adjacent to the fault.  It is likely that saline fluid migration along the fault plane is associated with the soil salinity anomaly, and may have contributed to the death of the cypress trees. 

The Bastian Bay fault (Fig. 17) parallels the Empire fault (Fig. 18), and both extend from the eastern flank of the Lake Washington salt dome.  The fault plane has been mapped in the subsurface with well logs and 3-D seismic data, and it can be extrapolated to a surface trace that coincides with a distinct lineation in the marsh.  A profile of well log correlations across the fault shows increasing fault throw with depth throughout the Pleistocene, and a vertical displacement at the top of the Pleistocene can be reasonably estimated at about 40 feet.  Martin (2006) also documented expansion indices across the fault in the same depth range that indicate continual fault movement.  The magnitude of the vertical offset at the top of the Pleistocene also suggests that it may be the result of larger and/or more frequent fault slip events that those previously discussed here.  The nature of one of these events may be revealed in a comparison of the marsh surface across the area in images from ProPublica (2014).  In the period between 1973 and 1975 a large area across the hanging wall of the Bastian Bay fault submerged and converted from mostly saline marsh to open water.  Gagliano et al (2003) estimated that the vertical displacement on the fault at the surface was 3.0 to 3.5 feet, and reported anecdotal evidence from oyster fishermen and camp owners.

This pattern of rapid subsidence across a large area would be consistent with a fault slip event, but it may be possible that the “event” lasted for several weeks or months.  In this type of scenario the entire magnitude of the fault slip would not be contained in a single catastrophic slip event, but it may have distributed over time as a series of smaller slip events or an extended period in which the rate of a continuous slow-slip motion was accelerated.  Much more investigation is needed to understand the potential for this type of fault movement in the delta plain environment, but it should be considered in an evaluation of the Ironton fault.

The Empire fault (fig 18) was named by Gagliano et al (2003).  The same fault has also recently been referred to as the Adams Bay fault, but the original name is used here to avoid confusion.  The Empire fault is parallel to and just north of the Bastian Bay fault.  The Empire fault plane has been mapped in the subsurface with well logs and 3-D seismic data, and it can be extrapolated to one of the most distinct surface escarpments on the delta plain.  Expansion indices across the fault indicate continual movement throughout the Pleistocene (Martin, 2006)  Gagliano et al (2003) constructed an auger boring profile across the fault to illustrate a vertical offset across the fault of 3.0 to 3.5 feet at the surface.  Martin (2006) also quoted anecdotal evidence from Greg Linscombe of the Louisiana Department of Wildlife and Fisheries that the marsh break associated with the fault escarpment formed between 1976 and 1977.  The magnitude and pattern of this fault slip event are similar to that associated with the Bastian Bay fault, but they appear to be separated in time by 3 or 4 years.  The relationship between these two apparent fault slip events may provide insights into the nature of the connection between elements in the linked tectonic system.  It may be possible that the Bastian Bay fault slip event changed the stress fields in the near-surface in a way that affected and contributed to the cause of the Empire fault slip event.  Much more work is needed to understand these relationships, but the possibility for other components of the linked tectonic system to affect the Ironton fault should be considered in the evaluation.

The Magnolia fault (Fig. 19) appears to be linked to the Ironton fault through a connection with the Lake Hermitage salt dome.  The Magnolia faults extends out of the eastern flank of the dome toward the Potash salt dome, but it not clear if it connects directly to the Potash dome from available data coverage.  The plane of the Magnolia fault has been mapped with well logs and 3-D seismic data (Bullock et al, 2018) and the fault plane has been extrapolated to a surface trace which coincides with a fairly distinct lineation in the marsh.  Bullock et al (2018) used a sediment core profile to illustrate the near surface offset of the fault and increased thickness of sedimentary layers on the hanging wall side of the fault.  The upper layer of accumulated peat is thicker on the hanging wall of the fault, and higher average subsidence rates over about the last 1,000 years have also been interpreted by the incorporation of carbon dating of the organic sediments. 

The surface trace of the Magnolia fault crosses the Lake Hermitage Marsh Creation Project.  A reasonable peat thickness map can be constructed from the geotech cores from this project and cores taken by Bullock et al (2018).  Peat thickness is greater on the hanging wall side of the fault, and it is likely that this reflects a cause and effect relationship with fault-induced subsidence.  This configuration also provides valuable insights into the effect of sediment loading across the fault, as emplaced fill from marsh creation project extends across portions of the footwall and hanging wall sides of the fault.  Simoneaux et al (2016) reported that “the magnitude of lateral displacement of these soft organic soils after fill material placement (aka ‘mudwaving’) was grossly underestimated.”  It is not clear if lateral displacement of organic soils was greater on the hanging wall side of the fault, but that would be a reasonable assumption given the evidence of their thickness.  This phenomenon should be further investigated, and some effort should be made to evaluate the potential for the lateral displacement of thick deposits of highly organic soils in the vicinity of the MBSD Project the as a result of mineral sediment deposited by the MBSD Project.  This will be discussed in more detail in a subsequent section.

Subregional geological structure

To meet the objective of providing insights into the historical relationships between sediment loading, fault slip events, and fault-induced subsidence that may be incorporated into predictive models for land elevation and land area gain in the MBSD project area, this evaluation will review the sub-regional geological structure in the vicinity of the project area. It will also examine the interactions between the structural system and four major Miocene delta systems.  Fig. 20 shows an expanded scope of the linked tectonic system around the MBSD project area with a map of fault plane contours in yellow and salt dome contours in green.  The contours are not labeled, but the contour interval is 1,000’ feet, and each of the faults extends to the surface with a 0’-depth contour.  Figs. 21-23 show the extent of the mapped horizons at the Mid-Miocene, Pliocene and Pleistocene respectively.  The Pleistocene map as derived from subsurface depth contours on the maps in the Coastal Plain section of the USACE Engineering Geology Mapping website.  Faults that appear to reach the surface were integrated with the contours, and the contours were adjusted to reflect reasonable amounts of throw at the depth of the Pleistocene.  Four sub-regional profiles are indicated on each map.  These profiles are shown in block diagram form in Figs. 24-27. 

Profile 1 crosses the Lafitte and Bay de Chene salt domes at the western ends of the Ironton and Barataria faults.  The profile is just west of the surface trace of the Ironton fault, and the fault can be seen on the profile extending up from an intersection with the Lafitte salt dome, but not reaching the Pliocene mapped horizon.  The salt domes exhibit a pattern that reflects a history in which they were probably squeezed up from an original Jurassic layer of salt under the weight of late Cretaceous and Paleogene sediments.  Their tilted alignment may also suggest some association with down to the north faults that are related to “toe structures” that are antithetic to the Baton Rouge fault system.

The Lafitte graben is expressed above the top of the salt dome.  “Graben” is derived from the German word for grave, and it refers to the down-dropped area between sets of opposing faults.  Movement on these faults is directly associated with halokenetic movement on the salt dome.  The fact that these faults appear to extend to the surface suggests some degree of geologically recent salt movement.  Profile 1 very closely coincides with the boring profile constructed by Frazier (1967).  The inset in the upper left corner of Fig. 24 shows the boring profile with the interior faults of the Lafitte graben superimposed.  The black layer at the top of the graben indicates accumulated peat within the graben.  This would further suggest recent fault movement and related subsidence at the Lafitte graben.

The profile also shows the dramatically expanded upper Miocene section on the hanging wall of the Barataria fault.  This is reflected in the expansion index graph for the fault in Fig. 31.

Profile 2 crosses near the center of the Ironton and Barataria Bay fault planes.  It clearly shows the listric nature of the faults.  With the ability to seismically image below the 20,000’ depth of the profile, it would be possible to see the deeper extents of these fault planes.  It is likely that the faults would continue to flatten and to eventually merge into a nearly horizontal glide plane or decollment surface, as suggested by the dashed lines.  This probable deep connection between the faults is another aspect of the linked tectonic system that is not presented in the maps and profiles of this evaluation.  Dokka et al (2006) referred to this configuration as the “southeast Louisiana allochthon”, an extensional complex driven by gravity instabilities.  They used GPS data to assign rates of vertical and lateral motion associated with movement on this tectonic system.  (Also see Karegar et al (2015), Fig. 33)

Profile 3 crosses the Magnolia fault and the Lake Washington salt dome.  Lake Washington exhibits a pattern that suggests a strong component of lateral movement through the middle Miocene followed by a period of vertical diapiric movement.  The top of the dome is at a depth of about 2,000’ and it intersects the Pliocene mapped horizon.  There appears to be a genetic relationship between the Lake Washington salt dome and the Magnolia fault.  Expansion on the fault appears to be greatest in the middle Miocene.  The thickness of the upper Miocene increases into the center of structural syncline that is probably related to salt withdrawal in the early stages of vertical diapiric movement on the dome.  The extent of this syncline can be seen in map view on Fig. 21.  It is probable that the downward movement of the syncline due to salt withdrawal would have contributed to movement on the Magnoila fault throughout the upper Miocene and Pliocene.

Throughout the Cenozoic Era the faults moved in response to sedimentary loading.  Some of the smaller faults on the profile that were active in the upper and middle Miocene ceased their movement, and became covered with enough sedimentary overburden to prevent future movement.  The major faults continued to move episodically throughout the span of geological time, which has allowed them to propagate to the surface.  It is likely that, if properly imaged, these faults would exhibit evidence of movement in the recent geologic past and in the historical past over the last few centuries.  It is more probable that these faults are still capable of active movement in the present, than they that they lost that capability sometime in the recent past.

Profile 4 crosses the Delacroix Island fault, the Potash salt dome, the Empire fault and the Bastian Bay fault.  Interval thickening across the fault is greatest in the middle Miocene at Delacroix Island and upper Miocene at Bastian Bay.  The Bastian Bay fault also significantly expands the Pliocene interval.  As will be discussed in the next section, the changes in expansion index for any of these faults can be related to their relationships with the active deltas that prograded across southeast Louisiana in the Miocene.  There is a very clear relationship between sediment loading during active deltaic deposition and the magnitude of fault movement.  Insights from these relationships could be used to formulate a predictive framework to assess the impacts of sediment loading at the MBSD Project.

Fault-Delta Interactions

Curtis (1970) provided detailed delineations of the Miocene delta systems of southeast Louisiana and a generalized conception about their relationships with faults that were active on the continental margin at the time.  The general pattern over the period of several million years was a progressive advancement of the continental shelf edge to the south.  Delta systems tended to cluster into phases, and each phase had a relationship with a different set of faults.  Fig. 28 from Curtis (1970) begins with the advancement of deltas of the Mid-Miocene transgressive phase.  The block diagram on the right shows the engagement of the delta systems with a theoretical fault system in the center of the block.  Mid-Miocene regressive phase deltas fully engaged with this fault system, and deltaic sediments are thicker on the hanging wall side of the fault.  Fault activity is implicitly caused by sedimentary loading from the delta, and fault-induced subsidence creates accommodation space that allows for the accumulation of more sediment.  Deltas in the Late Miocene regressive phase prograded beyond the area of influence of the fault system, and engaged with another fault system further to the south and more capable of accepting the sedimentary load.  Through this process each fault system has an expansion index that reflects its engagement with a delta system of a certain age.

Six faults in the vicinity of the MBSD Project are evaluated here to consider the relationships between sedimentary loading associated with Miocene deltas and fault movement. Each of these faults labeled in Fig. 29 is a major fault that is considered to be a part of a linked tectonic system with other faults and salt domes. Each fault has been active since at least the middle Miocene, and all appear to extend to the surface and show some evidence of activity in the recent past. Fig. 29 represents a generalized map of fault traces and salt domes in the subsurface. It is not intended to represent any particular subsurface stratigraphic horizon, but it is a representation of an approximate location of the fault trace at a depth of about 10,000 feet. The map is a compilation of interpretations from numerous sources. It is primarily intended to provide more detail than fault maps currently available in the technical literature, such as

Wallace (1962).  The exact location of faults in the subsurface should be determined from horizon structure maps, such as those in Figs. 21 and 22.  This generalized representation is accurate enough to be used in comparison to the paleogeographic reconstructions in Figs. 30-32.  Expansion index graphs for each fault were constructed in exactly the same manner as that for the Ironton fault in Fig. 4.  On each graph in the succeeding figures age in million years before present is on the vertical axis and expansion index is on the horizontal axis.

The Delta Farms, Ironton and Delacroix Island faults align with the boundary between the continental shelf and the upper continental slope in middle Miocene between 12 and 13 million years before present.  The time interval is highlighted in yellow on the stratigraphic chart in the upper right and on each of the expansion index graphs.  These shelf edge faults are an integral part of the paleogeographic reconstruction.  The outlines of the active delta systems are derived from Curtis (1970) by relating the transgressive-regressive sequences to global sea level cycles.  Eustatic curves for global sea levels were not established at the time of the publication of Curtis (1970), but the patterns of the transgressive-regressive cycles used at the time can be correlated to the eustatic curves with reasonable accuracy.  A regional interval isopach (thickness) map for the same stratigraphic interval reveals two discrete depositional centers in which deltaic deposits of that age are measurably thicker than in surrounding areas.  A generalized cross section across any of these faults at the time of deposition would be similar to the center block diagram in Fig. 28.

The expansion index graph for each fault reveals maximum expansion on the fault during this time interval.  The faults are most active at the time of depositional loading, and fault-induced subsidence provides the accommodation capacity that keeps the delta engaged with the tectonic system during the span of deposition.  These relationships underscore the observation that faults are as much a part of a delta system as the channels and sand bars.  Fault movement caused by depositional loading should be an expected response in an active delta environment.

Fig. 31 shows the paleogeographic reconstruction for the end of the middle Miocene between 10 and 12 million years before present.  The shelf edge and the active delta complexes have prograded to the south and the active depositional centers have shifted to another set of faults that include the Barataria and Magnolia faults.  The expansion index graphs on these faults indicate maximum fault movement during this time interval.  The graphs in Fig. 30 exhibit expansion indices greater than 1 during this time period This indicates that those faults also interacted with delta systems of this time, but with significantly lower magnitudes of fault activity.

The paleogeographic reconstruction for the upper (or late) Miocene in Fig. 32 shows the major depositional centers positioned along the northern boundary of the Terrebonne Trough.  The Trough is defined by the alinement of down to the south faults along its northern boundary (including the Barataria and Bastian Bay and Montegut faults), and down to the north faults and salt domes along its southern boundary.  The accommodation capacity of this structural system kept the major delta systems engaged throughout the remainder of the Miocene and into the Pliocene.  The magnitude of accumulation of relatively thick and dense sedimentary layers within this basin created a regional density anomaly that is obvious on maps of the Earth’s gravitational field.  The density differential across the area has been a contributing factor to fault movement and subsidence since the end of the Miocene. 

Framework assessments for fault slip and subsidence rate models

The objectives of this evaluation are to provide a framework for predictive models for the magnitude and frequency of fault slip events and the relationships between fault slip, subsidence, elevation and land gain associated with the MBSD Project.  This type of framework can now be considered in the context of the characteristics other faults presented here.  The Ironton fault is a part of a linked tectonic system which allows for comparison with other faults in the system to help derive some preliminary values for the magnitude and frequency of fault slip, the rates of subsidence associated with the fault, and the probable response of loading sediment across the fault.  The principal metric that allows for comparison among the faults is the throw of the fault at the top of the Pleistocene horizon.  This value is estimated to be 6 feet on the Ironton fault based on the interpretation of the Conveyance Structure CPT profile presented here in Fig. 9.  The value of throw at this horizon on other faults in the area established here is 3 feet on the Vacherie fault (Fig. 11), about 20 feet on the Gentilly fault (Fig. 14), and about 40 feet on the Bastian Bay fault (Fig. 17).  Fisk (1944) documented a recent slip event on the Vacherie fault with 8 inches of vertical displacement, and Gagliano provided evidence for a recent slip event on the Bastian Bay fault of 3.0 to 3.5 feet of vertical displacement.

The intention of the recommendations made in McLindon et al (2017) was to collect the data necessary to evaluate the potential for fault slip in the vicinity of the MBSD Project.  In the absence of the data necessary to fully develop a probabilistic model for future fault slip events, the values provided in this evaluation can be used to make some framework estimates.  The expansion index graph on the Ironton fault in Fig. 5 gives some insight into the historical distribution of fault slip events.  If the full historical distribution could be known, it would be very likely to exhibit a log-normal pattern for the magnitude of the events.  This would mean that the distribution would consist of very many small events and very few large events.  This type of size distribution is commonly associated with magnitude of earthquakes, which are the result of fault slip events in tectonic regions.  The 6 feet of vertical offset at the top of Pleistocene on the Ironton fault is the cumulative offset of all slip events that have occurred since the beginning of Holocene deposition in the area.  Frazier (1967) dated the basal transgressive deposit on the top of the Pleistocene in St. Bernard Parish at about 7,000 years before present.  The orange sandy layer on the CPT profile in Fig. 9 is likely to represent a similar basal transgressive unit, and it is reasonable to assume that it has a similar age.  This would mean that 6 feet of vertical offset has occurred over the last 7,000 years.  If fault slip events occurred an average of once every century over that time span, then there would have been 70 discrete events with an average magnitude of about 1 inch of offset.  If the magnitude of the slip events were log-normally distributed, as should be expected, then the distribution would reflect many small events of perhaps a fraction of an inch of offset and a few large events with magnitudes of possibly 1 foot of offset or more.  This type of distribution would be consistent with the implications of the relationships between slip events on the Vacherie fault and crevasses of the river at the site of the fault (Fisk, 1944).  Given that 8 inches of vertical offset in 1943 was not adequate to crevasse the river, it is reasonable to assume that a larger magnitude event would have been required, which would support a high-end value of about 1 foot vertical displacement at Ironton. 

These framework estimates indicate that if a probabilistic distribution for future slip events were developed, it would include a non-zero value for the probability of the occurrence of a slip event on the Ironton fault during the time span of the MBSD project, and a smaller, but still non-zero value for the occurrence of an event with a magnitude of 1 foot of vertical offset.  The acquisition of high resolution seismic data and high resolution dating of sediment cores across the fault should contribute to a significant improvement in the development of probabilistic models, and the recommendations of McLindon et al (2017) are reiterated here. 

Estimates for subsidence rate models for the vicinity of the MBSD Project can be derived from observations about the relationships between subsidence rates, the thickness of the Holocene interval, and the organic content of the Holocene sediments in the areas examined here.  The depth to the top of the Pleistocene (or the thickness of the Holocene) can be estimated at each of the faults examined here.  The organic content of the soils has been estimated by the thickness of accumulated peat at the Gentilly fault, the Montegut fault and the Magnolia fault.  A fairly detailed map of the thickness of accumulated peat and organic clays has been made with data from geotech cores taken for the MBSD Project and the Bayou Dupont Marsh Creation Project (Fig. 35).  Byrnes et al (2019a & 2019b) found a “compelling relationship between subsidence and the age, composition and thickness of Holocene deltaic deposits” in the Breton and Barataria hydrologic basins, which includes the MBSD Project area.  An estimate of total Holocene thickness could be derived from the top of Pleistocene map (Fig. 23), however the Holocene “topstratum” contains the more highly organic and compactable sediments of the Holocene and the topstratum isopach map (Kulp, 2000) is more informative in evaluating subsidence.

Fig. 33 combines Holocene topstratum thickness contours from Kulp (2000) with measurements of horizontal and vertical (subsidence) velocities from Karegar et al (2015) and surface fault traces where blue are down to the south faults and red are down to the north faults.  Land motion velocities are measured at stations in the Continuously Operating Reference System using GPS technology.  The relationship between the values of vertical velocities measured at these stations and the thickness of the Holocene is obvious.  The three stations with subsidence values near 7 mm/yr lie along the axis of the Terrebonne Trough which coincides with the axis of thick Holocene sediments.  Karegar et al (2015) also found that the horizontal vectors of land motion “may reflect slow downslope movement on a series of listric normal faults due to gravitational sliding”.  Some consideration should be given to the potential impacts of horizontal land motion due to movement on the Ironton fault on the MBSD Project. 

A patchwork compilation of interpretations by Gould & Morgan (1962), Kosters (1989) and in this evaluation using geotech cores from the MBSD Project, the Bayou Dupont Marsh Creation Project, and the Lake Hermitage Marsh Creation Project in Fig. 34 provides insight into the probable thickness distribution of highly organic soils across the rest of the area.  There is a general tendency for thicker peats to accumulate off the natural levee flanks of distributary channels and on the hanging wall of faults that appear to extend to the surface.  Keucher et al (2001) indicated that the consolidation of highly organic and clay rich-facies in recent delta deposits is a primary component of subsidence.  Zou et al (2015) measured a subsidence rate anomaly from geodetic data that coincided with the thick peats between the natural levees of the Mississippi River and the Bayou Sauvage distributary channel and on the hanging wall of the Gentilly fault.  Maximum values of subsidence in this area were over 15 mm/yr.  Dixon et al (2006) documented similar values in the same area using InSAR technology.  A closer examination of peat thickness in the vicinity of the MBSD Project in Fig. 35 shows thickness contours constructed from measurements of thickness of the upper layer of organic clay and peat in geotech cores.  The pattern of the contours conforms to the general concept of thick peats accumulating off the natural levee flanks of distributary channels (taken from maps on the USACE Engineering Geology Mapping website) and on the hanging wall of a fault.

Thick peats have accumulated between the distributary channels Cheniere Traverse Bayou to the north Bayou Dupont in the middle and Bayou des Families to the southwest and the hanging walls of the Ironton fault and in the Lafitte graben.  There are no direct measurements of subsidence in these areas.  Predictive modeling of elevation and land gain for the MBSD Project should allow for subsidence rate values as high as 15 mm/yr  above the areas of thick accumulations of organic clay and peat based on the analogy with the Gentilly fault area.  Some qualitative insights into the patterns of subsidence in this area can be gained from the patterns of wetlands loss shown in Fig. 36.  Reconstructions of what the marsh surface looked like in 1956 and 2009 from ProPublica (2014) indicate areas of wetlands loss due to subsidence in relationship to the thick peat accumulations.  The compelling correlation suggests that compaction of the upper layer of organic clays and peats is the primary mechanism of subsidence.  It is probable that the episodic slip events on the faults has contributed to historical subsidence that allowed for the thicker accumulation of peat on the hanging wall of the fault, but the contribution of fault slip to the current subsidence rate is unknown.  Predictive modeling for the MBSD Project should provide for an accelerated rate of compaction of the upper organic soils in response to the deposition of mineral sediment caused by the MBSD Project, which is currently in the models described in the Draft EIS (indicated by the red outline on Fig. 35).  The models should also allow for lateral displacement of the soft organic soils (mudwaving) in response to

sediment accumulation based on analogous configurations of a fault and thick peats at the Lake Hermitage Marsh Creation Project (Fig. 19).

The profile in Fig. 36 is constructed from geotech cores from the MBSD Project and the Bayou Dupont Marsh Creation Project it shows the thickening of the organic clay and peat across the Ironton fault.  The thick accumulation of highly organic soils on the hanging wall is very similar to the pattern at the Gentilly fault.

Morton et al (2009) concluded that the extraction of oil and gas from the Lafitte Field has been the primary cause of subsidence over the last few decades in the vicinity of the MBSD Project.  They provided no discussion of a causal process by which fluid extraction at the field would have induced subsidence, nor did they provide any evidence that the conditions necessary for such a process existed at the field.  Although never stated in Morton et al (2009), the implicit mechanism for extraction-induced subsidence at the field was compaction of the produced reservoirs caused by the change in volume due to the extraction of fluids.  Chan and Zobak (2007) documented that the magnitude of subsidence at the surface due to the extraction of fluids in the subsurface is a function of pressure change in the reservoir at depth.  The reservoirs that they studied in Lapeyrouse Field in Terrebonne Parish had all experienced significant reductions in reservoir pressure due to the extraction of natural gas.  Their calculations of reservoir compaction were based on the reduction in reservoir pressure.  Their models showed that if reservoir compaction did occur, the expression of subsidence at the surface would be in the form of a bowl-shaped depression immediately above the reservoir.

The reservoirs at Lafitte Field are almost entirely oil reservoirs that are portions of very large saline aquifers.  The oil is more buoyant than the saline pore fluid, and so oil accumulations tend to be in reservoirs around anticlinal structures, such as the Lafitte salt dome.  The size of any individual reservoir is very small relative to the size of the entire aquifer.  As oil is extracted from these reservoirs, the natural expansion of the aquifer is allowed by a slight compressibility of the pore fluids due to dissolved gases.  The saline fluids naturally flow into the pore spaces from which oil has been extracted in a process called “water drive”.  There is no significant change in pore pressure, nor is there any meaningful change in the volume of the aquifer.  The submerged marshes in this area are well-removed from the produced reservoirs at Lafitte field.  It is considered to be highly improbable that oil and gas extraction at Lafitte Field contributed to subsidence in the vicinity of the MBSD project.  There is compelling evidence that subsidence in this vicinity is due primarily to the compaction of the upper layer of organic clays and peats.

Reiteration of Recommenations

McLindon et al (2017) recommended:

1. A review the subsurface geology using oil and gas industry 3-D seismic data. This may be accomplished through a collaborative engagement with owners, licensees and interpreters of the 3-D seismic surveys in the area. Such a collaborative engagement may be facilitated with the assistance of the New Orleans Geological Society, the Louisiana Mid-Continent Oil and Gas Association, or the Louisiana Oil and Gas Association.

2. The acquisition of high resolution seismic data in the immediate vicinity of the diversion structure. This should necessarily include land-based acquisition along both banks of the river and marine acquisition in the river channel, as indicated in Figure 6.

3. The acquisition of sediment core profiles across potential faults. The arrangement of these core profiles should be of adequate density to allow for the interpretation of faults by the vertical offset and variations in thickness of the sedimentary layers. The evaluation of core profiles should include detailed stratigraphic analysis and age-dating of the sedimentary layers to allow for estimates of historical subsidence rates and rates of fault movement.

4. The addition of subsidence measurement capabilities similar to those of the Myrtle Grove Superstation at several additional locations in the vicinity of the diversion. These stations should be positioned with advance knowledge of the location of faults in the area to allow for the direct measurement of variations in subsidence velocities across the faults.

5. The integration of detailed variations in subsidence rate and estimates of fault slip rate into predictive subsurface geological models including models for the response to sediment loading associated with diversion operations.

These recommendations are reiterated here and should be enhanced to insure that the acquisition of high resolution seismic data is based on survey design that provides for quality imaging to at least 200 feet and vertical resolution of at most a few feet.  Sediment cores should be subjected optically stimulated luminescence and carbon dating techniques.

It is further recommended here that the Draft EIS should consider:

1. The potential impacts of an episode of fault slip on the MBSD Project infrastructure based on a predictive model for the magnitude and frequency of future episodes

2. The potential for accelerated rates of subsidence due to sedimentary loading associated with the MBSD Project,

3. The potential for an induced fault slip event due to sedimentary loading associated with the MBSD Project.

REFERENCES

Akintomide, A. O. and Dawers, N. H., 2019, Spatial and Temporal Variation of Fault Activity in the Terrebonne Salt Withdrawal Basin, Southeastern Louisiana: Response to Salt Evacuation and Sediment Loading, presentation, AAPG Annual Meeting, San Antonio, Texas.

Armstrong, C., Mohrig, D., Hess, T., George, T., Straub, K.M.,  2014, Influence of growth faults on coastal fluvial systems: Examples from the late Miocene to Recent Mississippi River Delta, Sedimentary Geology, v. 301, p. 120-132

Bullock, J. S., Kulp, M. A., McLindon, C. D., 2018, Evaluation of the Magnolia growth fault, Plaquemines Parish, southeastern Louisiana, poster session, GSA Annual Meeting, Indianapolis, Indiana.

Byrnes, M. R., Britsch, L. D., Burlinghoff, J. L., Johnson, R., Khalil, S., 2019a, Recent Subsidence rates for Barataria Basin, Louisiana, Geo-Marine Letters, 14 p. doi.org/10.1007/s00367-019-00573-3.

Byrnes, M. R., Britsch, L. D., Burlinghoff, J. L., Johnson, R., Khalil, S., 2019b. Determining Recent Subsidence Rates for Breton Sound and Eastern Pontchartrain Basins, Louisiana: Implications for Engineering and Design of Coastal Restoration Projects. Final Report prepared for Louisiana Coastal Protection and Restoration Authority. Contract 4400009020, Task 8, 58 p.

Chan, A.W. & Zoback, M.D., 2007. The role of hydrocarbon production on land subsidence and fault reactivation in the Louisiana coastal zone. Journal of Coastal Research, 23(3), 771–786

Coastal Protection and Restoration Authority (CPRA), 2014a, Mid-Barataria Sediment Diversion - Final Draft Executive Summary Report 30% Basis of Design, State of Louisiana.

Culpepper, D.B., McDade, E. C., Dawers, N. H., Kulp, M. A., Zhang, R., 2019, Synthesis of Fault Traces in SE Louisiana Relative to Infrastructure,  TranSET Project No. 17GTLSU12

Curtis, D. M., 1970, Miocene Deltaic Sedimentation, Louisiana Gulf Coast, S.E.M.P. Special Publication 15, Deltaic Sedimentation, Modern and Ancient, p. 293-308

Dixon, T.H., Amelung, F., Ferretti, A., Novali, F., Rocca, F., Dokka, R., Sella, G., Sang-Wan, K., Wdowinski, S., Whitman, D., 2006, Subsidence and flooding in New Orleans, Nature, v. 441, p. 587-588

Dokka, R. K., G. Sella, and T. H. Dixon, 2006, Tectonic control of subsidence and southward displacement of southeast Louisiana with respect to stable North America, Geophys. Res. Lett., 33, L23308, 5 p.

Dokka, R.K., 2011, The role of deep processes in late 20th century subsidence of New Orleans and coastal areas of southern Louisiana and Mississippi, Journal of Geophysical Research, v. 16, 25 pgs.

Fisk, H.N., 1944, Geological Investigation of the Alluvial Valley of the Lower Mississippi River, Mississippi River Commission.

Frank, J. P., 2017, Evidence of fault movement during the Holocene in Southern Louisiana: integrating 3-D seismic data with shallow high resolution seismic data, MS Thesis, University of New Orleans, 91 p.

Frazier, D.E., 1967, Recent deltaic deposits of the Mississippi River: their development and chronology, Trans. G.C.A.G.S., v. 17, p. 287‐315

Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., 2003. Active Geological Faults and Land Change in Southeastern Louisiana. Prepared for U.S. Army Corps of Engineers, New Orleans District, Contract No. DACW 29-00-C-0034.

Gould, H. R. and Morgan, J.P., 1962 Coastal Swamps and Marshes, Geology of the Gulf Coast and Central Texas, p 287-341

Hopkins, M., Lopez, J., Songy, A. 2018, Subsidence rates from faulting determined by real-time kinematic (RTK) elevation surveys of bridges in Lake Pontchartrain,  presentation, State of the Coast Conference 2018, New Orleans, Louisiana.

Johnston, A., Zhang, R., Gottardi, R., Dawers, N. H., 2017, Investigating the relationships between tectonics and land loss near Golden Meadow, Louisiana by utilizing 3-D seismic and well log data, poster session, GSA Annual Meeting, Seattle, Washington.

Karegar, M.A., Dixon, T.H., Malservisi, R., 2015, A three-dimensional surface velocity field for the Mississippi River Delta: Implications for coastal restoration and flood potential, Geology, v. 43, p. 519-522

Kosters, E. C., 1989, Organic-Clastic Facies Relationships and Chronostratigraphy of the Barataria Interlobe Basin, Mississippi Delta Plain, Journal of Sedimentary Petrology, v. 59, no. 1, p. 98-113

Kuecher, G.J., Roberts, H.H., Thompson, M.D. and Matthews, L., 2001, Evidence for Active Growth Faulting in the Terrebonne Delta Plain, South Louisiana: Implications for Wetland Loss and the Vertical Migration of Petroleum., Environmental Geosciences, v. 8, p. 77‐94

Kulp M. 2000. Holocene stratigraphy, history, and subsidence of the Mississippi River delta region, north-central Gulf of Mexico. PhD thesis. Univ. Kentucky, Lexington. 283 pp.

Levesh, J. L,, Kulp, M. A., McLindon, C. D., 2019, Fault-slip history of the Delacroix Island fault system and its effect on Holocene salt marshes of the Mississippi River delta plain,  presentation, GSA Annual Meeting, Charleston, South Carolina

Lopez, J., Penland, S., Williams, J., 1997, Confirmation of Active Geologic Faults in Lake Pontchartrain in Southeast Louisiana, GCAGS Transactions, v. 47, p. 299-303

Martin, E., 2006, Fault Induced Subsidence Near Empire and Bastian Bay, Louisiana, MS Thesis, Tulane University, 163 p.

McLindon, C.D., 2017, History of fault slip and interaction with deltaic deposition from the middle Miocene to the Present – Barataria fault, coastal Louisiana, poster session, American Geophysical Union, annual meeting, New Orleans, Louisiana.

McLindon, C.D., Dawers, N.A., Culpepper, D., Kulp, M.A., and McDade, E., 2017, Comments to the U.S. Army Corps of Engineers, New Orleans Division in reference to the The Environmental Impact Statement for the Mid-Barataria Sediment Diversion.  11 pg.

Morton, R.A., Bernier, J.C., and Kelso, K.W., 2009, Recent subsidence and erosion at diverse wetland sites in the southeastern Mississippi delta plain: U.S. Geological Survey Open-File Report, 2009–1158, 39 p., plus app. (p. 41-221).

ProPublica, 2014, Losing Ground

Simoneaux, R., Richard, B., Cantu, K., and Roy K., 2016, The Impacts of In-Situ Soil Displacements on Settlement and Consolidation of Marsh Creation Fill – Lake Hermitage Marsh Creation, Abstract – State of the Coast Conference, New Orleans, LA

Wallace, W.E., 1962, Fault Map of South Louisiana (Scale 1:500,000), Gulf Coast Association of Geological Societies Transactions Vol. XII, p. 194.

Zou, L., Kent, J., Lam, N. S.-N., Cai, H., Qjang, Y., and Li, K., 2016, Evaluating Land Subsidence Rates and Their Implications for Land Loss in the Lower Mississippi River Basin, Water, v. 8, 15 p.

The Gentilly Fault and Subsidence in the Central Wetlands Unit

The most tangible evidence of subsidence in the Central Wetlands Unit near Chalmette, Louisiana is the elevation of the roadbed of Old Paris Road. WVUE News anchor John Snell stood on the roadbed with the City of New Orleans in the background for one installment of his “Coast In Crisis” series. Old Paris Road was built in the 1930s. It is now nearly two feet under water.

Independent studies of subsidence using different measuring techniques found subsidence rates between 15 and 20 millimeters per year (about 5 to 8 inches per decade) in the area of the Central Wetlands Unit, making it one of the “hot spots” of local subsidence on the Louisiana coast. Zou et al, 2015 used data from a US Coast and Geodetic Survey project to resurvey elevation benchmarks to derive rates of change in elevation. Dixon et al, 2006 used InSAR satellite technology, which measures changes in elevation by reflecting radar off of hard surfaces. It is nearly impossible to measure changes on the marsh surface with either technology, but the buildings along New Paris Road provided enough of a reflective surface to provide measurements.

LSU Geologist Dr. Harold Fisk used boring profiles to evaluate the vicinity of the Central Wetlands Unit in his 1944 publication “Geological Investigation of the Alluvial Valley of the Lower Mississippi River” for the Corps of Engineers. In one of the profiles seen here Fisk identified a fault offsetting the layers of accumulated sediment. The top of the Pleistocene surface, which underlies the entire region, and is used as a piling anchor for nearly all construction, was offset by about 20 feet by the fault. The layer of humus composed of peats and organic clays was also measurably thicker on the downthrown or hanging wall side of the fault. This fault has since been named the Gentilly fault, and it appears to play an important role in the subsidence hot spot at the Central Wetlands Unit.

Fisk later produced a map of the thickness of accumulated peat in the subsurface around New Orleans. A comparison of the peat thickness map with the Dixon subsidence map shows a significant correlation. The peat layer beneath the Central Wetland Unit and along the downthrown side of the Gentilly fault is the thickest and largest in the area. The common wisdom is that the thick peat layers are more compactable, and therefore cause the higher rates of subsidence. A consideration of why the peat layers are thicker is some areas than others suggests both a cause and effect relationship between peat thickness and subsidence. Some areas are subsiding at higher rates because the underlying peats are thicker, but the peats are also thicker because the area is subsiding. Subsidence rates have been accelerated even further within the levee system around New Orleans as canals and drainage pumps have dried out and oxidied the peat layers causing them to compact even faster.

Gould and Morgan, 1962 provided another set of profiles across the area revealing more about the patterns of peat accumulation. The marshes between the distributary channels of the Mississippi delta system maintain their elevation by organic growth with periodic minor influxes of mineral sediment from river flooding and tidal flux. In areas where subsidence rates are higher the rate of organic growth must also be higher to maintain elevation. The marsh literally builds upon itself as the plant material is submerged below the surface.

The relationships between the Gentilly fault, subsidence, and the thick accumulation of peat on the downthrown side of the fault is a good example of the feedback loop between faults and accumulating sediments. Subsidence events may be caused by the episodic tectonic movement of faults. The magnitude and frequency of these episodic slip events is poorly understood in coastal Louisiana. Larger fault slip events may occur once every few centuries and may cause vertical offsets at the surface of a few inches to a few feet. Faults may also move in very small incremental slip events that are closer to a creeping movement. The vertical movement on the Gentilly fault is evidenced by the offset at the top of the Pleistocene and the thicker accumulation of sediment on the downthrown side. The total vertical displacement on the fault since the end of the Pleistocene is likely to be the result of a combination of larger episodic slip events and cumulative slow creep movement. Downward movement of the hanging wall side of the fault relative to the foot side creates accommodation space that allows for the accumulation of more sediment on one side than the other. This differential has two effects. The greater thickness of compactable organic material on the hanging will create a higher rate of subsidence due to compaction. The differential in subsidence rates across the fault may be taken up as induced fault slip that is exclusive to the near-surface interval, and is not associated with deeper fault slip. The accumulation of greater thicknesses on the hanging wall also means that the there is a weight differential across the fault, which would tend to cause a fault slip response. These processes combine to create a loop in which the generation of accommodation space allows for more sediment accumulation on one side of the fault, which propagates fault movement.

The plane of the Gentilly fault can be mapped in the subsurface using standard oil and gas industry techniques. The surface trace of the fault coincides with the 0-depth contour of the fault plane map. The two profiles across the area from Fisk and Gould and Morgan are shown in relative perspective. What appears to be a westward extension of the Gentilly fault was also mapped by Dokka, 2011 using a LIDAR digital elevation model. The offset of the fault can be seen as a change in elevation at the surface. Dokka also related the surface expression of the fault to cracks in the roadway.

Understanding the relationships between fault movement and subsidence is critical to the long term sustainability of the Louisiana coastal zone. Much more scientific investigation is needed.

REFERENCES

Dixon, T.H., Amelung, F., Ferretti, A., Novali, F., Rocca, F., Dokka, R., Sella, G., Sang-Wan, K., Wdowinski, S., Whitman, D., 2006, Subsidence and flooding in New Orleans, Nature, v. 441, p. 587-588

Dokka, R.K., 2011, The role of deep processes in late 20th century subsidence of New Orleans and coastal areas of southern Louisiana and Mississippi, Journal of Geophysical Research, v. 16, 25 pgs.

Fisk, H.N., 1944, Geological Investigation of the Alluvial Valley of the Lower Mississippi River, Mississippi River Commission.

Gould, H. R. and Morgan, J.P., 1962 Coastal Swamps and Marshes, Geology of the Gulf Coast and Central Texas, p 287-341

Zou, L., Kent, J., Lam, N. S.-N., Cai, H., Qjang, Y., and Li, K., 2016, Evaluating Land Subsidence Rates and Their Implications for Land Loss in the Lower Mississippi River Basin, Water, v. 8, 15 p.

Interactions of the Bayou des Families delta and the Barataria linked tectonic system

The Bayou des Families delta lobe is the seventh of sixteen lobes delineated by Exxon research geologist David Frazier in 1967.  This lobe was a part of the St. Bernard Delta Complex, and was active between 3,300 and 2,000 years ago.  The delta lobe probably got its start as a crevasse of the west bank of the main river channel near the location of the modern city of New Orleans.  The flow of water through the crevasse likely increased over time as the new opening provided a gradient advantage, or a shorter route to sea level than the previously active delta lobe, Bayou Terrebonne (Frazier #6).

As the channel progressed southward toward the Gulf of Mexico, it interacted with elements of a linked tectonic system of faults and salt domes.

The Bayou des Families delta lobe can be divided into three segments. The northern segment extends from the original crevasse of the main channel to the Lafitte salt dome and graben system. The channel appears to have interacted with the dome and graben in a pattern that suggests a terminal delta at this location for some period of time. The middle segment runs in a straight line along the hanging wall of the Lake Five fault and adjacent to the axis of the tilted Bay de Chene salt dome. The southern segment is the fully developed terminal delta on the hanging wall of the Barataria fault and in the basin between the Bay de Chene and Lake Washington salt domes. All of these structural elements are portions of a tectonic system, and they are linked to each other in the subsurface.

Annual flooding over the centuries in which the Bayou des Families delta lobe was active left behind sand-rich natural levee deposits along the banks of the channel. Finer silts and clays were carried further away from the channel during floods, providing sediment to the freshwater marshes. The elevation of the natural levees may have been 20 feet above sea level when the channel was active. They have subsided over time, but still maintain elevations of 8 to 10 feet above sea level, as seen on the LIDAR digital elevation model. The freshwater marshes maintained an elevation of one or two feet above sea level throughout the history of the delta lobe by a delicate balance of subsidence and sediment accretion. The accumulation of organic material and fine mineral sediment left behind accumulations of peat that are up to 20 feet thick. One of the unintended consequences of the flood protection levees on the west bank of the modern Mississippi River can be seen on the LIDAR model. The organic peats inside the levees have been exposed to oxidation and desiccation by canals and pump stations. The levees also obstruct the natural sediment accretion that continues to feed marshes outside the levee during high tides and tropical storms. As a result, surface elevations inside the levee system are several feet lower than those outside of the levees.

JuxtaposeJS Embed

The Louisiana Geological Survey measured the thickness of peat accumulations in the Barataria Basin (Kosters, 1989). Thick accumulations are found on the flanks of the natural levees.

The Bayou des Families channel appears to have interacted with the Ironton fault and the Lafitte salt dome and graben in a manner that suggests a terminal delta. The finger-like splay of Bayou Dupont and Cheniere Traverse Bayou may be vestiges of distributary channels of a former delta. If there were a terminal delta here for some time, it was eventually by-passed as the main channel extended along the hanging wall of the Lake Five fault toward the Gulf.

The word “graben” is derived from the German word for grave. It refers to the down-dropped central portion between two faults that face each other. The seismic profile (Green, 1998) shows the leaning salt mass of the Lafitte dome and the opposing red and yellow faults that define the graben. The vertical yellow lines represent wells in the Lafitte oil field. The faults can be seen to extend to the surface, and their surface traces are represented on the map.

The primary structural elements of the southern segment of the Bayou des Families delta lobe are the Barataria fault and the Lake Washington and Bay de Chene salt domes. They can be seen here in a subsurface 3-D model. The terminal delta of the lobe slays out across the hanging wall of the fault between the two domes between about 800 and 300 BC. The delta lobe was abandoned at this point when the river changed course to the St. Bernard delta lobe. The Bayou des Families delta retreated back to the a marsh edge coincident with the Barataria fault over the next 300 years.

Remnants of the terminal delta were imaged using high resolution seismic data , including an incised channel (Flocks et al, 2006), which can be seen on the video.

The trace of the Barataria fault can be superimposed on images from Frazier, 1967 and Flocks et al, 2006 to suggest the interaction of the fault and the delta. Delta deposits (#7) can be seen to be vertically displaced across the fault on the Frazier profile, and are now 20 feet below the surface on the hanging wall side of the fault. The Flocks et al diagram shows the delta building out across the fault and then retreating to a marsh edge coincident with the fault after abandonment.

If the marsh edge was coincident with the surface trace of the Barataria fault shortly after abandonment about 2,000 years ago, it is no longer. Just to the east the edge of the marsh platform is almost perfectly coincident with the surface trace of the Adams Bay fault. Martin, 2006 reported that this edge was formed between 1976 and 1977 “… according to Greg Linsombe, a biologist for the Louisiana Department of Wildlife and Fisheries who was flying low transects over the area during this time for the purposes of making counts of migratory waterfowl”. There is a general lineation along the current edge of the marsh platform that roughly parallels the Barataria fault trace. It may be possible that if this lineation is the result of the progression of edge erosion over 2,000 years since the edge and the trace were coincident. This may provide some insight into the relationships between edge erosion and sediment accretion on the marsh platform.

Wilson and Allison, 2008 provided a graphic representation of the relationships between edge erosion and accretion on the marsh platform. Vertical accretion caused by re-deposition of sediment eroded from the marsh edge allows the platform to maintain elevation in the face of subsidence.

Rates of erosion along the marsh edge north of the surface trace of the Barataria fault were measured by Sapkota and White, 2019. Subsidence rates were estimated from the USGS Tide Gauge in Barataria Bay by Byrnes et al, 2019. Rates of sediment accretion and surface elevation change are reported on the Coastwide Reference Montioring System website. As Wilson and Allison suggest, the volume of sediment provided by edge erosion allows for vertical accretion rates that are generally greater than the rate of subsidence, and in several cases greater than the rate of subsidence plus sea level rise.

Sediment redistribution on fault-bounded marsh platforms may provide insights into effective coastal sustainability projects.

REFERENCES

Byrnes, M. R., Britsch, L. D., Burlinghoff, J. L., Johnson, R., Khalil, S., 2019, Recent Subsidence rates for Barataria Basin, Louisiana, Geo-Marine Letters, 14 p. doi.org/10.1007/s00367-019-00573-3.

Flocks, J.G., Ferina, N.F., Dreher, C., Kindinger, J.L., Fitzgerald, D.M, Kulp, M.A., 2006, High-Resolution Stratigraphy of a Mississippi Subdelta-Lobe Progradation in the Barataria Bight, North-Central Gulf of Mexico, Journal of Sedimentary Research, v. 76, p. 429-443

Frazier, D.E., 1967, Recent deltaic deposits of the Mississippi River: their development and chronology, Trans. G.C.A.G.S., v. 17, p. 287‐315

Green, A.L., 1998, Successful Utilization of 3D Seismic at Lafitte Field Southeast Louisiana, GCAGS Special Publication - 3-D Seismic Case Histories from the Gulf Coast Basin, p 159-171

Kosters, E. C., 1989, Organic-Clastic Facies Relationships and Chronostratigraphy of the Barataria Interlobe Basin, Mississippi Delta Plain, Journal of Sedimentary Petrology, v. 59, no. 1, p. 98-113

Martin, E., 2006, Fault Induced Subsidence Near Empire and Bastian Bay, Louisiana, MS Thesis, Tulane University, 163 p.

McLindon, C.D., 2017, History of fault slip and interaction with deltaic deposition from the middle Miocene to the Present – Barataria fault, coastal Louisiana, poster session, American Geophysical Union, annual meeting, New Orleans, Louisiana.

Sapkota, Y. & White, J.R., 2019, Marsh edge erosion and associated carbon dynamics in coastal Louisiana” A proxy for future wetland-dominated coastlines world-wide, Estuarine, Coastal and Shelf Science, v. 226, 9 p.

Wilson, C.A., & Allison, M.A., 2008 An equilibrium profile model for retreating marsh shorelines in southeast Louisiana, Estuarine, Coastal and Shelf Science, v. 80, p. 483-494

The Montegut fault - A conduit for fluid migration

The sedimentary layers underlying south Louisiana are saturated with fluid, and those fluids are in constant motion.  Saltwater makes up the overwhelming majority of subsurface fluids.  Most sediment deposition takes place in shallow seas along the delta coastline.  Sediments have their maximum fluid content at the time of deposition.  Flocculated clays can be up to 80% water and sands can have porosities approaching 40%.  Compaction begins immediately after a sedimentary layer is buried by a younger layer.  The weight of the overburden squeezes the underlying sediment.  This increases the grain-to-grain contact of the mineral matrix, making the pore spaces smaller and forcing the pore fluid out.  Compaction progresses with depth of burial as long as fluids can be expelled.  In deep marine (or abyssal) sediments sands are thinner and less continuous.  The free flow of fluids is inhibited in these sedimentary layers.  The inability to expel pore fluids prevents continued compaction, and the fluids become highly pressured (or geopressured). 

William Galloway of UT Austin and the Texas Bureau of Economic Geology defined three major regimes of subsurface fluid flow.  The abyssal or thermobaric regime consists of highly pressured marine sediments.  Hot, high pressured fluids are forced out primarily along faults.  The movement of pressured fluids along fault planes is an integral part of the mechanics of fault slip.  The elisian or compactional regime lies above the top of geopressure.  Interstitial fluids expelled from within the sediments in this regime follow natural pathways of fluid flow.  These are usually through the porous and permeable sand layers, but fluids can also move vertically up faults.  The fluids moving up from the abyssal regime may also enter the pathways in the elisian regime.  These fluids are not only hot and pressured, they may be hypersaline and carrying oil and natural gas cooked out of organic-rich source rocks.  Most of the oil and gas produced in south Louisiana comes from accumulations in sand layers in the elisian regime just above the top of geopressure.  The shallowest sedimentary layers may be exposed at the surface further inland.  These layers make up the meteoric or infiltration regime.  The saltwater in the pores of sands in this regime has been flushed out by rainwater that moves by artesian flow through the aquifers. 

Many faults in south Louisiana extend into the meteoric regime.  Segments of the Baton Rouge fault system commonly form boundaries between freshwater and saltwater aquifers.  Hypersaline fluids can also migrate from the faults into the freshwater aquifers.  Ronald Stoessell and Lesley Prochaska at the University of New Orleans studied brackish water found in wells in St. Tammany Parish.  They determined that the brackish water was “derived from deep migrating formation fluids from dissolved halite migrated up fault planes”.   Gerald Kuecher and Harry Roberts at LSU studied faults in Terrebonne Parish.  They also found evidence for the migration of saline fluids up those fault planes.  Their diagram of fluid flow up a fault plane into a reservoir sand illustrates the fractured nature of a fault plane.  It is often envisioned that faults create a zone of rubble along the plane that is both porous and permeable.  Mark Rowan’s block diagram shows how faults are a part of a linked tectonic system.  Along with salt (in red) and salt welds, which are the remnant pathways of salt movement, faults provide essential conduits for fluid flow.  Arrows of fluid movement are overlain on Rowan’s original diagram.

The Montegut fault is in the area studied by Kuecher and Roberts.  The fault plane is displayed as a colored surface extending from a depth of 15,000 feet along the dark blue band up to the visible trace of the fault at the surface.  The Montegut fault is a part of the linked tectonic system across coastal Louisiana called the Terrebonne Trough.  Faults along the northern boundary of the trough are dipping to the south.  Faults along the southern boundary are dipping to the north.  Sedimentary layers are thicker within the bounds of the trough.  The surface expression on most of these faults is visible on the USGS Land Area Change Map (Couvillion et al, 2017).  Fault traces and the distributary channel network have been overlain on the original map.  Hot spots of land loss shown in red and orange tend to lie along the downthrown sides of the faults at the points where the distributary channel networks branch out.  Each of these branching channel networks represents the architecture of one of the Holocene deltas of the Mississippi River.  It appears that the Mississippi River moved back and forth across the Terrebonne trough constantly seeking out maximum accommodation capacity for its sedimentary load.  The Montegut fault was an integral part of the Bayou Terrebonne delta system, which was active between about 800 and 500 years ago.

A three-dimensional diagram shows the contours of the fault plane dipping to the south.  The surface trace of the fault crosses the branching network of the former distributary channels of the delta system.  The natural levees of these distributary channels provide the elevation necessary for human inhabitation in this area.

At a depth of about 10,000 feet below the surface the Montegut fault intersects a  Miocene sand layer.  The New Orleans Geological Society (NOGS) identified this sand as the “Big 2 Sand” for its association with a microscopic foraminifera called Bigerina 2.  Foraminfera found in sediments and recovered from the cuttings of a drilling well are the most common method for age-dating Miocene sediments.  The NOGS subsurface structure map on the Big 2 Sand shows the concentric contours of the anticlinal structure of Lirette Field in Terrebonne Parish.  The northern edge of the anticline is crossed by the trace of the Montegut fault.  Gas reservoirs accumulated in the Big 2 sand across the top of the anticline are shown in red.  A study of the subsurface controls on historical subsidence rates by the USGS (Morton et al, 2002) noted that the discovery of Lirette field was aided by the detection of gas seeps at the surface.  The study found that “The seeps indicate that before commercial development began, fluids were migrating vertically along deep fault planes that intersect the subsurface gas reservoirs”.

Lirette Field was studied by Leigh Anne Flournoy and Ray Ferrell at LSU.  Several whole cores of reservoir sands taking during drilling operations allowed them to do detailed examination of the mineral matrix of the sand layers.  They determined that a sequence of diagenetic events had occurred over time in association with fluid movement through the sands.  These included both the deposition of minerals that were precipitated from fluids and the dissolution of other minerals by fluid flow.  Flournoy and Ferrell identified likely routes of fluid movement from the faults into the reservoir sand layers. A profile across the Montegut fault shows the patterns of fluid movement.  The movement of hotter fluids migrating up the fault plane into the sands is also evidenced by the displacement of the 200° F isotherm along the fault.

The surface trace of the Montegut fault was documented in the study “Synthesis of Fault Traces in SE Louisiana Relative to Infrastructure” (Culpepper et al, 2019).  The surface trace determined by mapping the fault with 3-D seismic data coincided with the surface expression of the fault determined by shallow borings by Gagliano et al, 2003.  A profile of borings across the fault shows thicker accumulations of peat on the downthrown side of the fault relative to the upthrown side.  The accumulation of plant material that makes up a peat layer is evidence of the marsh’s attempt to keep up with subsidence by organic growth.  This profile is evidence of subsidence caused by slow slip movement on the Montegut fault over the centuries since the Terrebonne Bayou delta system was abandoned.  The elevation change across the fault at the surface causes a sharp transition from emergent marsh to open water.  This coincides with the area of recent land loss on the USGS Land Area Change Map, which may evidence a recent fault slip event.

Kuecher and Roberts examined soil salinities across southeastern Louisiana.  A colored recreation of their map shows a distinct anomaly in soil salinity (measured as total dissolved solids in parts per thousand) in the vicinity of the Monetgut fault.  They determined that this anomaly could not be explained by traditional means of saltwater encroachment.  A photo from Gagliano et al, 2003 shows the proximity of an area of dead cypress trees to the Montegut fault.  It may be possible that the increased soil salinities along the trend of the fault may have played a role in the death of the trees.

The Montegut fault exhibits evidence of continual episodic movement since the middle Miocene.  There is also evidence of fluid migration associated with fault movement from that time until the present.  Understanding the relationships between fault movement, fluid migration, subsidence and wetlands loss will be essential to the ultimate success of long-term sustainability planning in coastal Louisiana.

REFERENCES

Couvillion, B.R., Beck, H., Schoolmaster, D., and Fischer, M., 2017, Land area change in coastal Louisiana 1932 to 2016, U.S. Geological Survey Scientific Investigations Map 3381, 16 p. pamphlet, doi.org/10.3133/sim3381.

Culpepper, D.B., McDade, E. C., Dawers, N. H., Kulp, M. A., Zhang, R., 2019, Synthesis of Fault Traces in SE Louisiana Relative to Infrastructure,  TranSET Project No. 17GTLSU12

Flournoy, A.L. and Ferrell, R.E., 1980, Geopressure and diagenetic modifications of porosity in the Lirette Field area, Terrebonne Parish, Louisiana, GCAGS Transactions, v 30, p 341-344

Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., 2003. Active Geological Faults and Land Change in Southeastern Louisiana. Prepared for U.S. Army Corps of Engineers, New Orleans District, Contract No. DACW 29-00-C-0034.

Galloway, W. E., 1982, Epigenetic Zonation and Fluid Flow History of Uranium-Bearing Fluvial Aquifer Systems, South Texas Uranium Province: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 119, 31 p.

Kuecher, G.J., Roberts, H.H., Thompson, M.D. and Matthews, L., 2001, Evidence for Active Growth Faulting in the Terrebonne Delta Plain, South Louisiana: Implications for Wetland Loss and the Vertical Migration of Petroleum., Environmental Geosciences, v. 8, p. 77‐94

Morton, R.A., Buster, N.A., Krohn, M.D., 2002, Subsurface Controls on Historical Subsidence Rates and Associated Wetland Loss in Southcentral Louisiana, GCAGS Transcations, v. 52, p. 767-778

New Orleans Geological Society, Oil and Gas Fields of Southeast Louisiana Volume 1, 1963

Stoessell, R. K. & Prochaska, L., 2005, Chemical Evidence for Migration of Deep Formation Fluids into Shallow Aquifers in South Louisiana, GCAGS Transactions, v. 54, p. 794-808

St. Rose Fault

The St. Rose fault offers one of the best examples of surface expression not associated with a significant elevation change in southeast Louisiana.  The surface traces of the Baton Rouge fault system are easily imaged with lidar digital elevation models.  The surface escarpments, which may exhibit 10 to 15 feet of elevation change show up as sharp lineations on lidar (Shen et al, 2017).  The St. Rose fault does not create an elevation escarpment, but it does create a sharp lineation that is visible on satellite imagery.  The tree line in the swamps of St. Charles Parish clearly shows the fault.  More investigation is needed to determine exactly why that is.

The St. Rose fault is one of network of surface fault traces that have been mapped with oil and gas industry seismic and subsurface data in preparation for the construction of a fault atlas.  The fault traces to the north have been published in peer-reviewed technical journals.  The fault traces to the south have been published in university research projects and assembled on the LA Dept of Transportation website.  The publication of the results of these research projects in peer-reviewed journals is anticipated in the coming months. 

A profile across southeast Louisiana shows the complex interrelationships between faults, salt domes and the sedimentary layers that have accumulated to thickness of up to 40,000 feet over the last 50 million years.  The St. Rose fault is at the northern boundary of the thick Miocene accumulations that dominate the subsurface of southeast Louisiana.  Movement on the faults has continually interacted with ductile, low density salt, which has been squeezed up from a layer originally deposited during the Jurassic.  The ancestral deltas of the Mississippi River continually delivered their sedimentary load to the geological basins formed by the faults and salt.  Loading of delta sediments in these basins certainly triggered episodes of fault movement.  The faults also acted as conduits for fluid migration as the sediments compacted and expelled pore waters.  The organic-rich shales of the Cretaceous also expelled oil and gas, which migrated up the faults and into reservoirs in the deltaic sands of the Miocene.

Evidence for recent movement can be found on other faults in the vicinity of St. Rose.  The Vacherie fault ruptured the surface of the Earth in an episode of fault slip in 1943.  A high-resolution seismic profile shot by the USGS in Lake Pontchartrain shows the recent offset of Pleistocene sediments on the Frenier fault.  This type of high-resolution seismic data is essential to accurately mapping faults and determining patterns of recent movement, but it is extremely rare.

The smooth planar surface of the St. Rose fault is exhibited by a map of subsurface depth contours.  The color code of the fault plane grades from cool to warm based on depth.  The contour lines indicate the depth of the fault below the surface in 1,000-foot increments.  The 0-depth contour is coincident with the surface trace of the fault plane, and the tree line in the swamp.

The fault roughly parallels a reach of the Mississippi River west of New Orleans and appears to run under the New Orleans International Airport.  The surface trace is just west of the interchange of Interstate Highway 310 and US Highway 61 (Airline Highway). 

Subsurface geological structure maps are the essential tools for oil and gas exploration.  A subsurface depth contour map on a Middle Miocene sand layer shows the geological structure.  The subsurface trace of the fault is mapped where the fault plane intersects the mapped horizon – in this case at about 9,000 feet below the surface.  The St. Rose fault offsets the Middle Miocene sand layer by about 500 feet at this depth.  It is the offsetting of the sand layer below the surface that creates the trap where natural gas accumulates.  The gas accumulations in the Middle Miocene sand on the downthrown side of the St. Rose fault make up the St. Rose gas field.  Wells drilled into the gas reservoirs in this field have produced significant volumes of natural gas.  The concentric contours of the Good Hope oil field to the north of St. Rose indicate that it is rooted by a salt dome.

A profile constructed from wireline logs taken in wells drilled in the vicinity of St. Rose field shows the St. Rose fault offsetting the sedimentary layers.  Sand and shale layers can be correlated between the well logs used to construct the profile.  The depths of each layer can also be used to construct a subsurface depth map on that horizon.  The profile shows that offset of the layers extends up through the Upper Miocene and Pliocene intervals and into the Quaternary.  Direct determination of fault offset in the Quaternary cannot be made with oil and gas industry data.  In order to make this determination it would be necessary to construct a profile of shallower borings or cone penetrometer readings across the fault.  This type of profile could be combined with the acquisition of high resolution seismic for the most definitive determination.

The importance of attempting to determine recent fault movement, or more importantly to estimate associated rates of fault slip, is based on the proximity of the surface trace of the St. Rose fault to critical infrastructure.  The visible surface trace of the fault appears to cross LA State Highway 626, the Kansas City – Southern Railroad, Airline Highway and the St. Rose levee and drainage structure.  Extrapolations of the possible surface fault trace to the east and west of the visible trace could cross I-310 and may even reach the airport at a possible eastern extent of the fault trace and the Mississippi River levees at the possible western extent.  It is also possible that movement on the fault may affect rates of settling around the I-310 – Airline Interchange and the floodwall and levee reach to the south of the fault.  An elevation survey across the Highway 11 Bridge fault by the Lake Pontchartrain Basin Survey (Hopkins et al, 2018) showed that the drop in elevation across the fault affected the entire southern span of the bridge, not just the section on the immediate downthrown side of the fault.

Potential indications of the effects of fault movement can be seen at two locations along the visible trace of the fault.  An elevation survey on Highway 626 shows a distinct offset at the point where the fault crosses the highway.  An offset crack in the T-Wall structure at the St. Rose drainage structure also appears to be almost exactly coincident with the surface trace of the fault.  These potential impacts of fault movement are consistent with the type of slow-slip movement that is believed to typify faults in southeastern Louisiana most of the time.  The potential for a more significant fault slip event, such as that which occurred on the Vacherie fault is the real source of concern.

While the sharp tree line in the swamp clearly delineates the surface trace of the St. Rose fault, it is not clear why it does.  The tree line exists because trees on the downthrown or hanging wall side of the fault have died.  This may be because fault slip has lowered the elevation of the base of the swamp increasing the water depth on the downthrown side, effectively causing the trees to drown.  Cypress trees are generally tolerant of a range of water depths, however, and it seems unlikely that a change in water depth significant enough to have killed them exists along this fault.  A more likely explanation may be offered by the role that faults have played throughout geological history as conduits for fluid migration.  The migration of saltwater up faults and into freshwater drinking aquifers has been documented along the Baton Rouge fault trend.  Kuecher et al, 2001 interpreted salinity anomalies along the surface trace of the Montegut fault in Terrebonne Parish, and Gagliano et al, 2003 related saltwater migration along this fault to the death of cypress trees in the area.  This may be the case along the St. Rose fault, but more investigation is needed. 

Much more study is needed to understand the St. Rose fault and its potential impacts on the natural environment and manmade infrastructure in the area.  A complete investigation of the fault should include detailed mapping of the fault at depth with 3-D seismic data, determination of recent fault movement with borings, cone penetrometers and high-resolution seismic, and determination of possible saltwater migration along the fault.  It would not be wise to wait for a catastrophic event associated with fault slip to begin this type of investigation. The area around the visible trace of the fault would also be an excellent place to attempt to measure subsidence rates. GPS and/or InSAR velocity measurement techniques could be employed. The integration of the subsurface geological interpretation with the measurement of land surface motion should be employed across the coastal plain.

References

Gagliano, S.M., et.al., 2003. Active Geological Faults and Land Change in Southeastern Louisiana. Prepared for U.S. Army Corps of Engineers, New Orleans District, Contract No. DACW 29-00-C-0034.

Hopkins, M., Lopez, J., Songy, A. 2018, Subsidence rates from faulting determined by real-time kinematic (RTK) elevation surveys of bridges in Lake Pontchartrain,  presentation, State of the Coast Conference 2018, New Orleans, Louisiana.

Kuecher, G.J., Roberts, H.H., Thompson, M.D. and Matthews, L., 2001, Evidence for Active Growth Faulting in the Terrebonne Delta Plain, South Louisiana: Implications for Wetland Loss and the Vertical Migration of Petroleum., Environmental Geosciences, v. 8, p. 77‐94

Shen, Z., Dawers, N. H., Tornquist, T. E., Gasparini, N. M., Hijma, M. P., and Mauz, B., 2017, Mechanisms of late Quaternary fault throw-rate variability along the north central Gulf of Mexico coast: implications for coastal subsidence, Basin Research, v. 29, p. 557-570

Breton Island

The Breton National Wildlife Refuge was formed in 1904 by executive order of President Theodore Roosevelt.  It is the second oldest refuge in the National Wildlife Refuge system.  Looking at this video of Roosevelt’s visit to Breton Island in 1915, it’s not hard to understand why.  The visit was sponsored by the Audubon Society and they played an instrumental role in preserving ecosystems that support wildlife. 

Breton Island is at the southern end of a chain of barrier islands that form an arcuate perimeter to Breton and Chandeleur Sounds.  The sand that makes up these islands was deposited in bars at the mouths of historical lobes of the Mississippi River over 2,000 years ago.  Geologists Bryan Rogers, Mike Miner, and Mark Kulp at the University of New Orleans reconstructed the patterns of the historical delta lobes.  Their research suggests that Breton Island may have gotten its start as early as 4,000 years ago from delta lobe 5 at about the time the Pyramids of Giza were being constructed.  The ancestral Mississippi River revisited the area several times, and the deposits of delta lobes 5, 8, 9 and 13 are all preserved in the shallow subsurface.  While the swamps and marshes of those deltas have subsided below the surface, the sand remains near the surface as barrier islands.  Breton Island is different than the rest of the chain.  The long linear pattern of the islands from Hewes Point to Curlew Island gives way to a distinctly rounded shape at Breton Island.

The Louisiana Barrier Island Comprehensive Monitoring (BICM) program was established by Louisiana Coastal Protection Restoration Authority (CPRA).  The goal of the program is to provide long-term data on the barrier islands of Louisiana that could be used to plan, design, evaluate, and maintain current and future barrier-island restoration projects. CPRA partnered with the the U.S. Geological Survey (USGS) and the University of New Orleans to help achieve these goals. The BICM program used both historical and newly acquired data to assess and monitor changes in the aerial and subaqueous extent of islands, habitat types, sediment texture and geotechnical properties, environmental processes, and vegetation composition.  This program is an excellent example of the value of data collection as an essential component of coastal sustainability planning.

The detailed bathymetric maps around Breton Island produced by the BICM program show the mound-shape accumulation of sand that supports the island.  The island itself appears to wrap around the mound, suggesting that the sediment supply is from the northeast.  A comparison of the 1870 and 2007 maps shows that while the island has suffered significant degradation, the supporting mound has remained fairly stable.  If the sediment supply to the island was primarily littoral drift along the island chain from the northeast, it may be possible that the dredging of the Mississippi River Gulf Outlet channel caused an interruption of the natural sediment supply, which contributed to the degradation of the island.  The hook-shaped configuration of Breton Island that appears to be wrapping around a point of resistance is interesting because the island is underlain by a salt diapir.

Subsurface geological interpretation reveals the dome-shaped top of a salt diapir at depth of about 18,000 feet directly beneath the dome-shaped bathymetric feature that supports the island.  The diapir has a distinct convex upper surface typical of the conceptual “salt dome”, but it has a concave lower surface with the suggestion of a “tail”.  This indicates that the diapir was squeezed up from deeper source of salt to the northwest. 

The southern face of the salt diapir is cross by a fault.  The depth contours of the fault plane map indicate that the fault is slightly deflected by the diapir, and it is likely that there is a genetic relationship between the emplacement of the salt feature and the formation of the fault.  The fault appears to extend to the surface, and the projected surface trace of the fault appears to arc around the bathymetric mound.

Natural gas accumulations were found in Miocene sand layers beneath Breton Island in the 1940s.  The well logs from the drilling that followed has allowed for detailed mapping of the Miocene sands.  Subsurface geological structure maps show that the gas accumulations are trapped in anticlinal structures the directly overlie the salt diapir.  The concentric depth contours on the Miocene 1 and Miocene 2 sands, at depths of about 3,800 and 5,800 feet respectively, define the anticline feature.  The relationship between the anticline and the fault is that of a classic “growth fault rollover”.  The subsurface structure maps on the Miocene sands and the well profile across the area show that the anticline nests within the arcuate shape of the fault plane.  The thickness of the sedimentary layers is greater on the downthrown side of the fault, indicating that the fault was actively slipping during the deposition of the Miocene sediments.  The sedimentary layers also get thinner onto the crest of the anticline and then thicken into the fault on the downthrown side.  This is indicative of active fault slip during deposition, but it also indicates a relative upward movement of the crest of the anticline.  The fact that the area of relative upward movement on the rollover anticline directly coincides with the underlying salt diapir suggests the salt feature has continued to exert a structural influence on the overlying sedimentary structure long after the upward movement of active diapirism had ceased.

An analog for this type of influence from below may be found in the art installation “The Impact of a Book” by Jorge Mendez Blake.  This work uses the physical expression of a wall of bricks stacked across a book to create a powerful metaphor for the intellectual and sociological impacts of a book.  The physical expression itself also provides a useful image for the geology of Breton Island.  To make the analogy complete imagine that instead of placing the book on the floor and stacking bricks on top of it, a moveable foundation is set at the level of the top of the wall.  The book is placed on this foundation, and as each row of bricks is laid down, the foundation is lowered.  In this configuration the progressive downward movement of the foundation is equivalent to subsidence, the book is equivalent to salt diapir, and the addition of each new row of bricks is equivalent to sediment deposition.  The impact of the diapir is continually expressed at the surface as each new layer is added on, and the structural expression of the diapir is propagated all the way to the surface.  This appears to be the relationship between the Breton Island salt dome and the Breton Island bathymetric feature.

A panel of four geological reconstructions through time shows the progression of structural development and the continual influence of the salt diapir.  At the end of the Paleogene Period (the Paleocene Epoch through the Oligocene Epoch) the salt diapir was near the end of its period of active diaprism.  The salt appears to have been squeezed up from the northwest under the weight of a gradually advancing sedimentary load.  The area was in a deep water depositional environment which the huge influx of terrigenous sediments from the Rocky Mountains had yet to reach.  The initial upward movement of the salt diapir and the formation of the fault may have been related to the compressional “toe” feature of a fault system further to the north, but this is not entirely clear.  The influx of course terrigenous sediments into this area began in the lower Miocene.  The rate of sediment accumulation was so great that the continental margin prograded southward, and by the middle Miocene a structural graben had formed across the crest of the diapir in the upper continental slope environment.  The antithetic fault component of the graben ceased its motion at the end of the middle Miocene, and the primary fault developed into a normal growth fault by the upper Miocene.  This coincided with the progradation of the continental shelf across the area.  The outer shelf deltas of the ancestral Mississippi River during the upper Miocene deposited the sand layers that would become the reservoirs for the natural gas accumulations.  The transition of the structure across the top of the diapir from a graben to a fault rollover happened during this time.  The red box at the right of the panel shows the interval covered by the well profile.

It appears that the rollover anticline continued to be expressed throughout the Pliocene and Pleistocene Epochs.  The shallowest horizon that can be mapped with well log data is a Pleistocene sand at a depth of about 800 feet below the surface.  This map shows the rollover anticline at this horizon indicating that the fault was active and that the impact of the diapir was being expressed.  The indication of fault slip during the Pleistocene generally classifies the fault as being an “active fault”.

The evolution of the Holocene delta across this area was primarily driven by the interplay of deposition and subsidence.  When delta lobe 9 was active emergent wetlands covered most of what is now Breton and Chandeleur Sounds.  The mouth bars of this delta formed the arcuate fringe of the delta lobe that would eventually evolve into the barrier island chain.  That evolution began with the abandonment of the delta lobe when the river changed course and began building delta lobe 10 in Barataria Bay.  Radiocarbon dating on an in situ cypress stump encountered in a sediment boring near Breton Island indicates it is a remnant of this delta environment from about 2100 years ago.  The stump was found at a depth of about 30 feet below the surface.  Given that it was at the surface 2100 years ago, this indicates an average subsidence rate of about 5 millimeters per year over that time span.  This is same rate of current subsidence that was measured in a recent study of the Breton Basin by CPRA. 

Breton Island appears to have maintained a surface expression at nearly the same place over the same 2100 year time span.  The influence of a salt diapir over 18,000 feet below the surface may have played a role in maintaining the surface expression of the island.

REFERENCES

Frazier, D.E., 1967, Recent deltaic deposits of the Mississippi River: their development and chronology, Trans. G.C.A.G.S., v. 17, p. 287‐315

Rogers, B.E., Kulp, M.A., Miner, M.M., 2009, Late Holocene chronology, origin, and evolution of the St. Bernard Shoals, Northern Gulf of Mexico, USA, Geo-Mar Lett, v. 29, p.379-394

The Bastian Bay fault

The best inputs to a scientific investigation always come from the people in the field.  Woody Gagliano spent many years in the marshes of coastal Louisiana studying the patterns of change.  He collected scientific data, but also spoke to the local oyster fishermen and camp owners.  It was Pete Hebert that told Gagliano about his camp built in the 1960s in Plaquemines Parish west of Buras.  The camp was located on the banks of Bayou Ferrand, but by the mid-1970s it was standing in open water.  Hebert reported that the land surface under the camp sank by 3 to 3 ½ feet over that time span.  Gagliano realized that the submergence of Hebert’s camp was due to movement on the Bastian Bay fault, which he had found by constructing profiles of the shallow subsurface from auger borings.  Gagliano later referred to the Bastian Bay fault as the “Rosetta Stone” for deciphering relationships among fault movement, subsidence and land loss in southeast Louisiana.  Subsequent scientific investigation of the Bastian Bay fault by geologists at Tulane and the University of Texas at Austin confirmed the location of the fault using seismic data (Armstrong et al, 2013; Dawers and Martin, 2006)

The Bastian Bay fault is near the eastern end of the Terrebonne Trough.  The fault is one of a series of down-to-the-south faults (highlighted in blue) that bound the northern rim of the trough.  They appear to have a conjugate relationship with a set of northern-dipping faults (highlighted in red) and salt domes that bound the southern rim.  Miocene, Pliocene and Quaternary sedimentary layers are all thicker within the trough, and the weight differential created by these thick piles of sediment appears to have driven subsidence and fault movement for at least the past 15 million years. 

Most of the major faults bounding the Terrebonne Trough appear to extend to the surface where recent fault movement has played a critical role in causing wetlands loss.  The figure below shows the subsurface depth contours of the Bastian Bay fault plane in yellow and their connection to the Lake Washington salt dome shown in green.  The zero-depth contour coincides with the surface trace of the Bastian Bay fault.  Similar fault plane contour maps for each of the other faults that reach the surface could be constructed using seismic data and well logs. 

Like most of the other faults along the Terrebonne Trough, the Bastian Bay fault controlled the formation of geological structures at depth.  Most of these structures control the accumulation of oil and gas in the Miocene, Pliocene and Pleistocene sands.  Bastian Bay Field is an example of a “rollover fault structure” in which an anticlinal structure is created by the thickening of sedimentary layers into the fault at the time of deposition.   At Bastian Bay Field the producing gas reservoirs are found in sand layers that were deposited by ancestral deltas of the Mississippi River during the upper Miocene.  The subsurface structure map on the “O” Sand published by the New Orleans Geological Society shows the concentric contours of the anticlinal structure, which has a crest at depth of 12,400 feet below the surface.  The red area indicates the extent of gas accumulation at the top of the rollover structure.  The Bastian Bay fault runs along the northern edge of the anticline and forms a boundary of the gas reservoir.  A small synthetic fault running parallel to the Bastian Bay fault forms the southern boundary of this gas reservoir.  Wells drilled to a depth of 12,400 feet or greater within the red area encountered gas in the “O” Sand.

A cross section of well logs from the field shows the arrangement of the fault and the producing gas reservoirs.  The “throw” of the fault at any horizon is the difference in elevation across the fault.  At the upper blue Miocene horizon the throw is about 1,500 feet.  At the “X” Sand the throw is over 3,000 feet.  The interval thickness between each of the labeled sand layers is greater on the downthrown (or hanging wall) side of the fault than it is on the upthrown (or footwall) side.  This indicates continuous fault movement during the deposition of the deltaic sands, and it is why faults like this are referred to as “growth faults”.  

A simplified model for the history of the Bastian Bay fault can be constructed by isolating the fault in a depositional sequence in which all other faults and salt domes have been removed and sedimentary layers are considered to be flat and continuous away from the fault.  The reality is that the complex arrangement of structural features in the subsurface have all been interacting with each other throughout the deposition of the sedimentary layers, but this is very difficult to model.

The Bastian Bay fault probably began as slide structure on the lower continental slope during the lower Miocene.   A study of the mechanics of faults by geologists W. Crans, G. Mandl and J. Harembore at Shell Research Laboratory found that when loose sediment is deposited on a gently inclined slope (about 3 degrees), its weight has a surface-parallel component that tends to pull it down the slope.  As long as this force is balanced by the reactive shear stress along the slope, the sediment will stay on the slope in stable equilibrium.  As sedimentation continues and the height of the sediment layer increases, both the “pulling” weight component and the reactive shear stress increase proportionally.  The equilibrium will remain stable as long as the slope-parallel shear stress remains below the limit determined by the shear strength of the sediment.

The shear strength of the sediment may be reduced when pore fluids become “geopressured” meaning that they support a portion of the overburden.  The greater the magnitude of geopressure, the lower the shear strength of the sediments.  At a critical point when the driving weight component exceeds the friction along the base of a sedimentary layer it may begin to slide along the basal slip plane.  In this model the initial lateral slip of the fault results in a negative vertical component of slip in the active plastic region at the head of the fault and a positive vertical component in the passive plastic region at the toe of the fault.  The accommodation capacity created at the head of the fault allows for a differential accumulation of sediment on the hanging wall of the fault, which will continue to drive fault movement in a feedback loop.

The influx of terrigenous sediments into the Terrebonne Trough during the middle Miocene triggered episodes of dramatic fault movement on all of the bounding faults including the Bastian Bay fault.  It is significant to note that these faults tended to act as an interconnected system of faults throughout this time.  It is probable that a triggering event on one fault could result movement that rippled throughout the system.  The simplified model on the Bastian Bay fault during this period shows that the original lateral slip plane of the fault begins to arc upward to a steeper angle.  This is the result of more competent normally pressured sediments. The curving arc of the fault plane defines the nature of a listric fault.  The model also shows that fluids migrate along the fault plane, including hydrocarbons.  It is almost certain that the gas accumulated in the Miocene reservoirs at Bastian Bay Field migrated up the Bastian Bay fault and into the sand layers at some time after their deposition. 

Movement on the Bastian Bay fault continued to propagate throughout the Pliocene and the Quaternary.  The movement of fluids up the fault plane continued throughout this time period and into the present.  It is probable that saline fluids reach the surface along these major faults.  This may be responsible for some of the salinity anomalies that have been measured in the coastal wetlands (Keucher et al, 2001).

A subsurface structure map on the top of the Pliocene stratigraphic interval shows the trace of the Bastian Bay fault.  The fault trace is constructed on this map by marking the intersection of the depth of the mapped horizon and the depth of the fault plane.  In the vicinity of the Bastian Bay Field the top of the Pliocene horizon is at a depth of about 2350 feet on the hanging wall and about 2250 feet on the footwall.  The two sides of the fault trace on the map are coincident with the equivalent 2350-foot and 2250-foot contours on the fault plane map.  This means that the throw of the fault in this area at the top of the Pliocene is about 100 feet.  Because the elevation of the surface in south Louisiana is effectively zero, a map on the depth of the top of the Pliocene is also a map of the thickness of the Quaternary (the Pleistocene and the Holocene).  The Quaternary is therefore 100 feet thicker on the downthrown side of the Bastian Bay fault than it is on the upthrown sides.  This indicates that the fault continued to be active in the Quaternary, which generally qualifies it as an “active fault”. 

A detailed well log cross section of the Quaternary across the Bastian Bay fault shows that the fault remained continually active throughout the interval.  Individual stratigraphic intervals in the Pleistocene are vertically offset by the fault and they are generally thicker on the hanging wall side than they are on the footwall side.  This pattern extends into a tentative interpretation of the base of the Holocene interval from well log correlation.  It is apparent from this interpretation that there is stratigraphic evidence that the Bastian Bay fault was active during the Holocene. It is probable that late Holocene activity on the fault would have been associated with the deposition of sediments in the Bayou Robinson delta lobe between 950 and 300 years ago (Kulp et al, 2005).  Bayou Ferrand was originally formed as a distributary channel in this delta lobe.

Gagliano realized that the submergence of the marshes around Pete Hebert’s camp was a result of the latest episode of movement on the Bastian Bay fault.  It is the nature of all faults to move episodically in (geologically) short bursts of activity.  In the tectonic regions of California these episodes are generally very brief, and fault slip happens dramatically in a matter of seconds.  In a delta system where the forces are all driven by the weight of the deltaic sediments fault slip can happen much more slowly and will generally not create a detectable seismic signal.  The most recent episode of fault slip on the Bastian Bay fault appears to have happened over a period of about a decade between the early 1970s and the early 1980s.  Imagery of the area taken from the ProPublica publication “Losing Ground” is used in this short video sequence to show the submergence of the marsh on the hanging wall side of the Bastian Bay fault during that time period.  The interconnected nature of the system of faults bounding the Terrebonne Trough throughout their geological history strongly suggests that similar patterns of movement happened on most of these faults during the same time period.  This is likely to be the principal cause of the episode of wetlands loss that occurred between the 1970s and 1980s in southeastern Louisiana.

REFERENCES

Armstrong, C., Mohrig, D., Hess, T., George, T., Straub, K.M., 2014, Influence of growth faults on coastal fluvial systems: Examples from the late Miocene to Recent Mississippi River Delta, Sedimentary Geology, v. 301, p. 120-132

Crans, W., Mandl, G., Haremboure, J., 1980, On the Theory of Growth Faulting: A Geomechanical Delta Model Based on Gravity Sliding, Journal of Petroleum Geology, v. 2, p. 265-307

Dawers, N. H. and Martin, E., 2006, Fault Related Changes in Louisiana Coastal Geometry, Louisiana Governor’s Applied Coastal Research and Development Program, GACRDP Technical Report Series 05-000, 21 p.

Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., Sabate, R. W., 2003 Neo-Tectonic Framework of Southeast Louisiana and Applications to Coastal Restoration, Transactions G.C.A.G.S., v. 53, p. 262-272

Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., 2003. Active Geological Faults and Land Change in Southeastern Louisiana. Prepared for U.S. Army Corps of Engineers, New Orleans District, Contract No. DACW 29-00-C-0034

Kulp, M. A., Fitzgerald, D., Penland, S., 2005, Sand-Rich Lithosomes of the Holocene Mississippi River Delta Plain, SEPM Special Publication No. 83, SEPM (Society for Sedimentary Geology), ISBN 1-56576-113-8, p. 279–293.

The Vacherie fault

At 2:00 o’clock in the morning on April 12th, 1943 a fault ruptured the surface of the earth at a sugarcane plantation along the west bank of the Mississippi River near Vacherie, Louisiana. Given that it was spring, the river was at medium flood stage, and the north end of the fault was only about three-tens of a mile from the levee, the New Orleans District of the Corps of Engineers dispatched Harold Fisk to investigate.  Fisk was an Associate Professor of Geology at LSU and was putting the finishing touches on his seminal work GEOLOGICAL INVESTIGATION OF THE ALLUVIAL VALLEY OF THE LOWER MISSISSIPPI RIVER.  He found that the surface rupture was composed of a series of parallel open cracks arranged in an en echelon pattern.  Together they were over a mile long, on an orientation of about 30 degrees west of north, and the western side of the fault was down-dropped relative to the eastern side.  There as a maximum displacement of about eight inches near the center of the fault trace.

This episode brought together three seemingly disparate timeframes of geological history.  The movement of the fault was an instantaneous geological event.  Fisk’s investigation would show that the fault had probably been moving episodically for centuries.  Later exploratory drilling would find that this same fault was a critical structural component of the Vacherie oil field and had been moving interactively with the Vacherie-Hester salt dome for millions of years.  Most people tend to think of geological processes in terms of things that happened over long periods of time in the distant past.  It is not until the occurrence of an instantaneous event that we consider the implications of geology in the here and now.  The importance of the Vacherie fault rupture is that it shows that studying geological processes in the past is the best way to predict and prepare for the occurrence of instantaneous (and potentially catastrophic) events in the future.

Exhibit 1 of Docket No. 76-432 at the Geological Oil and Gas Division of the Louisiana Office of Conservation includes a subsurface structure map and fault plane map submitted as evidence in a unitization proposal filed at Vacherie Field.  The outline of an approved geological unit determines the ownership and revenue sharing of the oil and gas production from the unit.  The subsurface structure map on the Hurley Sand shows a gas reservoir (colored red on the map above) in a wedge-shaped structural trap between two faults on the southwestern flank of the salt dome.  The depth of the reservoir sand below the surface is indicated by the values of the contour lines, which can be read like a subsurface topographic map.  The fault traces at this horizon are colored light brown and the triangles indicate the downthrown direction.  The fault plane map on Fault “C” shows the connection between the deep geological fault that forms the trap in the 20 million-year old gas reservoir and the surface trace that ruptured in April of 1943.  The values labeled in black next to the well symbols indicate the depth at which the fault plane crosses each well.  “Picking fault cuts” on well logs is an element of basic subsurface geological interpretation.  The vertical displacement on Fault “C” is measured to be 700 feet at a depth of 7,100 feet near to the dome.  Displacement progressively decreases at shallower depths until it reaches the value of 8 inches measured by Fisk on the surface rupture.

The most significant of Fisk’s findings was that the fault rupture line appeared to coincide with the channel of the historical crevasse of the Mississippi River at Vacherie.  It is reasonable to conclude from this coincidence that the crevasse of the river was caused by an episode of fault movement at some time in the past few centuries.  This episode would have caused a surface rupture that did reach the channel of the river causing the crevasse to occur.  Fisk took a series of sediment borings across the fault to construct a profile of the Holocene and upper Pleistocene sedimentary layers.  Section A-A’ shows that the Holocene strata are offset by the fault, and Fisk measured 3.5 feet of displacement in the upper Pleistocene.  If this displacement represents the total offset caused by episodes of fault movement during the Holocene and each episode contributed the same 8 inches of offset as the most recent episode did, then it is logical that there were five or six episodes of fault movement during the Holocene.  The overall size of the topographic ridge that extends from the point of crevasse to Lake Des Allemands and the relative complexity of the crevasse channel network suggests that they may be the result of multiple crevasse events during the late Holocene.

It is important to note that the movement of this fault did not create a seismic event that was detectable at the seismography station at Loyola University about 50 miles away.  This would suggest that movement experienced at the surface was not directly associated with deep tectonic movement.  Understanding the processes associated with this type of aseismic or “slow slip” fault movement is likely to prove to be very important to gaining a broader understanding of fault movement in south Louisiana in general.  In the particular case of the fault at Vacherie one possibility may be the dissolution of the Vacherie-Hester salt dome.  Brent Bray and Jeff Hanor of the LSU Geology Department studied pore water salinities in the vicinity of the dome and found a significant plume of hypersaline fluids emanating from the salt stock.  They concluded that this plume indicates that salt is being dissolved and migrating away from the dome along faults.  Hanor has found this phenomenon at multiple salt domes across south Louisiana and he has estimated that in total several cubic kilometers of salt has been lost to dissolution.  The dissolution of salt creates an obvious space problem in the subsurface, and it may be possible that fault movement around these domes is in part a response to salt dissolution.

The map below shows several places where surface fault traces appear to intersect the channel of the Mississippi River.  Surface fault traces are shown in black with squares indicting the downthrown direction of the main faults. These traces represent the current state of a compilation of published works (Armstrong et al, 2014; Gagliano et al, 2003; Dokka, 2011; Kuecher et al. 2001) and an active, on-going interpretative process involving research at UNO, Tulane and ULL (Akintomide et al, 2019; Bullock et al, 2018; Frank, 2017; Johnston et al, 2017; Levesh et al, 2019).  The map also reflects compilations of research funded by the RESTORE Act Centers of Excellence, the TranSET transportation consortium and the Louisiana Department of Transportation and Development.  Access to the oil and gas industry seismic data used in the university research has been coordinated through the Louisiana Geological Survey Coastal Geohazards Atlas project.  In addition to Vacherie,  sites at Darrow, St. Rose, Fort St. Phillip and The Jump are associated with historical crevasses or avulsions, or in the case of Fort St. Phillip, an active crevasse that formed in 1973.  Surface fault traces also appear to intersect the river at Ironton and Magnolia.

The potential for the movement of faults that intersect the channel of the Mississippi River is an obvious area for more research.  The first objective should be to get a complete and accurate map of all surface fault traces near the river.  The publication of a complete and accurate map of surface fault traces is one of primary objectives of the LGS Coastal Geohazards Atlas project.  In order to achieve this objective, it will be necessary to adequately fund the Atlas project and to negotiate a more comprehensive data-sharing arrangement with the oil and gas industry.  This should be a priority for the multiple state and federal agencies that would be impacted by a crevasse of the river.

References

Akintomide, A. O. and Dawers, N. H., 2019, Spatial and Temporal Variation of Fault Activity in the Terrebonne Salt Withdrawal Basin, Southeastern Louisiana: Response to Salt Evacuation and Sediment Loading, presentation, AAPG Annual Meeting, San Antonio, Texas.

Armstrong, C., Mohrig, D., Hess, T., George, T., Straub, K.M.,  2014, Influence of growth faults on coastal fluvial systems: Examples from the late Miocene to Recent Mississippi River Delta, Sedimentary Geology, v. 301, p. 120-132

Bullock, J. S., Kulp, M. A., McLindon, C. D., 2018, Evaluation of the Magnolia growth fault, Plaquemines Parish, southeastern Louisiana, poster session, GSA Annual Meeting, Indianapolis, Indiana.

Bray, B., Hanor, J., 1990. Spatial variations in subsurface pore fluid properties in a portion of Southeast Louisiana: implications for fluid migration and solute transport. Trans. Gulf Coast Assoc. Geol. Soc. 40, 53–64.

Dokka, R.K., 2011, The role of deep processes in late 20th century subsidence of New Orleans and coastal areas of southern Louisiana and Mississippi, Journal of Geophysical Research, v. 16, 25 pgs.

Fisk, H. N. 1943. Geological Investigation of Faulting at Vacherie, Louisiana. Vicksburg, MS: U.S. Army Engineers Waterways Experiment Station.

Fisk, H. N. 1944. Geological Investigation of the Alluvial Valley of the Lower Mississippi River. Vicksburg, MS: U. S. Army Corps of Engineers, Mississippi River Commission.

Frank, J. P., 2017, Evidence of fault movement during the Holocene in Southern Louisiana: integrating 3-D seismic data with shallow high resolution seismic data, MS Thesis, University of New Orleans, 91 p.

Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., 2003. Active Geological Faults and Land Change in Southeastern Louisiana. Prepared for U.S. Army Corps of Engineers, New Orleans District, Contract No. DACW 29-00-C-0034.

Johnston, A., Zhang, R., Gottardi, R., Dawers, N. H., 2017, Investigating the relationships between tectonics and land loss near Golden Meadow, Louisiana by utilizing 3-D seismic and well log data, poster session, GSA Annual Meeting, Seattle, Washington.

Kuecher, G.J., Roberts, H.H., Thompson, M.D. and Matthews, L., 2001, Evidence for Active Growth Faulting in the Terrebonne Delta Plain, South Louisiana: Implications for Wetland Loss and the Vertical Migration of Petroleum., Environmental Geosciences, v. 8, p. 77‐94

Krinitzsky, E. L., 1950, Geological Investigation of Faulting in the Lower Mississippi Valley, U. S. Army Corps of Engineers Technical Memorandum 3-311, Vicksburg, MS

Leblanc, R. J., 1996, Harold Norman Fisk as a consultant to the Mississippi River Commission, 1948-1964 - an eyewitness account, Engineering Geology, v. 45, p. 15-36

Levesh, J. L,, Kulp, M. A., McLindon, C. D., 2019, Fault-slip history of the Delacroix Island fault system and its effect on Holocene salt marshes of the Mississippi River delta plain,  presentation, GSA Annual Meeting, Charleston, South Carolina

McLindon, C.D., 2017, History of fault slip and interaction with deltaic deposition from the middle Miocene to the Present – Barataria fault, coastal Louisiana, poster session, American Geophysical Union, annual meeting, New Orleans, Louisiana.