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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Louisiana's Oil and Gas Industry - The Missing Link In Coastal Sustainability

This article was originally published in the Quarterly journal of the Society of Independent Professional Earth Scientists

The sustainability of coastal zones is becoming a global priority.  Nearly half of the world’s population lives within 100 miles of the ocean, and half of them live within 50 miles.  Population centers tend to be in cities close to the ocean that are built on coastal plains or river deltas.  South Louisiana is often recognized as a model for sustainability in coastal zones.  What is generally not recognized is the critical role that the oil and gas industry must play in that model.  The concept of sustainability has evolved over the past few decades.  Many of the early ideas of trying to restore coastal ecosystems to a pristine state that preceded the industrial revolution have given way to a managed sustainability that balances ecology with economy.  Real sustainability allows for the utilization of the natural resources of the coast while working to protect and preserve the essential ecological components.  Fisheries, transportation, ports and oil and gas production can all coexist with a productive natural environment, if effectively managed.  The truth is that the viability of the economic component of a coastal system in many ways determines the viability of the ecological component.  The worst areas of pollution and coastal degradation tend to be in countries with the poorest economies.

Louisiana’s plan for sustainability is laid out in the document “Integrated Ecosystem Restoration & Hurricane Protection: Louisiana’s Comprehensive Master Plan for a Sustainable Coast”.   The 2021 Fiscal Budget for the Master Plan was recently published by the Louisiana Coastal Protection and Restoration Authority (CPRA).  It is based on $3.34 billion in revenue in the successive three-year period 2021-23.  81% of that revenue is derived from the oil and gas industry.  A significant portion of this revenue comes from settlements from the Deepwater Horizon disaster, but almost $400 million comes from the Gulf of Mexico Energy Security Act (GOMESA), which provides revenue from offshore leasing, rentals and royalties.  Another $100 million comes from the Coastal Protection and Restoration Trust Fund, which collects revenues from leases, rentals, royalties and severance taxes in onshore Louisiana.  The economic viability of Louisiana’s plan for coastal sustainability is directly tied to the economic viability of the oil and gas industry along the coast of Louisiana.

The real value of the oil and gas industry’s contribution may lie in the science of sustainability.  Regardless of the magnitude of its expenditure, the success of the Master Plan will ultimately be determined by the degree to which it is based on an accurate assessment of the natural processes in the wetlands.  Earth science lies at the foundation of understanding the natural processes of the coast, and the oil and gas industry can provide a wealth of earth science data and knowledge base.  The application of industry data to studying coastal Louisiana began in the first years of this century.  Dr. Woody Gagliano started his career at the LSU Coastal Studies Institute before he founded his own company, Coastal Environments, Inc.  Dr. Gagliano is a recipient of the Coalition to Restore Coastal Louisiana’s Lifetime Achievement Award.  In a 2003 publication that accompanied his report to the US Army Corps of Engineers he stated “A century of oil and gas activity has made the subsurface of Southeastern Louisiana, which lies in the Gulf Coast Salt Dome Basin (Gulf Basin), one of the most understood geological provinces on earth.  Thousands of wells have been drilled; hundreds of thousands of seismic lines have been run; and the geological literature is voluminous.  The success of the oil and gas industry in this and other similar sedimentary basins throughout the world attest to the validity of the process-response models of sediment loading, compaction, salt movement and fault adjustments developed by geologists and geophysicists working in the Gulf Basin.   A working premise of the present study was to use this solid foundation of research as a basis for understanding and predicting land form and environmental change.”  Gagliano had recognized that subsurface geological processes played a fundamental role in controlling surface processes including subsidence and wetlands loss, and that oil and gas industry knowledge base was essential to understanding the geology.

Gagliano’s study pre-dated the use of 3-D seismic for coastal research.  It was not until 2014 that the first 3-D survey was used in a university research project on coastal geology.  Geologists at Tulane University and the University of Texas at Austin used a 530-square mile 3-D survey in Plaquemines Parish which was donated by Western Geophysical.  Their research produced the publication “Influence of growth faults on coastal fluvial systems: Examples from the Late Miocene to the Recent Mississippi River Delta”, which documented 28 faults mapped with the 3-D survey.  The study found that “Most of the seismically imaged faults appear to extend up to the modern land surface and some affect the modern delta morphology. … several of these faults correspond to abrupt shifts from emergent wetlands to fully submerged areas of open water on the delta surface.”  Their study found that the submergence of wetlands along the downthrown sides of faults that reached the surface was major factor in causing wetlands loss.  Since then, seven 3-D surveys and one 2-D survey have been made available for university research through direct donations and internships at oil and gas companies.  These data have been used in four MS theses at the University of New Orleans, three MS theses at the University of Louisiana at Lafayette and one PhD dissertation at Tulane University (Figure 1).  Each of these research projects mapped faults that were projected to extend to the surface and to affect surface coastal processes.  Publications from this research will be entering the scientific literature over the next few years.  The first summary of this research “Synthesis of Fault Traces in Southeast Louisiana Relative to Infrastructure” was published in 2019 by the Transportation Consortium of South-Central States (TranSET).  The surface fault traces are now available in GIS format on the Louisiana Department of Transportation and Development website.  The TranSET study stated that “Highly detailed 3D seismic reflection data from the energy sector was donated for use by local universities to map geologic features identified on seismic data to help predict areas of future land loss. It is hypothesized that the locations of geologic features as deep-seated faults and salt domes will prove to be closely related to many areas of coastal land-loss in south Louisiana. As a result of this collaboration, coastal restoration, flood-control and sustainability initiatives that are based upon this knowledge should be optimized to deal with future relative sea-level rise.”

Ironically, no meaningful consideration is given to the impact of faults in modern coastal sustainability planning.  This is the missing link in the sustainability models.  The paradigm under which the concepts about the causes of change in Louisiana’s coastal wetlands were developed did not include geology.  Nobody has described this better than Dr. Len Bahr, who ran Louisiana’s coastal restoration program from 1993 to 2008.  He wrote is in his blog post of May 2013  “Since its very inception in 1989 Louisiana’s coastal restoration program has been dominated by coastal wetland ecologists like me, folks who deal in relatively short-term surface processes, not the long term geophysical and riverine processes that underlie the delta. In other words, the planning expertise has been dominated by those who deal primarily with surface processes on the visible veneer of the delta, not the riverine hydrodynamics and sedimentary processes that created the delta and the underlying tectonic processes and shallow and deep subsidence to which the delta ultimately responds.”  Louisiana’s coastal sustainability planning has been very slow to integrate a consideration of the subsurface geological processes to which the delta ultimately responds.

It is the nature of science that new data is constantly changing our collective understanding of natural processes.  This is true for coastal Louisiana.  It turns out that many of the initial conceptions about wetlands loss were incorrect, but many of the popular narratives that developed early in the paradigm still persist.  The average person in coastal Louisiana would be likely to tell you that the wetlands are in a state of ecological crisis.  They would note that this is evidenced by the fact that the coast is losing land at the rate of a football field an hour.   They would also note that the principal causes of loss are the lack of a sediment supply caused by the levees and erosion that is caused by “saltwater intrusion”.  None of these contentions are scientifically accurate.  Correcting the misconceptions about the coast is essential to the long-term success of sustainability.  New data is beginning to challenge the popular narratives of the old paradigm, but in order to be fully integrated into coastal sustainability planning new data must be understood in the context of subsurface geology.

CPRA and the USGS monitor a wide array of data from the coastal wetlands.  These data include rates of subsidence, sediment accretion, surface elevation change and land area change.  Data are measured at nearly 400 stations in the Coastwide Reference Monitoring System (CRMS).  Figure 2 shows a map of the CRMS network and data being collected at some typical sites.  Most CRMS stations measure rates of sediment accretion that are greater than subsidence rates, and in many cases greater than subsidence plus sea level rise rates.  CPRA has reported that 70% of the stations in the delta region measure “positive elevation trajectories”, meaning that the marsh surface is gaining elevation relative to a fixed rod at the station.  Land loss rates were high in the mid-1970s when the popular narratives were formulated, but rates have been continually decreasing.  In fact, a 2019 study from the University of Arkansas found that “Remarkably, during the recent period … coastal Louisiana gained a modest amount of land”.  CPRA determined in a 2019 study that subsidence rates used in the formulation of the 2017 Master Plan were over 1.5 times higher than the rates actually being measured by modern technology.  This is the scientifically factual assessment of coastal Louisiana.  The coastal marshes actually have a fairly robust sediment supply.  Sediment accretion rates are high relative to subsidence, and much of the wetlands have been able to maintain surface elevation (Figure 3).  The result has been over a decade of stability in total net wetlands area.

Geological interpretation using oil and gas industry data offers the best explanation for the revelations of the new body of data.  The university research has produced a fairly extensive coverage of surface fault trace maps.  These have shown conclusively that historical wetlands loss over the past few decades has been concentrated along the downthrown sides of the faults (Figure 4).  Historical data from several tide gauges along the coast have been used to document a subsidence event in the 1970s that coincided with the land loss event (Figure 5).  The most probable explanation for the coincidence of the timing of these events and the concentration of loss along the faults is that there was an episode of fault slip that spanned about two decades.  This is what Dr. Woody Gagliano believed.  LSU Geology professor Dr. Roy Dokka also believed that fault slip was a significant contributor to subsidence along the coast.  He referred to the gradual progressive slip over time as being like “decades-long earthquakes”.  This type of slow-slip movement on faults has been documented in tectonic regions.  It will require more detailed study to fully document this phenomenon in the gravity-driven faulting of the delta region.

The rapid submergence of large areas of coastal marsh over these decades would have resulted in a substantial addition of mineral sediment to the coastal hydrologic system.  While high rates of land loss were being measured at the marsh surface, sediment was being redistributed in the coastal bays and estuaries.  Several studies have shown that significant sediment accretion happens during tropical storms.  Tidal flux carries sediment from the bays and estuaries onto the marsh platforms.  The high rates of sediment accretion and surface elevation change measured by CRMS can be best explained as a redistribution of sediment from marsh material that was lost to subsidence during the fault slip event.

The integration of interpretations from oil and gas industry seismic data with data from CRMS points to a positive outlook for the long-term sustainability of the Louisiana coast.  It appears that the natural processes of accretion are working to maintain the elevation of resilient marsh platforms in the face of subsidence and sea level rise.  Many of the edges of these marsh platforms are coincident with the surface traces of faults.  The most successful sustainability plans will work to supplement the natural processes of sediment accretion – perhaps by depositing dredged material from the Mississippi River in the bays adjacent to the marsh.  The most effective of these plans can only be developed with a more complete understanding of the subsurface geology that is controlling the surface process.  This can only be achieved by finding a way to integrate oil and gas industry data and knowledge base into the planning process.  That is the missing link in coastal sustainability.

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