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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