New Data Points to Good News for Coastal Louisiana

The past few years have yielded a bumper crop of new data on the Louisiana coastal wetlands.  Wise investments in data collection infrastructure are now providing detailed and accurate data on rates of subsidence, sediment accretion, surface elevation change and land area change.  These data paint a much more optimistic picture for the sustainability of the wetlands than what is generally portrayed in the popular narratives and media coverage.

Newly acquired data has revealed that current subsidence rates are less than half of the values used in formulating the 2017 Coastal Master Plan.  Significant vertical accretion rates have been measured across most of the coast.  Surface elevation change trajectories have been positive for 75% of the measurement stations in the delta region.  Contrary to the ubiquitous narrative that coastal Louisiana is “losing a football field an hour”, a 2019 study at the University of Arkansas found that “Remarkably, during the recent period … coastal Louisiana gained a modest amount of land” (Sanks et al, 2019).

Making sense of these data requires understanding two basic principles about coastal Louisiana that are rarely discussed.  First, there was a significant land loss event in the latter part of the 20th century, but it was just that, an event.  After reaching a peak in the mid-1970 land loss rates have steadily declined until they hit the small reversal noted by the Arkansas study in the first decade of the 21st century.  Second, there is a robust process of sediment accretion in the coastal marshes that is not dependent on river-borne sediment.  Accretion studies have shown that is sediment carried into brackish and saline marshes from the bottom of the bays and sounds by tidal exchange.  Across most of the coastal wetlands rates of sediment accretion and positive surface elevation change rates are greater than combined values of subsidence and global sea level rise.  Taken together, subsidence and global sea level rise are commonly called “relative sea level rise”.  This means that natural processes of accretion are doing a pretty good job of maintaining elevation and land area in the face of subsidence and rising sea levels.

Two of the main players in the data renaissance have been the Louisiana Coastal Protection and Restoration Authority (CPRA) and the U.S. Geological Survey (USGS).  CPRA and USGS co-manage the Coastwide Reference Monitoring System (CRMS).  The nearly 400 data collection stations in this system have yielded the decades-worth of data necessary to do meaningful statistical analysis.  CRMS was establish under the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA), which passed in 1990.  Also called the “Breaux Act” for its primary sponsor Louisiana Senator John Breaux, CWPPRA is funded by excise taxes on fishing equipment and small engine and motorboat fuel taxes.  Data collection from CRMS is proving to be one of the most valuable investments of CWPPRA funding.

CPRA has also taken on the task of getting accurate measurements of subsidence.  Working with experts from Applied Coastal Research and Engineering (ACRE) and C.H. Fenstermaker and Associates, CPRA has produced detailed subsidence studies in two of the coast’s hydrologic basins.  These studies include evaluations of the rates, causes and patterns of subsidence.  Work continues toward finishing a study of the entire coast.  Together these factors – subsidence, accretion, surface elevation change and land area change - are the essential data necessary to understand historical changes to the wetlands and to model predictions for future change. 

A typical CRMS marsh location is supported by elevated boardwalks in an H-shaped configuration.  Four components of each station are set up to measure vertical accretion, surface elevation change, pore water salinity and hydrographic data.  All this data plus measurements of land area change around each station are compiled and available on the CRMS website.

Accretion data is measured by a set of procedures in which a white feldspar powder is spread on the surface of the marsh at designated locations around the boardwalks.  At regular intervals sediment samples are recovered by inserting a tipped slim copper tube into the soil column and filling it with liquid nitrogen.  This process freezes the sediment around the tube and allows for the extraction of an undisturbed core-like sample.  The surface created by the feldspar powder is obvious as a white layer within the core.  The thickness of sediment accreted at the site can be measured above the white layer.  Over time multiple feldspar markers at multiple locations around each site provide enough data to allow for accurate estimates of long- and short-term accretion rates through statistical analysis.  The figure above illustrates the essential elements of accretion measurement process and a typical graphical representation of the data from one site.  This body of data shows that sediment accretion is a significant recent process across the coastal plain.  Accretion appears to be happening in freshwater, intermediate, brackish and saline marshes.  Sediment sources are likely to be some combination of river-borne suspended load and sediment carried in from estuaries by tidal flux.  Accretion in most of the saline marshes in the Terrebonne, Barataria and Breton Basins appears to be primarily sourced by the latter process.  Long-term accretion data from CRMS can presented using a standard gridding algorithm to show patterns across the delta region.

The methodology for measuring surface elevation change was established by Cahoon et al, 2000.  This methodology has been employed at most CRMS stations to record data over the past two decades.  As shown in the figure above, a rod surface elevation table (R-SET) is established by driving the rod to refusal and surveying its elevation.  A set of fiberglass rods are suspended from an arm which is perpendicular to the rod.  This allows for the direct measurement of the elevation of the surface of the marsh relative to the R-SET.  Changes in surface elevation are not measurements of subsidence (positive or negative velocities relative to a fixed datum such as sea level); they are strictly elevation measurements relative to the surveyed elevation of the R-SET.  In order to relate changes in surface elevation measured at a CRMS site to subsidence the site would have to be surveyed on a repeated basis to determine changes in elevation relative to the fixed datum, or the site would have to be equipped with a GPS device.  An evaluation of CRMS surface elevation change data was presented by Leigh Anne Sharp and Camille Stagg at the 2018 State of the Coast Conference.  They found “… 332 [CRMS sites] have surface elevation data that can be assessed along with land change data to infer which processes are influencing recent land change … In the Deltaic Plain land loss, in general, is not associated with elevation loss, suggesting that erosion, not subsidence, may be responsible for continuing land loss there. Over the last decade, wetland surface elevation trajectories have been positive at 75% of sites, including those in coastal areas near the Mississippi River delta that have seen the most historic land loss. Land change data reveal continuing land loss at just 14% of CRMS sites and land gain at 10% of CRMS sites including those in the birdsfoot delta and in the upper Mermentau basin. Furthermore, at least eight sites that were classified as floating marsh at project inception are now attached, including sites downstream from diversions and near the Atchafalaya delta. In general, CRMS data reveal that the coast of Louisiana is dynamic, that much of it has been stable in recent years, and that restoration efforts in concert with natural processes have increased stability in some areas”.

Land area change is measured at each CRMS site by the USGS.  The methodology employed by the survey for each one-square kilometer area around a site is identical that used in definitive publication on land area change -  Scientific Investigations  Map 3381 LAND AREA CHANGE IN COASTAL LOUISIANA 1932 TO 2016 (Couvillion et al, 2017).  This study does very careful analysis of imagery data to ensure the alignment of the various vintages of the imagery, and very careful statistical analysis to get the best fit to a curve that provides a meaningful representation of the data.  The Land Area Change Map in the sequence below shows the patterns of land area change in southeastern Louisiana since 1932.   The associate graph of data from the report shows the rate of change in coastal wetlands area over time calculated from fitted models using finite difference approximation (positive values indicating rates of land loss).  This graph suggests that land area change in coastal Louisiana can best be understood as an event that peaked in the mid-1970s.  Estimates of the rate of change of land area have been decreasing since then.  SIM 3381 notes that a “coastwide net ‘stability’ in land area has been observed over the past 6-8 years [prior to 2016]”. 

The 2019 CPRA/ACRE reports produced the first data set of accurate and internally consistent measurements of recent subsidence rates.  They did it by evaluating Geodetic GPS elevation measurements for CORS primary benchmarks and CPRA/NGS secondary benchmarks to determine subsidence velocities. Water-level gauge measurements were evaluated for documenting subsidence relative to eustatic sea-level rise estimates for the northern Gulf of Mexico. These measurements along with a similar set gathered using the same methodology for the Barataria Basin also provided some very interesting insights into the patterns of subsidence rates across the surface of the basins.  The study determined that Holocene geology and sediment consolidation are primary factors controlling subsidence.  Specifically the report found that  “Spatial variability in subsidence indicates a compelling relationship between subsidence rates and the age, composition, and thickness of Holocene deltaic deposits” and “Consolidation of Holocene deposits is considered the primary contributor to subsidence in the Mississippi River deltaic plain … Primary consolidation occurs as soil volume is reduced due to dewatering under the weight of overlying sediment.  Oxidation of organic matter through chemical reactions also reduces soil volume.  Fine-grained deposits with high water content characterize the Louisiana coastal zone.  Thicker sediment deposits contain more interstitial water available for removal, which leads to high rates of subsidence as they consolidate.  Older deltaic deposits have undergone primary consolidation for a longer period and therefore should exhibit lower subsidence rates that recently deposited sediments”,

It has long been recognized that subsidence measured at the surface is the result of myriad geological processes such as lithospheric flexure (the deep response to the weight of sediments) and glacial isostatic adjustment (the “yoga mat effect” of coastal areas subsiding after being bulged up by the weight of a massive ice cap on central North America).  The ACRE study demonstrated that these factors are small relative to the compaction of the Holocene sediments.  The recognition of a relationship between the thickness and composition of the Holocene delta sediments and the current rate of subsidence also accounts for the impacts of faults.  The Holocene is thicker within the geological basins that are bounded by faults.  Movement on the faults has almost certainly contributed to the variation of thickness in the Holocene.  Recent fault movement may be contributing to recent rates of subsidence, but that effect is captured in the relationship between subsidence and the thickness of the Holocene.

The mathematical relationships between subsidence and the thickness and composition of the Holocene delta sediments are captured by this graph of the data.  The locations used in the ACRE studies can be grouped into “marsh sites” and “levee sites”.  The profile diagrams show that natural levee sites tend to have greater mineral content (sand and silt) while marsh sites have greater organic content.  These relationships indicate that the rate of subsidence for any location in the marsh should be determined by the equation [subsidence rate = 0.0255 x (Holocene thickness) + 2.0246] and the rate of subsidence for any location on a natural levee should be 70% of the rate for the same thickness in the marsh.  This should mean that maps of the thickness and composition of the Holocene delta sediments could be used to create a map of current subsidence rates for coastal Louisiana.

The map of Holocene thickness was published by Dr. Mark Kulp of UNO in his 2000 PhD dissertation.  The map of the composition of Holocene sediments was published as the Map of Soil Subsidence Potential by the Louisiana State Planning office in 1976.  Kulp used data from approximately 400 cores and borings to construct this map, and it is generally the most widely cited resource in the investigation of the Holocene in coastal Louisiana.

The Map of Soil Subsidence Potential was created to establish the variation in subsidence potential in Louisiana’s coastal wetlands.   The zones  established by this map range from the green areas in which there is no anticipated subsidence potential due to organic  content  to  the  vermilion  areas  that represent areas that have greater than 51 inches of accumulated organic layers by this evaluation, and are expected to have the highest subsidence rate potential.  The green areas are coincident with the natural levees of the historical distributary channels of the Mississippi River system, and they generally contain little organic content.  The vermilion areas are generally coincident with freshwater marshes where organics have been accumulating for thousands of years.   The orange zones are coincident with the intermediate to saline marsh, which generally correspond to the settings of the marsh stations measured by ACRE report and represented on the graph.  The relationship between the subsidence rate values from the marsh stations and the subsidence rate values from the natural levee stations is presented by the equation [natural levee value = marsh value x (0.7)].  This equation sets the correction value for subsidence in the orange zone on the map at 1.0 and the correction factor in the green zone at 0.7.  The black contour lines overlain on the Soil Subsidence Potential Map show the subsidence correction factor values that are intermediate between orange and green.  The vermilion zones are assigned a correction factor value of 1.1 meaning that it is estimated that anticipated subsidence rates for a given thickness of Holocene sediment in the vermilion zones will be 1.1 times the value for an equal thickness in the orange zone.  These correction factor values are used to generate the subsidence correction factor grid surface.  This process of determining a correction factor for subsidence rate based on the organic content of the Holocene sediments is considered to be the most effective way to account for differences in organic content using available data.

the Subsidence Correction Factor grid surface can be applied to the Subsidence Rate from Holocene Thickness map to generate a corrected subsidence rate map based on both the thickness and composition of the Holocene deltaic sediments.  A map of the Relative Sea Level Rise Rate map is generated by adding a value for eustatic or global sea level rise to the corrected subsidence grid.   The Relative Sea Level Rise Rate map uses this methodology and a rate of global sea level rise of 3.0 mm/yr.  This is a generally accepted average rate.  The ACRE report established that the rate of sea level rise for the Gulf of Mexico to be 2.0 mm/yr, so this map has a built-in accounting for some amount to future sea level rise.  Any value of anticipated future sea level rise could be added to the corrected subsidence grid to generate a future rate of relative sea level rise across coastal Louisiana.

The Long-Term Accretion Rate map shows that these rates are generally greater than 10 mm/yr across most of the coast of southeast Louisiana.  The Long-Term Surface Elevation Change Rate map shows similar values across much of the coast.  It appears that sediment accretion is resulting in the positive trajectories of surface elevation change.  Conversely, the Relative Sea Level Rise Rate map shows that most of the coast of southeast Louisiana is experiencing rates that are less than 10 mm/yr.  The Accretion Minus Relative Sea Level Rise Rate map shows some relatively small areas in blue where the rate of relative sea level rise is greater than the rate of sediment accretion.  For most of the area the greens and yellows indicate that accretion is substantially greater than relative sea level rise.

The fact that accretion and surface elevation changes rates are greater than relative sea level rise rates for most of the southeastern coast is the most reasonable explanation for the relative stability in land area change.

The range of land area change measured at CRMS sites across southeastern Louisiana indicates that most of the sites are relatively stable, experiencing minimal changes in land area. In general the belt of sites that are experiencing net gains (red-yellow) are inland of the belt of sites that are experiencing net losses (blue). This pattern would tend to support the idea that gains in land area and the relative stability of the coast are a result of sediment being transported landward by tidal flux. These processes are discussed in more detail in the February 8th and February 15th blog posts.

The good news for coastal Louisiana is that land loss rates are currently low, and natural sediment accretion seems to be helping to maintain stability. Coastal sustainability projects that work with these natural processes of accretion are likely to be the most successful.

References

Applied Coastal Research and Engineering (ACRE), 2019. Determining Recent Subsidence Rates for Breton Sound and Eastern Pontchartrain Basins, Louisiana: Implications for Engineering and Design of Coastal Restoration Projects. Final Report prepared for Louisiana Coastal Protection and Restoration Authority. Contract 4400009020, Task 8, 58 p.

Bianchette, T. A., Liu, K-b., Qiang, Y., Lam, N. S.-N., 2015, Wetland Accretion Rates Along Coastal Louisiana: Spatial and Temporal Variability in Light of Hurricane Isaac’s Impacts, Water, v. 8 no. 1, 16 pgs. doi:10.3390/w8010001

Blom. R., Chapman, B., Dokka, R., Fielding, E., Ivins, E., Lohman, R., 2009, Gulf Coast Subsidence, Crustal Loading, Geodesy, and InSAR, GCAGS Transactions, v. 59 p. 101-114

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

Cahoon, D. R., Marin, P. E., Black, B. K., Lynch, J. C., 2000, A Method for Measuring Vertical Accretion, Elevation and Compaction of Soft, Shallow-Water Sediments, Journal of Sedimentary Research, v. 70, p. 1250-1253

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.

Ivins, E.R., Dokka, R.K., Blom, R.G., 2007, Post-glacial sediment load and subsidence in coastal Louisiana, Geophysical Research Letters, v. 34, L16303, 5 p.

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

Kolker A.S., Allison, M.A., Hameed, S. 2011. An evaluation of subsidence rates and sea-level variability in the northern Gulf of Mexico. Geophys. Res. Lett. V.38:L21404

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

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

Meckel, T. A. 2008, An attempt to reconcile subsidence rates determined from various techniques in southern Louisiana, Quaternary Science Review, v. 27, p. 1517–1522

Penland, S., Ramsey, K.E., McBride, R.A., Mestayer, J.T. and Westphal, K.A., 1989, Relative Sea Level Rise and Subsidence in Louisiana and Gulf of Mexico, Coastal Geology Teck Report No. 3, Louisiana Geological Survey, Baton Rouge, Louisiana, 65 p.

Penland, S., Ramsey, K.E., McBride, R.A., Mestayer, J.T. and Westphal, K.A., 1988, Relative Sea Level Rise and Delta Plain Development in the Terrebonne Parish Region: Coastal Geology Tech. Report. No. 4, Louisiana Geological Survey, Baton Rouge, Louisiana. 121 p.

Sanks, K. M., Shaw, J. B., Naithani, K. J., 2019, Field-Based Estimate of the Sediment Deficit in Louisiana, Journal of Geophyscial Research – Earth Surface, preprint, 34 p.

Sharp, L. A., and Stagg, C., 2018, Surface Elevation Change and Land Change Observed Using CRMS Data, abstract, State of the Coast Conference, New Orleans, Louisiana.

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

The Bastian Bay fault

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

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

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

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

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

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

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

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

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

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

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

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

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

REFERENCES

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

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

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

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

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

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

The submergence of Fort Proctor

Fort Proctor is a pre-civil war military installation on the shore of Lake Borgne southeast of New Orleans. Historical records state that the fort was constructed 150 feet inland from the shore of the lake just north of the mouth of Bayou Yscloskey. This was also the site of Proctorville, a rail depot at the terminus of the Shell Beach Branch of the New Orleans and Gulf Railroad, which ran along the east bank of the Mississippi River to the town of Poydras, then down the natural levees of Bayou Terre aux Boeufs, Bayou La Loutre and Bayou Yscloskey to the shore of Lake Borgne. While it can logically be assumed that the original elevations of the fort and the railroad depot were necessarily at least a few feet above sea level, neither of the architectural studies of the fort conducted by Tulane or Louisiana State Universities appear to include any definitive values for the land elevation at the time of construction.

The fort is about 1,000 feet from the Shell Beach Continuously Operating Reference Station (CORS) of the National Geodetic Survey. This station is a part of the Global Navigational Satellite System that provides data for the three-dimensional Global Positioning System (GPS) network. In addition to surface positioning data this station provides a measurement of the vertical movement of the earth’s surface, which in this case can be used to estimate a rate of subsidence.

Evaluation of data from the Shell Beach CORS by the LSU Center for GeoInformatics indicates a current rate of subsidence at this location of about 6.263 millimeters per year (mm/yr) or about 2.5 inches per decade. The premise of the illustrations of the impacts of subsidence over time that follow is that this subsidence rate can be used to estimate the elevation of Fort Proctor at various points of time in the past relative to its current elevation. This relative approximation is made without knowledge of the absolute value of the elevation at any time. If data for the elevation of the fort at some point of time in the past were known, it could be used to calibrate the estimated relative rates of change based on current subsidence data.

An elevation profile of the fort taken from a Historic American Building Survey published in the report FORT PROCTOR: A CONDITIONAL PRESERVATION by Ursula Emery McClure and Bradley Cantrell of the LSU Coastal Sustainability Studio in 2013 is used as the basis for this illustration. The profile has a proportional vertical scale and a reference for the elevation of “high tide”, but it is not otherwise referenced to a defined benchmark elevation. For the purposes of illustrating the relative effects of subsidence and sea level rise the high tide level is taken to be current sea level.

For the purposes of constructing the illustration below the rate of global sea level rise and the rate of local subsidence at Fort Proctor are assumed to be constant for the time period from 1856, when the fort was constructed, to the present. A constant subsidence rate of 6.23 mm/yr for this 160 time span results in a total elevation change at the site of the fort of 39.45 inches.  The CSRIO Marine and Atmospheric Research website published a graph of long-term sea levels showing a average rate of sea level rise of 1.7 mm/yr.  This results in a change in global sea level of 10.71 inches during the same time period. This means that in 1856 the elevation of the foundation of the fort was about three and a half feet higher than it is today and sea level was almost a foot lower than it is today. The total combined “relative sea level rise” experienced at the site of Fort Proctor between 1856 and the present has been 50.16 inches based on the extrapolation of these data. In other words the site of Fort Proctor was over four feet higher than it is today relative to sea level at the time of construction. For the purposes of constructing illustrations of the relative changes of land elevation and sea level over time the elevation of the “high tide” line on the architectural drawings of the fort that follow is assumed to be current sea level. This assumption is certainly not accurate, but it allows for tying relative changes in elevation to a scaled vertical elevation profile of the fort, and it is not intended to represent an actual elevation.  It is also important to note that recent rates of sea level rise are greater than the 1.7 mm/yr long-term average used here.  Recently published research by Byrnes et al, 2019 set the current rate of sea level rise in the Gulf of Mexico at 2.0 mm/yr.  A generally accepted rate of sea level rise for the rest of the world since the 1990s is 3.0 mm/yr.