Wells to Monuments Project Proposal - Phase 1

Wells to Monuments Program

Phase 1 Proposal

Purpose and Justification

Introduction

The underlying concept for the Wells to Monuments Program was conceived by Dr. Elizabeth McDade of Chinn-McDade Associates, LLC.  Dr. McDade is also a co-author of the TranSET study referenced here.  This proposal is made by the group of collaborators listed below.  The intent of the program is to repurpose orphaned oil and gas wells to be used as geodetic monuments equipped with Global Navigation Satellite System (GNSS) equipment in a manner consistent with protocols of the U.S. National Geodetic Survey.  The most prevalent GNSS is the Global Positioning System (GPS) is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services.  It may also be possible that these monuments could be attached to corner reflectors design to reflect radar signals from an interferometric synthetic aperture radar (InSAR) satellite.  Geodetic monuments provide a survey base for the state and support mapping, boundary determination, property delineation, infrastructure development, resource evaluation surveys and scientific applications.  In addition, monuments equipped with GPS and/or InSAR reflectors allow for the measurement of vertical and horizontal velocities of the Earth’s surface to which the monument is anchored.  Orphaned oil and gas wells generally consist of steel casing that is anchored many thousands of feet below the surface.  This means that monuments created from repurposed wells will measure velocities that are not impacted by near-surface compaction (e.g., Keogh and Törnqvist, 2019).  The ability to compare vertical and horizontal velocities between new deep-anchored, as well as existing shallow-anchored GNSS and tide gauge monuments would provide new insights that both increase the spatial resolution of coastal monitoring, but that also enable separation of shallow and deep contributions to coastal subsidence (vertical velocity) and all forms of infrastructure design and planning, which may be impacted by both vertical and horizontal velocities.  Spatial patterns in the new data may also enable quantification of slow creep along coastal fault systems.

Collaborators

Ahmed Abdalla

Assistant Professor Research, LSU Center for GeoInformatics

Reda M. Amer

Adjunct Professor, Tulane University School of Science & Engineering

Asst. Professor, GIS Remote Sensing Director, Lamar University

Mark Byrnes

Principal Coastal Scientist, Applied Coastal Research and Engineering, Inc.

J. Anthony Cavell

Surveyor, LSU Center for GeoInformatics

Cynthia J. Ebinger

Marshall-Heape Chair Professor, Tulane University School of Science & Engineering

Jeffrey Freymueller

Endowed Chair for Geology of the Solid Earth,  Michigan State University Department of Earth and

Karen Luttrell

Associate Professor, LSU Department of Geology & Geophysics

Chris McLindon

Geologist, McLindon Geosciences, LLC

Cliff Mugnier

Chief of Geodesy, LSU Center for GeoInformatics

Randy L. Osborne

Network Manager, LSU Center for GeoInformatics

Diana Carolina Hurtado Pulido

Ph.D. Candidate, Tulane University, School of Science & Engineering

The Wells to Monuments Program Proposal consists of four phases:

1.       Purpose and Justification

2.       Engineering Design and Cost Estimation

3.       Bureaucratic Procedure

4.       Implementation

This document is Phase 1 of the program proposal.  It is intended to outline the scientific purpose and justification for the program, and it will be followed by separate documents that define the subsequent phases.  Increasing the density of geodetic monuments in Louisiana is considered to have implicit value beyond the scientific purposes discussed here.  Louisiana currently has relatively sparse coverage of monuments in the National Geodetic Survey CORS (Continually Operating Reference Station) program.  The State of North Carolina, by comparison, has made a substantial investment in its network of geodetic monuments, and has much more robust coverage.  The benefits of the increased density of coverage in Louisiana that would result from the implementation of this proposed program extend to all phases of infrastructure design and planning including transportation, navigation, ports and coastal restoration.

In addition to providing measurements of subsidence and enhancing positioning applications, other useful information can be extracted from the GPS signals. For the wells that are located over open water, reflectometry techniques can be applied to estimate the height of the GPS antenna above the water surface from the GPS receiver’s recorded signal to noise ratio, effectively turning the GPS site into a tide gauge (e.g., Larson et al., 2013; Larson et al., 2017; Larson, 2019). In addition, all of the sites may be usable to estimate precipitable water vapor (PWV) from the atmospheric delays on the GPS signals (Bevis et al., 1992; Bevis et al., 1994; Moore et al., 2015; Shuanggen et al., 2015). PWV observations from GPS (e.g., https://www.suominet.ucar.edu) have been incorporated in improved weather forecasts, and have been used to study severe weather events and intense rainfall events (e.g., Basivi et al., 2015; Sapucci et al., 2018).

It is the intention of this proposal that the data generated by the monuments created through its implementation should be open access.  This offers the opportunity to build on current in-state data collection that is managed by the LSU Center for GeoInformatics.

Proposed Wells

Six wells were chosen from the Louisiana Department of Natural Resources (DNR) Orphaned Well Program for consideration in the proposal.  The selection of these wells was based on three factors:  1) a distribution of locations that would effectively increase the density of geodetic monuments in the coastal zone, 2) locations that are likely to provide a range of velocity signals based on subsurface geology, and 3) the utility of the well in the program based on the conditions of the well and associated infrastructure at the surface.  The wells are shown here referenced by their American Petroleum Institute (API) well serial number, which is a unique identifier for each well, and to which all records of the well are referenced.  The well locations are shown in context to the existing CORS network.

According to the DNR program the term “orphaned” refers to the current status of a particular well site and indicates that the operator of record is no longer a viable responsible party.  Following a specific notification procedure, well sites are declared officially orphaned after being sent to the State Register to be published.  Wells are removed from the Orphaned Well Program through the Louisiana Oilfield Site Restoration Program, which was created in 1993. The focus of the program is to properly plug and abandon orphaned wells, and to restore the site to approximate pre-well site conditions by cutting well casing below the surface and removing all surface facilities and infrastructure associated with the well.  Revenue for the Oilfield Site Restoration Program is generated from a fee on oil and gas production in the state which is paid quarterly by Louisiana oil and gas operators.  The fund has collected on average about $4.5 million each year.  The program has plugged and abandoned and restored 2306 wells since 1993 at a cost of $64 million.  There are currently 2833 wells remaining in the program.  The intention of this proposal is to use funds from the Oilfield Site Restoration Program to properly plug and abandon each well below the surface but to leave the well head and surface infrastructure in place to be repurposed as a deeply-anchored geodetic monument.

Subsidence

The primary objective of this proposal is to provide new infrastructure for the direct measurement of subsidence velocities in the Louisiana coastal zone.  The National Academies Consensus Study Report (2018) recognized subsidence as an essential research gap in understanding the long-term evolution of the coastal system “The causes, rates, and patterns of subsidence along the Gulf Coast are not sufficiently well understood to allow for accurate prediction at the local to regional scale.”  The reasons for this insufficiency are both a paucity of subsidence measurement infrastructure and a poor comprehension of the relative contribution of the various causal processes.  According to the most recent study of subsidence published by the Louisiana Coastal Protection and Restoration Authority (Byrnes et al, 2019) “Understanding the causes and rates of subsidence is critical to successful planning and implementation for Louisiana Coastal Master Plan Projects”.  The study recognized that subsidence measured at the earth’s surface is the result of natural causes such as the consolidation of Holocene, Pleistocene, and Tertiary age sediments, fault-induced elevation changes due to basin tectonics, and down-warping of the underlying lithosphere due to sediment loading, and human-induced causes such as lowering the groundwater table, overburden associated with flood protection levees, and marsh settling due to altered hydrology.  Other studies have recognized Glacial Isostatic Adjustment (GIA) as a contributing cause of subsidence (Love et al, 2016).  Some of these processes are rooted in geological processes that are happening deep within the earth, such as down-warping of the lithosphere, and some are rooted in very shallow processes, such as near-surface consolidation.  The study found that previous attributions of deep-seated geologic processes as the primary mechanisms of subsidence measured at the surface by Dokka (2006,2011) assumed that all measured elevation changes reflected movement deeper than the base of piling or rods.  Byrnes et al found that unanchored and/or unsleeved benchmark rods or pilings in Holocene sediment are affected by the downward force exerted by the consolidating deltaic sediment.

Keogh and Törnqvist (2019) highlighted the importance of understanding the depth at which benchmark monuments are anchored.  Tide gauges, which have been are the primary source of data used to calculate multi-decadal- to century-scale rates of relative sea-level change, are each associated with an elevation benchmark.   Keogh and Törnqvist showed that depth at which these monuments are anchored varies substantially across the coast relative to the top of the Pleistocene surface.  Holocene sediments above this surface are highly compactable.

Using deeply anchored wells as monuments would provide a true direct measurement of subsidence velocities that are not affected by the consolidation of Holocene deltaic sediments.  This proposal recommends that each deeply-anchored monument created from a repurposed well should be co-located with a shallow-anchored monument.  Each of these monuments could be equipped with both a GPS device and an InSAR corner reflector.  This configuration would allow for the most accurate assessment of subsidence signals.

Horizontal velocities

These figures from Karegar et al (2015) show values of vertical and horizontal velocities measured by CORS stations across Louisiana.  The inset in the graph on the left shows the relationship between vertical velocities and the thickness of Holocene sediments, as measured by Kulp (2000).  Byrnes et al (2019) similarly found that the spatial variability in subsidence has a “compelling relationship” with the thickness of Holocene deltaic deposits.  Karegar et al state that the horizontal velocities in the graph on the right “may reflect slow downslope movement on a series of listric normal faults due to gravitational sliding but could also represent the horizontal component of differential compaction”.  Velocity data collected from deeply-anchored monuments could provide valuable insights into horzontal motion in coastal Louisiana.  This vector of land motion is rarely considered in infrastructure design and planning.

Subsurface Geology

Every significant recent study of vertical and horizontal land motion in coastal Louisiana (Byrnes et al, 2019; Dokka et al (2006); Jones et al (2016); Karegar et al (2015); Zou et al (2015)) has recognized the potential contribution of fault slip.  However, there has been limited access to accurate maps of subsurface fault planes or surface fault traces.  The first accurate map of surface fault traces in coastal Louisiana was published by Armstrong et al (2014).  Researchers  at  Tulane  and UT-Austin used an oil and gas industry 3-D  seismic survey in Plaquemines Parish to project the surface traces of about 20 faults.  Since then, on-going work by research groups at Tulane, UNO and ULL have significantly expanded the coverage of surface fault trace interpretation using seismic data.  This research has been funded in part by grants from the U.S. Department of Transportation’s University Transportation Centers Program and through the Louisiana CPRA Center of Excellence Research Grants Program (the RESTORE Act) administered by The Water Institute of the Gulf.  Preliminary interpretations have been published by the Transportation Consortium of South Central States (Culpepper et al, 2019), and are available in GIS form on the LA Department of Transportation Website.  These published interpretations have been integrated here with unpublished interpretations by McLindon Geosciences that are the result of subsurface geological interpretation in coastal Louisiana utilizing seismic and well log data over the past 40 years.  The surface fault trace maps presented here follow the color scheme used by Armstrong et al and are considered to be the most comprehensive and accurate interpretations available.

These maps illustrate the strong relationship between vertical velocities measured by existing monuments and the thickness of Holocene deltaic sediments from Kulp (2000) that was noted by Karegar et al (2015) and Byrnes et al (2019).  It is important to recognize, however, that the axis of maximum thickness of the Holocene coincides with that of the regional geological feature commonly called the Terrebonne Trough.  Maximum Holocene thickness lies between sets of conjugate faults that form the northern and southern boundaries of the trough.  Progressive fault slip throughout the Holocene could have contributed to the increased thickness of the Holocene along this axis.  Recent fault movement could be contributing to the subsidence velocities that may otherwise appear to be solely dependent on Holocene thickness.  Deeply-anchored monuments could provide data that could help to differentiate the relative contributions of these two factors.  Horizontal velocities of the CORS sites reported by Karegar et al (2015) appear to be consistent with expected fault movement based on these interpretations.  If the horizontal velocities were entirely due to a horizontal component of compaction, the station at Grand Isle should measure a more northerly vector of motion, toward the center of the compacting basin.

The CORS network and the proposed wells are shown in context to stations in the Coastwide Reference Monitoring System (CRMS), which is operated by CPRA and USGS.  CRMS has collected a wealth of valuable data over the past two decades.  A typical station measures vertical sediment accretion, surface elevation change, land area change and soil salinity.  Currently, the highly accurate surface elevation change data does not record measurements relative to a fixed datum like sea level.  Elevation change measurements are made relative to a fixed Rod Surface Elevation Table (RSET), which is probably subsiding at an unknown rate.  An increased density of geodetic monuments among the CRMS stations would improve the estimations of subsidence rate at each RSET and would thereby allow for a more accurate estimation of the surface elevation change relative to sea level.  Eventually InSAR corner reflectors may be attached to the RSET at CRMS stations and calibrated to nearby geodetic monuments.  A direct measurement of subsidence rates at CRMS stations would allow for detailed evaluation of the relationships between accretion, subsidence and land area change.  Many CRMS stations that are measuring high sediment accretion rates are also measuring net gains in land area.  It is likely that the rate of accretion is greater than the rate of relative sea level rise (subsidence plus eustatic sea level rise) in these areas.

The geological interpretations presented here indicate that the horizontal vectors of land motion in coastal Louisiana are primarily affected by movement associated with the network of faults and salt features including salt domes and salt welds.  Hedec and Jackson (2011) modified the genetic block diagram from Rowan (1999), which shows the relationships among these features.  Schuster (1995) mapped outlines of the ridges of salt bodies that parallel the coast and the directions of horizontal movement along the salt welds (evacuated allochthonous salt).  The implied vectors of horizontal motion are consistent with those being measured by the existing geodetic monuments.  The increased density of monuments provided by the proposed program would allow for a more detailed understanding of these relationships.

Geological overview of proposed well locations

Three of the proposed well locations are shown in relationship to the surface fault traces published by Culpepper et al (2019).  Subsurface depth contours of the associated salt domes are shown in green.  These locations offer a range of settings relative to the subsurface geology that are likely to produce a range of land motion velocity signals.  The higher vertical velocities should be expected at wells 170578814100 to the south, which is near the center axis of the Terrebonne Trough and well 170572144800 which is in the Golden Meadow graben.  Well 170572297200 is north of the bounding faults of the trough and adjacent to the Clovelly salt dome.  This location should measure lower vertical velocity than those to the south.  It may also measure some component of velocity that is influenced by the dome, which rises to less than 400 feet below the surface.  Research in this area continues at Tulane with the addition of a 3-D seismic survey to the south.  Future publications resulting from this research should significantly expand the scope and detail of geological interpretations in this area.  The addition of land motion data from these proposed locations would offer a significant enhancement to understanding the results of the ongoing geological research.  The ultimate goal is to better understand relationships between subsurface geological processes and surface morphological and ecological processes.

These three proposed locations are shown in relationship to surface fault traces and salt domes mapped by McLindon Geosciences.  A portion of these fault traces are included in the publication by Armstrong et al (2014).  The eventual publication of ongoing research at UNO will provide more detailed interpretation of faults in this area.  Maximum land motion velocities values would be expected from well 170750164600 to the south, which is anchored on the hanging wall block of the Bastian Bay fault.  The lowest values would be expected from well 1707523036200 to the north, which is on the footwall block of the Ironton fault.  Well 170512090000 in the center is in a complex location involving the Barataria fault, the Lake Five fault and the Bay de Chene salt dome and would be expected to produce intermediate velocity values.  Each of these locations will be considered in the context of more detailed geological interpretations in the pdf version of the full proposal document.

Well 170750164600 is of particular interest because of its proximity to Pete Hebert’s camp. Gagliano et al (2003) collected anecdotal evidence of subsidence from local oyster fishermen and camp owners.  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 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 investigations of the Bastian Bay fault using seismic data were conducted by Armstrong et al (2014) and Dawers and Martin (2006).  The episode of fault movement implied by the rapid sinking of Pete Hebert’s camp would coincide with the formation of the Fort St. Phillip crevasse during the 1973 flood.  It may be the case that the crevasse was caused by the fault movement.  Historical crevasses at Darrow, Vacherie and Davis Pond appear to be associated with faults.  It may also be the case that movement on the fault during a major flood event was not coincidental.  The dilation of shallow aquifers during high river stages is likely to change the stress field across the surface and may contribute to triggering fault slip.  Episodic fault movement would be likely to produce variable velocity signals from a monument in this location.

Summary and Conclusions

The Wells to Monuments Program offers an opportunity to significantly increase the density of geodetic monuments in the Louisiana coastal zone by making use of existing infrastructure that would otherwise incur costs to be removed.  The array of proposed well locations offers several sites that appear to be well suited for the co-location of deep- and shallow-anchored monuments that could make use of both GPS and InSAR technologies.  Collecting velocity data from locations that were specifically chosen for the geologic setting is the most expedient way to determine the relationships between subsurface geological features and land motion at the surface – and ultimately between subsurface geology and surface coastal processes.

References

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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.

Geological underpinnings of the Modern Mississippi Delta

On May 8, 1541 the expedition of Hernando de Soto encountered the Mississippi River near the current site of Memphis.  At the time the mouth of the river was just west of Grand Isle.  The modern “birdfoot delta” had not yet been formed.  The earliest navigational charts of coastal Louisiana postdate the inception of the latest delta lobe, so the timing is not certain.  It is likely that the river avulsed to a new course directed toward what is now lower Plaquemines Parish some time in the 18th century.  For the previous 4,000 years or so that river had changed is course roughly every 500 years.  These changes or the patterns of the delta lobes that they created were not random.  The river sought out accommodation capacity for its sedimentary load with the efficiency of a living organism.

Accommodation capacity in a delta is a direct function of subsidence.  For a delta lobe to have stayed in one place over the 500-year span that it took to develop, the geological structure of the area had to be able to give way to the accumulating sedimentary load.  A recreation of the history of delta lobes in southeastern Louisiana by David Frazier shows that the river methodically sought out the salt and fault basins.  The river tended to settle on sites underlain by geological synclines generally bounded by conjugate pairs of faults that down-dropped to the center of the syncline. 

Since the time of the Pyramids of Giza the river repeatedly returned to the elongate syncline called the Terrebonne Trough that runs beneath the coast of Louisiana.  Over time the accumulated thickness of the Holocene delta sediments revealed the shape of the Terrebonne Trough with its bounding conjugate faults.  At the eastern end of the trough the alignment of the faults and the axis of the sedimentary thick takes a distinct turn to a more north-south orientation.  This is the portion of the syncline sought out and occupied by the Mississippi River some time in the 18th century.

The fault and salt map of the area represents the salt domes with colors indicating the depth to top of salt.  The warmer colors indicate shallower depths.  The large Lake Washington dome in the northwest corner rises up to about 2,000 feet below the surface.  The major faults that reach the surface are represented by fault plane contour maps.  The blue surface fault traces coincide with the 0-depth contour of the fault plane.  The basin occupied by the modern delta is bounded by salt domes on its east and west sides.  The Bastian Bay, Main Pass and Garden Island Bay faults arc around the northern and eastern flanks to form very distinct boundaries.  At depth these faults are matched by conjugate sets of faults that create a graben structure.  These faults can be seen in profile view, but they do not extend to the surface and are not represented on this map.  The central axis of the graben clearly parallels the bounding faults to the east.

The river found maximum accommodation capacity along the central axis of the graben.  The course of the main channel is straight across the center of basin directly overlying the graben axis.  It appears that the tri-furcation of the main channel at Head of Passes occurred where the main channel encountered the down to the north Head of Passes fault.  Faults have been documented to have bathymetric expression at the surface in this area.  It is possible that an escarpment on the bottom of the channel caused by this fault could have provided enough resistance to flow to cause the river to separate its flow.

Two profiles across the basin reveal the geological underpinnings.  Profile 1 crosses the large Main Pass fault.  It can be seen extending to a depth of about 27,000 feet below the surface where it meets a smaller conjugate fault.  Together they form the central graben.  Sediments in the Upper Miocene, Pliocene and Quaternary are clearly thicker on the downthrown side of the Main Pass fault indicating that it has been continually active throughout that time.  The graben probably formed some time in during the Lower Miocene, and offset the underlying Paleogene sediments.  In the early stages of fault movement, the lateral component can be greater than the vertical component.  The lateral component of slip on the faults in this graben caused the Paleogene section to be offset to the extent that the central axis of the graben is defined by a belt in which no Paleogene section is found.  This belt directly coincides with the course of the main channel of the river.

Profile 2 crosses the Garden Island Bay fault.  The similarity in orientation and growth history of this fault with the Main Pass fault indicated that they are genetically related, but they are not the same fault.  As can be seen on the fault plane map, the contours do not connect. 

Three dimensional renderings of the delta show the relationships between the channels, lobes and faults from two different perspectives.   The first one from the northwest shows the main channel running parallel to the bounding faults of the central graben.  It also reveals how the channel spread out when it encountered the Head of Passes fault. 

The perspective from the southwest suggests a relationship between faults and each of the main sub-lobes/channels on the east side of the delta.  There is an undeniable relationship between the architecture of the delta and the subsurface geological structure.

Andrew Tweel and Eugene Turner at LSU’s College of the Coast and Environment studied the relationships between watershed land use, river engineering, wetlands formation and wetland loss in the Mississippi River delta.  In a 2012 study they reported an anthropogenically driven increase in mean suspended sediment concentrations in the Mississippi River below New Orleans between the end of the 18th century and late 19th century, followed by a sharp reduction, and then a period of stabilization after 1962.  The rise of agriculture across the Mississippi River watershed in the 18th and 19th centuries, and the inefficient practices associated with it caused an increase in erosion and a dramatic rise in the suspended sediment load of the river.  By the early part of the 20th century engineering of the river including levees, locks and dams reversed that dramatic rise.

Tweel and Turner noted importantly that “The rapid growth of the Mississippi River birdfoot delta until the 1930s, which has been used as a reference for one of the world’s largest wetland restoration efforts, may not be a suitable archetype for the majority of the coast.”  In other words the patterns and rates of land area growth in the modern delta were substantially impacted by anthropogenic factors, and are not indicative of the natural patterns and rates associated with deltas prior to the European occupation of North America.

Navigational charts from 1838 forward have allowed for a reasonable reconstruction of the modern delta of the Mississippi River in ten-year increments.  This reconstruction suggests the relationships between the patterns of formation of the delta and the underlying geology, as well as the influence of changing sediment supply throughout the time span.

REFERENCES

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

Tweel, A. W. and Turner, R. E., 2012, Watershed land use and river engineering drive wetland formation and loss in the Mississippi River birdfood delta, Association for the Sciences of Limnology and Oceanography, v. 57, p. 18-28

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.