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

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

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

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

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

JuxtaposeJS Embed

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

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

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

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

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

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

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

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

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

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

REFERENCES

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

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

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

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

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

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

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

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

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

The Montegut fault - A conduit for fluid migration

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

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

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

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

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

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

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

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

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

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

REFERENCES

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

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

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

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

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

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

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

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

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

St. Rose Fault

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

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

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

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

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

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

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

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

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

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

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

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

References

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

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

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

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

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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Changing Course - Historical Avulsions and the Old River Control Structure

The Mississippi River has been delivering its sedimentary load to the Gulf of Mexico for the past 50 million years. The site of river’s delta during that time span has varied widely across the coastal plain from an area just southeast of the present day town of Denham Springs to a shelf edge delta 200 miles south of Lake Charles.  Since the end of the last ice age that demarks the Holocene Epoch the Mississippi delta system has spread out across present day southeast Louisiana.  The river channel has been meandered back and forth across the floodplain that stretches nearly 50 miles between Baton Rouge and Lafayette.  The colorful maps of the historical channels of the Mississippi River created by Harold Fisk for the Corps of Engineers in the 1940s and 1950s offer a striking visualization of the river channel moving freely across the flood plain over the past several thousand years of the late Holocene.  Fisk also showed that the delta of the Mississippi River moved freely across the coastal plain during this time period with major changes in course or “avulsions” of the river channel.

Historical Avulsions

The construction of the southeast Louisiana delta plain over the past 7,000 years was chronicled in the seminal work of David Frazier in 1967.  Using interpretation from hundreds of cores taken across the coastal plain Frazier reassembled the process by which the land area of coastal Louisiana was built up by historical deltas of the Mississippi River.  Frazier mapped and age-dated sixteen historical deltas that were active during this time period.  Each component delta followed a cyclical pattern from initiation when a change in course in the major channel of the Mississippi system brought its sedimentary load to a new area, thorough development and eventual abandonment when the river changed course to a new location.  Abandoned deltas progressively subsided below the surface creating an open body of water which would provide the site for an eventual reoccupation by the river and the building of a new active delta.  The formation of the deltaic wetlands of southeast Louisiana is therefore best understood as a continual switching back and forth of the river channel across the coastal plain like a garden hose spraying water across a patch of lawn.

Frazier logically grouped the sixteen historical deltas into four primary complexes, which he called the Teche, St. Bernard, Lafourche and Plaquemine Delta Complexes.  Each component delta is numbered in chronological order.  The chart below shows the life span of each delta, and the maps show the location of the delta complexes and each component delta.  The first major avulsion of the Mississippi River during the late Holocene occurred about 4800 years before present when the primary channel of the river at the time, which is represented today by the course of Bayou Teche, was abandoned in favor of a more easterly course that approximated the modern course of the river to an area near what is now New Orleans.  The site of this avulsion was very near to the current location of Old River.  This site appears to have allowed for the river to switch back and forth between the Teche and St. Bernard Complexes over the next few hundred years as Deltas number 4 and 5 developed.

About 3600 years before present a second major avulsion of the river occurred at what is the modern day location of Donaldsonville.  This event initiated the formation of the Lafourche Delta Complex and the development of the primary channels of the river that are represented today by Bayou Lafourche and Bayou Terrebonne.  The third major avulsion location developed at about this same time with formation of Delta 7 in the St. Bernard Complex.  From that point forward until the very recent past the Mississippi River switched back and forth between these two points.  Deltas number 10, 12, 14 and 15 fanned out below the avulsion point at Donaldsonville, and Deltas number 8, 9, 13 and 16 fanned out from the avulsion point just south and east of New Orleans.  Delta number 11, which formed the modern day marshes that surround the city of New Orleans, formed between the two avulsion points about 1900 years before present.  The entire process of building the coastal wetlands with delta deposits of the Mississippi River over the past 7,000 years was controlled by three principal avulsion nodes.  Each of these nodes was formed by a major avulsion of the river into a significantly new hydrologic basin.  These major avulsions occurred only three times during the late Holocene.  Once the node sites had been established they provided the sites of delta lobe switching within each of the major delta complexes.  The chart shows that delta lobe switching occurred about once every 500 years throughout the late Holocene.  Any given vertical line on the chart can be taken to represent a point in time in the past.  If, for example, one were to vertically trace a line equivalent to 2,500 years before present, it would intersect the lifespans of Deltas number 6, 7, and 8.  This means that not only was the river switching back and forth between delta locations at each of the nodes, but that the flow of the river must have also been divided between two or more delta systems at the same time.  In other words while the major avulsions tended to represent a complete shift of the flow of the river from one hydrologic basin to another, the smaller delta lobe switches may have been much more transitional in nature.

Avulsions and delta lobe channel switches occur at the time and place where three necessary initial conditions exist simultaneously:  

1) The change in course of the river must offer a significant gradient advantage, i.e. a route from the current elevation of the channel at the point of avulsion to sea level that is significantly shorter than the current route of the river,

2) the route of the new course must consist primarily of substrate that is easily erodible to allow for the initial scour at the site of avulsion and the development of the new channel by erosion in the direction of the gradient advantage,

3) a trigger event, such as a large flood, or possibly an earthquake, that facilitates the change in course.

The vestiges of the earlier river channels in the flood plain seen in Fisk’s map are the point bar and natural levee deposits that define the shape of the abandoned channels.  Most of the elements of the distributary channel networks of the abandoned deltas on the coastal plain are recognized today as bayous and tidal channels, which are also flanked by natural levee deposits.  The point bar and natural levee deposits of the abandoned elements of the Mississippi River system consist of significant percentages of sand and sandy soils.  This concentration of easily erodible sandy soils in the meander belts and distributary courses of the older river channels offers an explanation as to why the river has tended to continually reoccupy previous pathways by way of a select few avulsion nodes.  Changes in course of the river therefore tend to occur during flood events at the sites of one of the established avulsion nodes when there is a route from that node to sea level that offers a significant gradient advantage over the current course of the river.

The Old River Avulsion

The initial conditions that favored the next major avulsion on the Mississippi River began to develop in the 16th century at the site of the original late Holocene avulsion, which had occurred about 4,800 years before present.  The maps below show a time sequence of active channel configurations taken from Aslan et al. 2005 and overlain on Google Earth imagery.  The latest in the sequence is in the lower right corner and shows a representation of the active channels of the Mississippi River in red, the Red River in green and the Atchafalaya in orange.  The image is dated 1800 AD and shows the point in time at which a westerly-migrating meander loop of the Mississippi had fully intercepted the course of the Red River, and the two river were exchanging flows.  What had been the recently established southern course of the Red River that flowed into Atchafalaya Bay became recognized as the Atchafalaya River, which was by then receiving flow contributions from both the Mississippi and the Red River.  The northern and southern reaches of the meander loop of the Mississippi, which had begun interacting with the Red River 200 years earlier, were named Upper and Lower Old River.

The Lower Old River channel became the primary conduit of flow, and it maintained a bi-directional flow into the early 20th century.  Some of the time water from the Mississippi flowed westward through the Lower Old River channel and exited to the Gulf of Mexico through the Atchafalaya, and some of the time flow from the Red River flowed eastwardly through the channel and combined with the flow of the Mississippi in its course to the Gulf.  The maps above also show in the three other images the sequence that meander belts of these river systems had crisscrossed the point of avulsion several times in the past leaving behind sand-rich and easily erodible deposits that helped to facilitate the most recent change in course.  5,000 years before present the active channel of the Mississippi River shown in blue in the upper left image followed a course to the Gulf that is represented by Bayou Teche today.  The first major avulsion of the river occurred about 200 years later with a shift of the active channel to the east.  The general course of this channel is still shown to be the active channel 2,000 years before present represented as the green meander belt in the upper right image.  This channel was flowing into the St. Bernard Delta complex at this point in time, but the avulsion node at Donaldsonville had been established, and for the next 2,000 years the river switched back and forth between the St. Bernard/Plaquemine Complexes and the Lafourche Complex.  The image in the lower left shows that about 900 years before present the Red River shown as the purple channel crossed over the floodplain and joined the active channel of the Mississippi shown in orange.  At some point in the next few hundred years a crevasse in the Red River initiated a flow into the low-lying Atchafalaya Basin, an area between the elevated meander belts of the historical Bayou Teche and Mississippi channel courses.  This flow of the Red River into the Basin had a significant gradient advantage to the Gulf of Mexico, but it did not encounter the sandy soils of either of the bounding meander belts.  This lower arm of the Red primarily cut through clay-rich organic soils that had been deposited in the backswamps of the older river systems.  This accounts for the remarkably straight course of what is now the Atchafalaya River, and it may ultimately have been a factor in capturing the flow of the Mississippi River because the lower channel offered such a straight course to sea level.

It may also be true that the relative difficulty of eroding the clay-rich soils of the Atchafalaya Basin may have accounted for the extended transitional period over which the progression toward a complete avulsion developed.  The top graph below from Mossa, 2013 shows that the flow of the Atchafalaya measured as a percentage of the combined flows of the Atchafalaya and Mississippi Rivers at Old River progressively increased throughout the first half of the twentieth century.  At the same time the percentage of the total flow of the Mississippi that went through Old River went from oscillating between positive and negative during the period of bi-directional flow to progressively increasing in the direction of flow going down the Atchafalaya, as shown in the middle graph.  The bottom graph shows that the cross sectional area of the Atchafalaya measured at three stages of river elevation also grew steadily during this time period.  The combination of this data illustrates that progression of avulsion was dependent on the interrelationship between the amount of water that moved from the Mississippi River into the Atchafalaya and the size of the channel south of the conjunction at Old River.  A survey of the rivers in 1950 by Latimer and Schweizer for the Mississippi River Commission recognized the fundamentals of these trends, and they predicted that 43% of the flow of the Mississippi would be discharging into the Atchafalaya River by 1965.

Old River Control Structure

As a result of the survey of Latimer and Schweizer and the work of Harold Fisk, the U.S. Congress passed the Flood Control Act of 1954, which authorized the construction of the Old River Control Structure, which was completed in 1963.  The lower arm of the original meander loop of the Mississippi that had intercepted the course of the Red River had become the channel component known as Lower Old River.  The upper arm of this loop, which had been known as Upper Old River, was eventually cut off from the flow of the river primarily because of the dredging of a channel cut in 1831 called “Shreve’s Cut”, which offered a more direct course for flow of the river across the neck of the original meander loop.  The initial construction of the Control Structure included the installation of a lock and dam on Lower Old River that effectively prevented flow in either direction, but allowed for boat and barge traffic moving between the rivers.  It also included the creation of the Outflow Channel located north of the original Upper Old River Channel, and the installation of the Low Sill Structure, which was used to regulate flow from the Mississippi into the Atchafalaya.   It is the stated objective of the Old River Control Structure to maintain the “latitude flow” of both rivers measured at the latitude of Red River Landing (30°56’20.4”) below the Structure at a proportionate ratio of 70% in the Mississippi channel and 30% in the Atchafalaya channel.  The integrity of the Old River Control Structure was tested during the spring of 1973 in a major flood event.

One of the principal initial conditions for avulsion of the Mississippi River – the easy erodibility of the sand soils at the site of the avulsion – is also a principal reason for the near failure of the Old River Control Structure during the 1973 Flood, as well as the high probability that this historical avulsion node will be the site of the ultimate failure of the Structure and the change in course of the river.  The figure below is a profile across the site of the Old River Control Structure compiled using the interpretation from borings taken along the path of the Outflow Channel prior to its construction.  The profile documents the very sandy composition of the soils beneath the Structure down to depths of 100 feet and greater.  The upper layers of sandy soils are the natural levee and point bar deposits of the numerous channels of the river that have migrated back and forth across the flood plain.  The abandoned channels are evidenced by the clay-filled plugs they leave behind that dissect the plain.  The channels cut into a substratum of undifferentiated  sands  and  gravels  that  are the  deposits  of  the  early Holocene  that  filled  in  the Mississippi River valley as sea level rose with the melting of the last ice caps.  These layers of sandy soil are also the surface aquifers of the Mississippi River Valley alluvium.  There is good evidence that water from the Mississippi River flows into these surface aquifers, and that flow is increased during high river stages.  It may be possible that this subterranean flow of water from the river into the sandy soils formed as deposits of the meander belts of older river channels may precede, or even predict, the flow of the current of the river that cuts the path of a new avulsion of the river.

The flow of water from the Mississippi River reached its highest level since the construction of the Old River Control Structure during the flood of 1973.  The estimated rate was 2 million cubic feet per second – about twenty times the flow of Niagara Falls – and it stayed at that level for almost three months.  The flux of water from the river into the alluvial aquifers and the turbulence from the flow through the Low Sill Structure scoured out the foundation, and cut deeply into the sandy substratum beneath it.  The figure below shows that the current of the river completely undermined the Low Sill Structure cutting down to a depth of 130 feet below sea level.  The upper 50 feet of the 90-foot long steel pilings supporting the structure were exposed, and it came close to complete failure.  The U.S. Army Corps of Engineers constructed the Auxiliary Structure in an effort to reduce the flow across the Lower Sill Structure during a flood.  This structure shown in figure 6 was completed in 1986.  The Corps also dumped tons of rip rap into the scoured opening after the flood, but there is nothing in place to insure that the flow of water will not undermine the structure again in a future flood event.  A failure of this type at the Old River Control Structure during a major flood is the most likely occurrence that will precipitate an avulsion of the Mississippi River.

Impacts of avulsion at Old River

The immediate impacts of an avulsion of the Mississippi River caused by a failure of the Old River Control Structure would depend in large part of the nature of the failure and the severity of the triggering event whether it be a flood or an earthquake.  Scenarios for the impacts discussed by Johnson, 1990 include destruction of the Interstate 10 and U.S. Highway 190, as well as railroad bridges and natural gas pipelines crossing the Atchafalaya Basin.  Johnson also noted the potential for severe impacts due to flooding on Morgan City and the possibility for widespread flooding across much of coastal Louisiana south of New Orleans.  The longer term and sustained impacts of an avulsion would be seen along the channel of the Mississippi River between New Orleans and Old River.  The abandonment of the current channel of the Mississippi River would have profound long term impacts on the capacity of the channel as a navigational conduit and as a source of freshwater for industrial and residential use along all reaches of the river south of point of avulsion.

The figure below shows an elevation profile for the Mississippi River channel from the mouth of the river at the Southeast Pass jetty to Lake Providence in northern Louisiana.  The elevation of the base of the channel is below sea level for the full extent of the river from the mouth to a point north of Old River.  This means that without the flow of freshwater moving down the channel saltwater from the Gulf of Mexico would flow into all portions of the channel below sea level creating a “slackwater estuary”.  All points along the river south of Baton Rouge would be completely filled with saltwater, and there would be no short term source of fresh water for industrial and residential use along this stretch.  It is probable that without the flow of river water in the channel the elevation of the river surface would also drop to sea level for all points south of the point of avulsion.  With this drop in the elevation of the river and the loss of the scouring effects of the river’s current the channel north of Baton Rouge may become impassible to boat and barge traffic without program of regular dredging. 

REFERENCES

Aslan, A., Autin, W. J., Blum, M.,  2005, Causes of River Avulstion Insights from the Late Holocene Avulsion History of the Mississippi River, USA, Journal of Sedimentary Research v. 75, p. 650-664

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

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

Fisk, H. N., 1952, Geological investigation of the Atchafalaya basin and the problem of the Mississippi River diversion. Vicksburg, MS: US Army Waterways Experiment Station, Mississippi River Commission

Johnson, D. B., 1980, A Change in the Course of the Lower Mississippi River: Description and Analysis of Some Economic Consequences, LA Water Resources Research Institute, Bulletin 12B

Latimer, R. A., & Schweizer, C. W., 1951, The Atchafalaya River study: A report based upon engineering and geological studies of the enlargement of Old and Atchafalaya rivers. Vicksburg, MS: US Army Corps of Engineers

Mossa, J., 2013, Historical changes of a major juncture: Lower Old River, Louisiana, Physical Geology, v. 34, p. 315-334

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.

Breton Island

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

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

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

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

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

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

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

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

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

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

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

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

REFERENCES

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

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

The Bastian Bay fault

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

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

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

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

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

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

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

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

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

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

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

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

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

REFERENCES

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

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

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

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

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

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

The Vacherie fault

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

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

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

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

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

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

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

References

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.