Geological underpinnings of the Modern Mississippi Delta

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

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

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

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

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

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

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

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

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

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

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

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

REFERENCES

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

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

The 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.