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.
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