Central Wetlands Unit — Blog — McLindon Geosciences, LLC

The most tangible evidence of subsidence in the Central Wetlands Unit near Chalmette, Louisiana is the elevation of the roadbed of Old Paris Road. WVUE News anchor John Snell stood on the roadbed with the City of New Orleans in the background for one installment of his “Coast In Crisis” series. Old Paris Road was built in the 1930s. It is now nearly two feet under water.

Independent studies of subsidence using different measuring techniques found subsidence rates between 15 and 20 millimeters per year (about 5 to 8 inches per decade) in the area of the Central Wetlands Unit, making it one of the “hot spots” of local subsidence on the Louisiana coast. Zou et al, 2015 used data from a US Coast and Geodetic Survey project to resurvey elevation benchmarks to derive rates of change in elevation. Dixon et al, 2006 used InSAR satellite technology, which measures changes in elevation by reflecting radar off of hard surfaces. It is nearly impossible to measure changes on the marsh surface with either technology, but the buildings along New Paris Road provided enough of a reflective surface to provide measurements.

LSU Geologist Dr. Harold Fisk used boring profiles to evaluate the vicinity of the Central Wetlands Unit in his 1944 publication “Geological Investigation of the Alluvial Valley of the Lower Mississippi River” for the Corps of Engineers. In one of the profiles seen here Fisk identified a fault offsetting the layers of accumulated sediment. The top of the Pleistocene surface, which underlies the entire region, and is used as a piling anchor for nearly all construction, was offset by about 20 feet by the fault. The layer of humus composed of peats and organic clays was also measurably thicker on the downthrown or hanging wall side of the fault. This fault has since been named the Gentilly fault, and it appears to play an important role in the subsidence hot spot at the Central Wetlands Unit.

Fisk later produced a map of the thickness of accumulated peat in the subsurface around New Orleans. A comparison of the peat thickness map with the Dixon subsidence map shows a significant correlation. The peat layer beneath the Central Wetland Unit and along the downthrown side of the Gentilly fault is the thickest and largest in the area. The common wisdom is that the thick peat layers are more compactable, and therefore cause the higher rates of subsidence. A consideration of why the peat layers are thicker is some areas than others suggests both a cause and effect relationship between peat thickness and subsidence. Some areas are subsiding at higher rates because the underlying peats are thicker, but the peats are also thicker because the area is subsiding. Subsidence rates have been accelerated even further within the levee system around New Orleans as canals and drainage pumps have dried out and oxidied the peat layers causing them to compact even faster.

Gould and Morgan, 1962 provided another set of profiles across the area revealing more about the patterns of peat accumulation. The marshes between the distributary channels of the Mississippi delta system maintain their elevation by organic growth with periodic minor influxes of mineral sediment from river flooding and tidal flux. In areas where subsidence rates are higher the rate of organic growth must also be higher to maintain elevation. The marsh literally builds upon itself as the plant material is submerged below the surface.

The relationships between the Gentilly fault, subsidence, and the thick accumulation of peat on the downthrown side of the fault is a good example of the feedback loop between faults and accumulating sediments. Subsidence events may be caused by the episodic tectonic movement of faults. The magnitude and frequency of these episodic slip events is poorly understood in coastal Louisiana. Larger fault slip events may occur once every few centuries and may cause vertical offsets at the surface of a few inches to a few feet. Faults may also move in very small incremental slip events that are closer to a creeping movement. The vertical movement on the Gentilly fault is evidenced by the offset at the top of the Pleistocene and the thicker accumulation of sediment on the downthrown side. The total vertical displacement on the fault since the end of the Pleistocene is likely to be the result of a combination of larger episodic slip events and cumulative slow creep movement. Downward movement of the hanging wall side of the fault relative to the foot side creates accommodation space that allows for the accumulation of more sediment on one side than the other. This differential has two effects. The greater thickness of compactable organic material on the hanging will create a higher rate of subsidence due to compaction. The differential in subsidence rates across the fault may be taken up as induced fault slip that is exclusive to the near-surface interval, and is not associated with deeper fault slip. The accumulation of greater thicknesses on the hanging wall also means that the there is a weight differential across the fault, which would tend to cause a fault slip response. These processes combine to create a loop in which the generation of accommodation space allows for more sediment accumulation on one side of the fault, which propagates fault movement.

The plane of the Gentilly fault can be mapped in the subsurface using standard oil and gas industry techniques. The surface trace of the fault coincides with the 0-depth contour of the fault plane map. The two profiles across the area from Fisk and Gould and Morgan are shown in relative perspective. What appears to be a westward extension of the Gentilly fault was also mapped by Dokka, 2011 using a LIDAR digital elevation model. The offset of the fault can be seen as a change in elevation at the surface. Dokka also related the surface expression of the fault to cracks in the roadway.

Understanding the relationships between fault movement and subsidence is critical to the long term sustainability of the Louisiana coastal zone. Much more scientific investigation is needed.

REFERENCES

Dixon, T.H., Amelung, F., Ferretti, A., Novali, F., Rocca, F., Dokka, R., Sella, G., Sang-Wan, K., Wdowinski, S., Whitman, D., 2006, Subsidence and flooding in New Orleans, Nature, v. 441, p. 587-588

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

Gould, H. R. and Morgan, J.P., 1962 Coastal Swamps and Marshes, Geology of the Gulf Coast and Central Texas, p 287-341

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.