CAUSES AND EFFECTS OF A LATE 20TH CENTURY FAULT SLIP EVENT IN COASTAL LOUISIANA

Wetlands loss in the delta plain of coastal Louisiana is best understood as an episode that peaked in the late 20^th century, and has been generally declining for the past three decades. Losses were concentrated in the Barataria and Terrebonne hydrologic basins, as well as some intense hot spots in the Mississippi River Delta. This land loss episode had a strong temporal correlation with an episode of higher subsidence rate estimates derived from tide gauge records (Penland et al, 1988) and elevation benchmark leveling surveys (Shinkle and Dokka, 2004). Many of the major hot spots of wetlands loss associated with this episode are located along the surface traces of major faults, and it appears that a late 20^th century fault slip event was a principal cause of increased rates of subsidence. Wetlands loss was primarily a function of the submergence of marshes on the hanging walls of the faults.

A possible cause of the fault slip event in the Louisiana delta plain is suggested by the sites of the subsidence hot spots relative to the distributary channels of the last few historical deltas and the surface traces of the major faults. The timing of the fault slip event suggests a slow build-of surface stresses due to abnormal sedimentary loading in the 18^th and 19^th centuries and a release of that stress caused by a triggering event in the last three decades of the 20^th century. Sediment loading would have been anomalously high across the delta plain in the 19th and early 20th centuries due to high suspended sediment loads in the Mississippi River associated with the spread of agriculture across the basin. A trigger mechanism may have been provided by a period of high river stages in the 1970s which would have dilated surface aquifers throughout the distributary channel network causing changes in stress fields where the distribuary channels crossed the major faults.

The implications for coastal sustainability are striking. The hot spots of wetlands loss are for the most part hot spots of higher subsidence rates associated with faults. The objective of most coastal sustainability projects is to build and maintain the elevation of the marsh surface. There is a much lower probability of maintaining elevation in areas with higher subsidence rates. Conversely, stable areas outside of the subsidence hot spots have been effectively maintaining elevation for decades through natural sediment accretion. Sustainability projects that seek to work with nature by supplementing natural accretion on stable marsh platforms will be the most effective.