HR: 1330h
AN: G43A-06    [Abstracts]
TI: Estimates of historic, present, and future rates of surface displacement due to hydrodynamic autocompaction of Holocene sediments in the Louisiana delta plain
AU: * Meckel, T A
EM: tmeckel@usgs.gov
AF: United States Geological Survey, Woods Hole Science Center 384 Woods Hole Road, Woods Hole, MA 02543-1598 United States
AU: ten Brink, U
EM: utenbrink@usgs.gov
AF: United States Geological Survey, Woods Hole Science Center 384 Woods Hole Road, Woods Hole, MA 02543-1598 United States
AU: Williams, S
EM: jwilliams@usgs.gov
AF: United States Geological Survey, Woods Hole Science Center 384 Woods Hole Road, Woods Hole, MA 02543-1598 United States
AB: Subsidence and resulting rapid relative sea level rise and wetland loss in Louisiana have been attributed to natural geologic (isostatic sediment loading, faulting, sediment compaction, dewatering) and human (subsurface fluid withdrawal, salt mining) processes, but we lack quantitative estimates of the relative contributions of each factor, and their role in future subsidence is largely unknown. Such information is critical for ongoing regional-scale wetland restoration plans. We attempt to isolate the component of subsidence attributable to compaction (gravity-driven reduction in porosity and resulting surface displacement) of sediments deposited above the Lower Wisconsin unconformity (<~18 k.y.) in the delta region. One-dimensional, multi-lithology compaction models based on Darcy flow and Terzaghi effective stress principles are solved using a finite difference technique, allowing calculations of cumulative sedimentation and surface displacement, as well as historic, present, and future (in the absence of further loading) rates of vertical surface displacement. Detailed modeling of the USGS/LGS P-1-90 boring (47 m recovered, 13 radiocarbon dates), at the LUMCON facility in Cocodrie, revealed relationships among sediment type, geotechnical parameters, depositional rate, time, and compaction rate. Results indicate that, regardless of variations in depositional history, present rates of surface displacement can be constrained by two end-member stratigraphic models: 100% sand and 100% mud. Shallow (0-200 m) borehole data from the U.S. Army Corps of Engineers have been used to interpolate a regional isopach of Holocene sediments for the Mississippi River alluvial valley and adjacent delta plain and to extend lithologic generalizations (topstratum/substratum) and suitable geotechnical parameters from the USGS boring. Regional-scale analysis allows comprehensive testing of the hypothesis that higher subsidence rates in the alluvial valley can be attributed to compaction of thicker Holocene deposits.
DE: 1208 Crustal movements--intraplate (8110)
DE: 7843 Numerical simulation studies
DE: 8105 Continental margins and sedimentary basins
DE: 8107 Continental neotectonics
SC: Geodesy [G]
MN: 2005 Joint Assembly