HR: 09:15h
AN: G21D-06    [Abstracts]
TI: Numerical constraints of current rates of subsidence due to compaction of shallow sediments in the Louisiana coastal plain
AU: * Meckel, T A
EM: tmeckel@usgs.gov
AF: United States Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AU: ten Brink, U
EM: utenbrink@usgs.gov
AF: United States Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Williams, S J
EM: jwilliams@usgs.gov
AF: United States Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AB: The conversion of wetlands to open water occurs at alarming rates in coastal Louisiana, where relative sea level rise is ~1cm/yr. The value of wetlands for buffering storm surge and providing habitat has prompted efforts to identify and mitigate subsidence (and other) processes that cause wetland loss. Quantitative estimates of the relative contributions of geologic and anthropogenic processes to subsidence of coastal Louisiana are largely unknown. Such information is critical for ongoing regional wetland restoration. We use a stochastic methodology (see related abstract) to estimate the probable contribution of shallow (<200 m) natural compaction to present subsidence rates in the Louisiana coastal plain. Knowledge of present stratigraphic thickness and accumulation time at a location allows the cumulative distribution and probable maximum and minimum rates of surface elevation change due to compaction to be predicted. We then convert present thickness data above the late Wisconsin unconformity in coastal Louisiana to probable ranges of present rates of surface displacement due to compaction, assuming a constant accumulation rate. If regional average accumulation rates have been ~10 mm/yr, 90% of our calculated rates of land surface elevation change due to compaction of post late-Wisconsin sediments are <~2 mm/yr. If regional average accumulation rates have been closer to 40 mm/yr, 90% of compaction rates are likely <~4 mm/yr. We provide graphic solutions for assessing maximum probable surface-displacement rates due to compaction at any site of interest, provided fundamental stratigraphic characteristics are moderately well known. Our methods enable the identification of locations where observed subsidence rates are unlikely to result from post late-Wisconsin unconformity sediment compaction alone for further investigation. Observed subsidence rates greater than the 90^th percentile of the calculated compaction rate distributions likely reflect processes in addition to compaction of post-late Wisconsin sediments.
DE: 1641 Sea level change (1222, 1225, 4556)
DE: 1861 Sedimentation (4863)
DE: 1890 Wetlands (0497)
DE: 4556 Sea level: variations and mean (1222, 1225, 1641)
SC: Geodesy [G]
MN: Fall Meeting 2005