HR: 0800h
AN: G11A-1200    [Abstracts]
TI: A Stochastic Computational Method for Determining Present Rates and Stratigraphic Controls of Shallow Compaction in Coastal Deltaic Environments
AU: Williams, S J
EM: jwilliams@usgs.gov
AF: United States Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
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
AB: Natural sediment compaction in coastal environments influences subsidence, coastal geomorphology, and relative sea level rise. The most accurate calculations of present compaction rates require detailed knowledge of subsurface geotechnical properties and depositional history, neither of which is often readily available. We numerically model the incremental accumulation and compaction of stochastically-generated stratigraphies to capture the anticipated range of present compaction rates for an array of thicknesses and accumulation times. Our stochastic methodology is easily modified for any modern delta or coastal environment. We use Monte Carlo simulations to construct stratigraphies that are randomly composed of five facies typical of the modern Louisiana coastal plain (peat, natural levee, pro-delta mud, bay mud, bar sand). Each facies has a constant suite of previously measured geotechnical parameters (initial porosity, compressibility, grain density, and constants relating permeability to porosity). Synthetic stratigraphies are generated incrementally by randomly selecting a facies with layer thickness and accumulation rates derived from predefined exponential and gamma distributions (respectively). The simulations solve for vertical displacement rate of the uppermost surface for one-dimensional compaction models based on Darcy flow and Terzaghi effective stress principles using a finite difference technique. Obvious potential stratigraphic influences on present compaction rates include facies amounts (proportions), accumulation rates, and facies order. We performed detailed analyses of modeled stratigraphies with present compacted thickness ~100 m that accumulated in ~10 k.y. We are surprised to observe that, for these stratigraphies, modeled present compaction rates do not correlate with the proportion of total depositional thickness for any facies (strongest correlation coefficient for peat facies=0.25), nor with recent accumulation rates. In general, the fastest and slowest compacting modeled stratigraphies are not associated with a characteristic facies order either. The abundance of peat appears to be the most consistent characteristic of fast and slow compacting stratigraphies, but is not diagnostic.
DE: 1641 Sea level change (1222, 1225, 4556)
DE: 1861 Sedimentation (4863)
DE: 1890 Wetlands (0497)
SC: Geodesy [G]
MN: Fall Meeting 2005