Geologic Framework and Sedimentology of Recent Mud Deposition on the Eastern Chenier Plain Coast and Adjacent Inner Shelf, Western Louisiana

By

ROBERTS, H.H., BENTLEY, S., COLEMAN, J.M., HSU, S.A., HUH, O.K., INOUE, M., ROUSE, L.J., JR., SHEREMET, A., STONE, G., WALKER, N., and WELSH, S.

Coastal Studies Institute, Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA

Field data collected in May 2001 by a multidisciplinary team of researchers addressed both physical and sedimentological processes responsible for shoreline progradation and shelf accretion along the eastern Chenier Plain coast. High resolution acoustic data (side-scan and chirp sonar) determined that the sedimentary architecture of the inner shelf is characterized by thin, compensationally stacked mud lobes that show surficial evidence of soft sediment deformation on various scales. Offshore, acoustically opaque muds 1-2 m thick overlie an irregular reflective surface. Multisensor core logger profiles indicate a significant increase in density below the reflective surface suggesting older, dewatered muddy sediment. However, bulk density of these sediments ~ 1 m below the seabed is still < 1.5 gm/cc. Measurements of ⁷Be in the surficial sediments of the inner shelf identify event-driven units 10-20 cm thick that are interpreted as representing spring flood deposition. Internal sedimentary structure of this surficial unit, as determined by X-ray radiography, is consistent with a single depositional event. In contrast, sediments of the newly accreted mudflats that front the eastern Chenier Plain coast are comprised of stacked depositional increments that vary in thickness from 5-15 cm. Multisensor core logger profiles, X-ray radiographs, and grain size measurements indicate that each depositional unit is composed of a mud-sand/silt couplet. Measurements of ⁷Be activity was determined in 5 cores acquired on a shore-normal transect across a mudflat fronting the study area coast. Results describe a newly accreted wedge of muddy sediment 20-22 cm thick at the seaward edge, 9-10 cm thick in a mid-flat position, and wedging out completely at the landward limit of the mudflat. Multisensor core logger profiles and X-ray radiographs indicate stacked depositional units that have decimeter-scale stratigraphy that can be correlated across the mudflat. These onshore depositional units are thought to be largely forced by cyclic process events related to the multiple passages of winter cold fronts.