Evaluating Bayhead Delta Back-Stepping Mechanisms by Quantifying Sediment Accommodation and Accumulation Throughout the Holocene in Mobile Bay, Alabama

Antonio Rodriguez¹, John Anderson², Alexander Simms³, and D. Lawrence Greene^4
1Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, NC
²Department of Earth Science, Rice University, Houston, TX
³T. Boone Pickens School of Geology, Oklahoma State University, Stillwater, OK
⁴ConocoPhillips, Houston, TX

The rate of creation of sediment accommodation and the rate of sediment accumulation are the primary variables that define the evolution of depositional environments, but are seldom quantified in studies of coastal evolution. From a detailed map of antecedent topography, a sea-level curve, and measured and modeled sedimentation rates these variables are quantified for Mobile Bay, Alabama throughout the Holocene. The timing of recorded rapid changes in depositional environments is compared to the calculated changes in sediment accommodation and sediment accumulation to infer causality.

Comparing cumulative changes in sediment accommodation with cumulative sediment-volume changes shows that the estimated volume of sediment accretion in the estuary kept pace with and slightly exceeded sediment accommodation until around 8.20 ka. Prior to 8.20 ka, Mobile Bay was an intertidal to supratidal marsh, likely part of the delta-plain environment. Between 8.68 and 8.20 ka the bay shoreline transgressed up the axis of the study area at a rate of ~100 m/yr while the extensive delta plain and marsh was eroded and replaced by a central-basin environment. At 8.20 ka, parts of the study area were initially submerged and the central-basin depositional environment was created. After 8.20 ka, the locations of the depositional environments of Mobile Bay were relatively static and aggraded throughout the remainder of the Holocene rise in sea level. A threshold was crossed during the early Holocene in which low-gradient antecedent topography was rapidly flooded by sea-level rise. This promoted sediment accretion landward of the study area and facilitated erosion in the study area through inundation and ravinement, which is reflected as a dramatic decrease in the rate of estuarine-basin sediment accumulation between 8.68 and 8.40 ka.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas