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Seismic Stratigraphy and Late Quaternary Evolution of Mobile Bay and Eastern Misssissippi Sound, Alabama: A Record of Large- and Small-Scale Fluvial Systems through Multiple Sea-Level Cycles

D. Lawrence Greene
The University of Alabama, Tuscaloosa, AL

Examination of the Mississippi and Alabama shelf, mapping of offshore incised-valleys and shelf-edge deltas, and determination of their feeder systems has been heavily examined over the last twenty years largely focusing on the Mobile River with variation among investigations. In attempt to end this controversy while examining the relationship of piedmont versus coastal-plain fluvial systems to sea-level fluctuations, approximately 750 km of high-resolution seismic data and 18 drill cores were collected from Mobile Bay, the Mobile Bay-head Delta, eastern Mississippi Sound, to map the headward components of previously published offshore valleys and to compare the incised-valley model.

Seismic data show that the Late Quaternary stratigraphy is composed of at least four unconformity-bound stacked seismic units. This study focuses on the upper Seismic Units 1 and 2. The youngest unconformity is an exposure surface sampled in cores and associated with the last lowstand in sea-level (Oxygen Isotope Stage 2). The underlying surface is a high relief exposure surface sampled in cores and interpreted as the Oxygen Isotope Stage 6 Sequence Boundary. Mapping of these sequence boundaries reveals a complex network of channels through Mobile Bay and Mississippi Sound that re-incise in approximately the same location through multiple sea-level cycles.

Lithologic data show that the valley fill sequences consist of the same facies in both Mississippi Sound and Mobile Bay and roughly conform to the idealized model. Variations in valley fill are attributed to terrace morphology. Thus, the two estuaries while different in their orientations have similar evolutionary histories.