--> ABSTRACT: Late Quaternary Stratigraphic Architecture of a Sand-Rich Shelf and the Origin of Linear Shoals: Northeastern Gulf of Mexico, by R. A. McBride, M. R. Byrnes, L. C. Anderson, and B. K. S. Gupta; #91021 (2010)

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Late Quaternary Stratigraphic Architecture of a Sand-Rich Shelf and the Origin of Linear Shoals: Northeastern Gulf of Mexico

McBRIDE, RANDOLPH A., and MARK R. BYRNES,  LAURIE C. ANDERSON, and BARUN K. SEN GUPTA

The late-Quaternary geology of the northeastern Gulf of Mexico shelf between Mobile Bay, Alabama and Pensacola Bay, Florida was investigated using 48 vibracores, 3,000 line-km of high-resolution seismic data, 47 14C dates, and biostratigraphic identifications. Major shelf features include: (1) shore-oblique sand ridges; (2) shore-parallel, linear sand shoals, and (3) shelf-edge deltas. This study focuses on the stratigraphic signature and depositional history of the last rise of eustatic sea level within a sequence stratigraphic framework.

Six primary lithofacies and two erosional surfaces characterize the shelf deposits. Facies I is a Pleistocene soil horizon. Facies 1 is truncated by a major erosional unconformity formed by subaerial exposure during the last sea level fail (Type 1 sequence boundary) and the bay ravinement process (first flooding surface) during the subsequent post-glacial rise. The erosional unconformity is overlain by fine-grained estuarine deposits represented by Facies 2, 3 or 4 (lower transgressive systems tract [TST]). The lower TST is truncated by a shoreface ravinement diastem (second flooding surface). The diastem is overlain by a graded shell-bed (Facies 5) dominated by marine mollusks that were concentrated at the base of shoreface. Facies 5 grades into Facies 6, which is a fine to coarse quartz sand with open marine foraminifera. Facies 6 typically fines upward and represents a shelf sand sheet. Together, Facies 5 and 6 are up to 5.5 m thick and characterize the upper TST.

The morphostratigraphy of the linear shoals consists of post-transgressive deposits because: (1) the foraminiferal and molluscan assemblages are dominated by open, shallow-marine species, and (2) the deposits lie above the shoreface ravinement diastem. However, the linear form and orientation of the shoals are dictated by the underlying transgressive topography (i.e., escarpments) that was cut into the Pleistocene substrate contemporaneous with sea level rise. Hence, the upper TST is completely reactivated by storm processes and it drapes the transgressive topography. No in-situ or degraded barrier deposits are found.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.