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R. Van Richmond1, Philip J. Bart2, John B. Anderson3
(1) Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA
(2) Louisiana State University, Baton Rouge, LA
(3) Department of Geology and Geophysics, Rice University, Houston, TX

Abstract: Late Quaternary sequence development on a slowly subsiding, ramp margin: offshore Alabama/northwest Florida

Approximately 2600 km of high-resolution single channel seismic data were analyzed as part of a larger ongoing project investigating variability in sequence stratigraphic architecture across the northern Gulf of Mexico margin. To date, this project has demonstrated tremendous lateral variability in stacking patterns. This variability has been attributed to differences in drainage basin size, surface geology, vegetative cover, shelf physiography and subsidence rates, which affect creation and infill of accommodation. At this point in the project, we are evaluating the isolated influence of these factors. The Florala study area is unique because it is characterized by small, sandy coastal plain drainage systems with minimal floodplain storage, a narrow ramp-style shelf, and low subsidence rates. Furthermore, the margin receives minimal influence from growth faulting and salt tectonics.

Seismic stratigraphic correlation to a Main Pass 303 drill site indicates that the strata investigated were deposited during the last glacio-eustatic cycle (i.e. oxygen-isotope stage 5 to 1). Time-structure and time-thickness maps show a thin, but broad clastic shoreline that rapidly prograded across the shelf as sea level fell and accommodation diminished. During the stage 4 lowstand, large fluvial deltas locally prograded the shelf margin. Topset truncations of high angle foresets indicate numerous delta lobe switching events and major shelf-margin progradation continued through to the stage 2 lowstand. The margin remained constructive throughout the lowstand. During the transgression, the shelf-margin deltas and adjacent shoreline deposits were severely ravined. Ravinement resulted in the formation of a relatively thick (maximum thickness=10 m) transgressive sheet sand.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana