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Abstract: Surficial Sediments and Morphology of the Southeastern Alabama/Florida Panhandle Shelf

Randolph A. McBride

Analysis of surficial sediments and associated morphology was performed for the southeastern Alabama/Florida Panhandle shelf using 48 cores and digital National Ocean Service (NOS) hydrographic data. The study area lies on the eastern margin of the Gulf Coast geosyncline and is blanketed by a surficial sand layer known as the Mississippi-Alabama-Florida (MAFLA) sand sheet. Surficial sediment samples collected from the study area show a definite size and sorting trend to the west from moderately sorted, medium-grained quartz sand to moderately well sorted, fine-grained quartz sand.

The study area can be separated into three geomorphic zones. From north to south, Zone 1 ranges from 0 to 20 m water depth and is dominated by shoreface sand ridges. These ridges form oblique angles to the adjacent shoreline (azimuth 70°) and open to the east. They are generally over 700 m long, have relief up to 5 m, side slopes that average less than 1°, and are 1 to 4 km wide with wavelengths of 1 to 8 km. Zone 2 encompasses the middle to outer shelf between 20 and 50 m water dept The seafloor in this area is dominated by two, long, linear, semi-shore-parallel shoals that can exceed 5 m in relief. Water depths over both sand bodies shallow to less than 25 m on the northeastern end and slowly deepen to the southwest to a depth of about 36 m on the seaward-most shoal. A sho e-parallel bathymetric low occurs landward of each linear shoal. Moreover, a distinct, shore-normal bathymetric low (oriented NW/SE) cuts across both linear shoals and continues updip (landward) toward Perdido Bay, Florida. The southwestern end of each linear shoal is also truncated by a much broader bathymetric low, suggesting that the linear shoals are best developed on interfluves between relict fluvial drainage systems. Zone 3 extends seaward to 125 m water depth and includes the outer shell/shelf break area. This zone has steeper slopes than Zones 1 and 2, and is dominated by 10 to 25 km wide, shelf margin bulges that occur along strike. The largest shelf margin bulge occurs in the southwest corner of the study area and exists in water depths that range between 65 and 200 m. To the ortheast (along the western margin of the DeSoto Canyon), the bulges are much smaller and their seaward extent occurs in water depths averaging about 125 m. The largest shelf margin bulge, the broad bathymetric low, and the outer linear shoal found to the southwest have similar morphologic characteristics to the modern Mobile Bay and barrier system that lies directly updip. In addition, the smaller shelf margin bulges to the northeast are located seaward of the narrow, shore-normal bathymetric low. This spatial relationship between these two geomorphic features suggests that, during the last lowstand, the Perdido and Escambia drainage systems possibly converged on the inner to middle shelf and debouched along the shelf margin as a single fluvial system.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana