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Surficial Sediments and Morphology of the Southwestern Alabama/Western Florida Panhandle Coast and Shelf
Randolph A. McBride, Mark R. Byrnes
Surficial sediments and associated morphology were analyzed for the southwestern Alabama/Florida Panhandle coast and shelf (between Mobile and Pensacola Bays) using 35 cores, digital National Ocean Service (NOS) hydrographic data, and existing sedimentologic studies. The study area lies on the eastern margin of the northern Gulf Coast depocenter and is blanketed by a surficial sand layer known as the Mississippi-Alabama-Florida (MAFLA) sand sheet. Surficial sediment samples collected from the shelf show little textural variation and are composed of >90% sand, <2.7% mud, and <2% granules. Median grain size for shelf sediments ranges from 0.14 to 0.46 mm or fine to medium sand. Shelf sediments show a definite size and sorting trend in a westerly direction from a moderately sorted, medium-grained quartz sand to a moderately-well sorted, fine-grained quartz sand. In contrast, the estuaries (Mobile, Perdido, and Pensacola) are characterized by a much greater range in mean grain size, fluctuating between 0.002 and 0.63 mm or clay and coarse sand.
The estuarine systems can be divided into two geomorphic zones that consist of a narrow marginal platform and a deeper, central basin. The shallow platform projects out to about the 1.8 m contour and has an average width of less than 1 km. Central basin depths range from 1.8 to 11.6 m, and the bay floor is characterized by a fairly flat central area that gently slopes toward the central bay axis and the Gulf of Mexico. In contrast, the continental shelf 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 sand ridges. These ridges form oblique angles to the adjacent shoreline (azimuth 70°) and open to the east. Zone 2 encompasses the middle to outer shelf between 20 and 50 m water depth. The seafloor in this area is dominated by two long, linear, shelf break-parallel shoals (North and South Perdido shoals) that can exceed 5 m in relief. A 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 inter-fluves between relict fluvial drainage systems. Zone 3 extends seaward from 50 to 150 m water depth and includes the outer shelf/shelf break area. This zone has steeper slopes than Zones 1 and 2, and is dominated by 10 to 25 km wide, shelf margin lobes that occur along strike. The largest shelf margin lobe occurs in the southwest corner of the study area, whereas the three lobes to the northeast (along the western margin of the DeSoto Canyon) are much smaller. The largest shelf margin lobe, 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 lobes are located seaward of the narrow, shore-normal bathymetric low. These features are probably related to the lowstand fluvial systems (i.e., incised valleys and shelf edge deltas) of the Mobile and Perdido/Escambia Rivers. The morphology of the eastern Alabama/western Florida Panhandle shelf likely is produced by a combination of pre-transgressive, transgressive, and post-transgressive processes.
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