Gulf of Mexico Origin, Waters, and Biota: Volume III, Geology

10. Recent Geologic Framework and Geomorphology of the Mississippi–Alabama Shelf, Northern Gulf of Mexico

157 ␥ been the focus of numerous studies that identify geomorphologic features in the context of aggregate resources or marine habitat. High-resolution, single-channel seismic profiles (HRSP) and sediment cores collected from the shelf by various research institutions are shown in Figure 10.2. Because of their focus, the studies typically are constrained to distinct geographical areas (Table 10.1), with some conducted on a regional scale (Ludwick 1964; Doyle and Sparks 1980). The near-surface stratigraphy of the Mississippi–Alabama shelf is the product of fluvial-deltaic sedimentation, driven by sea-level oscillations during the late Pleistocene and into the Holocene. Delta progradations during high sea-level stands produced stacked, offlapping sedimentary sequences derived from the Mississippi, Pearl, Pascagoula , Tombigbee, Alabama, and west Florida river watersheds . During periods of lower sea level, the coastal plain was incised by rivers that cut across the shelf stratigraphy (Kindinger 1989). Kindinger et al. (1994) and Bartek et al. (2004) described 2 major incised valley complexes originating near the present-day position of the Mobile and Pascagoula rivers that extended across the shelf to the shelf break (Fig. 10.3). During the late Pleistocene, deltaic loads from the fluvial systems initiated differential rates of shelf subsidence. This is suggested to be the primary mechanism for fluvial orientation and shelf geometry (Bartek et al. 2004). At the end of the Last Glacial Maximum, ensuing sea-level rise flooded the incised valleys, which backfilled with terrigenous material as the bay-head deltas This paper provides a summary of previous studies and a synthesis of the surficial geology of the Mississippi–Alabama shelf, located between the modern Mississippi River Delta and the Florida carbonate platform. Presently, sedimentation processes on the shelf are a function of prevailing winds and currents; however, in the past, the shelf was the focus of numerous delta cycles. Major episodes of deposition and erosion on the shelf have been primarily in response to oscillations in sea level. As sea level fell during the last ice age, deltas moved across the shelf to the shelf edge, incising river valleys across the middle shelf and creating stacked delta sequences on the slope. The delta complexes regressed during the last sea-level rise, infilling valleys while also providing sediments for erosion . Shoals were formed throughout these processes and are found along the shelf and modern shoreline. Data collected from the shelf, incorporated into this summary, include bathymetric, geophysical, and sediment cores. The purpose of the report is to integrate past studies with archived data to provide a comprehensive overview of the geology and geomorphology of the shelf. In addition, areas of further study are identified in the summary as a bulleted list of future needs and goals. The Mississippi–Alabama shelf is bound to the west by landforms associated with the Mississippi River Delta, to the north by the barrier island systems of the Mississippi –Alabama shoreline, and to the east by the De Soto Canyon (Fig. 10.1). This portion of the northern Gulf of Mexico is described as a slowly subsiding, passive continental margin (Sydow and Roberts 1994). The shelf has 10 Recent Geologic Framework and Geomorphology of the Mississippi– Alabama Shelf, Northern Gulf of Mexico James G. Flocks, Nicholas F. Ferina, and Jack L. Kindinger 158 ~ Flocks, Ferina, and Kindinger The St. Bernard Delta was abandoned by the Mississippi River about 2000 yr BP (Otvos and Giardino 2005), and direct fluvial deposition onto the shelf has ceased. Presently, the surficial sediment of the western shelf is predominated by fine-grain material associated with prodelta deposition from the Mississippi and Mobile rivers. During the Holocene sea-level rise, carbonate buildups retreated upstream (Bart and Anderson 2004). Preservation of these valley-fill deposits provides the most complete stratigraphic history of the shelf. As sea level reached its present position, delta building on the western periphery of the shelf covered the backfilled stratigraphy. These delta deposits represent the St. Bernard Delta progradation of the Mississippi River (Fig. 10.3). Figure 10.1. Bathymetric map of the Mississippi–Alabama shelf, northern Gulf of Mexico (data from National Geophysical Data Center [NGDC]). Figure 10.2. Locations of some of the sediment cores (circles) and high-resolution, single-channel seismic profiles (lines) available in the study area. Recent Geologic Framework ~ 159 represent shelf-edge deltas, which were deposited during sea-level lowstands (Sydow and Roberts 1994). Regional Geology The continental shelf and shoreline of the...