Seafloor Morphology, Geologic Framework, and Sedimentary Processes of a Sand-Rich Shelf Offshore Alabama and Northwest Florida: Northeastern Gulf of Mexico.
Date of Award
Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
Harry H. Roberts
Late-Pleistocene and Holocene geology of the northeastern Gulf of Mexico shelf offshore Alabama and northwest Florida was investigated using 38 vibracores, 47 radiocarbon dates, foraminiferal and macrofauna assemblages, and bathymetry data. The morphologic and stratigraphic signatures of the last rise of eustatic sea level was examined along a passive continental margin. Major shelf features include shore-oblique sand ridges, mid-shelf linear shoals, and shelf-edge deltas. Surficial shelf sediments consist of $>$90% sand, $<$2.7% mud, and $<$2% granules and fine in a westerly direction from a medium to fine sand. The sharp boundary that demarcates these two sand types (Apalachicola and Mobile subprovinces) was identified for the first time in this study. Six facies and two erosional surfaces characterize shelf stratigraphy. Facies 1 is a Pleistocene soil horizon. This facies is truncated by a major erosional unconformity (Type 1 sequence boundary) created by subaerial exposure during the last sea level lowstand and the bay ravinement process during the ensuing transgression. Fine-grained estuarine deposits (Facies 2, 3, or 4 (lower transgressive systems tract)) overlie the unconformity. Facies 3 and 4 are truncated by a shoreface ravinement diastem (flooding surface) and overlain by a marine shell-bed (Facies 5; lower shoreface). Facies 5 grades into Facies 6, a quartz sand with marine foraminifera. Facies 5 and 6 comprise the upper transgressive systems tract (up to 5.5 m thick). Compared to a eustatic sea level curve, mollusk dates from estuarine shell beds show a time-transgressive trend, whereas marine shell beds are time-averaged. Transgressive and post-transgressive processes (strong cold fronts, tropical cyclones) concentrate marine mollusks above the shoreface ravinement diastem. Consequently, linear shoals are not in-situ or degraded barriers because marine species dominate the foraminiferal and molluscan assemblages, and deposits lie above shoreface ravinement diastem. Although shelf morphology is similar to modern barrier island geomorphology, shelf morphostratigraphy (linear shoals) is related to transgressive and post-transgressive processes. Shoal form and orientation are dictated by underlying transgressive topography (escarpments) that was cut into the Pleistocene substrate during the Holocene transgression. During transgression, erosional shoreface retreat produced a trailing sand sheet that draped the transgressive topography.
Mcbride, Randolph Alexander, "Seafloor Morphology, Geologic Framework, and Sedimentary Processes of a Sand-Rich Shelf Offshore Alabama and Northwest Florida: Northeastern Gulf of Mexico." (1997). LSU Historical Dissertations and Theses. 6434.