Late Pleistocene and Holocene deposits of the northeastern Gulf of Mexico shelf were examined using 48 vibracores, X-ray radiography, grain-size analysis, and macrofaunal and foraminifera identifications. The facies succession is similar throughout the study area. The composite section is characterized by five facies and two erosional unconfommities. These preserved facies and surfaces reflect the stratigraphic signature of the last major fall and rise of eustatic sea level. At the base, Facies 1 is a yellowish, burnt orange and grey, massive to highly bioturbated, dense, oxidized clayey quartz sand (Pleistocene soil horizon) that is capped by a distinct erosional unconformity. The unconformity is overlain by Facies 2 or Facies 3. Facies 2 is a tan clayey sandy silt to silty fine quartz sand with subtle bioturbation and characterized by an estuarine foraminiferal assemblage. Commonly incorporated at the base of Facies 2 are well-developed, yellowish burnt orange and grey rip-up clasts (3X6 cm) and/or a low abundance of matrix supported, estuarine mollusks. The thickness of this facies ranges from 0.2 to 1.0 m. In contrast, Facies 3 is a dark grey clay with subtle bioturbation throughout but only a few distinct burrows. The base of this facies can contain well-preserved estuarine mollusks. This unit also includes thin (1-5 cm) shelly fine-to-medium quartz sand layers interlaminated within the clay. Facies 3 ranges from <1 to 4 m thick. Both Facies 2 and 3 are truncated by another distinct erosional unconformity (shoreface ravinement surface). Facies 4 is a well-developed shell bed containing a primarily shallow-marine molluscan assemblage. The shell bed is up to 0.50 m thick with fine-to-medium quartz sand matrix and some quartz granules and pebbles. In addition, many shell beds are graded with large (up to 6 cm) bioclasts crudely stratified at the base which fine upward into horizontally laminated to massive, shelly (<0.25 cm) fine-to-medium quartz sands. As shell content decreases upward, Facies 4 grades into Facies 5, which is a tan, massive to horizontally laminated, fine-to-coarse quartz sand (0.13 to 0.95 mm) containing open-marine foraminifera and scattered shell fragments (MAFLA sand sheet). Vertical grain size trends for Facies 5 typically fine upward or show no trend. Total thickness of Facies 5 ranges from 2 to 5.5 m.

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The late-Quaternary geology of the northeastern Gulf of Mexico shelf between Mobile Bay, Alabama and Pensacola Bay, Florida was investigated using 48 vibracores, 3,000 line-km of high-resolution seismic data, 47 (super 14) C dates, and biostratigraphic identifications. Major shelf features include: (1) shore-oblique sand ridges; (2) shore-parallel, linear sand shoals, and (3) shelf-edge deltas. This study focuses on the stratigraphic signature and depositional history of the last rise of eustatic sea level within a sequence stratigraphic framework. Six primary lithofacies and two erosional surfaces characterize the shelf deposits. Facies I is a Pleistocene soil horizon. Facies 1 is truncated by a major erosional unconformity formed by subaerial exposure during the last sea level fail (Type 1 sequence boundary) and the bay ravinement process (first flooding surface) during the subsequent post-glacial rise. The erosional unconformity is overlain by fine-grained estuarine deposits represented by Facies 2, 3 or 4 (lower transgressive systems tract [TST]). The lower TST is truncated by a shoreface ravinement diastem (second flooding surface). The diastem is overlain by a graded shell-bed (Facies 5) dominated by marine mollusks that were concentrated at the base of shoreface. Facies 5 grades into Facies 6, which is a fine to coarse quartz sand with open marine foraminifera. Facies 6 typically fines upward and represents a shelf sand sheet. Together, Facies 5 and 6 are up to 5.5 m thick and characterize the upper TST. The morphostratigraphy of the linear shoals consists of post-transgressive deposits because: (1) the foraminiferal and molluscan assemblages are dominated by open, shallow-marine species, and (2) the deposits lie above the shoreface ravinement diastem. However, the linear form and orientation of the shoals are dictated by the underlying transgressive topography (i.e., escarpments) that was cut into the Pleistocene substrate contemporaneous with sea level rise. Hence, the upper TST is completely reactivated by storm processes and it drapes the transgressive topography. No in-situ or degraded barrier deposits are found.

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

Late Pleistocene and Holocene geology of the northeastern Gulf of Mexico shelf offshore Alabama and northwest Florida was investigated using 47 vibracores, foraminiferal and macrofaunal assemblages, and bathymetric data. The morphologic and stratigraphic signatures of the last rise of eustatic sea level were examined along this passive continental margin characterized by low subsidence. 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 medium to fine sand. The sharp boundary that separates these two surficial sand types (Apalachicola and Mobile subprovinces) is identified for the first time in this study. Six facies and two erosional surfaces characterize the shelf stratigraphy. Facies 1 is a Pleistocene soil horizon. This facies is truncated by a major erosional unconformity (Type 1 sequence boundary [SB]) that was created by subaerial exposure during the last sea-level lowstand and during the bay ravinement process (flooding surface [FS]) of the ensuing transgression (FS/SB). Fine-grained estuarine deposits (Facies 2, 3, or 4 [lower transgressive systems tract]) overlie the unconformity. Facies 3 or 4 are truncated by a shoreface ravinement diastem (flooding surface) and are overlain by a marine shell-bed (Facies 5; lower shoreface). Facies 5 grades into Facies 6, a quartz sand with open marine foraminifera that represents a shelf sand sheet. Facies 5 and 6 comprise the upper transgressive systems tract, which is up to 5.5 m thick. The mid-shelf is characterized by two long (30-120 km), narrow (<6 km), shore-parallel to subparallel sand shoals that average 4 m thick. North Perdido Shoal is located 15-25 km offshore at the 20-25-m isobath, whereas South Perdido Shoal lies 20-70 km offshore at approximately the 35-m isobath. Both shoals trend southwest-northeast. The linear shoals are not in situ or degraded barriers (Stubblefield et al., 1984a, b), offshore shelfridge (bar) complexes (Tillman and Martinsen, 1984, 1987; Gaynor and Swift, 1988), or lowstand/transgressive incised shoreface deposits (Bergman, 1994; Walker and Wiseman, 1995) because the sediments that comprise the shoals lie above the shoreface ravinement diastem, and open marine species dominate the foraminiferal and molluscan assemblages. Although shelf morphology is similar to modern barrier island geomorphology, shelf morphostratigraphy 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 post-glacial transgression. During transgression, erosional shoreface retreat produced a trailing sand sheet that draped the transgressive topography. Consequently, 1) the linear nature of the shoals is derived from their formation along the shoreface (i.e., depositional strike) at lower stands of sea level during an overall transgression; 2) sediment transport from the present shoreline across the shelf appears to have little influence on shoal development; 3) the interplay between relative sea-level changes and sediment supply caused translation of the shoreface profile, thus dictating the position of the linear shoals; and 4) post-transgressive reworking and subaqueous landward migration in response to storm processes are integral parts of shoal evolution.