Regional sediment transport patterns on the continental shelf seaward of Cumberland Island, Georgia and Amelia Island, Florida are documented using historical shoreline position and bathymetry data. Spatial variability in the net rate of shoreline change is considerable due to jetty construction at St. Marys Entrance in the early 1900s. Net average shoreline progradation is documented for both islands (1.5 m/yr for Cumberland and 0.4 m/r for Amelia), however, localized areas of shoreline retreat are recorded along Amelia Island, especially for the southernmost 5 km of beach where erosion has been chronic since 1871. Qualitative descriptions of net sediment transport were integrated with quantitative results to produce a model of large-scale coastal evolution for the study area. From this analysis, net sediment transport in this coastal compartment is controlled by inlet and shelf hydraulics, and littoral zone processes have minimal impact on net long-term coastal change.

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.

No Abstract Avaliable

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.