A subsurface investigation of the coastal areas in the Apalachicola delta region on the northwest Florida coast indicates that the Pleistocene sediments, which thicken to the southwest, have been deposited on an uneven Miocene surface of variable age. The thickest Pleistocene section, west of the mouth of the Apalachicola River, consists of two superimposed sequences of terrigenous clastic sediments. Each sequence grades upward from coarse to fine, and represents two major Late Pleistocene sea-level fluctuations. Radiocarbon dates and stratigraphic relationships indicate that the upper of these sequences probably represents a mid-Wisconsin transgression of the sea, and the lower represents a Sangamon transgression. The thickest section of Recent sediments lies in the old incised valley of the Apalachicola River which was cut during the last lowering of sea level. These deltaic, prodeltaic, and bay sediments represent the gradual filling of the drowned river valley, which was inundated by the Recent rise in sea level. The bays, barrier islands, and spits are the only other areas of any significant Recent sedimentation with the exception of one offshore basin to the southwest of the present river mouth. Radiocarbon dates, stratigraphic relationships, and environmental interpretations suggest that there was a relatively high stand of the sea that was very near present sea level sometime between 24,000 and 40,000 years B.P. Geomorphologic evidence in the area indicates that this high stand may have been slightly higher than present sea level and probably corresponds to the Silver Bluff shoreline of Florida and Georgia. No evidence has been found for a Recent higher sea-level stand, and radiocarbon dates and sediments indicate that sea level was approximately 10 to 15 feet below its present level sometime between 4000 and 4500 years ago. A rather rapid rise probably occurred after this time to a position slightly below present sea level, from which it has gradually risen to its present position.

Abstract Shelf deposition following lowstand delta building at the shelf edge has been documented for the northeast Gulf of Mexico between the Mississippi River and Apalachicola River deltas. Fifty-two vibracores, foraminiferal data, and bathymetry data have been used to detail the quartz-rich terrigenous clastic sediments that dominate the entire coastal to shelf depositional system. Because of low subsidence rates that characterize most of the study area, reworking and hydrodynamic winnowing occurred during repeated cycles of sea-level rise and fall in response to Pleistocene glaciation and deglaciation, producing sandy coastal-plain and continental-shelf deposits. Moreover, during the postglacial rise and present highstand in sea level, the eastern two-thirds of the shelf has been sediment starved, enabling additional reworking of the shelf sands during the passage of strong cold fronts and hurricanes, thus concentrating there a nearly uniform thickness of clean, multicyclic quartz sand known as the Mississippi–Alabama–Florida (MAFLA) shelf sand sheet. In the northeast Gulf of Mexico, variations in shelf width and morphology (e.g., shoals and shelf-edge deltas) are a function of glacio-eustatic changes in sea level, relative river discharge, size of drainage-basin area, and river location and frequency. Four surficial sediment types characterize the shelf: the MAFLA sand sheet, the St. Bernard prodelta deposit, the Chandeleur sand deposit, and outer-shelf carbonates. The MAFLA sand sheet dominates, covering about 75% of the shelf surface, and consists of a fine- to medium-grained quartz sand. Although relatively thin (3.5 to 5.5 m thick), the areal extent of the sand sheet is continuous and extensive (at least 400 km along strike; 60 km along dip), producing a sand volume of ∼ 7.2 × 10 10 m 3 . Furthermore, five major shoal complexes are located in the study area: Cape St. George shoals, Cape San Blas shoals, South Perdido shoal trend, North Perdido shoal trend, and St. Bernard shoals. The St. Bernard shoals consist of the Chandeleur sand deposit, whereas the other four formed within and consist of the MAFLA sand sheet. A composite stratigraphic column was compiled for the late Quaternary geology of the northeastern Gulf of Mexico shelf, which is up to 35 m thick and has seven depositional environments (Units 1–7) and five erosional surfaces. This composite stratigraphic column synthesizes the modern transgressive and highstand systems tracts preserved on the shelf. Unit 1 is a Pleistocene strandline deposit capped by a well-developed soil horizon, and represents the top of the last highstand and/or falling-stage systems tract. This unit is truncated by an erosional unconformity (Type 1 sequence boundary) produced when the entire shelf was subaerially exposed during the last sea-level lowstand, at about 18 ka. This unconformity was further eroded and reworked during the ensuing transgression to form a flooding surface (bay ravinement), thus creating a combined erosional surface (SB–FS). Overlying the sequence boundary is Unit 2, a fine-grained estuarine unit with occasional rip-up clasts and shell layers. The estuarine unit was planed off to form a regional transgressive surface of erosion (shoreface ravinement). Unit 3, a shelf sand sheet (MAFLA), sits on top of the transgressive surface of erosion and is dominated by fine-to-medium quartz sand up to 5.5 m thick with a distinctive shell bed and quartz pebbles at its base. The maximum flooding surface (MFS) overlies the MAFLA sand sheet and represents the boundary between the transgressive systems tract below (Units 2 and 3) and the highstand systems tract above (Units 4–7). Unit 4 is a prodelta deposit dominated by laminated silty clay that ranges in thickness from 12 to 16 m. The prodelta deposit grades upward into delta-front sediments (Unit 5) that are characterized by interlaminated silty sand and silty clay averaging 8 to 10 m thick. The delta-front unit is cut by channel-base diastems caused by the erosional scour of distributaries. Distributary sands (Unit 6) are 4 to 7 m thick and tend to be oriented shore-normal. Together, Units 4, 5, and 6 are coarsening-upward, shallow-water deltas, and represent parasequences within the modern highstand systems tract. In the western part of the study area, the shallow-water deltas overlie the MAFLA sand sheet, the result of the eastward progradation of the St. Bernard delta complex of the Mississippi River. The deltaic avulsion process (autocyclic) caused the St. Bernard delta complex to become abandoned, thus creating a local transgressive surface of erosion (parasequence boundary). The composite section is capped by a shelf sand shoal (Unit 7), which is a retrogradational parasequence up to 3.5 m thick within the modern highstand systems tract. The MAFLA sand sheet serves as an actualistic modern-day analog for shallow marine sandstones deposited under regional transgression in the ancient sedimentary record. These sandstones—commonly known as “transgressive lags” or “sheet sands”—are poorly documented with respect to sedimentary characteristics of recognition, stratigraphic framework, and reservoir architecture. This study provides additional insight to the geologic characterization of shelf sand sheets. Transgressive shelf sandstones can be significant hydrocarbon reservoirs in certain sedimentary basins of North America and elsewhere. This study offers examples that suggest that the preservation and resource potential of transgressive shelf sandstones commonly are misinterpreted in reconstructions of ancient sedimentary successions.

ABSTRACT The distribution and composition of the coastal and inner-shelf clastic sediments of northwest Florida have been strongly influenced by the activity of the Apalachicola River. The largest river in the northeastern Gulf of Mexico, the Apalachicola has dominated sedimentation in the region since Neogene time. High-resolution shallow seismic investigations of the inner shelf, combined with coastal borehole records, indicate that the bulk of the clastic sediment cover of the coast and inner margin is late Quaternary in age, and probably Sangamonian or younger. Late Quaternary sea level fluctuations have exerted a major influence on the timing and character of sedimentation in the Apalachicola Embayment.