Late Quaternary Geology of the Northeastern Gulf of Mexico Shelf

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 10m 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 strandlinedeposit 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 estuarineunit 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 prodeltadeposit dominated by laminated silty clay that ranges in thickness from 12 to 16 m. The prodelta deposit grades upward into delta-frontsediments (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 diastemscaused by the erosional scour of distributaries. Distributarysands (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.

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