Holocene and Late Pleistocene Sedimentary Facies of a Sand-Rich Continental Shelf: A Standard Section for the Northeastern Gulf of Mexico

ABSTRACT A standard section is proposed for late-Pleistocene and Holocene deposits of the northeastern Gulf of Mexico shelf based on vibracores, grain-size analysis, and mollusk and foraminifera identifications. The standard section is characterized by four facies and two erosional unconformities. These preserved facies and regional 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, oxidized clayey quartz sand (Pleistocene soil horizon) that is capped by a distinct erosional unconformity (bay ravinement). The unconformity is overlain by Facies 2, a tan clayey, sandy silt to silty fine quartz sand with subtle bioturbation and characterized by an estuarine foraminiferal assemblage. Incorporated at the base of Facies 2 are well-developed, yellowish-burnt-orange and grey rip-up clasts. Facies 2 is truncated by a second distinct erosional unconformity (shoreface ravinement). Facies 3 is a well-developed shell bed containing a primarily shallow-marine molluscan assemblage. The shell bed is 0.88 m thick with a fine quartz sand matrix. In addition, the shell bed is graded with large bioclasts crudely stratified at the base, and fines upward into horizontally- laminated to massive, shelly, fine sand. As shell content decreases upward, Facies 3 grades into Facies 4, which is a tan, massive to horizontally-laminated, fine quartz sand (MAFLA sand sheet) containing open-marine foraminifera and scattered shell fragments. Facies 4 fines upward and is 2.7 m thick.

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