Abstract: Shelf Sedimentary Facies Offshore Southwestern Alabama and Western Florida Panhandle: Northeastern Gulf of Mexico

Randolph A. Mcbride, Mark R. Byrnes, Laurie C. Anderson

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 unconformities. 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 gray, 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 si ty 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 gray rip-up clasts (3 × 6 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 gray 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 o 5.5 m.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas