--> ABSTRACT: Late Quaternary Lowstand and Transgressive Systems Tracts of the Northeastern Gulf of Mexico: Surfaces, Facies, and Stratigraphy, by Randolph A. McBride, Mark R. Byrnes, Laurie C. Anderson; #91020 (1995).

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Late Quaternary Lowstand and Transgressive Systems Tracts of the Northeastern Gulf of Mexico: Surfaces, Facies, and Stratigraphy

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

The late Pleistocene and Holocene depositional history of the northeastern Gulf of Mexico shelf was investigated using 3,000 line km of high-resolution seismic reflection data, 48 vibracores, 39 radiocarbon dates, and macro- and micro-fossil identifications. The study area is located between two large sediment depocenters, the Mississippi and Apalachicola River deltas. Major geomorphic features of the shelf and upper slope include: (1) shore-oblique, shoreface sand ridges; (2) middle to outer shelf sand shoals that are long, linear, and shore-parallel and interrupted by several shore-perpendicular bathymetric lows, and (3) several well-developed shelf margin deltas. This investigation focuses on the stratigraphic signature of the last fall and subsequent rise of sea level within a sequence stratigraphic framework.

Cores show a distinctive facies succession throughout the study area (~75 km2). At the base, Facies 1 is a yellowish burnt orange and grey, massive to highly bioturbated, dense, oxidized clayey quartz sand that is capped by a distinct erosional unconformity. The erosional unconformity is overlain by Facies 2, a dark grey clay or a tan silty to fine-grained quartz sand, with subtle bioturbation throughout that destroys bedding. Incorporated within this unit are well-developed, yellowish burnt orange and grey rip-up clasts. This facies ranges from 0.25 to 1.0 m thick and is truncated by another distinct erosional unconformity. Facies 3 is a graded shell bed up to 0.50 m thick with a fine to medium quartz sand matrix and some quartz granules. As shel content progressively decreases upward, Facies 3 grades into Facies 4, which is a tan, massive to horizontally laminated, fine to coarse quartz sand (0.13 to 0.95 mm) with low concentrations of shell fragments (0.13 to 3.0 mm). Total thickness of Facies 4 ranges from 2 to 5 m.

In a sequence stratigraphic framework, Facies 1 represents a Pleistocene soil horizon that was subaerially exposed during the last sea level lowstand, approximately 18,000 yrs. B.P. As a result of subaerial erosion, a Type 1 sequence boundary was produced on top of the highstand systems tract. Shelf margin delta deposition, indicated by well-developed clinoforms, occurred downdip contemporaneously with sequence boundary formation. The subsequent rise in sea level generated a marine flooding surface (bay ravinement) that reworked the top of the sequence boundary thus producing rip-up clasts (Facies 2). Therefore, the contact between Facies 1 and 2 represents the first flooding surface. Facies 2 represents a finer-grained estuarine environment. As tra sgression continued, the outer shoreline translated landward through erosional shoreface retreat producing an erosional unconformity (ravinement surface) that corresponds to the bottom of the shell beds (second flooding surface). The graded shell beds (Facies 3) probably formed by storm processes that vertically mixed and concentrated bioclasts. The overlying sandy unit (Facies 4) is either the upper part of storm deposits forming a thick graded bed (combine Facies 3 and 4) or a surficial sand sheet. Within this framework, Facies 2, 3, and 4 are part of the transgressive systems tract (TST). In this case, the shelf sand sheet has a volume of 5 billion m3 thus providing an excellent petroleum reservoir within the TST.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995