Stratigraphic Setting and Controls on Occurrence of Ancient Carbonate Beach Deposits: Examples from the Lower Cretaceous of the Gulf of Mexico Region

By

KERANS, CHARLIE, and LOUCKS, ROBERT G.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Carbonate beach deposits are superb reservoirs and excellent shoreline indicators. However, fossil beaches in carbonate strata are sparse, and a disproportionate number of documented examples are from caprinid rudist-rich Cretaceous carbonates. Examples of ancient beach deposits from the Cow Creek Limestone of central Texas and the Devils River Formation of south Texas are used to describe features and evaluate stratigraphic setting and controls on preservation of these deposits. Two explanations for paucity of fossil beach deposits are (1) lack of recognition and (2) lack of preservation in the fossil record. We address the latter hypothesis.

The Cow Creek occurrence contains many features of a beach-foreshore deposit, including parallel wedge sets, moderate- to low-angle seaward-dipping swash lamination, small-scale avalanche bedding, and carbonate beach rock. Its stratigraphic setting is in the highstand systems tract directly below the latest Aptian sequence boundary. The Devils River Formation examples contain similar beach facies, but the larger outcrop and detailed sequence framework allows documentation of their distribution within the overall base level/preservational setting. Within the Albian 6 composite sequence, three out of the four high-frequency, highstand systems tracts contain beach deposits within 20 ft of the overlying sequence boundary. These deposits appear to have been lithified prior to subsequent transgression.

The most important features for development and preservation of beach deposits appear to be (1) availability of coarse-grained sediment to the shoreline and (2) early lithification to prevent transgressive ravinement of the deposits. The low-accommodation, late-highstand setting also is common between these occurrences.