Abstract

The morphology of a typical beach is clearly shown by bedding in the Cow Creek Formation, which crops out as a downdip-thickening, 0- to 15-m limestone wedge on the southeastern flank of the Llano Uplift. The inferred beach deposits comprise the upper part of the formation over an area of at least 600 sq km (≃240 sq mi) and represent a minimum shoreline progradation of 40 km.

The formation is comprised of three superposed, laterally equivalent facies described and interpreted as follows: A basal unit of fine-to-coarse, silty calcarenite and an overlying unit of fine-grained silty calcarenite are attributed to deposition in progressively shoaling waters; the coarsest grained coquina facies at the top of the formation is representative of beach deposition. The latter facies contains three diagnostic types of cross-bedding: (1) a lower section displaying festoon cross-beds, (2) an intermediate sequence of offlapping, planar cross-beds with consistent southerly dips, and (3) upper, locally developed northerly dipping beds. These three types of cross-beds are related, respectively, to the beach-toe, foreshore, and backshore depositional zones of a beach profile. The beach deposits are overlain disconformably by the Hensel Formation which contains basal fluviatile or estuarine clays updip and nodular caliche downdip, with locally associated calcareous dune(?) deposits.

The sequence of festoon cross-beds overlain by planar cross-beds is the most significant key to recognition of beach deposits as well as reconstruction of geomorphic and geographic aspects of the depositional environment. Abundant orientation measurements from both types of cross-beds favor the interpretation that Cow Creek beach accretion originated in a shoreline re-entrant sheltered from the erosive influence of vigorous, southwesterly flowing longshore currents. Within this depositional regimen, coarse shell debris furnished by slackened currents was reworked by waves refracted against a curving shoreline and deposited as seaward-prograding planar cross-beds of a shifting foreshore zone. Beach pro-gradation apparently terminated just downdip from the outcrop area once the shoreline was stabilized by filling of the re-entrant.

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