F. L. STRICKLIN, JR. Shell Oil Company, P.O. Box 60775, New Orleans, Louisiana 70160 C. I. SMITH Department of Geology and Mineralogy, University of Michigan, Ann Arbor, Michigan 48103 Environmental Reconstruction of a Carbonate Beach Complex: Cow Creek (Lower Cretaceous) Formation of Central Texas ABSTRACT shoreline re-entrant sheltered from the erosive influence of vigorous, southwesterly flowing The morphology of a typical beach is clearly longshore currents. Within this depositional shown by bedding in the Cow Creek Forma- regimen, coarse shell debris furnished by tion, which crops out as a downdip-thickening, slackened currents was reworked by waves 0- to 15-m limestone wedge on the southeastern refracted against a curving shoreline and flank of the Llano Uplift. The inferred beach deposited as seaward-prograding planar cross- deposits comprise the upper part of the forma- beds of a shifting foreshore zone. Beach pro- tion over an area of at least 600 sq km (~240 gradation apparently terminated just downdip sq mi) and represent a minimum shoreline from the outcrop area once the shoreline was progradation of 40 km. stabilized by filling of the re-entrant. The formation is comprised of three super- posed, laterally equivalent facies described and INTRODUCTION interpreted as follows: A basal unit of fine-to- Modern beaches are often regarded as coarse, silty calcarenite and an overlying unit of ephemeral shoreline veneers destined for fine-grained silty calcarenite are attributed to destruction by future marine transgressions. deposition in progressively shoaling waters; Indeed, relatively limited descriptions of the coarsest grained coquina facies at the top of ancient beach deposits in the geologic literature the formation is representative of beach does tend to substantiate this assumption. deposition. The latter facies contains three Without refuting the contention that beach diagnostic types of cross-bedding: (1) a lower deposits are predisposed to destructive marine section displaying festoon cross-beds, (2) an agencies, the chances of preservation would ap- intermediate sequence of offlapping, planar pear to improve under certain protective con- cross-beds with consistent southerly dips, and ditions. Such conditions, perhaps unusual (3) upper, locally developed northerly dipping judging from limited references to ancient beds. These three types of cross-beds are re- beach deposits, would include burial beneath a lated, respectively, to the beach-toe, foreshore, protective cover of either continental or low- and backshore depositional zones of a beach energy marine deposits or early cementation profile. The beach deposits are overlain discon- into beach rock. Carbonate beach deposits formably by the Hensel Formation which con- would be favored for preservation in the latter tains basal fluviatile or estuarine clays updip case. and nodular caliche downdip, with locally Another reason for the deficient number of associated calcareous dune(?) deposits. ancient beach examples appears to stem from The sequence of festoon cross-beds overlain difficulty in recognizing the deposits. This is by planar cross-beds is the most significant key probably particularly true of quartz beach to recognition of beach deposits as well as re- sands because of their chance of similarity to construction of geomorphic and geographic adjacent deposits and the subtle expression of aspects of the depositional environment. cross-bedding developed on foreshores of gentle Abundant orientation measurements from both dip. Carbonate beach deposits—favoring ac- types of cross-beds favor the interpretation that cretions of a steep foreshore built on coarse Cow Creek beach accretion originated in a shell debris—are probably easier to recognize Geological Society of America Bulletin, v. 84, p. 1349-1368, 14 figs., April 1973 1349 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1349/3418292/i0016-7606-84-4-1349.pdf by guest on 01 October 2021 1350 STRICKI.IN AND SMITH because of a greater likelihood of contrast wi th proximately 30 km eastward from the Cow associated deposits and ease of identification of Creek outcrop area, Lower Cretaceous deposits wave and current cross-bedding of greater local are downthrown into the subsurface along the relief. Regardless of composition or age, the key Balcones Fault Zone which is marked on the to identification of ancient beach deposits ap- surface by a prominent erosional escarpment pears to hinge on recognition of a characteristic extending through San Antonio and Austin. sedimentary sequence or package herein refer- Although several outlying faults of the zone red to as a "beach complex." traverse the investigated area, structure is The primary purpose of this paper is to relatively uncomplex with eastward formation present a model of a carbonate beach complex dips of less than Io. reconstructed from Lower Cretaceous out- Cow Creek and age-equivalent deposits crops in central Texas. To achieve this goal within the United Stal es are exposed only in and provide the groundwork for concluding the investigated area. Outcrops, present along interpretations, the regional geologic setting of drainage basins of the Guadalupe, Blanco, the Cow Creek Formation is first described Pedernales, and Colorado Rivers, are ter- followed by details of lithology and sedi- minated down stream by normal down-to-the- mentary features. Interpretations of environ- east faults which limit the breadth of exposures mental processes and geometry are presented to widths varying from a few km to up to 27 last. km (Fig. 2). The most extensive outcrops, This paper is respectfully dedicated to R. J. which expose a complete section of the Cow Russell, outstanding geomorphologist, and Creek, are located along the Colorado and recently deceased Boyd Professor of Louisiana Pedernales Rivers. State University, who devoted much of his life The Cow Creek is one of several formations to the study of landforms resulting from included in the lowermost Cretaceous Trinity fluvial and marine processes. To the senior Division (Lozo and Scricklin, 1956; Stricklin author, who received much of his early training and others, 1971) of central Texas (Fig. 3). from Professor Russell, development of the Collectively, the division is comprised of three Cow Creek environmental model meant not terrigenous-carbonate subdivisions referred to only studying rock types and sedimentary as Lower, Middle, and Upper Trinity, each of features and equating these with water depth which represents a sed-.mentary cycle of marine and degree of circulation but also attempting to transgression and regression. The Cow Creek gain a geomorphic impression of the over-all Limestone and underlying Hammett Shale, depositional setting. This led to efforts to work which comprise the Middle Trinity, constitute out the changing configuration of the shoreline the thinnest of the three subdivisions. The and bathymetry of the sea floor through time cyclic character of the Middle Trinity rock and the nature and topography of the bordering wedge is demonstrated by bounding discon- land with its attendant effects on deposition. formities, onlap relations of the Hammett It is hoped that these goals have been achieved to some degree and that the reader is presented a rudimentary picture of a Cretaceous land- seascape as well as stratigraphic details of a carbonate beach complex. REGIONAL SETTING The Cow Creek Formation crops out on the southeastern flank of the Llano Uplift on a low- relief structural promontory known as the San Marcos Arch (Fig. 1). The latter, which struc- turally divides the East Texas and South Texas Basins, was a stable area through much of Cre- taceous time as indicated by over-all thinning of section, the presence of numerous discon- formities, and sedimentary facies representing shallow-water to supratidal depositional en- vironments (Adkins, 1933; Imlay, 1945; Dur- ham, 1956, 1957; and Rose, 1968, 1972). Ap- Figure 1. Geologic setting of beach deposits. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1349/3418292/i0016-7606-84-4-1349.pdf by guest on 01 October 2021 RECONSTRUCTION OF A CARBONATE BEACH COMPLEX, TEXAS 1351 Shale, and offlap relations of the Cow Creek pioneer investigations conducted in central Limestone. These characteristics, discussed in Texas by the United States Geological Survey. more detail within the context of total Trinity It was subsequently differentiated in the stratigraphy (Stricklin and others, 1971), leave Colorado-Pedernales drainage basin by Lozo little doubt that Middle Trinity deposits and Stricklin (1956) into three lithologic units record the advance and retreat of an early which are herein designated as lower, middle, Cretaceous sea. and upper members (Fig. 4). Aggregate thick- ness of these members varies from zero at the COW CREEK FORMATION updip edge to almost 15 m in outcrops downdip with most thickening resulting from deposi- Nature of Exposures tional expansion of the upper and lower mem- Cow Creek outcrops within the Colorado bers (Fig. 4). The lithology, faunal content, and and Pedernales drainage basins are comprised of bedding characteristics of the three members grey to white limestone ranging from a pinch- are described as follows: out edge in updip exposures to 15-m-thick Lower Member. This basal member of the downdip. Topographically, the formation is Cow Creek is composed primarily of fossilifer- developed as a resistant ledge which contrasts ous, fine to coarse