Environmental Reconstruction of a Carbonate Beach Complex: Cow Creek (Lower Cretaceous) Formation of Central Texas

Home , Hensel Formation

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

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

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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 environments (Adkins, 1933; Imlay, 1945; Dur- ham, 1956, 1957; and Rose, 1968, 1972). Ap- Figure 1. Geologic setting of beach deposits.

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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 thickness 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 silty calcarenite ranging markedly with slope-forming continental red- from 0 to 5 m thick. Individual beds of the beds of the overlying Hensel Formation and member are comprised of alternations of with the underlying Hammett Formation. The resistant, massive limestone with thicknesses of latter formation is comprised primarily of up to 1 m and thin shaly receding oyster beds. poorly consolidated buff-colored marine shale The base of the member is marked updip by a which offers little resistance to erosion and persistent bed of Ostrea fran^lini, a horizon characteristically forms a slope widened by recognized by pioneer geologists (Taft, 1892; stoping beneath the resistant limestone. Fresh Hill, 1901), and distinguished downdip by an exposures of Hammett Shale are unusual due to equivalent shale bed without oysters. Grain obscurity by massive Cow Creek slump blocks components of the member are dominantly and a tendency to weather rapidly to soil and rounded oyster and other mollusk fragments support vegetative cover. The Hammett Shale and uniformly distributed quartz silt cemented disconformably overlies sandstone or con- by calcite. No cross-bedding has been observed. glomerate of the Sycamore Formation, which Individual fossils in the member consist of usually weather to form a continuous slope the oyster, Ostrea franklini, large ribbed clams with the shale. such as Trigonia, occasional ammonites (Dufre- Particular outcrops of Cow Creek Limestone noya and Cheloniceras), and numerous thick- must be selected to study individual beds. shelled, high-spired gastropods (Nerinea) as Generally, the massive bluffs do not lend them- much as 25 cm long. selves readily to detailed scrutiny because ac- Middle Member. The middle member is a cessibility is limited, and because the develop- dolomitic, thin-bedded, fine-grained calcarenite ment of a caliche-like weathering rind obscures with abundant terrigenous silt and fine sand bedding planes and lithologic properties. in a section up to 4.5 m thick. Some beds are Stream channels or low positions along stream distinctively characterized on the outcrop by bluffs where beds are scoured clean and stand in the irregular development of white to cream differential relief as a result of slight variations colored spheroidal limestone concretions (Fig. in erosional resistance provide the most favor- 5) up to one-third meter in diameter and set in able outcrops for study. a leached buff to tan dolomitic siltstone matrix. These concretions, according to petro- Stratigraphy and Lithology graphic studies by D. L. Amsbury (1971, oral The following descriptions apply only to the commun.) were formed by calcite cementation northern outcrop area where most of the in- prior to dolomitization of the host matrix and vestigation effort was concentrated. Cow thus preserve the original composition and Creek deposits in the more confined outcrops texture of the rock. of the Guadalupe and Blanco Rivers are briefly Silt and fine sand within the member in- summarized later for purposes of general com- creases up to as much as 75 percent within the parison. leached, dolomitized beds and are often con- The Cow Creek Formation was recognized centrated along bedding planes or in thin beds and named by R. T. Hill (1901) as a result of or laminae. Amsbury states that organic grain

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Figure 2. Geologic map of area of investigation.

components in the undolomitized beds con- fossils in the member include thin-shelled sist of rounded oyster fragments, small hard mollusks, gastropods, and locally numerous pellets, echinoid fragments, lignite, calcareous juveniles of the ammonite Dufrenoya. Bedding forams, and ostracods and that the cement is consists of near-horizontd, even laminae often typically very fine crystalline calcite spar. Large obscured by irregular surfaces caused by dif-

NW REFERENCE SE OUTCROP SECTION LLANO UPLIFT BALCONES FAULT ZONE 1=^

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y GLEN ROSE LIMESTONE z ^ 1UFPER1 o / o: UJ \ V) /- C0R8ULA BED Q. — \ a. > / \ GLEN ROSE LIMESTONE o / (LOWER)

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Figure 3. Stratigraphic diagram of the Trinity Division.

ferential weathering. Small-scale cross-bedding fragments, recrystallized fragments of other has been observed locally. molluscs, and varying but subordinate quanti- Upper Member. This member, up to ties of terrigenous sand and gravel, frequently nearly 7 m thick, consists of coarse calcarenite concentrated along bedding surfaces. The larger and coquina and is distinctively characterized pebbles are typically well rounded igneous, by the development of large-scale cross- metamorphic, and Paleozoic sedimentary rock bedding throughout its extent. Two types of fragments probably derived from the nearby cross-bedding are present in vertical sequence: Llano Uplift. The cross-bedded layers are gen- (1) basal festoon cross-beds with diversely erally highly leached with larger pores resulting dipping laminae which are seen in plan view on from removal of shell fragments; fern and moss bedding surfaces to fill plunging, ellipsoidal lines developed at the base of the sequence troughs (Fig. 6), and (2) upper planar cross- along bluffs and around waterfalls attest to the beds of relatively uniform dip (Fig. 7). Beds of porosity and permeability of the interval. (2) locally flatten or reverse dip direction at the D. L. Amsbury (1971, oral commun.) reports top of the sequence immediately below a that cement within the member consists of thin highly irregular upper contact. The sequence rims of fibrous drusy calcite around many grains of planar cross-beds ranges up to 4.5 m in with coarser calcite spar filling, or partially thickness, whereas the basal festoon cross-beds filling, intergranular areas. have not been observed to exceed a thickness The terrigenous fraction in the upper mem- of 2.5 m. Dips of the planar beds average 7° in a ber comprises 25 to 35 percent of the beds and predominantly southerly direction and are gradually grades from dominantly quartz sand normal or subnormal to dip attitudes of the downdip to coarse chert and quartz gravels festoon troughs. The implications of these dip updip. Sizes of the latter rarely exceed 2.5 cm orientations are considered in later discussions downdip but are commonly 5 to 10 cm in of depositional environments. Ripply marks diameter near the landward pinchout. Typi- are atypical of the sequence but are locally cally admixed with the coarse updip gravels are developed on surfaces of planar beds with large, pholad-bored boulders of Ellenburger crests trending approximately normal to dip dolomite and scattered pebbles of Smithwick direction (Fig. 8). Shale. Constituent grains of the calcarenite and Large, whole fossils within the member in- coquina consist dominantly of rounded oyster clude oysters and the thick-shelled clams,

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 Figure 4. Stratigraphie section o£ Middle Trinity deposits.

Figure 5. Typical view of spheroidal limestone concretions in middle member of Cow Creek Formation.

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 Figure 6. Oblique plan views of festoon troughs in lain by planar cross beds and oriented toward right side basal upper Cow Creek Member. Trough in (A), which of photograph. Multiple, overlapping troughs in (B), shows curved, truncated edges of internal cross beds which also show internal cross beds, are oriented parallel and a cross-bed face outlined by recent erosion, is over- toward upper left of photograph.

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 Figure 7. Beach stratification in Cow Creek Lime- and intervening beach cres t. (B) View down channel of stone, in channel of Cow Creek, 1 km east of road Cow Creek showing offl ipping sequence of dipping crossing to Hensel Ranch House, Travis County, foreshore beds. Texas. (A) Dip view of foreshore and backshore beds

Trigonia and Protocardia(?), locally concen- indicated (Fig. 4). According to Thompson trated along bedding planes. Scattered am- (1937), who investigated several modern Cali- monites occur at the top of the sequence. A fornia beaches, the beach foreshore is con- striking local faunal component in the vicinity tinually modified by a changing combination of Coe Hollow (Fig. 4, Locality 3) includes of variable factors, probably the most im- numbers of large, reworked coral heads ranging portant of which are the size and direction of up to 25 cm in diameter. The growth site of approach of waves, tidal variations which cause these corals, considered unusual not only be- the zone of wave action to shift back and forth, cause of local occurrence but also because of and the character of material available for proximity to the Paleozoic landmass and transportation and deposition. The relatively association with coarse 5-to-10-cm gravels, is steep dips of the accretions, which locally in- unknown. Further investigation may reveal increase above the 7° average to as much as 14°, that the corals are associated vertically with a are attributed primarily to the. angle of repose local development of festoon and planar cross- of coarse shell fragments reworked by vigorous bedded coquina in the underlying Hammett waves. Foreshore deposits of the Cow Creek Shale (Fig. 4, Locality 3), much like the show local variations in size of components and sequence of the Upper Cow Creek Member. dip that seem to be consistent with varying Conceivably, the corals are related in dis- dynamic conditions of a shoreline environ- tribution to a relict topographic prominence ment. Throughout the outcrop, the directional produced above these deposits by differential attitude and pronounced dip of the accretion cross beds, as revealed by 243 determinations, compaction, but such a relation requires veri- indicate rather uniform southerly beach growth fication. over an area of at least 600 sq km (~;240 sq mi; GEOMORPHIC INTERPRETATION OF Figs. 10 and 11). THE COW CREEK AND OVERLYING Horizontal beds or beds of reverse dip de- DEPOSITS veloped locally at the top of the Cow Creek add Over-all lithologie, faunal, and stratigraphie the final geomorphic dimension to the succes- character of the Cow Creek suggests a deposi- sion of beacli deposits. Because these beds re- tional origin resulting from accreting beach and verse abruptly in dip at a probable beach crest contemporaneous shallow-water carbonates or berm (Fig. 7A), they are interpreted as that built out from a Paleozoic landmass. The backshore beds deposited on a surface of beach constituent members are interpreted as time- erosion. Similar opposing dips of foreshore and equivalent facies corresponding to deposition backshore beds are reported by McKee (1957) in waters of increasing depth offshore. Geo- from several beaches along the Pacific coast of morphic and environmental implications of California and Mexico and at Mustang Island, these facies and diagnostic stratigraphie details Texas, bordering the Gulf of Mexico. Scattered are discussed in the section that follows. intact shells of pelecypods and ammonites suggest that Cow Creek backshore deposits are not Beach and Off-Beach Deposits as thoroughly reworked as those of the fore- The type and sequence of cross beds in the shore, implying that they are possibly storm upper member offer striking geomorphic evi- deposits flung up by unusually large waves and dence of deposition along a typical beach pro- tides onto higher parts of the beach. These file such as that shown in Figure 9. Most con- backshore deposits are the final addition to a vincing in this regard are the offlapping planar sequence recording gradual shoaling of the sea cross beds which are interpreted as successive floor and eventual conversion to land by sea- seaward-built accretions of a smooth, relatively ward shifting beach zones of sedimentation. steep beach foreshore. Individual cross beds of The festoon cross-beds in the base of the up- the planar sequence are ordinarily less than 30 per member are interpreted as deposits of an cm thick, are separated by well-developed off-beach zone of vigorous scour and fill. Al- bedding planes, and exhibit crude internal though vertical successions of these beds show laminations parallel to major bedding or at a much diversity in dip of laminae, individual steeper angle. Since offlap relations are observ- festoon accumulations display garlandlike or able in outcrops (Fig. 7B) throughout the crescent-shaped patterns in both plan view and lateral extent of the member, extensive growth cross sections perpendicular to trough axes. of the beach over an area of at least 40 km is These properties are consistent with de-

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Figure 8. Ripple marks of approximate 4-cm of transition of dipping foreshore beds to near hori- amplitude exposed on a planar cross-bed surface. Dip zontal backshore beds coincident with a beach berm. flattening in upper center of photograph reveals point

scriptions of festoon cross-bedding by Knight sediment waves may have been either con- (1929, 1930). Knight (1930) attributes the tinuous in the form of irregular megaripples or formation of such beds to an orderly process of broken into individual cusp-shaped megarip- scour and fill dominated by the following ples. In either case, however, the resulting events: (a) scouring of plunging, elongate, sediment-filled troughs should be oriented in ellipsoidal troughs followed by (b) filling of the the direction of current flow with the laminae troughs by laminae concordant with the trough showing a dominant downcurrent component floor, and finally (c) partial truncation of the of dip. The vertical and lateral development of laminae by later scouring. Apparently, current- Cow Creek festoon cross-beds, with laminae driven sediment waves are responsible for the filling individual festoons up to > 1 m thick and scour-and-fill sequence of events indicated by occupying troughs as wice as 10 m, indicates such cross-bedding, and it seems likely that the that the currents responsible were very power-

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STRATIGRAPHIC DIP SECTION Figure 10. Outcrop map of Cow Creek Limestone graphic dip section showing facies interpretation of Cow in Colorado-Pedernales drainage basin and strati- Creek members. Compare with Figure 11.

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SCHEMATIC DE POS IT IOMAL DIAGRAM Figure 11. Map showing plots of dip orientation of Schematic diagram summarizes interpretative relations festoon troughs and planar (foreshore) cross beds with of facies members and directional features to offshore inferred trends of successive beach face accretions, depositional profile. Compare with Figure 10.

ful. Orientation measurements of 46 festoon distance of slightly less than 40 km. However, troughs in the Cow Creek indicate that the the pronounced inclination of the foreshore troughs agree closely in spatial attitude and are beach deposits indicates that the slope was aligned primarily in a southwesterly direction not uniform but steeper near the shore. From parallel or subparallel to restored beach faces the average inclination of the foreshore beds, (Fig. 11). Therefore, these festooned cross beds which is above 7°, the slope at or nearest the are interpreted as deposits of vigorous, south- store was about 4.5 m in a distance of slightly westerly flowing longshore currents. less than 38 m. Thus, the over-all depositional slope was concave with respect to sea level, the Water Depths and Depositional Profile of greatest concavity being along shoreward Cow Creek Deposits extremities. Some workers might tend to Facies of the middle and lower members of interpret occasional beds of coarse calcarenite in the Cow Creek are interpreted as the result of the lower member as the result of winnowing or deposition in progressively deeper waters pri- preferential habitation of organisms on low- marily by their vertical and lateral relations to relief offshore bars. the beach beds and progressive downward reduction in grain size. Gradual seaward shift of Nature of the Upper Contact and the depositional zones is indicated by progradation Hensel Formation of the beach. An attempt to relate the facies to Basal continental deposits of the Hensel the bathymetric profile at any instant in time is Formation and the morphology of the contact as shown in Figure 11. are briefly discussed because these deposits Water depths at the time of deposition can be further document conversion of sea to land approximated because the superimposed facies through regressive sedimentation. are successive aggradations of a depositional Hensel deposits rest disconformably on suc- profile which built up to slightly above sea cessively younger beds in a downdip direction level. Therefore, water depths of the various as a result of seaward beach progradation. Some depositional zones correspond approximately to erosion or weathering of abandoned beach the vertical distance of the respective facies segments is implied by chaotic surface topog- beneath the top of the formation. For example, raphy with local relief of up to several feet in Section 7 (Fig. 10), the distance of the three (Fig. 12A). Such disorderly topography does facies below the top of the Cow Creek are as not suggest the integrated drainage of a stream follows: upper member, 0 to 6.5 m (beachbeds, and a resulting planated or incised surface but 0 to 3.5 m and off-beach beds, 3.5 to 6.5 m); may be the result of reworking of uncon- middle member, 6.5 to 10 m; lower member, solidated backshore sediments by storm waves 10 to 15 m. With the exception of the northern- and irregular solution of a limey terrane by most sections (1, 2, and 3), which may not be infiltrating ground water. Wind erosion and complete and representative of a fully de- deposition may have been an additional com- veloped depositional sequence, water depths of plicating factor, because in some places the the several zones are indicated to be relatively beach deposits are overlain by calcareous constant. In arriving at these depths, tidal mounds containing shell fragments (Fig. 12B) variations and slight aggradation of the beach that may have originated as eolian dunes of above mean sea level are disregarded. Depth sediment swept from the Cow Creek beach. figures are considered minimal because of some Continental Hensel deposits consist of a but probably slight limestone compaction that downdip basal limey layer up to 4 m thick, accompanied burial. followed by red, green, and maroon clay, and The slope of depositional profiles inferred an upper sandy section with coarse gravel (Figs. from Cow Creek facies can be approximately 4 and 10). Both the clay and sand-gravel determined using a similar line of reasoning. sequences thicken updip and appear to be an On the basis of the restored profiles shown in alluvial fan built out from the Llano region Figure 10, which have been reconstructed onto a low-lying coastal plain. However, origin essentially parallel to the base of the Cow of the downdip limely layer only becomes ap- Creek so that the facies are laterally equivalent, parent from its position in the sedimentary the average slope is about one-third meter per sequence. The layer is irregular and non- kilometer; for example, the lowermost profile stratified, composed of nodular fine-grained shows an over-all slope of about 13 m in a limestone and dolomite, and contains vertically

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 Figure 12. Features along upper Cow Creek surface conformity overlain by continental deposits of caliche of marine offlap, in channel of Cow Creek, 1 km east of and alluvium. (B) Calcareous mounds or possible dunes road crossing to Hensel Ranch House, Travis County, resting on foreshore beds. Relief of mounds is up to one Texas. (A) Irregular, subaerially eroded surface of dis- and two-thirds meters.

oriented pockets and cracks filled with clay A proposed hypothesis to account for the identical in color to that of the overlying beds. origin and growth of the beach emphasizes the The clay likely fills a network of irregular depositional interplay between southwesterly crevasses produced by downward percolating flowing longshore currents and waves refracted ground waters. These characteristics bear some against a curving shoreline. The shifting pattern similarity to calcareous soils forming in certain of beach growth between early and late stages coastal regions of low rainfall, as in South Texas and the disparity between the indicated beach where a caliche mantle of the Lissie Formation faces and the earliest, most landward shoreline is ascribed by Price (1958) to the leaching and (drawn along the onlap limit of Cow Creek on concentration of carbonates by evaporation of Paleozoic rocks) suggest that beach growth surface and soil waters. Possibly similar solu- began along a shoreline re-entrant sheltered tion-reprecipitation products of subaerial dia- from the erosive influence of vigorous long- genesis of carbonate terrane are also reported shore currents (Fig. 13B). The curvature of on raised Pleistocene reef tracts of Barbados Cow Creek isopach contours in the northern (Harrison and Matthews, 1972). The basal outcrop area of beach deposits (Fig. 13A) may limey deposits of the Hensel Formation are in reflect such a shoreline re-entrant. This the proper sequential position to have been postulated re-entrant in the shoreline was ap- exposed to weathering and soil-forming proces- parently maintained and propagated south- ses, since they cover an abandoned beach and ward over the outcrop area during the deposi- lie at the base of an alluvial fan. tion of the foreshore planar cross beds (Fig. 13C). These beds, comprised largely of car- The Hensel deposits of the Guadalupe- bonate detritus probably dropped by slackened Blanco outcrop areas record a southward facies longshore currents in the lee side of the re- change as the formation there is an entirely entrant and reworked by vigorous currents of marine, 9-m thick sequence of sandy, fossilifer- breaking waves, appear to reflect the influence ous dolomite beds. From a detailed study of the of wave refraction along a curving shoreline. Hensel Formation in the Spring Branch Area, For example, dip directions of these planar Comal and Kendall Counties, Cooper (1964) cross beds show a progressive change from reports a persistent bed of Exogyra quitman- southeasterly to southerly away from the ensis in the basal part of the formation a few initial shoreline of Paleozoic rocks. The dis- meters above medium-grained oolitic calcaren- parity between the southeasterly-flowing long- ites of the upper Cow Creek. Although the shore currents, as determined from festoon contact between the two formations is stained trough orientations in the off-beach beds, and by limonite and locally pholad-bored, no the reconstructed, east-trending beach faces planar cross beds are present in the upper Cow suggest that longshore currents were deflected Creek. The absence of planar cross-bedding in into the re-entrant and flowed subparallel to the Cow Creek of the southern outcrop area beach faces. If the proposed hypothesis is cor- plus the entirely marine overlying Hensel rect, the Cow Creek beach deposits originated Formation serve to establish a southerly limit in a depositional regimen created by a long- of beach growth and progradation of succeeding shore current barrier at the headland of a continental deposits. southward-migrating re-entrant.

RECONSTRUCTION OF Recent analogs of the Cow Creek beach BEACH GROWTH model possibly exist on a local scale in the The numerous dip direction measurements Persian Gulf. This is suggested by aerial views of the planar foreshore cross beds, as shown in (Fig. 14) of the western shoreline showing Figure 11, indicate that growth of the Cow carbonate beach accretions forming in re- Creek beach began in the northern part of the entrants sheltered from erosion by longshore Colorado-Pedernales outcrop area and accreted currents flowing in a southerly direction (E. A. initially in a southeasterly direction. An Shinn, 1971, oral commun.). These beach abundance of highly consistent dips indicates deposits have not been studied, but Shinn that beach accretions shifted to a near southerly reports that headlands on the upcurrent side of direction except near the Paleozoic shoreline the re-entrants are developed on resistant and ultimately advanced seaward over a dis- Eocene dolomite. tance of at least 40 km beyond the southern- A somewhat similar mechanism of beach most outcrops. growth has been proposed by Ball (1967) to

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B C Figure 13. Reconstn ion of beach deposits. account for spit beach accretions extending the bination of deposition of sediment by currents capevvard ends of certain Bahama Islands. and reworking by wave action has led to ac- There, wind-driven currents are deflected cretionary leeward growth of capes and a around the island capes where they slacken and sequence of festoon and planar cross-beds much deposit much of their sediment load. The com- like that of Cow Creek bea ch deposits.

Figure 14. Aerial views of beach accretions filling rent flow is from upper middle to lower right of each current sheltered re-entrants along western shore of photograph. Scale in foreground is 1 cm equal to ap- Persian Gulf, 50 to 75 km north of Doha, Qatar. Gulf proximately 70 m. Photographs by E. A. Shinn. is to right in both views and direction of longshore cur-

A detailed study of the high-energy near- lunate megaripple, and asymmetric megaripple) shore of southern Oregon has led to recognition are described as the result of two opposing cur- of five facies with distinctive types of cross- rents set up by seaward and landward surge of bedding (Clifton and others, 1971). There waves. Analogous multiple facies are not de- facies (inner planar, inner rough, outer planar, veloped in Cow Creek beach deposits probably

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due to the combined effect of a steeper fore- water depths based on thickness of deposits can shore and the influence of strong longshore cur- be assigned to respective depositional zones. rents. Under such conditions some of the facies These facies, which are well exposed in broad, as described in Oregon might merge and others easily accessible outcrops, ai e offered as an en- be replaced by festoon cross beds resulting from vironmentally controlled model for future longshore currents. diagenetic, paléontologie, and environmental The postulated approximate limits of the studies. It is hopefully anticipated that such Cow Creek beach deposits beyond the outcrop studies will s.ied more light tin postdepositional area are as shown in Figure 13C. These limits, histories of carbonate alteration and cementa- which incorporate the re-entrant concept of tion and on water depth and ecologic dis- beach growth, are based on the downdip re- tribution of faunas and w:.U provide further placement of Hensel continental beds by details on the paleocaliche and related con- marine deposits in the nearby subsurface (as tinental beds overlying the beach deposits. indicated by well and core hole data shown in The sedimentary sequence of festoon cross- Figure 13C south of the exposed beach de- beds overlain by planar cross-beds is a diagnostic posits) and in the Guadalupe-Blanco outcrop key to recognition of beach deposits, regardless areas. In addition, the Cow Creek in the south- of lithic composition, becaust: current and wave ern outcrop areas is an offshore medium-grained dynamic agencies operate uniquely in beach oolitic calcarenite with no upper development construction and growth. Once the sequence is of beach planar cross-bedding. recognized, and this is facilitated if continental deposits are preserved, detailed study of CONCLUSIONS stratigraphie relations and sedimentary fea- A shoreline re-entrant sheltered from the tures can be combined, with the aid of modern erosive influence of southwesterly flowing long- analogs, to arrive at the geomorphology and shore currents is the reconstructed environ- geography of the depositional setting. mental setting of Cow Creek beach accretions. Shell debris furnished by longshore currents ACKNOWLEDGMENTS was reworked by waves refracted against a During the course of field work, numerous curving shoreline and deposited as foreshore people assisted in the investigation. The authors planar cross beds from which extensive sea- wish to thank their many colleagues of Shell Oil ward growth of the beach accretions has been Company who visited the outcrops during reconstructed. The morphology of a beach is training excursions and contributed helpful manifest not only in the foreshore planar beds ideas and suggestions for additional study. but also in basal festooned cross-beds, laid down Special gratitude is extendeo to: F. E. Lozo, by vigorous longshore currents at the toe of the who supervised the study as part of a research beach, and backshore beds of reversed dip project by Shell Development Company; locally preserved on top beneath a discon- R. N. Ginsburg, who, based on extensive field formity. Although no large deltas were present work in the Florida-Bahamas carbonate plat- in the immediate areas as indicated by pre- form area, pointed out significant analogies and dominance of carbonate components, the invoked helpful discussions of sedimentary beach accretions are overlain by a blanket of processes; F. J. Pettijohn, John Hopkins Uni- continental clay—the initial deposits of a thin versity, who initially suggested the probability alluvial fan which progressively encroached of beach deposits in the Cow Creek during a upon and buried abandoned beach segments. field trip; V. E. Barnes, Bureau of Economic The protection afforded by the clay from later Geology, Texas, who pointed out significant stream and marine erosion is considered a key strat:graphic relations in the beginning stage of factor in preservation of the beach deposits. field work and also contributed a map of Cow The beach deposits accreted seaward over a Creek outcroos in the Coiorado-Pedernales distance of 40 km and ultimately covered an Rivers area; aid D. L. Amsbury, who volun- area of at least 600 sq km ( ~240 sq mi) prior to teered ideas and particularly helpful informa- stabilization of the shoreline. Because the tion on rock composition derived from beach accretions terminate a regressive marine pétrographie studies. Last, nut not by any sequence built up to mean sea level, the under- means least, :he authors ar: especially ap- lying carbonate members are recognized as preciative to Shell Oil Company for permission contemporaneous deeper water facies, and to publish the paper.

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