ABSTRACT

Carbonate beach deposits are both a superb reservoir facies as well as excellent shoreline indicators. However, the record of fossil beaches in carbonate strata is sparse, and a disproportionate number of the documented examples are from Lower Cretaceous carbonates associated with caprinid rudist or oyster deposits. We use examples of ancient beach deposits from the Cow Creek and Lower Glen Rose limestones of central Texas and from the Devils River Formation of the Pecos River Canyon, south Texas, to evaluate common features, stratigraphic setting, and controls on preservation of these deposits. The two most obvious explanations for the paucity of described fossil beach deposits are (1) lack of proper recognition, or (2) lack of preservation in the rock record. We will address the latter hypothesis. The Cow Creek beach complex within the Pearsall Formation and Pipe Creek beach complex within the Glen Rose Formation have been well documented and contain all the basic features of a beach-foreshore deposit including planar wedge-set lamination, moderate- to low-angle seaward-dipping swash lamination, small-scale avalanche bedding, back-beach bedding, carbonate beachrock, and eroded-beachrock lithoclasts.

The stratigraphic setting of the Cow Creek beach deposits is in the highstand systems tract of a latest Aptian sequence directly below the sequence boundary, and the stratigraphic setting of the Pipe Creek beach deposits is in the highstand systems tract of an early Albian highstand progradational event. The Pecos River Canyon examples contain similar beach facies, but the larger outcrop and detailed sequence framework allow documentation of the distribution of these facies within the overall base level/preservational setting. Of the four high-frequency sequences within the Albian-6 composite sequence, only the three highstand sequences contain beach deposits, and in each of these sequences, the beach deposits occur within the upper 20 ft, immediately below the sequence boundary. No good caliche textures are present but a pitted top surface of several of the beach deposits suggests lithification and erosion at the sequence boundary prior to subsequent transgression.

The most important features for development and preservation of the Lower Cretaceous beach deposits studied here are (1) the availability of coarse-grained sediment to the shoreline setting, which in this example was the caprinid rudist and oyster debris, and (2) early lithification to prevent transgressive ravinement of the deposits. The low-accommodation, late highstand setting also is common between these occurrences, and most likely reflects dominance of wave-related processes in highstand versus transgressive settings coupled with sequence-boundary-related exposure and early lithification.