AN ENIGMATIC LIMESTONE CONGLOMERATE IN THE PREDOMINATELY SILICICLASTIC ANTLERS FORMATION, TAYLOR COUNTY, TEXAS

The Antlers Formation is poorly studied in west-central Texas. It is typically discussed in relation to other units within the Trinity Group (Paluxy Sandstone) or for its significance as an aquifer. It is a Cretaceous-aged sandstone found in southern Oklahoma, northeastern Texas, and Arkansas. In this study, a distinctive channel conglomerate in the upper Antlers, just above of the middle (red) Antlers is investigated. The Steamboat Mountain Outcrop is a hillside exposure along County Road 650, east of Buffalo Gap, Texas. The surrounding sandstone has a tan color with some red staining. The channel is defined, at the base, by a scour surface overlain by flattened boulders and cobbles. Samples were collected, above and below the scour, split into gravel and sand portions, and textural analysis of both fractions were completed. Below the scour, gravel-sizes range from -1.653 φ to -3.113 φ and sand-sizes range from 1.86 φ to 2.74 φ. Both the sands and gravels are strongly fine-skewed and poorly sorted. This unit is tan in color and dominated by quartz. The abundance of limestone fragments in the gravel fraction increases upward. There are no fossils recovered from this unit. The range of gravel-size for the channel is -3.37 φ to -5.09 φ and the sand-size ranges from 1.67 φ to 1.953 φ. Both the gravels and sands are strongly fine-skewed and very poorly sorted. This unit is tan or red from staining and is composed primarily of limestone fragments. Poorly preserved crinoids, bivalves, and gastropods (including Turritella sp.) have been recovered. Bivalves and gastropods are internal molds. The limestone fragments collected are powdery and fine-grained, not characteristic of the bulk of the Antlers. The source of the limestone is unknown, but its powdery, fine-grained, texture is similar to the underlying Permian limestones of the region. The invertebrate fossils are poorly preserved and not diagnostic of a specific age. The underlying sand indicates a general increase in energy as the sand and gravel fractions coarsen upward and become more poorly sorted. The conglomerate represents a longitudinal bar in a fast-moving channel, as is supported by its large grain-size, well-defined scour, boulder lag, and overall poor sorting.