DEPOSITIONAL SETTING OF THE BASAL PRESIDIO FORMATION (LOWER CRETACEOUS), AT THE RIO GRANDE MINING COMPANY SHAFTER SILVER MINE, WEST TEXAS

Located at Shafter, Presidio County, West Texas, the Lower Cretaceous Presidio Formation unconformably overlies the Middle Permian Mina Grande Formation and records the first transgressive marine depositional cycle of the Cretaceous. The formation has previously been described as a basal sandstone and conglomerate with fossiliferous limestone interbeds.

Examination of outcrop and 400 complete mineral exploration diamond cores shows that the conglomerate is not one unit, but rather many individual beds and lenses that may be absent at the unconformity surface. The lowest occurrence of the conglomerate at Shafter is a few to hundreds of feet above the Permian-Cretaceous unconformity. The Presidio Formation contains a higher frequency of discrete conglomerate units south of the Presidio Mine. The lithology of the basal unit varies throughout the mine property and more commonly is observed to be a brecciated nodular limestone in a silty matrix.

The conglomerate clasts include Permian limestone with fusulinids, and the matrix contains Cretaceous mollusk fragments. Some clasts have algal coatings indicating a shallow subaqueous environment, possibly in a fanglomerate. A Cretaceous conglomerate in Pinto Canyon, northwest of Shafter, also contains clasts with similar algal-coated rims. Equivalent-aged basal conglomerate samples collected from the Shutup Conglomerate Formation at the Solitario in Big Bend Ranch State Park, approximately 40 miles southeast from the Presidio Formation contain Paleozoic clasts of Maravillas chert and Caballos Novaculite which indicate a source from the Marathon Uplift. An absence of similar chert clasts in the conglomerate units at Shafter points to an unrelated source.

The Cretaceous-Permian unconformity is obscured in many cores by mineralization, but the unconformity consists in part of a marine hardground as well as possible karst and paleosols. The hardground surface is well exposed in outcrop and exhibits closely spaced, gourd-shaped lithophagid bivalve borings into Permian limestone. These borings are infilled and silicified and can be etched free from the limestone.