Abstract

A spectacular brecciated limestone is exposed one mile south of Shafter, Presidio County, Texas, along US 67, in the Shafter Mining District. The breccia exposure in the road cut is 25 meters wide, and the sides are vertical. It is unique to the area and only occurs on one side of the road cut. The breccia is within the Late Cretaceous-age Presidio Formation, which un-conformably overlies the Permian Mina Grande Formation, and in the southern Shafter area is overlain by the Cenozoic Perdiz Conglomerate.

The beds are composed of skeletal wackestones and packstones, as well as some fluvial inputs and several beds of large black oysters. The fluvial material is composed of medium grained, well sorted, sub-angular quartz; which is present in most of the beds in the outcrop as well as comprising most of the material in the sandstones that are present.

Breccia clasts of limestone and sandstone are as large as 2 meters and are cemented by calcite with some void spaces remaining. Distinctive beds of Orbitolina, large oysters and oolitic limestone make up the Presidio Formation here. Beds on either side of the breccia appear to be offset with the east side dropped by 11 meters.

Although the strata in the area have been deformed by Cenozoic folding and faulting, as well as igneous intrusions, the breccia does not appear to be a tectonic fault, because it is not continuous and shows no sign of slick-ensides. The breccia is concluded to be a karst feature, specifically, a breccia pipe underlying a collapse structure trending about 110 degrees. Breccia pipes contain large amounts of angular blocks, and can show offset due to dissolution collapse. The lithology of the breccia clasts is a mixture of the Presidio Formation ’s oyster beds and oolitic limestone, so vertical displacement was not great. No trace of the overlying Perdiz Conglomerate, which consists of only volcanic clasts, is present. Speleothems and flowstones indicate formation in vadose conditions, and lack of mechanically emplaced clastic infill suggests a depth of formation greater than about 10-20 meters.

Although karst features are known in the underlying Permian carbonates, this appears to be the only karst feature in the local Cretaceous strata.