Stratigraphy and deposiotinal environment of the Lower Cretaceous basel Presidio Formation at the Shafter Silver Mine, Presidio County, Texas.

摘要

Located in Shafter, Presidio County, West Texas, the Lower Cretaceous (AptianAlbian) Presidia Formation unconformably overlies the Middle Permian Mina Grande Formation and records the first transgressive marine depositional cycle of the Cretaceous. The base of the Presidio Formation has previously been described as a sandstone and conglomerate with fossiliferous limestone interbeds.;Data from 412 continuous diamond drill cores as well as outcrops within the Shafter Mining District indicate that the conglomerate unit is located up to 150 feet above the unconformity and is rarely at the basal contact, Lenticular conglomerate beds are common and the unit is interpreted as a braid-delta.;Equivalent-aged basal conglomerates of the Shutup Conglomerate Member of the Yucca Formation exposed in the Solitario in Big Bend Ranch State Park contains mostly Paleozoic, pebble-sized clasts of angular to sub-angular Caballos Novaculite and Maravillas chert, which indicate a source from the Marathon Uplift. An absence of similar chert clasts in the conglomerate units at Shaffer 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 and karst. The hardground surface is well exposed in outcrop and exhibits closely spaced, gourdshaped Gastrochaenolites bivalve borings into Permian limestone. These borings are infilled.;The basal Presidia Formation is concluded to be a fan-delta deposited on a karsted Permian terrain as the Cretaceous sea transgressed the region. The basal unit is composed of sandstone, marl, and breccias/gravels of poorly-sorted, pebble-to cobble-size chert and limestone clasts in a mud, silt, and clay matrix with calcite cement present.