Petrology of the early Cretaceous Yucca Formation, southern Quitman Mountains and vicinity, Trans-Pecos Texas

Abstract

The Yucca Formation of Neocomian to Aptian age extends from the geosynclinal region of the Chihuahua Trough in Mexico, eastward to the Diablo Platform in Trans-Pecos Texas. At Quitman Gap, in the Quitman Mountains, the Yucca Formation is 5,390 feet thick and its base is not exposed; the formation can be divided into lower, middle, and upper members. Beds of silty fine-grained limestone and sandstone, marl and mudstone, comprise most of the upper and lower members, and locally these beds contain charophyte gyrogonites, thin-shelled ostracods and pelecypods, beaded-shell turtles, and ornithischian dinosaurs, as well as cerithiid, littorinid, and valvatid gastropods. Fragments of a fish and abundant oyster-shell debris occur near the top of the formation. The beds of the upper and lower members were probably deposited in fresh- to brackish-water environments of lagoons and bays. The middle member of the Yucca Formation in the southern Quitman Mountains, like the Yucca in the Red Hills and Indio Mountains, consists of alternating sets of medium- and fine-grained siliciclastic strata. Basal sandstone beds are commonly conglomeratic, cross-stratified, and rich in quartz, chert, metamorphic rock fragments, and locally, clasts of carbonate rocks. These sandstones are normally overlain by beds of mudstone and siltstone in which the framework grains are embedded in a matrix of sericite, hematite, and authigenic chlorite, calcite, and dolomite. Very fine-grained sandstones and siltstones commonly exhibit horizontal or cross-lamination. Petrified coniferous logs and desiccation cracks occur in some of the mudstone and siltstone beds. A thin limestone bed in the Red Hills contains naticoid gastropods associated with blue-green(?) algal stromatolites and bone fragments. The basal sandstones of the sedimentary sets probably represent point bar deposits of meandering streams; the overlying relatively fine-grained units were probably deposited in floodbasin environments. Petrographic data suggest a low to moderately rugged source terrain developed on sedimentary and low-grade metamorphic rocks. Maximum grain-sizes and channel orientations indicate a transportation direction to the northwest. The source area was probably the Diablo-Coahuila Platform and the southern portion of the Wichita Paleoplain. Fine-grained silty limestones, calcarenites, and oncolitic limestone-pebble conglomerates of the Yucca Formation in the Devil Ridge area were probably deposited in fresh- to brackish-waters of lagoons and bays. These environments were periodically swept by strong river currents and the carbonate materials were dispersed with the siliciclastic sediments carried by the meandering streams.