FACIES ANALYSIS AND STRATIGRAPHY OF JURASSIC SEDIMENTS, CHIHUAHUA BASIN, TRANS-PECOS, TEXAS

Complex Jurassic carbonates and clastics exist in outcrop in the frontier Chihuahua basin of Trans-Pecos Texas that record the deposition of sediments in response to a marine transgression. Having said this, uncertainty exists with respect to their age and regional and global context. To mitigate these uncertainties and separate fact from inference, advanced geologic mapping and sampling was carried out to begin the process of developing a robust depositional, petrologic, and stratigraphic framework to springboard future scientific and economic endeavor.

The results of this effort indicate that the Jurassic unconformably overlies the Permian, represented by an observed angular unconformity, and is conformably overlain by Cretaceous carbonates. Hand samples show that the Jurassic is characterized by a basal conglomerate, made of closely packed cobbles and pebbles with matrix supported clasts in places. Directly overlying the basal conglomerate, is limestone composed of biomicrite and biosparite, with interbeds of calcareous siltstone. Considering this, the conglomerate was deposited in a high energy beach environment; however, low energy beach and fluvial conglomerate deposits are present in the interval as well. With respect to the interbedded limestone, the initial interpreted depositional environment is above the shelf edge in extremely shallow water, evidenced by reef-like characteristics and mollusc-rich biostromal layers. Having said this, the 3-D variability of the sequence is somewhat obscured by the structural complexity of the mapping area and the depositional geometry of the deposits is hard to judge. The impact of this work and the work that follows, will advance the community’s understanding of the sedimentary history of the North American Jurassic by providing insight into the paleogeographic and physical, chemical, and biologic conditions necessary for framebuilding organisms.