New Mexico Geological Society Annual Spring Meeting — Abstracts

The Osha Canyon Formation is an Early Pennsylvanian unit with its 22 m-thick type section in Guadalupe Box, where it is underlain by Mississippian strata and overlain by the Atokan Sandia Formation. Lithology and marine invertebrate fossils from an 8 m-thick sequence of fissile gray shale and thin limestones along U.S. Forest Road 376, about 6 km north of Guadalupe Bix, indicate that this is the upper part of the Osha Canyon Formation, the younger Madera Formation as mapped by previous workers (e.g., Woodward, 1987). These strata can be traced a short distance to the west, where they are seen to directly overlie a prominent mass of reddish Precambrian granite, indicating that the limestone-rich lower part of the Osha Canyon as well as Mississippian strata are missing at the locality. This locality is known as a source of some of the youngest and largest edrioasteroid echinoderms (Sumrall and Bowsher, 1996), but the identity of the formation, its exact age, and the composition of the fauna have not previously been reported.

The fauna of the upper Osha Canyon at this locality is abundant, diverse, and well preserved. Based on more than 4000 specimens collected, it is dominated by brachiopods, with crinoid fragments, solitary rogues corals, echinoid fragments, and bryozoans being important subsidiary elements. Molluscs are uncommon, representing only 1.5% of the total brachiopod + mollusk specimens; however, the large bivalve Wilkingia is conspicuous. Brachiopods are represented by 27 species. By far the most abundant brachiopod is Neochonetes platynotus (59% of total brachiopod specimens), followed in abundance by Punctospirifer morrowensis (8%), Hustedia gibbosa (7%), Anthracospirifer newberryi (5%), and Linoproductus nodosus (3%). Other less common age-diagnostic species include Antiquatonia coloradoensis, Sandia welleri, Plicochonetes arkansanus, Spirifer gorei, Rhipidolmella trapezoida, and Composita deflecta. Based upon well-established brachiopod zones in the Sangre de Cristo Mountains, NM, and in Oklahoma and Arkansas, the age indicated by the other of the tiny biconvex genus Nucloespira. The latter genus is widespread in the Mississippian of the North American but very rare in the Pennsylvanian; this is the first report of Nucleospira in the southwestern United States.

The fauna at this locality adds considerably to the record of the taxa in the upper part of the Osha Canyon Formation, at this interval is poorly exposed or covered with float material at and near the type section. Although some elements of this fauna are also present in the limestones of the lower Osha Canyon, the two faunas differ significantly in composition as well as in the relative abundances of common taxa. These differences are not related to age, but to a change from relatively high-energy marine environments represented by the lower limestones, to lower energy, more highly siliciclastic substrates of the upper part of the Osha Canyon.