New Mexico Geological Society Annual Spring Meeting — Abstracts

Virgilian strata exposed in Bruton Canyon, northern Sierra Oscura, contain an assemblage dominated by the brachiopods Composita subtilita and Enteletes hemiplicatus. Thompson (1942) described the Missourian-Virgilian sequence in this area as consisting of 8 formations assigned to 4 groups. We regard these strata (excluding Thompson's Bruton Fm., which is mostly equivalent to the Bursum Fm..) as being the Atrasado Formation (111 m thick here), and use Thompson's thin but easily recognized formation-rank units as members of the Atrasado. The assemblage discussed here occurs in situ and on the slope below a gray nodular limestone bed a little below the middle of the Del Cuerto Member, Atrasado Formation (Madera Group), about 30 m below the Atrasado-Bursum contact, along the northeastern side of Bruton Canyon (sec. 31, 32, T5S, R6E).

This assemblage is dominated by brachiopods, with surprisingly few molluscs (about 1% of number of brachiopod specimens), moderately common solitary rugose corals, and sparse crinoid, fenestrate, and other bryozoan fragments. Based on a total of 1200+ specimens, brachiopods include 18 species, of which C. subtilita (41 %), E. hemiplicatus (31 %), Phricodothyris perplexa (9%), Hystriculina sp. (8%), and Beecheria sp. are the most abundant. C. subtilita is common in Pennsylvanian assemblages throughout New Mexico, and most of the other taxa are typical of Late Pennsylvanian faunas in the state. However, E. hemiplicatus has been cited previously in only a few faunal lists in New Mexico and its codominance, with Composita, makes this Del Cuerto assemblage unique among those known from the Pennsylvanian of the state. E. hemiplicatus is a small (up to 25 rum long), biconvex, strongly plicate orthid, which ranges from the early Virgilian to early Wolfcampian in the Midcontinent region and is limited to the early Virgilian in the Appalachian Basin. In New Mexico, E. hemiplicatus is replaced in the early Wolfcampian (Laborcita Formation) by E. bowsheri, a larger species having more plications. The rarity of molluscs in this assemblage is also unusual, and probably is explained both by low numbers in the original community and by some preservational attrition. The paleoenvironment of this assemblage was normal marine, with no salinity fluctuations, shallow, and perhaps some distance offshore. The abundance of Enteletes in this assemblage, when it is absent in otherwise similar Late Pennsylvanian assemblages in similar lithologies elsewhere, appears to be related to ecological factors as yet too subtle to be discerned from the existing lithologic record.