New Mexico Geological Society Annual Spring Meeting — Abstracts

Although Upper Pennsylvanian (Missourian-Virgilian) strata near Jemez Springs are profusely fossiliferous, these diverse faunas have been little studied; only the brachiopods of the uppermost unit (Jemez Springs Shale Mbr.) are well known. Extensive collections (7000+ specimens) of marine invertebrates were made from 3 shale units west of Hummingbird Camp, 7.8 km north of Jemez Springs town. The lowest fauna, from a yellow-brown shale unit (HC-1) about 65 m below the base of the Abo Fm., is dominated by a high-spired, large (25 mm high), probable new species of the gastropod Stegocoelia (Hypergonia), by the bivalve Myalina (Orthomyalina) slocombi, and by dense rhomboporoid bryozoan fragments. Snails (Goniasma, several bellerophontids), the bivalve Nuculopsis girtyi, and the brachiopods Composita subtilita and Crurithyris planoconvexa comprise important subsidiary elements of this fauna, which is dominated by molluscs and overall of relatively low taxonomic diversity.

A higher shale unit (HC-H, 27-33 m below the Ab6, about at the Missourian-Virgilian boundary) is, reddish gray with minor limestone beds. It contains a much more diverse, mollusc-dominated fauna, including about 20 species each of gastropods and bivalves, 18 species of brachiopods, and low to moderate numbers of scaphopods, echinoid and crinoid remains, small, solitary rugose corals, trilobites and bryozoans. Among gastropods, bellerophontids (e.g., Bellerophon n. sp., Euphemites n. sp., Retispira eximia, R. tenuilineata. Knightites n. sp.), Glabrocingulum (G.) n. sp., Trepospira aff. illinoisensis and Amphiscapha subrugosa are most albundant. No single bivalve species is unusually common; characteristic taxa are Astartella. Polidevcia arata, Septimyalina perattenuata and M. (O.) slocombi. About 50% of the brachiopods are C. subtilita and C. planoconvexa but Neospirifer. Derbyia, Antiquatonia, Linoproductus, Neochonetes and Juresania are moderately common.

The Jemez Springs Shale Mbr., within the uppermost 15 m of the Madera Formation, contains abundant brachiopods, with fewer gastropods, bivalves, bryozoans, echinoderms, rugose corals, fusulinids and trilobites. Sutherland and Harlow (1967) described the 18 species of brachiopods, of which Hystriculina armata, Composita subtilita, Neospirifer pattersoni and Antiquatonia jemezensis are most abundant. At least one thin interval of dense M. (O.) slocombi shells occurs in this unit, but bivalve diversity is restricted to only a few species. Gastropods mainly include species (chiefly bellerophontids and Glabrocingulum) that are present in unit HC-11, but total diversity is much lower.

Each of these shale units represents early phases of marine transgressions, but with somewhat different faunal assemblages. These differences reflect both varied nearshore marine depositional environments and temporal changes in some elements of these marine communities. Although Late Pennsylvanian brachiopod/bryozoan/echinoderm/fusulinid faunas in limestones are conspicuous in the Jemez Springs area, careful examination of shale units also reveals rich molluscan faunas. Most of these bivalve and gastropod species also occur in the Midcontinent and eastern U.S., but some are endemic to the Rocky Mountains region or are new species.

This study was supported by an NSF Research Experiences for Undergraduates grant to the senior author.