New Mexico Geological Society Annual Spring Meeting — Abstracts

Thirty-five limestone beds of the Bar B Formation at the Derry Hills and McLeod Hills (Caballo Mountains, Sierra County, south-central New Mexico) were sampled for fusulinids. Data from analyses of 140 fusulinids and 46 petrographic thin-sections indicate that the Bar B Formation ranges in age from middle Desmoinesian to lower Wolfcampian. Stage boundaries based on fusulinid data indicate that the Nakaye/Bar B contact is mid-Desmoinesian, and that the top of the Bar B is correlative with Bursum equivalents below the Abo Formation throughout south-central New Mexico. These results differ from those of Kelley and Silver (1952), who restricted the age of the Bar B Formation in the Caballo Mountains to the Virgilian.

Bar B sediments were deposited on the Late Paleozoic Robledo Shelf, the western "inactive" margin of the Orogrande basin. Petrographic, fusulinid, and other faunal evidence suggest that Bar B sediments were deposited in facies belts (FB) 6-9 of Wilson (1975). Bar B sections contain several repetitive shallowing-up sequences similar to those described by James (1986). The combination of these sequences and bioclast data infer that fusulinid habitats ranged from carbonate shoals to supratidal zones, with most fusulinids preferring to live in carbonate mud (tidal) flats, shallow lagoons, interdistributary bays, or tidal channels.

Fusulinid-bearing clasts from conglomerates in the upper Derry Hills and McLeod Hills sections indicate late Virgilian/early Wolfcampian reworking of Middle Pennsylvanian-Wolfcampian strata as well as the presence of Virgilian unconformities in both sections. Clast data support Singleton's (1990) interpretation of Wolfcampian tectonic activity on the Robledo Shelf of the Orogrande basin. An eastward migration of the Caballo Arch (Singleton, 1990) on the Robledo Shelf during the Missourian-Wolfcampian is inferred from thinning or absence of Virgilian and Missourian strata in the McLeod Hills, Derry Hills, and I Red Hills Tank sections of the Bar B.