New Mexico Geological Society Annual Spring Meeting — Abstracts

We measured seven overlapping sections on different fault blocks to construct a complete Pennsylvanian section in the Cerros de Amado of Socorro County. At the base of the Pennsylvanian section, the Sandia Fm unconformably overlies Precambrian basement and is a 162-m-thick cyclic succession of siliciclastics (notably quartzose sandstone and conglomerate) and limestones (mostly coarse-grained bioclastic wackestone/packstone). The overlying Gray Mesa Fm is 164 m thick and, following Rejas, we divide it into three members named by Thompson: Elephant Butte Mb (117 m of limestone and shale with a prominent 10-m-thick sandstone bed above the Warmington lst. at the base), Whiskey Canyon Mb (13 m of very cherty limestone) and Garcia Mb (57 m of diverse limestone, conglomerate, sandstone and shale). The overlying Atrasado Fm is ~ 300 m thick of interbedded silicilclastic (mostly shale and arkosic sandstone) and varied limestones. We recognize the formation rank units of Thompson and Rejas as members of the Atrasado Fm (ascending order): Bartolo, Amado, Coane, Adobe, Council Springs, Burrego, Story, Del Cuerto and Moya. The Atrasado Fm is mostly shallow marine facies with some nonmarine siliciclastics. The Bursum Fm caps the Pennsylvanian section in the Cerros de Amado and is as much as 105 m of red-bed siliciclastics and limestones.

Fusulinids are not common in the Pennsylvanian section in the Cerros de Amado, so we undertook sampling for conodonts. They indicate the following ages: (1) Atokan—47 m above the base of the Sandia Fm; (2) early Desmoinesian (Cherokee)—Elephant Butte, Whiskey Canyon and lower Garcia members of Gray Mesa Fm; (3) late Desmoinesian (Marmaton)—upper Garcia Mb of Gray Mesa Fm and Bartolo Mb of Atrasado Fm; (4) early Missourian—Amado Mb of Atrasado Formation; (5) late Missourian—Coane, Adobe, and Council Spring members of Atrasado Fm; (6) middle Virgilian—Del Cuerto and Moya members of Atrasado Fm. The conodont ages are consistent with the few fusulinid horizons we located, and fusulinids indicate that the Bursum Formation in the Cerros de Amado is early Wolfcampian in age. A striking aspect of the entire Cerros de Amado Pennsylvanian section is the degree to which coarse siliciclastics are present at various levels, indicating a strong tectonic influence on local sedimentation throughout the Middle-Late Pennsylvanian.