New Mexico Geological Society Annual Spring Meeting — Abstracts

The Atrasado Formation crops out extensively in the Cerros de Amado, east of Socorro, but little detailed information on the stratigraphy and paleontology of the formation has been published. Fossil assemblages low in the 72-m-thick Atrasado section along Arroyo de los Pinos (sec. 25, T2S, RIE) document a transition from nonmarine to marine environments. Near the base of this section, less than 10 m above the base of the Atrasado Formation, a 4-m-thick interval of thinly laminated, dark-gray shale contains plants, fish scales, conchostracans and malacostracans indicative of a lacustrine environment. Near the top of this unit a thin bed of fissile "paper shale" is largely composed of vast numbers of densely-packed, decalcified, complete valves of the bivalve Dunbarella. The presence of both juvenile and adult valves, and abnormally high density of the shells, suggest that this may represent a mass-kill event, possibly from a sudden influx of marine water into the brackish environment in which these bivalves lived. The transition to a marine environment is marked by a thin overlying bed of gray shale and profuse irregular encrusting algal growths with a fauna dominated by the brachiopods Derbyia and Crurithyris, representing a pioneering marginal marine community. The overlying 2 m of interbedded thin gray limestones and shales contain a diverse, stenohaline fauna distinguished by large numbers of the brachiopods Hustedia and Hystriculina, with lesser abundances of other brachiopods (Juresania, Cancrinella, Rhipidomella, Composita, Punctospirijer, Beecheria, Cleiothyridina, Crurithyris). The brachiopod Chonetinella flemingi, a widespread Missourian index species, supports the early to medial Missourian age (fusulinacean zones MC-l to MC-2) indicated by fusulinaceans (Triticites) about 60 m higher in the section. Solitary rugose corals, fenestrate bryozoans, and crinoid fragments are also common, but molluscs and trilobites are rare. This phase of marine deposition ended with deposition of a thin, sparsely fossiliferous gray limestone unit bearing a fauna essentially limited to solitary rugose corals, Crurithyris, and crinoid debris, possibly representing a slightly hypersaline lagoon.