New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Pennsylvanian strata of the Atrasado Formation (Madera Group) at NMMNH locality 4629 in the Cerros de Amado, Socorro County, include an unusual lacustrine deposit. The stratigraphic succession at this locality can be assigned to five units (in ascending order): (1) 4.1 m of trough-crossbedded, coarse-grained and conglomeratic sandstone that sharply overlies marine shale; (2) 0.8 m of muddy siltstone and nodular limestone; (3) 3.5 m of thinly laminated black shale with abundant conchostracans, darwinuloid ostracods and sparse plants (seed ferns); (4) 0.7 m of hematitic shale packed with shells of the brachiopod Derbyia; and (5) 1.9 m of interbedded limestone and shale with a marine fossil assemblage dominated by productoid brachiopods and fenestrate bryozoans. The conchostracans from unit 3 represent a single taxon characterized by round margins, a smooth and small umbonal area and 27-40 growth lines in the adult stage; they are readily assigned to Pseudestheria sp. At present, and in the fossil record, conchostracans only occur in freshwater to mesohaline brackish-water environments. Their abundance in unit 3, the presence of darwinuloid (freshwater) ostracods, land plants, lack of marine indicators and the lithology and geometry of the deposit suggest it represents a lake. The presence of Derbyia but absence of other typical marine invertebrates in unit 4 indicates the onset of nearshore marginal marine conditions. Indeed, the section at locality 4629 is analogous to a classical cyclothem. Thus, fluvial/deltaic deposits (unit 1) grade upward (unit 2) into a coastal lake (unit 3) that is then overlain (transgressed over) by marginal marine (unit 4) and normal marine (unit 5) environments. However, this cycle is not repeated either above or below the· locality 4629 deposit, suggesting it may reflect a local, unique event on the Late Pennsylvanian coastline in central New Mexico.