Depositional Environments, Conodont Biostratigraphy, and Porosity Diagenesis of Montoya Group (Upper Ordovician), Sacramento Mountains, South-Central Mexico: ABSTRACT

Abstract

Intensely dolomitized and siliceous marine carbonates comprise Upper Ordovician strata in the Sacramento Mountains, New Mexico. Pray subdivided these units into lower Montoya, upper Montoya, and Valmont formations. These strata are more appropriately designated the Montoya Group which includes 3 formations: Second Value, Aleman, and Cutter. The subrounded, medium to coarse-grained sandstone rests disconformably on the Lower Ordovician El Paso Formation. This thin sand (Cable Canyon Member of the Second Value Formation) represents a End_Page 457------------------------------ transgression following Middle Ordovician erosion. The transition into the overlying Upham Member of the Second Value is gradational, but can be locally abrupt. The massive, finely crystalline dolostone was originally coral (tabulate and rugose forms) and crinoidal wackestone-packstone. Fossils are poorly preserved by chalcedony replacement. The transition from the relatively shallow-marine sediments into the deeper water strata of the Aleman Formation occurs over several meters. The very finely crystalline, cherty dolostone hosts rynchonellid and dalmanellid brachiopods and bryozoan colonies. Ribbon cherts developed around clusters of fossils. The Aleman changes sharply into chert-free, thin to medium-bedded Cutter. The argillaceous dolomicrite is nonfossiliferous except for conodont , isolated brachiopods, and a Favosites-type coral horizon. Tidal channels, intraclasts, and cyclic bedding indicate peritidal deposition during Cutter deposition. Erosion preceded Fussleman (Silurian?) deposition. Conodont faunas represent shallower conditions than the Montoya faunas of Sweet, but compare favorably for correlation. Panderodus and Belodina faunas characterize the shallow-marine Second Value Formation; deeper water Plectodina and Phragmodus characterize the Aleman; and very shallow-water Rhipidognathus characterizes the Cutter. Ages for the Second Value, Aleman, and Cutter Formations are late Edenearly Maysville, Maysville-early Richmond, and middle-late Richmond, respectively. Dolomitization interrupted early silica replacement of shells, matrix, and sulfates. Mosaic dolomite and epitaxial rims on dolomite cement virtually destroyed all effective porosity. End_of_Article - Last_Page 458------------