New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper cretaceous marine strata in southwestern New Mexico have long been referred to as Colorado Shale (Formation), even though the term Colorado was abandoned elsewhere in the state more than 40 years ago. Indeed, Hayden's (1876) term Colorado Group has generally been supplanted by more precise local terminology throughout the Western Interior. In New Mexico, the terms Mancos, Graneros, Greenhorn, Carlile and Niobrara have replaced Colorado. In southwestern New Mexico, the term Mancos shoUld be used instead of Colorado when referring to Upper cretaceous marineI strata.

The Mancos Formation of southwestern New Mexico is of "Greenhorn age" (late Cenomanian early middle Turonian), is as much as 100 m thick and is dominated by olive-gray and gray siltstone and shale with lesser amounts of gray micritic limestone and yellowish orange calcarenite. It includes a medial, Bridge Creek Member which is well exposed in the southern Cooke's Range (sec. 18, T21S, R8W; sec. 13, T21S, R9W), near Virden (sees. 17 and 21, T18S, R20W) and in the Big Burro Mountains (sees. 2 and 11, T18S, R11W). The Mancos rests unconformably on early Cenomanian or late Albian strata of the Sarten Formation (includes the Beartooth Formation of some workers). In some sections (e.g., near Virden), the Mancos is overlain by the Atarque Sandstone, which in turn is overlain by fluvio-deltaic strata of the Moreno Hill Formation.

Late Albian-early Cenomanian sedimentation in southwestern New Mexico occurred in the Chihuahua trough seaway, a northern extension of Tethys, not in the Western Interior seaway. Therefore, the basal transgressive-regressive deposits of the Western Interior seaway, the Albian-Cenomanian Dakota Group, are absent in southwestern New Mexico. Their homotaxial equivalent, a thin (<2 m thick) late(?) Cenomanian transgressive sandstone, is present at the base of the Mancos Formation in the southern Cooke's Range. However, it is not a mappable unit, and therefore the name Dakota should not be applied to it.