PENNSYLVANIAN DEPOSITIONAL SYSTEMS AND PALEOGEOGRAPHY OF THE PECOS SHELF, NORTHERN NEW MEXICO

Stratigraphic, sedimentologic, and field mapping data from the Pecos River Canyon of northern New Mexico were used to interpret Pennsylvanian depositional systems and their relation to Ancestral Rockies tectonics. The Pennsylvanian La Pasada and Alamitos Formations record primarily subtidal and shallow water marine sedimentation on the Pecos shelf at the edge of an actively subsiding tectonic basin. Evidence of tectonism is reflected in the abundance of coarse clastic sediment. Paleocurrents show south transport in tabular, trough and foreset bedded mixed bioclastic/siliciclasitic sediment as well as arkosic conglomerates, sandstones, and siltstones. The arkosic composition of sandstones and abundance of granitic and metamorphic clasts in the conglomerates reveal basement uplift of igneous and metamorphic provenance. Stacked channels, reactivation surfaces, and sediment coarseness indicate high-energy sediment pulses and rapidly shifting channels in close proximity to the basement uplift. However, the presence of open marine invertebrate fossils and trace fossils in many of these clastic deposits show deposition in a marine realm. Thus, these sediments are interpreted as fan-delta deposits on the subsiding carbonate Pecos shelf, south of the Uncompahgre highlands of the Ancestral Rocky Mountains. Using existing stratigraphic data, a subsidence history calculation indicates that ~450 m of tectonic subsidence occurred at the Pecos shelf throughout the Pennsylvanian. This study provides insight into the sedimentology and stratigraphy of the paleo Pecos shelf adjacent to the Ancestral Rocky Mountains in northern New Mexico.