PENNSYLVANIAN DEPOSITIONAL SYSTEMS AND GEOMORPHIC DEVELOPMENT OF THE PECOS RIVER CANYON, 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, their relation to Ancestral Rockies tectonics, and Cenozoic geomorphic processes. The Pennsylvanian La Pasada and Alamitos formations record subtidal and shallow water marine sedimentation on the Pecos shelf at the edge of an actively subsiding tectonic basin. Evidence of this tectonism is reflected in the abundance of coarse clastic sediment. Paleocurrents show south transport of thick arkosic conglomerates, mixed bioclastic/siliciclasitic sediment, and tabular and trough cross-bedded arkosic sandstones and siltstones. These sediments are interpreted as deltaic deposits on the subsiding carbonate Pecos shelf, south of the Ancestral Rocky Mountains. Subsidence history calculations indicate that ~450 m of tectonic subsidence occurred at the Pecos shelf throughout the Pennsylvanian. A well-studied stratigraphic section at Dalton Bluff, 5 km north of the study area, contains 81 m of correlative strata.

Geologic mapping reveals folding and faulting that is likely associated with the Laramide Orogeny and Rio Grande Rift respectively. Formation of the Pecos River Canyon has resulted in rockfall and large landslides on the east side of the canyon. GPR data show that the landslides are at least 7 m deep. Pedimentation likely occurred as a result of Pleistocene climate fluctuation and regional uplift. Alluvial sedimentation, colluvial creep, and rockfall are processes that continue in the canyon today. This study enhances our understanding of Ancestral Rocky Mountain tectonics, and provides insight into regional geomorphic and structural styles.