| Description |
Stratigraphic, sedimentologic, and field mapping data from the Pecos River Canyon of northern New Mexico are used to interpret Pennsylvanian depositional systems, their relation to Ancestral Rocky Mountain paleoGeography;, the general geologic history, and Cenozoic geomorphic processes. The Pennsylvanian La Pasada and Alamitos Formations record primarily subtidal and shallow water marine sedimentation on the Pecos platform at the edge of an actively subsiding tectonic basin. The 203 m of strata in the study area consist of 59% siltstone, sandstone, and conglomerate, 25% carbonate, and 16% shale. Seven shoaling upward cycles and eight gravity flow pulses observed in the studied section reflect an interplay of intrabasinal and extrabasinal controls. Using existing stratigraphic data, subsidence history calculation based on an estimate of isostatic subsidence indicates that ~450 m of tectonic subsidence occurred at the Pecos platform throughout the Pennsylvanian (~165 m during the Desmoinesian). The large amount of tectonic subsidence during deposition of the studied interval and decrease in number of shallowing upward cycles proximal to the basin bounding fault show that tectonics had an influence on the succession of strata at Pecos. East to southeast lateral thinning of strata and south directed paleocurrents indicate that the Ancestral Rocky highland was located northwest of the study area. This highland location supports the idea that the Pecos-Picuris fault, a major fault approximately 6 km northwest of the study area, may have been an active Pennsylvanian basin-bounding fault. Paleocurrents show bidirectional north and south transport in tabular and trough crossbedded mixed carbonate-siliciclastic sediment with mud drapes and reactivation surfaces indicating reworking by tidal currents. Stacked channels and large clasts in conglomerates indicate high-energy sediment pulses from flood plumes and rapidly shifting channels in close proximity to the Uncompahgre Uplift. However, the presence of open marine Pennsylvanian invertebrate fossils and trace fossils in many of these conglomeratic deposits and interbedded carbonates shows deposition on a carbonate shelf. The arkosic composition of sandstones and abundance of granitic and metamorphic clasts in the conglomerates reveal basement uplift of igneous and metamorphic provenance. Thus, the clastic sediments are interpreted as tidal influenced fan-delta deposits on the carbonate Pecos platform, southeast of the Uncompahgre highlands of the Ancestral Rocky Mountains. Geologic mapping revealed previously unmapped faults, folds, and surficial deposits. The folding and faulting are likely associated with the Laramide Orogeny and Rio Grande Rift respectively. A large anticline and smaller superimposed folds show northwest to southeast crustal shortening. Northeast striking steep normal faults with an average offset of ~9 m parallel major normal faults associated with formation of the Rio Grande Rift. Geologic structure exerts important controls on geomorphology of the Pecos River Canyon and surrounding area. The canyon is asymmetric with the east rim higher than the west rim due to a southwest structural dip across the canyon. Because the east rim of the canyon is ~60-150 m higher than the west rim, the east side of the canyon is subject to a higher rate of physical weathering. A large area (~1.9 km2) below the east rim contains a combination of landslide, rockfall, and colluvial material. This study enhances our understanding of the sedimentology and stratigraphy of the Pennsylvanian Pecos shelf adjacent to the Ancestral Rocky Mountains in northern New Mexico and provides insight into geomorphic and structural styles of the Pecos River Canyon and the surrounding region. |
