Pennsylvanian depositional systems and geology of the Pecos River Canyon, Northern New Mexico
Creators: name: Heuscher, Sonja
Publisher: name: University of Utah
Year: 2012
Subjects: Pecos River Valley
Geology, Pennsylvanian
Stratigraphy

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.