Sedimentology and tectonic significance of Wolfcampian (Lower Permian) conglomerates in the Pedregosa basin: Southeastern Arizona, southwestern New Mexico, and northern Mexico

Abstract

A single interval of red chert-clast conglomerate and associated strata (RCC/CRCC interval) occurs within the Earp Formation (Pennsylvanian-Lower Permian) at many localities in southeastern Arizona, southwestern New Mexico, and northern Mexico; this interval records a middle Wolfcampian erosional event in the northern Pedregosa basin and adjacent shelf. The RCC and CRCC intervals are probably proximal and distal braided-stream deposits, respectively, consisting of conglomeratic longitudinal and transverse bar deposits; fluvial-channel sandstone; and interfluvial fine-grained sandstone, siltstone, and shale, and they are interstratified within a much thicker sequence of shallow-marine deposits. The RCC/CRCC interval is uniformly thin (average 5 m thick), yet it is characterized by sheet-like lateral continuity, indicating that deposition occurred on an extensive, low-lying surface on which fluvial channel migration and channel switching were important processes. Paleoflow directions were generally southeastward. Biostratigraphic evidence suggests that middle and lower Wolfcampian strata below the RCC/CRCC interval were beveled northward. Much clastic material of the RCC/CRCC interval may be composed of reworked residuum of the beveled strata, with additional contributions possibly from regions north and northwest of the Pedregosa shelf. Middle Wolfcampian emergence of the northern Pedregosa basin and shelf and accompanying deposition of the RCC/CRCC interval were probably caused by tectonics. Other less important factors, such as eustatic sea-level fluctuations, may have amplified the regression. Evidence favoring a tectonic cause of regression are synchronous timing of tectonism and synorogenic sedimentation throughout a wide area in the Ouachita foreland. The RCC/CRCC interval was probably deposited on a forebulge peripheral to the Pedregosa foreland basin. Stratigraphic evidence indicates that the southern Pedregosa basin evolved rapidly from a slowly subsiding platform to a deep foreland basin during early or middle Wolfcampian time. Subsidence of the Pedregosa foreland basin was presumably caused by downflexure under northward-verging thrust sheets in the Chihuahua region during the Ouachita orogeny. Downflexure of the Pedregosa foreland basin evidently caused flexural forebulging and subaerial exposure of large areas cratonward. Flexural models predicting deflection of lithosphere under isostatic thrust and sediment loads agree satisfactorily with the forebulge concept for the origin of the RCC/CRCC interval.