RED CHERT-CLAST CONGLOMERATE IN THE EARP FORMATION (PENNSYLVANIAN-PERMIAN), SOUTHEASTERN ARIZONA: STRATIGRAPHY, SEDIMENTOLOGY, AND TECTONIC SIGNIFICANCE

Abstract

A single interval of red chert-clast conglomerate and associated strata (RCC/CRCC interval) occur within the Earp Formation (pennsylvanian-Permian) at many localities in southeastern Arizona, southwestern New Mexico, and northern Mexico, and record a middle Wolfcampian erosional event in the Pedregosa shelf and northern basin. The RCC and CRCC intervals are respective proximal and distal braidplain deposits, in contrast to the Earp Formation exclusive of the RCC/CRCC interval, which consists of interbedded carbonate and fine-grained siliciclastic strata that were deposited in mostly shallow- and marginalmarine environments. Deposition of stream channel, gravel bar, and interfluvial shale beds of the RCC/CRCC interval occurred on a broad, low-lying surface with negligible local topography. Paleocurrents were generally southward. Biostratigraphic evidence suggests that lower Wolfcampian strata below the RCC/CRCC interval were beveled northward. Much of the chert present- in the RCC/CRCC interval is probably residual material from the beveled strata, as well as from a region just north of the Pedregosa shelf. The evolution of the Pedregosa shelf and northern basin during depoSition of the Earp Formation is illuminated by identification of facies belts for three time intervals: (1) restricted shelf, inner shelf, and open-marine shelf facies belts during Virgilian through early Wolfcampian ttme, (2) proximal and distal braidplain facies belts during middle Wolfcampian time, and (3) restricted shelf, estuarine-marginal marine, and tidal-flat facies belts during middle through late(?) Wolfcampian time . The middle Wolfcampian erosional event caxnpanying the deposition of the RCC/CRCC interval was probably related to the Ouachita orogeny. Stratigraphic evidence suggests that the southern Pedregosa basin in Chihuahua, Mexico, evolved rapidly to a deep foreland basin during early or middle Wolfcarrpian tine because of downflexure under northward overthrusts during the Ouachita orogeny. Flexural subsidence of the Pedregosa foreland basin was accanpanied by peripheral forebulging, causing subaerial exposure of large parts of the Pedregosa shelf and northern basin. Deposition of the FCC/CFfX interval probably occurred on the subaerially exposed forebulge. Flexural mxlels predicting the deflection of the lithosphere under isostatic thrust and secliIrent loads agree satisfactorily with the forebulge concept for the origin of the RCC/CRCC interval.