UNROOFING OF THE PEDERNAL UPLIFT: IDENTIFICATION OF REVERSE STRATIGRAPHY IN THE OROGRANDE BASIN, SOUTH-CENTRAL NEW MEXICO

This study identifies reverse stratigraphy in the Orogrande basin as it relates to the unroofing of stratal units atop the Pedernal uplift prior to erosion to crystalline basement. Three measured sections along strike of the basin margin adjacent to the uplift are utilized to assess lateral variation in uplift denudation. Samples from these three sections were subjected to modal petrographic analysis and whole-rock geochemical analysis in order to assess clastic sediment provenance, and relate that provenance to the suspected stratal units removed during denudation of the uplift. Specifically, erosion through the lower Paleozoic strata during early denudation resulted in quartz-rich sandstone deposits of the lower part of the Gobbler Formation in the Orogrande basin. Upward in the Gobbler Formation and into the lower part of the Beeman Formation, elevated TiO₂, elevated Ca/Na ratio, and increase in plagioclase are interpreted to reflect provenance from the Debaca sequence which consists of marble, gabbro, and volcanic clastic rocks, consistent with the mineralogical and geochemical changes observed. Most cover strata was removed atop the Precambrian core of the Pedernal uplift by the time of deposition of the Beeman Formation in the Late Pennsylvanian The inferred reverse stratigraphy is intercalated with well-defined biostratigraphy and thus erosion rates can be calculated by estimating the thickness of eroded strata and the duration of deposition of the reverse sequence. Erosion rates calculated for the Pedernal uplift range from 70.6 to 76.1 m/my, comparable to modern Colorado Plateau erosion rates. Characterizing the clastic sediment of the eastern basin margin, in conjunction with erosion rates, indicates the paleogeography of the Orogrande basin and Pedernal uplift as this part of the ARM province evolved during the Pennsylvanian.