Sedimentologic evidence for a major paleogeographic change at 8.5–6.5 Ma near San Antonio, south-central New Mexico
— Daniel J. Koning, Kevin M. Hobbs, Andrew P. Jochems, and Richard M. Chamberlin

West of San Antonio, in southern Socorro County, the 8.5 Ma basalt of Broken Tank is used to document a broad, westward- sloping piedmont that extended across preexisting, east-tilted fault blocks that now comprise the core of the Chupadera Mountains. Prior to 8.5 Ma, fingers of the west-sloping piedmont filled shallow paleovalleys cut across hilly paleotopography of the early Chupadera Mountains. The coarse piedmont sediment is part of the Popotosa Formation, which extends ~460 m below and ~180 m above the basalt of Broken Tank. In Nogal Canyon, 7 km west of San Antonio, we measured a stratigraphic section and collected paleoflow, clast composition, and clast size data in the Popotosa Formation within 35 m below and 65 m above the 40 m thick basalt. These data indicate a change in piedmont provenance and paleoflow after the emplacement of the basalt. Crystal-poor rhyolite clasts are slightly more abundant (about 10%) below the basalt and are similar to upper Luis Lopez “moat rhyolite” lava locally exposed below the Popotosa Formation. Paleoflow was clearly southwest-directed in strata below the basalt. Immediately above the basalt lies a 1.5 m bed of silty very fine– to fine-grained sandstone (possibly eolian), which is overlain by 8 m of a coarsening-upward pebbly sandstone to pebbly conglomerate. The 55 m of overlying strata coarsen upwards from a pebble conglomerate–pebbly sandstone, interbedded with two fine-grained beds interpreted as basin-floor facies, to a boulder-bearing conglomerate. This coarsening-upward trend, combined with northeast to southeast paleoflow indications, is consistent with eastward piedmont progradation. These data and observations indicate a major paleogeographic change after the basalt of Broken Tank was emplaced at 8.5 Ma. Prior to the basalt, volcanic gravel and sand derived from the Quebradas region, with an inferred additional contribution from a postulated paleotopographic high near the modern-day village of Luis Lopez, was deposited on a southwest-sloping piedmont. After the basalt emplacement, sometime between 8.5–6.5 Ma, the southwest-flowing piedmont was replaced by a southeast-sloping piedmont sourced in the eastern Magdalena Mountains, consistent with the provenance of a unique clast type in the upper Lemitar Tuff. The cause of this “piedmont reversal” is ascribed to increased slip rates on faults east of the Rio Grande, which caused collapse and burial of the southwest-sloping piedmont and the inferred Luis Lopez paleotopographic high; this reversal also resulted in an eastward shift in basin floor facies toward the modern Rio Grande. This tectonically driven paleogeographic change would have lowered the southern sill of the late Miocene playa presumed to be the terminus of the ancestral Rio Grande, facilitating the southward integration of the Rio Grande in the latest Miocene–earliest Pliocene.