Clues from the Santa Fe Group for Oligocene-Miocene paleogeography of the southeastern Colorado Plateau near Grants, NM
— Daniel J. Koning, Matthew Heizler, and Andrew Jochems

Progressive erosion of the southeastern Colorado Plateau during Miocene deposition shed notable amounts of sediment into the western Albuquerque basin, where it constitutes the Santa Fe Group. We use gravel compositions, clast ages, and paleoflow data from the middle Santa Fe Group to infer the Oligocene-Miocene paleogeography of the southeastern Colorado Plateau. This paper focuses on gravelly strata in the Cerro Conejo Formation (Santa Fe Group) mapped in the central Rio Puerco valley (within 25 km north of I–40), which are assigned to a new informal unit called the Benavidez member (14-8 Ma). These gravelly strata interfinger eastward with the Navajo Draw Member (Arroyo Ojito Formation) and the main, non-gravelly body of the Cerro Conejo Formation. The Benavidez member can be divided into three gravel-based petrosomes: volcanic-dominated, chert-dominated (≤5% volcanics), and mixed volcanic-chert (5-50% volcanics). Minor quartzite, quartz, and Proterozoic metarhyolites are found with the chert gravel. Only the chert-gravel petrosome is present north of latitude 35°9’N. To the south, the volcanic-gravel petrosome lies at the base of the Benavidez member, with stratigraphically higher Benavidez strata consisting of interfingering mixed-gravel and chert-gravel petrosomes. Paleocurrent data from the volcanic-gravel and mixed-gravel petrosomes are slightly more easterly (medians of 112 and116°) than the chert-gravel petrosome (median of 138°), supporting a more northerly source for the latter. ⁴⁰Ar/³⁹Ar radiometric analyses of four ignimbrite clasts in the volcanic-gravel petrosome returned ages and K/Ca values matching the Vicks Peak Tuff (28.77±0.01 Ma) and the La Jencia Tuff (29.00±0.01 Ma)—consistent with lithologic similarities observed in hand samples. Based on the spatial distribution of the Benavidez petrosomes and their associated paleocurrents, we interpret that a volcaniclastic apron sourced from the Mogollon-Datil volcanic field extended northward to ~35°20’ N in the Oligocene, coinciding with the northern edge of the younger Mount Taylor volcano (Plio-Pleistocene). This apron interfingered with the Chuska Sandstone and chert-rich gravel derived from the Zuni uplift. During the Pliocene through Pleistocene, exhumation caused a progressive southward retreat of the northward extent of the volcaniclastic sediment, culminating with its complete removal north of the Rio Salado.