New Mexico Geological Society Annual Spring Meeting — Abstracts

The Española basin is central to studies of the Rio Grande rift because it is home to the type Santa Fe Group—superb badland exposures of rift-basin fill rich in vertebrate faunas and dateable ash beds. New 1:24,000-scale geological maps (mostly NMBGMR supported by USGS-STATEMAP), aeromagnetic data (USGS), and subsurface stratigraphic studies of the Pajarito Plateau (LANL) have enhanced geologic understanding over the last decade. Stratigraphic studies and lithofacies mapping by Dan Koning build upon earlier interpretations by Ray Ingersoll to show that three primary sediment sources filled the basin: erosion of granitic basement and Paleozoic rocks of the Santa Fe Range on the east, recycling of volcaniclastic debris along with quartzite-rich basement detritus derived from the north, and recycling of volcaniclastic debris along with Paleozoic-sedimentary detritus that entered the basin from northeast. This latter sediment pathway may relate to an ancestral Rio Embudo with headwaters far to the east of the modern river, as suggested by upper Miocene basalt flows that probably entered the basin from the Ocate volcanic field. The Santa Fe River also had a larger watershed in the past that included the modern upper Pecos valley. This larger drainage produced coarsegrained, hydrologically important channel deposits rich in Paleozoic sedimentary detritus and quartzite that covered a broad area of the southern basin during the Miocene. Beheading of the earlier Santa Fe River basin may have resulted from rise of the Santa Fe Range as a ruptured hanging-wall hinge zone uplift that paradoxically defines higher elevations than the footwall region along the western side of the Española basin. Research in two areas implies that basin subsidence was underway by Oligocene to early Miocene. Near Santa Fe, volcaniclastic strata of the Bishop’s Lodge Mbr. of the Tesuque Fm. correlate to the Espinaso Fm. and include 30 Ma tephra. These deposits overlie ~400 m of conglomerate derived from Paleozoic and Precambrian rocks and deposited on basement formerly denuded during Laramide uplift. These relationships indicate Oligocene foundering of the earlier uplift. Within the Abiquiu embayment in the NW part of the basin, pre-25 Ma strata of the lowermost Santa Fe Gp. thin westward across a staircase of east-facing faults to the rift margin.