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Geologic map of the San Antonio Mountain area, northern New Mexico and southern Colorado

November 5, 2018

The geologic map of the San Antonio Mountain area in northern New Mexico and southern Colorado is located along the west-central part of the San Luis Valley. The San Luis Valley is the geomorphic expression of the San Luis Basin, an extensional basin associated with the northern Rio Grande rift. Deposits within the map area record volcanic, sedimentary, and tectonic processes over the last ~33 million years. Oldest exposed deposits include Oligocene volcanic rocks associated with the southeast San Juan Mountains locus of volcanism within the Southern Rocky Mountains volcanic field. Overlying deposits of the Southern Rocky Mountains volcanic field are volcaniclastic sedimentary rocks interbedded with predominantly basaltic lava flows of Oligocene to Miocene age. Basalt to rhyolite volcanic rocks of the Pliocene to Pleistocene Taos Plateau volcanic field unconformably overlie Oligocene to Miocene volcanic and sedimentary deposits. Superposed on the Tertiary deposits are Pleistocene to Holocene alluvial and colluvial deposits.

North- to northwest-trending faults displace rocks within the map area. Magnitude of deformation is broadly correlative with age of the deposits inasmuch as Oligocene to Miocene rocks display a greater degree of fault displacement and east tilting than Pliocene volcanic rocks. Within the map area, faults displace Oligocene to Miocene deposits 10–30 meters with generally down-to-west offset, and the units dip eastward 3–7 degrees. Pliocene volcanic rocks exhibit shallower eastward dips inferred primarily from the slope of upper lava flow surfaces that dip eastward from 1–3 degrees and lava flows are generally displaced less than 5 meters.