Geologic map of the Ute Mountain 7.5' quadrangle, Taos County, New Mexico, and Conejos and Costilla Counties, Colorado

The Ute Mountain 7.5' quadrangle is located in the south-central part of the San Luis Basin of northern New Mexico, in the Rio Grande del Norte National Monument, and contains deposits that record volcanic, tectonic, and associated alluvial and colluvial processes over the past four million years. Ute Mountain has the distinction of being one of the largest intermediate composition eruptive centers of the Taos Plateau, a largely volcanic tableland occupying the southern portion of the San Luis Basin. Ute Mountain rises to an elevation in excess of 3,000 m, nearly 700 m above the basaltic plateau at its base, and is characterized by three distinct phases of Pliocene eruptive activity recorded in the stratigraphy exposed on the flanks of the mountain and in the Rio Grande gorge. Unconformably overlain by largely flat-lying lava flows of Servilleta Basalt, the area surrounding Ute Mountain records a westward thickening of basin-fill volcanic deposits interstratified in the subsurface with Pliocene basin-fill sedimentary deposits derived from older Tertiary and Precambrian sources to the east. Superimposed on this volcanic stratigraphy are alluvial and colluvial deposits derived from the flanks of Ute Mountain and more distally-derived alluvium from the uplifted Sangre de Cristo Mountains to the east, that record a complex temporal and stratigraphic succession of Quaternary basin deposition and erosion. Pliocene and younger basin deposition was accommodated along predominantly north-trending fault-bounded grabens. These poorly exposed fault scarps cutting lava flows of Ute Mountain volcano. The Servilleta Basalt and younger surficial deposits record largely down-to-east basinward displacement. Faults are identified with varying confidence levels in the map area. Recognizing and mapping faults developed near the surface in young, brittle volcanic rocks is difficult because: (1) they tend to form fractured zones tens of meters wide rather than discrete fault planes, (2) the relative youth of the deposits has resulted in only modest displacements on most faults, and (3) some of the faults may have significant strike-slip components that do not result in large vertical offsets that are readily apparent in offset of sub-horizontal contacts. Those faults characterized as “certain” either have distinct offset of map units or had slip planes that were directly observed in the field. Lineaments defined from magnetic anomalies form an additional constraint on potential fault locations and are indicated as such on the map sheet.