Open-file Geologic Map - 271
Geologic Map of the Cuchillo 7.5-Minute Quadrangle, Sierra County, New Mexico

Located northwest of downtown Truth or Consequences, the Cuchillo quadrangle occupies a key location for understanding the geologic history of the Truth or Consequences area. We refine previous mapping, which produced a robust geologic map of the bedrock geology of the Mud Springs Mountains (Maxwell and Oakman, 1990), and focus most of our effort on mapping the internal stratigraphy and structure of the Santa Fe Group—building from the detailed, admirable work of Foster (2009) and Mack et al. (2012). The Cuchillo quadrangle includes the entire Mud Springs Mountains, a northeast-tilted fault block underlain by Ordovician-Pennsylvanian strata composed mainly of limestones, dolomites, and shales—with reddish, fluvial mudstones and sandstone present on the northeastern flank of these mountains. Surrounding this bedrock high are sandstones, with minor siltstones and conglomerates, of the Plio-Pleistocene Palomas Formation, whose age is well constrained at 5 to 1 Ma using magnetostratigraphic and previous biostratigraphic and ⁴⁰Ar/³⁹Ar geochronologic studies. The Palomas Formation comprises the upper basin fill unit of the Santa Fe Group that filled two adjoining basins, the Engle Basin to the northeast and the Palomas Formation to the southwest, during subsidence of the Rio Grande rift. The aggradational surface of the Santa Fe Group is well-preserved on this quadrangle and is called the Cuchillo surface (McCraw and Love, 2012). Near the town of Cuchillo, located in the northwest corner of the quadrangle in the valley of Cuchillo Negro Creek, are exposures of finer-grained, highly tilted, pre-5 Ma strata of the Santa Fe Group.