Circular 133—Middle to Late Tertiary geology of Cedar Hills-Selden Hills area, Doña Ana County, New Mexico

By W. R. Seager and R. E. Clemons, 1975, 23 pp., 2 tables, 14 figs., 2 appendices, sheets.

Emphasis on rock units and structures genetically related to Cedar Hills vent zone or critical to interpretation of the evolution of late Tertiary rifting. Rocks in the Cedar Hills and Selden Hills provide an excellent record of (1) Oligocene volcanism in the Cedar Hills vent zone, (2) the timing of initial rifting in the Rio Grande rift, and (3) the Miocene-Pliocene evolution of the rift. Volcanic activity in the Cedar Hills vent zone about 35 m. y. ago was concentrated along a north-trending zone at least 20 mi long and as much as 4 mi wide. Initial rhyolitic tephra eruptions were closely followed by emplacement of an ash-flow tuff sheet from fissures within the vent zone, and by the formation of tuff cones, diatremes, and collapsed areas. Volcanic activity in the zone was climaxed by the intrusion of at least 21 flow-banded rhyolite intrusions. The vent zone, together with other north-trending Oligocene faults and basins, all generally parallel to the Rio Grande rift, suggest that these features may be precursors of the rift.

The transition into active faulting within the Rio Grande rift is marked by outpouring of Uvas Basaltic Andesite. Initial rifting is indicated by eruption of basaltic andesite from a cinder cone constructed across the Cedar Hills fault, one of the large, through-going rift faults of the region. The interfingering of basaltic andesite flows with fanglomerate derived from the uplifted block suggests rifting was in progress at least 26 m. y. ago. Late Tertiary evolution of the rift is characterized by the development of deep, broad downwarps in Miocene time that were filled with Bolson deposits, and then segmented into smaller intrabasin uplifts and grabens in Pliocene time. Many of the uplifts of the region appear to have formed initially in Pliocene time, while others originated as early as earliest Miocene or latest Oligocene.

The Cedar Hills vent zone is located within the Cedar Hills and the Rough and Ready Hills in west-central Doña Ana County, New Mexico. The area shown on the tectonic map is about 20 mi northwest of Las Cruces, and encompasses about 100 mi² of canyon, badland, and hill country west of Radium Springs. Access through the northeastern part of the area is provided by US-85, which follows the Rio Grande through Selden Canyon. Jeep trails are scattered throughout the Cedar Hills and are accessible from the Faulkner or Selden Canyon. Access north of the Rio Grande is limited to a few jeep trails starting at the resort in Radium Springs on the east side of the river.

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