Geology of East Potrillo Mountains and vicinity, Dona Ana County, New Mexico
Authors: Seager William R. , Mack Greg H. 
Published: March 2023

Situated just north of the Mexico border in south-central Dona Ana County, New Mexico, the East Potrillo Mountains area reveals important details about Laramide, middle Tertiary, and late Tertiary deformation in the region, as well as data on Permian, Lower Cretaceous, and Cenozoic stratigraphy. The oldest rocks exposed in the area are limestone, dolomitic limestone, and minor silty or sandy beds, at least 300 m thick, that are believed to be correlative with the middle Permian Yeso and San Andres Formations of south-central New Mexico. Disconformably above these, Lower Cretaceous shelf limestone, siltstone, sandstone, and conglomerate are approximately 570 m thick in the northern part of the range, thinning southward. These rocks are Aptian-Albian in age and correlative with the Hell-to-Finish and U-Bar Formations of southwestern New Mexico. Orthoquartzite, possibly correlative with the Sarten or Mojado Formations, crops out locally. Cenozoic rocks are poorly exposed around the margins of the East Potrillo Mountains and on the flanks of the Mt. Riley-Mt. Cox volcanic domes. They include Upper Cretaceous and/or lower Tertiary conglomerate and sandstone (possible correlatives of the Love Ranch-McRae Formations), middle Tertiary intermediate-composition lavas and domes (correlative in part with the Rubio Peak-Palm Park Formations), and upper Oligocene or lower to middle Miocene fanglomerate. The latter rocks are thought to have been deposited in a broad "early-rift" basin extending 25 km or more to the west of the East Potrillo Mountains. Quaternary alluvial fans, basin-floor deposits, maar-rim tuffs, basalt flows, and cinder cones mantle older bedrock across much of the map area. Altogether approximately 1,350 m of Permian, Lower Cretaceous, and Cenozoic rocks are exposed.

Laramide, middle Tertiary, and late Tertiary structural features are exposed in the East Potrillo Mountains. Laramide thrust faults above overturned, thrust-faulted synclines trend N30 deg. W +/- 10 deg.; drag folds indicate tectonic transport to the northeast. Erosion is not sufficiently deep to demonstrate whether thin-skinned overthrusting or rigid basement-block uplift has produced the observed shallow structure. Middle Tertiary deformation involved northeast or perhaps nearly north-south extension resulting in a system of closely spaced normal faults trending from N30 deg. W to N60 deg. W that cut Laramide structures. These faults presently dip from 0 to 50 deg. (average approximately 30 deg.) to the northeast or southwest. Original dips of major fault segments were as low as 20-25 deg. east and, this geometry as well as relationships in allochthonous and autochthonous rocks suggest that the structurally lowest parts of the system may have been initiated as a detachment fault zone. All faults in the system were rotated eventually approximately 25 deg. southwestward, domino style, to their present attitude as middle Tertiary extension progressed. Late Tertiary uplift of the East Potrillo horst and subsidence of the adjoining grabens resulted from movement on modern, steep, range-boundary faults, trending from north-south to N20 deg. W. Piedmont scarps along the range-boundary faults indicate continual movement into the late Quaternary. A symmetric positive gravity anomaly above the East Potrillo horst suggests that its uplift was not accompanied by significant rotation.