Geology of the Fra Cristobal Range, south-central New Mexico
Eric P. Nelson
The Fra Cristobal Range is a horst along the east edge of the Rio Grande rift near Truth or Consequences. Precambrian crystalline rocks, including the granitic Fra Cristobal pluton and its metamorphic wallrocks, are unconformably overlain by at least 1,200 m of Paleozoic strata. These strata include the Cambrian– Ordovician Bliss Formation, the Ordovician El Paso and Montoya Groups, the Pennsylvanian Magdalena Group, and the Permian Abo, Yeso, and San Andres Formations. These strata, which include elastic, carbonate, and evaporite rocks, were deposited near the southeast margin (present coordinates) of the North American craton in shallow marine, near-shore, and continental environments. Disconformities, or very low-angle angular unconformities, below the Magdalena Group, and possibly below the Montoya Group, represent epeirogenic movements in the Paleozoic. Mesozoic strata in the area include the Dakota, Mancos, Mesaverde, and McRae
Formations. These strata are exposed mainly at the southern end of the range, where they dip into the Cutter sag to the south. The McRae Formation is also exposed in depositional contact with Precambrian rocks at the north end of the range, and in a klippe bounded by low-angle normal faults in the west-central portion of the range.
Structures are of Precambrian, Laramide, and Rio Grande rift age. Precambrian structures include foliation in the granite and tectonite fabrics in the metamorphic rocks. Thick-skinned Laramide structures include basement-cored folds and thrust faults carrying basement and Paleozoic strata. Thin-skinned Laramide structures include thrust faults bounding plates with generally upright, north-trending folds (commonly with box geometry). Rift-related structures include normal faults and related fractures, both of which are locally silicified and mineralized (fluorite fissure-vein deposits).
- Nelson, Eric P., 1986, Geology of the Fra Cristobal Range, south-central New Mexico, in: Truth or Consequences region, Clemons, R. E.; King, W. E.; Mack, G. H., New Mexico Geological Society, Guidebook, 37th Field Conference, pp. 83-91.