Geology of the Northern Part of the Fra Cristobal Range, Sierra and Socorro Counties, New Mexico

The northern part of the Fra Cristobal Range contains rocks of Precambrian, Cambrian, Pennsylvanian, Permian, Cretaceous, Tertiary and Quaternary age. One of the northernmost exposures in New Mexico of the Cambrian Bliss formation occurs in the central part of the range. Here the bliss formation wedges out due to widespread pre-Pennsylvanian erosion which removed all the other lower Paleozoic rocks. The Pennsylvanian Magdalena group, which is predominately limestone, rests uncomformably on Precambrian rock and the Bliss Formation. It forms the greatest part of the sedimentary sequence and is divided into three formations; the Red House, Nakaye, and Bar B formations which represent marine transgression, maximum transgression, and marine regression respectively. Permian time is represented by the predominately clastic Abo and Yeso formations. Triassic, Jurassic, and early Cretaceous rocks are not present in the mapped area. At the north end of the Fra Cristobal Range, the Jose Creek beds of the Upper Cretacious McRae formation form a course of fanglomerate of gneissic boulders on the Precambrian granite gneiss. Northward the fanglomerate grades into the ordinary Jose Creek beds of sandstone and mudstone. These relationships indicate profound deformation and deep erosion in pre-McRae time. This area probably provides the best evidence found so far of a pre-McRae age for the beginning of strong orogeny in south-central New Mexico. At least three periods of orogeny have affected the area. These took place in Precambrian, late Cretaceous and middle and late Tertiary time. Little is known of the Precambrian deformation. The structural features which are considered to be of Laramide age form a northward-trending belt of intense deformation along the west side of the range. This belt consists of overturned folds and associate thrust faults which have been highly modified by subsequent normal and faulting erosion. The tertiary structures include open folds and high-angle normal faults trending predominately northwesterly. Later movement on the Hot Springs and Fra Cristobal faults elevated the range to its present prominence. No mining is being done within the mapped area at the present time; however, mineral deposits including a manganese deposit adjacent to the Hot Springs fault and an old mine at the head of Spring Canyon which probably produced copper. Galena is present in some of the quartz veins