Bulletin 57—Geology of the central Peloncillo Mountains, Hidalgo County, New Mexico, and Cochise County Arizona

By E. Gillerman, 1958, 152 pp, 2 tables, 1 fig., 14 plates, 1 index.

In recent years a great impetus has been given to the study of the stratigraphy and igneous geology of southwestern New Mexico. The ever-increasing search for mineral and petroleum deposits has made necessary a reevaluation of the geology of the region. Areas have been restudied, and areas which previously had been geologically bypassed have been investigated. As part of this "new look" the geologic map of New Mexico is being revised, and the New Mexico Bureau of Mines and Mineral Resources is engaged presently in completing the geologic reconnaissance mapping of southwestern New Mexico for the new map. This report is one of a series representing detailed studies of areas regarded as critical both with respect to geologic information and to knowledge of mineral deposits.

The central Peloncillo Mountains, situated as they are on the New Mexico-Arizona boundary, are critical in a comparison and correlation of the geology of these two States. The mountains contain a relatively complete sequence of lower Paleozoic strata, similar to strata in portions of New Mexico farther east, but the older rocks of the sequence closely resemble rocks in Arizona. The upper Paleozoic strata are similar to rocks of southeastern Arizona and indicate a closer relationship with western areas. The Cretaceous rocks are closely allied to rocks to the south, in Mexico, and represent the northernmost exposures of the thick Lower Cretaceous sequence of southeastern Arizona, southwestern New Mexico, Sonora, and Chihuahua, with its included volcanic rocks.

The long volcanic history of the region is well represented in the area. Lower Cretaceous, Upper Cretaceous or lower Tertiary, middle and upper Tertiary, and Quaternary volcanic rocks are exposed. The distinction between the different periods of volcanic activity is particularly important in the search for mineral deposits, because no important base-metal deposits have been found in southwestern New Mexico in the younger volcanic rocks.