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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.

The central Peloncillo Mountains consist mostly of Late Cretaceous and late Tertiary volcanic rocks; in addition, Cretaceous and Paleozoic marine sedimentary rocks and Precambrian granite are exposed in fault blocks bounded by northwest-trending steeply dipping faults. Dikes and sills of Tertiary and probably Cretaceous age intrude the layered rocks. The central part of the mapped area is structurally high with reference to the northern and southern parts, because it occupies the crest of the northwest trending Peloncillo arch, which, with its associated northwest trending faults, probably took shape in the Tertiary period. A horst of Precambrian granite bounded by faults trending east-northeast lies transversely across the middle of the range in the vicinity of Granite Gap.

The volcanic rocks include an earlier sequence of much altered andesite flow rock and breccia, which are characterized by abundant epidote, and a younger sequence of mostly rhyolitic rock, the extrusion of which was probably accompanied by explosions, for pyroclastic debris is common. Nuées ardentes eruptions resulted in the thick sequence of ignimbrite which covers much of the southern part of the area. A few small copper, lead, zinc, and silver deposits, several of which are contact metasomatic deposits, are associated with Late-Cretaceous or early-tertiary quartz monzonite porphyry.

In recent years a great impetus has been given to the study of the stratigraphy and igneous geology of southwestern NM. 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 NM is being revised, and the NM Bureau of Mines and Mineral Resources is engaged in completing the geologic reconnaissance mapping of southwestern NM 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 NM-AZ 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 NM farther east, but the older rocks of the sequence closely resemble rocks in AZ. The upper Paleozoic strata are similar to rocks of southeastern AZ 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 AZ, southwestern NM, 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 NM in the younger volcanic rocks.

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