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Bulletin 96—Geology of the Little Hatchet Mountain, Hidalgo and Grant Counties, New Mexico

By R. A. Zeller, Jr., 1970, reprinted 1979, 23 pp., 2 plates (including colored geologic map, scale 1:31,680).

Thick Early Cretaceous sequence, modified from the S. G. Lasky, is a key to Mesozoic stratigraphy in the Southwest. It consists, in ascending order, of unnamed beds, Hell-to-Finish, U-Bar, Mojado, and Ringbone Formations, overlain by the early Tertiary Hidalgo volcanics, and by younger volcanic rocks. Intrusive rocks are of Precambrian, Laramide, and Tertiary age; Laramide thrust faults and Tertiary normal faults characterize the structure. Copper, lead, silver, zinc, molybdenite, tungsten, and gold ores are favorable for exploration.

The stratigraphic sequence in the Little Hatchet Mountains is, in ascending order, the Pennsylvanian–Permian Horquilla Limestone; the Permian Earp Formation; a 1,500 ft thick unnamed unit of thin-bedded limestone, dolomite, gypsum, and shale of probable Early Cretaceous age, gradational upward into the Hell-to-Finish Formation, which is a Lower Cretaceous sequence, as much as 6,000 ft thick, of red beds, arkose, limestone-cobble conglomerate, sandy limestone, and some lenses of andesite breccia; the Lower Cretaceous U-Bar Formation, conformable with adjacent formations, about 4,000 ft thick and consisting of fossiliferous thin-bedded limestone, reef limestone, gray to brown shale, and some arkose; the Lower Cretaceous Mojado Formation, as much as 5,000 ft thick, consisting mainly of gray to tan quartz sandstone with shale interbeds and some thick lenses of conglomerate; the Upper Cretaceous or lower Tertiary Ringbone Formation, unconformable on older strata, as much as 7,500 ft thick, consisting of limestone-cobble conglomerate, black shale, gray shale, thin lenses of arkose, much fossil wood, chert conglomerate, and, in the upper beds, basalt and andesite flows and breccias; and the Hidalgo Volcanics, of probable early Tertiary age, unconformable on older rocks, as much as 5,500 ft thick, composed of andesite flows, breccias, lower cobble conglomerates, and upper volcanic sandstone and shale.

Rhyolitic and latitic pyroclastics with interbedded tuffaceous sandstone and clay, of middle and late Tertiary age, are angularly unconformable on older rocks. Intrusive rocks are: Precambrian granite; Laramide stocks, dikes, and sills of diorite, monzonite, and quartz monzonite; a Tertiary granite stock; and dikes of Tertiary felsite, rhyolite, and latite. Laramide orogeny was marked by differential uplift, thrust-faulting, and intrusive activity; whereas block-faulting characterized the Tertiary structural movements. Limestone-replacement lead-silver-zinc-copper are near Old Hachita, low-grade copper in altered monzonite stocks, copper-molybdenite-tungsten ore in skarns near large granitic to dioritic stocks, and some gold-quartz veins are favorable for mineral exploration.

The chief purpose of this project was the detailed geologic mapping of the Little Hatchet Mountains on a scale of 1:31,680. About five months of field mapping was done between June 1967 and February 1968. Prior mapping had been done and various geologic studies made in parts of the range as a project for the NM Bureau of Mines and Mineral Resources; this earlier work was incorporated into the present project. The geologic mapping was done in detail. In critical places the mapping was in considerably greater detail than can be shown on the map. Unresolved problems remain, but the major problems of structure and stratigraphy were solved. More detailed study of some areas would result in refinement of the map but would not change the gross picture of the geology. Only field identifications were made of igneous rocks; these should be supplemented with petrographic identification.

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