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Bulletin 70—Geology of the Knight Peak area, Grant County, New Mexico

By D. L. Ballmann, 1960, 39 pp, 3 tables, 10 figs., 1 plate, 1 index.

Precambrian granite, intruded by dikes of Precambrian diabase, is present on the northeast and southwest sides of the Knight Peak area. Outliers of Cambrian–Ordovician Bliss Sandstone and Ordovician El Paso Limestone, of approximately the same thickness as these same units elsewhere in the Silver City region, suggest that the uplift of the Big Burro Mountains did not begin in Precambrian time.

An arkose, possibly of the Lobo Formation of Cretaceous age, is present locally above the Precambrian granite. Tertiary volcanic rocks, consisting of a lower rhyolitic series of lavas, tuffs, and agglomerate, early andesitic lava and tuff, and middle rhyolitic breccias, lava, and tuffs, accumulated to a thickness of over 5,000 ft Andesite was intruded locally along minor faults. Two large perlite bodies are among the early rhyolitic rocks.

Erosion in nearby areas furnished detritus for the lower beds of Gila conglomerate. Faulting along the Taylor fault on the northeast side of the area began at this time. Continued downward movement along this fault produced a basin in which the upper beds of the Gila accumulated. Greater displacement in the northern half of the area produced hinge faulting along the Malone fault, which bounds the northern half of the area on the west. Several phases of late volcanic activity in Quaternary time produced rhyolitic cones and intrusive bodies, dacite lava, and andesitic lava and tuff, which overlie the tilted rocks of the Knight Peak area with an angular unconformity. Later Quaternary events include the deposition of high-level gravels, pediment gravel, and alluvium. Dissection of the mountains and some crossfaulting continue to the present.

This project has as its purpose the investigation of the structure and stratigraphy of the Knight Peak area. Special emphasis has been placed upon deciphering the Tertiary volcanic history of the area. At the time the study was made, no topographic maps of most of the area were available. The U.S. Geological Survey is preparing topographic maps of the northern third of the Knight Peak area in its 1:24,000 series as a part of the Steeple Rock quadrangle, and these should be available soon. A topographic map of the entire southwestern corner of NM was published by the USGS in its 1:250,000 series in 1958.

For this study of the geology of the Knight Peak area, geologic features were plotted on uncontrolled aerial photographs taken in 1946 and available through the Soil Conservation Service of the U.S. Department of Agriculture. This information was later projected onto the Grant County highway map. Elevations of points for structure profiles were determined with two aneroid barometers. Some traverses were measured with a tape and Brunton compass.

  In the central and western portions of the area, exposures are sufficiently good that contacts can be traced easily. In the extreme northern and southern ends of the area and over much of its eastern half, soil and gravel cover make accurate plotting of contacts difficult. The summers of 1955 and 1957 and part of the summer of 1956 were spent in the field. The laboratory studies were conducted at the University of Illinois during the winters of 1955–1958.

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