Structural relationships and mylonites in Proterozoic rocks of the northern Pedernal Hills, central New Mexico
Paul W. Bauer and Michael L. Williams
The Pedernal Hills form an isolated, Precambrian-cored, north-trending structural high about 60 km east of Albuquerque on the eastern edge of the Estancia Valley (Fig. 1). The uplift represents a deeply eroded basement high of the "Ancestral Rocky Mountains" and is the southern termination of the Sangre de Cristo uplift of Laramide age.
Proterozoic metamorphosed sedimentary, volcanic, and plutonic rocks are exposed in the gentle, rolling, sparsely vegetated hills of the Pedernal. They are unconformably overlain by flat-lying, upper Paleozoic elastic sediments. The Precambrian units in the Pedernal Hills were first mapped by Gonzales (1968). Woodward and Fitzsimmons (1967) described the iron-rich quartzites in the northern Pedernal area. Armstrong and Holcombe (1982) studied the deformation, structure and chemistry of a sequence of complexly deformed metavolcanic rocks in the central Pedernal Hills.
Based on mineral assemblages in metavolcanic schists of the central Pedernal Hills, Armstrong and Holcombe (1982) concluded that metamorphic conditions ranged from lower to middle greenschist facies. The presence of staurolite (Gonzales, 1968) and kyanite in the quartzite of the northern Pedernal suggests that these rocks have experienced amphibolite-facies temperatures of at least 500°C and 3.75 kb. Rb/Sr ages of 1493 ± 30 m.y.B.P. and 1416 ± 100 m.y.B.P. have been repoted for a metavolcanic rhyodacite and a deformedreportede, respectively, from the Pedernal uplift (Armstrong and Holcombe, 1982). Armstrong and Holcombe (1982) suggested that these dates have been partially reset during metamorphism.
This report focuses on the northern part of the Pedernal uplift, which contains well exposed, massive, resistant orthoquartzites plus subordinate, less resistant mica schists. The contact between this metasedimentary terrain and the adjacent metavolcanic terrain is of particular interest. Studies of similar sedimentary—volcanic sequences elsewhere in New Mexico suggest that the contact between the metasediments and metavolcanics may not be a simple stratigraphic succession. Also of interest is the degree to which Pedernal stratigraphy and structure correlate with stratigraphy and structure of similar rocks exposed in the Manzano and Manzanita Mountains to the west, and possibly with other Proterozoic rocks in the southwestern United States.
- Bauer, Paul W.; Williams, Michael L., 1985, Structural relationships and mylonites in Proterozoic rocks of the northern Pedernal Hills, central New Mexico, in: Santa Rosa-Tucumcari region, Lucas S. G.; Zidek, J., New Mexico Geological Society, Guidebook, 36th Field Conference, pp. 141-145.