Proterozoic rocks of the Pilar Cliffs, Picuris Mountains, New Mexico
P. W. Bauer
The Cenozoic Embudo fault zone has juxtaposed the crystalline rocks of the Picuris Mountains against the Pliocene rocks of the San Luis Basin near the village of Pilar. A combination of north-down faulting and late Cenozoic erosion by the Rio Grande has produced several miles of spectacular exposures of south-dipping Paleoproterozoic rocks along the Pilar cliffs. An impressive ductile shear zone separates the Ortega Formation quartzite from the Glenwoody Formation schist. The Glenwoody Formation is composed of feldspathic quartz-muscovite and quartz-eye schist that probably represents metamorphosed, altered, phenocrystic, felsic volcanic rocks. The Ortega Formation of the Hondo Group is a remarkably clean, 1-kmthick, metamorphosed quartz arenite that is overlain by a metamorphosed siliclastic sequence of interlayered quartzites and pelitic schists of the Rinconada Formation. All supracrustal rocks have undergone a progressive ductile deformational history that involved the formation of pervasive penetrative foliations and lineations, major fold sets, and a variety of ductile shear zones. The Pilar cliffs section forms the north limb of the large, shallowly plunging, east-trending, overturned Hondo syncline. The south-dipping contact (Pilar shear zone) between the Glenwoody Formation and the overlying, upright Ortega Formation in the Pilar cliffs is a ductile shear zone preserving evidence for both early north-vergent shearing and late south-vergent shearing. The Pilar shear zone is probably not a major tectonic break, but instead is one of the south-dipping imbrication structures that are recognized in and around the Ortega Formation in northern New Mexico. In the southern Picuris Mountains, the Hondo Group section is bounded by a second south-dipping ductile shear zone (Plomo fault) along which the overturned Vadito Group has been thrust northward over the Hondo Group. The Ortega and Glenwoody Formations belong to the northeast-trending Yavapai-Mazatzal transition zone, and are part of the 1.70-1.69 Ga rhyolite-quartzite association of the Southwest that is thought to have accumulated in volcanic/sedimentary basins on juvenile, but relatively stable, continental crust. The preferred stratigraphic/structural model states that the Glenwoody Formation is the youngest Vadito Group unit exposed in the range, and was faulted out in the southern Picuris Mountains during the Mazatzal orogeny when the older Yavapai volcanogenic basement and its quartzitic cover were thrust northward at ca. 1.65 Ga as continental crust to the south collided with the Laurentian craton. Later collapse of the mountain belt (or 1.4 Ga intracontinental tectonism) resulted in south-directed movement of the Ortega Formation over the Glenwoody Formation.
- Bauer, P. W., 2004, Proterozoic rocks of the Pilar Cliffs, Picuris Mountains, New Mexico, in: Geology of the Taos Region, Brister, Brian S.; Bauer, Paul W.; Read, AdamS.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 55th Field Conference, pp. 193-205.