New Mexico Geological Society Annual Spring Meeting — Abstracts

Thick successions of Proterozoic and lower Paleozoic quartz arenite are broadly distributed along the juvenile accretionary margins of southern Laurentia. These packages are anomalous in their maturity, thickness and purity and are often stratigraphically associated with metarhyolites. Recent research in the Southwest has identified what appear to be two distinct pulses of quartzite deposition at ~1.70 and ~1.66 Ga. The first pulse coincides with the end of the Yavapai orogeny (1.70 Ga) and is characterized by deposition of the rhyolite-quartzite assemblage directly on unroofed basement (Karlstrom et al., 2004). Circa 1.7 Ga quartzites are present throughout Colorado and northern New Mexico, in the Colorado Front Range, Gunnison region, Sangre de Cristo, Tusas and Needle Mountains. While the majority of these quartzite packages characteristically overly metarhyolites, the Uncompaghre Group of the Needle Mountains is in tectonic contact with exhumed 1.78-1.69 Ga basement. Although there is some shearing along the contact, a basal conglomerate and paleosol horizon suggest an original angular unconformable relationship between Yavapai province basement and quartzite “cover”. Another question concerns the association of the Uncompaghre Group with the nearly Vallecito Conglomerate. We plan to test whether the Vallecito Conglomerate correlates with the basal facies of the Uncompaghre Group; if so, the Needle Mountains preserve a unique sedimentary succession not in other 1.7 Ga quartzite localities. We will evaluate the nature of these relationships through a combination of detailed structural analysis and detrital mineral geochronology in order to refine our understanding of the rhyolite-quartzite phenomenon.