NEOGENE EXHUMATION OF THE NORTHERN SAN JUAN MOUNTAINS RECORDED BY ANCIENT RIVER GRAVEL COMPOSITIONS AND DETRITAL ZIRCON U-PB GEOCHRONOLOGY, UNCOMPAHGRE PLATEAU, WESTERN COLORADO

Comparisons between modern and ancient river gravel gravel compositions and U-Pb detrital zircon geochronology of samples located on the Uncompahgre Plateau of western Colorado, record the late Cenozoic erosional history of the northern San Juan Mountains. Following cessation of San Juan volcanic field (SJVF) caldera-forming eruption ca. 23 Ma, rivers draining the volcanic field began to incise the widespread blanket of Oligo-Miocene volcanic debris burying western Colorado. Volcanic-rich deposits of these Neogene rivers include spectacular gravel deposits preserved along the crest of the present-day Uncompahgre Plateau.

The oldest Neogene river deposit is represented by sand and gravel at Columbine Pass, which is located on the crest of the Uncompahgre Plateau at an elevation ~1.1 km above the modern Uncompahgre River. Compared to the modern Uncompahgre River, Columbine Pass river gravels deposits contain a greater percentage of rhyolite and basalt clasts, and fewer Proterozoic quartzites and Mesozoic-Paleozoic sandstones. These compositional differences reflect Neogene unroofing of the San Juan volcanic field. During the evolution of the SJVF, older andesitic rocks were truncated and buried by rhyolitic and basaltic rocks representing younger phases of bimodal volcanism. Miocene erosion began with removal of the younger rhyolite and basalt, and was followed by Quaternary river incision through pre-volcanic sedimentary and metamorphic rocks.

River gravel compositional differences are similarly reflected by significant differences in young (<40 Ma) detrital zircon U-Pb age populations. Young grains are rare in samples of the modern Gunnison (n=2) and Uncompahgre (n=4) rivers, which drain the SJVF, whereas they are abundant in Columbine Pass river deposits (n=36). This difference is interpreted to reflect the greater proportion of zircon-rich rhyolitic rock that was present in the region during the Miocene. In comparison, the abundance of SJVF andesitic outcrops within watersheds of the modern rivers is responsible for the small number of <40 Ma detrital zircons in the modern Uncompahgre and Gunnison River sediments. Stratigraphic and geomorphic considerations suggest that as much as 2.0-2.5 km of volcanic and older rocks have been eroded from the northern San Juan Mountains since the Miocene.