CLIMATIC AND TECTONIC IMPLICATIONS OF LATE PALEOZOIC TROPICAL GLACIATION IN THE ANCESTRAL ROCKY MOUNTAINS, COLORADO

Two lines of evidence support an interpretation of late Paleozoic upland glaciation in the Uncompahgre uplift of the Ancestral Rocky Mountains (ARM, western equatorial Pangaea): (1) the age and geomorphology of the Precambrian inner gorge of Unaweep Canyon (western Colorado), and (2) the sedimentology of upper Paleozoic fill within the gorge, and of the Cutler Formation exposed at the western end of Unaweep Canyon (Gateway, Colorado). A minimal late Paleozoic age for the inner, Precambrian-cored gorge of Unaweep Canyon is confirmed by the presence of strata, within the gorge, bearing late Paleozoic palynomorphs and an exclusively Precambrian provenance. The inner gorge displays numerous cirques and hanging side valleys and is remarkably U-shaped; the geomorphology is inconsistent with fluvial incision, and instead suggests fluvial exhumation of a pre-existent, glacially carved valley. Furthermore, the upper Paleozoic sediment fill within the gorge, and the Cutler strata display features consistent with ice-associated deposition. Strata within the gorge are diamictites containing angular and rarely striated boulders; the Cutler Formation near Gateway includes >1000 m of crudely stratified sandy gravel, decimeter-scale graded beds in stacks several tens of meters thick, and matrix- and clast-supported boulder conglomerate and pebbly mudstone. This succession has previously been interpreted as the result of sedimentation in an arid alluvial-fan system, but many of the facies are inconsistent with such a setting and instead suggest proglacial deposition within an ice-contact lake and outwash system. Key facies consistent with the latter include subaqueously emplaced mass flows and dropstones that record lacustrine deposition, and large-scale cross-bedded gravels that record deep, laterally extensive flood events. Preservation of this apparent mountain glacier system in the equatorial ARM implies one or a combination of two, equally remarkable corollaries: (1) the Uncompahgre uplift was high enough to sustain equatorial glaciation, and subsequently collapsed to enable preservation of Unaweep’s inner gorge, or (2) the Uncompahgre uplift was of only moderate elevation, and the Pangaean icehouse was substantially cooler than previously thought.