LATE PALEOZOIC GLACIATION IN WESTERN EQUATORIAL PANGAEA

Late Paleozoic continental glaciation in high-latitude Gondwanaland is well documented, but emerging evidence indicates that glaciation extended to low-latitude upland regions as well. Key evidence exists in the Uncompahgre-Paradox system of the Ancestral Rocky Mountains (western equatorial Pangaea), in the form of a preserved Permo-Pennsylvanian glacial valley (ancestral Unaweep Canyon) and associated glacial and proglacial strata (proximal Cutler Formation).

Unaweep Canyon is a deep, enigmatic, Precambrian-cored gorge that orthogonally bisects the western Uncompahgre Plateau, and is occupied by two underfit creeks flowing from a divide in its midst. The geomorphology of the inner gorge appears strikingly glacial, but the elevation is too low for Quaternary ice. A minimal late Paleozoic age for the inner gorge is confirmed by the presence, within the gorge, of a Precambrian-sourced diamictite that yields late Paleozoic palynomorphs and rare striated clasts, and new well data indicates the gorge has a thick sedimentary section. Furthermore, the Permo-Pennsylvanian Cutler Formation onlaps, in a nonfaulted contact, the Precambrian basement at the western mouth of the gorge, and contains numerous facies of proglacial (glaciolacustrine and glaciofluvial), rather than arid-alluvial fan, origin. Key facies include (1) subaqueously emplaced sediment density flows, ice-rafted dropstones, and large slump blocks that all record glaciolacustrine deposition, and (2) prevalent massive to crudely stratified gravels and large-scale, laterally extensive cross-bedded gravels that record common and high-magnitude flood events typical of glacial outwash.

The Cutler facies here represent the proximal part of a system that ultimately extended to sea level in the distal Paradox basin. Maximum reasonable depositional gradients on this system yield maximum elevations of 500-1000 m for the glacial terminus, implying substantially cooler temperatures than the Quaternary and more widespread glaciation than previously considered. Given the close facies similarities between non-glacial fan systems and proglacial outwash, late Paleozoic highland-proximal systems worldwide merit scrutiny as the potential products of proglacial and periglacial processes.