Ice, Dust and Cold in the Late Paleozoic Tropics

Gerilyn S. Soreghan¹, Michael J. Soreghan¹, Michael A. Hamilton², and Dustin E. Sweet^1
1School of Geology and Geophysics, University of Oklahoma, Norman, OK
²Department of Geology, University of Toronto, Toronto, ON, Canada

The late Paleozoic (300 Ma) is the last time that Earth experienced large-scale glaciation. The Pangaean tropics have long been thought to record ‘normal’ tropical conditions like today’s icehouse. Emerging evidence in the form of periglacial, proglacial and glacial facies, however, suggests the possibility of anomalously cold conditions, even in the tropics. For example, the largest documented loess deposits are those of the late Paleozoic, best known from western equatorial Pangaea. Key attributes of the loess, e.g. its large volume, immaturity, minimal chemical alteration, and first-cycle provenance, are all most compatible with glacial or periglacial derivation. Although seemingly unlikely, we hypothesize that glaciation in the Pangaean tropics provides the simplest explanation for this non-uniformitarian distribution of loess. Moreover, we infer the Unaweep-Cutler system of the proximal Paradox basin (Colorado) to be a paired late Paleozoic glacial valley and proglacial depositional system that together record upland ice that persisted to relatively low elevation. Unaweep Canyon exhibits the geomorphology of a glacially carved canyon, and harbors upper Paleozoic diamictite containing striated clasts. The Cutler depositional system that onlaps the canyon (long interpreted as an alluvial fan) contains dropstones, turbidites and clastic varves typical of a proglacial lacustrine system. Periglacial weathering conditions are also recorded by frozen-ground phenomena preserved in the Pennsylvanian Fountain Formation (Colorado). Although these data seem at odds with features classically taken to record tropical warmth such as carbonates, evaporites and redbeds, such indicators are more reflective of moisture and/or nutrients than temperature. Accordingly, we suggest that the late Paleozoic tropics may have been, at times, much colder than commonly presumed.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas