Paper No. 12
Presentation Time: 11:15 AM
THE CASE FOR UPLAND ICE IN LOW-LATITUDE LATE PALEOZOIC PANGAEA
The Late Paleozoic archives the most expansive and longest glaciation of the Phanerozoic, marked by extensive evidence for continental glaciation grounding at sea level in the southern polar (Gondwanan) continents. In contrast, the low-latitude tropics have long been considered uniformly warm. We hypothesize that upland glaciation and periglacial conditions persisted in orogenic regions of Pennsylvanian-Permian western tropical Pangaea. Within the Ancestral Rocky Mountains (ARM), evidence includes a hypothesized late Paleozoic paleovalley containing diamictite, (b) coarse-grained lacustrine and fluvial clastics onlapping this paleovalley that preserve lonestones and abundant flood deposits, (c) polygonal cracking (in other locales) that may record freeze-thaw phenomena, and (d) quartz grains with high-stress microtextures (including microstriae) in fluvial clastics of ARM-piedmont regions. Additionally, vast deposits of loess occur, with provenance signaling derivation from basement regions in the cores of the ARM and Appalachian-Ouachita orogenic systems. Tropical glaciation is permissible even in warm tropics, given sufficient elevation of uplands. However, depositional systems emanating from ARM uplifts terminated at sea level, and estimates for ice terminus elevations derived from applying maximum reasonable fluvial slopes result in likely maximum values (<1200-1800 m), significantly lower than those of the Last Glacial Maximum. If our hypotheses are valid, then this result implies globally cool temperatures at least episodically, during the intervals recorded by upland glacial and periglacial conditions. This result also implies the probability of widespread glaciation in Permo-Pennsylvanian uplands, both tropical and subtropical, given the global orogenesis and thus expansive highlands associated with Pangaean assembly. More evidence should thus exist for upland glaciation globally, but identification of such deposits is elusive owing to lack of accommodation that precludes preservation of proximal-most strata containing ice-contact indicators. Hence, evidence must be sought in preserved piedmont regions, in fluvial and eolian deposits formed in (possible) periglacial settings.