THE PARADOX OF CYCLONIC STORMS IN A COLD PENNSYLVANIAN EQUATORIAL REGION

The climatic conditions of Earth during the Pennsylvanian (late Paleozoic Ice Age) have been widely debated. Traditionally, it was believed that high-latitude glaciers existed during the Pennsylvanian while the equatorial regions remained warm. However, over the past two decades, this view has been challenged: recent studies suggest that widespread cold conditions, including glaciation at low altitudes, were prevalent regionally within the equatorial Ancestral Rocky Mountains of Pangea during the Pennsylvanian–Permian. Pennsylvanian sections in Colorado record a variety of coarse grained alluvial through fan delta deposits derived from the Ancestral Front Range. Volumetrically minor intervals of marine strata exist in these units, including the middle Fountain Formation from the eastern side of the Front Range uplifts and the Minturn Formation on the western side. In this study, we present a paleohydraulic analysis of preserved storm-generated, hummocky bedforms in these marine intervals within upper prodeltaic (Minturn) and nearshore (Fountain) deposits. The maximum measured spacings of HCS allowed for reconstruction of wave orbital diameters, and these in turn provided wave periods for each site (5.5–11 s for the Minturn, and 7–12 s for the Fountain). These values yield fetch distances of 10–35 km for the Minturn Basin and 560–~1000 km for the mid-continental sea east of the Ancestral Front Range.

Existing paleogeographic maps place the Minturn Formation at ~5° latitude and the Fountain Formation at ~2.6–3.6° latitude. Hummocky bedforms are deposited under storm-generated complex waves or combined flows (waves and currents), which require strong winds and resulting large gravity waves. The large-scale HCS would have required cyclonic storms driven by high Coriolis vorticity and high sea surface temperatures, which challenges both the idea of a cold equatorial climate during the middle Pennsylvanian, and published paleolatitude estimates for the central Rocky Mountain region.