PROTRACTED GLACIAL CONDITIONS IN EASTERN GONDWANA MAINTAINED BY STRENGTHENED UPWELLING DURING MARINE HIGHSTANDS

Outsized clasts and glendonites, indicators of freezing depositional conditions, are prominent in the Permian succession of eastern Australia. Paradoxically, these cold climate indicators occur in transgressive and highstand units and persist in strata deposited after the postulated mid-Sakmarian collapse of glacial conditions in Gondwana. This paper explores the hypothesis that protracted glacial conditions along the eastern margin of Gondwana were maintained by strengthened upwelling of cold, abyssal waters during sea level highstands. Several aspects of lonestones point toward a depositional origin involving rafting into the marine environment by floating ice. The outsized clasts, which range from pebble to boulder (up to 2 m diameter) size, penetrate and depress underlying sediments and are draped by overlying sediments. Lonestones are polymictic, vary widely in shape and roundness, and tend to be randomly oriented. A limited number of specimens preserve striations. The lonestones commonly occur in association with glendonites. Glendonites are calcite pseudomorphs after the metastable mineral ikaite, a calcite hexahydrate. Ikaite is stable only under a narrow range of geochemical conditions, including near-freezing temperatures. In outcrop, glendonites occur in discrete horizons with variable spatial density ranging from one to more than 20 glendonites per square meter. Sedimentologic and geochemical data indicate that ikaite deposition was favored by the organic-rich nature of the sediments that likely led to the development of a shallow redox interface between sulfate reduction and methane oxidation. A scenario involving strengthened upwelling of cold, nutrient-rich waters along the eastern Gondwanan margin during marine highstands is consistent with recent paleoclimate models for the Permian. Furthermore, this scenario explains several enigmatic aspects of the eastern Australian Permian record, including: (1) evidence for protracted cold conditions along the eastern Gondwanan margin following the decline of glacial conditions elsewhere; (2) the restriction of cold-climate indicators to transgressive and highstand deposits; and (3) the prominence, distribution, and geochemistry of glendonites.