LATE PALEOZOIC SEA-LEVEL FLUCTUATIONS: A CLOSER LOOK AT GLACIOEUSTASY IN AN ICEHOUSE WORLD

A comprehensive literature review shows that the magnitude of glacioeustatic fluctuations varied systematically throughout the Carboniferous and Permian. Plotting previously published estimates of eustatic change versus time shows that at least seven distinct subdivisions developed in response to spatial and temporal distribution of glacial ice in Gondwana.

Erosional relief and facies juxtapositions in paleoequatorial successions record eustatic fluctuations of 20-25 m, and possibly up to 60 m, took place throughout the early Mississippian (Tournaisian) – a widely recognized period of Gondwanan glaciation. The sedimentological record from middle Mississippian (Chadian to mid-Brigantian) shallow marine successions contains indicates that eustatic fluctuations probably did not exceed 30 m, a decrease in maximum value that corresponds to the absence of coeval glacial deposits in Gondwana. Late Mississippian (mid-Brigantian) to earliest Pennsylvanian (Langsettian) strata are commonly cyclic and record eustatic fluctuations of 50-100 m, an architecture caused by the return of glacial conditions to the polar regions. Middle Pennsylvanian (Duckmantian to mid-Asturian) glacial deposits are present in eastern Australia, but paleovalley depths in coeval strata suggest that eustatic fluctuations were typically less than 30 m. Glacioeustatic fluctuations of 60-120 m have been widely reported from Late Pennsylvanian (mid-Asturian) to earliest Permian (mid-Sakmarian) paleoequatorial regions, an increase that corresponds to the growth of large ice sheets in Gondwana. In eastern Australia, depth of fluvial incision and facies juxtapositions in early to middle Permian (mid-Sakmarian to Wordian) successions indicates that eustatic fluctuations decreased to a maximum of 40-70 m as ice volumes decreased. Erosional relief in paleoequatorial carbonates suggests that eustatic changes of 20-30 m accompanied the final collapse of the Late Paleozoic ice age during the Late Permian (Capitanian).

This review confirms that far-field cyclic successions recorded changing glacial conditions in Gondwana, that generalizations from small temporal intervals are not representative of the entire Late Paleozoic Ice Age, and that previous sea-level and coastal onlap curves for this interval might not be accurate.