TIMING AND POSSIBLE CAUSES OF LATE PALEOZOIC ICE SHEET(S) ADVANCE AND RETREAT CYCLES

Gondwana glaciation has long been proposed as the driving force for late Paleozoic global climate change, eustatic sea-level fluctuations, and development of northern hemisphere cyclothems. The glaciation is commonly interpreted as a single long-term event. However, analysis of Gondwana strata allows recognition of three non-overlapping glacial episodes ( I, II, and III) rather than one continuous glaciation. Of the three episodes, none coincide with the timing of the smallest-scale cyclic sedimentation during the Late Carboniferous (Pennsylvanian), and only episode III (latest Westphalian/Stephanian to Sakmarian) was widespread with glacigenic deposits accumulating in basins throughout Gondwana.

The location and abundance of episode III topographically expressed sedimentary basins, which were not present during episode I and II glaciation, probably facilitated widespread growth, rapid mass balance fluctuations, and ice sheet(s) collapse. Basins influence glacial dynamics by: 1) providing moisture (derived from seawater filling the lows) for glacial growth; 2) promoting glacial advance by reducing total ablation areas in basins where ice fronts from multiple spreading centers merge; 3) intensifying surge-like advance and retreat because of enhanced development of ice streams, and 4) increasing the chance of glacial decay by creating favorable conditions for the development of ice shelves that are vulnerable to rapid grounding-line retreat.

Gondwana basins were located along the margins of the reconstructed ice sheet(s) of glacial episode III. Paleocurrent data indicate that ice from multiple spreading centers merged in the basins, while lithofacies data suggest that glaciterrestrial deposits are replaced down the basins by glacimarine deposits. The abundance of the basins and their distribution surrounding the episode III ice sheet(s) probably amplified the basins' influence on the growth, fluctuations, and decay of the ice sheet(s). In contrast, the paucity of large basins during the localized glaciations of episodes I and II would not have promoted widespread glacial growth.