Early Cretaceous Ice Rafting and Climate Zonation in Australia

Lower Cretaceous (Valanginian to Albian) strata of the southwestern Eromanga and Carpentaria basins of central and northern Australia, respectively, provide evidence of strongly seasonal climates at high paleolatitudes. These include dispersed clasts (lonestones) in fine sediments and pseudomorphs of calcite after ikaite (glendonites), the latter being known to form only at temperatures below about 7° C.

Rafting is regarded as the transport mechanism for clasts up to boulder size (lonestones) enclosed within dark mudrocks; this interpretation rests on rare occurrences of penetration by clasts into substrate layers. Driftwood and large floating algae are eliminated as possible rafts because fossil wood is found mainly concentrated in nearshore areas of the basins and large algal masses have not been observed. Rafting by icebergs is considered unlikely in view of the global lack of tillites and related glacial deposits of this age. Our interpretation is that seasonal ice, formed in winter along stream courses and strandlines, incorporated clasts which, during the melt season, were dropped into muddy sediments in both basins. Eromanga fine-sediment and lonestone clasts in places were reworked subsequently by storm activity, producing lag concentrations of large clasts and associated sand lenses, both lying above local erosion surfaces. In the Carpentaria Basin, local dumping of sediment from raft surfaces resulted in accumulation of pods of small clasts.

Three zones can be identified for the Early Cretaceous climate of eastern Australia: (1) a very cold southern region, at latitudes above about 72 ° S, characterized by meteoric waters possibly originating as Antarctic glacial meltwaters; (2) a zone of strongly seasonal climates, with freezing winters and warm summers, between about 72° and 53° S. Lat.; and (3) a mid-latitude zone (below about 50° S. Lat.), where freezing temperatures were not common. However, owing to scant biostratigraphic control on the successions, the time span of specific climates in both the cold and the seasonal zones cannot be determined and probably existed only for short intervals.