Late Paleozoic Cold-Water Carbonate Sedimentation in Tasmania: DISCUSSION

Abstract

We read the recently published account of Tasmanian Late Paleozoic cold-water carbonate sedimentation (Rao, 1981) with considerable dismay. It is not that we disagree with a cold-water origin for the Berriedale Limestone (and other Tasmanian Late Paleozoic rocks), for this has long been the view of most Tasmanian geologists. Rather we indicate below what we consider to be fundamental errors in Dr'. Rao's understanding of the stratigraphy of Tasmanian Late Paleozoic rocks, and their bearing on what we consider to be a totally untenable model for Berriedale Limestone deposition. In Tasmania rocks of Late Paleozoic age are widespread and display an unmistakable glacial imprint. They constitute the Lower Parmeener Super-Group (Forsyth et al., 1974) which is ahnost everywhere subhofizontal, and rests with profound landscape unconformity on a folded basement of Precambfian and earlier Paleozoic strata intruded by Late Devonian to Early Carboniferous granites (McDougall and Leggo, 1965). Basement surfaces are often scoured, plucked and striated, with exhumed roches moutonndes in rare instances. The subglacial surface had a relief in excess of one thousand meters. Vestiges of this relief remained until their final burial late in the Triassic Period. Without doubt, sometime late in the Carboniferous Period ice covered most of Tasmania. When the ice retreated, thick sequences (sometimes in excess of 500 meters) of massive and unbedded mixtite, tillite, glaciolacustrine rhythmite claystane, outwash conglomerate, and minor turbidite sandstone were deposited in areas of low relief within the basement (David, 1908; GuUine, 1977; Banks, 1981). In the light of recent recommendations concerning the placement of the Carboniferous-Permian boundary (Kemp et al., 1977; Balme, 1980), all these glacigene rtx:ks are wholly Late Carboniferous in age (Truswell, 1978). Following the major ice retreat, most of Tasmania was rapidly flooded by a transgressive sea from the south and east, the initial eustatic rise in sea-level being far more rapid than any isostatic readjustment. Thereafter Tasmania suffered repeated periods of transgression and regression throughout Asselian-Kazanian times, when several hundreds of meters of shallow-water and essentially marine platform sediments were deposited. No subsequent transgression was as widespread areally as the initial post-glacial flooding. Dropstones and lonestones are ubiquitous and can only be satisfactorily explained by ice-rafting. Very rich faunas of very low taxonomic diversity (Clarke and Banks, 1975; Clarke and Farmer, 1976) support the existence of cold-water conditions. Nevertheless, there is no evidence that any part of what is now Tasmania was occupied by ice following the major