Transgressions related to the demise of the Late Paleozoic Ice Age: Their sequence stratigraphic context
Oscar R. López-Gamundí, 2010. "Transgressions related to the demise of the Late Paleozoic Ice Age: Their sequence stratigraphic context", Late Paleozoic Glacial Events and Postglacial Transgressions in Gondwana, Oscar R. López-Gamundí, Luis A. Buatois
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The Gondwanan Icehouse Period spanned between the mid-Carboniferous and Early Permian waning by the early Late Permian. Early postglacial sea-level rise related to the final stage of the Late Paleozoic Ice Age favored creation of accommodation space with preservation potential for productive anoxia events in the newly inundated shelves and peat-forming conditions favored by rapid water table rise in updip positions in the basin. The combined effect of fast glacioeustatic sea-level rise and subsidence along basin margins led to a drastic landward facies shift; the newly created space was sufficient to accommodate a transgressive systems tract (TST) that, irrespective of the age of the glacial episode, exhibits common characteristics across Gondwana. High fresh-water discharges related to the retreat of glaciers resulted in associated reduction in coastal salinity. Therefore, fjord-like settings as part of early postglacial inland seas seem to be a valid analogue for many of these TSTs. The examples of glacial-postglacial transitions analyzed in this contribution are present in a variety of basin types, namely, those ranging from backarc foreland basins to rifts. In all of them a clear retrogradational stacking pattern is detectable in the transition from glacially dominated settings to glacially influenced early postglacial environments. Examples from South America (Calingasta-Uspallata and Paganzo Basins), South Africa (Karoo Basin), Peninsular India (several Gondwana basins), and eastern Australia (Tasmania Basin) help define two basic types of TSTs: (1) complete TSTs, with a basal part of clast-poor, massive to poorly stratified diamictites, thinly bedded diamictites, shales with ice-rafted debris (IRD) and IRD-free shales, and an upper part dominated by open-marine shales representing the maximum flooding of the shelf; and (2) base-cut TSTs in which the basal transgressive portion is mostly omitted, and the TST is exclusively represented by open-marine shales generally devoid of IRD. Whereas the complete TSTs are common in cases in which high sediment supply rates via rain-out, ice rafting, and settling of fines prevail during the early phase of deglaciation, the base-cut TSTs, on the other end, reflect the dominance of drastic sealevel rises related to fast glacier retreats.