The Geological Evidence for Triassic to Pleistocene Glaciations: Implications for Eustasy
Published:January 01, 1998
Paul J. Markwick, David B. Rowley, 1998. "The Geological Evidence for Triassic to Pleistocene Glaciations: Implications for Eustasy", Paleogeographic Evolution and Non-Glacial Eustasy, Northern South America
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Evidence for Mesozoic and early Cenozoic glaciations is ambiguous at best. Most of the purported evidence including faunal changes, glendonites and stable isotope paleothermometry imply cold conditions but do not explicitly require ice-sheets. In the absence of established indicators of glaciation, such as striated pavements, geomorphic features and tillites, only erratic bearing deposits, if interpreted as ice-rafted dropstones, provide direct evidence of large-scale terrestrial ice. It is shown here that such erratics can be equally well explained by non-glacial processes such as organic-rafting. Erratics in the Chalk of southern England and the middle Cretaceous of Australia are representative of the problems of “glacial” indicators in the Mesozoic.
The interpretation of high-frequency, large-magnitude, eustatic sea-level oscillations requires the existence of large ice sheets throughout much of the Mesozoic and Cenozoic. Although a temporal and spatial relationship between erratic-bearing deposits and transgressions is noted, the lack of demonstrable evidence for glaciation suggests that the amplitude of interpreted sea-level changes may have been exaggerated. We suggest that during non-glacial times the magnitude of any glacio-eustatic signal would have been no greater than 10-20 m and generally much less. Such oscillations would have been swamped by intra-basinal effects. We therefore suggest that global correlation of derived “3rd-order” or equivalent eustatic curves is untenable. We further suggest that beach front erosion explain the origin of erratic-bearing deposits than continental ice-sheets
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Paleogeographic Evolution and Non-Glacial Eustasy, Northern South America
Paleogeographic Evolution and Non-Glacial Eustasy Northern South America - Published eustatic cycle charts commonly call for eustatic fluctuations of more the 40 m every few million years or less. These cycles are interpreted as eustatic, but, so far, waxing and waning of continental glaciations is the only known mechanism which clearly has the ability to drive such large, short-term eustatic fluctuations. High-magnitude, high-frequency ?glacio-eustatic cyclicity? may be a valid concept for times of continental glaciations, but what about times when such glaciations was absent from Earth? Why do cycle charts have a similar form and style for time periods with and without glaciation? Is it that we have missed the identification of a fundamental driving cause which is as important as glaciation and which might have operated during non-glacial times? Or, is it that we are confusing local and eustatic drivers of relative sea-level change? These persistent questions, and others, continue to cast doubt on the entire subject of sequence correlatability. The papers in this book collectively address these questions.