SHALLOW MARINE ORGANIC-RICH FACIES: RELATIONSHIPS TO PLATFORM ONLAP, TROPICAL CONTINENT DISTRIBUTIONS AND A NEED FOR QUANTITATIVE MODELING

Shallow marine deposition of organic-rich facies, including black mudstones, has long been known. However, their nearly paradigmatic interpretation even on interior platforms is as deep-water facies reflecting upwelling and/or increased productivity. With platform and back arc settings limited to ca. 80 m in modern settings, the inability of upwelling to extend far across a platform because of frictional constraints, and occurrence of organic-rich/black mudstones as transgressive facies on unconformities, alternative models are needed that include development as a preservational facies. Widespread black muds in the Late Cambrian–earliest Ordovician (Avalonia, Baltica) and Late Ordovician obviously preclude their interpretation, as in the later Devonian, as somehow reflecting the rise of terrestrial vegetation. Eustatic rise could bring marginal OMZ water onto the platform, but this seems an unlikely way to explain partial or widespread organic-rich facies that can cover wide areas of continental platforms (Baltica, Laurentia). Platform organic-rich intervals in the terminal Edicacaran–Mesozoic seem associated with onlap of low latitude paleocontinents. Thus, onlap of reflective continents by less reflective sea water led to global hyperwarming with intensified tropical temperatures, very warm epeiric seas with low oxygen solubility, and consequent organic preservation possibly enhanced during intervals (at least through the middle Paleozoic) of estimated lower oxygen levels. Low bottom water oxygenation likely accompanied development of hypnoclines reflecting fresh-water run-off from subaerial regions, production of dense hypersaline water, and limited circulation by wave action with lower latitudinal temperature differences. Low bottom water oxygenation likely appeared de novo with onlap and did not require expansion of an anoxic “puddle” in deeper platform areas as suggested by Wignall in 1991. Recently determined, high Cambrian global temperatures may explain black mud deposition on high temperate paleocontinents (Avalonia, Baltica). Most of the terminal Ediacaran–Mesozoic lacked polar glaciation, but extensive Late Ordovician–Early Silurian black mudstones on low latitude paleocontinents seem to mark deglaciations and higher temperatures.