| Abstract |
Many organic-rich marine mudstones, which are key hydrocarbon sources, were deposited on continent margins in mid-water oxygen-minimum zones (OMZs) that expanded and intensified during oceanic anoxic events (OAEs). Other marine hydrocarbon sources include platform and forearc black shales that record trans-continental, long-erm anoxic/dysoxic environments with no modern analog. Their explanation as recording deep-water, Black Sea-type basins or low-oxygen upwelling is not satisfactory for occurrences on shelves that lack significant epeirogenic activity, while modern studies show that upwellings do not cross the shelf break. The alternative is the Bakken model, which concludes that regionally extensive shelves and forearc organic-rich mudstones are shallow-water facies. These Bakken facies reflect hyper-warming conditions with high sea-levels, high water temperatures with increased insolation and low oxygen solubility, turbid water due to algal blooms and mud eroded from orogenic highlands, and possible LIP activity. Early Paleozoic black shales indicate that increased nutrients presumed to accompany the Devonian appearance of forests with deep roots that enhanced weathering simply cannot explain older Cambrian–Ordovician shelf anoxia/dysoxia. Shallow-marine deposition by the Bakken model is mandated by black shales deposited on subaerial unconformities that show high-energy facies (wave cross beds, HCS) and common bioturbation. The Bakken model explains shallow anoxia/dysoxia with high Paleozoic sea levels and tropical distribution of large continents. It is based on the Upper Devonian–lower Mississippian Bakken Formation (western U.S. and adjacent Canada). Rising temperatures, diminished oxygen solubility, and eustatic rise with deglaciation accompany modern climate change and mean that near-future platform seas will feature the reappearance of low-oxygen Bakken facies and environments. |
