ANOXIA NOT OBLIGATORY – THE MYTHS AND REALITIES OF BLACK SHALE DEPOSITIONAL SETTINGS (Invited Presentation)

Judging by information promulgated in geology textbooks, encyclopedias, and online resources like Wikipedia, there is still a widespread assumption that anoxic and reducing environments, often associated with stagnant water columns and quiescent bottom waters, are essential to the formation of black shales. Yet, there are numerous features that can be observed in black shales that suggests that although many of them did accumulate under oxygen limited conditions, their environment of deposition nonetheless was dynamic and energetic enough to largely preclude anoxic bottom waters.

These features can be physical in nature, such as ripple lamination due to bottom current activity, true x-bedding at the dm-scale (bedload transport of mud aggregates), scour & drape stratification, lag deposits, bone beds, rip-up clasts, as well as cryptic sequence boundaries and stacking patterns. Benthic fauna, though not always readily apparent, can provide important clues to oxygenation, such as fossils on bedding planes and in the shale matrix, classical bioturbation in a low contrast medium, exceedingly subtle meioturbation, layers of agrichnia burrows that require CT-scans for recognition, and miniscule benthic foraminifera. Chemical analyses (redox-sensitive trace metals) as well, when precisely target at specific sediment layers (rather than averaging over many thousand years of history), can be instructive, as are considerations of the formative conditions of pyrite concretions and pyritized burrows (the issue of Fe mobility) and secondary marcasite (re-oxidation of seafloor sediments).

When considering black shale accumulation in in epicontinental seas, stratigraphic considerations tend to place black shales in shallow water settings where wave action and storms pretty much insured that bottom water anoxia due to thermal or density stratification were seasonal rather than long lived conditions. Truly anoxic black shales that record bottom water anoxia that persisted long enough to leave an imprint in the rock record that can be examined in thin sections and via geochemical proxies, say for thousands of years or longer, appear to be exceedingly rare and requiring unusual combinations of circumstances for their formation.