Widespread shallow-marine anoxia in Late Ordovician epicratonic seas of eastern Laurentia: Evidence from lithological and geochemical proxies

The Late Ordovician experienced dramatic episodes of globally widespread accumulation of organic-rich marine sediments as represented by black shales and their carbonate equivalents, often associated with ocean anoxia. During the mid-Katian, a widespread lithofacies of black, laminated, organic-rich marlstones referred to as BLORMs (new term ) in eastern Laurentia contain abundant benthic shelly fossils, such as brachiopods and large trilobites that are incompatible with anoxic water conditions previously suggested for the facies. Here, we present lithological, paleontological, and geochemical data to demonstrate that the BLORM facies accumulated in shallow epicratonic seas under predominantly suboxic conditions punctuated by frequent intervals of anoxia. Organic-rich laminae were the result of frequent eutrophication events associated with nutrient enrichment from weathering inputs enhanced by Taconic orogenic phases, and cool-water upwelling from the Appalachian Foreland Basin. The transitions from the normal-marine limestone of the Cobourg Formation to the Collingwood BLORM facies, and then to clay shale of the Blue Mountain Formation in southern Ontario represented oceanographic signals or changes resulting in widespread shallow-marine anoxia and a regional collapse of the carbonate factory in eastern Laurentia. The drastic increase in clay influx and decrease in TOC contents in the clay shale of the middle-upper Blue Mountain Formation mark the end of BLORM depositional conditions. The variations in the weathering (Rb/Sr, Al/Ti, ∑REE), redox (Cr, V, Mo, U, Th/U) and primary productivity (TOC, δ 15 N, Zn, P) proxies reflect the fluctuations of sea level and the influence of the Taconic orogenic pulses on siliciclastic inputs into the marine environment. The TOC, δ 15 N and P org profiles show distinct shifts that peak at a stratigraphic level correlated with the base of the Blue Mountain Formation and a significant increase in siliciclastic inputs as reflected by the climbing Rb/Sr, Al/Ti and ∑REE profiles, which is consistent with sedimentological evidence. Highlights : • Lithological evidence suggests extensive shallow-water anoxia during the Late Ordovician. • Organic-rich Katian marine sediments retain near-primary inorganic geochemical proxy signatures. • Geochemical proxies document the paleoenvironmental changes recorded by the sedimentary facies.