EPEIRIC SHALE DEPOSITION AT THE EDGES OF ANOXIA AND DYSOXIA; ORDOVICIAN AND DEVONIAN EXAMPLES FROM EASTERN IOWA

Bottom oxygenation within the Paleozoic epeiric seas of North America varied in response to changes in seaway circulation patterns, surface-layer organic productivity, and the effectiveness of vertical mixing. Seaways with large-scale quasi-estuarine (QEC) circulation systems had the capacity to develop lower water masses with reduced or depleted oxygen, especially when seaway inflow was connected with oceanic anoxic or oxygen-minimum waters. In some cases, epeiric stratification beneath a pycnocline would separate a lower dysoxic to anoxic water mass from the oxygenated surface layer (zone or organic productivity), provided that water depths were sufficient to preclude effective mixing. Epeiric facies deposited under anoxic bottom conditions in Upper Ordovician and Upper Devonian examples from eastern Iowa are characterized by organic-rich brown to black shales and the absence of benthic macrobiota (including burrowers); fine laminations are commonly preserved. A spectrum of increasing oxygenation within dysoxic environments (across the pycnocline) is reflected by decreasing organic content, a shift from brown to green-gray shale hues, decreased pyrite content, increased burrowing, and the progressive appearance of certain shelly benthos. Alternations between dysoxic and anoxic deposition is marked by development of particular facies. The Ordovician lower Maquoketa Formation shows dark brown to green-gray shales associated with phosphatic and pyritic lithologies (commonly containing diminutive molluscan faunas), interpreted to have been deposited at the interface between fluctuating anoxic (to euxinic) and dysoxic water masses. The Devonian (Famennian) Grassy Creek Shale of southeast Iowa contains intervals that display fine to coarse alternations between organic brown shale and green-gray shale. Such alternations are interpreted to reflect fluctuating bottom conditions at the margins of the anoxic lower water mass within the vast Upper Devonian “black shale sea.”