PALEOLAKE NONESUCH: A PALEOENVIRONMENTAL AND BIOLOGICAL RECORD AT THE END OF THE MIDCONTINENT RIFT

Following a prolonged interval of voluminous volcanic activity within the North American Midcontinent Rift, sedimentation within a thermally subsiding basin led to the deposition of sedimentary rocks of the Oronto Group. The Oronto Group commences with the Copper Harbor Conglomerate, which represents terrestrially-deposited alluvial fan and fluvial sediments. The Copper Harbor Conglomerate is conformable with the overlying lacustrine shales, siltstones and sandstones of the Nonesuch Formation (Fm.), exposed along a >250 km long belt in northern Michigan and Wisconsin. After the lacustrine deposition represented by the Nonesuch Fm., the lake shallowed into the overlying Freda Fm., dominantly comprised of channelized sandstones and overbank siltstones representing a prolonged terrestrial fluvial environment. Rock magnetic, petrographic, and stratigraphic observations of sections in Wisconsin and Michigan indicate the presence of an oxycline based on distinct iron mineralogical facies. The presence of pyrite and very low quantities of magnetite in the deep-water sediments are interpreted to indicate anoxic conditions either in the water column or restricted to sediment pore water. Sediments deposited in intermediate water depths contain both detrital hematite and magnetite indicating oxic to suboxic conditions rather than anoxia. Shallow waters were fully oxygenated, based on the identification of both detrital and pigmentary hematite in these lithologies. This predominantly oxic lacustrine water column contrasts with interpretations of mid-Proterozoic oceanic environments, which are thought to be dominantly anoxic with potential consequences for life and its evolution. The Nonesuch Fm. contains a well-preserved record of organic-walled microfossils comprised of more than 50 different species. New data allow us to link the environmental conditions of Paleolake Nonesuch to microfossil assemblages and palynofacies to better understand paleoecology and community architecture, providing insight into early eukaryote diversification in terrestrial versus marine environments. Our paired magnetic and microfossil analyses of the Nonesuch Fm. offer a distinct viewpoint into lacustrine sedimentology, redox conditions, and eukaryotic evolution during mid-Proterozoic time.