END-DEVONIAN SUCCESSION IN NORTHWEST PENNSYLVANIA: NEW DEVELOPMENTS CONCERNING POTENTIAL UNIT RELATIONSHIPS TO GLOBAL HANGENBERG EVENTS

Recent mapping of stratigraphic units (top-Riceville Formation-into-basal Mississippian succession) across Crawford County, Pennsylvania now allows temporal ordering of end-Devonian divisions in this part of northwest Pennsylvania. With recent confirmation that the Corry Formation, at its westernmost extent, distinctly underlies easternmost deposits of the Cussewago-Berea Sandstone succession, all regional end-Devonian units can be placed in stratigraphic sequence for comparison to successions elsewhere. Three unconformity-bounded depositional packages are recognized: a basal division (top-Riceville Formation, West Mead Bed, Drake Well Formation, Tidioute Shale, “unnamed sandstone”) below the regionally angular base-Corry disconformity; a middle succession (Corry Siltstone and post-Corry shale unit) below the regionally angular base-Cussewago disconformity, and an upper division (Cussewago Sandstone, Hayfield Shale, upper Berea succession) below the base-Mississippian disconformity.

The Cussewago Sandstone is believed by some workers to be a lowstand deposit linked to the global late Hangenberg “icehouse” event, recorded by glacial diamictites in Gondwana and apparently also in the Appalachian region. Hence, deposits corresponding to the earlier Hangenberg “super greenhouse” event, recording widespread associated black shale deposition and mass extinction, should be below the Cussewago. Discovery of black shale partings within the Corry Formation and lenses of detrital pyrite and fish bone debris along its base, suggest that this unit marks transgressive overspread of dysoxia and reworking of lag debris which could signal this event. It is suspected that a newly discovered base-Corry fish fauna is of a pre-extinction faunal composition.