PRECESSIONAL-DRIVEN CLIMATE CYCLICITY IN PENNSYLVANIAN BENWOOD FM, DUNKARD BASIN, EASTERN OHIO

In the Dunkard Basin of eastern Ohio, early Late Pennsylvanian glacio-eustatic marine to terrestrial cyclothems of the Appalachian Basin give way to cyclic continental - dominated successions in the late Late Pennsylvanian. This cyclic non-marine succession is coeval with marine limestones cyclothems in the Midcontinent, suggesting an allocylic control on both. Field investigation of the Benwood Fm., Monongahela Group of the Dunkard Basin reveals repeated m-scale packages of Fe-clay-rich mudstones and shales overlain by micritic carbonates and microbial bindstones capped by dessication cracks and breccias and incipient soil development. The Benwood is bound below by the Sewickly Coal and the incised Fulton Green Shale channel-fill from above and is interpreted to be analogous to 400 kyr marine cyclothems of similar age in the Midcontinent. Within the available age constraints, the m-scale stratigraphic packages in the Benwood appear to be precessional-scale cycles. Late Paleozoic climate simulations suggest that precessional forcing would have dominated the climate of tropical Pangaea leading to sub-precessional periods of wet conditions alternating with drier seasonal climate. Benwood carbonates are hypothesized to have precipitated during drier seasonal intervals, while the mudstones were deposited during wet climate intervals.

In order to test this hypothesis we integrated petrographic, geochemical and X-ray diffraction (XRD) analysis of samples collected at a dm-scale from an outcrop transect in order to evaluate the hydrologic, salinity, and organic productivity conditions of the depositional area. Integration of these results with Sr isotopic analysis of fish fin rays, ostracode shells, and calcite cements support a predominantly continental environment with carbonates forming in highly evaporative conditions. However, the stable and radiogenic isotopic compositions do permit the potential for influx of seawater during some stages of Benwood deposition.