PALEOENVIRONMENTAL CLUES ARCHIVED IN LIMESTONES OF THE MONONGAHELA AND DUNKARD GROUPS, CENTRAL APPALACHIANS

A field and petrographic study of limestones in the late Pennsylvanian Monongahela and the Permian(?) Dunkard Groups, central Appalachian Basin, indicate that these limestones are dominated by ostracod-peloidal, muddy carbonates with trace to moderate amounts of siliciclastic clay and silt. Limestones range from laminated wackestones with abundant articulated ostracods and phosphatic fish teeth and spine fragments to highly bioturbated wackestones and packstones dominated by disarticulated ostracod shells. In some, ostracods are concentrated into ‘shell-bed’ pavements intercalated with mudstone to wackestone laminae. Nested ostracod shells, similar to those inferred from the fossil record to have formed in low-energy estuaries or lagoons, are common.Overall, very low diversity ostracod assemblages within micritic limestones that exhibit minimal evidence of transportation suggest deposition under non-marine to very low salinity conditions in quiet water environments- the abundance of carbonate peloids in the Waynesburg and Washington Fm limestones suggest warm shallow waters within the photic zone. Greene Formation limestones exhibit evidence for formation under deeper water conditions.

Early diagenetic glauconite cements occur in the Benwood limestone between moderately to uncompacted fish fragments. The presence of pre-compaction glauconite suggests reduced pore waters during early burial in organic-rich sediments and overall low sedimentation rates, which are requisite for glauconite mineral precipitation. Although glauconite cements can indicate brackish to marine waters, they could alternatively indicate alkaline lacustrine conditions during Benwood deposition, consistent with paleocologic and sedimentologic characteristics. Several limestones in the Dunkard Gp. (e.g., Redstone, Waynesburg and Windy Gap limestones) contain rhizoliths and elongated to vuggy secondary porosity, lined with micritic cements, nonluminescent calcite cements and rare geopetal silts. These features along with microbrecciation indicate periodic exposure of Monongahela and Dunkard limestones that led to vadose diagenesis and incipient soil formation.