ALLOCYCLIC CONTROLS ON THE ORIGIN OF THE DUNKARD GROUP

The Permian (?) Dunkard Group consists of the Waynesburg, Washington, and Greene Formations. Allocyclic processes in a continental basin controlled the stratigraphy of the Dunkard Group. Variations in precipitation controlled variations in the following: 1) terrestrial organic productivity, 2) lacustrine base levels, 3) basin weathering, 4) pedogenesis, 5) sediment supply, and 6) sedimentary geochemistry. Tectonics controlled accommodation space whereas eustasy was of little consequence.

Paleosols, delineating regional unconformities, show a gradual upward transition from humid climate underclay paleosols in the late Pennsylvanian Monongahela Group to petroclacic paleosols indicative of climate drying during deposition of Dunkard strata. As in the Monongahela Group, the Waynesburg and Washington Formations contain multi-bedded nonmarine lacustrine limestones with subareal exposure features that grade laterally into petroclacic paleo-Vertisols. Subareal exposure features are uncommon in decimeter-scale micritic limestones in the Greene Fm; equivalent up-dip paleosols have been lost to erosion. Coal bed continuity, thickness, and quality, also decrease up-section where thin (cm scale) coal beds unconformably overlie the paleosol/underclay/limestone complexes in the Greene Fm. Lacustrine shales and impure fluvio-lacustrine sandstones, commonly with a weak pedogenic overprint, overly coal bed horizons.

Unconformities at the tops of multiple regional paleosols, overlain by lacustrine strata, suggest repeated rise and fall of lacustrine conditions on a fan-delta in a basin perhaps analogous to the Okavango basin and fan in Namibia, southern Africa. During maximum lake levels, progradation of fluvio-deltaic systems resulted in laminated shale conformably overlain by flat-bottomed distributary mouth bar siltstones and sandstones. Prograding distributaries and/or fluvial channels subsequently incised the flat-bottom mouth bar sands. During low lake levels, anastomosing fluvial systems marked by a weak pedogenic overprint spread across a low gradient (~20cm/Km) fan surface. Deposition of Dunkard strata appears to have been coeval with at least part of the10 My transition from relatively humid conditions in the Late Pennsylvanian to Middle Permian aridity in North America.