An overview and interpretation of autocyclic and allocyclic processes and the accumulation of strata during the Pennsylvanian–Permian transition in the central Appalachian Basin, USA

Abstract

Autocyclic and allocyclic processes controlled the lithostratigraphy of strata that accumulated in the central Appalachian foreland basin during the Pennsylvanian and the transition to the Permian. The transition strata, from bottom to top, include the Late Pennsylvanian Pittsburgh and Uniontown formations of the Monongahela Group, the Waynesburg and Washington formations of the Dunkard Group and the Permian Greene Formation of the Dunkard. Variations in the amount of precipitation were the predominant allocyclic control on stratigraphic variations in terrestrial organic productivity, lacustrine base levels, basin-scale weathering, water table and pedogenesis, sediment supply, and sedimentary geochemistry. Tectonic subsidence controlled accommodation space and basin configuration. Eustasy had little or no effect on the stratigraphy of strata deposited in this continental basin during the Pennsylvanian–Permian transition. Autocyclic processes were the predominant spatial control on architecture of alluvial plain aggradation.Paleosols, delineating regional unconformities, show a gradual upward transition from kaolinitic underclay paleosols indicative of a humid climate in the Late Pennsylvanian Monongahela Group to petrocalcic paleosols indicative of a dry subhumid to semiarid climate during deposition of Dunkard strata. As in the Monongahela Group, the Dunkard Group Waynesburg and Washington Formations contain multi-bedded nonmarine lacustrine limestones with subareal exposure features. These limestones grade laterally into petrocalcic paleo-Vertisols in the up-dip alluvial plain. Subareal exposure features are uncommon in centimeter-scale micritic limestones in the Permian Greene Formation; equivalent up-dip paleosols have been lost to erosion. Coal bed continuity, thickness, and quality, also decrease up-section from the very thick (>m) and laterally continuous Pittsburgh Coal at the base of the Monongahela Group to thin (cm scale) and laterally discontinuous coal beds that unconformably overlie the paleosol/underclay/limestone complexes in the Greene Formation. Lacustrine shales and impure (wacke) fluvio-lacustrine sandstones, commonly with a weak pedogenic overprint, generally overlie coal beds.During accumulation of Monongahela and Dunkard (MDG) strata within the basin center, unconformities at the tops of regional paleosols, overlain by lacustrine strata, suggest allocyclic-induced repeated rise and fall of lacustrine conditions in a lacustrine-fan-delta complex analogous to the Okavango basin and fan in Namibia, southern Africa, and/or the Pantanal in southern Brazil. During maximum lake levels, progradation of fluvio-deltaic systems resulted in laminated shale conformably overlain by dark shale and flat-bottomed distributary mouth bar siltstones and sandstones. Prograding distributaries and/or fluvial channels subsequently incised the flat-bottom mouth bar sands. Where basin margin strata are preserved, south of the basin center, depositional environments consist of aggrading alluvial plain sequences with paleosols, fluvial channel sands, and flood plain deposits. Anastomosing fluvial systems prograded across a low gradient (~1ft/mile; ~20cm/km) alluvial plain into the basin center. A weak pedogenic overprint, marked by ubiquitous root penetrations, occurs throughout most basin-centered fluvial deposits.The subtle but continuous decline in the repetition of cyclic lithostratigraphy up-section in the MDG, particularly in the Greene Formation, appears to be the result of a 10-myr-climate transition from the humid to dry subhumid climate cycles of the Late Pennsylvanian to the equable semiarid to arid climate of the Middle Permian in North America. Cyclothems, common in Pennsylvanian strata, become less distinct up-section in the Dunkard because of decreasing development of underclays, coal, and limestone. This 30-myr period of transition from the humid Pennsylvanian to the arid Permian has been referred to as the Dyassic Period.