Orbitally Driven Lacustrine - Palustrine Deposition in Upper Pennsylvanian Nonmarine Cyclothems: An integrated Study of the Benwood Limestone, Monongahela Group, Appalachian Basin

Decades of study of Permo-Carboniferous cyclic successions indicate that 'classic' mixed marine and terrestrial siliciclastic---carbonate cyclothems archive eccentricity driven glacioeustasy during the Late Paleozoic Ice Age, analogous to the late Pleistocene. Our understanding of their terrestrial equivalents, however, is far less well developed with several unresolved issues including the depositional environments in which the nonmarine limestones formed and the relative roles that tectonics, autocyclic depositional processes, and climate played in their formation. This integrated sedimentologic, mineralogical, petrographic and geochemical study of the Late Pennsylvanian Benwood cyclothem of the northern Appalachian Basin was carried out to: (1) develop a depositional model for the Benwood Limestone, (2) delineate the multiple scales of stratigraphic and geochemical cyclicity, (3) evaluate the role that climate played in deposition, and (4) quantitatively constrain the hydrochemistry of the depositional waters.;The Benwood cyclothem, defined from the base of the Sewickley Coal to the base of the Fulton Green Shale, consists of sublittoral, littoral, palustrine and fluvial and swamp facies assemblages that define a hierarchy of stratigraphic cyclicity. The overall dominance of littoral and palustrine facies records deposition in a low-gradient low-energy lacustrine environment. Results of a fluid mixing model reveal that the Benwood lake(s), if sourced dominantly by freshwater surface runoff, could have accommodated between 5 to 30% seawater, while a purely continental system is permitted if evolved saline groundwater was the primary source. Small-scale cycles in the Benwood exhibit decimeter-scale alteration of siliciclastic and carbonate facies capped by exposure and pedogenic features. The estimated average frequency for these cycles falls within the range of astronomical (Milankovitch) spectra for Permo-Carboniferous precessional cycles. Small-scale cycles bundle into three large-scale (multi-meter) packages, which along with corresponding carbonate delta delta18 and delta delta13 compositions, record the hydrologic evolution of the Benwood lake(s) throughout one long-eccentricity (405 kyr) cycle. The inferred changes in the net water balance throughout Benwood deposition are in excellent agreement with modeled Pangaean continental climate in response to eccentricity modulation of precessional forcing of low-latitude insolation, indicating that climate was the dominant driver of environmental and depositional conditions during Benwood deposition.