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Chronostratigraphy, Correlation, and Depositional History of the Marcellus Shale in the Central Appalachian Basin: A Study of Inorganic Geochemistry, Stable Isotopes, and Magnetic Susceptibility Data From Pennsylvania and West Virginia

Abstract

Five drill cores of Middle Devonian sedimentary rock from the central Appalachian Basin have been sampled at 1.5 foot intervals for inorganic (XRF) and stable isotope (δ13Corg) geochemistry along with bulk magnetic susceptibility data collected at 0.5 foot intervals. This data is employed here to resolve the depositional and diagenetic character of the uppermost Onondaga limestone, Marcellus shale, and overlying “Hamilton” units in West Virginia and southern Pennsylvania. A hierarchical cluster analysis reveals two major element associations, one likely controlled by clastic mineralogy (Na, Zr, Ti, K, Al, Ga, Rb) and another reflecting authigenic components such as biogenic apatite (P), carbonates (Ca, Mg, Mn), barite (Ba, S), and proxies for organic matter and low oxygen depositional environments (V, Mo, U, Cu, Ni). The abundance of silica is also correlated these redox sensitive indicators, though the reasons for this remain unclear. It may relate to diagenetic mineralization by low pH pore-waters; a finding consistent with exceptionally high DOP and low clay content. A detailed chemostratigraphic correlation has been generated based on key element abundances and ratios (Th/U, Na/Al, Zr/Al, Cr/Al, and the relative enrichment of U). This framework suggests that lower subdivisions onlap the Onondaga Formation to the northwest. Exceptionally low δ13Corg in this lower section (Union Springs) are consistent with deposition during the late Eifelian Kačák Event. Within this chronostratigraphy, upward-rising bulk magnetic susceptibility values that likely reflect the shift from transgression to highstand in the early Givetian, a pattern echoed by Th/U values. These quantitative data may be useful for calibrating high resolution sequence and chronostratigraphic models for the Marcellus shale.