SEDIMENTOLOGY AND CHEMOSTRATIGRAPHY OF UPPER SILURIAN–LOWER DEVONIAN MARINE STRATA AT WINFIELD QUARRY, CENTRAL PENNSYLVANIA

This study of a quarry outcrop in central Pennsylvania documents the sedimentology of strata spanning the Silurian-Devonian boundary, an interval of recent interest throughout the Appalachian Basin but rarely exposed in Pennsylvania. Detailed sedimentology is accompanied by d¹³C_carb chemostratigraphy related to the global Klonk Event.

Winfield Quarry, in the Appalachians of central Pennsylvania, exposes 113 m of Upper Silurian–Lower Devonian strata. Based on lithofacies, ichnofacies, and paleontology of a bed-by-bed stratigraphic section, these strata record two transgressive-regressive cycles. The Tonoloway consists of laminated micrites with evaporites and mudcracks deposited in an intertidal–supratidal setting. Keyser biomicrites with stromatolitic sponge-coral biostromes were deposited in slightly deeper subtidal settings above normal wave base. Uppermost Keyser micrites with mudcracks and tidal bedding indicate a return to an intertidal–supratidal setting. In situ stromatolites, crossbedded sandy instrabiosparite, and biomicrite in the New Creek-Corriganville indicate minor deepening to subtidal settings above normal wave base. Mandata shales indicate abrupt deepening to settings below storm wave base. Shriver micrites indicate minor shallowing within this environment. The Mandata and Shriver contain evidence for deposition in oxygen-limited settings (in order of increasing oxygenation): Mandata barren black shales, Mandata dark gray shales with Chondrites burrows, and Shriver micrite with Zoophycos burrows. Ridgeley biomicrites with dominantly unabraded normal marine fossils indicates continued shallowing to below normal wave base.

Analyses of 63 samples show a ~7.9‰ positive shift in d¹³C_carb. Values rise from a -4.27‰ low in the Tonoloway to a high of about +3.6‰ (max. +4.42‰) in the uppermost Keyser then fall to a low of about -0.6‰ in the Mandata. Minimum carbon isotope values as low as -4.2‰ occur in barren black shales of the Mandata, which record the most oxygen-limited conditions. The excursion was previously documented in West Virginian and New York (Saltzman, 2002; Husson et al., 2016), and correlation with these sections places the peak of the excursion at Winfield stratigraphically below the peak in New York and above the peak in West Virginia.