REASSESSMENT OF THE TIMING AND SYNCHRONICITY OF THE GICE, M4-M5 SEQUENCE BOUNDARY, AND PLECTODINA TENUIS ZONE IN THE CENTRAL APPALACHIANS BASED ON IDENTIFICATION OF THE MILLBRIG K-BENTONITE IN WEST VIRGINIA

Understanding the timing and synchronicity of geological events and the possible relationships between those events is vital to determining which processes (i.e., tectonic vs. climate) are dominate in forcing environmental changes. Key to this over long distances is the identification and precise correlation of widely distributed, closely related marker beds such as K-bentonite beds. Differing correlations exist for stratigraphically closely related events recorded in rocks of the late Sandbian to early Katian (early Late Ordovician) interval of the central Appalachians, such as the GICE, the M4-M5 sequence boundary, and the base of the Plectodina tenuis conodont zone. Here we present new data for late Sandbian and early Katian strata from Germany Valley, Pendleton County, West Virginia and Hagan, Lee County, Virginia obtained from 313 LA-ICP-MS analyses of apatite phenocrysts from K-bentonites, which allows for a more comprehensive integration of the Arc Hollow and Dolly Ridge, WV sections into the regional stratigraphic framework.

Apatite phenocryst geochemistry from a previously unreported K-bentonite ~13 m above the base of the Dolly Ridge Fm. at Arc Hollow exhibits geochemical trends similar to analyses of the Millbrig K-bentonite at Hagan and is geochemically consistent with previous apatite-based identifications of the Millbrig at the Dolly Ridge, Union Furnace, PA, and Reedsville, PA locations. The stratigraphic position of the Millbrig suggests that the base of the Dolly Ridge and Salona formations are similar in age in WV and central PA, while the base of the Trenton Limestone at Hagan is somewhat younger. Timing of the initial expression of the GICE occurs well below the Millbrig at the WV and PA locations, but is younger at the Hagan location. In addition, timing of the M4-M5 sequence boundary, first appearance of Plectodina tenuis, and Trenton type carbonate deposition is variable between locations according to their relationship to the Millbrig K-bentonite, which suggests that expression of these events along the southeastern margin of Laurentia was largely controlled by Taconian tectonics rather than by changes in the early Katian climate.