Zircon Geochronology of Ash Beds in the Marcellus Shale of the Appalachian Basin

Volcanic ash horizons, produced by arc magmatism in the Acadian orogeny, are interbedded with the Marcellus Shale and Onondaga Limestone of the Appalachian basin. Zircon grains from the ash layers are suitable for SIMS U-Th-Pb geochronology due to their resistant behavior against weathering and ability to record age of the eruptions.;A total of 24 Tioga ash layers within cores of the Middle Devonian Marcellus and Onondaga from six wells located in West Virginia and Pennsylvania were collected for U-Pb dating. The ash horizons were recognized as thinly laminated tuffaceous shale with graded beds of white mica and pyrite, ranging in thickness between 0.5 – 14 cm. Zircon grains were extracted from 16 of the ash horizons and analyzed using the Sensitive High Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG) method.;The zircon grains mounted on epoxy rounds were inserted in the SHRIMP RG to calculate Pb, U, and Th concentrations and obtain precise age constraints. 206Pb/238U ages of all ash layers range between 380.9±2 Ma to 403.8±4 Ma (Frasnian to Emsian). Zircons containing inherited cores reflect ages of greater than 1.1 Ga and are interpreted to originate from the regional basement. In some cases, zircons on the same ash horizon produced ages ranging from 390 to 400 Ma. With no evidence of Pb loss, it is interpreted that zircons reflect the longevity of the magmatic system and the younger age clusters represent eruption ages. Three distinct age groups are found from examination of population density plots including major peaks at 384.7, 390.9, and 394.95 Ma.;The age distribution over the foreland basin shows that the base of the Marcellus is diachronous. The ages of the lowermost ash horizons dated are mapped on the six well locations showing an age distribution trend. The base of the Marcellus is 402.1 Ma in a well in northwestern West Virginia and becomes progressively younger to the southeast. In our well closest to the Appalachian front, the basal Marcellus is about 380.9 Ma. The Marcellus deposition spanned at least 380.9±2 Ma to 403.8±4 Ma (Frasnian to Emsian).;Rare earth element and trace element analyses show that the zircons originate from one source, with multiple eruptions occurring from 383.4 Ma to 402.85 Ma. The zircons are interpreted to originate from a continental granitoid origin, which is consistent with the concept of continental arc magmatism. The direction of the source is from the southeast with respect to the foreland basin, consistent with at the assumed paleo-wind directions in the Middle Devonian time.