U-Pb zircon geochronology of the Tioga ash layers within the Marcellus Shale, Appalachian Basin: Implications for basin evolution during the Middle Devonian

Numerous thin volcanic ash layers are found interbedded within the upper Onondaga Formation and lower Marcellus Shale of the Appalachian basin. These ashes are believed to have been sourced from continental arc magmatism along the Acadian orogen during the Middle Devonian and form key stratigraphic markers, allow for geochemical analysis of parent magma, and most importantly, provide geochronologic dates used in determining depositional rates and chronostratigraphic relations. Zircons were extracted from ashes found in 6 well cores in Pennsylvania and West Virginia. The ashes range in thickness from 0.5-14 cm and are recognized by their buff color, abundant mica and pyrite, and high U/Th ratios in spectral gamma ray logs. The U-Pb age of each ash layer was determined by analyzing 12-18 spots on individual zircon crystals at the USGS-Stanford SHRIMP-RG laboratory. Recycled or reworked older zircon crystals are absent from the samples suggesting a primary origin as air-fall deposits. Concordant 206Pb/238U ages from zircon crystals within a single ash vary over a range of as much as 23 My which could represent the residence time of the zircon crystals within the Acadian magmatic system. Typically there is a cluster of younger ages that we interpret as the best estimate of the age of eruption and coeval with depositional age in the basin. This indicates that either the basal Marcellus Shale was deposited diachronously or that ashes were not preserved uniformly across the study area. Wells on the western-most side of the study area in northern West Virginia yield consistently older ages than the rest of the wells. However, this age distribution is inconsistent with the simplistic model of Appalachian basin fill where the oldest sediments are expected closer to the eastern margin. Some wells also showed stratigraphically younger ashes yielding older ages than underlying ashes, although most overlap within age error, possibly due to complexity in basin structure and bathymetry influencing sedimentation patterns. An estimate of compacted Marcellus sedimentation rates, from the Winner well where ashes were 9 feet apart, was around 1.2 ft/my (36.6 cm/my). Rare Earth Element (REE) data provide evidence that our zircons were sourced from continental arc magmatism with slightly greater inputs from magmatic rocks than crustal anatectic melts. Chondrite-normalized REE graphs show similar trends to a diorite melt source and unique Ce trends may help in identifying the Acadian-age parental pluton body or eruption source. Petrographic studies show a unique diagenetic signature for the ashes altering to K-bentonite and show significant reworking of mica grains. The addition of TIMS geochronology results and more geochemical data will help in refining our interpretations.