U-PB ZIRCON GEOCHRONOLOGY FROM VOLCANIC ASH LAYERS WITHIN MARCELLUS FORMATION, APPALACHIAN BASIN: UNDERSTANDING 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 for regional correlation, allow for geochemical analysis of parent magma, and can provide geochronologic dates used in determining depositional rates and stratigraphic 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 muscovite 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. 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 yield older ages than the rest of the wells which is inconsistent with the simplistic model of Appalachian basin fill where the oldest sediments are expected closer to the eastern margin. 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 zircons were sourced from a pluton in which there were inputs from both magmatic rocks and crustal anatectic melts which is consistent with continental arc magmatism. Petrographic studies show a unique diagenetic signature for the ashes altering to K-bentonite and show significant reworking of grains. Additional geochemical data and TIMS geochronology will help in refining our interpretations.