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U-Pb Geochronology of Zircon from Volcanic Ashes in the Marcellus Formation, Appalachian Basin

Toro, Jaime *1; Weislogel, Amy 1; Hayward, Jessica 1; Parrish, Chantelle 1; Wooden, Joseph 2
(1) Geology & Geography, West Virginia University, Morgantown, WV.
(2) Stanford University, Stanford, CA.

Numerous thin volcanic ash layers are found interbedded with 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. We have dated zircons extracted from ashes taken from cores of 6 wells and from 1 outcrop in Pennsylvania and West Virginia. The ashes range in thickness from 0.5-14 cm and are recognized by their buff color, abundant biotite, and high U/Th ratios in spectral gamma ray logs. Sampling targeted the uppermost and lowermost ashes when possible. The age of each ash layer was determined by analyzing zircon at the USGS-Stanford SHRIMP-RG laboratory. For each sample, 12-18 spots on individual zircon crystals were analyzed. Recycled or reworked older zircon crystals are absent from the samples suggesting a primary origin as air-fall deposits. Each ash typically contains euhedral Middle Devonian volcanic zircons that are either prismatic or equant in habit and exhibit oscillatory zoning in cathodoluminescence images. Equant zircon crystals tend to contain Proterozoic inherited cores that record the age of the crust underlying the arc. Individual concordant 206Pb/238U ages from zircon crystals within a single ash may vary over a range of as much as 23 My, but typically there is a cluster of younger ages that we interpret as the best estimate of the age of eruption, which would be coeval with depositional age in the basin. Mean eruption ages of ahses near the base of the Marcellus Shale range from 402.0 ± 1.8 Ma to 388.0 ± 1.5 Ma (Emsian to Givetian) in 5 wells. This indicates that either the basal Marcellus Shale was deposited diachronously across the study area or that ashes were not preserved uniformly across the study area. This result should be tested by dating zircon from ashes within the upper Onondaga to determine if the younger basal Marcellus Shale deposits are underlain by Onondaga strata that preserve older ashes. If age-equivalent ashes are found in basal Marcellus and upper Onondaga deposits, this can place important constraints on lateral facies variations that could complicate lithostratigraphic correlations and formation contact picks based on well log data. Estimates of Marcellus sedimentation rates, from wells where the uppermost and lowermost ashes are 7-9 feet apart, are from 0.6-1.6 ft/my (19-49 cm/my). We will refine our dating by using high-resolution TIMS geochronology on selected zircons.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California