Insights into the Appalachian Basin Middle Devonian Depositional System from U-Pb Zircon Geochronology of Volcanic Ashes in the Marcellus Shale and Onondaga Limestone
Parrish, Chantelle; Toro, Jaime; Weislogel, Amy; Hayward, Jessica; Wooden, Joe
Numerous thin volcanic ash layers are found within the upper Onondaga Limestone and lower Marcellus Shale of the Appalachian basin. These ashes were sourced from continental arc magmatism along the Acadian orogen during the Middle Devonian. They 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 white mica, 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. Older detrital 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 life span of the Acadian magmatic system. Typically, there is a cluster of younger ages that we interpret as the best estimate of time of eruption and deposition in the basin. Conodont biostratigraphy for the Marcellus Shale in the outcrop belt indicates an Eifelian age (398-392 Ma, 2009 Geologic Time Scale). However, our ash ages near the base of the Marcellus range from 381±2 Ma to 402±2 Ma (middle Emsian to middle Frasnian); this indicates that the basal Marcellus Shale was deposited diachronously in the basin.
Wells on the western-most side of the study area, in northern West Virginia, yield consistently older ages than the rest of the wells. This age distribution is inconsistent with the simplistic model of prograding Appalachian basin fill where the oldest sediments are expected to be found closer to the eastern margin. The first Marcellus black shale was deposited in the distal part of the basin at the same time that Onondaga limestone was being laid down closer to the Acadian orogen. An estimation of decompacted Marcellus sedimentation rates from the Winner well, where ashes were 9 feet apart, is 3.8 ft/my. This value is two orders of magnitude slower than present-day black shale deposition in the Black Sea.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013