Insights into the Appalachian Basin Middle Devonian Depositional System from U-Pb Zircon Geochronology of Volcanic Ashes in the Marcellus Shale and Onondaga Limestone.

摘要

Numerous thin volcanic ash layers are found within the upper Onondaga Limestone 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. Ash layers can form key stratigraphic markers, allow for geochemical analysis of parent magma, and most importantly, provide radiometric dates used in determining depositional rates and chronostratigraphic relations.;Zircons were extracted from ashes found in 8 well cores and 2 outcrops in Pennsylvania and West Virginia. Ashes range in thickness from 0.5-14 cm and are recognized by their buff color abundant white mica in core, and high U/Th ratios in spectral gamma ray logs. The U-Pb age of each ash layer was determined by analyzing 12-15 spots on individual zircon crystals at the USGS-Stanford SHRIMP-RG laboratory. The youngest concordant 206Pb/ 238U age populations were used to calculate average eruption ages also assumed to be coeval with the depositional age of each ash bed.;Conodont biostratigraphy for the Marcellus in the outcrop belt indicates a mid-upper Eifelian to lower Givetian age. However, our ash ages near the base of the Marcellus range from Late Emsian (394 +/-5 Ma) to Late Eifelian (389 +/-3 Ma), indicating that the basal Marcellus Shale was likely deposited diachronously. A trend is also apparent where the basal Marcellus is oldest in the western study area and becomes younger moving east, giving an age distribution which is inconsistent with the simplistic model of prograding Appalachian basin fill where the oldest sediments are expected closer to the eastern margin. We suggest complexity in basin bathymetry as a possible cause for earlier deposition of the Marcellus Shale within the western, more distal, area of the basin prior to major or widespread sea level rise and/or loadinginduced subsidence.;Rare Earth Element (REE), trace element and XRD geochemical data as well as petrography from the ash beds provide evidence that zircons were sourced from continental arc magmatism. These data have similarities in values, trends, and mineralogy to a pluton source composed of plagiogranite and/or granodiorite that may have been peraluminous.