COLLABORATIVE RESEARCH: Integrated Biostratigraphy and K-bentonite Chronostratigraphy in the Middle Ordovician of the Northern Appalachian Basin
Charles Mitchell Principal Investigator
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The late Middle and early Upper Ordovician carbonate and siliciclastic rocks of the Trenton Group in the northern Appalachian foreland basin enclose a large number of altered volcanic ash beds (K-bentonites) deposited from Plinian eruptions on an off-shore volcanic arc. These strata also form part of the standard reference section of the North American Ordovician time scale as well as the reference standard for the conodont and graptolite biozonations commonly used for temporal correlation in this interval. The goal of the research is to employ geochemical fingerprinting of unaltered phenocrysts within the K-bentonites together with new and detailed studies of the ranges of graptolite and conodont species to integrate these separate stratigraphies into a unified chronostratigraphic network. We propose to accomplish this by 1) establishing precise ties between the graptolite zones and the North Atlantic and midcontinent conodont zones based on detailed collecting of range data from numerous measured sections that each span the facies transition from Trenton Group carbonates into the Utica Shale and 2) establishing precise ties between the laterally equivalent siliciclastic and carbonate facies based on the identification of synchronous time planes through geochemical fingerprinting of the K-bentonites. The techniques to be employed for K-bentonite correlation include analysis of 1) major and trace element composition of melt inclusions within quartz phenocrysts. Additionally, we propose to use high precision U-Pb zircon geochronometry to calibrate our chronostratigraphy. The resulting chronostratigraphy will provide a temporal framework, calibrated in years, that will provide a direct test of the accuracy of the existing graptolite and conodont biostratigraphies as well as sequence stratigraphic and coenocorrelation studies of these rocks. This chronostratigraphy will constitute the temporal framework necessary for studies of the rate of evolution among graptolites and other organisms that occur in the studied sections. Simultaneously, the K-bentonite geochemical and geochronological data will provide unique insights about the tectonic provenance of these altered volcanic ash beds and the nature and duration of Taconic orogenic events.