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Inorganic Geochemistry of the Middle-Upper Ordovician Utica Shale Defining Clastic Trends Including a Change of Source Provenance Based on XRF Data

Abstract

The Middle-Upper Ordovician Utica Shale was deposited during a time when atmospheric conditions did not favor the establishment of land plants resulting in clastic input different from what conventional wisdom holds for organic-rich shales. A chemostratigraphic analysis of a Utica Shale core from eastern New York using handheld XRF technology reveals two organic-rich intervals depleted in detrital elements including Al, Si, Ti, and K reflecting the lack of clastic input including the abundance of clay. Perhaps the most noticeable feature of both organic-rich intervals is the enrichment of calcium, which is thought to be detrital as opposed to authigenic or biogenic calcium carbonate. Not only are the two organic-rich intervals enriched in Ca, however, the Ca profile displays a steady increase from the lower organic-rich interval to the upper 33m thick interval. Calcium's steady increase, including the 33m calcareous interval, may be due to a change in the tectonic stress regime resulting in an increase in the amount of carbonate sediment shed from a peripheral forebulge into the basin. Above both organic-rich intervals a sharp increase of detrital elements Al, Si, Ti, and K reflect pulses of sediment delivered to the basin indicating a change of environment and dilution of organic matter. Al, which is a robust proxy for clay displays cyclicity throughout much of the core influenced by sea level fluctuations and may reveal 3rd or 4th order cycles. Superimposed on to the cyclic deposition of the Utica is a change of source provenance indicated by a sharp contact of zirconium, nickel, and vanadium. The lack of a land plant root system during the Ordovician may have resulted in lower sediment input including clay and resulting low algal productivity.