Inorganic geochemistry of the Trenton Limestone-Utica shale contact based on XRF data
Saboda, Steve R.
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The contact of the Upper Ordovician Utica Shale and underlying Trenton Limestone is normally recognized in well logs as a sharp increase in gamma-ray (GR) response, apparently reflecting the increased abundance of organic matter from the carbonate to overlying shale. Though conventional wisdom holds that organic-rich black shales can be identified by characteristic GR log signature, this may not be the case for the Utica Shale. Chemostratigraphic analysis of the Utica Shale core 75NY-2 using handheld XRF technology in tandem with a suite TOC data indicates that the Trenton-Utica contact based on GR response actually falls within the Utica Shale. Gamma-ray response principally reflects the abundance of K, Th, and especially U. Indeed, enrichment of U above crustal levels in the presence of abundant organic matter and related bacterial sulfate reduction is typical of those environments in which black shale accumulates. However, U values of the lower carbonaceous interval of the Utica Shale remain close to or even depleted relative to crustal values, essentially unchanged from those concentrations of the underlying Trenton Limestone. Of particular interest is the roughly 6m thick interval of calcareous dark black shale containing > 10% TOC at the bottom of the Utica Shale. The apparent dichotomy of elevated TOC and minimal U concentration is obvious from an artificial GR log generated from our chemostratigraphic data that displays a profile similar to GR signatures documented from Utica wells. These results suggest that the sharp increase in GR response normally recognized as the Trenton-Utica contact actually delimits the top of the organic-rich interval within the bottom of the Utica. The level at which the GR increases is observed to be the boundary of organic-rich shale and overlying organic-lean (TOC ∼ 1%) grayish black shale. The contact of the organic-rich and organic-lean shale correlates with an increase of U somewhat in excess of crustal levels. Although authigenic U concentrations can be diluted by increased clastic flux, measured abundances of detrital proxies, including Al, Si, Ti, and Zr, are inconsistent with such an explanation. The U-depleted nature of the most organic-rich deposits of the Utica Shale may reflect the impress of Middle Ordovician global anoxia and consequent of drawdown of the global aqueous U inventory.