FAUNAL AND GEOCHEMICAL EVIDENCE FOR SIGNIFICANT RESTRICTION OF THE BASAL UTICA SHALE

The Utica Shale is a package of strata deposited during an Ordovician accretionary orogenic event known as the Taconic orogeny. Emplacement of the Taconic allochthons along the Laurentian margin resulted in the formation of the Taconic foreland basin. The initiation of this event is marked by the transition from passive margin to active margin carbonate, which in New York deepened upward to into the calcareous Utica Shale.

The basal portion of the Utica Shale is a thin geochemically unique unit, 3-6 m in thickness. Identification of this unit stemmed from the discovery of a uranium-depleted, but very organic rich unit in drill core 75NY-2 from the eastern flank of the Taconic foreland basin. Analyses of the basal Utica Shale of core 74NY-10 from nearly 36 kilometers to the NW show that this uranium depleted unit is not isolated to the deep portion of the basin. Other elements correlated to sediment influx (Cr, Zn, Fe, Th) are also depleted in the basal Utica Shale of both cores indicators of deposition in low oxygen conditions, including substantial enrichments in molybdenum. The top of this thin unit corresponds to a sharp increase in clastic sediment influx and increases in uranium concentrations to that of more typical dysoxic to anoxic depositional conditions.

Geochemical signatures observed in the basal Utica Shale interval suggests strong isolation and severely reduced influx of crustally-derived sediments. Lessened restriction beyond this interval corresponds to the disappearance of a limited graptolite species (Climacograptus? rugosus) and the appearance of Corynoides americanus Biozone fauna. This C? rugosus-containing interval is found at the base of the Utica Shale of various drill cores and surface exposures from core 75NY-2 westward to Canajoharie Creek. Although geochemical data of this interval is limited to the two aforementioned cores and a few samples at Canajoharie Creek, this basal interval is distinctly sensitive to erosion among field exposures within this interval across the basin, suggesting the presence of unique geochemistry or sedimentology that extended across the basin. Local changes in tectonism may have opened the Taconic foreland basin of the Mohawk Valley to the nearby seas, introducing a new graptolite community to the region and causing the local extinction of C? rugosus.