GEOCHEMICAL CONSTRAINTS ON THE DISTRIBUTION OF Climacograptus? rugosus WITHIN THE LATE SANDBIAN-AGED BASAL UTICA SHALE; IMPLICATIONS ON TACONIC FORELAND BASIN DEVELOPMENT AND GRAPTOLITE COMMUNITY TURNOVER

Abstract

Within the basal portion of the Utica Shale, there are significant shifts in the geochemical composition of the rock. These shifts in both bulk element and trace metal concentrations represent rapid changes in the sediment source composition or depositional environment (e.g., redox conditions), or both, most likely resulting from changes in local tectonism and eustasy. These geochemical changes correspond with the disappearance of Climacograptus? rugosus and the appearance of Corynoides americanus Zone graptolite species such as Diplacanthograptus caudatus and Corynoides americanus. This correlation between faunal transition and a sharp change in the sediment geochemistry, together with the change in basin conditions implied by those geochemical features, suggest that the disappearance of C? rugosus and appearance of C. americanus fauna in the Taconic Foreland Basin are controlled by the opening of a strongly restricted basin, perhaps with the bathymetry and hydrography of a silled basin. The timing of lessened restriction of this region appears to be nearly coincident with the punctuated evolution of C. americanus Zone fauna in the entirety of the Taconic Foreland Basin, suggesting some degree of regional to global changes on top of local tectonism.