THE EARLY UPPER ORDOVICIAN TACONIAN FORELAND BASIN SEDIMENTARY SUCCESSION: INTEGRATED OUTCROP AND DRILL CORE PERSPECTIVES IN THE MOHAWK VALLEY REGION, EASTERN NEW YORK

The Late Ordovician Taconian convergent/collisional event initiated diachronous, cratonward flexural subsidence along the margin of Laurentia. In eastern New York, (Mohawk Valley region), this drowning was facilitated by westward, stepwise, subsidence along high angle faults as well as by flexural processes resulting in diachronous collapse of a neritic shelf (Trenton Group carbonates) with concurrent expansion of dysoxic slope settings (ribbon limestone facies) and more severe exaerobic basin conditions represented by the black, organic-rich Utica Shale. A key feature in fault-bounded basin sections is the development of one or more sharp erosional/corrosional, drowning unconformities that seperate underlying neritic or dysoxic facies from the overlying Utica Shale; ongoing work is directed to establishing the timing of valley segment subsidence as reflected in the position of various geochronological markers relative to such contacts.

The present authors, assessing the regional diachroneity of contacts and facies in this succession, proceed through the correlation of regionally widespread sedimentary and bioevent features in sections. This approach is currently being used to compare outcrop sections with new drill core information from boreholes at varying distances from sections; planned work will involve direct Gamma ray logging of cores followed by matching of core-based (ground-truth) log standards with existing borehole logs. Recent examination of the sub-Utica unconformity in cores from the Schenectady-Saratoga area between the Hoffmans and Saratoga faults, shows a basal Utica (Flat Creek Shale) succession that may be older than Utica deposits visible further west in the Mohawk Valley. More importantly, long cores this far east, extending from the basement into post-Utica turbiditic molasse, provide a base-line for comparison of the autochthonous succession with coeval parautochthonous successions in nearby Taconic thrusts.