SEDIMENTARY GEOCHEMISTRY AND GRAPTOLITE FAUNAS IN THE UTICA SHALE OF NEW YORK STATE RECORD THE INTERACTION OF LATE ORDOVICIAN EUSTACY AND FAULT-DRIVEN BASIN SUBSIDENCE

The Utica Shale was deposited in a subsiding foreland basin in response to crustal loading associated with Late Ordovician emplacement of the Taconic Allochthons. We gathered new geochemical data via XRF, determined TOC, and logged graptolite species occurrence and lithology from upper Trenton Group and lower Utica Shale in two cores (74NY10, 75NY11), and integrated those with data from core 75NY2, in an effort to distinguish regional relative sea-level and local, structural effects on accommodation. The Utica basin developed across Saratoga to Little Falls region without measurable diachroneity (relative to K-bentonites), despite substantial local differences in unit thickness and facies. Nevertheless, regionally extensive changes in relative water depth and oceanographic conditions are also recognizable within the basin.

The lower most 3-7 m of the Utica Shale (Unit 1) is a laminated, highly-calcareous, silty shale. Redox and productivity-related proxies indicate deposition under dysaerobic to anoxic conditions. However, TOC is only locally elevated in this interval and the expected U enrichment is not present. Compositional similarities to the subjacent Trenton carbonates suggest these sediments were derived from cratonic sources. Several species within the low diversity graptolite fauna are limited to this thin unit. These observations suggest a relatively restricted basin.

Unit 2 is siltier and less calcareous sediment deposited in more oxic conditions. Zr, Cr, Th, and U concentrations increase sharply across the base of the unit. Detrital components, especially Cr abundance, suggest derivation of this sediment from the Taconic thrust belt to the NE. The more cosmopolitan C. americanus Zone graptolite fauna appears in the lower part of this unit. At the more craton-ward 74NY10 site, Unit 2 displays 10 cycles in clastic component concentrations and grainsize that may represent 100Ka-duration Milankovitch cycles.

Unit 3, high in the C. americanus Zone interval, records abrupt deepening under environmental conditions similar those of Unit 1, with an increase in graptolite abundance followed by a faunal turnover late in the transgressive interval. At the more proximal 75NY2 site, the Unit 2 detrital component persists into Unit 3, whereas at 74NY10, sediment was again derived from a cratonic source.