USING STABLE CARBON ISOTOPES OF ORGANIC-MATTER, ELEMENTAL RATIOS, AND TRACE-METAL GEOCHEMISTRY TO INFER ORGANIC-MATTER SOURCES, ACCUMULATION RATES, AND REDOX CONDITIONS DURING THE DEPOSITION OF THE MIDDLE DEVONIAN GENESEO FORMATION OF NEW YORK

The Middle Devonian Geneseo Formation of New York is a mudstone-dominated succession that records the westward progradation of the Catskill delta during the third tectophase of the Acadian Orogeny. Recent facies characterization and detailed petrographic analysis has yielded differentiation of distinct lithotypes that suggest an overall shallowing with increasing energy conditions upsection. This preliminary study uses stable carbon isotopes of organic-matter, elemental ratios, and trace-metal geochemistry (i.e., Ni, V, Zn, Cr) to better understand sources of organic-matter, sedimentation rates, and redox conditions during deposition of the Geneseo Formation.

In central New York, the Geneseo Formation consists of dark gray, pyritic and banded black shales, homogenized gray mudstones, and gray silty mudstones/muddy siltstones with intercalated carbonate concretionary horizons. Variations in δ¹³C_org appear to reflect shoreline proximity, and thus, can be used to infer depositional environment. Carbon isotope values of organic-matter consist of a mixture of a terrestrial component with δ¹³C values of about -26.5‰, and a marine component of about -29.5‰. Heavier values are typically associated with elevated Ti/Al and Si/Al ratios, decreased Fe_T/Al, and lower trace-metal concentrations in more organic-lean, moderately to strongly bioturbated gray mudstones. Lighter isotopic values are typically associated with decreased Ti/Al and Si/Al ratios, increased Fe_T/Al, and trace-metal enrichment in organic-rich, sparsely to weakly bioturbated pyritic and banded black shales.

The Geneseo Formation exhibits several shallowing upwards cycles (parasequences) that can be identified in the carbon isotope record. Heavier values reflect shallower water conditions and higher accumulation rates, under dysoxic to oxic water-column conditions. This is confirmed with the decreased trace-metal concentrations, and elevated Ti/Al and Si/Al ratios, suggesting basinward migration of coarser-grained facies belts. Lighter isotopic values and enrichment of trace-metals suggest deeper water conditions and lower accumulation rates, under sub-oxic to dysoxic water-column conditions.