USING BIOTURBATION TO EVALUATE PALEOREDOX CONDITIONS AND IDENTIFY CYCLICITY IN THE MIDDLE DEVONIAN GENESEO FORMATION OF NEW YORK

Middle to Upper Devonian black shales of the Catskill deltaic complex have been intensively studied to better understand the relative influence of sea-level fluctuations and paleoredox conditions on cyclic sedimentation, and formation of laterally extensive black shales in the eastern U.S.

The Middle Devonian Geneseo Formation of New York is composed of black and dark gray shales that are interbedded with concretionary carbonates and siltstone beds in the upper portion. Standard depositional models for promoting organic-matter preservation in the Geneseo Formation call on a major transgression (the Taghanic Onlap) to induce sediment starvation and a stratified anoxic-euxinic water column.

Detailed core description and optical petrography was used to derive depositional parameters and ichnofacies. The lower Geneseo Formation consists of organic-rich banded black shales that macroscopically appears to lack bioturbation. Cryptic bioturbation, however, is clearly visible on polished slabs and in thin sections. Upsection, the lower Geneseo Formation exhibits several dark gray to black shale coarsening upwards cycles. Further, dark gray mudstones are observed in the uppermost portion of the lower Geneseo Formation, that contain wave and current ripples and display an increase in macroscopically visible bioturbation, biodeformational structures, and escape traces. A silt-rich muddy concretionary carbonate unit (Fir Tree Member) is found in the middle Geneseo Formation. It contains three coarsening upwards cycles with increasing bioturbation intensity towards the top of each consecutive cycle. The upper Geneseo Formation is comprised of dark gray to gray silty mudstones that show abundant wave and current ripples. Upsection the bioturbation intensity and silt content increases.

Globally, the time interval represented by the Geneseo Formation indicates continuous eustatic sea-level rise, but the sedimentary record in New York suggests two separate aggradational-progradational cycles, with a general shallowing upsection as the shoreline migrated basinward. As sedimentation filled previously generated accommodation, rapid shallowing led to increasing energy levels and oxygenation, and caused an upwards decrease in TOC due to the combined effects of dilution and increasing consumption.