GENESIS OF BLACK SHALE-ROOFED DISCONTINUITIES ASSOCIATED WITH THE ORDOVICIAN UTICA SHALE IN NEW YORK STATE; A REVIEW OF EXISTING AND NEW CONCEPTS

Three dark gray-to-black shale-roofed unconformities, respectively designated in upward-succession; top-Glens Falls, Thruway, and Honey Hill discontinuities occur at or near the base of the Middle/Late Ordovician Utica Shale in eastern New York. The Thruway and Honey Hill contacts are characterized both by westward regional onlap of Utica strata and by evidence of corrosion of underlying beds and associated lag debris. Dissolution is most pronounced where black, near-anoxic facies overlies contacts.

Dissolution occurred where cold, lower dysoxic to anoxic bottom water was present above a sediment-starved, east-facing carbonate slope on the cratonward side of the growing Taconian foreland basin. Carbonate undersaturation and periodic oxidation of bacteriogenic monosulfide to sulfuric acid acted to destroy carbonate allochems, leaving phosphate-rich insoluble placers on the sea bed. Regional westward-younging of Utica strata above the Thruway and Honey Hill surfaces reflects tectonic expansion of the foreland basin and cratonward migration of the belt of submarine erosion. Transgressive westward migration of pycnoclines associated with basin stratification may explain the widespread nature of these surfaces; long-term shoaling of pycnocline-generated internal waves onto a regionally sloped seafloor could partly explain the diachronous onlap pattern.

New evidence from the Thruway Discontinuity shows that intrastratal dissolution beneath a thin, insoluble veneer led to destruction of unexposed carbonate. Hence, both surface and subsurface dissolution processes account for the distinctive corrosional textures observed both at and below this horizon.