Characterization of the Union Springs Formation, Finger Lakes Region, NY

Karaca, Ceren; Jordan, Teresa E.

The Devonian Marcellus "Shale" of the Appalachian Basin is currently being exploited widely for natural gas production. Conflicting interpretations of its depositional environment have been advanced, and no consensus has been reached yet. This study uses a high resolution sedimentological approach to show the heterogeneity of the Union Springs Formation (within the Marcellus Shale) and attempts to determine the local depositional conditions.

Details are documented in 32 samples collected from fresh, unweathered surfaces of an active rock quarry (Seneca Stone Co.) in Seneca Falls, NY [42.85573°N, 76.78544°W] in the northern extent of the outcrop belt. Utilizing a combination of outcrop observations, hand sample descriptions of textures and bioturbation, optical and scanning electron petrography and microfossil examination, TOC data, and major and trace element variations, three macroscopic lithofacies and thirteen microfacies have been identified based on their allochthonous, autochthonous and authigenic components.

In the study area, macroscopically, the Union Springs Formation is merely 3.6 m thick, with a lower bioclastic limestone, a middle organic-rich laminated mudstone, and an upper concretionary limestone. Microscopically, the main agent diluting the organic matter appears to be carbonates (styliolinids, brachiopods, calcisilt grains). During the deposition of the Union Springs Formation, primary biological production was high, as revealed by the prevalence of marine algal cysts in all of the organic-rich microfacies.

Whereas the small thickness of the Union Springs Formation and the study site's position towards the western end of the clastic Catskill wedge might lead to the expectation that here the Union Springs Formation would represent a distal basin margin, instead the microfacies reveal that even this highly condensed section experienced oxygen-poor sub-wavebase environmental conditions. The microfacies distribution defines a small-scale (<50 cm) stratigraphic stacking pattern, which reveals that this location experienced initial deepening of marine water depth and then shallowing. During part of the time of accumulation of the middle black shale lithofacies, the bottom waters were anoxic-sulphidic. Nearly all deposition occurred below wavebase. The local pattern of Transgressive Systems Tract overlain by Highstand Systems Tract fits well within existing sequence stratigraphic models of the large-scale Devonian Appalachian Basin.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013