MARCELLUS BLACK SHALE FACIES: CONSTRAINTS AND PERSPECTIVES ON WATER DEPTH

Long have debates come and gone about the depth of Devonian black shale deposition. In New York, black shale facies of the Marcellus subgroup have generally been interpreted to be deposited in relatively deeper water. Some recent hypothesis have viewed NY Marcellus black shales as shallow, even the shallowest facies, formed during eustatic lowstands. Many lines of evidence indicate difficulties with this model, however.

Marcellus black shales were deposited in an actively subsiding foreland basin system, associated with renewed tectonic loading in the Acadian orogenic belt (onset of “Neoacadian” orogeny). And at this time, global eustatic sea levels were at a significant high point. In central Laurentia, both the Iowa basin and even the transcontinental arch were submerged for the first time in the Devonian, as were other basins globally. During Marcellus time, carbonate ramps formed on the cratonward margin of the foreland basin system (central OH and adjacent Ont.), 200 to 400 km west of the locus of black shale in central to western NY. In eastern NY, ca. 580 meters of compacted Marcellus marine sediments accumulated before the foreland basin was overfilled, and upper Marcellus terrestrial facies were deposited. Within this succession, four major sandstone tongues prograded progressively basinward (Hudson Valley to central Finger Lakes). In each case the sand bodies thin to a feathers edge, rather than thickening as expected if encountering a rising sea floor at the toe of the prodelta.

Thin, condensed black shales in the foreland basin center (central to western NY) can be expected considering modern understanding of the difficulties of transporting largely flocculated marine muds far across a basin floor. Furthermore, if central to western NY was shallow during Marcellus time, especially over a sharply pronounced forebulge, relatively shallow marine carbonates should have formed – as happened coevally on the shallow margin of the basin in Ohio and Ontario, and as happened during the initial shallow and sediment-starved stages of 3^rd +/- 4^th order transgressions (e.g., Hurley-Cherry Valley and post-Marcellus Stafford limestones).

These and other lines of evidence make it difficult to argue for shallow water deposition of Marcellus black shales in the central body of the Appalachian foreland basin system.