Influence of Basin Dynamics on Upper Devonian Black Shale Deposition, Western New York State and Northwest Pennsylvania

Lash, Gary G.¹ (1) SUNY Fredonia, Fredonia, NY

Black shale is ubiquitous to the Upper Devonian succession of the Appalachian Basin of western New York and northwest Pennsylvania. The spatial distribution of the organic-rich deposits appears to have been partly controlled by Acadian basin dynamics. Middle Devonian tectonic quiescence reflected by the Tully limestone was terminated by development of an Acadian forebulge. The resulting Taghanic unconformity achieved its maximum hiatal extent in Chautauqua and Cattaraugus counties, New York, extending south into Pennsylvania. Uplift may have been controlled by basin structures, including the Clarendon-Linden Fault (CLF), which lies close to the eastern zero-thickness line of the Tully limestone. Subsidence of the forebulge is marked by onlap of the Upper Devonian Geneseo black shale onto the eroded Tully limestone and underlying Middle Devonian shale. However, the fact that the Geneseo is not present in extreme western New York and northwest Pennsylvania suggests that the forebulge had migrated to the west. The forebulge was no longer a positive area in New York when the uniformly south- and west-thinning Middlesex black shale accumulated. However, the CLF appears to have accommodated down - to - east offset during deposition of the Rhinestreet black shale resulting in abrupt eastward thinning of the radioactive facies proximal to the CLF. Subsidence to the east of the CLF, perhaps a consequence of further migration of the Acadian thrust complex, ponded the more clastic facies of the Rhinestreet in this area of the basin; the organic-rich deposits accumulated in the more elevated region west of the fault. The Upper Devonian Dunkirk black shale attains a maximum thickness in northwest Pennsylvania reflecting subsidence of the forebulge perhaps induced by advance of the Acadian thrust terrane.