Fault Controls on Devonian Black Shales In the Northern Appalachian Foreland Basin

Five thick black shales were deposited in western New York and northern Pennsylvania during the Middle and Late Devonian. Traditional models show the regional maximum black shale thickness successively steps farther west with the development of the gentle, structurally inactive clinoform. However, in the northern region of the Appalachian Foreland Basin, many of the areas of thickest black shale deposition coincide with areas of active, faulting that are semi-independent of the basin axis. From our outcrop studies in New York State and well-log analyses in New York and Pennsylvania we have observed thickening of several of the black shales coincident with active faults that extend up from basement structures, primarily the Clarendon-Linden Fault System and Iapetan-opening/Rome Trough structures. For example the regionally minor black shales the of Pipe Creek and the Hume formations are typically 1 meter or less in thickness and appear inconsequential as a petroleum target. However, within the extent of the Clarendon-Linden Fault System, the Hume Formation averages 36m (120ft) thick, and the Pipe Creek Formation reaches 5.5m (18ft). Additionally, thick accumulations of the Geneseo (~45m/150ft) and Middlesex (~61m/200ft) formations coincide with basement structures of reactivated Clarendon-Linden and the Iapetan-opening/Rome Trough, respectively.

We suggest that the combined stress of the Neo-Acadian collision and accompanying sediment loading reactivated the older basement structures, generating variable accommodation within the vicinity of the fault zones. In addition to the increased localized accumulation of organic-rich shale, later fault reactivation would increase local fracturing, increasing the potential of these black shales as reservoirs and source rocks.