The Middle Devonian Marcellus Shale - a Record of Eustacy and Basin Dynamics
The stratigraphy of the Middle Devonian Marcellus Shale of the Appalachian Basin reflects the interplay of Acadian-related flexural loading of the Laurentian craton and two rapid sea level rises. Accumulation of the Union Springs Shale, the lower unit of the Marcellus Shale, over the variably eroded Onondaga Limestone was coincident with a Middle Eifelian rise in sea level (T-R Cycle 1d) and the onset of Tectophase II of the Acadian Orogeny. The result of both events was a thin, highly radioactive, pyritiferous condensed interval at the base of the Union Springs. Further, isopach patterns reveal that the Union Springs was completely eroded along a NE-SW-trending region of western New York and western Pennsylvania, a likely consequence of development of a tectonic welt or forebulge induced by thrust loading. The overlying Cherry Valley (Purcell) Limestone likely reflects a brief stabilization of sea level. The Cherry Valley displays an isopach pattern similar to that of the underlying Union Springs Shale, suggesting that the tectonic welt was reactivated soon after accumulation of the carbonate sediment. Deposition of the organic-rich Oatka Creek Shale, the upper unit of the Marcellus Shale, reflects a Late Eifelian eustatic rise in sea level (T-R Cycle 1e). This event, in tandem with continued load-induced subsidence, resulted in accumulation of a carbonaceous basal interval that passes upward into less organic-rich shale. The Oatka Creek Shale, like the underlying units of the Marcellus Shale, was eroded along a NW-SE-oriented axis in western New York. However, the locus of erosion had shifted somewhat to the east, perhaps recording a brief period of tectonic relaxation at the end of Marcellus time.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009