The Influence of Basin Dynamics from Depositional Patterns of the Middle Devonian Marcellus Shale, Western New York and Northwestern Pennsylvania

Gary Lash
Geosciences, SUNY Fredonia, Fredonia, NY

The subsurface stratigraphy of the Middle Devonian Marcellus shale of the western New York - northwestern Pennsylvania region of the Appalachian Basin, an increasingly attractive exploration target, is partly reflective of Acadian thrust tectonics. The Oatka Creek shale, the upper unit of the Marcellus shale in this area, thickens both to the east and west of what appears to have been a north-northeast trending axis. However, the change in thickness is recorded only in the silty, less-organic upper part of the Oatka Creek shale. The basal organic-rich interval of the Oatka Creek shale reveals a general east-to-west decrease in thickness across the region. Further, the lower unit of the Marcellus shale, the Union Springs black shale, pinches out against the eastern flank of the axis. These relations suggest that the north-northeast trending axis was a topographic high during Marcellus time, likely induced by Acadian thrust loading. Indeed, this region of the Appalachian Basin appears to have experienced intermittent tectonic active throughout the Silurian and much of Early Devonian time. The basinward extent of the Union Springs shale was probably controlled by the topographic barrier. An inferred eustatic rise in sea level at the onset of Oatka Creek shale deposition, in tandem with thrust-load induced regional subsidence of the basin (Third Tectophase of the Acadian Orogeny), resulted in westward thinning of the basal black shale interval. Soon after this, the topographic barrier, making its existence known once again, influenced depositional patterns of the upper, silty interval of the Oatka Creek shale by favoring accumulation of a greater thickness of these deposits in deeper areas of the basin adjacent to the topographic high.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas