The Marcellus Shale

Overview

The Middle Devonian Marcellus Shale (or Marcellus Formation) has been investigated to determine its role in the history of the Appalachian Orogenic Belt. Numerous geological papers were analyzed to examine the impact of regional orogenic events and how the environment was shaped for sedimentary deposition. Thickness and stratigraphic trends in the Marcellus Shale indicate that two main mechanisms influenced its deposition: a) Acadian thrust loading as Avalonia collided with Laurentia, and b) base-level fluctuations due to transgressive-regressive oceanic sequences. The collision of Avalonia with Laurentia formed the Acadian Mountains and continued thrust loading led to crustal bending and the development of a foreland basin. This foreland basin became the space of accumulation for the Marcellus Shale as the continued advancement of the Acadian orogenic front produced fold-thrust sheets where Acadian Mountain rocks were uplifted, eroded, then added to the basin. Transgressive-regressive marine cycles also contributed to the sediment deposition in the basin. Transgressive systems dominantly deposited carbonate and organic-rich layers whereas regressive systems deposited mostly clay layers in the basin. High erosion and sedimentation rates during orogenesis were key to the formation of the Marcellus Shale as they generated low oxygen availability in the water and thus, anoxic water conditions aiding in the burial of organic carbon layers. Variations in thickness of this sedimentary succession can be accredited to reactivated basement faults that further shaped the foreland basin as sediment was added. It can be stated that the Marcellus Shale formed both as a result of orogenic events and eustasy in the Middle Devonian. This research on the Marcellus Shale helps paint the picture of the history of the Appalachian Orogenic Belt and reinforces how complex some depositional systems can be.

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