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Tectonic control on organic-rich mudstone deposition and associated unconformity development in the Utica, Marcellus and Geneseo Shales, Northern Appalachian Basin, USA


The most organic-rich intervals in the Ordovician Utica Shale, Middle Devonian Marcellus Shale and Upper Devonian Geneseo Shale of the northern Appalachian foreland basin overlie and onlap unconformities developed on the underlying units. It is here interpreted that these unconformities developed due to uplift, exposure and erosion on peripheral bulges that formed to the present-day west due to thrust loading and mountain building farther to the east. The organic-rich shales were deposited in relatively shallow water on the flanks and sometimes across the crests of the tectonic highs. This shallow marine setting on the peripheral bulge of a foreland basin where dilution from the clastic source is low appears to be conducive to organic-rich shale development.

The Utica Shale in New York has two main organic-rich units - the Flat Creek, which is equivalent to the Point Pleasant in Ohio, and the Lower Indian Castle, which is equivalent to what is called Utica in Ohio. The Flat Creek overlies an unconformity developed on the underlying Beekmantown and Trenton Formations and the richest part is commonly immediately above the unconformity. The most organic-rich part of the Lower Indian Castle Formation overlies an unconformity that formed on top of the Trenton to the west while there is a conformable contact to the east.

The contact between the Marcellus and the underlying Onondaga is also disconformable across the western part of the basin and conformable to the east. The Marcellus onlaps the unconformity to the west and the most organic-rich intervals of the Marcellus overlie the unconformity. Similarly, the contact between the Tully Limestone and the Geneseo Shale is conformable to the east and disconformable to the west where the most organic-rich interval commonly immediately overlies the unconformity. There is deep erosion into the underlying shale below this unconformity.

In all three cases, uplift to the present-day west driven by thrust loading and high subsidence to the east led to the development of erosional unconformities. The uplift occurred along pre-existing basement structures. As the Taconic thrust sheet moved westward during Utica time, the area of uplift also shifted westward as a series of pre-existing faults were reactivated. During Marcellus and Geneseo times, Acadian thrust-loading led to uplift along the Findlay-Algonquin and Cincinnati Arches which were basement highs throughout much of the Paleozoic.

The presence of shallow-marine facies below the unconformities, karst, lag deposits with shallow marine fossils, the documented time missing, and the field relations all suggest that these unconformities were subaerially exposed rather than submarine erosion surfaces. This model also applies to the overlying Rhinestreet, Dunkirk, Ohio and Chattanooga Shales and to other organic-rich shales deposited in similar settings across North America and around the world.