Tectonics of New York during the laterOrdovician


Figure modified from Bradley and Kidd, 1991.

During the Middle to Late Ordovician, the eastern edge of the North American plate was being subducted below the European plate along the Taconic Front. As a result, the edge of the North American plate (the Study Area) was warped downward. In addition to the thrust faulting of the volcanic arc over the North American plate as the plate was subducted, the flexural stress on the plate caused it to break along normal faults, roughly parallel to the contact between the two plates.


Figure modified from Bradley and Kidd, 1991.

As it moved downward into the Earth's hot interior, a fraction of the material in the North American plate melted. Most remained within the volcanic arc and solidified to form igneous rocks, but some reached the surface to erupt as volcanoes. The Trade Winds then blew the ash westward, where it settled out of the air and formed ash layers. In time, this material was altered into clay - bentonites. Being weathered ash layers, they were deposited in a matter of days, and form one of the most precise markers of time available to geologists. The bentonites in the Mohawk River Valley form the basis for the time-corrected lithostratigraphic cross-sections presented for the taxa.

As the Taconic Mountains rose in the East, material was eroded. The material eroded from the west-facing slope made its way into the trench where the two plates met. Some was moved even further westward, into the Study Area, where it formed the Utica Shale and the shale interbeds of the Dolgeville Formation.