ALONG-STRIKE DIFFERENTIAL SUBSIDENCE AND THE CREATION OF THE MOHAWK-VALLEY ARCH

The cause of the thinning and facies changes in the carbonate shelf strata of the Mohawk Valley has been an underappreciated aspect of the Cambro-Ordovician carbonate bank. These changes have caused many nomenclatural issues in the Champlain and St. Lawrence valleys and with stratigraphic correlations with areas to the south and east.

The Mohawk Valley was clearly a zone of nearshore deposition and frequent hiatuses throughout late Cambrian and early Ordovician time, with the overlying middle Ordovician strata being totally absent. This pattern suggests deposition atop an arch oriented roughly perpendicular to the Laurentian margin. It is proposed that the reduced subsidence of the Mohawk Valley was due to subsidence to the north and south controlled by localized deposition of sand. The only period with no apparent differential subsidence occurred during the Black River (earliest Cincinnatian) when there was no apparent sand delivery to the Champlain Valley shelf.

The axis of the adjoining basin to the north is the axis of the thick Potsdam sand section. This Potsdam sand section is largely fluvial with increasing marine character toward the southeast in the Champlain Valley. It is proposed that this thick Potsdam sand section represents a river system that persisted throughout the Cambrian and Early and Middle Ordovician and served as the source of the sand in the Champlain Valley carbonate shelf system. The continuous accumulation of sand along this river system served to create an axis of preferential subsidence that resulted in the corresponding upward flexure of the adjoining Mohawk Valley region. A similar, though less voluminous, coeval sand depositional axis existed in Pennsylvania, creating the subsidence symmetry that resulted in the linear arch within the Mohawk Valley system.