ORDOVICIAN CANYONS, SHALES, AND THE DEFORMATION OF THE APPALACHIAN CAMBRO-ORDOVICIAN CARBONATE SHELF MARGIN

The outer portions of the Cambro-Ordovician carbonate shelf along the Laurentian margin was cut by a series of submarine canyon systems, some extending downward nearly through the entire shelf sequence. Some of the trunk canyons existed throughout the Early and Middle Ordovician, but incision during the Knox Unfonformity eustatic lowstand (middle Middle Ordovician) greatly enlarged the extent of the canyon systems and dramatically altered their final geometry. These canyons were then infilled with turbiditic clastics marking the onset of Taconian-Caledonian (a.k.a. Salinian) orogenesis.

During the subsequent Paleozoic deformational episodes, the canyon fill acted as incompetent zones separating relatively rigid carbonate bank sections. Longitudinal (roughly parallel to the shelf margin) canyon systems localized thrust ramps and transverse (perpendicular to the shelf margin) canyons created along-strike structural discontinuities. Deformation of the shales, especially in the longitudinal canyon fills, is often reminiscent of Thomas' mushwads.

A first-order restoration of the canyon systems from the Champlain Valley southward to Pennsylvania indicates that many of the abrupt along-strike and cross-strike structural boundaries can be attributed to the presence of the canyons. Transverse trunk canyons subdivide the carbonates into a string of northwest-trending strips, each with its own structural characteristics. Longitudinal canyons isolated seaward portions of the carbonate bank and pinched connections between inner shelf portions and the continuous shelf of the continental interior. The thrust systems of the Champlain Valley and the Pennsylvania-New Jersey Great Valley sequences show evidence of the isolation of carbonate platform blocks, whereas the Pennsylvania Valley-and-Ridge carbonates show evidence of only partial isolation of carbonate platform segments. We believe that recognition of these canyons is necessary for a true understanding of the structural history of the outer portions of the Appalachians.