Geodynamical interpretation of a major unconformity in the Taconic Foredeep: Slide scar or onlap unconformity?

The Middle Ordovician ribbon limestones of the Dolgeville Formation (Trenton Group) and the overlying black shales of the Utica Group were deposited as the Laurentian continental plate impinged upon a Taconic trench. A prominent unconformity separates the two units in the central Mohawk Valley region (the "Thruway unconformity"), and this unconformity locally bevels the crests of soft-sediment folds. The Thruway unconformity has been described both as a locally developed feature and as a major boundary underlying a transgressive systems tract. Paleoflow measurements in the central Mohawk Valley region demonstrate that in the Dolgeville Formation the paleoflow was not directed easterly; rather, it was directed primarily southwesterly, parallel to syndepositional faults. Paleoflow directions in the overlying Utica Group remain the same as those in the Dolgeville Formation in locations removed from the syndepositional faults, but change direction at several sites close to faults. These prominent changes in paleoflow direction can be ascribed to (1) fault block rotation that redirected the regional paleoflow, and (2) fault scarps that redirected the regional paleoflow. Slump folds occur immediately below the unconformity at most sites in the central Mohawk Valley region. The different slump trajectories across the region and the different relative timing of the slumps suggest that they represent several discrete slumps. Regional chronostratigraphy also demonstrate that the hiatus at the unconformity differs in duration among the fault blocks. That the unconformity represents different discrete time intervals at different sites, and that at most sites slump folding lies directly beneath the unconformity, strongly suggests that the unconformity in the central part of the Mohawk Valley first developed as a series of slide (slump) scars, rather than as a direct response to changes in the eustatic sea level. The sediment slides (or slumps) probably developed in response to changes in slope from fault block rotations and to increased seismic activity along the nearby faults. These effects were, in turn, results of the plate bending as it entered the trench. The local reversal in fault motion, as documented by growth fault geometries and inferred paleoslopes, as well as the ubiquity of slumps, suggests that the Thruway unconfirmity records a time of unusual fault activity. Such activity may have been related to a changing stress field in response to plate motion (e.g., changing convergence rates or plate location relative to the peripheral bulge).