STRUCTURAL EVOLUTION OF THE ORDOVICIAN QUASSAIC GROUP, MARLBORO MOUNTAINS, NY

The Mid-to-Late Ordovician Quassaic Group is an autochthonous package of Taconic flysch phase turbidites in eastern Ulster County, NY. The Quassaic is preserved as an outlier in the core of the Marlboro Syncline, bound by the Esopus Thrust to the east, and grades into (or is thrust onto) the Bushkill shale to the west. Structures within the Quassaic record at least two contractional events. Reconnaissance 1:10,000 scale mapping and geometric/kinematic structural analysis was employed for a portion of the Marlboro Syncline. Bedding dips moderately (~20-40°) east in the western limb and is vertical to slightly overturned in the eastern limb. Folding is geometrically compatible with a predominant east-dipping axial-planar cleavage likely developed late during folding, as well as larger faults such as the eastern bounding Esopus Fault, and the Black Creek Fault which strikes along the axial trace of the Marlboro Syncline. The Marlboro Syncline is kinematically compatible with small-scale faults and flexural slip suggesting a WNW shortening direction. Sigmoidal veins associated with the Black Creek Fault also suggest top to the west displacement. Re-folding of the Marlboro Syncline is defined by the folding of axial planar cleavage into a moderately east plunging antiform, as well as a WSW striking high angle reverse fault (Illinois Mountain Fault) which uplifts Illinois Mountain, giving an apparent sinistral offset to the Esopus Fault. Kinematic analysis of the Illinois Mountain fault zone supports this NNW-SSE shortening. Structural evolution of the Marlboro Mountains therefore involves two major phases. Taconic E-W contraction resulted in the west-vergent folding of the Quassaic Group, possibly as a fault propagation fold, and displacement of the Quassaic along the Esopus and Black Creek Faults. Later Alleganian (?) N-S shortening resulted in the re-folding of the Marlboro Syncline and axial planar cleavage and the formation of the Illinois Mountain Fault. It is likely that Hudson Valley Fold and Thrust Belt style deformation, present to the north, is limited here due to a décollement situated at a higher structural level.