REPORT ON A SEGMENT OF A MAJOR THRUST FAULT IN THE MARTINSBURG GROUP (MEDIAL ORDOVICIAN), LOWER WALLKILL VALLEY, SOUTHEASTERN NEW YORK

Detailed study of exposures in a region extending about 6.5 km (4 mi) between N.Y. Route 299 and the Shawangunk Kill, a tributary of the Wallkill River, has revealed three structural zones trending NNE in Martinsburg strata. To the east is a complex of easterly dipping and younging strata in slices apparently imbricating to the west. Central is a zone of steeply dipping to overturned strata younging west. This distinct zone, about 1.2 km (4000 ft) wide, appears to have formed in response to tectonic forces from the east resulting in brittle failure along an overlying low angle thrust which appears to dip no less than 7 degrees to the east. The Central zone grades rapidly westward into a zone of open folds with minor faulting. The thrust inferred here appears to be part of a much larger post-Taconic dislocation trending about 44 km (27 miles) from Middletown to Rosendale, New York, possibly the most extensive fault in the lower Wallkill Valley.