Hudson Valley Fold-Thrust Belt

The Hudson Valley fold-thrust belt lies between the Hudson River on the east and the Catskill Mountains on the west. The fold-thrust belt involves Silurian-Devonian (~370-415 million years old) rocks that were deformed during the Acadian orogeny (~330-360 million years ago). The deformation gave rise to imbricate thrust faults, duplexes, and associated folds that developed in Devonian rocks with a basal decollement in the Upper Silurian Rondout Formation. In addition, a suite of small scale structures developed that include contraction and extension faults, spaced solution cleavage, vein arrays, and fibrous slickensides.

The entire area was folded again at a later time so that both the folded Silurian decollement and the underlying Taconic unconformity are exposed at the surface. The Taconic unconformity formed during the late stages of the Taconic orogeny (~ 440 million years ago) and separates the strongly deformed Ordovician Austin Glen flysch below from the Silurian-Devonian rocks above.

The undergraduate Structural Geology class regularly visits the outcrops of this belt along Rte 23 at Catskill, NY as part of a class exercise. Here are some pictures from some recent field trips.

Geology

The Taconic Angular Unconformity, with steeply dipping Ordovician Austin Glen below it (right), and Silurian Rondout (brown) and Devonian Manlius (grey) above it (left). These rocks are exposed in the east limb of the Tollbooth syncline	Close-up of the Rondout Detachment Zone with a zone of asymmetric folding (above) and a zone of duplexing (below), both indicating top to the west shearing. The detachment is ~2.5 m above the Taconic unconformity
An imbricate thrust fault placing Devonian Manlius in the hanging wall on Devonian Kalkberg in the footwall, a stratigraphic separation of ~ 50 meters. The fault is exposed in the west limb of the Tollbooth syncline	The Taconic Unconformity is near vertical in the west limb of the Eastern anticline
Fibrous slickenside surface on a bedding plane fault on the steeply dipping west limb of the Eastern anticline.	Antiformal stack duplex (the Central Anticline) just east of Catskill Creek
The Rip Van Winkle anticline (on the north side of Rte 23), just west of the NY Thruway. This is a fault propagation fold with a steep west limb; the tip of the fault is exposed on the south side of Rte 23, and plunges northward under the fold on the north side	Close-up of the west limb showing vertical beds with a contraction fault and refraction of solution cleavage seams. Also note vein-filled feather joints on small contraction fault (on right)