THE STRUCTURE OF TRANSITIONS BETWEEN CULMINATIONS AND DEPRESSIONS IN THE LEEDS-ROBERTS HILL SEGMENT OF THE HUDSON VALLEY FOLD-THRUST BELT, EASTERN NEW YORK

Results of 1:10,000-scale geologic mapping (supported by the USGS EDMAP program) in the Leeds to Roberts Hill segment of the Hudson Valley Fold-Thrust Belt (HVB) provide insight into the relation between structural culminations and depressions. The HVB is a north-south trending, 2 to 4 km-wide belt of 50 m-high ridges west of the Hudson River between Kingston and Albany. The ridges are the eroded limbs of folds involving Siluro-Devonian clastic and carbonate strata. The eastern edge of HVB is delineated by a 20 to 50 m-high cliff called the Helderberg Escarpment. Along the length of our 24 km-long map area, a flat-roofed duplex involving Siluro-Devonian strata occurs above the Rondout detachment. The detachment is located near the base of the escarpment and directly above the Taconic unconformity, which juxtaposes Siluro-Devonian and Ordovician strata. We also recognize two culminations (at Leeds and Coxsackie) that are separated by a depression (at Limestreet). The Leeds culmination is underlain by a folded duplex that brings the basal Helderberg Group up to ground level near the foreland edge of the HVB. In the adjacent depression, the Helderberg Group remains at depth and the Tristates Group is exposed. The transition between the Leeds culmination and the Limestreet depression is characterized by an increase in first-order fold wavelengths and a decrease in the number of exposed thrusts. In map view, fold axes protrude toward the foreland at the apex of the culmination, resulting in structural trends at either end of the culmination that differ from regional trends. A lateral ramp near Greens Lake accommodates differential shortening between the Leeds culmination and the Limestreet depression. North of Leeds, fault traces define an en echelon pattern, suggesting that displacement on individual ramps dies out to the north and is transferred to new ramps in a relay structure. Of note, the Helderberg Escarpment in the Leeds-Roberts Hill area is not parallel to structural trends in the HVB. Here, the escarpment trends more northerly, suggesting that its trend is joint controlled. New high-resolution digital elevation maps of segments of the HVB near Kingston and Rosendale support this proposal. Of note, glacial striations found at a few localities along the escarpment, emphasizing that it was modified by glacial erosion.