GEOREFERENCED CROSS-SECTION ANALYSIS AND PRELIMINARY THREE-DIMENSIONAL GEOMETRIC MODELING OF THRUST DUPLEX IN THE FEURA BUSH QUARRY, SELKIRK, NY

The Feura Bush Quarry in Selkirk, NY exposes a duplex geometry- the Feura Bush Duplex. This duplex structure is related to the structures in the northern part of the Hudson Valley Fold-Thrust Belt (HVB). The HVB is a north-south trending, 2-4 km wide region of deformed Silurian through Devonian strata that extends from Kingston, NY to Albany, NY. Studies along the HVB indicate a reduction in the deformation intensity along-strike towards the north. In this context, the location of an intensely deformed duplex towards the northern end of the HVB is enigmatic. To address the significance of the Feura Bush Duplex’s position in the northern HVB, we chose to study the geometry and the strain observed in the duplex. Three nearly perpendicular walls of the quarry expose the internal geometry of the duplex and therefore provide an excellent opportunity to study the three-dimensional geometry of the duplex and to measure strain accommodated along the faults and fault-related-folds exposed in the quarry. Overlapping images of each quarry wall were recorded and stitched together to create a composite image of each quarry wall, which were then stitched together to develop a composite fence diagram of the duplex structure. The composite fence diagram was then georeferenced in ArcGIS^TM to produce a georeferenced cross-section that will be stitched into New York State Geological digital map. Using the composite fence diagram, a three-dimensional model of the duplex was developed using Sketch-up^TM. Geometric analysis of the detachment faults exposed along the quarry walls indicate that deformation within this duplex was accommodated along at least three local detachments. The northern wall exposes numerous thrust ramps between the lowest and the intermediate detachments. Cut-off points of bedding across these thrust ramps were identified in the field and on images, and the distance between them were measured to estimate slip along the faults. Line-length measurements of fault-related folds between the thrust ramps were measured to determine shortening accommodated by the folds. The geometry of the detachments, the measurements of slip along thrust ramps, and the shortening measurement of folds indicate a decreasing deformation intensity to the west within the duplex which is consistent with the overall architecture of the HVB.