GEOMETRIC AND KINEMATIC ANALYSIS OF THE FEURA BUSH QUARRY DUPLEX IN THE NORTHERN HUDSON VALLEY FOLD-THRUST BELT, SELKIRK, NY

The Hudson Valley Fold-Thrust Belt (HVB) is a 2-4 km wide region of westerly verging deformed Silurian-Devonian strata that trends north-south from Kingston, NY to Albany, NY. Geologic mapping along-strike of structures in the southern and central HVB indicate a decrease in deformation intensity towards the north. The Feura Bush Quarry is situated in the northernmost part of the HVB and exposes intensely deformed structures. The goal of this study is to perform a kinematic analysis on the enigmatic structures exposed in this quarry.

The quarry reveals a duplex along its four main walls: northern wall, northwestern wall, western wall, and southern wall. Geometric analysis indicates that deformation is accommodated along three local detachments, which we refer to as the lower, intermediate, and upper detachments, along with numerous thrust faults and fault related folds. The northern wall of the quarry contains thrust ramps terminating between the lower and intermediate detachments which creates well-defined horses. The bedding within these horses show tight asymmetrical folds.

For this project, we quantified the minimum shortening accommodated by the exposed northern wall of the Feura Bush Quarry and the nested duplex within the northern wall by measuring (a) the spacing between the thrust faults in the nested duplex from west to east, (b) the shortening accommodated by the asymmetrical folds within the horses in the nested duplex, (c) the aspect ratio of long and short axis of the best-fit shape ellipse, (e) the minimum shortening along the northern wall, and (f) the depth to the underlying blind Rondout detachment.

The spacing between the thrust faults show that the intensity of deformation pulses regularly throughout the duplex from west to east with a dying of deformation intensity in the western 34% of the duplex. The shortenings calculated from asymmetrical folds in the western, central, and eastern parts of the nested duplex were -52.3%, -40.3%, and -97.5% respectively, suggesting a pulsing in fold shortening through thrust weakness variation. From west to east, we calculated aspect ratios of 37.8, 33.2, and 13.27 on three separate horses. The minimum shortening along the northern wall was calculated to be about 9.5% shortening.