PRELIMINARY STRAIN MEASUREMENTS WITHIN 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 strain 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. Along the northwestern wall, the detachments are folded and show an apparent dip of 27°. The western wall exposes the three local detachments but no thrust ramps or folds. The southern wall exposes the local detachments and macroscopic folds. We interpret (i) the folding of the local detachments to be related to deformation of the underlying Rondout detachment and (ii) the local detachments to terminate against the Rondout detachment.

To calculate the minimum strain accommodated between the lower and intermediate detachments in the northern wall of the quarry, we measured (a) the spacing between the thrust faults from east to west and (b) the shortening accommodated by the asymmetrical folds within the horses. The thrust fault spacing measurements an increasing in fault spacing in the western 35% of the duplex. Measurements of fold shortening within a horse indicated a 52% shortening. Measurement of change in length to original detachment length indicated a 9.5% shortening. Future work will involve additional measurements of fold shortening within horses, determining the aspect ratio of the best-fit strain ellipse of individual horses, and a Rf/phi analysis of the aspect ratios.