Abstract
The Silurian Shawangunk Fm., a quartz rich pebble conglomerate outcropping in east-central New York State, records west to west-northwest shortening attributed to either the Acadian or Acadian and Alleghenian orogenies, however the timing of deformation is subject of much debate. Previous studies of the Shawangunk Fm. have identified an apparent post-fold flattening fabric; however, these studies lack the microstructural characterization needed to definitively bracket the timing of the observed deformation. Using transmitted light (TL) and scanning electron microscopy with cathodoluminescence imaging (SEM-CL) with the EllipseFit software we calculate both the moment ellipse from 300-350 digitized grains, as well as fully map the distribution and cross-cutting relationships between the component fabric forming elements in 11 field oriented and 2 unoriented samples of the Shawangunk Fm. We hypothesize that the Shawangunk Fm. does record an episode of post-fold flattening and further hypothesize that its development is associated with exhumation and deposition of material during the final pulse of the Alleghenian Orogeny. Our results show that the Shawangunk Fm. records relatively low penetrative strain (Rxy, Rxz, and Ryz equal to 1.06 ± 0.03, 1.21 ± 0.04, and 1.14 ± 0.03 respectively) with a predominant bedding normal strain. A fold test was used to calculate the degree of clustering of Z-axes through progressive unfolding and shows the maximum degree of Z-axis alignment is at approximately 36% unfolding, suggesting the flattening fabric developed either syn- or post-folding. We identify brittle microfractures and fluid inclusion planes, pressure solution features, and inherited crystal-plastic deformation features as the dominant fabric forming mechanisms. The distribution and density of these fabrics in thin section largely supports burial as the principal fabric forming mechanism, however, the cross-cutting relationships alone are insufficient to resolve the timing of the fabric. It is likely that a deeper understanding of strain partitioning between these fabric elements is needed to understand if the flattening occurred syn- or post-folding.