QUANTIFYING THE PENETRATIVE DEFORMATION OF A LIMESTONE PEBBLE CONGLOMERATE FROM THE KOOTENAY ARC, NE WASHINGTON

This study applies R_f/phi analysis to high resolution (1200dpi) scans of three mutually perpendicular cuts from an oriented sample of deformed limestone pebble conglomerate, with the ultimate goal of producing a strain ellipsoid approximation. Similiar analysis was previously performed on the same sample, where ellipses were measured by taking the longest dimension of the pebble as the long axis, and its perpendicular bisector as the short axis (Stewart 1996). A different protocol for ellipse measurement (largest internal-fit ellipse) was used in this study to determine the extent to which the protocol for fitting ellipses to irregular pebble shapes may affect the results. Statistical analysis using the x² goodness-of-fit test (assuming random initial distribution of long-axes; Chew, 2003) was chosen to determine the range of most probable aspect ratios of the strain ellipse for the study plane, instead of the graphical approach of using the best-fit initial-axial-ratio envelope (Lisle, 1985). Comparison of results from the same cuts indicate that the difference in protocol does not make a signficant difference in outcome. Homogenous deformation was initially assumed because both pebbles and matrix are calcitic, suggesting negligible ductility contrast. However, new high resolution scans allow the measurement of very small pebbles (<2.5 mm²), a different population than either outcrop pebble data (the sizes of which are not constrained by the size of the sample) or previous measurement of the same cuts using lower resolution scans, which raises the question of a size-dependent variation in deformation behavior. Important questions under consideration include the soundness of the assumption that the pebbles deformed passively and the extent to which the analysis can produce robust results applicable on a regional scale.

Part of the Windermere supergroup, this deformed conglomerate originated during the late Proterozoic in a prograding passive margin that subsequently became shortened and deformed during Mesozoic Cordilleran orogenesis. This study contributes to research aimed at quantifying penetrative deformation in the hinterland of the Candian Cordilleran thrust belt, which can be compared to shortening estimated from palinspastic reconstructions of balanced cross-sections (Price, 1981; 1986).