COMBINING RECTIFIED HIGH-RESOLUTION GIGAPAN IMAGES AND AN INTERACTIVE STRAIN PROGRAM TO TEST THE RF-PHI METHOD FOR ANALYSIS OF CONGLOMERATES

Deformed pebbles are commonly used by researchers to estimate strain in rocks, and by instructors to teach fundamental principles of strain. The Rf-Phi method is widely used to measure strain in deformed pebbles. It relies on the assumption that pebbles were deposited with no initial preferred orientation of long axes. We tested this fundamental assumption in the undeformed polymictic Mount Toby Conglomerate from Sunderland, MA. We created high-resolution Gigapans, and used precisely embedded markers to rectify the stitched photos. The rectified images were then imported into Geoshear, an interactive Java program that allows users to explore visually how deformation of a set of elliptical objects appears on Cartesian and polar Rf-Phi plots. We digitized the pebbles based on lithology to measure the clast aspect ratios and orientations. In faces perpendicular to bedding, Rf-Phi plots showed a significant initial preferred orientation of long axes parallel to bedding when all clast lithologies were used. Clasts with higher aspect ratios showed a stronger preferred orientation in accord with the so-called delta configuration of Elliott (1970). On surfaces parallel to bedding, Rf-Phi plots showed no initial preferred orientation of long axes. Results from the Rf-Phi analysis of three joint faces were combined (Mookerjee and Nickleach, 2011) to determine an apparent ‘strain ellipsoid’. When all clast lithologies were used in the analysis, the apparent ‘strain ellipsoid’ for two locations was oblate with normalized axial ratios of 1.2:1:0.55 and 1.3:1:0.56, with a maximum initial axial ratio of 2.1 and 2.3 in the X-Z plane, and apparent octahedral shear strain of 0.57 ± 0.09 and 0.58 ± 0.10, respectively. When we restricted the Rf-Phi analysis to only quartz pebbles, however, faces perpendicular to bedding revealed only a weakly preferred orientation of long axes. When only quartz pebbles were used to compute apparent ‘strain ellipsoids’ the normalized axial ratios were 1:1:0.87 and 1.2:1:0.92, with a maximum initial axial ratio of only 1.1 and 1.3 in the X-Z plane, and apparent octahedral shear strain of 0.12 ± 0.06 and 0.18 ± 0.06, respectively. Thus, the assumption of no initial preferred orientation of pebble long axes is approximately valid for the quartz-pebble population.