PREFERRED ORIENTATION OF COBBLES AND BOULDERS IN UNSTRAINED ALLUVIAL FAN DEPOSITS: IMPLICATIONS FOR RF-φ ANALYSIS OF DEFORMED CONGLOMERATES

Deformed conglomerates are commonly used for strain analysis because clasts can be approximated as ellipses. The Rf-φ method uses the orientation and axial ratio of elliptical objects to estimate finite strain, but it assumes an initially random orientation of clast long axes. This assumption has been tested in sandstones, but not in cobble and boulder-sized conglomerates. In sandstones, initially preferred orientations of long axes can significantly affect estimated strain values and can be difficult to detect (Patterson and Yu, 1993).

Our work focused on undeformed conglomerates in the Deerfield Basin in MA. The Jurassic Sugarloaf Arkose and Mount Toby Conglomerate were deposited near a normal fault at the eastern margin of the basin. We measured 13 joint faces from 3 large outcrops. Exposures include both clast and matrix supported fabrics. Clast size ranges from pebble to boulder, and clasts are angular to well rounded. Each outcrop contains one or two joint faces parallel to bedding and two to four faces approximately perpendicular to bedding.

We used a Gigapan mount to take high-resolution photographs of the joint faces. We included a 2x1 m grid to correct for image distortion using Photoshop. We imported the photos into Geoshear, a strain analysis program (Karabinos and Warren, 2010). Using Geoshear, clasts are digitized and both Cartesian and polar Rf-φ plots created. We measured an average of 247 clasts on each surface, ranging from 85 to 955 clasts.

In faces perpendicular to bedding, Rf-φ plots show an initially preferred orientation of long axes parallel to bedding, and clasts with higher axial ratios are more likely to be oriented parallel to bedding; this is the so-called ‘delta’ configuration of Elliott (1970). Rf-φ plots for faces parallel to bedding showed no preferred orientation of clast long axes. The virtual tools allowed us to measure numerous clasts directly from rock faces, and to create a visual record of our measurements readily available for review by other geologists. Geoshear also allows us to run deformation ‘experiments’ to evaluate how accurately we can estimate strain imposed on samples with an initially preferred orientation, and thereby assess the importance of preferred orientations in typical conglomerates.