A Comparison of the Pebble Orientations in Ice and Deposits of the Matanuska Glacier, Alaska

Abstract

Depositional processes and their sediment source determine the orientation of pebbles in the deposits of the Matanuska Glacier and the relationship of this orientation to the direction of ice flow. Pebble fabrics in ice-derived deposits differ from those in resedimented deposits. The plunge direction of the long axes of prolate-shaped pebbles and the ab-plane dip of blade- and plate-shaped pebbles parallel the local ice flow direction in basal zone ice and in melt-out till derived from that ice. The pebble fabric in deposits from sediment flow, ablation of exposed basal zone ice, and the slumping and spalling of ice-cored slopes does not correspond to the ice flow direction, but is developed by these depositional processes. Pebbles in basal ice and melt-out till show a unimodal distribution of orientations with individual observations only slightly dispersed about the mean axis. Pebble fabrics in other deposits are polymodal with a significantly larger amount of dispersion about the mean axis. The regional pattern of mean axes of basal zone ice and melt-out till pebble fabrics approximates the local and regional trends of ice flow, but pebble imbrication in ice and sediment does not necessarily indicate the direction from which the glacier flowed. A small number of measurements of pebble orientations at many sites and the analysis of these data by the eigenvalue method appear to be suitable techniques for examining the pebble fabric of glacial deposits, but additional sedimentological data are needed to define the origins of these deposits.