COMPARISON OF MICROTEXTURES ON QUARTZ SAND GRAINS FROM GLACIAL SUB-ENVIRONMENTS IN SOUTHERN ALASKA

A suite of 181 quartz sand grains was selected from three sub-environments of temperate alpine glaciers in southern Alaska. Samples were collected from outwash sediments proximal to the glacial termini of Matanuska Glacier and Turner Glacier, a small tributary to Disenchantment Bay, and from a diamicton layer in a basal iceberg from Hubbard Glacier. Individual quartz grains were then selected, mounted, sputter coated, and imaged with a scanning electron microscope (SEM) in order to examine the morphology and micro-textures of the grains. Minerology was verified with Energy Dispersive X-Ray (EDX). Previously published studies suggest that surface textures and characteristics of the grains can indicate the glacial environment in which they were transported. From each photomicrograph of a grain, the frequency of occurrence of fourteen micro-textures was recorded. Micro-textures on the grains that can be attributed to crushing include high relief, conchoidal fracture, fracture faces, and sublinear features. Grains that have been transported by meltwater have rounded edges and percussion marks.

Analysis of the grains indicates variability in the frequency of occurrence of textures on the grains. Grains from Matanuska Glacier have the strong imprint of glacial crushing and little evidence of meltwater transport, Hubbard Glacier grains have a moderate glacial imprint, and grains from Turner Glacier have little evidence of glacial transport. Turner Glacier does, however, have a strong signal indicating fluvial transport, and measurement of Wadell Roundness of the grains indicates that the population of grains from Turner Glacier is on average significantly rounder than those with glacial micro-textures. Most grains from these sub-environments within this temperate setting show evidence on the quartz grain surface of weathering prior to glacial transport.