Analysis of microtextures on quartz sand grains from lower Cook Inlet, Alaska

Abstract

The present-day distribution of unconsolidated sediment over the floor of lower Cook Inlet, Alaska, implies that original Pleistocene deposits in northern areas have been winnowed and those to the south have been covered with a blanket of redeposited sand. Surface microtextures on sand-sized quartz grains typically reveal relict glacial features, in most cases modified by chemical or mechanical action. Quartz grains from samples within the central area with large sand waves are characterized by an abundance of microtextures that are due to mechanical impact. The presence of such microtextures implies high-energy bedload activity, which is confirmed by underwater TV observations. Outside of the central area, where bedforms are small or absent, chemical alteration of grain surfaces is characteristic; this indicates relatively low-energy bedload activity. At a few local areas in the north, grains with nearly unaltered glacial morphology are abundant; their presence implies that although these have been sites of winnowing, many of the remaining grains have in some way been sheltered from overwhelming chemical or mechanical action. The distribution of grain sizes and microtextures of the sediment in lower Cook Inlet shows that this sediment is approximately in equilibrium with the present current regime. Also, widespread net transport of bed material has not been common.