The Distribution and Character of Sediments in a Tidewater Glacier, Southern Baffin Island, N.W.T., Canada

Abstract

The distribution, shape, and grain-size characteristics of sediments in a Baffin Island tidewater glacier are examined. At the glacier terminus a 0.8- to 2.9-m thick zone of basal debrisrich (14 to 57%7o by volume) ice underlies clean ice. Marginal ice flow is extending, and highlevel debris-rich bands are largely absent. Little supraglacial sediment is present except as isolated rockfalls. Freezing-on near the glacier margin probably accounts for the relatively thick basal zone. Measured sediment characteristics were (1) clast roundness; (2) Fourier shape of quartz sand grains; (3) grain-size distribution. Modal shapes of basal ice and rockfall clasts were subrounded and angular, respectively. Fourier analysis showed quartz sand grains from clean ice and rockfall were more angular and microtextured than those from basal ice. Grain size envelopes for debris in basal ice, meltwater streams, and nearshore environments are described, and are compared with those from fiord glaciomarine sediments. The distribution of sediments within the tidewater glacier controls which mechanisms of debris release are important. This in turn influences the depositional facies found. Sediment is released mainly by (1) direct melting and (2) subglacial streams and associated density overflows. Calving is relatively unimportant. Acoustic sounding revealed three morainal banks offshore of the glacier terminus, probably relating to melt-out of basal debris. A calculated rate of basal sediment supply by melting of 6.1 to 10 m3 m-1 yr-l is similar to that required to build such banks.