Nature of Esker Sedimentation
Broad questions of esker sedimentation are reviewed in this paper. Two main environmental factors, nature of the conduit through which the esker stream flowed, and site of deposition, control esker sedimentation and commonly can be determined from the sedimentary succession. Interaction of these two factors permits definition of three different models of esker sedimentation: open-channel, tunnel and deltaic. Morphology of the esker ridge, sedimentary structures, facies relationships and paleocurrent variability are important parameters of proposed sedimentation models. The models are discussed on the basis of field data from eskers at Peterborough, Ontario and at Windsor, Quebec.
Sediments of the Peterborough esker were deposited largely in an open channel bordered laterally by ice walls. Backset beds related to antidunes are preserved at places. A common environment was deltaic, where dunes and ripples delivered sediment to avalanche faces; progressively downstream from the large foresets were regressive, sinusoidal, and progressive ripples, respectively. These in turn pass into graded beds and then into lacustrine rhythmites.
Tunnel sedimentation is illustrated by sediments in single steep-sided ridges in the Windsor esker. Sheetlike cross-bedded and parallel-bedded gravel and sand units persist downstream without facies change and are arranged in vertically stacked cycles that may be annual. Flow depth in the tunnel was 1 to 4 m and accumulation of sediment was accommodated by a melting upward of the ice roof.
Deltaic sedimentation is illustrated by beads in the Windsor esker that were deposited annually as subaqueous fans in the water body at the mouth of the subglacial tunnel. Cobble and pebble gravel at the proximal end of the bead intertongues over a few meters in a downstream direction with ripple-laminated fine sand, units of “structureless'' fine and medium sand, and graded beds.
Figures & Tables
Glaciofluvial and Glaciolacustrine Sedimentation
This publication is the outgrowth of a symposium on Glacial Sedimentology that was held in Buffalo, New York, March 1972. The great interest generated in glacial phenomena during the nineteenth century had important implications and repercussions for the infant field of sedimentology. It provided its fair share of the background stimulus necessary to establish sedimentology as a separate branch of the earth sciences in the twentieth century. The time for reciprocity is now at hand; feedback from the expertise gained in the burgeoning field of sedimentology can greatly help the Quaternary specialist solve particular field problems. The last decade has witnessed a growing interest in the sedimentology of the Quaternary, and it seems appropriate now to summarize progress in the study of stratified drift, to present results of some recent studies, and to focus attention on avenues of research that should be explored in the near future.