Proglacial Fluvial and Lacustrine Environments
This paper reviews the hydrology and hydraulics of high energy, particularly proglacial, riverine and deltaic environments, and discusses some of the consequences for resultant patterns of sediment movement and deposition.
The hydrology of proglacial rivers is under strong thermal influence and exhibits a singular pattern of flow, both seasonally and diurnally. Moderate flood flows are common. Sediment is frequently entrained and deposited, so that rapid evolution of fluvi u sedimentary features occurs on outwash plains. The possibility exists for extraordinary jokullhlaup floods to occur in front of many glaciers. The hydraulic behavior of proglacial rivers features frequent upper regime flow and rapid adjustment of channel resistance to accommodate the wide variations in discharge and sediment transport.
Sediment entrainment is reviewed in sime detail, and theconceptsof “overloose” and “underloose” boundary are introduced. Sediment transport theory is reviewed and recommendations made for assessing total sediment yield. The sediment transport in proglacial rivers is anomalously high by comparison with that in nonglacial environments, because of the large volumes of drift delivered to the glacier margin. Flood deposits on outwash plains and surficial patterns of sediment texture are described. The character of the surface is conditioned by selective deposition of sediment in a simple, aggradational context.
Depositional bedforms are classified as small forms (scale controlled by flow depth or lesser flow dimensions) and large forms (scale controlled by channel width). The former reflect purely local flow conditions, whereas the latter are influenced by the total flow pattern of the river. The persistence of bedforms as sedimentary structures in the stratigraphical record is considered. Gravels commonly exhibit only rudimentary plane bedding and imbrication, whereas fine materials commonly feature a wide variety of sedimentary structures. This is a consequence of the vertical distance available for deposition as compared to particle size, and the energy status of the depositional environment.
River channels in coarse, noncohesive materials are wide and shallow, so that boundary resistance to flow is high. When large volumes of sediment are being transported in flood, total resistance may become too high to permit passage of the water plus sediment load; deposition and selective scour then produce narrower, deeper channels that are hydraulically more efficient. The morphological result is the occurrence of channel braiding, which is a frequent characteristic on outwash. The long profiles of proglacial rivers and outwash surfaces are concave upward as a consequence of persistent aggradation.
Where proglacial rivers enter standing water bodies, classical, high-angle deltas develop. Sediment transported as bed load is deposited on the delta surface, or is deposited on the foreset wedge by avalanching over the delta lip. Turbidity flows (underflows) and slumps move coarse material farther into the water body. Finegrained material is carried in suspension into the standing water and settles to the bottom to form varves.
Examples illustrating the application of principles and characteristic conditions are drawn from the literature on glaciofluvial and glaciolacustrine environments.
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.