Deglacial valley fill sedimentation, Pelly river, Yukon territory, Canada

Abstract

A thick sequence of deglacial valley fill was studied along the Pelly River, Yukon Territory. Late Wisconsinan deglaciation was by regional stagnation of the Cordilleran Ice Sheet, resulting in complex sequences of sediments deposited in glacial lakes, many above stagnant ice. More than 30 sections were described and measured and eight facies were recognized and interpreted. The distribution of sediment reflects control by several systems: (1) ice depositing basal tills; (2) debris flows in glacial lakes depositing diamicton; (3) meltwater streams depositing sorted sediment; and (4) melting of underlying ice remobilizing and deforming previously deposited sediment. A model is constructed that helps to explain sedimentation along Pelly River. Material was deposited into lakes from melting ice, valley slopes and adjacent valleys. Basal tills are relatively rare with most diamictons representing deposition by sediment gravity flows. The presence of large thicknesses of highly disturbed sediments indicates that the sediments were deposited onto ice and subsequently failed. Undisturbed glaciolacustrine sediments were deposited either in more distal parts of the basin or after most of the ice had ablated. The adjacent topography and the height of the glacial limits also influenced sedimentation: steeper topography provided coarser sediments for deposition; while low elevation of glacial limits reduced the availability of sediment even if steep slopes were present. This study further contributes to models of supraglacial sedimentation by documenting that sedimentation was in several lake basins, that lateral gradations in grain size are present, and that fairly significant amounts of diamicton can be present.

Introduction

Many glaciated valleys contain thick sediment successions deposited during deglaciation. The style of sedimentation of deglacial valley fills is influenced by the type of deglaciation: (1) where deglaciation is gradual with an orderly retreat of the ice front; and (2) where deglaciation is rapid and the ice sheet melts by regional stagnation. Gradual deglaciation has been described from various parts of the world and commonly the sequence consists of till overlain by glaciofluvial outwash gravel (Rutter, 1972; Bergersen and Garnes, 1983; Lundqvist, 1983; Sollid and Reite, 1983). During regional stagnation there is no orderly retreat of the ice front and abundant glacial lakes commonly form, resulting in thick sequences of glaciolacustrine sediment (e.g. Fulton, 1965, Fulton, 1967). In some cases deposition occurred in supraglacial lakes or in close association with stagnant ice, resulting in complex sediment sequences that have been deformed by the melting of supporting ice (Shaw and Archer, 1979; Eyles et al., 1987; Fitzsimons, 1992). The Pelly River in central Yukon is a region where deglaciation was by regional stagnation and detailed sedimentological studies are lacking. The purpose of this paper is to further document the pattern, character and origin of deglacial valley fill associated with regional stagnation, where sedimentation was determined to have occurred in supraglacial lakes.