Deglacial valley fill sedimentation, Pelly river, Yukon territory, Canada
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.
Section snippets
Previous work
Regional stagnation was typical of Late Wisconsinan deglaciation throughout the Interior System of the Canadian Cordillera (Fulton, 1991; Jackson et al., 1991) and many studies have described thick, occasionally deformed glaciolacustrine sediments (Fulton, 1965, Fulton, 1967; Shaw, 1975, Shaw, 1977; Shaw and Archer, 1978, Shaw and Archer, 1979; Fulton and Archard, 1985; Eyles et al., 1987; Eyles and Clague, 1991; Ryder et al., 1991). Most of these studies indicate that at least some of the
Physiography
The study area contains three physiographic regions of Yukon: the Tintina Trench, the Yukon Plateaus, and the Glenlyon Range of the Kaska Mountains (Fig. 1) (Mathews, 1986). The Tintina Trench, the northern extension of the Rocky Mountain Trench, is a long structurally controlled valley that trends SE–NW across central Yukon. In the study area, Tintina Trench is up to 4 km wide with an average valley floor elevation of 600 m asl. The Yukon Plateaus have moderate relief, a variable topography and
Methods
More than 30 sections were described and measured along the Pelly River and adjacent creeks over a distance of 100 km from the area of Anvil Creek to the confluence of the Pelly and Macmillan rivers (Fig. 1). Elevation of the sections was established by altimetry tied to bench marks and spot elevations from 1 : 50,000 maps. Elevations are estimated to have an accuracy of ±2–3 m, except at section 7, where the elevation was determined by the average river gradient calculated from contours and is
Facies
Sediments were grouped into eight facies (Table 1) based on objective criteria: grain size (Fig. 2), sedimentary structures (Fig. 3), clast fabric (Fig. 4, Table 2) and presence or absence of deformation features (cf. Walker, 1984). Facies are described in more detail in the descriptions of the sections. Because of the regional nature of this study, many of these facies contain a wide range of sediment types and structures. Where possible, the main diagnostic characteristic of each facies is
Discussion and reconstruction of sedimentary environment
The distribution of sediment reflects control by several systems: (1) active and stagnant 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.
The till (Facies A) and debris flow deposits (Facies B) represent two contrasting types of diamicton. The till formed from a combination of lodgement and meltout processes, below both
Comparison to other models
The distribution of sediments described in this study is similar to the published models by Eyles et al. (1987), Eyles and Clague (1991) and Shaw (1988). A notable similarity is the abundance of material that was deposited above stagnant ice in a supraglacial lacustrine environment resulting in the deformation of the sediments. A clarification offered by this study is since it is of Late Wisconsinan sediments, surface topography is preserved so that it can be determined that sedimentation was
Conclusion
The deglacial valley fill sediments described along the Pelly river represent complex deposition in supraglacial lakes. Abundant supraglacial lakes formed because of the regional stagnation style of deglaciation. This study adds to previously developed models of supraglacial sedimentation. Sedimentation along the Pelly river was into several supraglacial lake basins rather than one large lake. The adjacent topography, the elevation of glacial limits and the relative location, both spatial and
Acknowledgements
This paper forms part of the requirements for a Ph.D. thesis by the senior author at the University of Alberta. This project was funded by the Geological Survey of Canada under the direction of Dr. Lionel Jackson. P. Beck, D. Lye and C. Rutter provided assistance in the field. The authors have benefited from discussions with J. Shaw and W. Nemec. N. Catto, D. Liverman and M. Reasoner reviewed an earlier version of this paper. Journal reviewers D. Naldrett and I. Spooner made many useful
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