Deglacial valley fill sedimentation, Pelly River, Yukon Territory, Canada

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 interprete d. The distribution of sediment reflects control by several systems: (1) ic e depositing basal tills: (2) debris flows in glacial lakes depositing diam icton; (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 adj acent valleys. Basal tills are relatively rare with most diamictons represe nting deposition by sediment gravity flows. The presence of large thickness es of highly disturbed sediments indicates that the sediments were deposite d onto ice and subsequently failed. Undisturbed glaciolacustrine sediments were deposited either in more distal parts of the basin or after most of th e ice had ablated. The adjacent topography and the height of the glacial li mits also influenced sedimentation: steeper topography provided coarser sed iments for deposition; while low elevation of glacial limits reduced the av ailability of sediment even if steep slopes were present. This study furthe r contributes to models of supraglacial sedimentation by documenting that s edimentation was in several lake basins, that lateral gradations in grain s ize are present, and that fairly significant amounts of diamicton can be pr esent. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved.