Abstract

Neoglacial Lake Alsek is an ice-dammed lake in the southwest Yukon that forms when Lowell Glacier rapidly advances from a side valley to block the Alsek River. A sedimentary record of past fillings and catastrophic drainages is preserved in small lakes lying within the Lake Alsek basin. Sediment cores from some of these small lakes were retrieved and studied. The cores were subjected to facies analysis involving description, classification, and interpretation of depositional processes, leading to assessment of depositional environment. Despite a relatively sparse set of sedimentary data, five distinctive deposits and associated environments can be recognized: (1) matrix-supported diamicton interpreted as a deposit of iceberg-rafted sediment; (2) sand and coarse silt interpreted as tractive current deposits; (3) massive silt and clay interpreted as rapidly deposited, lake margin derived sediment; (4) laminated silt and clay interpreted as glaciolacustrine deposits; and (5) carbonaceous muds interpreted as eutrophic pond deposits. Facies sequence analysis reveals that the sedimentary sequences are nonrandomly ordered, but cyclicity of the deposits was not found. Erosional unconformities reflecting gaps in the sediment record and difficulties with basin-wide stratigraphic correlations hamper further definition of the Lake Alsek filling and draining chronology. These problems could be overcome in future studies if either greater exposures could be studied or sufficient chronostratigraphic information could be obtained.

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