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Sedimentology and chronology of neoglacial Lake Alsek, Yukon Territory

University of British Columbia

Neoglacial Lake Alsek has formed many times during the last 3000 years when Lowell Glacier has blocked the flow of Alsek River. Although the basin is now empty, evidence for former lakes includes: valley-side beaches, driftwood strandlines, ice-rafted erratics, vegetation discontinuities, and unusually large (outburst-flood related) bedforms. The largest fillings covered approximately 463.0 km² of the valley bottom and were approximately 28.37 km³ in volume. The associated outburst floods produce instantaneous peak discharges estimated at 1.09 x 10⁵ m³ s⁻¹. Alsek valley sediments were examined for a record of past fillings and drainings. Physical, stratigraphic, and lithofacies analyses were conducted on sediment cores. Five facies assemblages are defined and used to interpret depositional environments: (1) matrix supported diamicton interpreted as a deposit of iceberg-rafted sediment, (2) sands interpreted as tractive current deposits, (3) massive muds interpreted as rapidly deposited lake margin derived sediment, (4) laminated muds interpreted as distal glaciolacustrine deposits, and (5) carbonaceous muds interpreted as eutrophic pond organic detritus deposits. Facies sequence analysis indicates non-randomly ordered sedimentary sequences. Cyclic sedimentation is not indicated. Many occurrences of erosional unconformities indicate either depositional hiatuses or an unknown amount of missing sediment. A single radiocarbon date of 2840 ¹⁴C years B.P. underlies 1.04 m of Lake Alsek sediment. This dated material overlies approximately 1.5 m of sediment presumably associated with Lake Alsek. The potential for absolute dating of the Lake Alsek stratigraphy has been shown to be quite high.

Text

2010-06-30

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http://hdl.handle.net/2429/26068

Geography

University of British Columbia

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