Thermokarst‐lake‐basin sediments, Tuktoyaktuk Coastlands, western arctic Canada

Abstract

ABSTRACTThree stages of deposition are distinguished in thermokarst‐lake‐basin sequences in ice‐rich permafrost of the Tuktoyaktuk Coastlands, western arctic Canada: (1) widespread retrogressive thaw slumping around lake margins that rapidly transports upland sediments into thermokarst lakes, forming a distinctive basal unit of impure sand and/or diamicton; (2) a reduction or cessation of slumping‐because of the pinching out of adjacent ground ice, slump stabilization or climatic cooling, that reduces the input of clastic sediment, permitting reworking of sediment around lake margins and suspension settling, principally in basin centres; (3) lakes drain and deposition may continue by gelifluction and accumulation of in situ peat or aeolian sand.Radiocarbon dating of detrital peat and wood from a progradational sequence (basal unit) defines a lateral younging trend in the direction of progradation. A progradation rate is calculated to be ∼ 4 cm yr−1, consistent with rapid deposition during stage (1) above. The nonuniform nature of the trend is attributed to episodic influxes of old organic material by slumping and reworking by waves and currents.In comparison with thermokarst‐lake‐basin sequences previously described in Alaska, Canada and Siberia, the middle unit of those in the Tuktoyaktuk Coastlands is similar, whereas the basal unit is generally thicker and, by contrast, often contains diamicton. These differences are attributed, respectively, to larger‐scale resedimentation of upland sediments by retrogressive thaw slumping and debris‐flow deposition in thermokarst lakes in the Tuktoyaktuk Coastlands. Compared with the sediments within supraglacial lakes in areas of moderate to high relief, the middle unit of thermokarst‐lake‐basin sequences in the Tuktoyaktuk Coastlands lacks clastic varves and the basal unit is much thinner and texturally less variable. These differences are attributed to higher relief and larger volumes of meltwater and glacigenic sediment in supraglacial lakes, which promote more suspension settling and resedimentation of glacigenic sediment than in thermokarst lakes in the Tuktoyaktuk Coastlands.It may be impossible to distinguish glacial and periglacial thermokarst‐lake‐basin sediments in permafrost areas of incomplete deglaciation. Not only is it often difficult to distinguish intrasedimental and buried glacier ice, but the depositional processes associated with thaw of both ice types are presumably the same and the host sediments very similar.