CGRG Bibliography of Canadian Geomorphology
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Author : Beierle, B.
Date : 2000.
Title : Late glacial-early Holocene landscape evolution of the Chapman Lake area, Yukon Territory, Canada.
Publication : 30th International Arctic Workshop, Program and Abstracts, 2000. Institute of Arctic and Alpine Research, University of Colorado at Boulder
Issue :
Page(s) : 20-21.
Abstract
Detailed sedimentological, geochemical and macrofossil analysis of two lake sediment coresfrom Chapman Lake, Yukon (64°51’N, 138°22’W) are used to reconstruct landscape change in the Blackstone River Valley (BRV) over the past 13,000 years. Chapman Lake is the largest of a group of kettle lakes, and is currently a closed basin perched 10 m above present base level in the BRV. A layer of plant detritus and overbank sediments in the cores overlies a fluvial sand facies and is inferred to represent the land surface prior to subsidence of the lake basin after 13,210 +/-300 BP. Fluvio-lacustrine sediments deposited after this time fine upward and increase in LOI organic carbon, suggesting decreasing fluvial influence on Chapman lake until ca. 11,000 BP when clastic sedimentation drastically decreased and LOI organic carbon increased from <5 % to >40 %. Correlation to dates from a previous study of Chapman Lake (Terasmae and Hughes, 1966) as well as dates from this study suggest that net sedimentation rates decreased by a factor of 3 at this time, and indicate that the lithofacies change resulted mostly from decreased clastic sedimentation despite increased lake productivity. Surficial and aerial photographic mapping revealed the presence of paleo meandering channels superimposed on the lake system, indicating that the Blackstone River (BR) flowed across the area prior to lake formation. The overbank and fluvial sediments found in the Chapman Lake cores are correlated to this phase of fluvial activity, and indicate that it occurred prior to 13,200 +/- 300 BP. Terracing of the paleo-fluvial system suggests that the river downcut in response to formation of the lake basin and regional incision, with flow from the BR entering the lake from the south and exiting to the north, where it merged with flow from the East Blackstone River (EBR). The fining upwards sequence between ca. 13,200 and 11,000 BP in the lake cores is interpreted to represent formation of the lake, with progressively decreasing fluvial influence resulting in increasing deposition of silt and clay versus fine sand. The BR now enters the EBR upstream of Chapman Lake as a result of capture by the EBR. The transition from clastic to organic sedimentation in the Chapman Lake cores at ca. 11,000 BP is thought to represent this capture, with an immediate and dramatic reduction of fluvial sediment flux to the lake as flow was diverted. This drainage diversion also resulted in the abandonment of the northern outlet of the lake, which as a result no longer downcut in response to changing base level in the BRV. Thus, the difference between the current lake surface elevation and base level in BRV (~10 m) is a minimum estimate for post-capture incision of the BRV. Topographic profiling of the BR and EBR and the surface of the non-incised remnants of glacial valley fill indicate that the ~20 m of total incision in the BRV at Chapman Lake is a result of dissection of the terminal moraine in which Chapman Lake lies. The profile of the moraine surface is such that it would not have impounded a lake upstream in either the BR or EBR valleys, suggesting that incision of the moraine must have occurred immediately after its deposition. The occurrence of at least half the total incision of the BRV within the Holocene is consistent with a ca. 13,200 BP age of formation for Chapman Lake and a ca. 11,000 BP age for capture of the BR and abandonment of the northern outlet of the lake. The timing of these events suggests the moraine at Chapman Lake was likely deposited during the last glacial maximum (LGM). An LGM age for the terminal moraine at Chapman Lake is, however, inconsistent with the current understanding of regional glacial stratigraphy (Duk-Rodkin, 1996), which places its age at greater than 200,000 years (Berger et al., 1996). Previous research has suggested that the climate of the LGM was extremely arid in Eastern Beringia, resulting in the desiccation of lakes and deflation of lake sediments. This mechanism is thought to lead to truncated lake records which cannot be used to date the surfaces in which they lie. The relationship between Chapman Lake and the surrounding landscape does not preclude such arguments, but does strongly suggest that the age of Chapman Lake reflects that of the glacial deposits in which it lies and indicates that other lake records in eastern Beringia previously thought to be incomplete may in fact accurately date the landscape around them.
Bibliography of Canadian Geomorphology