ABSTRACT: Late Glacial and Holocene environmental change inferred from sedimentary archives of Kusawa Lake, Boundary Range Mountains, Yukon, Canada.

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Abstract

Modern Kusawa Lake (142 km²) drains a 4290 km² catchment, 4.7% of which is glacier covered, in the Coast Mountains of the Yukon and northern British Columbia (Fig. 1). It lies in the southern extension of Glacial Lake Champagne formed in the Late Pleistocene when westward-flowing glaciers in the Takhini valley coalesced with eastward-flowing glaciers from the St. Elias complex. Geophysical, geomorphic and lake sediment cores document deglaciation of the lake, the presence of Lake Champagne, and the postglacial sedimentary environment of the basin. During the main phase of Lake Champagne (ca.12-11 ka BP), the water level stood at 772 m in the northern part of Kusawa Lake and 756 m in northern Dezadeash valley, both probably controlled by a spillway floored at 756 m to the north into the Nordenskiold River. This indicates differential isostatic rebound of 0.2 m/km from south to north. At that time glaciers in the Primrose valley and Takhini trunk valley built large deltas into Lake Champagne. A trunk glacier occupied the southern portion of Kusawa Lake, depositing a thick sequence of sediment in the basin (Fig. 2). Subsequently, the level fell to three major stages: 744 m, 714 m and its present level of 671 m. Lake levels were controlled by a spillway and subsequent down cutting of the sediment plug at the outlet of Kusawa Lake (Region I). Late Glacial and early Holocene sediment inputs to the lake were characterized by wash-in from exposed valley sides leaving distinctive acoustic facies in the north-central portion of Kusawa Lake. In the southern portion of the lake (Region IV), acoustically well-layered sediments have accumulated to a thickness of at least 60 m (Fig. 2). A 2.8 m long submersible vibra-core taken from this region (Fig. 3) indicates average accumulation rates of 0.27 to 0.48 mm a-1 and thus the upper 3 m documents the whole of Holocene sediment inputs. These rates are in agreement with estimates made from cores taken in Region II of the lake and with other studies. The late glacial and very early Holocene sediment package comprises more than 95% of the accumulated sediments demonstrating thick and fast sedimentation in response to deglaciation and glacier-dammed lake drainage. Numerous and large lakes in the upper watershed trap some sediment resulting in the low Holocene sediment inputs despite significant modern glacial cover. However, sediments in the deep, medial, part of the lake (Fig. 3) show at least 15 major and 25 minor episodes of high-energy turbidity deposits (sand) throughout the Holocene. We speculate these are tied to: a) major, basin-wide, flood events generated from incursions of warmer and moist Pacific air masses; b) a mixture of sediment contributions from the best connected glacier-fed headwater rivers and from smaller drainage basins proximal to coring sites; and c) continuing instability of both the sub-aerial and sub-aqueous portions of Lake Champagne terraces.

Bibliography of Canadian Geomorphology