THE SEDIMENTARY RECORD OF MOOSE LAKE - IMPLICATIONS FOR GLACIER ACTIVITY IN THE MOUNT-ROBSON AREA, BRITISH-COLUMBIA

The postglacial sedimentary environment of Moose Lake was investigated using 3.5 kHz subbottom profiles and cores from the sediment surface. A late phase in the deglaciation of the upper Fraser River valley involved a valley glacier that flowed east to west and that terminated near the present-day outlet of Moose Lake. The overdeepened Fraser valley was infilled with ice-contact sediments, including terminal and recessional moraines, thereby damming westward drainage and forming Moose Lake. Eastward glacier retreat resulted in the deposition of ice blocks in the shallow west zone of the lake, where subsequent melting disturbed conformable glaciolacustrine deposits. The main Moose Lake basin records a transition from proximal to distal proglacial environments. However, late glacial and postglacial sediments are dominated by settling from suspension. Coarse, but localized, sediment inputs are derived from the Thunder Creek fan-delta and avalanche activity on the north-facing slope of the south shore. Depths to major acoustic reflectors and known modern accumulation rates of 2.7 mm . a-1 indicate that the upper 15.5 m of sediment fill represents deposition during the Holocene and accounts for less than one third of the total late glacial and postglacial sediment yield. Sedimentary structures visible in cores and C-14 dating confirm the varved nature of the upper 3 m of sediment. Extreme positive departures in varve thickness are related to above-average discharge to Moose Lake following persistent, glacier-melt-augmented summer flows. The Moose Lake varve chronology confirms the general pattern inferred from other regional evidence of climate deterioration after 1600 AD, which led to glacier advance and moraine emplacement in the Canadian Rocky Mountains.