Seismic stratigraphy of Waterton Lake, a sediment-starved glaciated basin in the Rocky Mountains of Alberta, Canada and Montana, USA

Upper and Middle Waterton lakes fill a glacially scoured bedrock basin in a large (614 km(2)) watershed in the eastern Front Ranges of the Rocky Mount ains of southern Alberta, Canada and northern Montana, U.S.A. The stratigra phic infill of the lake has been imaged with 123 km of single-channel FM so nar ('chirp') reflection profiles. Offshore sonar data are combined with mo re than 2.5 km of multi-channel, land-based seismic reflection profiles col lected from a large fan-delta. Three seismic stratigraphic successions (SSS I to III) are identified in Waterton Lake resting on a prominent basal ref lector (bedrock) that reaches a maximum depth of about 250 m below lake lev el. High-standing rock steps (reigels) divide the lake into sub-basins that can be mapped using lake floor reflection coefficients. A lowermost transp arent to poorly stratified seismic succession (SSS I, up to 30 m thick) is present locally between bedrock highs and has high seismic velocities (1750 -2100 m/s) typical of compact till or outwash. A second stratigraphic succe ssion (SSS II, up to 50 m thick), occurs throughout the lake basin and is c haracterised by continuous, closely spaced reflectors typical of repetitive ly bedded and rhythmically laminated silts and clays most likely deposited by underflows from fan-deltas; paleo-depositional surfaces identify likely source areas during deglaciation. Intervals of acoustically transparent sei smic facies, up to 5 m thick, are present within SSS II. At the northern en d of Upper Waterton Lake. SSS II has a hummocky surface underlain by collap se structures and chaotic facies recording the melt of buried ice. Sediment collapse may have triggered downslope mass flows and may account for massi ve facies in SSS II. A thin Holocene succession (SSS III, <5 m) shows very closely spaced reflectors identified as rhythmically laminated fine pelagic sediment deposited from interflows and overflows. SSS m contains Mt. Mazam a tephra dated at 6850 yr BP. (C) 2000 Elsevier Science B.V. All rights res erved.