DEGLACIATION OF THE CORDILLERAN ICE SHEET: EMERGING PATTERNS AND PROCESSES FROM INTERIOR BRITISH COLUMBIA

Ice thickness and bed topography are controlling factors of the patterns and processes of deglaciation. This is particularly true for ice masses, like the Cordilleran Ice Sheet (CIS), that developed over mountainous terrain.

At present, onset, growth and decay of the CIS is summarized by a conceptual model developed by Kerr (1934) and Davis and Matthews (1944). In this model, the CIS developed from cirque glaciers that filled valleys then overtopped plateaus. At Glacial Maximum, ice thickness in valleys far exceeded that of the plateaus. This ice sheet geometry affected deglaciation by favoring early disappearance of ice on plateaus and development of extensive deglacial lakes impounded by “dead ice" masses in the deep valleys.

In the Okanagan Valley, southern British Columbia, field observations do not support this model and suggest that revisions are required. Okanagan Valley is a linear, ~200km long valley incised in the southern Interior plateau of British Columbia. Tributary valleys link Okanagan Valley to the surrounding plateaus located ~ 450-500 m above. During the Wisconsin glaciation, Okanagan Valley was drowned by extensive lakes where up to 550 m of rhythmic sand and silt were deposited. Subaqueous fans and deltas at the mouth of tributaries record energetic flows and abundant sediment delivery from the plateaus.

The emerging picture hints at an ice sheet where valleys deglaciated before plateaus and where thick plateau ice remained to provide meltwater and sediment to deglacial lakes developing in the valleys. We emphasize the role of subglacial reservoirs in creating and maintaining thin ice in valleys. These reservoirs may have developed at the onset of glaciation, as ice advanced over existing water bodies. Deglaciation was facilitated by periodic drainage of these subglacial reservoirs and ice evacuation by fast flow due to ice-bed decoupling.