Timothy Johnsen

Timothy F. Johnsen (2004)

Late glacial lakes of the Thompson Basin, southern Interior of British Columbia : paleogeography and paleoenvironment [M.Sc. thesis]


During the decay of the Cordilleran Ice Sheet (CIS), ~10 to 12 ka 14C BP, numerous ribbon lakes developed within the moderately deep valleys of the Interior Plateau of British Columbia. A rich geomorphic and sedimentary record of these lakes remains. This study integrates geomorphology, sedimentology, aerial photographs, differential global positioning system data, ground penetrating radar data and a digital elevation model (DEM) in a geographic information system (GIS) to (i) investigate, survey and correlate paleolake levels, (ii) reconstruct paleolake geography, evolution and environment, and (iii) reconstruct glacio-isostatic rebound.

Two definable glacial paleolake levels were identified, associated with Glacial Lake Thompson and Glacial Lake Deadman. DEMs of paleolake levels, inferred lake bottom and modern topography were integrated in a GIS to quantify lake parameters. Lakes were ribbon-shaped (width to length ratio of ~3:100), deep (~140 and ~50 m, respectively), and of significant volumes (84 and 24 km3, respectively). Glacio-isostatic tilts of these lake shorelines (1.8 - 1.7 m km-1) are among the highest measured in the world and are related to a thin lithosphere, a low viscosity mantle and rapid deglaciation. Glacioisostatic depression in the interior was likely hundreds of metres.

The sedimentary record of these lakes reflects the adjustments of a landscape undergoing deglaciation. Seventeen glaciolacustrine lithofacies were identified and record deltas, subaqueous fans, high rates of sedimentation, numerous hyperpycnal flows and a diversity of sediment dispersal and deposition processes. High sedimentation rates and numerous hyperpycnal flows suggest that ribbon lakes likely received their meltwater and sediment supply from ice remnant on the plateau.

Glacial Lake Deadman drained catastrophically with the breach of an ice dam, producing drainage bedforms and erosional surfaces within the basin, and discharging ~20 km3 of water. It is possible that this event may have triggered the failure of glacial lakes downstream or upstream in the Fraser River system. Eventually the floodwaters reached the Strait of Georgia , a distance of ~250 km. Here exotic sediments dated between ~9,200 14C yr BP and 10,800 14C yr BP may record this j√∂kulhlaup.

Full thesis PDF (56 MB)