THE MISSING MORAINES OF THE SOUTHERN FRASER PLATEAU: GLACIODYNAMIC INFERENCES AND IMPLICATIONS FOR CORDILLERAN ICE SHEET RETREAT IN SOUTH-CENTRAL BRITISH COLUMBIA, CANADA

Fundamental differences between reconstructions of lateglacial decay of the interior Fennoscandian (FIS) and Cordilleran (CIS) ice sheets (fronto-marginal retreat (FIS) vs. large-scale regional stagnation (CIS)) have received considerable support from the perceived absence of large recessional moraines in interior British Columbia (BC). However, recent availability of higher resolution data for BC’s southern Fraser Plateau has allowed documentation of significant linear ridge systems interpreted to be moraines. We map, classify and interpret four moraine types based on their geomorphic context, composition and sedimentary architecture as identified in exposures and geophysical surveys. The orientation of over-ridden push moraines (1) corroborates ice advance from the northeast. Near-pristine hill-hole pairs (2) organized in subparallel sets across some areas of higher relief, and the presence of push moraines (3) in valleys of the southern Fraser Plateau are indicative of glaciotectonism associated with episodic ice-margin oscillation and suggest lateglacial decay included some component of orderly and active ice-margin retreat. Grounding-line moraines (4) formed across valleys occupied by ice-marginal lakes during ice decay. Moraine systems 2-4 define northwestward ice retreat across the southern Fraser Plateau. Such a systematic regional retreat is consistent with nearby trends in glacioisostatic adjustment, the spatial pattern of ice-marginal lake evolution on the plateau, and recent inferences of the lateglacial reconfiguration of the CIS based on the pattern of ice-marginal channels. Overall, the picture of lateglacial ice sheet decay across the southern Fraser Plateau emerges as one in which oscillations of an active ice margin combined with general ice-surface lowering to efficiently contribute to northwestward ice margin retreat.