FLOW PATTERNS AND GLACIAL DISPERSAL PATTERNS IN THE EASTERN LAURENTIDE ICE SHEET - IMPLICATIONS FOR SUBGLACIAL PROCESSES AND ICE STREAM DEVELOPMENT

Our reconstruction of ice dynamics in the central regions of the Eastern Laurentide Ice Sheet (ELIS) rests on three main lines of evidence: (1) ice-flow direction changes recorded by crossing sets of glacial striae and landforms, (2) polyphase glacial dispersal trains, and (3) temporal constraints imposed by the global ice volume record and by numerical modeling of drift dispersal data.

Flow re-orientations in the core region of the ELIS commonly reached 90°, but were somewhat less in the inner peripheral regions. In the outer peripheral regions of the ELIS, sudden and large flow pattern changes were produced by ice streams that developed episodically in major regional troughs and in smooth bed regions. Our reconstruction of ice-flow systems shows that major reorientations and migrations of ice divides and outflow centers were driven by changes in snow accumulation patterns as well as by ice-stream activity.

In central Québec, large dispersal trains derived from Proterozoic outliers provide a unique record of polyphase ice dynamics and glacial transport. These indicators, consisting of distinctive red sandstone and orange quartzite clasts, were successively dispersed along three main ice-flow directions. They form regional-scale composite dispersal trains consisting of long ribbon-shaped trains and palimpsest trains. The latest westward glacial movement, presumably LGM and post-LGM in age, as well as the late reorientation toward NE each produced ribbon-shaped trains extending over 200 km in opposite directions. Dispersal trains associated with earlier glacial movements were reworked during subsequent movements but are detectible in the form of palimpsest trains. A 150 km-long dispersal train derived from one of the outliers straddles the final position of the ELIS divide, demonstrating that the ELIS central divide migrated westward at least 120 km following its LGM position and prior to the late-glacial reorientation.

These dispersal trains, as well as several others from across Quebec-Labrador, provide insights on the peregrinations of the main ice divide of the ELIS and on subglacial bed conditions during the last glacial cycle. The combined record of glacial landforms and clastic dispersal data indicates that ice stream development was controlled not only by topographic depressions but also by vast smooth bed areas.