CARBONATE-BEARING TILLS ON THE SOUTHWESTERN CANADIAN SHIELD: IMPLICATIONS FOR THE DYNAMIC EVOLUTION OF THE LAURENTIDE ICE SHEET

Discontinuous carbonate-bearing tills overlie a significant portion of the Canadian Shield southwest of the Hudson Bay Lowlands (HBL) in northern Ontario. These tills are commonly interpreted to have been deposited during Late Wisconsin recession of the Laurentide ice sheet (LIS). However, an alternate explanation is that they were deposited significantly earlier in the glacial cycle and represent erosional remnants of a larger, more continuous subglacial carbonate-bearing till sheet. The Ontario till contains abundant Paleozoic carbonate and Proterozoic greywacke derived from the HBL and displays features consistent with a deforming bed origin. This suggests early advance of the Laurentide ice sheet out of the HBL was characterized by advection of soft sediment as a deforming subglacial layer. Tills predating and correlative with the last glacial maximum (LGM) in central Minnesota contain an abundance of HBL carbonate and greywacke clasts indicating long-distance transport by a deforming layer potentially as far as 1000 km from the HBL. Continuity of this deforming layer was dependent on sufficient flux of sediment from the HBL. Failure of deforming layer continuity as the LIS approached its maximum extent caused a transition to hard bed conditions in the interior portion of the ice sheet and consequent thickening and volumetric increase of the ice sheet, while the ice margin remained relatively stable or even receded. After long-distance sediment transport ceased, remobilization of subglacial drift around the LGM was followed by a transition during recessional phases of the LIS to short-distance transport of material eroded from crystalline basement rocks. During interglacial periods the unconsolidated sediment reservoir in the HBL was replenished allowing the sediment transport cycle to repeat with subsequent glaciations. Preglacial weathering regolith developed on crystalline shield rocks augmented sediment flux from the HBL during early glaciations. The progressive removal of regolith resulted in earlier initiation of hard bed conditions in successive glacial cycles. Although soft sediment deformation continued to play an important role early in all glacial cycles, ice sheet thickening and volumetric increase are favored earlier during later glaciations.