The depositional history of the bloomington complex, an ice-contact deposit in the Oak Ridges Moraine, Southern Ontario, Canada

Abstract

The Oak Ridges Moraine is a major physiographic feature of south-central Ontario, Canada, that is made up of a number of distinct depositional elements. One of these elements, the Bloomington complex, is a body of sand, gravel and diamict that was deposited largely within an east-west trending interlobate lake basin that formed as the continental glacier split into two lobes: a northern lobe termed the Simcoe lobe and a southern lobe termed the Ontario lobe. The oldest exposed sediment in the Bloomington complex is a sequence of stratified sand and gravel that is interpreted to have been deposited as coalesced, subaqueous outwash fans that developed where several glacial meltwater conduits emptied into the proglacial lake. These deposits are locally sharply incised by stratified sand and gravel that is interpreted as a glaciofluvial deposit, laid down when the lake basin drained as ice margin retreat caused outlets to open along the Niagara Escarpment, 60 km to the west. The fan complex and glaciofluvial deposits are overlain, in turn, by stratified sand, silt and clay that was deposited within the lake basin after the ice margins readvanced, closing the drainage outlets and allowing the lake to rise. The glaciolacustrine deposits pass gradationally upward into a heterogeneous deposit of sand, silt, clay and diamict that is interpreted as an ice-marginal deposit of the last stages of closing of the lake basin as the northern and southern ice lobes converged. Finally, a cap of diamict covers the area, deposited as glacial ice, advancing from the south, over-rode the study area. The complete package of sediment that makes up the Bloomington complex illustrates the effects of fluctuating ice margins on proglacial deposition in a glaciolacustrine setting.