The record of Neoproterozoic glaciation in the Windermere Supergroup, southern Canadian Cordillera
The Neoproterozoic Windermere Supergroup (WSG) is exposed over an area of 35,000 km2 in the southern Canadian Cordillera, and consists primarily of deep-marine meta-sedimentary rocks interpreted to have been deposited during rifting and subsequent post-rift thermal relaxation. The main exposures of the WSG occur within thrust panels and structural culminations of the eastern Cordilleran orogen. Within the thick stratigraphic succession (~9 km) are three units of glacigenic affinity: Toby, Vreeland and Old Fort Point (OFP) formations. The Toby Formation is composed of up to 2500 m of diamictite, interbedded with conglomerate, sandstone, mudstone, carbonate and mafic volcanic rocks. The Vreeland Formation ranges from 350 m to 2000 m thick and consists of diamictite, interbedded with mudstone, sandstone and conglomerate. The OFP ranges from 60 to 450 m in thickness and consists of a distinctive three-fold stratigraphic package of basal siltstone grading upward into limestone-siltstone rhythmite, organic-rich mudstone and an overlying heterolithic unit of diamictite, breccia, conglomerate, sandstone, siltstone to mudstone and limestone. A locally prominent unconformity marks the base of the OFP upper member. Both the Toby and Vreeland formations represent remobilized glacially-derived marine sediments deposited by sediment-gravity flows. Deposition of the Toby Formation was faultcontrolled during an active tectonic phase (rifting) whereas the Vreeland Formation accumulated during the subsequent quiescent phase (post-rift) with limited structural control. The OFP is interpreted to be a regionally extensive deep-marine post-glacial marker temporally associated with the glacigenic Vreeland Formation. Although direct geochronological ages for WSG units in southwestern Canada are generally absent, high-precision radiometric ages of underlying and overlying igneous events constrain the relative maximum and minimum timing of deposition from ca. 740-728 Ma to ca. 569 Ma. At the base of the WSG succession, the Toby Formation may be as young as ca. 685 Ma based on ages obtained from potential stratigraphic correlatives in the United States. There is no direct age constraint for the deposition of the Vreeland Formation; its minimum timing is based on its stratigraphic relationship with the post-glacial OFP. The middle member of the OFP was precisely dated at 607.8 ± 4.7 Ma using the Re-Os method placing it in the Ediacaran Period. Published geochemical and stable isotopic data are similarly limited for all three units with only some 34Spy values available from one section of the OFP. Recent work has focused on detailed sedimentologic and stratigraphic studies of the Toby and the OFP formations with future efforts being directed towards integrated geochemical and isotopic research. Additional geochronological constraints are needed to refine palaeogeographic models and strengthen regional correlations with other North American Cordilleran glacigenic units.