NEOPROTEROZOIC WINDERMERE SUPERGROUP DEPOSITION AND RIFT TECTONICS, PURCELL MOUNTAINS, SOUTHEASTERN BRITISH COLUMBIA

The Neoproterozoic Windermere Supergroup is widely exposed within the Cordillera of North America. In the Purcell Mountains, the Windermere Supergroup consists of the Toby and Horsethief Creek formations, which unconformably overlie the Mount Nelson Formation of the Purcell Supergroup. The Windermere Supergroup is thought to record rifting and passive margin subsidence associated with the break up of Rodinia and the opening of the Paleo-Pacific ocean. The age of this succession remains unclear. Recent published radiometric data on the overlying Old Fort Point Formation suggest deposition before 607.8 +/- 4.7 Ma (Re-Os age). The Toby Formation has been ascribed a ‘Sturtian age' (approx. 750-700 Ma) based on correlation with the Rapitan Group to the North but is also broadly correlative with successions to the South which were deposited between 709 and 667 Ma.

Rift-related structures are documented at several sites in the Purcell Mountains and shown to have had significant control on Windermere stratigraphy throughout the region. The Toby Formation unconformably overlies various members of the Mount Nelson Formation, with significant changes in the level of sub-Toby erosion across faults that are inferred to be of Windermere age. The Toby Formation consists of a variable thicknesses of diamictite, sandstone, mudstone and minor conglomerate, some of which are channel-fill deposits. Deposition appears to be dominated by sediment gravity flow processes in a relatively deep water setting. Evidence for a glaciomarine influence on sedimentation is limited to occasional striated and faceted clasts. Dolomicrite units and pisolites within the carbonate matrix of diamictite are locally abundant, suggesting that carbonate may have accumulated on depositional highs, at the same time that diamictite was accumulating in fault-related topographic lows. Synsedimentary faults are also shown to have a significant control on the thickness and facies of the lowermost part of the Horsethief Creek Formation. We conclude that deposition of the Toby Formation and Horsethief Creek Group in this region was primarily controlled by extensional tectonic activity, and that glaciers may have played a subsidiary role in supplying sediment to the basin during Toby deposition.