The Neoproterozoic succession of the central Rocky Mountains, Canada

Abstract

Abstract The Neoproterozoic succession exposed in the central Canadian Rocky Mountains comprises a thick (2 to 5 km) sequence of marine clastic rocks deposited after major subsidence associated with continental scale rifting. A major transverse south to north change in lithofacies separates the succession into southern more distal and northern more proximal sequences. The southern sequence (Miette Group) comprises the sandstone-conglomerate dominated McKale Formation (new) overlain by the argillite and siltstone dominated East Twin Formation (new). Locally, black pyritic slate and limestone of the Cushing Creek Formation (new) occurs at the base. The Cushing Creek Formation was deposited in a deep euxinic basin with only minor sandstone turbidite deposition. The lower part of the McKale Formation was deposited in basin-floor turbidite complexes, whereas the upper part, with its thick interlayered argillite intervals and uncommon slump structures, was deposited in lower slope environments. A thick varicoloured argillite unit separates the lower and upper McKale Formation in northeastern exposures. This unit, mappable into the lower part of the Framstead Formation and correlative with the Old Fort Point Formation, records an abrupt reduction in the supply of clastic material associated with the termination of glaciation. The East Twin Formation represents a transition from slope sedimentation at the base to shallow-marine sedimentation at the top. It records the reduction of the supply of coarse clastic material into the basin and the filling of the Neoproterozoic basin. The northern sequence (Misinchinka Group) comprises in succession: diamictite of the Vreeland Formation (new); argillite with local carbonate olistoliths, medial sandstone to conglomerate and, in the east, basal interbedded sandstone and argillite of the Framstead Formation (new); carbonate of the Chowika Formation; and argillite and siltstone of the Cut Thumb Formation (new). Locally interbedded siltstone, argillite and sandstone of the Paksumo Formation (new) occur at the base of the exposed succession. The Paksumo, Vreeland Formation and basal part of the Framstead Formation record glacially influenced slope sedimentation. The Paksumo Formation and basal part of the Framstead Formation are primarily turbidite deposits, whereas diamictites of the Vreeland Formation are primarily resedimented mass flow and glaciogenic ‘rain-out’ deposits. The lower part of the Framstead Formation records an abrupt reduction in the supply of clastic material associated with a post-glacial eustatic sea level rise. Large olistoliths of shallow-water carbonate in both the lower and upper parts of the Framstead Formation indicate deposition in a slope environment and instability of the adjacent carbonate platform. Sandstone to conglomerate turbidite-filled channels in the middle part of the Framstead Formation record a relative sea level drop and a temporary breakdown of the adjacent carbonate platform. The return to fine clastic slope sedimentation in the upper part of the Framstead Formation records a relative sea level rise. The overlying Chowika carbonate platform records a dramatic reduction in clastic sediment supply and a relative sea level drop. The abruptly overlying Cut Thumb Formation represents a relative sea level rise and a change to deeper water siliciclastic shelf sedimentation that is not reflected in the south. Tectonism near the end of Neoproterozoic Windermere sedimentation resulted in the inversion of part of the initial deep basin-floor to form an outboard high on the west and in the reactivation of the transverse facies-change zone, as a north-side-down rather than south-side-down feature.