Stratigraphic record in the transition from basin floor to continental slope sedimentation in the ancient passive-margin Windermere turbidite system

Well-exposed, vertically dipping, glacially polished outcrops of the Neoproterozoic Windermere Supergroup in the southern Canadian Cordillera include basin-floor deposits of the Upper Kaza Group overlain by slope channel complexes of the Isaac Formation. Within the 2·5 km thick Kaza and Isaac succession is an up to 360 m thick interval composed of diverse deep-water stratal elements including scour and interscour deposits, distributary channels, fine-grained turbidites, terminal splays, mass-transport deposits, erosional and levéed channels and avulsion splays, which collectively were formed during the development of an ancient passive-margin channel-lobe system. The proportion and vertical and lateral arrangement of stratal elements reveal three distinct complexes. The lower complex, consisting mostly of distributary channels and small and large scours, is interpreted to represent the detachment of lobes from an upflow levéed channel, wherein a well-developed channel-lobe transition zone was formed by efficient, siliciclastic flows during a period of sustained transport bypass and limited deposition coincident with the onset of falling relative sea level. The middle, comparatively thicker and more sandstone-rich complex, comprises distributary channel fills, fine-grained turbidites and lesser terminal splays that are interspersed with small scours, capped by a slope levéed channel filled with coarser-grained siliciclastic sediment. The abundance of basin-floor elements suggests negligible separation between the levéed channel and lobe, and therefore a poorly-developed channel-lobe transition zone, resulting from inefficient, siliciclastic-rich depositional flows that became dominant during lowstand and/or ensuing transgression. The stratal makeup of the upper complex resembles the lower detached complex, suggesting a return to efficient flows, and an abrupt change to mixed carbonate–siliciclastic sediments associated with highstand conditions. Accordingly, the stratigraphic architecture and stacking pattern of the Kaza–Isaac interval, which relate to the formation of multiple channel-lobe transition zones, were controlled by temporal changes in sediment supply and flow characteristics during the long-term progradation of the Laurentian continental margin.