ANATOMY AND EVOLUTION OF A SLOPE CHANNEL (ISAAC FORMATION, NEOPROTEROZOIC, CANADIAN CORDILLERA)

A detailed architectural analysis of a slope channel system (Channel 5) in the Neoproterozoic Isaac Formation was conducted along a superbly exposed 3.5 km-wide section in the southern Canadian Cordillera (east-central B.C., Canada).

Channel 5 is 75-100 m thick and consists of three vertically stacked channel complexes (8 to 30 m thick), each bounded by a high-relief erosion surface. Channel complexes are interbedded with laterally extensive (> 3 km) fine-grained units (<11 m thick) and debris-flow deposits (< 8 m thick) containing shallow-water carbonate clasts. Channel complexes consist of 4 kinds of channel-fill elements (CFE), each composed of a unique assemblage of facies associations and internal geometry. Amalgamated elements comprising thick-bedded Bouma Ta beds are the most common, and are interpreted to represent channel-axis deposition. Irregular CFE with abundant mud-clast breccias and dune cross-stratification indicate periods of dominant tractive flows. Parallel-sided and accretionary CFE consist of Ta divisions interstratified with Tbcde/Tcde turbidites and, respectively, are the product of weakly confined sedimentation (lobe formation?) and deposition on the inner bend of a laterally accreting channel.

Each channel complex was initiated by an episode of deep (up to 12 m) incision. Locally, remnants of debris flow deposits above the erosion surface suggest that incision may have coincided with an episode of shelf and/or upper slope instability. Subsequently, changes in flow parameters (decrease in flow size or increase in sand/mud ratio) triggered accumulation within the active channels. Although deposition of coarse sediment dominates the axis of channels, the variety of lateral and superjacent CFE illustrates the temporal and spatial complexity of channel-complex evolution. Cessation of sand-rich sediment gravity flows (updip avulsion?) led to accumulation of a sheet of upper division turbidites indicating channel-complex deactivation. Subsequent rejuvenation of the main sediment-transport pathway led to the development of the next channel-complex.

Results of this study have important implications for better understanding the evolution and complexity of slope channels and can help better characterize sand distribution and continuity in slope channel reservoirs.