Comparison of Stratigraphic Elements and Their Spatial Distribution in a Mixed Deep-Marine Continental Slope System, Windermere Supergroup, Canadian Cordillera, BC
The first Isaac carbonate (FIC) of the Neoproterozoic Windermere Supergroup is a mixed siliciclastic-carbonate unit within an otherwise siliciclastic dominated deep-water turbidite system in the southern Canadian Cordillera. Well-exposed, high-angle dipping strata at the Castle Creek (CC) and Milk River (MR) study areas, separated by ~20 km, provide a unique opportunity to compare the long-term stratigraphic trends in an ancient mixed deep-marine system and how facies, stratal architectures and stacking patterns, in addition to their areal distribution, are influenced by fluctuations in sea level, siliciclastic sediment supply and carbonate platform productivity and stability. Detailed logging and mapping at both study areas shows that the FIC accumulated during a long-term rise in relative sea-level marked by three laterally continuous 10-20 m units composed mostly of fine-grained calciturbidites and intercalated calcidebrites related to highstand shedding from a well-developed carbonate platform. Superimposed on this trend are shorter-term relative sea-level falls that formed Dm-thick channel complexes filled with amalgamated, carbonate-cemented, coarse-grained sandstone. Although the stratal make up and thickness of units is similar between the study areas, two notable exceptions occur at MR. These occur in the upper ¾ of the section and together cause the section to be ~40% thicker compared to CC -- both consist of thin-bedded siliciclastic turbidites deposited on the distal levee of active channel belts. Additionally, mass transport deposits are more common, thicker (up to ~14 m) and more laterally continuous at MR. The uniformity of the stratigraphy in the lower part of the FIC at both study areas suggests that channel belts were mobile and wandered across much of the study area. Stratigraphically higher, however, the spatial patterns of transport and deposition changed with significant bypass at CC, manifest as a thinner stratal pile, compared with significant deposition of mostly fine-grained off-axis turbidites at MR. Moreover, the commonality and well-developed nature of mass transport deposits at MR suggests enhanced slope instability and/or preservation. These spatial differences may be related to preferential transport through the CC area, which served to maintain an optimal, but comparatively lower-angle slope that promoted sediment bypass and erosion of local flow impediments like debrites, but also reduced upslope slope instability.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017