Stratigraphy and Architecture of Distal Basin-Floor Strata, Neoproterozoic Middle Kaza Group, British Columbia, Canada

Excellent exposures of the Middle Kaza Group provide a unique opportunity to study basin-floor terminal-splay/lobe deposits of the Windermere turbidite system. The stratigraphy of these basin-floor strata is characterized by three vertically stacked units: “precursor beds”, sandstone units, and laterally-continuous, fine-grained strata. Slumps/slides and debrite complexes, which are common elements in slope and proximal basin-floor strata, are absent, and thus consistent with a more distal basin-floor setting.

Precursor beds typically form packages 1-5 m thick consisting of massive, laterally continuous, medium-bedded, matrix-rich, medium-grained sandstone. These strata are interpreted to be deposited down flow of a hydraulic jump and represent the early stages of local lobe development and composed of sediment sourced from an adjacent, soon-to-be-deactivated lobe complex.

Precursor beds are abruptly overlain by a 5-55 m thick sandstone unit consisting of stacked, crudely graded sandstone beds up to 3 m thick. Extensive lateral continuity and homogeneity of these beds suggests that they were deposited from unconfined flows in a terminal-splay complex. About half of the sandstone units become finer-grained and more interstratified with thin-bedded turbidites upward, whereas others show little upward change. This dissimilarity probably reflects differences in position (distal versus proximal) on the lobe in relation to the main feeder channel system.

Sandstone units are sharply overlain and completely separated by laterally-continuous, fine-grained units 5-35 m thick. Typically, they consist of thin- to medium-bedded, very-fine sandstone turbidites interbedded with medium- to coarse-grained sandstone beds, and indicate local(?) abandonment of the lobe system due to upstream channel avulsion and lobe shifting.

Architecturally the three stacked units are continuous laterally for over 800 m, and are repeated 14 times in the exposed strata. The repetitive and consistent stacking of the three stratal units illustrates the dynamic, yet systematic nature of basin-floor lobe deposition.