Lifecycle of a Deepwater Depositional Lobe in the Windermere Supergroup, B.C., Canada: Correlating Architectural Elements in Space and through Time
Basin-floor strata crop out well in periglacial, kilometer-scale exposures of the Kaza Group, Neoproterozoic Windermere Supergroup, southern Canadian Cordillera. Here we compare and contrast the distribution and relative abundance of the different stratal elements that populate the proximal and more distal parts of distributive sand-rich basin floor systems, termed lobes, which then stack to form a larger scale, prograding basin floor fan.
Depositional lobes comprise an assemblage of architectural elements, including scours, matrix-rich sandstone units, feeder channels, distributary channels, terminal splays, inter- and intra-splay fine-grained turbidites. These elements are systematically and predictably arranged, both spatially (along a single depositional transect) and temporally (stratigraphically upward).
Lobes typically became initiated by channel avulsion, and hence deactivation of another pre-existing lobe. In the proximal part of the system scours up to several meters deep, several tens of meters wide are interpreted to have formed by erosion within a hydraulic jump downflow of the avulsion node. Erosion also formed the common large, tabular mudstone clasts, and charged the flow with fine-grained sediment. Further downflow rapid expansion and consequent deposition from successive flows built-up dm- to several meter-thick units consisting of distinctively matrix- and mudstone-clast-rich beds. These units are significantly more common in the proximal basin floor, and commonly are observed beneath distributary channel and terminal splay deposits, and indicate activation of sedimentation, at least locally.
Successive flows then exploited the basin-floor topography and on the proximal basin-floor carved an up to 15 m-deep feeder channel. It then expanded downflow into a network of shallow (several m deep) distributary channels, which eventually terminate in sandstone-rich terminal splays, each a few up to about 10 m thick. Both these elements commonly overlie the earlier deposited matrix-rich beds. During the lifetime of a single lobe the feeder channel remains fixed, but the downflow distributary channel network and its associated terminal splays wander, causing them to be intercalated vertically. Eventually an upstream avulsion shuts off local sediment supply, causing a new lobe to be initiated elsewhere on the fan, and the processes outlined above repeated.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California