Using Regional and Local Stacking Patterns of Architectural Elements to Model Internal Lobe Architecture in a Deep-Water Basin-Floor Fan: Windermere Supergroup, B.C., Canada
Terlaky, Viktor and Arnott, R. William C.
Basin-floor strata crop out well in periglacial, kilometer-scale exposures of the Kaza Group, Neoproterozoic Windermere Supergroup, southern Canadian Cordillera. Seven architectural elements are identified in these strata, including: scours, avulsion splays, feeder channels, distributary channels, terminal splays, debrites and fine-grained turbidite units. These architectural elements are defined by their basal contact, cross-sectional geometry and internal facies distribution. The characteristics of architectural elements vary little from proximal to more distal settings, but their relative abundance does. Here we describe the stacking pattern and relative abundance of architectural elements through the proximal and more distal parts of a distributive sand-rich basin-floor system, which then forms the basis for modeling the internal architecture of larger scale elements (termed lobes) on the basin floor.
Lobes typically comprise an assemblage of all, or subset of the abovementioned architectural elements, which then are systematically and predictably arranged in both space (along a single depositional transect) and time (stratigraphically upward). Lobes typically became initiated by channel avulsion. 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 down flow of the avulsion node. Erosion also formed the common large, tabular mudstone clasts, and charged the flow with fine-grained sediment. On the lateral margins and down flow of the energetic jet flows dm- to several meter-thick units consisting of distinctively matrix- and mudstone-clast-rich beds were deposited (avulsion splays). 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, which then fed the downflow depositional lobe. At the mouth of feeder channels flows expanded into a network of shallow (several m deep) distributary channels, which distally shallow and widen until they merge into sandstone-rich terminal splays, each a few up to about 10 m thick. During the lifespan 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 #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013