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Stratigraphic Architecture of a Large-Scale Deep-Water Slope Conduit Margin, Nanaimo Group, Hornby Island, Canada


Large conduits on continental margins focus sediment transfer to the deep-sea, characterized by protracted erosion and sedimentary bypass. In the axis of these conduits, erosion is most significant and amalgamated, coarse-grained channel fill deposits dominate the resultant stratigraphic record. Conversely, at the margins of these conduits, the preservation potential for a more complete record of erosion, sediment bypass, and deposition is higher. In order to consider the spectrum of processes that persist within long-lived deep-sea slope conduits, deposits of the Nanaimo Group (Late Cretaceous), Hornby Island, western British Columbia, are examined. Hornby Island is located within the Georgia Strait, which separates Vancouver Island from mainland British Columbia. The central portion of the island consists of a slope conduit axis, including extensive conglomerate and sandstone deposits that collectively compose a channelform sedimentary body that is approximately 150 m thick and 6000 m wide. The northwest and southeast edges of the island are largely defined by the lithologic transition from coarse-grained channel axis deposits to more heterolithic conduit margin units. Excellent exposures of the conduit axis to margin transition are exposed in the intertidal realm along the edges of the island, which are the focus of this research. Fifty measured sections, numerous photomosaic interpretations, and differential GPS surveys of stratigraphic surfaces were collected to help detect subtle changes in architecture. Measurements of imbricated clasts, sole marks and current ripples reveal that paleoflow was largely directed to the southwest. The conduit is characterized by a composite fill that consists of numerous channelform bodies 5-15 m thick. Clast-supported conglomerate, thick-bedded sandstone, thin-bedded sandstone and mudstone, and chaotically bedded units are the primary facies present; concave-up erosion surfaces range in scale from 1 m deep and 5 m across, to >10 m deep and 100s of m across. Thick successions of chaotic mass-transport deposits, including rafted sediment blocks up to 600 m2, characterize the conduit margin and floor locally. The complex stratigraphic architecture and facies arrangement is comparable, in some respects, to that of slope valley fills characterized in subsurface data from numerous petroliferous continental margins.