--> Protracted Processes of Erosion, Mass-Wasting and Deposition at the Margin of a Slope Valley-Scale Deep-Water Conduit, Cretaceous Geoffrey Formation (Nanaimo Group), British Columbia, Canada

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Protracted Processes of Erosion, Mass-Wasting and Deposition at the Margin of a Slope Valley-Scale Deep-Water Conduit, Cretaceous Geoffrey Formation (Nanaimo Group), British Columbia, Canada

Abstract

Submarine canyons and slope valleys represent significant pathways for coarse-grained sediment transfer across seascapes, providing a critical link between continents and the deep-sea. Abundant material is bypassed basinward, or deposited locally, recording protracted sediment transfer processes. From seafloor and seismic-reflection data, the edges of these conduits are evidently shaped by varied processes including erosion, mass-wasting and in some instances, onlap of fine-grained terrace or inner levee deposits. The stratigraphic expression of canyon or valley walls is presumably characterized by composite surfaces shaped by these innumerable and varied gravity flow events. To consider this hypothesis, the transition from conduit axis to edge is examined in outcropping deep-sea deposits of the Nanaimo Gp. on Hornby Island, BC. The present-day morphology of Hornby Island is shaped by resistant submarine conduit conglomerate and sandstone at its core, and corresponding recessive fine-grained margin deposits along its intertidal shorelines. The paleo-conduits, recorded by >500 m of gross strata, trend SW-W. An outcrop on the south side of the island at Downes Point features the transition from conduit axis to margin. The transition occurs over 500 m laterally and through 110 m of stratigraphy. It reveals a series of features consistent with protracted sediment transfer processes: (1) steep edges of conglomeratic channel fill punctuated by steps that record shifts of the formative channel over time; (2) strata between successive conglomerates dominated by lenticular thin- to thick-bedded sandstone and mudstone associated with scours and barforms; (3) locally abundant remobilized sedimentary blocks up to 30 m in diameter in a zone 125 m wide and 40 m thick present beneath and between channel fill and the conduit edge. The blocks originated from retrogressive slumping of a tabular sandstone package into the conduit, perhaps an inner levee. A series of near vertical failure planes are apparent, with the most axial evidently an important source of slump blocks. Towards the conduit margin, these failure planes are associated with progressively less deformed and rotated sedimentary units. This high-resolution perspective sheds critical insight into the meaning of the stratigraphic record, validating a complex history of sediment transfer, including erosion, mass failure, sediment bypass and deposition, at the composite margin of large-scale submarine conduits.