Shelfbreak gullies; Products of sea-level lowstand and sediment failure: Examples from Bowser Basin, northern British Columbia

The transition from cyclothemic shelf to shale-dominated slope deposits (Ca llovian to Oxfordian) that accumulated dong the northern convergent margin of Bowser Basin is delineated by spectacular, conglomerate-filled channels, or gullies, The gullies, representing the shelf-slope break, formed in two stages: (1) Slumping of upper slope-outer shelf sediment created topograph y, which determined (2) the course of fluvial, distributary-type channels a nd focused gravel-dominated sediment transport during relative sea-level lo w-stands. Gully fill consists predominantly of debris-flow deposits, locall y incorporated into small (lowstand) fan deltas that prograded from gully m argins, Lowstand fluvial channels overlie shelf cycles, incise the underlyi ng highstand deposits, and are overlain by fossiliferous, transgressive san dstone. However, the shelfbreak gullies deposits are up to 10 times thicker than those of the associated lowstand fluvial channels. Although the lowstand fluvial channels likely acted as bypass conduits for gravel and sand during times of low sea level, the initiation and "overdeep ening" of the shelfbreak gullies by slumping probably took place during bot h high and low relative sea levels. Deposition within gullies may also have continued during relative highstands, from sediment in storage on the oute r shelf. The implications for sequence stratigraphic models are: shelf-break gullies can form at any stage of sea level fluctuation; gully thickness may not be related to the magnitude of lowstand incision on the adjacent shelf; and g ullies formed by sediment gravity failure do not provide quantitative infor mation about the magnitude of sea-level fall.