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The Topset and Upper Foreset of the Modern Fraser River Delta, British Columbia, Canada
The Fraser River delta is the largest on the west coast of Canada. Deposits of the delta are entirely Holocene in age and have a maximum known thickness of 305 m. The topset of the delta varies from 10 to 40 m thick. It is dominated by a nearly continuous unit of sand that is 8 to 30 m thick and consists of one or more decametre-scale sharp-based fining-upward sequences. The sand unit underlies the upper delta plain and parts of the lower delta plain, and it is interpreted to be a complex of distributary channel deposits. It is gradationally overlain by finer sediments that were deposited in tidal flat, abandoned channel and floodplain environments. The distribution and geometry of the topset deposits have been controlled by the interaction of tidal and fluvial processes and the rise of sea level during the mid-Holocene. The upper foreset deposits (<50 m) consist of interbedded and interlaminated silts and sands that dip up to 7° seaward. The upper foreset is dominated by silts, although localized sand-dominated units occur and were deposited close to active river mouths. The silts and sands of the upper foreset are commonly organized into metre-scale coarsening-upward sequences that are interpreted to represent annual deposits. Most of the foreset deposits were deposited from suspension from plumes of sediment-laden fresh water discharged from river mouths, the suspended sediment concentration of which was regulated by tidal and fluvial processes. The sand-dominated units also include metre-scale fining-upward sequences interpreted to be sediment gravity-flow deposits.
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