Stratigraphy, diatom biofacies, and palaeogeomorphology of a mid Holocene distributary channel system, Fraser River delta, British Columbia, Canada

Abstract

A buried distributary channel system that delivered sediment to the currently inactive southern front of the Fraser River delta during the mid Holocene is described from 32 vibracores and 403 core logs. Sedimentary properties in core and cone logs as well as fossil diatom assemblages were used to distinguish channel fill from associated deltaic facies (overbank deposits, tidal flats, and peat bogs). Active-channel fill fines upwards from a coarse base into medium-fine sand capped by thin silt beds. It contains a sparse diatom assemblage dominated by heavily silicified freshwater planktonics. The channel fill deposited during the period of diminishing flow consists of silt with thin sandy interbeds deposited during flood events. Diatoms are abundant in these deposits. The diatom biofacies is dominated by sessile freshwater species at upstream sites and by brackish-marine species at channel mouths. The transition to organic-rich silt and peat and an aerophile diatom assemblage marks channel abandonment. The palaeochannel was 500–800 m wide and 18 m deep, slightly smaller than the present main distributary channel of the Fraser River. Calibrated radiocarbon ages suggest that the palaeochannel was active from around 8000 to 6000 BP and was finally abandoned by 5000 BP.