--> --> Tidally Influenced Asymmetric Deltas: A Case Study of the Fraser River Delta Front and Prodelta, British Columbia, Canada

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Tidally Influenced Asymmetric Deltas: A Case Study of the Fraser River Delta Front and Prodelta, British Columbia, Canada

Abstract

The Fraser River Delta displays asymmetry in sediment, burrow, and infaunal distributions on the delta front, and this reflects asymmetry in the magnitude of flood and ebb tidal currents. The updrift delta front (relative to the main distributary channel of the Fraser River) comprises sand-dominated beds, and displays low infaunal diversities (H` 0.9–1.5) and low bioturbation intensities (BI 0–3). Preserved sedimentary structures include parallel and low angle laminations, gas-escape structures, and soft-sediment deformation. The downdrift delta front comprises muddy bedsets that are intensely bioturbated (BI 3–6) and display a diverse community of infauna (H` 1.4–2.3). Where preserved, sedimentary structures in muddy sediments include parallel lamination and some soft-sediment deformation. The asymmetric character of the Fraser River Delta is sedimentologically similar, but ichnologically and biologically dissimilar to mixed river- and wave-influenced asymmetric deltas, and hence represents a distinctive form of delta asymmetry. The updrift delta front is sand-dominated, and the downdrift delta front is mud dominated; this trend is comparable to that of wave-dominated asymmetric deltas. However, infaunal and burrow distributions show a trend opposite to that of mixed river- and wave-influenced asymmetrical deltas in that the biology, trace diversity, and infaunal density are higher on the downdrift side of the delta front, not the updrift side. Utilizing these results, we propose a new model for tidally asymmetric deltas that considers tides as the main controlling mechanism on asymmetry. This model may have significant application to the rock record, because tides are not depth dependent and can control sediment distribution in deep-water settings. Waves are limited to the fair-weather wave base (∼30 m). Consequently, tidally influenced asymmetric deltas can potentially develop in deep-water settings and therefore, may be more common than the mixed river- and wave-influenced counterparts. Moreover, in settings where the vertical thickness of an asymmetric delta is greater than the depth of storm-wave base, it is possible for the upper and lower delta front to exhibit distinctive sedimentological and / or ichnological trends.