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A Synthesis of Depositional Trends In Intertidal and Upper Subtidal Sediments Across the Tidal–Fluvial Transition In the Fraser River, Canada
Sedimentological, neoichnological, palynological, and geochemical trends from upper subtidal and intertidal positions on channel bars in the lower Fraser River, Canada are synthesized into a single, coherent framework. From these data we define criteria for determining depositional position in shallow water depths in tide-influenced rivers. Three sedimentological trends are observed from the river mouth, through the locus of mud deposition (within the turbidity maximum zone (TMZ)), and into the freshwater-tidal zone. (1) The recurrence (per meter) and thickness of mud beds increase towards the TMZ and tapers in both the landward and seaward directions. (2) Muddy current ripples and graded current ripples are most abundant in the TMZ; they are less common with decreasing brackish-water influence and are absent in the freshwater river reach. (3) Heterolithic bedding (i.e., flaser, wavy, and lenticular) is common in the TMZ, less common seaward, and absent from the freshwater realm. In addition to the sedimentological trends, four ichnological trends parallel decreasing water salinity. With decreasing salinity, there is: (1) a decrease in bioturbation intensity from BI 2–3 to BI 0–1; (2) a decrease in the abundance of bioturbated beds; (3) a marked decrease in the diversity of traces from 5–6 forms to 1–2 forms; and, (4) a decrease in the diameter and length of traces. Traces are rare to absent in the tidal freshwater zone. Palynological and geochemical trends generally follow ichnological trends but are less obvious. Neither dinocyst abundance nor geochemical signature can be used to determine relative position in a tide-influenced river channel, although dinocyst abundances greater than 1% indicate significant marine influence, and 0–1% marine dinocysts indicate tidal influence.
Although it is not feasible to determine exact depositional position within the tidal–fluvial transition, our results suggest that it is possible to determine where sediments were deposited relative to the TMZ. In turn, predicting relative depositional position can assist in unraveling stratigraphy and in recognizing nested channels in architecturally complex sedimentary successions.
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