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Hydrodynamic Controls on Channel Bar Sedimentation Across the Tidal-Fluvial Transition, Lower Fraser River, BC, Canada


Sedimentation in the lower Fraser River is controlled by river flow and tidal flux, and the degree of saltwater-freshwater mixing. Local hydrodynamics in each channel determine the distribution of sand and mud, and the nature of bioturbation on channel bars. Vibracores, box cores, and sediment samples were collected from 9 bars. Bars were grouped into 1 of 3 zones that share similar hydrodynamics and salinity: tidally influenced freshwater zone, tidally influenced freshwater to brackish-water transition zone, and mixed tidal-fluvial with sustained brackish-water zone. Data was collected to identify sedimentological and ichnological characteristics that define channel-bar deposits within each zone, and that can be used to predictor depositional environments for similar deposits in the rock record. In the freshwater-tidal zone, bars dominantly comprise sand. Sands are m-scale, and consist of current- or climbing-ripples. Mud beds are rare, and muddy bedsets rarely exceed 5 cm thick. Bioturbation ranges from BI 0–1, and Planolites, in mud beds, is the only trace present. Near the freshwater to brackish-water transition, bars are mud dominated, and muddy bedsets range from cm- to m-thick. Discontinuous cm-scale sands are sporadic within the muds. The muds may reflect deposition from energetic currents (i.e. with high suspended-sediment concentrations and/or flocculation). Mud on ripple foresets possibly indicates bedload floccule transport. Bioturbation varies from BI 1–3, is sporadic, and consists of Polykladichnus, Skolithos, and Arenicolites. In the mixed tidal-fluvial zone with sustained brackish-water, bars have subequal sand and mud. Sands are dm- to m-scale, whereas muds are cm-scale. Current ripples, with common mud flasers characterize the sands. Muds are bioturbated or laminated. Bioturbation ranges from BI 0–3, consisting mainly of burrows in the mud beds (e.g. Skolithos, Arenicolites, Polykladichnus, and Planolites). A process-response facies model is proposed that links the sedimentology and ichnology of channel bars to the hydrodynamic conditions across the tidal-fluvial transition. Herein, we identify characteristics that reflect increasing tidal and brackish-water influence, including: 1) increasing bioturbation intensity; 2) increasing size and diversity of traces; 3) increasing thickness and lateral continuity of mud beds towards the brackish-water to freshwater transition; 4) increasing depositional cyclicity between sands and muds.