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Sedimentation on high‐energy sand flats in the Bay of Fundy: The record of tidal‐bore activity and deposition from high‐concentration suspensions of sand

The Cobequid Bay – Salmon River estuary, Bay of Fundy, does not possess particularly large tidal bores (maximum ca 30 cm). Nevertheless, detailed logging of 13 peels made from cores spread throughout the entire area influenced by tidal bores indicates that inferred tidal‐bore deposits comprise approximately 15% of the deposits in the upper‐flow‐regime sand flats in the inner part of the estuary, averaging more than 10 tidal‐bore layers per metre of core. A landward increase in tidal‐bore height is accompanied by an increase in the number and thickness of the tidal‐bore layers. The deposit of a single tidal bore begins with structureless sand (from <1 cm to >10 cm thick) that can be gradationally overlain by sand with indistinct parallel lamination, which in turn passes upward into the well‐defined parallel lamination that is the dominant sedimentary structure in these sand‐flat deposits. The bases of the structureless layers range from prominently erosional to passively draping over the ripples formed during the preceding ebb tide. The structureless and indistinctly laminated layers are interpreted to be the deposit of bore‐generated hyperconcentrated suspensions of sand that have similarities with turbidity currents in which near‐bed turbulence is extinguished or damped. The tidal‐bore Froude number (Frtb), which determines the nature of the tidal bore, together with the height of the tidal bore, influences the amount of sand in suspension, and presumably also controls the nature of the deposits. Tidal bores with high Froude numbers (Frtb = 2 to 3) that are breaking create deposits that are analogous to the deposits of high‐concentration turbidity currents; most bore deposits documented to date from other ancient and modern settings are of this high‐energy type. Tidal bores that are closer to the lower limit of bore occurrence (Frtb = 1) generate deposits that will be difficult to distinguish from the background sedimentation.