Abstract:

Inclined heterolithic stratification (interbedded sand and mud with depositional dip; IHS) is developed on an in-channel bar in the tide-influenced, fluvially dominated (brackish water) reach of the Fraser River, British Columbia, Canada. The vertical bar succession is characterized by a fining-upward profile with an increase in mud content and mud-bed thickness from the shallow subtidal zone to the upper intertidal zone. There is also an increase in the number and lateral continuity of mud beds from the upstream side to the downstream side of the bar. Sand beds are dominated by current ripples in the intertidal zone, and by current ripples and trough cross-beds in the shallow subtidal zone. The channel base is mantled by downstream (ebb)-oriented dune-scale bedforms. Mud beds are dominantly parallel laminated, although current ripples may develop in silt-rich and sand-rich mud beds. Current-generated bedforms are predominantly ebb-oriented.

Sediment deposition is seasonally controlled. Sand deposition occurs during periods of high discharge (snowmelt-induced freshet), and mud is deposited during waning freshet flow and low discharge (base flow). In mud-dominated deposits, current ripples may develop in sand-rich mud beds deposited during the freshet. Seasonal cyclicity in sediment deposition is also recorded in the ichnological characteristics of the IHS. Bioturbation is significantly more common in base-flow deposits (mud beds) than freshet deposits (sand beds). Burrows in sand beds typically subtend from overlying mud beds. Diminutive, vertical burrows dominate the trace suite, reflecting a very low diversity of infauna. Bioturbation is more common on the downstream side of the bar, associated with the thicker and more laterally contiguous mud beds.

Based on these observations, seasonal cyclicity in channel-bar deposits, formed in tide-influenced, fluvially dominated channels, can be identified using a combination of sedimentological and ichnological characteristics, particularly in mud-dominated successions and in IHS. In mud-dominated deposits, seasonal controls are best-expressed ichnologically through the interbedding of bioturbated and unbioturbated intervals. The dominance of a low-diversity suite of mainly vertical traces can also be considered indicative of brackish-water conditions in the channel. In IHS successions, interbedded sand and mud beds are the best indicators of seasonal cyclicity: sand beds are typically unburrowed, and mud beds are burrowed. In sand-dominated successions that lack mud interbeds, it is difficult to recognize seasonal cyclicity.

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