CHARACTERIZATION OF RAPIDLY DEPOSITED MUD: EXAMPLES FROM THE TIDALLY INFLUENCED FRASER RIVER, BC, CANADA

Mud is commonly regarded to accumulate exclusively from suspension settling in quiescent settings. Recent work has demonstrated, however, that mud also gets deposited in environments with relatively high and/or variable flow velocities. Such dynamic muds even occur where salinities and fine-grained sediment concentrations are low. The tidally influenced Fraser River provides an opportunity to observe the depositional signatures of dynamic mud accumulation because it has abundant suspended sediment, currents that fluctuate periodically in magnitude and direction, and salinities that vary from fresh to brackish in its lower reaches. Here, we consider hydrodynamic and chemical controls that lead to variations in deposited mud with longitudinal position in the Fraser River.

Three hydrodynamic zones occur in the Fraser River, each with a distinctive style of mud deposition. The freshwater tidal zone has rare mud confined to intertidal positions. Mud is deposited as thin (< 2 cm) structureless, unburrowed beds. The freshwater to brackish-water transition zone is characterized by increasing mud thicknesses and proportions towards the turbidity maximum zone (TMZ). Muds occur subtidally and intertidally, with mean thicknesses of 15 cm and sporadically distributed bioturbation. Fluctuations in flow strength and direction, as well as water chemistry, promote rapid contemporaneous deposition of mud and sand. Finally, in the sustained brackish-water zone, mud proportions decline with decreasing proximity to the TMZ. Mud is only deposited at upper subtidal and intertidal positions, with average bed thicknesses of 10 cm and more persistent bioturbation consistent with the brackish-water ichnological model. Heterolithic bedsets dominate, though muddy current ripples are less common than in proximity to the TMZ. Mud deposition in the sustained brackish-water zone is controlled by reversing currents and higher salinity water.

Detailed characterization of mud in the Fraser River allows establishment of process-response linkages between hydrodynamic controls and facies characteristics. Recognizing the distinctive mud facies in each depositional zone can be used to refine interpretations in the rock record and improve paleogeographic reconstructions of tidal-fluvial channels.