Modern and ancient perspectives on deposition across the tidal–fluvial transition in rivers

摘要

The tidal–fluvial transition (TFT) in rivers exhibits a complex distribution of sediments and bioturbation that results from the interaction between river flow and tides. In the Fraser River, British Columbia, Canada, inclined heterolithic stratification (IHS) accumulates in intertidal positions on channel bars across the longitudinal profile of the TFT. Correspondingly, this portion of the river is studied to gain insight as to how sediments are distributed. The Cretaceous-aged McMurray Formation of Alberta, Canada contains thick and widespread IHS successions in the southern Athabasca region that accumulated on channel bars in a large river. This thesis focuses on identifying physical manifestations of channel-bar deposits that reflect along-strike variations in depositional processes within the TFT. These observations are put forward as criteria that can be used to determine depositional position relative to the TFT for other river systems in the modern and rock record. Vibracores, box cores, and surface-sediment samples were collected from nine channel-bars across the Fraser River’s TFT. Results show that mud bed thickness and mud volume are highest in the freshwater to brackish-water transition zone, with bioturbation decreasing from seaward to landward across the TFT. Heterolithic bedding is formed where mud is deposited, and is limited to locations with persistent brackish-water conditions. In the freshwater and tidal realm, amalgamated sand beds dominate channel-bar successions, although sand-mud rhythmicity increases towards the river mouth and reflects seasonal variations in river discharge. Thirty-one subsurface cores of the McMurray Formation were analyzed to test the depositional trends defined from the TFT in the Fraser River. Quantification of sedimentological and ichnological parameters was undertaken to differentiate between channel-bar successions deposited towards the seaward, middle, and landward ends of the paleo-TFT. The core dataset utilized in this study is too limited to draw direct comparisons between the Fraser and the McMurray, yet, trends are identified that suggest a potentially broad TFT was present in the “A Valley” of the McMurray Formation in the southern Athabasca region. Further work is needed to confirm this assessment.