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Integrating Sedimentology, Ichnology, Palynology, and Geochemistry Towards a Process-Response Facies Model for Tidally Influenced River Deposits: Fraser River, BC, Canada

Abstract

The lower Fraser River is tidally influenced, and shows strong seasonal variability in discharge. The interaction between tides and river flow determines the upstream limit of a saltwater wedge into the distributaries, and controls deposition of sand and mud on channel bars. Herein we compare the palynology and geochemistry to the sedimentology and ichnology of intertidal sediments across the tidal-fluvial transition (TFT) to link hydrodynamic processes with facies characteristics. The results are a process-analog for similar deposits in the rock record, such as inclined heterolithic stratification (IHS) successions in the middle McMurray Formation, Alberta, Canada. Three zones with well-defined hydrodynamic and water salinity conditions occur across the Fraser River's TFT – freshwater-tidal, transitional fresh- to brackish-water tidal, and brackish-water-tidal. Several trends in the characteristics of IHS between these zones are observed. Deposits shift seaward from sand-dominated in freshwater zones towards mud-dominated at the turbidity maximum, and are mixed sandy and muddy near the river mouth where water is brackish. A paucity of bioturbation in the freshwater zone shifts to “brackish-water” suites with a patchy distribution in the brackish-water realm. The progressive changes in the ichnological character reflect the duration and magnitude of interstitial saline water, as well as deposition rates. Palynological suites show low dynocyst populations overall, and where present, marine and salinity-tolerant dinocyst populations increase in the seaward direction. Sediments are increasingly enriched with C13 in the seaward direction, reflecting higher concentrations of marine-derived organic carbon. The palynology and geochemistry of muds through the TFT suggest organic matter is dominantly terrestrial, with subordinate marine influence. Since the ichnology, palynology, and geochemistry of muds are particularly sensitive to water salinity, it's possible use these characteristics to establish the relative magnitude of brackish-water and tidal influence on deposition. Although more modern examples are needed to confirm these results, linking physical sediment characteristics with depositional conditions indicates that IHS variability can be predicted. In IHS-prone successions in the rock record, predicting this variability is important for refining reservoir models by understanding the controls on reservoir heterogeneity and compartmentalization.