Fluvial to tidal transition zone facies in the McMurray Formation (Christina River, Alberta, Canada), with emphasis on the reflection of flow intensity in bottom set architecture

摘要

12.1 INTRODUCTION The understanding of depositional processes and products of the fluvial to tidal transition (FTT) zone (Fig. 12.1) has grown significantly over the last 25 years (Allen, 1991; Cuevas Gozalo and De Boer, 1991; Dalrymple and Choi, 2007; Fischbein et al., 2009; Ghosh et al., 2005; Jablonski and Dalrymple, 2016; La Croix and Dashtgard, 2014; Martinius and Gowland, 2011; Martinius and Van den Berg, 2011; Sisulak and Dashtgard, 2012; Van den Berg et al., 2007). This is in part due to the realization that this is an area with a significant depositional footprint along the trajectory of a river channel. In the FTT unidirectional, freshwater (fluvial) flow interacts with the tides and the intruding salty and denser seawater. It connects to the inshore tidal zone (if present) and together they form the fluvial to marine transition (Fig. 12.1) in an estuary or a (tide-dominated) delta distributary. For a sedimentological definition, it is appropriate to include the recognition of this interaction. Consequently, the FTT zone is defined as that part of the river which lies between the landward limit of measurable effects of tidally influenced flow deceleration on fluvial cross-bedding at low river discharge, and the most seaward occurrence of a textural or structural fluvial signature related to high river discharge (cf. Van den Berg et al., 2007). In many instances, the effect of salinity changes on faunal and floral distributions in the FTT can be understood in significant detail, which supports any sedimentological interpretation. As shown by Martinius and Van den Berg (2011), some sedimentary structures point to more fluvial dominance and others to dominance of tidal processes. The simultaneous occurrence of these structures next and above each other is in fact an important characteristic of this transition zone, where the fluvial- and tide-dominated areas move up and down the depositional profile in response to short- and long-term fluvial discharge variations and the combined effect of the tidal cycles at various temporal scales. This interaction can be relatively simple but may also lead to a multifaceted hydrodynamic regime, involving salinity changes as well, resulting in a signal in the rock record that is not straightforward to interpret (Dalrymple et al., 2015; Dalrymple and Choi, 2007; Jablonski, 2012).