Sedimentation Across the Tidal–Fluvial Transition in the Lower Fraser River, Canada

Abstract

The Fraser River is the largest undammed river on the west coast of North America. In its lower reaches, a saltwater wedge intrudes up to 30 km inland during mixed semi-diurnal tidal cycles that range up to 5.3 m in height. Sediments deposited in the lower reaches of the Fraser River show distinctive characteristics that reflect the relative control of river versus tidal processes, as well as the persistence of saline water at each point along the channel. Grain-size trends along the river are controlled by the hydrodynamics in each distributary. Mud deposition is concentrated in the zone of saltwater-freshwater mixing. Coarse-grained sand and mud/fine-grained sand deposition is largely seasonally controlled, wherein bed material (diameter > 0.177 mm) is deposited during the waning freshet, and washload transported mud and fine-grained sand (< 0.177 mm) is deposited during the late-stage waning freshet flow and during base flow. The diversity and density of bioturbation changes according to the volume and residence time of brackish water at the bed. Higher salinity water and greater durations that saline water is sustained at any locale, supports a more diverse and uniformly distributed trace assemblage. With decreasing salinity, the trace assemblage decreases in diversity and bioturbation becomes more sporadically distributed. This results in a reduction of infaunal diversity from 100% on the nearly fully marine tidal flats in the abandoned part of the lower delta plain, to 14% in intertidal sediments of the brackish-water reach. The character of the sediments deposited across the tidal-fluvial transition provide criteria for differentiating sediments deposited within freshwater-tidal reaches, brackish-water-tidal reaches, and mixed tidal-fluvial distributaries. These data are presented as a process-based analog for tidal-fluvial sediments preserved in the rock record. The results can be used to predict changes in facies character across the tidal-fluvial transition of similar tide-influenced, river-dominated systems or their rock-record equivalents.

Stratigraphic Record of River-Dominated Crevasse Subdeltas With Tidal Influence (Lajas Formation, Argentina)

“…This architecture is consistent with the general case that distributary channels are usually straighter than their fluvial meandering counterparts (Payenberg et al 2003) and that their deposits have a lower width/thickness ratio (Fielding 1985;Payenberg et al 2003). The unidirectionality of sedimentary structures suggests river dominance, but rare rhythmic carbonaceous drapes are interpreted as indicating tidally modulated bedforms (Martinius and Gowland 2011) that form in the fluvial to marine transition zone (Dashtgard et al 2012) and not in purely tidal settings. However, the majority of carbonaceous drapes usually lack rhythmicity consistent with tidal processes (Kvale 2012;Longhitano et al 2012); thus a tidal origin cannot be inferred (Martinius and Gowland 2011).…”

“…Minor effects of tides on sedimentation are reported in the Wax Lake (Wellner et al 2005;Shaw et al 2013;Shaw and Mohrig 2014), Atchafalaya (Roberts 1998), Mississippi (Scruton 1956;Arndorfer 1973;Mikhailov and Mikhailova 2010), and Po (Falcieri et al 2014), and the Huang He example shows crevasse subdeltas coexisting with tidal channels and tidal flats (Xue 1993;Yin et al 1999;Hui and Haijun 2004). Present-day tide-dominated deltas such as the Fly (Harris et al 1993;Baker et al 1995;Dalrymple et al 2003), Ganges-Brahmaputra (Kuehl et al 2005), and Chang Jiang (Yangtze; Hori et al 2001), or mixed-energy fluvial-tidal deltas such as the Fraser (Dashtgard et al 2012) or the Mahakam (Storms et al 2005;Salahuddin and Lambiase 2013) do not show active crevasse subdeltas, probably because crevassing is inibited by the presence of tidal currents in distributary channels and because of more effective sediment redistribution processes in the bays. It appears therefore that crevasse-subdelta systems form preferentially in relatively sheltered settings where input from the rivers is predominant and marine processes are minor.…”

The ichnology of the fluvial–tidal transition

“…8.1). Examples of these parameters are salinity, turbidity, sedimentation rate, seasonal processes, tidal modulation, and oxygen content (e.g., Buatois et al, 1997;Carmona et al, 2009;Dashtgard et al, 2012;Johnson and Dashtgard, 2014;MacEachern and Gingras, 2007;MacEachern and Pemberton, 1994;Mángano and Buatois, 2004;Pemberton and Wightman, 1992). In the fluvial-tidal transition, numerous and fluctuating environmental parameters represent controlling factors on benthic communities, therefore regulating the distribution of the biogenic structures.…”

“…Subsequent studies have been undertaken in the Ogeeche Estuary of Georgia (D€ orjes Howard and Frey, 1975;, followed by more recent work in Willapa Bay of Washington Neoichnology of the fluvial-tidal zone. (Gingras et al, 1999) and the Fraser River Delta of British Columbia (e.g., Dashtgard et al, 2012;Johnson and Dashtgard, 2014). Note the maximum tide limit and the maximum salinity limit delineating, the inner end of the fluvial-tidal transition and the inner end of the brackish-water conditions, respectively.…”

“…In addition, during the last couple of decades significant progress has been made on the ichnology of freshwater and terrestrial settings (e.g., Buatois andMángano, 1995, 2004;Genise, 2004;Genise et al, 2000Genise et al, , 2004Genise et al, , 2010Melchor et al, 2012;Scott et al, 2012). However, there is a growing number of studies documenting examples of biogenic structures in the fluvial to tidal transition in both the fossil record (e.g., Buatois et al, 1997Buatois et al, , 1998aMángano and Buatois, 2004) and, more recently, in modern settings (e.g., Dashtgard et al, 2012;Johnson and Dashtgard, 2014). However, there is a growing number of studies documenting examples of biogenic structures in the fluvial to tidal transition in both the fossil record (e.g., Buatois et al, 1997Buatois et al, , 1998aMángano and Buatois, 2004) and, more recently, in modern settings (e.g., Dashtgard et al, 2012;Johnson and Dashtgard, 2014).…”