Sedimentation across the Tidal-Fluvial Transition in the Lower Fraser River, Canada

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. INTRODUCTION With increasing interest in recognizing differences in the sedimentological and ichnological character of sediments deposited across the tidal-fluvial transition, it is necessary to study a variety of modern analogs subjected to varying degrees of fluvial and tidal input. One such analog is the Fraser River, British Columbia, Canada, which in its lower reaches is affected by strong tidal flow. The grain sizes of sediments deposited in the channel are strongly linked to the degree of tidal flow relative to river flow, and to the extent and duration of saltwater intrusion at sites of sediment deposition. Consequently, the sedimentological and ichnological character of the sediments reflects the depositional conditions under which they were deposited, and hence, can be used as a proxy for establishing the volume and duration of saline water intrusion and the relative input of tidal versus river flow. Understanding sediment deposition across the tidal-fluvial transition has received increasing attention in recent years (e.g., Dalrymple and Choi 2007) in large part because subsurface reservoirs of the bitumen-hosting middle McMurray Formation in northeast Alberta are interpreted as paleo-estuarine channel bars (Pemberton and Wightman 1992; Ranger and Pemberton 1997; Musial et al. 2012). The lateral and vertical extent of mud beds and muddy bedsets interbedded with sand in the channel bars is the dominant control on reservoir compartmentalization, and can strongly impact the economic viability of exploiting hydrocarbons contained in these deposits (Strobl et al. 1997; Hein and Cotterill 2006). To better understand mud distribution in tide-influenced river channels, a range of modern analogues are being evaluated across the spectrum from tidedominated (Dalrymple et al. 2003; Choi et al. 2004; Pearson and Gingras 2006), to fluvially influenced, tide-dominated (Smith 1988; Gingras et al. 1999; Smith et al. 2009), to mixed tidal-fluvial (Johnson 2012), to tideinfluenced, fluvially dominated (Sisulak and Dashtgard 2012) to fluvially dominated (Smith et al. 2009; Smith et al. 2011) settings. The lower Fraser River (Fig. 1) is a natural laboratory for studying sediment deposition in tide-influenced, fluvially dominated channels and mixed tidal-fluvial channels. The availability of historical and real-time hydraulic data (e.g., Johnson 1921; Ages 1979; Thomson 1981), the fact that the Fraser River is undammed along its entire length, and the extensive studies of sediment transport in the river (e.g., Kostaschuk and Luternauer 1989; Church and Krishnappan 1995; Kostaschuk and Villard 1996; Kostaschuk et al. 1998; McLean et al. 1999; Kostaschuk and Best 2005) provide datasets that can be used to link sedimentological, ichnological, and architectural characteristics of the sediments to the hydrodynamic and chemical conditions under which they were deposited and colonized by infauna. THE FRASER RIVER The Fraser River drains 228000 km of mountainous terrain and has a mean annual river discharge of 2710 ms at Hope, BC, ~165 river km upstream of the delta front. Low flow rates in winter are ~1000 ms, and the annual peak flow ranges between 5130 and 15200 ms with a mean of 8642 ms (Water Survey of Canada, http://www.wateroffice.ec.gc.ca/, accessed Sept 7, 2012). At low flow, the saltwater wedge (SWW) intrudes ~30 km upriver (Kostaschuk and Atwood 1990). During high river flows,