Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

Abstract

Unit-bar deposits are ubiquitous components of river-channel deposits and strongly influence their hydrological properties, yet they are not easy to interpret. This paper concerns details of the internal structures of six unit bars from the South Saskatchewan River, Canada, that were investigated using trenches, epoxy resin peels, and 900 MHz ground-penetrating radar (GPR) profiles. The composition of unit bars depends on flow unsteadiness and superimposed bedforms. Flow unsteadiness causes changes in the mean grain size of the sediment in transport, but is expressed primarily as a change in the type and direction of migration of smaller ripple-and dune-scale bedforms superimposed on unit bars. Superimposed bedforms with heights that exceed 25% of the host bedform height reduce their host's slope and generate inclined sets. Host bedforms with smaller superimposed bedforms form angle-of-repose cross strata with a visible pre-sorting pattern. The relationship between the formative host-and superimposed bedforms and such pre-sorted cross strata can be used to interpret numerous aspects of the three-dimensional geometries of the bedforms. Such detailed interpretations rely on: (i) regularly spaced finegrained drapes deposited during the passage of troughs of superimposed bedforms, (ii) grain-size sorting patterns generated by sorting within the superimposed bedforms before deposition, (iii) grain-size sorting during deposition by grainfall and intermittent grainflows down the bar lee-slope, and (iv) the presence and nature of low-angle reactivation surfaces.The detailed interpretations revealed that the deposits of dam-related floods were significantly smaller than the deposits of individual unit bars in this study. The unsteady flow and sediment transport conditions are reflected by changes in the structural composition of the unit bars. Reactivation surfaces associated with flow unsteadiness, as opposed to those formed by large superimposed bedforms, were characterized by changes in mean grain size, the buildup of sediment at the base of the bar lee slope, and changes in structures formed by superimposed bedforms. Reactivation surfaces generated by large superimposed bedforms indicate that bedform preservation is likely increased in areas of flow deceleration. The trough deposits of exposed unit bars contained bubble sand, planar laminae due to upper-stage plane beds, and low-angle inclined laminae due to antidunes. These structures indicate that unit-bar troughs can act as ephemeral channels. The composition of the investigated unit bars thus emphasizes the interplay between flow unsteadiness and bed morphology in the formation of sedimentary structures in river channels.Sediment surfaces generate visible GPR reflections where the thickness and contrast in electrical properties of the sediment layers are sufficiently large. Most cross-stratified sets were represented only by reflections from the base of the set, and not by high-angle inclined reflections from the cross strata. The local absence of refle...

Bottomset Architecture Formed In the Troughs of Dunes and Unit Bars

“…Cross-stratified sets containing bottomsets are common. They have been described in many depositional environments: fluvial (e.g., Basumallick 1966;Boersma 1967;Boersma et al 1968;Tillman and Ellis 1968;Collinson 1970;Ashworth et al 2011;Reesink and Bridge 2011), lacustrine (e.g., Gilbert 1885; Jol and Smith 1991), marine (e.g., Terwindt 1971;Sohn et al 2003;Nielsen andJohannessen 2009), andeolian (e.g., Hunter 1977;Clemmensen and Abrahamsen 1983;Eastwood et al 2012). The examples presented herein formed at the base of unit bars and dunes migrating in rivers and flumes.…”

“…1, 2A), co-flow ripples (e.g., Boersma 1967;Boersma et al 1968;Martinius and Van den Berg 2011) (Fig. 1A), lower-or upper-flow-regime planar lamination (e.g., Reesink and Bridge 2011), antidunes (e.g., Reesink and Bridge 2011), massive sand (e.g., Røe and Hermansen 2006), muds (e.g., Reesink et al 2014), and coarse-grained (gravel) layers (e.g., Allen 1982;Kleinhans and van Rijn 2002;Kleinhans 2004;Reesink and Bridge 2011) (Fig. 2D).…”

“…1B). However, for the former, more complex foreset structures can develop, such as multiple down-climbing sets (e.g., Lunt and Bridge 2007;Reesink and Bridge 2011; often termed compound cross-stratification). Complex unitbar internal structures develop due to interactions between the host bar and superimposed bedforms (e.g., Reesink and Bridge 2011) or flow unsteadiness (e.g., Jopling 1963Jopling , 1966Lunt and Bridge 2007).…”

“…2D). Bottomset architecture may be particularly complicated if the bedform trough contained transverse flows (e.g., Boersma et al 1968;Reesink and Bridge 2011), if trough deposits were reworked before burial (e.g., reworking by low-stage channelization through bar troughs; Reesink and Bridge 2011) or where tides led to changes in flow direction (e.g., Visser 1980; Allenexample, the bidirectionality of stacked co-flow and back-flow ripples within bottomsets where the foreset was not preserved could be erroneously attributed to tidal influence.…”

“…Despite the wide range of potential variation and its ubiquity in fluvial deposits, there has been little reported on bottomset variation within fluvial deposits, although a few have observed it (e.g., Tillman and Ellis 1968;Collinson 1970;Reesink and Bridge 2011). This paper focuses on improving understanding of bottomsets by categorizing the controls on their development (antecedent, primary 2D, primary 3D, and secondary), describing commonly occurring internal structures, discussing how bottomsets can be identified in the rock record, discussing the role of flow unsteadiness on bottomset variation, and relating the variation to reservoir heterogeneity.…”