Type A beds: Wave-enhanced sediment gravity flows of mud
Type A beds encompass several types of normally graded bed, distinguished by subtle variation in the relative proportions of microfacies. The base of each bed is always sharp, sometimes irregular, never shows load casts and is cut in muddier sediment. Typically, beds comprise three divisions; in ascending order, well-sorted siltstone (microfacies 1), sharply overlain by silt-streaked claystone (microfacies 2), overlain by unstratified silty claystone (microfacies 3) and/or clay-rich mudstone (microfacies 4; Fig. 24A to G).
A sharp basal surface overlain by wave-rippled silt implies that wave-induced turbulence was responsible for initial scouring of the sea bed, leading to winnowing, re-suspension of weakly cohesive mud, and probably introduction of new silt. The sharp upward transition from rippled siltstone to silt-streaked claystone suggests a distinct change in flow character, from a turbulent or turbulence-enhanced state to transitional or quasi-laminar plug flow in which turbulence was suppressed to a greater or lesser extent (e.g. Baas et al., 2011). Such changes in flow state are commensurate with deposition from wave-enhanced sediment gravity flows in which the proportion of suspended clay is initially low, but over time increases to the point where flow structure is modified (Ogston et al., 2000; Fan et al., 2004; Friedrichs & Wright, 2004; Macquaker et al., 2010; Baas et al., 2011). During the passage of a storm across a shelf, waves would initially erode the sea bed, winnowing and concentrating silt from underlying silty mud. Wave action would also cause re-suspension of flocculated, river flood-sourced mud in the nearshore region (e.g. Fan et al., 2004), potentially forming a thin, near-bed fluid mud (typically having >10 g l−1 suspended sediment); this fluid mud would be strongly confined below the wave boundary layer (i.e. the lutocline; Traykovski et al., 2007). Fluid mud can become sufficiently dense that it will undergo downslope flow in response to gravity. Macquaker et al. (2010) suggested that the sedimentary record of this process will consist of a distinct ‘triplet’ bed that comprises a lower, sharp-based rippled sand (division ‘A’), a middle parallel laminated division of silt and clay (division ‘B’) and an upper structureless mud (division ‘C’). Rippled sand records initial wave-generated turbulent flow, followed by the formation and subsequent downslope flow of fluid mud, forming division B, followed by cessation of wave action and vertical deposition of structureless mud, division C (Macquaker et al., 2010; Ghadeer & Macquaker, 2011).
In the Dunvegan samples, the preponderance of south-east directed cross-lamination in silt layers indicates that these ripples, although experiencing oscillatory wave effects, also underwent preferential downslope migration; this is attributed most reasonably to the flow of bottom-hugging fluid mud (cf. Ogston et al., 2000; Macquaker et al., 2010; Fig. 25). Wave-rippled siltstone (microfacies 1) records a low suspended sediment concentration (probably <10 g l−1) and turbulent combined flow. The incursion of a dense fluid mud from upslope resulted in a change to transitional plug flow or even quasi-laminar plug flow, and the suppression of both turbulence and of ripple formation (Baas et al., 2011). Subtle velocity fluctuations in the near-bed region below the fluid mud are postulated to have resulted in a planar depositional surface on which was deposited silt-streaked clay (microfacies 2; cf. Wolanski et al., 1992; Teisson et al., 1993; Traykovski et al., 2007). The upward gradation from silt-streaked claystone to structureless silty claystone (microfacies 3) is analogous to the transition from divisions B to C of Macquaker et al. (2010). This facies change is interpreted to record the cessation of both oscillatory and unidirectional flow at the sea bed as the storm waned, wave base-lifted and wave-driven production of fluid mud ceased. Without energy input from waves, the suspended fluid mud collapsed vertically, depositing an unstratified mud, the cohesive properties of which would have inhibited the settling of suspended fine silt grains and clay aggregates (Mehta, 1991; cf. Ichaso & Dalrymple, 2009).
The inter-stratification of centimetre-scale sandy storm beds and millimetre-scale silt-clay storm beds in the upper, more proximal part of the studied section (Fig. 5, samples 7 to 11), shows that muddy, wave-enhanced sediment gravity flows operated across most of the prodelta. Based upon analogy with modern storm beds (Aigner & Reineck, 1982), it is inferred that, for any given palaeogeographic location, thin muddy beds record smaller storms, whereas thicker sandy beds record larger storms. Regardless of palaeogeographic position on the prodelta, however, silt ripples in all samples show a strong south-east directed mode that indicates that wave-enhanced sediment gravity flows experienced negligible Coriolis deflection as they travelled across the prodelta (Fig. 25). This finding is consistent with calculations (Traykovski et al., 2000), that showed, for a mid-latitude setting (such as the Dunvegan Formation), that the force exerted by gravity would greatly exceed the Coriolis effect, and hence wave-enhanced sediment gravity flows would have travelled directly down the bathymetric slope.
Although the normally graded, type A beds in the Dunvegan Formation superficially resemble distal muddy turbidites, they lack a number of key features, including an absence of load structures, a basal layer of planar-laminated silt, climbing and/or fading current ripples, and a gradational boundary between rippled siltstone and silt-streaked claystone; features that Schieber (1999) considered diagnostic of true turbidites. Therefore, type A beds are interpreted as the deposits of wave-enhanced sediment gravity flows rather than classical turbidites. This interpretation is supported by the fact that Dunvegan prodelta mudstone shows a relatively distinct lap-out onto the Fish Scales Upper (FSU) marker, which implies that wave-enhanced gravity flows stopped rather abruptly as they dropped below wave base. Had the flows been able to achieve true auto-suspension, it might be expected that they would have travelled further offshore, and generated a more extended toeset geometry.
The small-scale variability of microfacies successions in type A beds (Fig. 24A to G) is interpreted to record the considerable natural variability of the wind direction and duration of storms, the thermal and salinity structure of shelf waters, and the availability of river-supplied sediment, all of which would have affected the effectiveness of sediment re-suspension, the thickness and density of the resulting fluid mud layers, and the distance that they would have travelled downslope (Allison et al., 2000; Ogston et al., 2000; Fan et al., 2004; Neill & Allison, 2005; Traykovski et al., 2007).
Although downslope flow predominates, a minority of silt ripples indicate upslope bedform migration, particularly for samples 5 to 8 (Fig. 25A). It is possible that these bedforms record landward-directed asymmetrical flow beneath shoaling waves (e.g. De Raaf et al., 1977). If so, they imply waves approaching from the south-east, which is perpendicular to the direction determined for sand beds in the upper part of the section (Fig. 25B); unfortunately, it is not possible to determine crestal trends for silt ripples in thin section but, given the probability of a complex, combined-flow current regime, it is likely that they would have been strongly three-dimensional. The origin of these upslope directed ripples remains enigmatic.
Type B beds: Muddy gravity flows of indeterminate origin
Type B beds commonly lack a sharp basal contact and grade upward from clay-rich mudstone or silty claystone to silt-streaked claystone that may be capped by one or more thin, sometimes lenticular, laminae of well-sorted siltstone (Figs 16F and 24H and I). In some instances, silt-streaked claystone sharply overlies structureless clay-rich mudstone and shows an upward-thickening of siltstone laminae, capped by a thicker, apparently winnowed siltstone lamina (Fig. 24J). Instead of having a sharp top, reverse-graded beds may have a more symmetrical profile, with microfacies 2 grading upward to microfacies 3 and/or microfacies 4 (Fig. 24K).
Inversely graded beds suggest a gradual increase in flow strength, and a normally graded cap suggests a gradually waning flow. These types of bed show little or no evidence of turbulent flow or erosion at the base; instead the transition from microfacies 3 to microfacies 2 suggests gradual onset of flow, such as might be expected beneath a slowly accelerating muddy density current. The upward increase in the frequency and/or thickness of silt laminae in microfacies 2 suggests increasing flow velocity and more effective transport (and shear segregation) of silt grains. The layer of well-sorted silt at the top of some beds may show very subtle lenticularity, suggestive of very small wave ripples, but cross-lamination is not developed (Fig. 24J). Such silt layers might be interpreted as a terminal phase of flow that was sufficiently vigorous to rework and winnow the underlying silt and clay or, alternatively, a completely separate event characterized by reworking, rather than the introduction of new sediment (comparable to the ‘low concentration’ depositional regime of Fan et al., 2004).
Inversely graded, and inverse to normally graded beds of sand, silt and clay have been recognized as characteristic of hyperpycnites (Mulder et al., 2003), deposited from sustained density flows issuing from river mouths. Variations in the discharge of the river are recorded by subtle coarsening and fining patterns, and by minor intra-bed erosion surfaces. Soyinka & Slatt (2008) predicted that muddy hyperpycnites must be just as abundant as their sandy equivalents, but noted that their recognition would be hampered by fine grain size, lack of obvious sedimentary structures and greater susceptibility to surface weathering. Soyinka & Slatt (2008) made very detailed observations of sedimentary lamination and grain-size variability in millimetre to centimetre-bedded siltstones and mudstones in the Upper Cretaceous Lewis Shale of Wyoming. These authors recognized both normally graded and reverse-to-normally graded thin beds of silt and mud that were interpreted as flood-sourced hyperpycnites. Importantly, no evidence of wave ripples was observed in these ‘deep water’ mudstones.
Similarly, guided by the hyperpycnite model, Bhattacharya & MacEachern (2009) interpreted centimetre-scale normally graded sand-mud beds, as well as reverse-graded and reverse-to-normally graded beds of sand and mud in proximal Dunvegan prodelta deposits in terms of flood-generated hyperpycnal flows that may have issued from deltaic distributaries. Unlike the Lewis Shale, however, the sandstone beds illustrated by Bhattacharya & MacEachern (2009) are pervasively wave-rippled, suggesting that wave effects were important in generating and sustaining the flows.
Given the deltaic setting of Dunvegan allomember G, it is possible that type B beds are also the deposits of waxing and waning, river-sourced hyperpycnal flows. However, the prodelta gradient is estimated at ca 0·05° (see 'Discussion' below), which is much lower than the 0.7° necessary to sustain a river-fed hyperpycnal flow (Friedrichs & Scully, 2007). The absence of wave-rippled siltstone layers in type B beds suggests that wave action was not an important influence, at least at the site of deposition, yet the abundance of silt-streaked claystone is indicative of effective boundary-layer shearing, as would be expected beneath a flowing fluid mud. The simplest explanation may be that type B beds record deposition from flows of fluid mud that were generated by waves in shallow water, but subsequently underwent deposition close to effective wave base. At this depth, waves were insufficiently energetic to erode the sea bed and hence neither a scoured surface nor ripples were formed. Wave influence at the bed might have been further diminished because the density of the fluid mud suppressed the propagation of wave energy down through the lutocline, and hence to the bed (Traykovski et al., 2000). Upward-coarsening lamina-sets could be interpreted as the downslope record of the gradual onset of wave-supported hyperpycnal flow as winds strengthened, whereas upward-fining could record the gradual waning of the storm. Type B beds might therefore be interpreted as base-absent type A beds, deposited by relatively weak storms or below effective wave base.
Sand transport processes
The palaeoflow pattern for sand is radically different from that for silt. Wave ripple crests in sandstone beds that were deposited an inferred ca 10 km to ca 80 km offshore, trend shore normal (Fig. 25B). All but one of the observed sandstone beds contained combined-flow ripples that indicate ripple migration obliquely across the prodelta slope directed between south-west and south-east (Fig. 25B). This pattern of sand transport may be explicable in terms of storm winds that blew across the delta front from the north-east. Such winds would be expected for mid-latitude storms tracking west to east across the Interior Seaway, as modelled by Slingerland & Keen (1999). As storm systems began their passage across the Seaway, offshore winds blowing from the south-west, off of the leading edge of the storm, would have had little opportunity to build large waves because of limited fetch. Later, however, winds blowing from the north-east, off of the trailing edge of the storm, would travel unimpeded across hundreds of kilometres of water, building large waves. Storm winds from the north-east would entrain surface water that, under Coriolis deflection, would flow to the right of the wind, causing coastal set-up along the Dunvegan shoreline to the north-west. The resulting downwelling, geostrophic and combined flow would drive bottom sediment transport alongshore to obliquely offshore; i.e. to the south-west and south. This is the pattern observed (Figs 25B and 26).
Figure 26. Cartoon summarizing the palaeogeography and palaeobathymetry of the Dunvegan allomember G prodelta. The most effective storm winds blew from the NE and drove coastal set-up along the deltaic shore to the NW; resulting geostrophic and combined flows drove sand migration along-shore to obliquely offshore to a distance of ca 80 km. Storm waves also re-suspended river-borne mud on the inner prodelta, forming dense fluid mud that flowed downslope as wave-enhanced sediment gravity flows. Below effective wave base for mud (ca 70 m), these gravity flows settled, producing the distinct downlap pattern of the Dunvegan delta complex.
Download figure to PowerPoint