Downslope changes in gradient and confinement

Changes in gradient impact the behaviour of a sediment gravity flows, which directly controls the dispersal patterns of sand and the character of the deposits. In subsurface reflection seismic datasets, research has demonstrated that slope bathymetry has a key influence on reservoir quality, architecture and distribution (Prather, 2000, 2003; Booth et al., 2003; Gamberi and Rovere, 2011; Bohn et al., 2012; Prather et al., 2012). Due to the large scale of seismic datasets, bathymetrically complex slopes are categorised by variation in the overall slope profile (Fig. 2.30), and give a 3D and basin wide perspective on topographic confinement (e.g. sections 2.3.1, 2.3.2 and 2.3.3) often unavailable in outcrop studies. Prather (2003) characterise slopes as: above grade slopes with well-developed ponded accommodation with large amounts of mid- to upper-slope healed-slope accommodation (e.g. Gulf of Mexico); above grade slopes with stepped profiles, that lack well-developed ponded accommodation (e.g. Niger delta slope, Lower Congo, NW Borneo; and graded slopes without significant topography (e.g. eastern Gulf of Mexico). The angle of confining slopes interpreted from seismic datasets can be more subtle and complex than those identified from outcrop (e.g. Gervais et al., 2006; Heiniö and Davies, 2007; Hanquiez et al., 2010; Prather et al., 2012a).

Figure 2.30 Ternary diagram modified from Meckel et al. (2000), and Booth et al. (2002), showing slope

type end-member and key processes controlling graded to above-grade slope transition (from Prather, 2003).

Accommodation on the submarine slope is the gap between the sediment surface (the background slope surface) and the equilibrium profile (the slope profile of no net erosion or deposition) (Pirmez et al., 2000; Kneller, 2003). The gradient of the equilibrium profile responds to changes in the volume and composition of turbidity currents, and the position of base level (Pirmez et al., 2000; Prather, 2003).

Figure 2.31 Idealised ponded depositional sequence (I-VI) and idealised bypass deposition sequence (VII-

XI) (from Prather, 2000).

Ponded accommodation occurs with 3D closure of topographic lows (Fig. 2.31) (Prather et al., 1998; Prather, 2000) that forms within intraslope basins as the result of localized withdrawal of mobile substrates (i.e. salt or shale; Prather, 2003). Healed slope accommodation (Fig. 2.31) is the space between the top of ponded accommodation and below a 3D convex surface fit to

the rugose seafloor topography. Healed slope accommodation is more common and volumetrically greater than ponded accommodation in many slopes (Steffens et al., 2003). Smith (2004a) classified three broad classes of complex slope topography (Fig. 2.32): (1) Silled sub-basins (i.e. closed depressions), most commonly documented from in salt-withdrawal minibasins of the Plio-Pleistocene Gulf of Mexico slope (Diegel et al., 1995; Liu and Bryant, 2000); (2) Partially silled basins with lateral escape paths, e.g. the Chumash Fracture Zone (Normark et al., 1984) and the physiography present on the Brunei slope (Demyttenaere et al., 2000); (3)Tectonically induced bounding slopes that guide, but do not block, flow paths, which can vary from highly tortuous to close to linear and commonly exhibit segments of lower ('steps') and higher (between 'steps') gradients (e.g. Hay, 2012).

Figure 2.32 Schematic diagrams illustrating the importance of the areal extent of sediment gravity flows

relative to the areas of receiving depressions. (A) Silled sub-basin in which sand-transporting flows are small in volume relative to the scale of the receiving space. (B) Silled sub-basin in which sand-

transporting flows are large in volume relative to the scale of the receiving space. The diagram shows spill to the next sub-basin downslope with associated incision and bypass in the upper sub-basin. (C) Connected tortuous corridor in which sand-transporting flows are small in volume relative to the

potential flow path. A possible example is shown in figure 8 of Demyttenaere et al. (2000). (D) Connected tortuous corridor in which sand-transporting flows are large in volume relative to the potential flow path (from Smith, 2004a).

Systems can therefore be classified into two end members silled sub-basins and tortuous corridors (Fig. 2.32). In the cascade of silled sub-basins model, topographic barriers between sub-basins are effective in blocking at least the basal sand-rich portions of flows until fill is achieved (Smith, 2004a). When substantial portions of flows to travel beyond the former barrier, flow will accelerate on the steep slope downdip of the barrier, and lead to

downcutting and successive flows cut back into the fill of the updip sub-basin (Smith, 2004a) also known as up-dip migrating knickpoints (Pirmez et al., 2000).

Slopes with less extreme topographic controls include stepped slope profiles. Stepped slope profiles are classified as above grade slopes that exhibit subtle changes in depositional gradient that result in low gradient steps that are linked by high gradient ramps. Above-grade slopes are low relief systems that lack the 3D closure of ponded mini-basins (Meckel et al., 2002) and are characterised by complicated, connected flow pathways with varying

depositional gradients that are marked by alternating sections of erosion and bypass (Fig. 2.33) (O’Byrne et al., 2004; Smith, 2004; Hay, 2012). Step flats are areas of net accumulation and have a low or negative gradient, and are essentially toe of slope settings. Entry or exit ramps are zones of net sediment bypass, which will have a higher gradient (O’Byrne et al., 2004). Stepped slope profiles are dominated by healed-accommodation. The shape of this

accommodation varies but it is generally strike-orientated curvilinear elongated ellipsoids on mud-prone stepped-slope profiles (Prather, 2003).

Figure 2.33 Seismic section showing the stepped topography with numerous ramps and flats along the

middle Angolan continental slope (from Hay, 2012).

An example form offshore Angola (Fig. 2.33) from Hay (2012) demonstrates the evolution of stepped slope profiles through healing from a slope with distinct areas of net deposition and

net bypass (Fig. 2.34), lessening gradient changes, creating broader zones of erosion and finally creating a through-going bypassing channel system as the slope reaches equilibrium (Fig 2.34).

Figure 2.34 Conceptual model for depositional evolution along a stepped-slope profile (modified from

O’Byrne et al., 2004). Predicted slope evolution describes depositional and erosional response of turbidity currents to progressive slope build-up and associated accommodation reduction (from Hay, 2012).

If unobstructed, a slope will grade to equilibrium (Pirmez, 2000; Prather, 2003), but an important consideration in all of these settings is the rate/amount of structural growth versus the nature and rate of sediment supply and deposition or timing (Jackson et al., 2008; Mayall et al., 2010). For example, where structural growth is rapid compared to sediment supply, basinward transported sediments may be deflected or completely trapped on the slope (Jackson et al., 2008). In contrast, where structural growth is slow in comparison to sediment supply, any topographic variations associated with these structures may be smoothed-out, resulting in only minor or no re-routing and/or trapping of sediments (Jackson et al., 2008). Exceptions to this may be on steep slopes, where erosion at the base of out-of-grade channels with a constant supply of sediment may result in these systems incising into and cross-cutting even the most rapidly growing structures (e.g. Badalini et al., 2000; Pirmez et al., 2000; Heiniö and Davies, 2007).

Figure 2.35 Sketch illustrating the structural controls on depositional systems on shelf, slope, and base-

of-slope systems affected by gravity-driven tectonics. Terrestrial fluvial-delta systems in orange, sand- rich facies in yellow, deep-water fans in pale yellow, slope muds in grey, salt structures in pink. Note the complex and tortuous paths taken by slope channels around salt structures and folds. Sands can also pond in intraslope basins until the basin is filled and then channels continue down the topographic slope. Typical scale of 150 km (93 mi), modified from Mayall et al., 2010.

Clark and Cartwright (2009) reported four end-member channel interactions with structures (folds/faults) in the deep-water Nile Delta, including: deflection of channels to fold tips; diversion of channels by folds; confinement of channels between two parallel folds, and blocking of channels by folds (Fig. 2.35). The influence of flow dynamics and height of

topography at individual flow scale has been discussed above (section 2.2.2). At a system scale Mayall et al. (2010) demonstrates that a more complex interplay of characteristics can

influence channel response to topography (Fig. 2.34), including: the size, shape, and

orientation of the structure on the depositional slope; the timing and rate of structural growth compared to channel initiation and development (e.g., Morley, 2009); and the erosional power of the channel complex systems/ variation in substrate resistance to erosion (e.g., Mitchell, 2006) (Fig. 2.36). Where structures are large (e.g. diapir, salt wall or large fault) and located parallel or oblique to slope strike, channels make major diversions to continue downslope (Huyghe et al., 2004) (Fig. 2.35).

Figure 2.36 Summary figure showing the range of channel responses to growth-related seabed

bathymetry. The vertical axis goes from small structures to large structures orientated at a high angle to regional dip. The horizontal axis goes from structural growth, which predates channel development and/or low erosion of channels, through offset stacking due to growing structures to channels cutting through growing structures. Examples described in the text are plotted on this matrix. The direction of flow is shown by blue arrows except in the middle lower seismic line, where flow would be into the page (from Mayall et al., 2010).

Where a channel system traverses a part of an undeformed slope that later exhibits growth of topography a range of channel responses can be recognized depending on the rate of

structural growth and the erosive power of the flows crossing the growing structure (Fig. 2.36) (Mayall et al., 2010). Where the rate of growth of the structure is greater than the ability of a channel to erode through it, channel complex systems are deflected. When channels form prior to or at the onset of deformation the channel can appear to cut through the growing structure when the downcutting rate keeps pace with the structural growth. When structural

deformation is ongoing the erosive power of flows has been shown to be a major factor in determining channel incision across topography, with channel shear stress and flow velocity partly determining whether a channel erosion can keep pace with deformation (Jolly et al., 2017). Detailed study by Jolly et al. (2017) shows that submarine channels in the Niger Delta can keep pace with structural uplift rates of up to 70 m in 1.7 m.y., and that channel

entrenchment upstream of growing structures plays a major role in driving this process. When topography creates 3D topographic closure the ‘fill-and-spill’ model for silled sub-basins (Fig. 2.32) can be applied (Smith, 2004a).