The effect of varying origins, morphology and scales of slope and basin floor topography on stratigraphic architecture of deepwater successions is poorly understood. The principal aim of this thesis is to understand the effects that complex but subtle slope and basin floor
topography have on sediment gravity flow processes and resultant depositional architecture.
This will enhance our understanding of deepwater system evolution, as well as helping to bridge the gap between the small scale experimental studies assessing dynamics of single flows and the large scale studies of slope and basin floor architecture using seismic reflection and modern seafloor datasets. This study will also add to our knowledge of the Karoo Basin succession in the Laingsburg depocentre along with additional SLOPE 4 projects. Previous studies undertaken as part of the SLOPE project have investigated the regional and localised aspects of stratigraphic architecture of the Laingsburg depocentre, which have been utilized in this study to investigate the influence of varying scales of topography.
Figure 1.1 A) Models of simple and topographically complex slope profiles. (B) Examples of intraslope
“perched apron”, Einstein-Fuji slope Eastern Gulf of Mexico, Seismic attribute map (From Sylvester et al., 2012). (C) Intraslope “ponded apron” from the Brazos-Trinity intraslope basins, Gulf of Mexico (From Prather et al., 2017). (D) Examples of slope gradient variations (From Posamentier and Walker, 2006).
In this context, the thesis is focussed around key research questions that span data chapters 4-6 (Fig. 1.2). These are outlined in detail as follows, and will be returned to at the end of the thesis (Chapter 7):
Figure 1.2- Key results from data chapters 4, 5 and 6, colour coded in relation to each research question.
Beige relates to question 1, green to question 2 and blue to question 3.
Question 1: How does the orientation and gradient of slope to basin floor topography influence sediment gravity flow processes and resultant stratigraphic architecture?
Rationale: Gradient variations and topographic obstacles along deep water slope and basin floor profiles have been documented to have significant impact on gravity flow behaviour, with consequent effect on sedimentary facies (e.g. Baines, 1984; Kneller and McCaffrey, 1991;
Edwards et al., 1994; Haughton, 1994; Smith, 2004a; Hodgson and Haughton 2004; Stevenson et al., 2013; Spychala et al., 2017a) and depositional architecture (e.g. Prather, 2003; Deptuck et al., 2012; Mayall et al., 2010; Hay, 2012; Prather et al., 2012a, b; Moody et al., 2012; Wynn et al., 2012). The distribution, length scales and orientations of gradient variations can range widely, creating topographically complex slope to basin floor systems.
Many experimental and numerical studies have been undertaken with the aim of
understanding the effects of gradient and confinement changes and obstacles on turbidity current processes, which can result in the reflection, deflection or decoupling of flows (e.g.
Baines, 1984; Lawrence, 1986; Edwards et al., 1994; Meiburg and Kneller, 2009; Nasr-Azadani and Meiburg 2014; Wang et al., 2017). However, relating these process changes to deposits in the sedimentary record and scaling these processes to bed, package or system scale can be challenging (e.g. Pickering and Hiscott, 1985; Marjanac, 1990; Kneller and McCaffrey, 1991;
Edwards et al., 1994; Haughton, 1994; Smith, 2004a; Jackson and Johnson, 2009). The effects of slope gradient on flow processes is more notable in situations where gradient change is of higher magnitude (e.g. Pickering and Hilton, 1998; Sinclair, 2000; Sinclair and Tomasso, 2002;
Hodgson and Haughton, 2004; Marini et al., 2015) but more difficult to constrain where gradient change is subtle (<1°) (e.g. Smith, 2004b; Stevenson et al., 2013; Spychala et al., 2017a).
Key areas of topographic variability within slope to basin floor profiles include: slope failure, forming a concave basal shear surface, which can capture sediment routing systems, or pond deposits (e.g. Alves and Cartwright, 2010; Ortiz-Karpf et al., 2015; Kneller et al., 2016;
Fallgatter et al., 2017; Qin et al., 2017); the resultant remobilized deposits, which can deflect/reflect flows and pond deposits within the rugose top surface (Armitage et al., 2009;
Jackson and Johnson, 2009; Ortiz-Karpf et al., 2015; Kneller et al., 2016; Sobiesiak et al., 2016;
Fallgatter et al., 2017); intraslope basins or flats (Prather et al., 2003), forming intraslope accommodation, with the potential to pond deposits, forming intraslope lobes (e.g. Steffens et al., 2003; Deptuck et al., 2012; Prather et al., 2012a,b; Spychala et al., 2015) or weakly
confined channel systems (e.g. Beaubouef and Friedman, 2000; Pirmez et al., 2000; Deptuck et al., 2012; Moody et al., 2012); intraslope variations in gradient and orientation (e.g. Hay, 2012), which can lead to increased/ decreased flow velocity, and defection of flows, resulting in changes in system architecture, e.g. tortuous corridors (e.g. Steffens et al., 2003; Smith et al., 2004a; Burgreen and Graham, 2014); the base-of-slope, where a reduction in gradient and/or flow confinement can cause flows to undergo hydraulic jumps, transitioning from super- to sub-critical flow conditions (Mutti and Normark, 1987, 1991; Weirich, 1989; Kostic and Parker, 2006; Sumner et al., 2013) with the potential to create a sediment bypass dominated channel-lobe transition zone (e.g. Wynn et al., 2002a; Hofstra et al., 2015;
Pemberton et al., 2016); and, lateral basin margins or lateral intrabasinal slopes, which can affect basin-floor depositional systems by deflecting flows (e.g. Kneller et al., 1991; Sinclair, 1994; Kneller, 1995; Amy et al., 2004; Gamberi et al., 2014; Spychala et al., 2017b) with the potential to cause onlap geometries in lobes (e.g. Smith and Joseph, 2004; Bersezio et al., 2009; Marini et al., 2015). Intraslope and basinal gradient changes can be related to dynamic substrate i.e. mud and salt diapirism, active faulting, and folding (Jackson et al., 2008).
The studies presented in this thesis investigate relatively subtle gradient changes (<1° to a few degrees) within a range of depositional settings across a slope to basin floor setting in the Laingsburg depocentre, allowing for analysis and discussion of the effects of frontal, lateral and oblique orientated gradient increases and decreases on deepwater stratigraphy.
Question 2: How does topographic influence on turbidity currents vary and evolve i) During deposition of a single system, and ii) during multiple successive systems?
Rationale: The effect of topography on turbidite systems will inevitably depend of the scale and orientation of the topography as discussed in the rationale to Question 1, but also with variability in flow dynamics, both of which will vary temporally. Topographic variation will occur due to both active deformation of the slope (e.g. Barton, 2012; Deptuck et al., 2012;
Hay, 2012; Prather et al., 2012a, b) and the modifications each flow will make as it erodes and deposits (e.g. Normark et al., 1979, 2009; Pickering and Corregidor, 2005; Dakin et al., 2013;
Ortiz-Karpf et al., 2015; Spychala et al., 2015), therefore each individual flow will be interacting with a unique bathymetric configuration. Variability in flow dynamics will result from intrinsic and extrinsic controls. Extrinsic controls determine the initial thickness and volume of flows,
the sediment concentration and grain size distribution, and intrinsic factors are of influence throughout the flow pathway, causing flows to deposit or erode, and increase or decrease sediment concentration, flow velocity, stratification etc. (Kneller and McCaffrey, 1999; Kneller and Buckee, 2000). Therefore, each incoming flow is unique and the resultant effect of the same topography on flow processes will vary. Within the deposition of an individual system subsequent flows have the potential to increase (e.g. erode entrenched channel systems) or decrease topographical complexity (e.g. healing of intraslope accommodation), in an attempt to form a slope to basin floor profile that is at equilibrium (Pirmez, 2000; Prather, 2003).
Slope topography is likely to change more dramatically over the deposition of multiple systems. At this longer time scale, an actively deforming seabed has the potential to significantly alter the configuration of the slope and basin floor systems (e.g. Stewart and Clark, 1999; Lopez-Mir et al., 2014). Moreover, erosional or depositional relief of the preceding system or multiple stacked systems may be apparent on the seabed (e.g. Jackson and Johnson, 2005; Pickering and Corregidor, 2005; Spychala et al., 2015; Ortiz-Karpf et al., 2015) or result in topography generated by differential compaction (Færseth and Lien, 2002; Koša, 2007).
Therefore, the ability of flows to heal topography and reach an equilibrium profile can be outpaced, be equal to, or be surpassed by formation of new topography at the onset of each depositional system.
Basins undergoing frequent episodic sediment input have been demonstrated to form more ponded and healed basin successions, which can often be associated with more numerous hydrocarbon reservoir and seal pairs (Prather, 2000, 2003). Sixty nine percent of producing deepwater hydrocarbon reservoirs occur in slope accommodation (Prather et al., 2009), with seventy five percent of global Tertiary deepwater reservoirs deposited across stepped slope profiles (O’Byrne et al., 2004). Through understanding how the effects of topography change throughout deepwater system evolution, generic models can be established, to predict changes in system architecture and to aid the interpretation of lower resolution datasets.
Therefore, understanding the dynamics of these systems and being able to predict stratigraphic architecture is crucial for hydrocarbon exploration.
Question 3: How are topographically complex components transferred into the stratigraphic record i) as surfaces, and ii) as stratigraphic successions?
Rationale: The transfer of deepwater systems into the stratigraphic record can be complicated, especially in topographically complex areas where flow processes can be highly variable.
Individual surfaces or beds can represent a large range of time scales. Therefore, it is
important to consider the location of an outcrop within a system, whether the setting is overall aggradational or degradational, and the flow processes that formed the strata, when
interpreting resultant deposits in order to delineate the preservation potential. Modern seafloor datasets afford a single timeslice of a system showing a geomorphic palimpsest (e.g.
Wynn et al., 2002a), but if a surface is not actively aggrading or is later eroded it will not be preserved in the stratigraphic record. Distinguishing between stratigraphic surfaces that are time transgressive and composite, and geomorphic surfaces that are rarely preserved in the rock record can be challenging (e.g. Strong and Paola, 2009; Sylvester et al., 2011; Holbrook and Bhattacharya, 2012; Blum et al., 2013; Hodgson et al., 2016). This distinction between physiographic snapshot and stratigraphic transfer is important in understanding the preservation potential of all systems.
Moreover the stratigraphic record does not simply show surfaces and rock volumes as they were at initial deposition. Dewatering, compaction and deformation takes place, possibly through multiple cycles. Compaction has a significant impact on volumes of sediment, especially in lithologically variable successions, where differential compaction can greatly impact the resultant preserved geometry (e.g. Alves, 2010). This can occur after the entire succession is deposited, or when systems are still active, resulting in topographic highs and lows, and compensational stacking of sand-rich elements.
Therefore, in order to draw conclusions about original depositional topography from the rock record it is important to understand (i) how preserved sediments encapsulate formational processes, and (ii) what changes sediments undergo from deposition to exhumation.