Stacking patterns of lobes are proxies for the relationship between sediment supply and seabed topography during the evolution of lobe complexes and lobe complex sets (Piper & Normark, 1983; Mitchum & Van Wagoner, 1991; Schlager, 1993; Twichell et al., 2005; Picot et al., 2016). Seabed topography can be dynamic or static, which can change the degree of confinement experienced by a system over time. Confinement has been documented to be an important allogenic factor in the control of sediment dispersal patterns and lobe stacking patterns (e.g. Piper & Normark, 1983; Smith & Joseph, 2004; Amy et al., 2004, Twichell et al., 2005; Southern et al., 2015; Marini et al. 2015). Sediment supply is governed by climate and tectonics in the hinterland, sea level variations that change accommodation on the shelf (e.g. Mitchum & Van Wagoner, 1991; Schlager, 1993), and topography on the supply slope (e.g. Prather, 1998). Several types of stacking patterns have been documented from lobe successions (e.g. Gervais et al., 2006; Amy et al., 2007; Deptuck et al., 2008; Prather et al., 2012b; Prélat & Hodgson, 2013, Burgreen & Graham, 2014; Grundvåg et al., 2014; Picot et al., 2016): compensational, aggradational, and longitudinal, with either basinward (progradational) and landward (retrogradational) trends (Fig. 7.2). There can be a continuum between these stacking patterns during the growth of a lobe complex set or lobe complex. For example, parts of a lobe complex set or lobe complex can experience the influence of confinement that results in aggradational stacking patterns, whereas away from the confining structure compensational stacking can be prevalent.
Different stacking patterns can be interpreted from 1D datasets and extensive 2D/3D datasets, with different implications for the distribution of heterogeneities and reservoir potential for a system. Where only 1D data are available, interpretations and predictions have to be considered very carefully as recognised thickening/thinning upwards cycles can be biased (e.g. Hiscott, 1981; Anderton, 1995).
Lobes from the Karoo Basin show a range of stacking patterns reported in the literature. Below these stacking patterns and their controlling factors are presented:
7.2.1 Compensational stacking
Deposits of the Laingsburg and Tanqua depocentres are reported to show low to no influence of confinement by seabed topography and compensational stacking can be observed in several of the examined lobe successions, e.g. lobes of Unit A in the Laingsburg depocentre (cf. Chapter 4.7). Compensational stacking patterns can be identified by the paucity of thickening- and thinning-upward cycles, abrupt stratigraphic changes in lobe sub-environments, and lateral thickness variations between stacked lobes. Compensational stacking can be observed across many scales from bed to lobe complex scale (Prélat & Hodgson, 2013) to lowstand sequence sets (van der Merwe et al. 2014). Migration of beds and lobes can be dip parallel, strike parallel or oblique, depending on the exact location of a topographic low. As long as there is no confinement impeding free dispersal of sediment then compensational stacking will be the dominant stacking pattern through a lobe complex (Straub et al., 2009). Compensational stacking in lobe to lobe complex sets is governed by the avulsion of feeder-channels to redirect to a new topographic low after the creation of sufficient depositional relief in a lobe complex.
Figure 7.2 Schematic plan view of lobe stacking patterns. A: Compensational stacking; B: Aggradational stacking; C: Progradational stacking; D: Retrogradational stacking. The dashed blue line indicates the locus of deposition of the next lobe.
7.2.2 Aggradational stacking
In the Karoo Basin deposits, aggradational stacking patterns have been observed in settings where accommodation and/or confinement influences sediment dispersal, e.g. on the slope (D/E and E1, Fort Brown Formation; cf. Chapter 5) and through the existence of an intrabasinal lateral slope (Unit A; cf. Chapter 4). The main recognition criterion for aggradational stacking is the vertical superposition of the same lobe sub-
environment over the evolution of a lobe complex (E1, Unit A) and lobe complex set scale (Unit A), with little or no lateral offset. In E1, the axes of intraslope lobes are stacked in the axis of the available accommodation while fringes are stacked at its margin and show influence by confinement (deflected and reflected palaeoflow indicators; Chapter 4, 5). In the case of Unit A, a subtle intrabasinal slope influenced the stacking patterns (Chapter 4). While lobe deposits in the axis of the system show compensational stacking (see above), towards the slope and on the slope there is a vertical accumulation of lobe fringes (therefore termed aggradational lobe fringe). Aggradational stacking occurs where avulsion is not possible due to the scale of confinement (Burgreen & Graham, 2014) and is commonly observed with lobes deposited in highly confined settings, e.g. mini-basins and ponded basins (e.g. Burgreen & Graham, 2014). When accommodation is filled, the system will either prograde and sediment will spill into the next available mini-basin (fill and spill; e.g. Prather et al., 1998), or, in the case of stepped slope profiles, is transported farther downslope to the next available area of slope accommodation or the basin-floor (van der Merwe et al., 2014). If progradation is not possible deposits will migrate landwards as the break-of-slope successively moves up-slope.
7.2.3 Progradational stacking
Traditionally, the existence of thickening-upward cycles in lobes was interpreted as evidence of progradation (e.g. Mutti, 1974; Ricci Lucchi, 1975). Since then the interpretation of thickening upward cycles as the sole indicator for progradation has been challenged (Hiscott, 1981; Anderton, 1995; Chen & Hiscott, 1999; Macdonald et al., 2011). Hiscott (1981) stated that cycles have been over-interpreted by comparing them with thickening-upward cycles in delta-lobes even though processes of deposition are different. Macdonald et al. (2011) suggested that evidence of increased erosion and bypass needs to be identified in addition to repetitive thickening upward cycles to argue for progradational stacking over aggradational or compensational stacking.
Progradational stacking has been observed in combination with compensational stacking in the Karoo Basin (e.g. Unit A, Laingsburg Formation; Fan 4, Skoorsteenberg Formation), but never as the predominant stacking pattern.
Grundvåg et al. (2014) proposed that progradational stacking can be associated with 1) basin configurations that limit the space for lateral migration of lobes, 2) tectonic
activity that increases the basin-floor gradient, 3) increased rates of sedimentation due to shelf edge progradation and initiation of larger volume flows, and/or 4) high sediment supply rates resulting in rapid shelf-margin accretion. Essentially, progradational stacking has been attributed to high sediment supply rates by Picot et al. (2016), and a combination of shelf edge progradation and high sediment supple rates by Grundvåg et al. (2014). Macdonald et al. (2011) suggest that progradational stacking of lobe elements can be explained by autocyclic growth of the supply channels through progressive confinement. If similar mechanisms could work on lobe scale has not been determined yet. Progradational stacking patterns might also be more common in proximal and base-of-slope settings where there is less accommodation and less space for lateral compensation and the deposition/ preservation of lobe fringe deposits.
7.2.4 Retrogradational stacking
Unit E.2 of the Fort Brown Formation (Laingsburg depocentre; Chapter 5) is a prominent example of landward stacking of lobes caused by low accommodation through healing of a slide scar on the slope. Retrogradational stacking can be identified by the vertical succession of lobe axis deposits overlain by lobe-off axis and eventually frontal lobe fringe deposits. Care must be taken to distinguish this stacking pattern from lateral offset stacking due to autogenic avulsion processes. These findings conform to subsurface observations made in the Gulf of Mexico indicating temporal evolution of the locus of sedimentation (Prather et al., 2012b) and outcrop observations from highly confined basins e.g. the Peïra Cava Basin, France (Amy et al., 2007), where the shift in deposition has been inferred to be caused by aggradation in the depocentre and an up-slope migration of the slope break (Amy et al., 2007). In general, retrogradational stacking patterns can also be caused through decrease in sediment supply at the end of a depositional cycle (e.g. at the end of a LST) when sediment is trapped on the shelf rather than transported to the deep-water. The same stacking has been described at sequence set scale in Unit C by Di Celma et al. (2011), where the upper LST stacks retrogradationally with respect to the lower two LSTs within the Unit C lowstand sequence set.