The fringes of lobes can stratigraphically separate lobe axis and off-axis deposits as a function of compensational stacking, and can stack to form the fringes of lobe complexes. Lobe fringe deposits are the least well-studied sub-environments of lobes despite showing the widest range of facies configurations (Fig. 7.1a-d). Here, lobe fringe deposits from unconfined to weakly confined settings have been studied to capture the range of sedimentological and stratigraphic expression of lobe fringes (Chapters 3, 4, 5).
Several authors (e.g. MacPherson, 1978; Pickering, 1981, 1983) suggested that lobe deposits show different facies transitions down-dip and across-strike, which could lead to distinctive facies trends and characteristic facies associations for frontal and lateral fringes. This theory has been tested on Fan 4 of the Tanqua depocentre (Chapter 3). Characteristic facies associations and the flow processes are summarized and discussed below.
Figure 7.1. A: Range of lobe fringes within unconfined to subtly confined basin-floor settings. B: Frontal fringes are characterised by pinch-and-swell geometries and the occurrence of hybrid bed deposits. C: Lateral fringes are characterised by thin-beds with planar- and ripple-lamination and a tapering geometry. D: Aggradational lobe fringes are lateral fringes under the influence of subtle confinement resulting in modified sedimentology and stacking patterns, e.g. climbing bedforms.
7.1.1 Frontal lobe fringes
The frontal lobe fringe facies association is characterised by dewatered, structureless or planar laminated fine-grained sandstone (Fig. 7.1a, b; Chapter 3.7.2) associated with hybrid beds and rare debrites. In strike section, frontal fringes exhibit prominent depositional pinch-and-swell geometries at lobe scale, with laterally variable bed thickness. Beds thin or pinch out abruptly at the transition to the distal lobe fringe (Fig. 7.1a, b).
Generally, frontal lobe fringes are dominated by deposits from high-density turbidity currents and other high-concentration flows (structureless sandstones, debrites and hybrid beds). In frontal lobe fringes, there is evidence that relatively distal turbidity currents eroded and entrained substrate material, preserved as mudstone clasts and dispersed mud (Hodgson, 2009, Kane et al., in review) that damped turbulence and resulted in the collapse of the upper part of the flow (McCave & Jones, 1988; Kane et al., in review), leading to the deposition of hybrid bed deposits. Deposits of high- density turbidity currents are able to create their own pathways and become successively more elongated down-dip, forming finger-like bodies as observed in the Tanqua depocentre. These finger-like bodies are 1.5 to 2 km long and 200 to 300 m wide. Finger-like pinchouts of frontal lobes are observed within successive lobes of multiple different lobe complexes within the Tanqua depocentre (Bouma & Rozman, 2000; Rozman, 2000; Prélat et al., 2009; Groenenberg et al., 2010). Similar terminations have also been observed within other basin-floor lobe systems (Nelson et al., 1992; Twichell et al., 1992), albeit occasionally misinterpreted as channel-forms (e.g. Van der Werff & Johnson, 2003b) due to their elongated shape in planform view and their concave-up form in outcrop.
7.1.2 Lateral lobe fringes
The lateral lobe fringe facies association is dominated by thin-bedded (>0.2 m) heterolithic deposits of structureless or planar laminated siltstone and wavy, ripple and climbing ripple laminated very-fine grained sandstone (Fig. 7.1 a, c; Chapter 3.7.1). Lobe pinch out occurs over several kilometres through thinning and fining of the deposits away from the lobe axis and lobe off-axis environments (Fig. 7.1a,c). In outcrop, lateral lobe fringes commonly show tabular geometries at the scale of observation.
Generally, lateral lobe fringes are predominantely characterised by deposits from low- density turbidity currents. Luthi’s (1981) experiments show that flow velocities are
lowest in these flow marginal areas, and that the decrease in flow thickness is greatest laterally away from the central flow axis. The deposits of the low-density turbidity currents probably form laterally extensive radial deposits that are higher in proportion at the lateral fringe, owing to the forward momentum and lack of lateral spreading of the higher concentration flows.
The expression of lateral lobe fringes has been documented in a relatively unconfined basin-floor setting (Chapter 3.7.1). Studies from more confined areas in the Karoo Basin show that the presence of subtle seabed topography can influence the expression of lobe fringes and the stacking of lobe fringes in lobe complexes. Subtle seabed topography was formed by a lateral intrabasinal slope (Unit A, Laingsburg Formation; Chapter 4) and differential compaction on a stepped slope (E1, Fort Brown Formation; Chapter 5). The lobe fringe facies association in subtle confined environments differs from the lobe fringe facies association proposed from the unconfined Tanqua depocentre (see also Prélat et al., 2009), largely due to evidence for high sedimentation rates (climbing ripples and climbing bedforms; Fig. 7.1 a,d) and the persistent aggradational stacking of facies over tens of metres on lobe complex scale.
7.1.3 Aggradational lobe fringes
Aggradational lobe fringes (Fig. 7.1a, d) comprise a heterolithic facies association. Siltstones make up the bulk of the succession. Sandstone beds show stoss-side preserved climbing ripple-, planar, or wavy-lamination. Ripple morphology is preserved on bed tops, and in cross-section. Successions of these ripples form larger dune-like features. The heterolithic package comprises multiple event beds that stack in the direction of palaeoflow (climbing bedforms). Commonly, interbedded sandstones and siltstones form stacked, aggradational packages up to 10 m thick, which internally show no discernible trends in grain size or bed thickness. Palaeocurrents either show a narrow range parallel to the confining slope (Chapter 4) or deviation from the regional palaeocurrent trend that is interpreted to indicate deflection and reflection of turbidity currents off seabed topography (Chapter 5). The facies association of aggradational lobe fringes indicates rapid deposition from density-stratified turbidity currents (e.g. Kneller & Buckee, 2000; Peakall et al., 2000), with a thin basal sand-prone section and a thick mud-prone section, that interact with
the confining seabed topography. The sand-prone portion is confined and pinches out, whilst the fine-grained sediment is held aloft and deposited higher up the confining structure. Stoss-side preserved climbing-ripple lamination indicates deposition beneath flows with high aggradation rates (Allen, 1971a; Allen, 1982; Southard, 1991; Jobe et al. 2012). The lateral facies transition between lobe axis and off-axis to fringe is governed primarily by the height of the topography relative to the thickness of the flows (Muck & Underwood, 1990; Pickering & Hilton, 1998, Wynn et al., 2012).