Disconnected submarine lobes as a record of stepped slope evolution over multiple sea-level cycles

Abstract

The effects of abrupt changes in slope angle and orientation on turbidity current behavior have been investigated in numerous physical and numerical experiments and examined in outcrop, subsurface, and modern systems. However, the long-term impact of subtle and evolving seabed topography on the stratigraphic architecture of deep-water systems requires fine-scale observations and extensive 3-D constraints. This study focuses on the Permian Laingsburg and Fort Brown formations, where multiple large sand-rich systems (Units A-F) have been mapped from entrenched slope valleys, through channel-levee systems, to basin-floor lobe complexes over a 2500 km 2 area. Here, we investigate three thinner (typically <5 m in thickness) and less extensive sand-rich packages, Units A/B, B/C, and D/E, between the large-scale systems. Typically, these sand-rich units are sharp-based and topped, and contain scours and mudstone clast conglomerates that indicate deposition from high-energy turbidity currents. The mapped thickness and facies distribution suggest a lobate form. These distinctive units were deposited in similar spatial positions within the basin-fill and suggest similar accommodation patterns on the slope and basin floor prior to the larger systems (B, C, and E). Stratigraphically, these thin units represent the first sand deposition following major periods of shut-down in sediment supply, and are interpreted as marking a partial re-establishment of sand delivery pathways creating "disconnected lobes" that are fed mainly by flows sourced from failures on the shelf and upper slope rather than major feeder channel-levee systems.Thickness and facies patterns throughout the deep-water stratigraphy suggest seabed topography was present early in the basin formation and maintained persistently in a similar area to ultimately form a stepped slope profile. The stepped slope profile evolved through three key stages of development: Phase 1, where sediment supply exceeds deformation rate (likely caused by differential subsidence); Phase 2, where sediment supply is on average equal to deformation rate; and Phase 3, where deformation rate outpaces sediment supply. This study demonstrates that smaller systems are a sensitive record of evolving seabed topography and they can consequently be used to recreate more accurate paleotopographic profiles.

Clinoform architecture and along‐strike facies variability through an exhumed erosional to accretionary basin margin transition

“…Here, slumps and erosional features are present in the west (up-dip), and two 'thicks' (Figure 3c) coincide with basal sandstone packages in the FB6 and FB8 localities (Figures 4c, 7 and 8). The thickening of Unit G at FB6 is coincident with a zone of thinning of the mudstone package between top Unit F and base Unit G (Figure 3c), suggesting that underlying seabed topography played a significant role in the location of intraslope sandstone-dominated packages, as also described for the underlying units (Brooks, Hodgson, Brunt, Peakall, Poyatos-Moré et al, 2018;Spychala et al, 2015). The abrupt top of the intraslope sandstone body suggests that the feeder system might have abruptly propagated basinward or laterally to this area when the available accommodation was filled, to feed a proximal basin-floor fan beyond the base-of-slope near the FB8 locality (Figures 4c and 8).…”

“…These lower erosive beds are sharp-topped, highly discontinuous and tend to pass abruptly into the background siltstone-dominated sediments (Figure 7e). They are interpreted as sand-rich intraslope deposits in a bypass-dominated part of the slope (Brooks, Hodgson, Brunt, Peakall, Poyatos-Moré et al, 2018;Spychala et al, 2015), and are overlain by heterolithic lower foreset deposits of sub-unit G3.…”

“…The regional mudstone-rich packages between the deep-water units also thin basinward, resulting in a marked wedge-shaped geometry (Figure 10a). Thus, the Vischkuil Formation to Unit F succession shows a persistent area of sediment accumulation in the western part of the Laingsburg depocentre through time, which influenced sediment distribution through the continued generation of subtle seabed topography during deposition of the deep water units (Brooks, Hodgson, Brunt, Peakall, Poyatos-Moré et al, 2018, see also Figure 3b). Unit G itself marks a major transition in the stratigraphic arrangement of the basin fill.…”

“…Laugier & Plink-Björklund, 2016). In the Karoo Basin margin succession, this along-strike variability is inherited from the underlying units, persists through the whole deep-water stratigraphy (Brooks, Hodgson, Brunt, Peakall, Poyatos-Moré et al, 2018) and is also observed in overlying shelf units (Jones et al, 2015). This variability has different expressions and controls in different depositional environments.…”

“…Regional correlations show the Laingsburg depocentre displays a highly asymmetric sedimentary fill (Figure 10), indicating a differential subsidence pattern (higher in the W-SW, lower in the E-NE), particularly documented in the deep-water stratigraphy (Brooks, Hodgson, Brunt, Peakall, Poyatos-Moré et al, 2018;van der Merwe et al, 2014). These areas of greater subsidence were infilled by progressively lower gradient clinothems as sedimentation outpaced subsidence.…”