Abstract
Scours, and scour fields, are common features on the modern seafloor of deep-marine systems, particularly downstream of submarine channels, and in channel-lobe-transition-zones. High-resolution images of the seafloor have improved the documentation of the large scale, coalescence, and distribution of these scours in deep-marine systems. However, their scale and high aspect ratio mean they can be challenging to identify in outcrop. Here, we document a large-scale, composite erosion surface from the exhumed deep-marine stratigraphy of Unit 5 from the Permian Karoo Basin succession in South Africa, which is interpreted to be present at the end of a submarine channel. This study utilizes 24 sedimentary logs, 2 cored boreholes, and extensive palaeocurrent and thickness data across a 126 km2 study area. Sedimentary facies analysis, thickness variations and correlation panels allowed identification of a lower heterolithic-dominated part (up to 70 m thick) and an upper sandstone-dominated part (10–40 m thick) separated by an extensive erosion surface. The lower part comprises heterolithics with abundant current and sinusoidal ripples, which due to palaeocurrents, thickness trends and adjacent depositional environments is interpreted as the aggradational lobe complex fringes. The base of the upper part comprises 2-3 medium-bedded sandstone beds interpreted as precursor lobes cut by a 3–4 km wide, 1–2 km long, and up to 28 m deep, high aspect ratio (1:100) composite scour surface. The abrupt change from heterolithics to thick-bedded sandstones marks the establishment of a new sediment delivery system, which may have been triggered by an updip channel avulsion. The composite scour and subsequent sandstone fill support a change from erosion- and bypass-dominated flows to depositional flows, which might reflect increasingly sand-rich flows as a new sediment route matured. This study provides a unique outcrop example with 3D stratigraphic control of the record of a new sediment conduit, and development and fill of a large-scale composite scour surface at a channel mouth transition zone, providing a rare insight into how scours imaged on seafloor data can be filled and preserved in the rock record.
Highlights
This study describes a unique outcrop in Unit 5 of the Karoo Basin, South Africa, where a large (2 long × 4 wide km) and high aspect ratio (28 m deep) erosion surface can be mapped with three dimensional constraints
The scour surface marks a significant and abrupt change from a lower package of heterolithics to an upper package of amalgamated sandstones, which indicates a change in sediment supply to the area, reflecting either establishment of a new slope conduit, or an updip avulsion event
The underlying thick package of heterolithics is interpreted as aggradationally stacked lobe complex fringes that were deposited in an area of increased subsidence
Summary
Scours are readily recognized erosional bedforms on modern seafloor datasets in deep-marine systems (Wynn et al, 2002; Bonnel et al, 2005; Fildani et al, 2006; Macdonald et al, 2011; Maier et al, 2011, 2020; Covault et al, 2014; Carvajal et al, 2017; Droz et al, 2020) and have been imaged in many high resolution seafloor data, providing more detail about their distribution and geometry (Carvajal et al, 2017; Droz et al, 2020; Maier et al, 2020, 2018). The concretion marker bed and the base of the erosion surface from the logs, which showed that net erosion is up to 28 m (Figure 8A), and 2) mapping the erosion surface using photogrammetric models of the outcrop built from UAV imagery to provide 3D constraints on the shape (Figures 8B–D), and the elevation change from inside to outside the cut to constrain erosion depth The results of both methods showed that the area of maximum erosion is in the west of the study area, between logs SK03 and PK02 forming a deeper low aspect ratio heel of maximum erosion. Unit 5 in NS2 is ∼91 m thick, with a lower ∼30 m siltstone- and sandstone-prone heterolithic package (Figure 10) This a ∼25 m thick, very fine to finegrained, medium to thick-bedded sandstone package punctuates the succession, which is predominantly parallel and ripple laminated (Figure 10). Succession in Fan 4, and support a similar interpretation as the fringe of another lobe complex (Spychala et al, 2017a)
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