Stratigraphic hierarchy and three‐dimensional evolution of an exhumed submarine slope channel system

Abstract

Submarine slope channel systems have complicated three‐dimensional geometries and facies distributions, which are challenging to resolve using subsurface data. Outcrop analogues can provide sub‐seismic‐scale detail, although most exhumed systems only afford two‐dimensional constraints on the depositional architecture. A rare example of an accessible fine‐grained slope channel complex set situated in a tectonically quiescent basin that offers seismic‐scale, down‐dip and across‐strike exposures is the Klein Hangklip area, Tanqua‐Karoo Basin, South Africa. This study investigates the three‐dimensional architecture of this channel complex set to characterise the stratigraphic evolution of a submarine channel‐fill and the implications this has for both sediment transport to the deep‐oceans and reservoir quality distribution. Correlated sedimentary logs and mapping of key surfaces across a 3 km2 area reveal that: (i) the oldest channel elements in channel complexes infill relatively deep channel cuts and have low aspect‐ratios. Later channel elements are bound by comparatively flat erosion surfaces and have high aspect‐ratios; (ii) facies changes across depositional strike are consistent and predictable; conversely, facies change in successive down depositional dip positions indicating longitudinal variability in depositional processes; (iii) stratigraphic architecture is consistent and predictable at seismic‐scale both down‐dip and across‐strike in three‐dimensions; (iv) channel‐base‐deposits exhibit spatial heterogeneity on one to hundreds of metres length‐scales, which can inhibit accurate recognition and interpretations drawn from one‐dimensional or limited two‐dimensional datasets; and (v) channel‐base‐deposit character is linked to sediment bypass magnitude and longevity, which suggests that time‐partitioning is biased towards conduit excavation and maintenance rather than the fill‐phase. The data provide insights into the stratigraphic evolution and architecture of slope channel‐fills on fine‐grained continental margins and can be utilised to improve predictions derived from lower resolution and one‐dimensional well data.

The Origin and 3D Architecture of a Km-Scale Deep-Water Scour-Fill: Example From the Skoorsteenberg Fm, Karoo Basin, South Africa

“…Previous work has constrained the palaeogeographic context of the study area where there is a downdip architectural change from submarine channel complexes 25 km south of the study area (e.g. Wild et al, 2005;Bell et al, 2020) to lobe-dominated deposits mapped southwest of Skoorsteenberg (Hofstra et al, 2017).…”

“…Regional mapping of an overlying 12 m thick mudstone that correlated these sand-prone units led to the redefinition of Unit 5 (Wild et al, 2009). In proximal (southern) areas of Unit 5 at Kleine Hangklip, stacked W-E and SW-NE orientated submarine slope channel complexes have been interpreted (Wild et al, 2005;Bell et al, 2020) that overlie the updip pinchout of Fans three and 4 (Hansen et al, 2019). In distal (northern) areas of Unit 5 submarine fan deposits have been mapped southeast of the study area at Blaukop, where sand-rich channel-fills incise into proximal lobes (Hofstra et al, 2017).…”

The origin and 3D architecture of a km-scale deep-water scour-fill: example from the Skoorsteenberg Fm., Karoo Basin, South Africa

“…Previous work has constrained the palaeogeographic context of the study area where there is a down-dip architectural change from submarine channel complexes 25 km south of the study area (e.g. Bell et al, 2020;Wild et al, 2005) to lobe-dominated deposits mapped southwest of Skoorsteenberg (Hofstra et al, 2017).…”

“…Regional mapping of an overlying 12 m thick mudstone that correlated these sand-prone units led to the redefinition of Unit 5 (Wild et al, 2009). In proximal (southern) areas of Unit 5 at Kleine Hangklip, stacked W-E and SW-NE orientated submarine slope channel complexes have been interpreted (Bell et al, 2020;Wild et al, 2005) that overlie the updip pinchout of Fans 3 and 4 (Hansen et al, 2019). In distal (northern) areas of Unit 5 submarine fan deposits have been mapped southeast of the study area at Blaukop, where sand-rich channel-fills incise into proximal lobes (Hofstra et al, 2017).…”

Digital re‐evaluation of down‐dip channel‐fill architecture in deep‐water slope deposits: Multi‐scale perspectives from UAV‐SfM

“…However, outcrops are often non‐continuous, variably accessible, and limited to perspectives afforded by exposure orientation relative to depositional dip. Consequently, models of submarine channel evolution have primarily developed from limited views along depositional strike‐oriented perspectives (e.g., Figure 1B; Covault et al, 2016), while the variability of facies and stratigraphic architecture along depositional dip is rarely addressed (Bell et al, 2020; Malkowski et al, 2018; Plink‐Björklund et al, 2001).…”