Abstract
AbstractSubmarine 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.
Highlights
Submarine slope channels are conduits for some of the largest sediment transport events on Earth (e.g. Piper & Aksu, 1987; GonzalezYajimovich et al, 2007; Talling et al, 2007; Jobe et al, 2018), with individual flows up to an order of magnitude larger than the global annual flux of rivers to the ocean (Milliman & Syvitski, 1992; Talling et al, 2007; Sømme et al, 2009)
Most outcrop analogues are from relatively small foreland basins, with small catchments and coarse-grained sediment (Beaubouef, 2004; De Ruig & Hubbard, 2006; Jobe et al, 2010; Moody et al, 2012; Hubbard et al, 2014; Bain & Hubbard, 2016; Casciano et al, 2019), which are poor analogues for the comparatively large, fine-grained and mud-rich systems that are common in offshore passive margin settings with large drainage basins (e.g. Reading & Richards, 1994; Bouma, 2000; Stelting et al, 2000; Hubbard et al, 2005; Pickering & Corregidor, 2005; Prelat et al, 2010; Kane & Ponten, 2012)
The channel complex set is interpreted to consist of two channel complexes: stratigraphically, ‘KHKC’ and ‘KHKD’, each of which is made up of four channel elements
Summary
Submarine slope channels are conduits for some of the largest sediment transport events on Earth (e.g. Piper & Aksu, 1987; GonzalezYajimovich et al, 2007; Talling et al, 2007; Jobe et al, 2018), with individual flows up to an order of magnitude larger than the global annual flux of rivers to the ocean (Milliman & Syvitski, 1992; Talling et al, 2007; Sømme et al, 2009). The evolution and character of slope channels is challenging to decipher using subsurface data, as they are often characterised by complicated three-dimensional (3D) facies heterogeneity and depositional geometries at sub-seismic scale. Outcrop analogues can help to bridge this scale gap and provide data to populate 3D bodies mapped in seismic with stratigraphic and facies information (e.g. Bryant & Flint, 1992; Clark & Pickering, 1996; Campion et al, 2000; Sullivan et al, 2000; McCaffrey & Kneller, 2001; Hodgetts et al, 2004; Bakke et al, 2008, 2013; Hofstra et al, 2017). Most outcrop analogues are from relatively small foreland basins, with small catchments and coarse-grained sediment (Beaubouef, 2004; De Ruig & Hubbard, 2006; Jobe et al, 2010; Moody et al, 2012; Hubbard et al, 2014; Bain & Hubbard, 2016; Casciano et al, 2019), which are poor analogues for the comparatively large, fine-grained and mud-rich systems that are common in offshore passive margin settings with large drainage basins (e.g. Reading & Richards, 1994; Bouma, 2000; Stelting et al, 2000; Hubbard et al, 2005; Pickering & Corregidor, 2005; Prelat et al, 2010; Kane & Ponten, 2012)
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