Upward‐thickening patterns and lateral continuity of Permian sand‐rich turbidite channel fills, Laingsburg Karoo, South Africa

Abstract

Analysis of extensive exposures of the Permian Laingsburg Formation, Karoo basin, South Africa, have enabled a detailed reconstruction of the base of slope stratigraphy and palaeoenvironments in a deep‐water system characterized by a very narrow grain‐size range (fine sandstone). The deposits include an ≈ 4 km wide and 80 m thick channel complex, fringed by sandy sheet deposits that extend laterally for at least 6 km across depositional strike. Within the channel complex, individual channel fills are marked by shallow basal erosion surfaces draped by thin, parallel‐stratified beds of very fine sandstone and siltstone, interpreted as flow tails to largely bypassing flows. These thin beds are overlain by 0·4 to 5 m thick beds of structureless, fine‐grained sandstone that represent the majority of the channel fills. The basal packages may be partially to completely removed by localized scour in the axial zone of the channel complex but can be mapped laterally into overbank areas where they thicken and are dominated by rippled fine sandstones with intercalated siltstones. Axial confinement resulted from subtle topography on the basin floor, whereby the lower, dense parts of the initially erosive and bypassing flows were partially confined in the lows and the more dilute, slower moving upper parts of the flows deposited sheet‐like successions across slightly elevated overbank areas. The narrow grain‐size distribution prohibited the formation ofcoarse‐grained residual bypass deposits during the initial phases of channel formation. With decreasing magnitude, later flows became more depositional, filling remaining axial depressions with thick‐bedded structureless sandstone. The smaller volumes of late‐stage sediment were more axially focused, producing local scour‐and‐fill features and starvation of the overbank areas. Resulting grain‐size vertical profiles are complex. The basal flow tail packages and overlying massive deposits form a thickening and slightly coarsening‐upward trend in the channel fills. The overbank deposits show a thinning‐ and fining‐upward profile as a result of less bypass plus late‐stage starvation of sand. Application of traditional deep‐water facies models could therefore potentially lead to erroneous interpretations of the channel complex as a prograding lobe and the overbank sheets as channel‐fills.

Quantitative seismic geomorphology of a submarine channel system in SE Brazil (Espírito Santo Basin): Scale comparison with other submarine channel systems

“…Samuel et al, 2003;Deptuck et al, 2007;Catteral et al, 2010;De Ruig and Hubbard, 2006;Kolla et al, 2012;Jolly et al, 2016), and at outcrops (e.g. Bain and Hubbard, 2016;Grecula et al, 2003). In the Indus fan, the valley can be up to 10 km wide (Deptuck et al, 2003).…”

Integrating outcrop and subsurface data to assess the temporal evolution of a submarine channel–levee system

“…overlying Fort Brown Formation (Figure 2) is punctuated by five sandstone-rich units (units C-G; Flint et al, 2011) that comprise slope channel-levee systems separated vertically by extensive mudstone units (Grecula et al, 2003;Figueiredo et al, 2010;Di Celma et al, 2011;Hodgson et al, 2011;Morris et al, 2014a). These mudstone units constrain the stratigraphy and have been mapped for up to 90 km (56 mi) downdip (van der Merwe et al, 2014).…”

“…Deep-water deposition began with distal basin floor deposits of the Vischkuil Formation (van der Merwe et al, 2009Merwe et al, , 2010, which is overlain by basin floor and base-of-slope fan systems of the Laingsburg Formation (units A and B; Grecula et al, 2003;Sixsmith et al, 2004). The muddy slope succession of the 0.5-km-thick (1640-ft-thick) …”

Capturing stratigraphic and sedimentological complexity from submarine channel complex outcrops to digital 3D models, Karoo Basin, South Africa

“…2 for detail). The Laingsburg Formation is further subdivided into basal Fan A, a basin-floor fan system (Sixsmith et al 2004) overlain by Units B-F. Each unit represents a period of sandstone deposition in a mud-dominated, progradational system spanning basin-floor to upper-slope environments (Grecula et al 2003). The deep-water succession has a maximum grain size of fine sandstone so -for the purpose of reservoir modelling -very-fine-grained sandstone and coarser is considered as net.…”

“…The deep-water succession has a maximum grain size of fine sandstone so -for the purpose of reservoir modelling -very-fine-grained sandstone and coarser is considered as net. Ongoing field-based research has shown that additional minor sandstone-prone units are present which are key to regional correlation: the A/B interfan between Fan A and Unit B and the B/C interfan between Units B and C (Grecula et al 2003), as well as a basin-wide, 30 cm thick marker bed called the Thin Siltstone Interval (TSI) within slope mudstones beneath Unit B (see Fig. 2 for detail).…”