The Tanqua area of the Karoo basin, South Africa, contains five Permian deep‐water turbidite fan systems, almost completely exposed over some 640 km2. Reconstruction of the basin‐fill and fan distributions indicates a progradational trend in the 450 m+ thick succession, from distal basin floor (fan 1) through basin‐floor subenvironments (fans 2, 3 and 4) to a slope setting (fan 5). Fans are up to 65 m thick with gradational to sharp bases and tops. Facies associations include basin plain claystone and distal turbidite siltstone/claystone and a range of fine‐grained sandstone associations, including low‐ and high‐density turbidite current deposits and proportionally minor debris/slurry flows. Architectural elements include sheets of amalgamated and layered styles and channels of five types. Each fan is interpreted as a low‐frequency lowstand systems tract with the shaly interfan intervals representing transgressive and highstand systems tracts. All fans show complex internal facies distributions but exhibit a high‐frequency internal stratigraphy based on fan‐wide zones of relative sediment starvation. These zones are interpreted as transgressive and highstand systems tracts of higher order sequences. Sandy packages between these fine‐grained intervals are interpreted as high‐frequency lowstand systems tracts and exhibit dominantly progradational stacking patterns, resulting in subtle downdip clinoform geometries. Bases of fans and intrafan packages are interpreted as low‐ and high‐frequency sequence boundaries respectively. Facies juxtapositions across these sequence boundaries are variable and may be gradational, sharp or erosive. In all cases, criteria for a basinward shift of facies are met, but there is no standard ‘motif’ for sequence boundaries in this system. High‐frequency sequences represent the dominant mechanism of active fan growth in the Tanqua deep‐water system.

No abstract

No 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.

The Middle Permian Skoorsteenberg Formation is part of the Ecca Group (Karoo Supergroup) of South Africa. It is also known as the ‘Tanqua fan complex’ due to its origin as a deep-water sedimentation unit associated with a prograding deltaic system. The Skoorsteenberg Formation crops out over approximately 650 km2 along the western margin of the Main Karoo Basin. It thins out in a northerly and easterly direction and therefore has a limited extent with cut-off boundaries to the south and north. It is underlain by the Tierberg Formation and overlain by the Kookfontein Formation, the latter being limited to the regional distribution of the Skoorsteenberg Formation. The Skoorsteenberg Formation has a composite thickness of 400 m and comprises five individual sandstone packages, separated by shale units of similar thickness. The sandstones are very fine- to fine-grained, light greyish to bluish grey when fresh, poorly sorted and lack primary porosity and permeability. The Tanqua fan complex is regarded as one of the world’s best examples of an ancient basin floor to slope fan complex associated with a fluvially dominated deltaic system. It has served as analogue for many deep-water systems around the world and continues to be a most sought after “open-air laboratory” for studying the nature of fine-grained, deep-water sedimentation. The fan systems are essentially tectonically undeformed, outstandingly well exposed and contain an inexhaustible amount of information on the deep-water architecture of lower slope to basin floor turbidite deposits.