Anatomy, geometry and sequence stratigraphy of basin floor to slope turbidite systems, Tanqua Karoo, South Africa

The Tanqua area of the Karoo basin, South Africa, contains five Permian dee pwater turbidite fan systems, almost completely exposed over some 640 km(2) . Reconstruction of the basin-fill and fan distributions indicates a progra dational 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 settin g (fan 5). Fans are up to 65 in thick with gradational to sharp bases and t ops. Facies associations include basin plain claystone and distal turbidite siltstone/claystone and a range of fine-grained sandstone associations, in cluding low- and high-density turbidite current deposits and proportionally minor debris/slurry flows. Architectural elements include sheets of amalga mated and layered styles and channels of five types. Each fan is interprete d as a low-frequency lowstand systems tract with the shaly interfan interva ls representing transgressive and highstand systems tracts. All fans show c omplex 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 hig her order sequences. Sandy packages between these fine-grained intervals ar e interpreted as high-frequency lowstand systems tracts and exhibit dominan tly progradational stacking patterns, resulting in subtle downdip clinoform geometries. Bases of fans and intrafan packages are interpreted as low- an d high-frequency sequence boundaries respectively. Facies juxtapositions ac ross these sequence boundaries are variable and may be gradational, sharp o r 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. Hi gh-frequency sequences represent the dominant mechanism of active fan growt h in the Tanqua deep-water system.