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SIXSMITH, PETE, MARTIN GRECULA, STEVE FLINT, STRAT Group, Dept. of Earth Sciences, University of Liverpool, Liverpool, UK, and DeVILLE WICKENS, Georoutes, Kuilsrivier, Cape Town, Republic of South Africa

Abstract: Evolving Geometries and Architecture of Basin Floor to Slope Turbidite Systems: The Laingsburg Formation, Karoo Basin, South Africa

The Karoo basin of southern Africa is an elongate, retro-arc foreland basin, the initial part of which was filled during the Permian, following retreat of Late Carboniferous ice sheets. Deposits of the Prince Albert and Whitehill Formations (Fig 1) record anoxic conditions in relatively deep water. The superseding Collingham and Vischkuil Formations (Fig 1) record early Cape Fold Belt (CFB) tectonism shown by spectacular ash and mud interlayering and extensive slump zones. CFB deformation led to the creation of several basin floor highs prior to deposition of the Laingsburg Formation, an 800m thick succession of turbidite fans. Present work is indicating these highs were growing during deposition. The Laingsburg Formation comprises six turbidite fan systems, A-F. Within each fan system, cyclicity has been recognised on a variety of scales, with high frequency cycles superimposed on longer duration cycles, representing both allocyclic and autocyclic controls.

The lowermost Fan A complex is the thickest (350m) and comprises discrete fan systems separated by through-going shales. Internal division and palaeocurrent analysis reveals that initially two small, separate fan systems operated independently, 50km apart. As basin floor topography was overcome, progradation of the westernmost fan along the basin axis connected these two previously independent systems. Thickness and facies changes across strike are the result of east-west trending basin floor highs.The northernmost high (The Hexrivier Anticline) generated enough relief to cause slump zones of 20m thickness and at least 7km lateral extent.This basin floor high inhibited the transport of sand northwards and therefore sand-rich bodies on the south side of the basin floor highs correlate in time to sand depleted bodies dominated by ripple and wavy laminated siltstones on the northern flank. The complex flow behaviour related to these topographic features is also registered by palaeocurrent rotations upward through many beds.

Fan B is a very persistent sand-rich system extending continuously for at least 100km along the basin axis, with very few internal fine-grained horizons. Lateral thickness is relatively consistent (80-110m). In the proximal region, a channelized zone of up to 6km in width contains thick, massive sand bodies stacked vertically to 60m. This zone passes laterally into thin bedded, predominantly ripple laminated, very fine "overbank" sandstones, stretching up to 10km either side of the channel zone. Very little syn-sedimentary deformation is observed due to the lack of levee morphology, which we interpret as due to the lack of mud in the system. The channelized zone is moderately sinuous and migrates laterally. Migration of the channels to the north was restricted by the contemporaneously growing Hexrivier Anticline, and only overbank deposits are recorded here. Eastwards along the basin axis, where basin floor structures die out, an unconfined sheet-like, amalgamated succession of proximal lobe deposits is recorded.

Slump horizons in the mudstone intervals above the top of Fan B indicate an increase in the basin floor gradient (progradation of the slope?), and possible further reconfiguration of basinal topography. Overlying sandbodies are highly discontinuous, much thinner than both fan complexes A and B and display higher degrees of lateral variability. Channel-overbank and channel-lobe relationships are still recognised, but on an order of magnitude smaller scale than seen in lower fans. These fans pinch out more proximally than fan complexes A and B and are interpreted to represent a reduced sediment supply, filling subtle intraslope basins. Locally developed hummocky cross stratification within fans E and F indicate water depths within reach of storm waves.These deposits are overlain by pro-delta heteroliths of the Fort Brown Formation.

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