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Linking Deepwater Fan Morphometrics to Slope Valley Feeder Geometry


A key question in deep water exploration is how much sand is bypassed through slope channels to feed distributive fan systems and how far down dip is the fan? Exceptional regional-scale continuous exposures of the Permian Laingsburg deep water system in the Karoo basin, South Africa have allowed mapping of four complete deep water systems from middle/upper slope to distal basin floor fan. Each mapped sandstone-dominated unit is up to 80 m thick and interpreted as a lowstand sequence set. The overlying 25-35 m thick regional mudstone drape is interpreted as the linked combined transgresive/highstand sequence set. Depositional dip sections of up to 100 km and strike sections of 25 km, built from over 1100 sedimentary logs collected over 10 years have been used to build thickness isopach and facies distribution maps for lowstand sequence sets C, D E and F and their constituent sequences. All key surfaces were walked out between logged sections to ensure accuracy of correlation. The dataset allows estimation of minimum volumes of distributive lobe deposits to be calculated as facies/sandstone percentage trends to be quantified in 3D. In the Unit D composite sequence it has been possible to constrain a minimum volume of sediment in the distributive system (60 km3), all of which must have been bypassed through a single slope valley, 120 m deep and 1.5 lm wide. The fan volumes are compared to published reviews of late Tertiary to recent source to sink systems in terms of linkages between fan size/volume and parameters such as slope length and catchment area. Regional reconstructions and provenance studies indicate a Karoo source area in Patagonia, requiring fluvial transport of over 500 km to a shelf edge east of present day Cape Town, and a slope length of approximately 70 km. However, using the morphometrics database of Somme et al (2009), the fan size and volume of each Karoo composite sequence suggests a small catchment and a slope length in the 20 km range. These major differences in relationship between fan size/volume and up-dip delivery system may be because we are comparing lowstand systems with a database dominated by modern highstand deepwater systems. This indicates that caution is required when comparing modern and ancient deepwater system morphometrics.