Facies Prediction in Turbidite Fan Systems – Nature and Significance of ‘Linked Debrites' in Sand-Rich Versus Mixed Sand-Mud Systems

Peter Haughton¹, Chris Davis¹, and William McCaffrey^2
1 University College Dublin, Dublin, Ireland
² University of Leeds, Leeds, United Kingdom

The stacked deposits of co-genetic turbidity currents and debris flows are a key element of many turbidite systems, particularly in relatively distal and lateral fan fringe settings. They dominate the record of small, syn-tectonic, sand-rich fans, such as the hanging wall Upper Jurassic fans of the North Sea, where debrites repeatedly cap structureless and/or dewatered sandstones emplaced by forerunner turbidity currents. The 'linked' debrites increase in proportion and thickness down-dip, suggesting that they largely bypass sandier mid-fan sections. They are characterised by distinctive fabrics and textures indicative of bulking from the underlying just deposited sand, often in the form of sheared injections. Similar facies are identified in the lateral and distal parts of larger (100 km radius) mixed sand-mud fans such as the Tertiary Forties Fan in the North Sea. Although the disrupted debrite facies have been attributed to local remobilisation and failure induced by salt diapirism – the fan built out over active salt diapirs – analogies with facies in the smaller sand fans, the texture and composition of the debrites, and a comparison of diapir-associated and non-diapir distal/lateral successions argue that they represent far-travelled linked debris-flows. The mixed sand-mud linked debrites show less irregular contacts with the underlying sandstone, more homogenised textures and smaller clasts than the syn-tectonic Jurassic sand-rich fans. This may reflect linked debrite-generation via erosion on the unstable, tectonically-controlled slopes in the case of the small Jurassic fans, but intrafan erosion during incision of the inner fan in the case of the larger Forties system.