Facies Prediction in Turbidite Systems – the Impact of Up-Dip Incision

Peter Haughton¹, Simon Barker¹, Chris Davis¹, and William McCaffrey^2
1 University College Dublin, Dublin 4, Ireland
² University of Leeds, Leeds, United Kingdom

Proximal-distal facies transitions in turbidite systems are widely guided by a perception of downslope flow dilution with thin-bedded, low concentration turbidites deposited close to the flow runout limit. However, an increasing number of fans, including those in tectonically-active basins, do not fit this pattern, with distinctive turbidite-debrite couplets now recognised as important distal fan elements. In some cases (e.g. Jurassic fans in the North Sea), these couplets dominate the distal fan record throughout the growth and decay of the fans. In others, the couplets either abound in the early fan ‘switch-on' phase (e.g. Ross Sandstone Formation, Co. Clare), or occur as intrafan bundles enclosed by otherwise conventional low concentration basin floor turbidites (e.g. Marnoso-arenacea Fm.). The ‘linked' debrites that cap the turbidite sandstones to form the couplets are thought to record periods of significant up-dip erosion that choke and collapse part of a forerunner sandy turbidity current to form a relatively low velocity debris flow that propagates over the just-deposited wet sand. Where the debris flow caps dominate the distal fan stratigraphy throughout, this indicates repeated phases of up-dip erosion on steep, fault-maintained slopes. Where they are restricted to early fan successions, they record the opening up or re-opening of more permanent conduits that thereafter act to transmit flows with out significant slope erosion. Where developed as local intrafan bundles, this may relate to periods of intrafan deformation leading to alternating phases of localised erosion and normal flow transmission.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005