Slurry Flows Revisited – Re-interpretation of the Occurrence and Significance of Banding

Simon P. Barker¹, Peter D. W. Haughton², William D. McCaffrey³, and Bill Hakes^4
1 BG Group, Reading, United Kingdom
² University College Dublin, Dublin, Ireland
³ University of Leeds, Leeds, United Kingdom
⁴ Britannia Operator Ltd, Aberdeen, United Kingdom

“Slurry flow” emplacement of deep-marine clastic sediments is thought to occur from flows alternating cyclically between laminar and turbulent states, giving rise to a characteristic banding in the deposit, and locally chaotic ‘mixed slurry' facies. Banding and mixed slurry facies occurs in lateral association with more conventional turbidite sandstone facies, and the two facies associations have previously been interpreted to occur in axial succession. A re-evaluation of densely-cored slurry flow deposits in the upper part of the Cretaceous Britannia Sandstone Member (Outer Moray Firth, offshore UK) indicates the Britannia flows became strongly texturally-fractionated both longitudinally and transverse to flow, producing off-axis clay-prone slurry facies, locally overprinted by secondary remobilisation, and axial sandier deposits, capped by slurry facies. At times, episodes of slope failure produced more irregular sea floor topography with which the flows interacted, resulting in more complex facies and thickness distributions and localised flow ponding. Thus in assessing links between turbulent and laminar flow components, care must be taken to rule out overprinting due to syn- or immediately post-depositional modification by local gravity failures that can produce the mixed slurry facies. Transitional slurry facies (including banding) analogous to the Britannia system are relatively common in other systems. For example, banding occurs within the basal Ross Formation (Carboniferous of Co. Clare, Ireland), albeit in beds that are an order of magnitude thinner than those typical of Britannia, and records transient flow transformation in longitudinally-fractionated flows close to their runout limit.