Slurry-flow deposits in the Britannia Formation (Lower Cretaceous), North Sea: a new perspective on the turbidity current and debris flow problem

The Lower Cretaceous Britannia Formation (North Sea) includes an assemblage of sandstone beds interpreted here to be the deposits of turbidity current s, debris flows and a spectrum of intermediate flow types termed slurry flo ws. The term 'slurry flow' is used here to refer to watery flows transition al between turbidity currents, in which particles are supported primarily b y flow turbulence, and debris flows, in which particles are supported by fl ow strength. Thick, clean, dish-structured sandstones and associated thin-b edded sandstones showing Bouma Tb-e divisions were deposited by high- and l ow-density turbidity currents respectively. Debris flow deposits are marked by deformed, intraformational mudstone and sandstone masses suspended with in a sand-rich mudstone matrix. Most Britannia slurry-flow deposits contain 10-35% detrital mud matrix and are grain supported. Individual beds vary i n thickness from a few centimetres to over 30 m. Seven sedimentary structur e division types are recognized in slurry-flow beds: (M-1) current structur ed and massive divisions; (M-2) banded units; (M-3) wispy laminated sandsto ne; (M-4) dish-structured divisions; (M-5) fine-grained, microbanded to fla t-laminated units; (M-6) foundered and mixed layers that were originally la minated to microbanded; and (M-7) vertically water-escape structured divisi ons. Water-escape structures are abundant in slurry-flow deposits, includin g a variety of vertical to subvertical pipe- and sheet-like fluid-escape co nduits, dish structures and load structures. Structuring of Britannia slurr y-flow beds suggests that most flows began deposition as turbidity currents : fully turbulent flows characterized by turbulent grain suspension and, co mmonly, bed-load transport and deposition (M-1). Mud was apparently transpo rted largely as hydrodynamically silt- to sand-sized grains. As the flows w aned, both mud and mineral grains settled, increasing near-bed grain concen tration and flow density. Low-density mud grains settling into the denser n ear-bed layers were trapped because of their reduced settling velocities, w hereas denser quartz and feldspar continued settling to the bed. The result of this kinetic sieving was an increasing mud content and particle concent ration in the near-bed layers. Disaggregation of mud grains in the near-bed zone as a result of intense shear and abrasion against rigid mineral grain s caused a rapid increase in effective clay surface area and, hence, near-b ed cohesion, shear resistance and viscosity. Eventually, turbulence was sup pressed in a layer immediately adjacent to the bed, which was transformed i nto a cohesion-dominated viscous sublayer. The banding and lamination in M- 2 are thought to reflect the formation, evolution and deposition of such co hesion-dominated sublayers. More rapid fallout from suspension in less mudd y flows resulted in the development of thin, short-lived viscous sublayers to form wispy laminated divisions (M-3) and, in the least muddy flows with the highest suspended-load fallout rates, direct suspension sedimentation f ormed dish-structured M-4 divisions. Markov chain analysis indicates that t hese divisions are stacked to form a range of bed types: (I) dish-structure d beds; (II) dish-structured and wispy laminated beds; (III) banded, wispy laminated and/or dish-structured beds; (IV) predominantly banded beds; and (V) thickly banded and mixed slurried beds. These different bed types form mainly in response to the varying mud contents of the depositing flows and the influence of mud on suspended-load fallout rates. The Britannia sandstones provide a remarkable and perhaps unique window on the mechanics of sediment-gravity flows transitional between turbidity curr ents and debris flows and the textures and structuring of their deposits.