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 currents, debris flows and a spectrum of intermediate flow types termed slurry flows. The term ‘slurry flow’ is used here to refer to watery flows transitional between turbidity currents, in which particles are supported primarily by flow turbulence, and debris flows, in which particles are supported by flow strength. Thick, clean, dish-structured sandstones and associated thin-bedded sandstones showing Bouma Tb–e divisions were deposited by high- and low-density turbidity currents respectively. Debris flow deposits are marked by deformed, intraformational mudstone and sandstone masses suspended within a sand-rich mudstone matrix. Most Britannia slurry-flow deposits contain 10–35% detrital mud matrix and are grain supported. Individual beds vary in thickness from a few centimetres to over 30 m. Seven sedimentary structure division types are recognized in slurry-flow beds: (M1) current structured and massive divisions; (M2) banded units; (M3) wispy laminated sandstone; (M4) dish-structured divisions; (M5) fine-grained, microbanded to flat-laminated units; (M6) foundered and mixed layers that were originally laminated to microbanded; and (M7) vertically water-escape structured divisions. Water-escape structures are abundant in slurry-flow deposits, including a variety of vertical to subvertical pipe- and sheet-like fluid-escape conduits, dish structures and load structures. Structuring of Britannia slurry-flow beds suggests that most flows began deposition as turbidity currents: fully turbulent flows characterized by turbulent grain suspension and, commonly, bed-load transport and deposition (M1). Mud was apparently transported largely as hydrodynamically silt- to sand-sized grains. As the flows waned, both mud and mineral grains settled, increasing near-bed grain concentration and flow density. Low-density mud grains settling into the denser near-bed layers were trapped because of their reduced settling velocities, whereas denser quartz and feldspar continued settling to the bed. The result of this kinetic sieving was an increasing mud content and particle concentration 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 grains caused a rapid increase in effective clay surface area and, hence, near-bed cohesion, shear resistance and viscosity. Eventually, turbulence was suppressed in a layer immediately adjacent to the bed, which was transformed into a cohesion-dominated viscous sublayer. The banding and lamination in M2 are thought to reflect the formation, evolution and deposition of such cohesion-dominated sublayers. More rapid fallout from suspension in less muddy flows resulted in the development of thin, short-lived viscous sublayers to form wispy laminated divisions (M3) and, in the least muddy flows with the highest suspended-load fallout rates, direct suspension sedimentation formed dish-structured M4 divisions. Markov chain analysis indicates that these divisions are stacked to form a range of bed types: (I) dish-structured 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 currents and debris flows and the textures and structuring of their deposits.