No abstract

The outer parts of a number of small Late Jurassic sandy deep-water fans in the northern North Sea are dominated by the stacked deposits of co-genetic sandy and muddy gravity flows. Sharp-based, structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones, pseudonodular siltstones and mudstones. The contacts between the clast-rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection. The breccias contain fragments (up to metre scale) of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains (often coarser than in the immediately underlying sand bed). Markov chain analysis establishes that the breccias consistently overlie sandstones, and the character of the breccias and their external contacts rule out a postdepositional origin via in situ liquefaction, intrastratal flowage or bed amalgamation and disruption. The breccias are interpreted as debrites that rode on a water-rich sand bed just deposited by a co-genetic concentrated gravity current. As such, they are referred to as 'linked debrites' to distinguish them from debrites emplaced in the absence of a precursor sand bed. The distinction is important, because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath, and they ride on a low-friction carpet of liquefied sand. This explains the paradox presented by fan fringes in which there are common debrites, when conventional thinking might predict that deposits of low-concentration gravity currents should be more important here. In fact, evidence for transport by low-concentration turbidity currents is rare in these systems. Several possible mechanisms might explain the formation of linked flows, but the ultimate source of both sandy and clast-rich flow components must be in shallower water on the basin margin (the debrites are not triggered from distal slopes). Flow partitioning may have occurred by upslope erosion and retardation of the mudclast-charged portion of an erosional sandy density current, partial flow transformation of a precursor debris flow and/or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport. Linked debrites are not restricted to small sand-rich fans, and similar mechanisms may be responsible for the long runout of debris flows in other systems. The recognition of a distinct class of linked debrites is of wider importance for facies prediction, reservoir heterogeneity and even carbon fluxes and sequestration on continental margins.

This study documents the character and occurrence of hybrid event beds deposited across a range of deep-water sub-environments in the Cretaceous-Palaeocene Gottero system, north-west Italy. Detailed fieldwork (>5200 m of sedimentary logs) has shown that hybrid event beds are most abundant in the distal confined basin plain domain (>31% of total thickness). In more proximal sectors, HEBs occur within outer-fan and mid-fan lobes (up to 15% of total thickness), whereas they are not observed in the inner-fan channelised area. Six hybrid event bed types (HEB-1 to HEB-6) were differentiated mainly on basis of the texture of their muddier and chaotic central division (H3). The confined basin plain sector is dominated by thick (max 9.57 m; average 2.15 m) and tabular hybrid event beds (HEB-1 to HEB-4). Their H3 division can include very large substrate slabs, evidence of extensive auto-injection and clast break-up, and abundant mudstone clasts set in a sandy matrix (dispersed clay ca 20%). These beds are thought to have been generated by highly This article is protected by copyright. All rights reserved.energetic flows capable of delaminating the sea-floor locally, and carrying large rip-up clasts for relatively short distances before arresting. The unconfined lobes of the mid-fan sector are dominated by thinner (average 0.38 m) hybrid event beds . Their H3 divisions are characterised by floating mudstone clasts and clay-enriched matrices (dispersed clay >25%) with hydraulically-fractionated components (mica, organic matter and clay flocs). These hybrid event beds are thought to have been deposited by less energetic flows that underwent early turbulence damping following incorporation of mud at proximal locations and by segregation during transport. Although there is a tendency to look to external factors to account for hybrid event bed development, systems like the Gottero imply that intrabasinal factors can also be important; specifically the type of substrate available (muddy or sandy) and where and how erosion is achieved across the system producing specific hybrid event bed expressions and facies tracts. (A) INTRODUCTIONBed character and bed stack architecture are two key elements controlling the heterogeneity of deep-water turbidite systems. The former represents the depositional record of sediment gravity flows at a given location with the vertical sequence of grain-size, textures and sedimentary structures recording flow evolution in time and space (Bouma, 1962; Lowe, 1982; Mutti, 1992; Kneller, 1995; Kneller & McCaffrey, 2003). The latter is set by the longer term response of the system to variations in flow volume and concentration over many events modulated by the inherited seafloor topography (Prélat et al., 2010; Brunt et al., 2013b; Marini et al., 2015a). Both define the character and distribution of sedimentary sub-environments in deep-water systems.A wide range of sediment gravity flow deposits have been recognised in turbidite systems; they include the well-known Bouma-type graded sandstones an...

No abstract

Hybrid event beds comprising both clean and mud‐rich sandstone are important components of many deep‐water systems and reflect the passage of turbulent sediment gravity flows with zones of clay‐damped or suppressed turbulence. ‘Behind‐outcrop’ cores from the Pennsylvanian deep‐water Ross Sandstone Formation reveal hybrid event beds with a wide range of expression in terms of relative abundance, character and inferred origin. Muddy hybrid event beds first appear in the underlying Clare Shale Formation where they are interpreted as the distal run‐out of the wakes to flows which deposited most of their sand up‐dip before transforming to fluid mud. These are overlain by unusually thick (up to 4·4 m), coarse sandy hybrid event beds (89% of the lowermost Ross Formation by thickness) that record deposition from outsized flows in which transformations were driven by both substrate entrainment in the body of the flow and clay fractionation in the wake. A switch to dominantly fine‐grained sand was accompanied initially by the arrest of turbulence‐damped, mud‐rich flows with evidence for transitional flow conditions and thick fluid mud caps. The mid and upper Ross Formation contain metre‐scale bed sets of hybrid event beds (21 to 14%, respectively) in (i) upward‐sandying bed set associations immediately beneath amalgamated sheet or channel elements; (ii) stacked thick‐bedded and thin‐bedded hybrid event bed‐dominated bed sets; (iii) associations of hybrid event bed‐dominated bed sets alternating with conventional turbidites; and (iv) rare outsized hybrid event beds. Hybrid event bed dominance in the lower Ross Formation may reflect significant initial disequilibrium, a bias towards large‐volume flows in distal sectors of the basin, extensive mud‐draped slopes and greater drop heights promoting erosion. Higher in the formation, hybrid event beds record local perturbations related to channel switching, lobe relocations and extension of channels across the fan surface. The Ross Sandstone Formation confirms that hybrid event beds can form in a variety of ways, even in the same system, and that different flow transformation mechanisms may operate even during the passage of a single flow.