‘Linked’ debrites in sand-rich turbidite systems – origin and significance



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 post-depositional 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.