The stratigraphic record and processes of turbidity current transformation across deep‐marine lobes

Abstract

13Sedimentary facies in the distal parts of deep+marine lobes can diverge significantly from those 14 predicted by classical turbidite models, and sedimentological processes in these environments are 15 poorly understood. This gap may be bridged using outcrop studies and theoretical models. In the 16Skoorsteenberg Fm., a downstream transition from thickly+bedded turbidite sandstones to 17 argillaceous, internally layered hybrid beds is observed. The hybrid beds have a characteristic 18 stratigraphic and spatial distribution, being associated with bed successions which generally coarsen+ 19 and thicken+upwards reflecting deposition on the fringes of lobes in a dominantly progradational 20 system. Using a detailed characterisation of bed types, including grain size, grain fabric and 21 mineralogical analyses, a process+model for flow evolution is developed. This is explored using a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 and spatially isolated higher+quality reservoir sandstones (Zarra, 2007; Kane and Pontén, 2012). 67In this contribution, the spatial and stratigraphic distribution of the various sedimentary facies The dataset comprises 20 sedimentological logs collected in the field and correlated by walking out 84 individual beds (Fig. 1). These logs were collected at 1:20 scale with more detailed logs of 85 individual beds and packages of beds collected at 1:2 scale (Figs. 2+5). Aerial photographs supported 86 field correlation in areas that were difficult to access or were covered (Fig. 2). Data collected 87 include lithology, bed thickness, and palaeocurrent measurements from ripples, flutes and other sole 88 marks. In addition, the equivalent stratigraphic intervals within cores from 7 research boreholes were 89 logged at 1:20 scale (Fig. 3). During the Permian, the Karoo Basin is interpreted as either a retro+arc foreland basin developed 109 inboard of a fold and thrust belt

Sedimentological regimes for turbidity currents: Depth‐averaged theory

“…Unfortunately, since the theory of Eggenhuisen et al [2017] determines a threshold that is not strictly a function of Ro and Ri, no direct comparison of their threshold with the K-B threshold is possible. While the results of Kane et al [2017] are suggestive, we believe that it is premature to interpret any particular stratigraphic observations using our two-layer flow hypothesis at this time.…”

“…It is interesting to contrast these results with recent work on ''hybrid flows'' of Kane et al [2017], who use the turbulence collapse criterion of Eggenhuisen et al [2017] to identify a threshold in a submarine fan (the Skoorsteenberg Formation) beyond which both debris flow or bedload deposits and turbidites are both present. Unfortunately, since the theory of Eggenhuisen et al [2017] determines a threshold that is not strictly a function of Ro and Ri, no direct comparison of their threshold with the K-B threshold is possible.…”

Physical theory for near-bed turbulent particle suspension capacity

“…In many problems, this region extends only a few millimetres from the flow boundary, and the present theory is therefore especially useful as a concentration boundary condition at flow boundaries in simulations (e.g. Kane et al, 2017). Dynamic suspension support by turbulent stress gradients can generally be neglected in the bulk of the turbulent fluid because turbulent stress gradients are much smaller above the turbulence intensity maximum (Fig.…”

“…This will allow the inclusion of the physical control on turbulent sediment suspension in field studies when the solution of the full equations governing multiphase turbulent transport is beyond the scope of a given study (e.g. Kane et al, 2017). In order to achieve this objective, we establish a force balance parameter that compares turbulent forces near the boundary of a turbulent suspension to gravity and buoyancy forces acting on suspended particles.…”

Dispersion, Accumulation, and the Ultimate Fate of Microplastics in Deep-Marine Environments: A Review and Future Directions

“…Sedimentary facies broadly evolve from thick high density turbidites at the lobe axis, to thin low density turbidites at the lateral and distal fringes (e.g., Johnson et al, 2001;Hodgson et al, 2006). Transformation of turbidity currents into dense cohesive flows is common in these settings, as the fine cohesive material left on the seafloor is picked up and incorporated into incoming turbidity currents (Haughton et al, 2003;Hodgson, 2009;Talling et al, 2013;Kane et al, 2017;Southern et al, 2017;Spychala et al, 2017;Fildani et al, 2018). The deposits of these flows (hybrid beds) typically have an ungraded muddy-sand division which is rich in organic material; microplastics may be incorporated into these internal divisions and, being in a distal locality are less likely to be eroded by subsequent flows.…”