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

Abstract

Sedimentary facies in the distal parts of deep-marine lobes can diverge significantly from those predicted by classical turbidite models, and sedimentological processes in these environments are poorly understood. This gap may be bridged using outcrop studies and theoretical models. In the Skoorsteenberg Fm., a downstream transition from thickly-bedded turbidite sandstones to argillaceous, internally layered hybrid beds is observed. The hybrid beds have a characteristic stratigraphic and spatial distribution, being associated with bed successions which generally coarsen- and thicken-upwards reflecting deposition on the fringes of lobes in a dominantly progradational system. Using a detailed characterisation of bed types, including grain size, grain fabric and mineralogical analyses, a process-model for flow evolution is developed. This is explored using a numerical suspension capacity model for radially spreading and decelerating turbidity currents. The new model shows how decelerating sediment suspensions can reach a critical suspension capacity threshold beyond which grains are not supported by fluid turbulence. Sand and silt particles, settling together with flocculated clay, may form low yield-strength cohesive flows; development of these higher concentration lower boundary layer flows inhibits transfer of turbulent kinetic energy into the upper parts of the flow ultimately resulting in catastrophic loss of turbulence and collapse of the upper part of the flow. Advection distances of the now transitional to laminar flow are relatively long (several km) suggesting relatively slow dewatering (several hours) of the low yield strength flows. The catastrophic loss of turbulence accounts for the presence of such beds in other fine-grained systems without invoking external controls or large-scale flow partitioning, and also explains the abrupt pinch-out of all divisions of these sandstones. Estimation of the point of flow transformation is a useful tool in the prediction of heterogeneity distribution in subsurface systems.

Sedimentological regimes for turbidity currents: Depth‐averaged theory

“…It is interesting to contrast these results with recent work on “hybrid flows” of Kane et al . [], who use the turbulence collapse criterion of Eggenhuisen et al . [] to identify a threshold in a submarine fan (the Skoorsteenberg Formation) beyond which both debris flow or bedload deposits and turbidites are both present.…”

“…While the results of Kane et al . [] are suggestive, we believe that it is premature to interpret any particular stratigraphic observations using our two‐layer flow hypothesis at this time.…”

Variable character and diverse origin of hybrid event beds in a sandy submarine fan system, Pennsylvanian Ross Sandstone Formation, western Ireland

“…Many published models seek to explain the close association of turbidite, transitional flow deposit and debrite emplaced as part of the same event (Haughton et al ., ; Talling et al ., ; Kane & Pontén, ; Kane et al ., ). Experiments involving deceleration of clay‐laden flows in the laboratory have shown that it is possible to segregate basal sand from the suspension before the onset of cohesion and en masse deposition of sand and clay together to form the remainder of the bed (Baas et al ., ).…”

Turbidites facies response to the morphological confinement of a foredeep (Cervarola Sandstones Formation, Miocene, northern Apennines, Italy)

“…() in the Gottero Sandstone. In particular, the occurrence of slurry beds in Log 4 can be related to the deceleration of mud‐rich turbulent flows against the Pievepelago high which, through the increase in fallout rate and consequent turbulence suppression, tend to favour the development of mud‐rich slurries (bed type 6‐4) (see Barker et al ., ; Sumner et al ., ; Baas et al ., ; Tinterri et al ., ; Kane et al ., ).…”