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.
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
This is a repository copy of Spatial variability in depositional reservoir quality of deep-water channel-fill and lobe deposits.
The Middle Devonian Gauja Formation in the Devonian Baltic Basin preserves tide-influenced delta plain and delta front deposits associated with a large southward prograding delta complex. The outcrops extend over 250 km from southern Estonia to southern Lithuania. The succession can be divided into 10 facies associations recording distributary channel belts that became progressively more tide influenced when traced southwards towards the palaeo-shoreline, separated by muddy intra-channel areas where deposition was characterized by crevasse splays, delta plain lakes, abandoned channel deposits and tidal gullies. Tidal currents influenced deposition over the entire delta plain, extending up to 250 km from the contemporary shoreline. Tidal facies on the upper delta plain differ from those on the lower delta plain and delta front. In the former case, deposition from river currents was only occasionally interrupted by tidal currents, e.g. during spring tides, resulting in mica and mudstone drapes, and distinctive graded cross-stratification. The lower delta plain was dominated by tidal facies and tidal currents regularly influenced deposition. There was a change from progradation to aggradation from the lower to the upper part of the Gauja Formation coupled with a vertical decrease in tidal influence and a decrease in coarse-grained sediment input. The Gauja Formation contrasts with established models for tide-influenced deltas as the active delta plain was not restricted by topography. The shape of the delta plain, the predominant southward (basinward)-directed palaeocurrents, and the thick sandstone succession, show that although tidal currents strongly influenced deposition at bed scale, rivers still controlled the overall morphology of the delta and the larger-scale bedforms. In addition, there are no signs of wave influence, indicating very low wave energy in the basin. The widespread tidal influence in the Devonian Baltic Basin is explained by changes in the wider basin geometry and by local bathymetrical differences in the basin during progradation and aggradation of the delta plain, with changes in tidal efficiency accompanying the change in basin geometry produced by shoreline progradation.
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