Structural and Sedimentological Controls of Coal Deposition in the Nongoma Graben, Northern Zululand, South Africa
23
Citation
0
Reference
10
Related Paper
Citation Trend
Abstract:
The Nongoma graben in northern Zululand developed in response to crustal thinning and the first phase of extensional tectonics (rifting) prior to continental break-up and the separation of east and west Gondwanaland. Sedimentation contemporaneous with graben formation led to the deposition of a thick sequence of coal-bearing fluvio-deltaic Ecca Group (Permian) sediments, controlled mainly by episodes of tensional stress build-up and release in the crust, and not to factors inherent in the depositional system, as on the flanking craton to the north. A lower progradational deltaic phase of deposition is succeeded by a fluvial depositional phase characterized by fining-upward sequences which themselves show a gross fining-upward trend throughout the succession. Early fluvial deposition was dominated by high-gradient, low-sinuosity (non-braided) bedload channels. During later fluvial deposition, tectonic events controlling deposition were spaced increasingly further apart. Although low-sinuosity channels developed at first, stabilization of source and depositional site promoted lower gradients, increased production of fines and produced high-sinuosity channels due to denudation and possible drifting of the source area. Thus, late fluvial deposition is dominated by fining-upward depositional couplets, comprising low-sinuosity channel deposits overlain by high-sinuosity channel deposits. Fluvial deposition was terminated by a transgressive deltaic depositional phase. Economically important coals occur at or near the top of fining-upward sequences associated with the high-sinuosity channel facies. Within this facies thick, laterally persistent coal seams formed in extensive floodbasin peat swamps. Overbank flooding was infrequent, with the result that the coals are low ash, low volatile, bright coals containing few shale partings. Depositional modelling indicates that: coal trends are seldom influenced by the thick, laterally extensive sand sheets at the base of the low–high-sinuosity channel depositional couplets; the better-quality coals are of distal floodbasin origin; roof conditions are generally good and the effects of differential compaction minimal. Because of the strong structural imprint on sedimentation, the coals differ in depositional setting and physico-chemical properties from coals in the nearby northern Natal Coalfield where more stable conditions prevailed. This has wider implications for the structural evolution of this part of Africa and the location of coal elsewhere in the southern hemisphere.Keywords:
Sinuosity
Overbank
Deposition
Overbank
Aggradation
Cite
Citations (0)
D etailed seismic-geomorphologic, seismic-stratigraphic, and seismic vizualization analyses of 3D seismic data offshore Indonesia, Nigeria, and the Gulf of Mexico reveal the presence of extensive turbidite and debrite deposits. Key depositional ele- ments include: turbidity flow leveed channels, channel overbank sediment waves, frontal splaysldistributary channel complexes, and debris flow channels, lobes and sheets. These depositional elements will be described and the mode of formation discussed within the context of deepwater sedimentary process and inter- action with local bathymetry. Turbidity flow channel widths range from 2 km to less than 200 nl. Sinuosity ranges from moderate to high, and channel meanders are observed to migrate in a down-system direction. High- sinuosity channels are associated with extensive sediment wave development in proximal overbank settings, especially in association with outer channel bends. The long axes of these sediment waves are oriented normal to the inferred direction of turbidity flow.;. These sediment waves reach heights of 20 m and spacing of 3 km. Overbank thickness decreases systematically down-system. Near to where overbank thickness can no longer be resolved seismically, high-sinuosity isolated channels feed low-sinuosity distributary channel complexes/frontal splays. Low sinuosity distributary channel complexes are expressed a. lobate sheets, in excess of 5-10 knl wide and potentially tens of kilometers long. Notably, they appear to be characterized by channelized flow all the way to the edges of these systems.
Overbank
Sinuosity
Turbidity current
Aggradation
Channelized
Cite
Citations (0)
Most outcrop-based studies of fluvial successions predominantly focus on sand-prone channel complexes; less attention has been directed towards fluvial overbank successions. Crevasse-splay deposits represent an important component of the stratigraphic record of fluvial overbank systems and yield information about the size, form and behaviour of formative fluvial systems.
Quantitative facies and architectural-element analysis was undertaken on outcrop successions from the Morrison Formation (Upper Jurassic) and the Castlegate and Nelsen formations, Mesaverde Group (Upper Cretaceous), this was then supported by analysis of 10 modern fluvial systems to better constrain the planform variations in overbank areas.
Lithofacies arrangements are used to establish the following: (i) recognition criteria for splay elements; (ii) criteria for the differentiation between distal parts of splay bodies and flood plain fines; and (iii) empirical relationships with which to establish the extent (ca. 280-500 m long by 180-1000 m wide) and planform shape of splay bodies in the Morrison Formation (teardrop) and Castlegate and Neslen formations(semi-elliptical).
A nested, hierarchical stacking of the deposits of fluvial overbank successions are recognized and records accumulation of the following components: (i) lithofacies; (ii) individual event beds comprising an association of lithofacies; (iii) splay elements comprising genetically related beds that stack vertically and laterally and represent the deposits of individual flood events; (iv) splay complexes comprising one or more genetically related elements that have a common breakout point and represent the deposits of multiple flood events.
Splay accumulations occur as parts of larger successions in which floodplain-dominated intervals accumulate and become preserved in response to longer-term autogenic controls, such as rate of lateral migration and avulsion frequency of parent channels, and allogenic controls, such as changes in subsidence, climate, base-level and sediment supply. Sandy splays contribute ‘hidden’ volume to fluvial reservoirs and may form significant connectors that link otherwise isolated primary channel bodies, thereby contributing to reservoir connectivity.
Overbank
Outcrop
Transgressive
Cite
Citations (0)
Three sediment transport formulae have been applied to experimental data for sediment transport during inbank and overbank flow in meandering compound channels. The experimental tests were undertaken in the large-scale UK Flood Channel Facility and the small-scale Ulster Channel. The relative depth and channel sinuosity were taken as criteria to establish where sediment transport takes place. An attempt to find a calculation method capable of accurately predicting the mean velocity in the main channel and the total channel cross-sections has been made. Comparison between computed and measured sediment discharges indicates that theYang sediment transport formula applied with the proposed velocity calculation method gives the best predictions for sediment discharge
Overbank
Sinuosity
Bedform
Meander (mathematics)
Cite
Citations (12)
Abstract The geomorphic evolution of the Jordan River in recent decades indicates that interaction between incision and high‐magnitude floods controls sinuosity changes under increasing mouth gradients during base‐level fall. The evolution of the river was analyzed based on digital elevation models, remotely sensed imagery, hydrometric data, and a hydraulic model. The response varies along the river. Near the river mouth, where incision rate is high and a deep channel forms, overbank flooding is less likely. There, large floods exert high shear stress within the confined channel, increasing sinuosity. Upstream, near the migrating knickzone channel gradients also increase, incision is more moderate and floods continue to overtop the banks, favoring meander chute cutoffs. The resulting channel has a downstream well‐confined meandering segment and an upstream low‐sinuosity segment. These new insights regarding spatial differences along an incising channel can improve interpretations of the evolution of ancient planforms and floodplains that responded to base‐level decline. © 2018 John Wiley & Sons, Ltd.
Sinuosity
Overbank
Meander (mathematics)
Stream power
Cite
Citations (24)
Abstract An upward change in palaeochannel morphology from low sinuosity, possibly braided, to high sinuosity channels set in a matrix of overbank fines is deduced for the Scalby Formation. The resulting sandbodies are of sheet and ribbon morphologies, respectively. The early low sinuosity channels are thought to have carried much higher discharges than the later high sinuosity channels. The deduced upward changes in channel morphology and magnitude are thought to be due to initial hinterland upwarp causing high valley gradients and initial fluvial advance into the marine Yorkshire basin followed by rising sea level causing extensive alluviation, lowered gradients and final marine transgression of the Cornbrash sea over the defunct alluvial plains.
Sinuosity
Overbank
Saurashtra
Marine transgression
Palaeochannel
Cite
Citations (36)
Abstract The complex and non‐linear fluvial river dynamics are characterized by repeated periods of fluvial erosion and re‐deposition in different parts of the floodplain. Understanding the fluvial architecture (i.e. the three‐dimensional arrangement and genetic interconnectedness of different sediment types) is therefore fundamental to obtain well‐based information about controlling factors. However, investigating the fluvial architecture in buried floodplain deposits without natural exposures is challenging. We studied the fluvial architecture of the middle Weiße Elster floodplain in Central Germany, an extraordinary long‐standing archive of Holocene flooding and landscape changes in sensitive loess‐covered Central European landscapes. We applied a novel systematic approach by coupling two‐dimensional transects of electrical resistivity tomography (ERT) measurements and closely spaced core drillings with spatially resolved measurements of electromagnetic induction (EMI) of larger floodplain areas at three study sites. This allowed for (i) time and cost‐efficient core drillings based on preceding ERT measurements and (ii) spatially scaling up the main elements of the fluvial architecture, such as the distribution of thick silt‐clay overbank deposits and paleochannel patterns from the floodplain transects to larger surrounding areas. We found that fine‐grained sand and silt‐clay overbank deposits overlying basal gravels were deposited during several periods of intensive flooding. Those were separated from each other by periods of reduced flooding, allowing soil formation. However, the overbank deposits were severely laterally eroded before and during each sedimentation period. This was probably linked with pronounced meandering or even braiding of the river. Our preliminary chronological classification suggests that first fine‐grained sedimentation must have occurred during the Early to Middle Holocene, and the last phase of lateral erosion and sedimentation during the Little Ice Age. Our study demonstrates the high archive potential of the buried fluvial sediments of the middle Weiße Elster floodplain and provides a promising time and cost‐effective approach for future studies of buried floodplain sediments.
Overbank
Silt
Meander (mathematics)
Point bar
Cite
Citations (18)
Overbank
Aggradation
Cite
Citations (2)
The anastomosing fluvial system was considered as the same term of the braided fluvial system. Smith and others have recognized the difference between them. They described the anastomosing fluvial system as low energy complex of several interconnected channels of variable sinuosity crossing a wetland and forming many elongate vegetated islands, and established a depositional model of anastomosing fluvial system based on their great
Sinuosity
Cite
Citations (1)