An Alluvial Plain and Lacustrine Model for the Precambrian Witwatersrand Deposits of South Africa
23
Citation
0
Reference
10
Related Paper
Citation Trend
Abstract:
ABSTRACT A depositional model is presented for the gold-rich Precambrian deposits within the Witwatersrand basin. It is a closed basin inset in a now stable craton of the southern African Shield with an areal extent of approximately 39,000 km2 The clastic fill has a maximum thickness of 7,500 m and comprises laterally persistent conglomerates, sandstones and shales occurring within thick (up to 800 m), widespread, and continually alternating coarser and finer-grained sequences. The succession as a whole coarsens upward and is divided into a sandstone and shale dominated lower division and a sandstone and conglomerate dominated upper division. Mineralogical maturity also increases upward, the sandstones of the lower division being mainly sub-greywackes, those of the upper division be ng feldspathic quartzites. Analysis of the abundant sedimentologic data gathered during exploration and development of the gold deposits demonstrates that sedimentation within the basin fits a braided alluvial plain and lacustrine model. The origin and lateral relationships of the different rock types are explained by this depositional setting. The thick alternating coarser and finer grained sequences are explained by a shifting depositional system (sourceward and basinward) controlled by tectonism in the basin and source area. The upward increase in mineralogical maturity and grain size is explained by migration inward of the basin margins, contemporaneous with sedimentation, causing marginal reworking of the lower division sequence as well as regression of the sedimentary succession. The lower division become a distal environmental equivalent of the upper division.Keywords:
Alluvial plain
Alluvial plain
Alluvial soils
Soil series
Soil morphology
Soil horizon
Entisol
New horizons
Cite
Citations (1)
Abstract Recent studies reveal that low‐slope bedrock reaches (bedrock surface slope milder than ~5 m/km) are more common than previously thought and can be found in engineered rivers and densely populated deltas. Here we present a novel formulation of alluvial morphodynamics of low‐slope bedrock rivers transporting nonuniform bed material that accounts for the nonuniformity of the sediment size and the presence of small scale bedforms such as dunes and can thus be of aid to solve management/restoration problems in low‐slope bedrock rivers. The formulation is implemented in a one‐dimensional morphodynamic model. Numerical results are compared with laboratory experiments on equilibrium bedrock reaches downstream of stable alluvial‐bedrock transitions. The differences between experimental and numerical results are comparable with those obtained in the alluvial case. Model applications simulate (1) bedrock reaches with a stable bedrock‐alluvial transitions, (2) an alluvial‐bedrock transition subject to sea level rise, and (3) steep bedrock reaches. Upstream of a stable bedrock‐alluvial transition the flow decelerates in the streamwise direction with the formation of a stable pattern of downstream coarsening of bed surface sediment. In response to sea level rise, alluvial‐bedrock transitions migrate downstream and bedrock‐alluvial transitions migrate upstream. Opposite migration directions are expected in the case of sea level fall. When applied to steep channels, the model predicts gradual alluviation, but it fails to reproduce runaway alluviation.
Bedrock
Beach morphodynamics
Bedform
Alluvion
Cite
Citations (6)
Hydrosphere
Cite
Citations (3)
Mercury
Alluvial plain
Alluvion
Alluvial fan
Cite
Citations (39)
The paper generalizes the data of the alluvium accumulation and soil formation rates in the flood plains of the plain rivers. The method of assessment is based on facies analysis of flood plain deposits containing series of buried soils. The Middle Oka, the Moscow, the Upper Don, and the Nepryadva rivers were investigated. The correlation of paleogeographic events with the conditions of soil formation and human activity in the Holocene were revealed, the stages of soil formation were discovered. We established that at the rate of accumulation more than 0.25 cm/y the soil formation doesn’t change the alluvium, at the rate of 0.1-0.25 cm/y the alluvium have traces of the soil, at the rate of 0.03-0.1 cm/y the cumulative sod-layered soils appear, and at the rate of 0.01-0.03 cm/y the well-developed sod and meadow soils are forming.
Alluvial plain
Alluvial soils
Cite
Citations (3)