Uranium mineralization in the fluviatile Upper Permian Beaufort Group of the southwestern Karoo basin, South Africa, is mainly confined to one sandstone-rich unit. The reason for this stratigraphic confinement of the mineralization can be related to the geomorphic evolution of the basin and the deposition of the host sandstones at a time when gradients and sediment input were declining in response to weathering and denudation of an initially uplifted, volcanically active source area. As a result, orographic rainfall declined, promoting deeper, more intense weathering of the source rocks and slower rates of sedimentation at the depositional site. This provided optimum conditions for leaching and solution of metals while increasing the residence time of the mineralizing solutions at the depositional site, and the possibility of precipitating minerals from the solution on encountering a suitable reducing agent. Mineralization is mainly confined to the base of the thicker, more permeable channel sandstones trending down the regional paleoslope. Erosion of the deposits was effectively prevented by deposition of a thick overlying sequence of flood basin mudstones and by early compaction and diagenesis of the sediments rendering them impervious to later fluid movement. Thus, mineralization has retained its original character and was relatively unaffected by later remobilization and reconcentration processes.The uranium was probably leached from intrabasinal volcanic detritus in the host sediments by weathering and diagenesis in an arid climate, and was mobilized as uranyl carbonate complexes by oxygenated alkaline ground waters. These moved through the more permeable channel sand bodies, and on encountering bacterially generated H 2 S at sites of differential permeability, uranium was precipitated as coffinite and uraninite. Postburial changes in ground-water chemistry caused the primary oxidized sandstones to be reduced, a process that was concentrated in the more permeable sandstones leaving the mudstones relatively unaffected.Mineralization occurred shortly after deposition but before deep burial and final reduction of the host sandstones. The timing of mineralization is indicated by the replacement of undeformed plant structures by ore minerals, the presence of laumontite, and the estimated depths at which bacterial activity and decarboxylation can occur. These factors suggest an early diagenetic origin for the mineralization which must have occurred within a comparatively short period of time after deposition at temperatures of less than 50 degrees C and depths of less than about 1,400 m.
Upper Karoo sediments (Middle to Upper Triassic) in NE Swaziland consist of an upward-fining, fluvially dominated sequence (Molteno and Elliot Formations) followed by the mainly aeolian Clarens Formation. Diamonds and associated kimberlitic garnets are strata-bound in two thin upward-fining sequences (4–16 m) at the base of the Elliot Formation. These sequences comprise erosionally based, clast-supported conglomerate channel-fill, overlain by coarse, feldspathic, cross-stratified sandstone and rippled siltstone with gritty layers. Diamonds and garnets occur in the matrix of the conglomerate, but are five times more abundant in the coarse feldspathic sandstone. They also occur in the sandy mode in the overlying siltstone, but in greatly diluted concentrations. The diamonds were locally derived and released to the sedimentary system following a climatic change when conditions favoured more intense erosion of nearby kimberlite pipes. Diamonds and garnets released to the passing sediment were concentrated locally in thin (<2m), extensive channel sands deposited by overloaded ephemeral flood flows. Source area denudation and the increasing aridity of the climate during Upper Karoo times led to a gradual reduction in slope and sediment supply, with the result that river energy and bedload diminished, producing a gross upward-fining trend in the succession.
ABSTRACT The Upper Triassic (Carnian?) Molteno Formation in the main Karoo (Gondwana) Basin, South Africa forms a northerly thinning, intracratonic clastic wedge comprising sandstones, shales and coals occurring within thick (up to 140 m) laterally persistent fining‐upward sequences. These sequences were deposited by braided streams draining an alluvial plain which may have been built on to the distal slopes of alluvial fan complexes of glacial outwash type. Geometric relations between sequences indicate three phases of tectonic activity. The lowermost fining‐upward sequence in the south accumulated against a rising mountain front; cessation of movement and an eastward shift in the main locus of tectonism and sedimentation was followed by renewed uplift and basinwide progradation of the second fining‐upward sequence adjacent to a fault‐block granite terrain located close to the present south‐east coastline of South Africa. This is believed to be the granite at the eastern end of the Falkland Island Plateau, an interpretation consistent with its position on most continental reconstructions and the fracture zone marking its northern scarp face. Faulting is attributed to the first phase of extension prior to continental breakup. The sourceward recession and lack of gross fining‐upward trends shown by the uppermost fining‐upward sequences is accounted for by limited back‐faulting of the still active basin margin. Cessation of activity and further basin margin recession occurred with deposition of the overlying floodplain deposits (Elliot Formation) which were distal equivalents of the braided alluvial plain.
ABSTRACT ODP Leg 119 drilled a sequence of ?Permo-Triassic continental red beds 58 m thick containing paleosols in Prydz Bay, East Antarctica. The paleosols, mainly in overbank siltstones and mudstones, are characterized by extensive destruction of sedimentary structures and the presence of mottles, rootlets, and rare chalky caliche. Because of the shallow depth of burial and limited compaction and diagenesis, the paleosols have retained most of their original microstructure and fabric. Micromorphological analysis of the paleosols reveals the presence of phreatic monopodial root systems, nodules, and a variety of soil structures and fabrics as well as a micritic root structure with alveolar texture. Most of the paleosols are weakly developed, with small, incomplete ped structures and lack of illuviation. They resemble weakly developed alluvial soils with A-C and B(C) profiles, and are similar to modern base-deficient inceptisols. The nature of the paleosols suggests relatively high permeabilities and water circulation during soil formation and a relatively mobile water table, with little reduction of iron except locally on the less well drained parts of the alluvial plain. Extensive pedogenic carbonate formation was precluded by the soil-moisture regime and relatively wet climate. During Late Permian and Early Triassic times Prydz Bay, which lay some 30° south of the Equator, experienced climatic conditions broadly similar to parts of the contemporary tropics, dominated by a ubtropical high-pressure system, easterly winds, and seasonal rainfall.
La Formacion Mamuniyat del Ordovicico Superior (Ashgiliense) es el principal reservorio objetivo en los tres campos petroliferos (A, B y H) dentro del area de la Concesion NC511 de Repsol Oil Operations, en el flanco NO de la Cuenca Murzuq, en el SO de Libia. En el sudeste del area de concesion la Formacion Mamuniyat comprende arenas braided que cambian hacia el noreste dentro de la propia cuenca a sedimentos de plataforma de aguas marginales y someras que consisten, predominantemente, en areniscas
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.
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