Investigations to determine the long-term sustainable yield of the Karoo aquifer and the sustained availability of groundwater for small-scale irrigation projects, in Dendera area, Kwekwe District - Zimbabwe
- Authors: Njanike, Joseph Tendayi
- Date: 2001
- Subjects: Groundwater , Irrigation -- Equipement and supplies , Irrigation -- Kwekwe (Zimbabwe)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5093 , http://hdl.handle.net/10962/d1020853
- Description: In this thesis the long-term sustainable yield of the Karoo sediment aquifer unit occurring in Dendera area of Kwekwe District is investigated, with the object of providing quantitative data on the sustained availability of groundwater for small-scale irrigation projects. Archaean Basement Schists and Pre-Cambrian gneissic granites, the Basement Complex rocks, underlie the entire study area. Overlying these are Upper Karoo sediments. Aeolian Kalahari sands unconformably mantle higher interfluves, while redistributed sands occur along valleys of major rivers and streams. The Karoo sediments, which predominantly consist of loosely cemented, fine- to medium-grained sandstone alternating with red siltstone and mudstone, constitute the main aquifer. The thickness of the Karoo sediment unit ranges from 30m to 80m. The hydraulic parameters of the Karoo sediment aquifer were characterised in the field by constant discharge pumping tests and slug tests. Pumping tests indicated unconfined conditions and thus the Neuman's method of analysis has been used. Transmissivities from pumping tests are within the range 4.7 m²/d to 13.6 m²/d with an average of 8.9m²/d. The low transmissivities seem to be a major limiting factor in the exploitation of the groundwater resources. Thus the sustainable borehole yields tend to be small, mean values ranging from 33 m²/d to 253 m²/d. Specific yield could not be determined from the pumping tests due to the lack of observation boreholes. Low chemical concentrations render the water suitable for irrigation of all crops, while neither total nor any individual concentrations present health hazards to human or livestock. An average recharge value of 47.7 mm/y was inferred from water table fluctuation method. Chloride mass balance technique in the same area indicates recharge value in the order of 67.4 mm/y. Because the chloride mass balance gives a long-term mean annual recharge, the recharge figure of 67.4 mm/y was adopted for the study area. Based on the abstractable proportion of recharge, the sustainably exploitable volume of groundwater of the order of 2.68 x 10⁷ m³/y was established. This volume is more than 100 times the estimated current demand for groundwater (1.35 x 10⁵ m³/d), implying that there are large volumes of surplus water, which can be utilised for irrigation.
- Full Text:
- Date Issued: 2001
- Authors: Njanike, Joseph Tendayi
- Date: 2001
- Subjects: Groundwater , Irrigation -- Equipement and supplies , Irrigation -- Kwekwe (Zimbabwe)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5093 , http://hdl.handle.net/10962/d1020853
- Description: In this thesis the long-term sustainable yield of the Karoo sediment aquifer unit occurring in Dendera area of Kwekwe District is investigated, with the object of providing quantitative data on the sustained availability of groundwater for small-scale irrigation projects. Archaean Basement Schists and Pre-Cambrian gneissic granites, the Basement Complex rocks, underlie the entire study area. Overlying these are Upper Karoo sediments. Aeolian Kalahari sands unconformably mantle higher interfluves, while redistributed sands occur along valleys of major rivers and streams. The Karoo sediments, which predominantly consist of loosely cemented, fine- to medium-grained sandstone alternating with red siltstone and mudstone, constitute the main aquifer. The thickness of the Karoo sediment unit ranges from 30m to 80m. The hydraulic parameters of the Karoo sediment aquifer were characterised in the field by constant discharge pumping tests and slug tests. Pumping tests indicated unconfined conditions and thus the Neuman's method of analysis has been used. Transmissivities from pumping tests are within the range 4.7 m²/d to 13.6 m²/d with an average of 8.9m²/d. The low transmissivities seem to be a major limiting factor in the exploitation of the groundwater resources. Thus the sustainable borehole yields tend to be small, mean values ranging from 33 m²/d to 253 m²/d. Specific yield could not be determined from the pumping tests due to the lack of observation boreholes. Low chemical concentrations render the water suitable for irrigation of all crops, while neither total nor any individual concentrations present health hazards to human or livestock. An average recharge value of 47.7 mm/y was inferred from water table fluctuation method. Chloride mass balance technique in the same area indicates recharge value in the order of 67.4 mm/y. Because the chloride mass balance gives a long-term mean annual recharge, the recharge figure of 67.4 mm/y was adopted for the study area. Based on the abstractable proportion of recharge, the sustainably exploitable volume of groundwater of the order of 2.68 x 10⁷ m³/y was established. This volume is more than 100 times the estimated current demand for groundwater (1.35 x 10⁵ m³/d), implying that there are large volumes of surplus water, which can be utilised for irrigation.
- Full Text:
- Date Issued: 2001
Processes and products in the kimberlitic crater facies of the south lobe, Jwaneng Mine, Botswana
- Authors: Machin, Kimberley
- Date: 2001
- Subjects: Kimberlite -- Jwaneng (Botswana)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4995 , http://hdl.handle.net/10962/d1005607 , Kimberlite -- Jwaneng (Botswana)
- Description: The Pennian (~ 245 Ma) Jwaneng kimberlite, situated in southern Botswana, comprises three steep-sided pipes that coalesce approximately 100m below the present day surface to fonn a 54ha body. These pipes have been labelled the South, Centre and North Lobes. The kimberlites intruded a thick sequence of Proterozoic shales, dolomites and sandstones and a thin veneer of consolidated to poorly consolidated mudstones and siltstones of the Karoo Supergroup. Although the shapes of these pipes are comparable to other southern African pipes, they are filled predominantly with crater facies volcaniclastic kimberlite. No tuffisitic kimberlite breccia, the characteristic rock type of the diatremes of other southern African pipes, has yet been identified. The Jwaneng kimberlite thus represents an exception to the standard model for southern African kimberlites, implying that different processes need to be invoked to explain its fonnation. The present study involves a detailed volcanological and sedimentological analysis of the volcaniclastic fill of the Jwaneng South Lobe. Two principal and distinct lithofacies have been identified: the quartz-free RVK facies and the quartz-bearing QRVK facies. Both facies include fine to coarse grained, predominantly massive and subordinate chaotically bedded deposits. The volcaniclastic rocks have been classified as resedimented volcaniclastic kimberlite (RVK) , since their deposition is ascribed to mechanisms dominated by mass flow processes. Based on certain characteristics and differences between the two principal facies, and their spatial distribution within the pipe, they are interpreted as being the products of at least two separate eruption episodes. Certain characteristics (e.g. }hape, granularity~ of the juvenile· magma clasts III the volcaniclastic kimberlite suggest complete crystallisation and devolatilisation of the magma at depth prior to explosive fragmentation. A scenario in which this might have occurred, and which led to catastrophic explosive eruption and pipe excavation is proposed. Explosive eruption and associated tuff cone formation is followed by resedimentation of the material back into the pipe by mass flow processes. Mass flow processes are dominated by debris flow, with lesser grain flow, hyperconcentrated flow and subaqueous mud flow and suspension settling of muddy kimberlitic sediments. Geochemical analyses of the latter indicate a high degree of contamination and weathering, and mixing between pristine kimberlite and silicic shale/mud compositions. Failure and collapse of parts of the underlying pipe walls yielded megablocks of poorly consolidated Permian Karoo mudstone in the peripheral zone of the pipe. This source of the megablocks is supported by their bulk chemical composition. Minor phreatic/phreatomagmatic eruptions are suggested by the presence of rare accretionary and armoured lapilli within both the QRVK and RVK facies. Subsidence of the volcaniclastic pipe fill, inferred mainly from the oversteepened dips of the bedded QRVK and RVK facies, may be related to gravity-induced compaction, late-stage phreatomagmatic eruptions or eruption ofthe adjacent Centre Lobe.
- Full Text:
- Date Issued: 2001
- Authors: Machin, Kimberley
- Date: 2001
- Subjects: Kimberlite -- Jwaneng (Botswana)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4995 , http://hdl.handle.net/10962/d1005607 , Kimberlite -- Jwaneng (Botswana)
- Description: The Pennian (~ 245 Ma) Jwaneng kimberlite, situated in southern Botswana, comprises three steep-sided pipes that coalesce approximately 100m below the present day surface to fonn a 54ha body. These pipes have been labelled the South, Centre and North Lobes. The kimberlites intruded a thick sequence of Proterozoic shales, dolomites and sandstones and a thin veneer of consolidated to poorly consolidated mudstones and siltstones of the Karoo Supergroup. Although the shapes of these pipes are comparable to other southern African pipes, they are filled predominantly with crater facies volcaniclastic kimberlite. No tuffisitic kimberlite breccia, the characteristic rock type of the diatremes of other southern African pipes, has yet been identified. The Jwaneng kimberlite thus represents an exception to the standard model for southern African kimberlites, implying that different processes need to be invoked to explain its fonnation. The present study involves a detailed volcanological and sedimentological analysis of the volcaniclastic fill of the Jwaneng South Lobe. Two principal and distinct lithofacies have been identified: the quartz-free RVK facies and the quartz-bearing QRVK facies. Both facies include fine to coarse grained, predominantly massive and subordinate chaotically bedded deposits. The volcaniclastic rocks have been classified as resedimented volcaniclastic kimberlite (RVK) , since their deposition is ascribed to mechanisms dominated by mass flow processes. Based on certain characteristics and differences between the two principal facies, and their spatial distribution within the pipe, they are interpreted as being the products of at least two separate eruption episodes. Certain characteristics (e.g. }hape, granularity~ of the juvenile· magma clasts III the volcaniclastic kimberlite suggest complete crystallisation and devolatilisation of the magma at depth prior to explosive fragmentation. A scenario in which this might have occurred, and which led to catastrophic explosive eruption and pipe excavation is proposed. Explosive eruption and associated tuff cone formation is followed by resedimentation of the material back into the pipe by mass flow processes. Mass flow processes are dominated by debris flow, with lesser grain flow, hyperconcentrated flow and subaqueous mud flow and suspension settling of muddy kimberlitic sediments. Geochemical analyses of the latter indicate a high degree of contamination and weathering, and mixing between pristine kimberlite and silicic shale/mud compositions. Failure and collapse of parts of the underlying pipe walls yielded megablocks of poorly consolidated Permian Karoo mudstone in the peripheral zone of the pipe. This source of the megablocks is supported by their bulk chemical composition. Minor phreatic/phreatomagmatic eruptions are suggested by the presence of rare accretionary and armoured lapilli within both the QRVK and RVK facies. Subsidence of the volcaniclastic pipe fill, inferred mainly from the oversteepened dips of the bedded QRVK and RVK facies, may be related to gravity-induced compaction, late-stage phreatomagmatic eruptions or eruption ofthe adjacent Centre Lobe.
- Full Text:
- Date Issued: 2001
Sedimentology of the Karoo Supergroup in the Tuli Basin (Limpompo River area, South Africa)
- Authors: Bordy, Emese M
- Date: 2001
- Subjects: River sediments -- South Africa Sedimentology Limpopo river Sedimentology -- Limpopo river Limpopo river (South africa)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4999 , http://hdl.handle.net/10962/d1005612
- Description: The sedimentary rocks of the Karoo Supergroup in the Tuli Basin (South Africa) consist of various terrigenous clastic and chemical deposits (parabreccias, conglo-breccias, conglomerates, sandstones, fine-grained sediments, calcretes and silc~etes). Four stratigraphic units were identified: the Basal, Middle and· Upper Units, and the CI~rens Formation. The palaeo-environmental reconstructions of the four stratigraphic units are based on evidence provided by primary sedimentary structures, palaeo-flow measurements, clast size/shape analysis, petrographic studies, palaeontological findings, borehole data and stratigraphic relations. The facies associations of the Basal Unit are interpreted as colluvial fan and low sinuosity, braid~d river channel with coal-bearing overbank and thaw-lake deposits. The interpreted depositional environment implies a cold climate, non-glacial subarctic fluvio-Iacustrine system. The current indicators of the palaeo-river system suggest flow direction from ENE to WSW. The lithologies of the Basal Unit are very similar to the deposits of the fluvial interval in the Vryheid Formation (Ecca Group) of the main Karoo Basin. There is no indubitable evidence for glacial activity (e.g. striated pavements or clasts, varvites, etc.), therefore the presence of unequivocal Dwyka Group correlatives in the Tuli Basin remains uncertain. The sedimentary structures and palaeo-current analysis indicate that the beds of the Middle Unit were deposited by an ancient river system flowing in a north-northwesterly direction. A lack of good quality exposures did not allow the reconstruction of the fluvial style, but the available data indicate a high-energy, perhaps braided fluvial system. The lack of bio- and chronostr~~igraphic control hampers precise correlation and enables only the lithocorrelation of the Middle Unit with other braided river systems either in the Beaufort Group or in the Molteno Formation of the main Karoo Basin. The depositional environment of the Upper Unit is interpreted as a low-sinuosity, ephemeral stream system with calcretes and silcretes in the dinosaur-inhabited overbank area. During the deposition of the unit, the climate was semi-arid with sparse precipitation resulting -iFlhighmagnitude, low-frequency devastating flash floods. The sediments were built out from a distant northwesterly source to the southeast. The unambiguous correspondence between the Upper Unit and the Elliot Formation (main Karoo Basin) is provided by lithological similarities and prosauropod dinosaurs remains. The palaeo-geographic picture of the Clarens Fonnation indicates a westerly windsdominated erg environment with migrating transverse dune types. The ephemeral stream deposits, fossil wood and trace fossils are only present in the lower part of the Formation, indicating that the wet-desert conditions were progressively replaced by dry-desert conditions. Based on lithological and palaeontological evidence, the Formation correlates with the Clarens Formation in the main Karoo Basin. At this stage, it remains difficult to establish the exact cause of the regional palaeo-slope changes during the deposition of the Karoo Supergroup in the Tuli Basin. It is probable that foreland system tectonics, which affected the lower part of the Supergroup (Basal Unit and Middle Unit?), were replaced by incipient continental extension and rift related tectonic movements in the Middle and Upper Units, and Clarens Formation.
- Full Text:
- Date Issued: 2001
- Authors: Bordy, Emese M
- Date: 2001
- Subjects: River sediments -- South Africa Sedimentology Limpopo river Sedimentology -- Limpopo river Limpopo river (South africa)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4999 , http://hdl.handle.net/10962/d1005612
- Description: The sedimentary rocks of the Karoo Supergroup in the Tuli Basin (South Africa) consist of various terrigenous clastic and chemical deposits (parabreccias, conglo-breccias, conglomerates, sandstones, fine-grained sediments, calcretes and silc~etes). Four stratigraphic units were identified: the Basal, Middle and· Upper Units, and the CI~rens Formation. The palaeo-environmental reconstructions of the four stratigraphic units are based on evidence provided by primary sedimentary structures, palaeo-flow measurements, clast size/shape analysis, petrographic studies, palaeontological findings, borehole data and stratigraphic relations. The facies associations of the Basal Unit are interpreted as colluvial fan and low sinuosity, braid~d river channel with coal-bearing overbank and thaw-lake deposits. The interpreted depositional environment implies a cold climate, non-glacial subarctic fluvio-Iacustrine system. The current indicators of the palaeo-river system suggest flow direction from ENE to WSW. The lithologies of the Basal Unit are very similar to the deposits of the fluvial interval in the Vryheid Formation (Ecca Group) of the main Karoo Basin. There is no indubitable evidence for glacial activity (e.g. striated pavements or clasts, varvites, etc.), therefore the presence of unequivocal Dwyka Group correlatives in the Tuli Basin remains uncertain. The sedimentary structures and palaeo-current analysis indicate that the beds of the Middle Unit were deposited by an ancient river system flowing in a north-northwesterly direction. A lack of good quality exposures did not allow the reconstruction of the fluvial style, but the available data indicate a high-energy, perhaps braided fluvial system. The lack of bio- and chronostr~~igraphic control hampers precise correlation and enables only the lithocorrelation of the Middle Unit with other braided river systems either in the Beaufort Group or in the Molteno Formation of the main Karoo Basin. The depositional environment of the Upper Unit is interpreted as a low-sinuosity, ephemeral stream system with calcretes and silcretes in the dinosaur-inhabited overbank area. During the deposition of the unit, the climate was semi-arid with sparse precipitation resulting -iFlhighmagnitude, low-frequency devastating flash floods. The sediments were built out from a distant northwesterly source to the southeast. The unambiguous correspondence between the Upper Unit and the Elliot Formation (main Karoo Basin) is provided by lithological similarities and prosauropod dinosaurs remains. The palaeo-geographic picture of the Clarens Fonnation indicates a westerly windsdominated erg environment with migrating transverse dune types. The ephemeral stream deposits, fossil wood and trace fossils are only present in the lower part of the Formation, indicating that the wet-desert conditions were progressively replaced by dry-desert conditions. Based on lithological and palaeontological evidence, the Formation correlates with the Clarens Formation in the main Karoo Basin. At this stage, it remains difficult to establish the exact cause of the regional palaeo-slope changes during the deposition of the Karoo Supergroup in the Tuli Basin. It is probable that foreland system tectonics, which affected the lower part of the Supergroup (Basal Unit and Middle Unit?), were replaced by incipient continental extension and rift related tectonic movements in the Middle and Upper Units, and Clarens Formation.
- Full Text:
- Date Issued: 2001
Stratigraphy and geochemistry of the Makganyene formation, Transvaal supergroup, Northern Cape, South Africa
- Authors: Polteau, Stéphane
- Date: 2001
- Subjects: Geology, Stratigraphic -- South Africa -- Northern Cape , Geochemistry -- South Africa -- Northern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5003 , http://hdl.handle.net/10962/d1005616 , Geology, Stratigraphic -- South Africa -- Northern Cape , Geochemistry -- South Africa -- Northern Cape
- Description: The Makganyene Formation forms the base of the Postmasburg Group in the Transvaal Supergroup of the Northern Cape Province. The Makganyene Formation has diamictite as the main rock type, but siltstone, sandstone, shale, and iron-formations are also present. A glacial origin has been proposed in the past due to the presence of dropstones, faceted and striated pebbles. Typically, the Makganyene Formation contains banded iron-formations interbedded with clastic rocks (shale, siltstone, sandstone and diamictites) at the contact with the underlying iron-formations. This transitional zone is generally overlain by massive or layered diamictites which contain poorly sorted clasts (mainly chert) within a shaly matrix. Striated pebbles have been found during field work, and dropstones have been observed in diamictites and banded iron-formations during the study. The top of the Makganyene Formation contains graded cycles interbedded with diamictites and thin layers of andesitic lavas from the Ongeluk Formation. The basal contact of the Makganyene Formation with the underlying Koegas Subgroup was described as unconformable by previous workers. However field work localised in the Rooinekke area shows a broadly conformable and interbedded contact with the underlying Koegas Subgroup. As described above, banded iron-formations are interbedded with the clastic rocks of the Makganyene Formation. Moreover, boreholes from the Sishen area display the same interbedding at the base of the Makganyene Formation. This suggests that no significant time gap is present in the whole succession between the Ghaap and Postmasburg Group. The Transvaal Supergroup in the Northern Cape displays the following succession : carbonates-BIFs-diamictites/ lava-BIFs-carbonates. The Makganyene Formation is thus at the centre of a symmetrical lithologic succession. Bulk rock compositions show that the diamictites have a similar composition to banded iron-formation with regard to their major element contents. Banded iron-formations acted as a source for the diamictites with carbonates and igneous rocks representing minor components. Differences in bulk composition between the Sishen and Matsap areas emphasize that the source of the diamictite was very localised. The Chemical Index of Alteration (CIA) has been calculated, but since the source dominant rock was iron-formation, this index cannot be usefully applied to the diamictites. ACN, A-CN-K, and A-CNK-FM diagrams confer a major importance in sorting processes due to the separation between the fine and coarse diamictites. The interbedded iron-formations display little clastic contamination indicating deposition in clear water conditions. However, dropstones are present in one borehole from the Matsap area, indicating that iron-formation took place under ice cover, or at least under icebergs. Stable isotope studies show that the iron-formations, interbedded towards the base of the Makganyene Formation, have similar values to the iron-formations of the Koegas Subgroup. As a result of the above observations, new correlations are proposed in this study, relating the different Transvaal Supergroup basins located on the Kaapvaal Craton. The Pretoria Group of the Transvaal Basin has no correlative in the Griqualand West Basin, and the Postmasburg Group of the Northern Cape Basin has no lateral equivalent in the Transvaal Basin. These changes have been made to overcome problems present in the current correlations between those two basins. The Makganyene Formation correlates with the Huronian glaciations which occurred between 2.4 and 2.2 Ga ago in North America. Another Precambrian glaciation is the worldwide and well-studied Neoproterozoic glaciation (640 Ma). At each of these glaciations, major banded iron-formation deposition took place with associated deposition of sedimentary manganese in post-glacial positions. The central position of the Makganyene Formation within the Transvaal Supergroup in the Northern Cape emphasizes this glacial climatic dependence of paleoproterozoic banded iron-formation and manganese deposition. However these two Precambrian glaciations are interpreted in paleomagnetic studies as having occurred near to the equator. The controversial theory of the Snowball Earth has been proposed which proposes that the Earth was entirely frozen from pole to pole. Results from field work, sedimentology, petrography and geochemistry were integrated in a proposed depositional model of the Makganyene Formation occurring at the symmetrical centre of the lithologic succession of the Transvaal Supergroup. At the beginning of the Makganyene glaciation, a regression occurred and glacial advance took place. The diamictites are mostly interpreted as being deposited from wet-based glaciers, probably tidewater glaciers, where significant slumping and debris flows occurred. Any transgression would cause a glacial retreat by rapid calving, re-establishing the chemical sedimentation of banded iron-formations. These sea-level variations are responsible for the interbedding of these different types of rocks (clastic and chemical). The end of the Makganyene glacial event is characterised by subaerial eruptions of andesitic lava of the Ongeluk Formation bringing ashes into the basin. Banded iron-formation and associated manganese accumulations are climate-dependant. Glacial events are responsible for the build up of metallic ions such as iron and manganese in solution in deep waters. A warmer climate would induce a transgression and precipitation of these metallic ions when Eh conditions are favourable. In the Transvaal Supergroup, the climatic variations from warm to cold, and cold to warm are expressed by the lithologic succession. The warm climates are represented by carbonates. Cold climates are represented by banded iron-formations and the peak in cold climate represented by the diamictites of the Makganyene Formation. These changes in climate are gradual, which contradict the dramatic Snowball Earth event: a rapid spread of glaciated areas over low-latitudes freezing the Earth from pole-to-pole. Therefore, to explain low-latitude glaciations at sea-level, a high obliquity of the ecliptic is most likely to have occurred. This high obliquity of the ecliptic was acquired at 4.5 Ga when a giant impactor collided into the Earth to form the Moon. Above the critical value of 54° of the obliquity of the ecliptic, normal climatic zonation reverts, and glaciations will take place preferentially at low-latitudes only when favourable conditions are gathered (relative position ofthe continents and PC02 in the atmosphere).
- Full Text:
- Date Issued: 2001
- Authors: Polteau, Stéphane
- Date: 2001
- Subjects: Geology, Stratigraphic -- South Africa -- Northern Cape , Geochemistry -- South Africa -- Northern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5003 , http://hdl.handle.net/10962/d1005616 , Geology, Stratigraphic -- South Africa -- Northern Cape , Geochemistry -- South Africa -- Northern Cape
- Description: The Makganyene Formation forms the base of the Postmasburg Group in the Transvaal Supergroup of the Northern Cape Province. The Makganyene Formation has diamictite as the main rock type, but siltstone, sandstone, shale, and iron-formations are also present. A glacial origin has been proposed in the past due to the presence of dropstones, faceted and striated pebbles. Typically, the Makganyene Formation contains banded iron-formations interbedded with clastic rocks (shale, siltstone, sandstone and diamictites) at the contact with the underlying iron-formations. This transitional zone is generally overlain by massive or layered diamictites which contain poorly sorted clasts (mainly chert) within a shaly matrix. Striated pebbles have been found during field work, and dropstones have been observed in diamictites and banded iron-formations during the study. The top of the Makganyene Formation contains graded cycles interbedded with diamictites and thin layers of andesitic lavas from the Ongeluk Formation. The basal contact of the Makganyene Formation with the underlying Koegas Subgroup was described as unconformable by previous workers. However field work localised in the Rooinekke area shows a broadly conformable and interbedded contact with the underlying Koegas Subgroup. As described above, banded iron-formations are interbedded with the clastic rocks of the Makganyene Formation. Moreover, boreholes from the Sishen area display the same interbedding at the base of the Makganyene Formation. This suggests that no significant time gap is present in the whole succession between the Ghaap and Postmasburg Group. The Transvaal Supergroup in the Northern Cape displays the following succession : carbonates-BIFs-diamictites/ lava-BIFs-carbonates. The Makganyene Formation is thus at the centre of a symmetrical lithologic succession. Bulk rock compositions show that the diamictites have a similar composition to banded iron-formation with regard to their major element contents. Banded iron-formations acted as a source for the diamictites with carbonates and igneous rocks representing minor components. Differences in bulk composition between the Sishen and Matsap areas emphasize that the source of the diamictite was very localised. The Chemical Index of Alteration (CIA) has been calculated, but since the source dominant rock was iron-formation, this index cannot be usefully applied to the diamictites. ACN, A-CN-K, and A-CNK-FM diagrams confer a major importance in sorting processes due to the separation between the fine and coarse diamictites. The interbedded iron-formations display little clastic contamination indicating deposition in clear water conditions. However, dropstones are present in one borehole from the Matsap area, indicating that iron-formation took place under ice cover, or at least under icebergs. Stable isotope studies show that the iron-formations, interbedded towards the base of the Makganyene Formation, have similar values to the iron-formations of the Koegas Subgroup. As a result of the above observations, new correlations are proposed in this study, relating the different Transvaal Supergroup basins located on the Kaapvaal Craton. The Pretoria Group of the Transvaal Basin has no correlative in the Griqualand West Basin, and the Postmasburg Group of the Northern Cape Basin has no lateral equivalent in the Transvaal Basin. These changes have been made to overcome problems present in the current correlations between those two basins. The Makganyene Formation correlates with the Huronian glaciations which occurred between 2.4 and 2.2 Ga ago in North America. Another Precambrian glaciation is the worldwide and well-studied Neoproterozoic glaciation (640 Ma). At each of these glaciations, major banded iron-formation deposition took place with associated deposition of sedimentary manganese in post-glacial positions. The central position of the Makganyene Formation within the Transvaal Supergroup in the Northern Cape emphasizes this glacial climatic dependence of paleoproterozoic banded iron-formation and manganese deposition. However these two Precambrian glaciations are interpreted in paleomagnetic studies as having occurred near to the equator. The controversial theory of the Snowball Earth has been proposed which proposes that the Earth was entirely frozen from pole to pole. Results from field work, sedimentology, petrography and geochemistry were integrated in a proposed depositional model of the Makganyene Formation occurring at the symmetrical centre of the lithologic succession of the Transvaal Supergroup. At the beginning of the Makganyene glaciation, a regression occurred and glacial advance took place. The diamictites are mostly interpreted as being deposited from wet-based glaciers, probably tidewater glaciers, where significant slumping and debris flows occurred. Any transgression would cause a glacial retreat by rapid calving, re-establishing the chemical sedimentation of banded iron-formations. These sea-level variations are responsible for the interbedding of these different types of rocks (clastic and chemical). The end of the Makganyene glacial event is characterised by subaerial eruptions of andesitic lava of the Ongeluk Formation bringing ashes into the basin. Banded iron-formation and associated manganese accumulations are climate-dependant. Glacial events are responsible for the build up of metallic ions such as iron and manganese in solution in deep waters. A warmer climate would induce a transgression and precipitation of these metallic ions when Eh conditions are favourable. In the Transvaal Supergroup, the climatic variations from warm to cold, and cold to warm are expressed by the lithologic succession. The warm climates are represented by carbonates. Cold climates are represented by banded iron-formations and the peak in cold climate represented by the diamictites of the Makganyene Formation. These changes in climate are gradual, which contradict the dramatic Snowball Earth event: a rapid spread of glaciated areas over low-latitudes freezing the Earth from pole-to-pole. Therefore, to explain low-latitude glaciations at sea-level, a high obliquity of the ecliptic is most likely to have occurred. This high obliquity of the ecliptic was acquired at 4.5 Ga when a giant impactor collided into the Earth to form the Moon. Above the critical value of 54° of the obliquity of the ecliptic, normal climatic zonation reverts, and glaciations will take place preferentially at low-latitudes only when favourable conditions are gathered (relative position ofthe continents and PC02 in the atmosphere).
- Full Text:
- Date Issued: 2001
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