Colluvial deposits provide an interesting challenge for luminescence-dating techniques because of... more Colluvial deposits provide an interesting challenge for luminescence-dating techniques because of the short and varied light-exposure histories of the grains prior to deposition. Results of thermoluminescence (TL) and infrared stimulated luminescence (IRSL) measurements are presented for two stacked colluvial deposits from Natal. A radiocarbon date for bulk organic matter in the A horizon of the soil formed within the surface of the lower unit provides a means of assessing the suitability of various luminescence procedures for such material. The overlying colluvium contains grains bleached to varying extents, as shown by the range of EDs for the coarse-grain feldspar separate obtained using the single-aliquot IRSL method. For this sample all age estimates (either by IRSL or TL) are too large compared with the radiocarbon age, indicating that even the IRSL signal was not zeroed in all grains before they were buried. The underlying colluvium contains grains bleached more uniformly, resulting in a smaller range in the values of single-aliquot EDs; the agreement of the IRSL age with the radiocarbon age indicates that the IRSL signals of all grains were zeroed prior to deposition. The TL results for this sample suggest that the signals were not totally zeroed, but had experienced considerably more light at deposition than the grains from the overlying unit.
The Cenozoic stratigraphy of South Africa has developed over the past 166 years since geological ... more The Cenozoic stratigraphy of South Africa has developed over the past 166 years since geological mapping of the region was initiated. The current status of lithostratigraphy and the global chronostratigraphic framework is discussed in the context of the diverse Cenozoic regolith across the region. Geological mapping here utilizes lithostratigraphy to organise the Cenozoic deposits, although some extensive units are characterised informally using lithological descriptors. Although there are no formal biostratigraphic units, the allied use of “Land Mammal Ages” compiled from fossil type sites is described. An analogous archaeological cultural-historical “technocomplex” stratigraphy is outlined to subdivide stone age cultural material commonly associated with Quaternary deposits and has often been used as a relative dating framework. A summary table of Cenozoic regolith is presented, differentiating deposits into their terrain morphologically defined Geomorphic Province context as a me...
Detailed 1:10,000 scale engineering geological mapping of Pietermaritzburg covered an area of app... more Detailed 1:10,000 scale engineering geological mapping of Pietermaritzburg covered an area of approximately 670km 2 , some of which is experiencing rapid growth and development across a geologically varied area. This variation has resulted in a number of differing geological and geotechnical conditions, some of which are problematic and likely to have significant negative consequences for future developments. The mapping provided very detailed engineering geological coverage of the area and focused on delineating areas where hazardous geological or geomorphological conditions could impose environmental constraints or elevated cost implications on future infrastructure development. The use of digital elevation data has enabled the production of innovative map products that provide enhanced visualisation of the geotechnical and engineering geological data for the area. The geological and terrain morphological diversity of the Pietermaritzburg city bowl and surrounding hilly areas resu...
Abstract A detailed field survey identified a late Neogene sedimentary succession which accumulat... more Abstract A detailed field survey identified a late Neogene sedimentary succession which accumulated in ephemeral palaeolakes on the edge of the Kalahari Basin in the northwest of Transvaal Province, South Africa. Palygorskite and dolomite formed along lake margins which fluctuated between lacustrine and dry-land conditions. The sedimentary sequence presented an opportunity to investigate the interrelationships between these minerals and study their possible modes of formation, either by lacustrine sedimentation or pedogenic processes. Clay mineralogy, transmission and scanning electron microscopy and geochemical analysis revealed an increase in magnesium-enriched minerals, palygorskite and dolomite, within the pedogenically altered marginal lacustrine sediments. Field relationships showed that the dolomite is a replacive phase within the decomposing palygorskite, although the scanning electron micrographs do not display such unequivocal evidence for a direct replacive relationship. They suggest rather that the dolomite forms by neoformation from soil solutions, which included decomposition products released by the breakdown of the palygorskite. Seen in a regional context, subaerial pedogenic processes acted on marginal lacustrine sediments and resulted in the decomposition of smectite and neoformation of palygorskite. A change in climate to drier conditions encouraged the formation of calcrete in surrounding areas which increased the magnesium/calcium ratio in groundwaters. This increase was supplemented by evaporative concentration during the desiccation of the lacustrine environment. The high magnesium concentration led to the formation of the dolomite-rich dolocrete. This interpretive model is a means by which existing genetic models for calcareous pedocretes and those models that define the concentration of magnesium in groundwater and saline lacustrine environments can be combined. The pedogenic mineral relationships described are suggested as useful palaeoenvironmental indicators for the late Neogene/early Pleistocene of southern Africa.
A hierarchical classification, vegetation map, description and ecological interp retation of the ... more A hierarchical classification, vegetation map, description and ecological interp retation of the plant communities of the Tembe Elephant Park and surrounding areas are presented. The study area falls within the Maputaland Centre of Endemism, which is part of the Maputaland-Pondoland Region , a cen tre of plant diversity rich in endemic plants and anima ls. Sixty-four sample plots were distributed in a stratified manner throughout the study area , A TWINS PAN classification , refined by Braun-Blanquet procedures, revealed eight distinct, mainly woodland plant communities , Much of the vegetation distribution can be ascribed to vegetation dynamics and the geomorphological evolution of the region. Dynamics of water in the landscape either directly or indirectly, through its role in moisture levels and soil formation , plays a role in the determination of plant communities at the higher levels. The proposed classification and general descriptions can be used for
Hillslopes in central and western parts of KwaZulu-Natal, South Africa are often mantled by collu... more Hillslopes in central and western parts of KwaZulu-Natal, South Africa are often mantled by colluvial sediments of the Masotcheni Formation. These sediments have accreted in response to several cycles of deposition, pedogenesis and incomplete erosion. Climatic controls on these cycles are incompletely known. Results from fieldwork, micromorphology, stable carbon isotope analysis and Optically Stimulated Luminescence dating of Masotcheni Formation sediments from Okhombe valley in the Drakensberg foothills are combined. Deposition in the area had at least 11 phases, starting before 42 ka and ending before 0.17 ka. The first six deposits (from before 42 ka to after 29 ka) resulted from the interplay between slope processes and fluvial redistribution under cold conditions. Solifluction was the most important slope process. No deposits have been found from the Last Glacial Maximum, arguably because this period was too dry. The last five deposits (from about 11 ka to before 0.17 ka) resulted from fluvial redistribution of upslope material and older deposits under increasing precipitation. Current extreme gully erosion in the Masotcheni Formation indicates a lack of available upslope material, leaving downslope deposits as the only sediment source for fluvial redistribution. This model for landscape response to climate change may be able to explain how climate controlled landscape processes in other Masotcheni Formation sites in KwaZulu-Natal. In the research area and elsewhere, this proposition may be tested with numerical landscape evolution models.
The layer of soil - the Earth's 'living skin' - is thinner than we think. That extrao... more The layer of soil - the Earth's 'living skin' - is thinner than we think. That extraordinary ratio 1:12 700 represents its thickness in comparison to that of the rocks beneath. By way of illustration, the layer of paint on the outside of a house, relative to the size of the house, is ten times thicker!
Soil erosion is one of the most challenging environmental issues in the world, causing unsustaina... more Soil erosion is one of the most challenging environmental issues in the world, causing unsustainable soil loss every year. In South Africa, several episodes of gully erosion have been documented and clearly linked to the presence of Quaternary colluvial deposits on the Drakensberg Mountain footslopes. The aim of this study was to identify and assess the triggering factors of gully erosion in the upper Mkhomazi River basin in KwaZulu-Natal, South Africa. We compiled a gully inventory map and applied remote sensing techniques as well as field surveys to validate the gully inventory. The gullies were subdivided into slope gullies and fluvial gullies. We derived susceptibility maps based on the spatial distribution of gully types to assess the most important driving factors. A stochastic modeling approach (MaxEnt) was applied, and the results showed two susceptibility maps within the spatial distribution of the gully erosion probability. To validate the MaxEnt model results, a subset of...
We present a 1:50 000 scale geomorphological map of the upper Mkhomazi River basin, located in th... more We present a 1:50 000 scale geomorphological map of the upper Mkhomazi River basin, located in the foothills of the Drakensberg mountains in KwaZulu-Natal Province, South Africa. The subhorizontal strata of the Permo-Triassic Beaufort Group forms plateau interfluves with a concave valley slope morphology. Locally, thick sequences of late Pleistocene colluvial deposits and associated buried paleosols (Masotcheni Formation) infill first-order tributary stream valleys and extend across the adjacent lower slopes. Surface runoff processes preferentially incise into the poorly consolidated, highly erodible sediments causing severe gully erosion that is responsible for widespread land degradation and desertification phenomena. The main purpose of this work is to derive a geomorphological map of the study area focussing on the erosional landforms to understand their spatial distribution and their relation to the colluvial deposits. Finally, a local and regional stratigraphic correlation of colluvial deposits and associated buried palaeosol profiles is proposed.
Sets of sandy beach ridges and intervening swales define shoreline sections of the shallow St Luc... more Sets of sandy beach ridges and intervening swales define shoreline sections of the shallow St Lucia wetland system within the iSimangaliso Wetland Park, World Heritage Site in northern KwaZulu-Natal province, South Africa. The sets comprise 3-10 beach ridges, the most prominent being 80-150 m wide and rising 0.5-2 m above the adjacent swales. The highest beach ridge crests elevated 3.2-4.6 m above mean sea level are furthest from the present shoreline and the lowest ridges, rising about 0.5 m above the swales, occur closest to the present mean lake level shoreline. This investigation assesses the genesis of the sandy beach ridges on five strand plain remnants within the estuarine lake. The ridges were topographically surveyed and their ages estimated using optically stimulated luminescence (OSL) dating. OSL dating reveals that the oldest beach ridge formed around 6,240 years ago, with a sequence of beach ridges having accumulated during the period ~4,000 to 1,500 years ago
St Lucia's complex biodiversity is driven by varia-tions of salinity over time and space, wh... more St Lucia's complex biodiversity is driven by varia-tions of salinity over time and space, which control the occurrence and growth of plants and animals at any particular time and locality (Starfield et al. 1989; Taylor 1993). For management it is necessary to know how the biota survive ...
Geomorphological and lithostratigraphic mapping of dune systems on the Maputaland coastal plain o... more Geomorphological and lithostratigraphic mapping of dune systems on the Maputaland coastal plain of northeastern KwaZulu-Natal Province, South Africa has defined the relative-age relationships between the complex pattern of Quaternary parabolic and hummocky dunes, sand megaridges and the composite coastal barrier dune cordon. Here infra-red-stimulated luminescence (IRSL) ages are presented in the context of regional stratigraphic relationships and serve to bracket the period of accumulation of the main dune systems. The luminescence ages challenge the widely held belief that the north–south aligned dune pattern reflects a coastward-younging sequence. The framework of ages highlights the spatial variations in dune mobilization and long accretion history on some dune forms. Weathered dune and interdune wetland sediments spanning from prior to marine isotope stages (MIS) 11 and up to MIS 7 underlie the coastal plain. At least two generations of decalcified aeolianite of MIS 5 and 4 form the core of the complex coastal barrier dune. On the coastal plain, frequent sand mobilization events during MIS 3–2 resulted in the development of discrete complexes of highly extended, northward directed parabolic dune systems and reworking of the crest of the central sand megaridge. The luminescence-dated dune mobilization history brackets the period of formation of interdune and lacustrine peat and diatomite deposits. During the Holocene marine transgression at least four laterally extensive, complex transverse ridges of coalesced ascending parabolic dunes accreted against the aeolianite core of the coastal barrier. Polyphase dune formation and remobilization is discussed in the context of the regional groundwater and vegetation responses to global climatic changes, wind regimes and glacio-eustatic sea-level fluctuations.
Colluvial deposits provide an interesting challenge for luminescence-dating techniques because of... more Colluvial deposits provide an interesting challenge for luminescence-dating techniques because of the short and varied light-exposure histories of the grains prior to deposition. Results of thermoluminescence (TL) and infrared stimulated luminescence (IRSL) measurements are presented for two stacked colluvial deposits from Natal. A radiocarbon date for bulk organic matter in the A horizon of the soil formed within the surface of the lower unit provides a means of assessing the suitability of various luminescence procedures for such material. The overlying colluvium contains grains bleached to varying extents, as shown by the range of EDs for the coarse-grain feldspar separate obtained using the single-aliquot IRSL method. For this sample all age estimates (either by IRSL or TL) are too large compared with the radiocarbon age, indicating that even the IRSL signal was not zeroed in all grains before they were buried. The underlying colluvium contains grains bleached more uniformly, resulting in a smaller range in the values of single-aliquot EDs; the agreement of the IRSL age with the radiocarbon age indicates that the IRSL signals of all grains were zeroed prior to deposition. The TL results for this sample suggest that the signals were not totally zeroed, but had experienced considerably more light at deposition than the grains from the overlying unit.
The Cenozoic stratigraphy of South Africa has developed over the past 166 years since geological ... more The Cenozoic stratigraphy of South Africa has developed over the past 166 years since geological mapping of the region was initiated. The current status of lithostratigraphy and the global chronostratigraphic framework is discussed in the context of the diverse Cenozoic regolith across the region. Geological mapping here utilizes lithostratigraphy to organise the Cenozoic deposits, although some extensive units are characterised informally using lithological descriptors. Although there are no formal biostratigraphic units, the allied use of “Land Mammal Ages” compiled from fossil type sites is described. An analogous archaeological cultural-historical “technocomplex” stratigraphy is outlined to subdivide stone age cultural material commonly associated with Quaternary deposits and has often been used as a relative dating framework. A summary table of Cenozoic regolith is presented, differentiating deposits into their terrain morphologically defined Geomorphic Province context as a me...
Detailed 1:10,000 scale engineering geological mapping of Pietermaritzburg covered an area of app... more Detailed 1:10,000 scale engineering geological mapping of Pietermaritzburg covered an area of approximately 670km 2 , some of which is experiencing rapid growth and development across a geologically varied area. This variation has resulted in a number of differing geological and geotechnical conditions, some of which are problematic and likely to have significant negative consequences for future developments. The mapping provided very detailed engineering geological coverage of the area and focused on delineating areas where hazardous geological or geomorphological conditions could impose environmental constraints or elevated cost implications on future infrastructure development. The use of digital elevation data has enabled the production of innovative map products that provide enhanced visualisation of the geotechnical and engineering geological data for the area. The geological and terrain morphological diversity of the Pietermaritzburg city bowl and surrounding hilly areas resu...
Abstract A detailed field survey identified a late Neogene sedimentary succession which accumulat... more Abstract A detailed field survey identified a late Neogene sedimentary succession which accumulated in ephemeral palaeolakes on the edge of the Kalahari Basin in the northwest of Transvaal Province, South Africa. Palygorskite and dolomite formed along lake margins which fluctuated between lacustrine and dry-land conditions. The sedimentary sequence presented an opportunity to investigate the interrelationships between these minerals and study their possible modes of formation, either by lacustrine sedimentation or pedogenic processes. Clay mineralogy, transmission and scanning electron microscopy and geochemical analysis revealed an increase in magnesium-enriched minerals, palygorskite and dolomite, within the pedogenically altered marginal lacustrine sediments. Field relationships showed that the dolomite is a replacive phase within the decomposing palygorskite, although the scanning electron micrographs do not display such unequivocal evidence for a direct replacive relationship. They suggest rather that the dolomite forms by neoformation from soil solutions, which included decomposition products released by the breakdown of the palygorskite. Seen in a regional context, subaerial pedogenic processes acted on marginal lacustrine sediments and resulted in the decomposition of smectite and neoformation of palygorskite. A change in climate to drier conditions encouraged the formation of calcrete in surrounding areas which increased the magnesium/calcium ratio in groundwaters. This increase was supplemented by evaporative concentration during the desiccation of the lacustrine environment. The high magnesium concentration led to the formation of the dolomite-rich dolocrete. This interpretive model is a means by which existing genetic models for calcareous pedocretes and those models that define the concentration of magnesium in groundwater and saline lacustrine environments can be combined. The pedogenic mineral relationships described are suggested as useful palaeoenvironmental indicators for the late Neogene/early Pleistocene of southern Africa.
A hierarchical classification, vegetation map, description and ecological interp retation of the ... more A hierarchical classification, vegetation map, description and ecological interp retation of the plant communities of the Tembe Elephant Park and surrounding areas are presented. The study area falls within the Maputaland Centre of Endemism, which is part of the Maputaland-Pondoland Region , a cen tre of plant diversity rich in endemic plants and anima ls. Sixty-four sample plots were distributed in a stratified manner throughout the study area , A TWINS PAN classification , refined by Braun-Blanquet procedures, revealed eight distinct, mainly woodland plant communities , Much of the vegetation distribution can be ascribed to vegetation dynamics and the geomorphological evolution of the region. Dynamics of water in the landscape either directly or indirectly, through its role in moisture levels and soil formation , plays a role in the determination of plant communities at the higher levels. The proposed classification and general descriptions can be used for
Hillslopes in central and western parts of KwaZulu-Natal, South Africa are often mantled by collu... more Hillslopes in central and western parts of KwaZulu-Natal, South Africa are often mantled by colluvial sediments of the Masotcheni Formation. These sediments have accreted in response to several cycles of deposition, pedogenesis and incomplete erosion. Climatic controls on these cycles are incompletely known. Results from fieldwork, micromorphology, stable carbon isotope analysis and Optically Stimulated Luminescence dating of Masotcheni Formation sediments from Okhombe valley in the Drakensberg foothills are combined. Deposition in the area had at least 11 phases, starting before 42 ka and ending before 0.17 ka. The first six deposits (from before 42 ka to after 29 ka) resulted from the interplay between slope processes and fluvial redistribution under cold conditions. Solifluction was the most important slope process. No deposits have been found from the Last Glacial Maximum, arguably because this period was too dry. The last five deposits (from about 11 ka to before 0.17 ka) resulted from fluvial redistribution of upslope material and older deposits under increasing precipitation. Current extreme gully erosion in the Masotcheni Formation indicates a lack of available upslope material, leaving downslope deposits as the only sediment source for fluvial redistribution. This model for landscape response to climate change may be able to explain how climate controlled landscape processes in other Masotcheni Formation sites in KwaZulu-Natal. In the research area and elsewhere, this proposition may be tested with numerical landscape evolution models.
The layer of soil - the Earth's 'living skin' - is thinner than we think. That extrao... more The layer of soil - the Earth's 'living skin' - is thinner than we think. That extraordinary ratio 1:12 700 represents its thickness in comparison to that of the rocks beneath. By way of illustration, the layer of paint on the outside of a house, relative to the size of the house, is ten times thicker!
Soil erosion is one of the most challenging environmental issues in the world, causing unsustaina... more Soil erosion is one of the most challenging environmental issues in the world, causing unsustainable soil loss every year. In South Africa, several episodes of gully erosion have been documented and clearly linked to the presence of Quaternary colluvial deposits on the Drakensberg Mountain footslopes. The aim of this study was to identify and assess the triggering factors of gully erosion in the upper Mkhomazi River basin in KwaZulu-Natal, South Africa. We compiled a gully inventory map and applied remote sensing techniques as well as field surveys to validate the gully inventory. The gullies were subdivided into slope gullies and fluvial gullies. We derived susceptibility maps based on the spatial distribution of gully types to assess the most important driving factors. A stochastic modeling approach (MaxEnt) was applied, and the results showed two susceptibility maps within the spatial distribution of the gully erosion probability. To validate the MaxEnt model results, a subset of...
We present a 1:50 000 scale geomorphological map of the upper Mkhomazi River basin, located in th... more We present a 1:50 000 scale geomorphological map of the upper Mkhomazi River basin, located in the foothills of the Drakensberg mountains in KwaZulu-Natal Province, South Africa. The subhorizontal strata of the Permo-Triassic Beaufort Group forms plateau interfluves with a concave valley slope morphology. Locally, thick sequences of late Pleistocene colluvial deposits and associated buried paleosols (Masotcheni Formation) infill first-order tributary stream valleys and extend across the adjacent lower slopes. Surface runoff processes preferentially incise into the poorly consolidated, highly erodible sediments causing severe gully erosion that is responsible for widespread land degradation and desertification phenomena. The main purpose of this work is to derive a geomorphological map of the study area focussing on the erosional landforms to understand their spatial distribution and their relation to the colluvial deposits. Finally, a local and regional stratigraphic correlation of colluvial deposits and associated buried palaeosol profiles is proposed.
Sets of sandy beach ridges and intervening swales define shoreline sections of the shallow St Luc... more Sets of sandy beach ridges and intervening swales define shoreline sections of the shallow St Lucia wetland system within the iSimangaliso Wetland Park, World Heritage Site in northern KwaZulu-Natal province, South Africa. The sets comprise 3-10 beach ridges, the most prominent being 80-150 m wide and rising 0.5-2 m above the adjacent swales. The highest beach ridge crests elevated 3.2-4.6 m above mean sea level are furthest from the present shoreline and the lowest ridges, rising about 0.5 m above the swales, occur closest to the present mean lake level shoreline. This investigation assesses the genesis of the sandy beach ridges on five strand plain remnants within the estuarine lake. The ridges were topographically surveyed and their ages estimated using optically stimulated luminescence (OSL) dating. OSL dating reveals that the oldest beach ridge formed around 6,240 years ago, with a sequence of beach ridges having accumulated during the period ~4,000 to 1,500 years ago
St Lucia's complex biodiversity is driven by varia-tions of salinity over time and space, wh... more St Lucia's complex biodiversity is driven by varia-tions of salinity over time and space, which control the occurrence and growth of plants and animals at any particular time and locality (Starfield et al. 1989; Taylor 1993). For management it is necessary to know how the biota survive ...
Geomorphological and lithostratigraphic mapping of dune systems on the Maputaland coastal plain o... more Geomorphological and lithostratigraphic mapping of dune systems on the Maputaland coastal plain of northeastern KwaZulu-Natal Province, South Africa has defined the relative-age relationships between the complex pattern of Quaternary parabolic and hummocky dunes, sand megaridges and the composite coastal barrier dune cordon. Here infra-red-stimulated luminescence (IRSL) ages are presented in the context of regional stratigraphic relationships and serve to bracket the period of accumulation of the main dune systems. The luminescence ages challenge the widely held belief that the north–south aligned dune pattern reflects a coastward-younging sequence. The framework of ages highlights the spatial variations in dune mobilization and long accretion history on some dune forms. Weathered dune and interdune wetland sediments spanning from prior to marine isotope stages (MIS) 11 and up to MIS 7 underlie the coastal plain. At least two generations of decalcified aeolianite of MIS 5 and 4 form the core of the complex coastal barrier dune. On the coastal plain, frequent sand mobilization events during MIS 3–2 resulted in the development of discrete complexes of highly extended, northward directed parabolic dune systems and reworking of the crest of the central sand megaridge. The luminescence-dated dune mobilization history brackets the period of formation of interdune and lacustrine peat and diatomite deposits. During the Holocene marine transgression at least four laterally extensive, complex transverse ridges of coalesced ascending parabolic dunes accreted against the aeolianite core of the coastal barrier. Polyphase dune formation and remobilization is discussed in the context of the regional groundwater and vegetation responses to global climatic changes, wind regimes and glacio-eustatic sea-level fluctuations.
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Papers by Greg Botha