The identification and counting of diatoms in fluvial and lacustrine sediments is an important te... more The identification and counting of diatoms in fluvial and lacustrine sediments is an important technique in the reconstruction of past environments, allowing the identification of water depth, pH, salinity, temperature and turbidity. A number of studies have been carried out in northern Botswana over the past 30 years on the Okavango Delta and its associated palaeolakes, suggesting, on the basis of species associations, the presence of three major diatom ecosystemsthe acidic Okavango Delta, the neutral to slightly saline major lakes, and saline ephemeral lakes and pans. This paper reviews the Botswana studies and presents a check list of diatoms found in Botswana at the genus, or where possible, species level as an aid to future research.
The identification and counting of diatoms in fluvial and lacustrine sediments is an important te... more The identification and counting of diatoms in fluvial and lacustrine sediments is an important technique in the reconstruction of past environments, allowing the identification of water depth, pH, salinity, temperature and turbidity. A number of studies have been carried out in northern Botswana over the past 30 years on the Okavango Delta and its associated palaeolakes, suggesting, on the basis of species associations, the presence of three major diatom ecosystemsthe acidic Okavango Delta, the neutral to slightly saline major lakes, and saline ephemeral lakes and pans. This paper reviews the Botswana studies and presents a check list of diatoms found in Botswana at the genus, or where possible, species level as an aid to future research.
The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia... more The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia, on the southwest flank of Etna, is investigated using biota assay, sedimentology and geochemistry. Dating control has been provided by radiocarbon and optically stimulated luminescence (OSL). The Barcavecchia sediments, comprising a lower lacustrine unit, and an upper marsh unit, provide a late Quaternary history of local conditions following drainage disruption of the Simeto river by hawaiite lavas and the outflow of carbonate rich groundwater from the Adrano Escarpment after 24 ka. Initially a small, saline lake formed before 17 ka, which dried out around 14 ka leaving a depositional hiatus. Subsequent sedimentation in a saline marsh environment preserved a sequence of tephra layers of local, hawaiite origin, of which the lowest (and thickest) was deposited between 8-6 ka, coincident with the recurrent activity of the Leone eruptive center. The upper sections preserve further tephra layers and a discontinuous pollen record, and probably terminates around 3-2 ka. These ages accord well with those obtained by ROMANO et alii (1987) from radiometric dating. The Holocene vegetation of the site mirrors that of the contemporary upper marsh, although tree species largely disappeared in the late Holocene.
Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, c... more Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, can be clearly identified from Landsat imagery of northern Botswana. These basins are delineated by a series of beach ridges marking the former shorelines of an inundated area that, at its largest extent, encompassed 60,000 km 2. In the last 30 yr over 150 radiocarbon ages have been produced from three component basins that make up the Palaeo-Makgadikgadi system (consisting of the Ngami, Mababe and Makgadikgadi basins). The paucity of well preserved organic material in the region has led to these 14 C ages being derived from inorganic carbonates whose open system behaviour and polygenetic origin frequently result in unreliable dates from uncertain contexts and then only within the temporal limits of the radiocarbon method. The application of Optically Stimulated Luminescence (OSL) dating to beach ridge sediments from Lake Ngami [Shaw, P.A., Bateman, M.D., Thomas, D.S.G., Davies, F., 2003. Holocene fluctuations of Lake Ngami, Middle Kalahari: chronology and responses to climatic change. Quaternary International 111 (1), 23-35] improved chronological resolution, but was limited by the use of hand dug pits. The new application of OSL to samples derived from deep coring of beach ridges has allowed access to a much longer, three dimensional, geomorphic record than previously available. Seventyfour ages new OSL ages from shorelines are presented allowing, for the first time, the opportunity to unravel the complex history of lake stages in the Ngami basin. Multiple lake highstands are identified in the period ∼ 0.3 ka to ∼ 140 ka, with several causes of these events, including tropical and local climates and upstream channel avulsion, identified. To unravel the full paleaoclimatic significance of these events it will be necessary to identify the timing of high stands in the other terminal basins within the Okavango system.
Transactions of the Institute of British Geographers, 1985
ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features con... more ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features controlled by volume: area thresholds in the Okavango Delta region. Of particular interest are the 945 m (Lake Palaeo-Makgadikgadi) and the 936 m shorelines, the latter ...
Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. P... more Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. Previously, arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods. To redress this, optically stimulated luminescence (OSL) dating was applied to sediment samples taken from a range of aeolian landforms in the SW Kalahari. Four periods of dune building are identified in the late Pleistocene and Holocene. Holocene aeolian activity at 6 ka and 1-2 ka led to the construction of localised dune forms, in contrast to late Pleistocene times when the extensive field of linear dunes that dominates the region was emplaced. Late-Holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at Witpan. The record of aeolian activity is evaluated in the context of humid chronologies and an ocean-atmosphere model of Holocene climate change, with good agreement obtained. It is concluded t...
High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistoce... more High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistocene environmental changes in Botswana. Large shifts in δ18O and δ13C are observed between two main periods of deposition. The first period, between 51,000 and 43,000 yr B.P., was warm and humid and may have been associated with some C3 vegetation. The second period, between 27,000 and 21,000 yr B.P., had temperatures approximately 2°C lower, and vegetation dominated by drought-adapted C4 plants. The intervening period, between 43,000 and 27,000 yr B.P., bounded by two major hiatuses in stalagmite growth, produced discontinuous speleothem formation, probably under dry conditions. New 230 Th/234U mass spectrometry age determinations for the stalagmite generally agree with previously measured 230 Th/234U ages.
This paper describes models put forward to explain the development of silcretes within drainage l... more This paper describes models put forward to explain the development of silcretes within drainage lines at the distal end of the Okavango Delta system in the Middle Kalahari of Botswana, and proposes that they provide an analogue for the formation of sarsen stones within sarsen trains. The models describe silcrete formation in shallow pans situated within river valleys, with silicification resulting from the accumulation of fluvial inputs of clastic material, silica from groundwater and additions of silica phytoliths from aquatic vegetation in seasonal pools. It is suggested, on the basis of macro-and micromorphological comparisons, that sarsens in the Clatford Bottom area of Wiltshire formed by this mechanism. The sarsens would have originally formed a spatially-limited linear silcrete body and would have then accumulated within contemporary valleys during the course of landscape evolution. The implications of this model for the environmental conditions associated with sarsen formation and the likely timing of silicification are subsequently discussed. It is concluded that the geomorphological setting of sarsen formation may have been more important than climatic conditions at the time(s) of silicification.
... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Envir... more ... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Environmental Science,Luton College of Higher Education, Luton, England DAVID SC THOMAS and DAVID J. NASH Department of Geography, University of Sheffield, Sheffield S10 2TN, England ...
In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic fra... more In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic frameworks, Quaternary scenarios for the Kalahari have been developed only in the past 20 years based on deductive geomorphological studies. Landforms studied include palaeo-lakes, caves, pans, dry valleys and spring deposits, all indicative of wetter episodes. Attempts to establish the presence of episodes of aridity from the vast linear dune fields have not been successful, primarily because the function of linear dunes has been misunderstood. A chronological framework for the past 40,000 years has been established, based on a corpus of 239 radiocarbon dates, of which nearly half, using calcrete as the dating medium, require careful interpretation. One of the most important findings is that the entire Kalahari experienced a wetter episode from 16,000-13,000 B.P., followed by a period of lowering water tables and return to the present climatic mean. The Kalahari is thus out of phase with Africa north of the Equator, and with other major subtropical deserts. This has implications for global palaeo-climatic modelling, although the phenomenon could be explained by the orbital forcing hypothesis. The recent use of U/Th dating on cave sinters suggests this pattern extends back into earlier glacial/interglacial cycles.
Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts w... more Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts with a cement comprising a mixture of silica and CaCO3) have been widely identified in the geological, geomorphological and pedological literature, but have not, to date, been systematically described or classified. This paper presents a review of previous definitions of the end members of the silcrete-calcrete spectrum and subsequently identifies the major silica-carbonate relationships within intergrade types are identified on the the Kalahari of Botswana and Namibia. Three main intergrade types are identified on the basis of silica-carbonate associations: duricrusts where secondary silica occurs within a calcareous matrix; varieties where secondary carbonate occurs within a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in close succession. Within each of these three groups, sub-types are identified dependent upon whether secondary materials have replaced or been emplaced within a pre-existing duricrust. Finally, a practical procedure for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-section analyses.
Geografiska Annaler, Series A: Physical Geography, 1997
Recent studies of pan sediment stratigraphy in southern Africa have indicated the potential of ge... more Recent studies of pan sediment stratigraphy in southern Africa have indicated the potential of geophysical and geochemical analyses for elucidating the evolution of pans and the environmental conditions under which formation took place. We have investigated three cores, of 1.8 to 2.6 m in length, from Lebatse Pan in southeast Kalahari, Botswana, and compare the results with the few other similar studies available. Through the sedimentological characteristics of Lebatse we identify cycles of deposition, chemical precipitation and post-depositional alteration under shifting environmental conditions from c. 60,000 years ago to the present. Four periods are separated by depositional hiatuses, marking dry conditions during which deflation of the pan cannot be excluded. During the two oldest periods the sequence of finely laminated, magnesia-rich sediments suggests deposition in a permanent shallow waterbody under evaporative conditions, causing chemical precipitation of dolomite from groundwater. Less evaporative conditions are inferred for the third period of deposition between hiatuses H2 and H1. The youngest period, 14 C-dated to 29,000 years BP, is characterised by water-logged conditions and the development of reed beds. It is believed to have been laid down under fluctuating water levels. The cycles of deposition exhibited in the pan cores appear synchronous with wetter periods in the Southern Kalahari, indicated by the limited evidence currently available.
The identification and counting of diatoms in fluvial and lacustrine sediments is an important te... more The identification and counting of diatoms in fluvial and lacustrine sediments is an important technique in the reconstruction of past environments, allowing the identification of water depth, pH, salinity, temperature and turbidity. A number of studies have been carried out in northern Botswana over the past 30 years on the Okavango Delta and its associated palaeolakes, suggesting, on the basis of species associations, the presence of three major diatom ecosystemsthe acidic Okavango Delta, the neutral to slightly saline major lakes, and saline ephemeral lakes and pans. This paper reviews the Botswana studies and presents a check list of diatoms found in Botswana at the genus, or where possible, species level as an aid to future research.
The identification and counting of diatoms in fluvial and lacustrine sediments is an important te... more The identification and counting of diatoms in fluvial and lacustrine sediments is an important technique in the reconstruction of past environments, allowing the identification of water depth, pH, salinity, temperature and turbidity. A number of studies have been carried out in northern Botswana over the past 30 years on the Okavango Delta and its associated palaeolakes, suggesting, on the basis of species associations, the presence of three major diatom ecosystemsthe acidic Okavango Delta, the neutral to slightly saline major lakes, and saline ephemeral lakes and pans. This paper reviews the Botswana studies and presents a check list of diatoms found in Botswana at the genus, or where possible, species level as an aid to future research.
The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia... more The palaeoenvironmental history of a ~0.1 km2 delta-shaped tufaceous spring mound at Barcavecchia, on the southwest flank of Etna, is investigated using biota assay, sedimentology and geochemistry. Dating control has been provided by radiocarbon and optically stimulated luminescence (OSL). The Barcavecchia sediments, comprising a lower lacustrine unit, and an upper marsh unit, provide a late Quaternary history of local conditions following drainage disruption of the Simeto river by hawaiite lavas and the outflow of carbonate rich groundwater from the Adrano Escarpment after 24 ka. Initially a small, saline lake formed before 17 ka, which dried out around 14 ka leaving a depositional hiatus. Subsequent sedimentation in a saline marsh environment preserved a sequence of tephra layers of local, hawaiite origin, of which the lowest (and thickest) was deposited between 8-6 ka, coincident with the recurrent activity of the Leone eruptive center. The upper sections preserve further tephra layers and a discontinuous pollen record, and probably terminates around 3-2 ka. These ages accord well with those obtained by ROMANO et alii (1987) from radiometric dating. The Holocene vegetation of the site mirrors that of the contemporary upper marsh, although tree species largely disappeared in the late Holocene.
Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, c... more Large lake basins, now almost entirely devoid of water in the Kalahari's semi arid environment, can be clearly identified from Landsat imagery of northern Botswana. These basins are delineated by a series of beach ridges marking the former shorelines of an inundated area that, at its largest extent, encompassed 60,000 km 2. In the last 30 yr over 150 radiocarbon ages have been produced from three component basins that make up the Palaeo-Makgadikgadi system (consisting of the Ngami, Mababe and Makgadikgadi basins). The paucity of well preserved organic material in the region has led to these 14 C ages being derived from inorganic carbonates whose open system behaviour and polygenetic origin frequently result in unreliable dates from uncertain contexts and then only within the temporal limits of the radiocarbon method. The application of Optically Stimulated Luminescence (OSL) dating to beach ridge sediments from Lake Ngami [Shaw, P.A., Bateman, M.D., Thomas, D.S.G., Davies, F., 2003. Holocene fluctuations of Lake Ngami, Middle Kalahari: chronology and responses to climatic change. Quaternary International 111 (1), 23-35] improved chronological resolution, but was limited by the use of hand dug pits. The new application of OSL to samples derived from deep coring of beach ridges has allowed access to a much longer, three dimensional, geomorphic record than previously available. Seventyfour ages new OSL ages from shorelines are presented allowing, for the first time, the opportunity to unravel the complex history of lake stages in the Ngami basin. Multiple lake highstands are identified in the period ∼ 0.3 ka to ∼ 140 ka, with several causes of these events, including tropical and local climates and upstream channel avulsion, identified. To unravel the full paleaoclimatic significance of these events it will be necessary to identify the timing of high stands in the other terminal basins within the Okavango system.
Transactions of the Institute of British Geographers, 1985
ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features con... more ABSTRACT Lake Ngami and the Mababe Depression contain series of concentric shoreline features controlled by volume: area thresholds in the Okavango Delta region. Of particular interest are the 945 m (Lake Palaeo-Makgadikgadi) and the 936 m shorelines, the latter ...
Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. P... more Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. Previously, arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods. To redress this, optically stimulated luminescence (OSL) dating was applied to sediment samples taken from a range of aeolian landforms in the SW Kalahari. Four periods of dune building are identified in the late Pleistocene and Holocene. Holocene aeolian activity at 6 ka and 1-2 ka led to the construction of localised dune forms, in contrast to late Pleistocene times when the extensive field of linear dunes that dominates the region was emplaced. Late-Holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at Witpan. The record of aeolian activity is evaluated in the context of humid chronologies and an ocean-atmosphere model of Holocene climate change, with good agreement obtained. It is concluded t...
High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistoce... more High-resolution δ18O and δ13C analyses of a stalagmite from Lobatse II Cave reveal late Pleistocene environmental changes in Botswana. Large shifts in δ18O and δ13C are observed between two main periods of deposition. The first period, between 51,000 and 43,000 yr B.P., was warm and humid and may have been associated with some C3 vegetation. The second period, between 27,000 and 21,000 yr B.P., had temperatures approximately 2°C lower, and vegetation dominated by drought-adapted C4 plants. The intervening period, between 43,000 and 27,000 yr B.P., bounded by two major hiatuses in stalagmite growth, produced discontinuous speleothem formation, probably under dry conditions. New 230 Th/234U mass spectrometry age determinations for the stalagmite generally agree with previously measured 230 Th/234U ages.
This paper describes models put forward to explain the development of silcretes within drainage l... more This paper describes models put forward to explain the development of silcretes within drainage lines at the distal end of the Okavango Delta system in the Middle Kalahari of Botswana, and proposes that they provide an analogue for the formation of sarsen stones within sarsen trains. The models describe silcrete formation in shallow pans situated within river valleys, with silicification resulting from the accumulation of fluvial inputs of clastic material, silica from groundwater and additions of silica phytoliths from aquatic vegetation in seasonal pools. It is suggested, on the basis of macro-and micromorphological comparisons, that sarsens in the Clatford Bottom area of Wiltshire formed by this mechanism. The sarsens would have originally formed a spatially-limited linear silcrete body and would have then accumulated within contemporary valleys during the course of landscape evolution. The implications of this model for the environmental conditions associated with sarsen formation and the likely timing of silicification are subsequently discussed. It is concluded that the geomorphological setting of sarsen formation may have been more important than climatic conditions at the time(s) of silicification.
... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Envir... more ... Southern Kalahari, southern Africa PAUL A. SHAW' School of Geology and Environmental Science,Luton College of Higher Education, Luton, England DAVID SC THOMAS and DAVID J. NASH Department of Geography, University of Sheffield, Sheffield S10 2TN, England ...
In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic fra... more In the absence of closed sites, fossiliferous strata and regional or even local stratigraphic frameworks, Quaternary scenarios for the Kalahari have been developed only in the past 20 years based on deductive geomorphological studies. Landforms studied include palaeo-lakes, caves, pans, dry valleys and spring deposits, all indicative of wetter episodes. Attempts to establish the presence of episodes of aridity from the vast linear dune fields have not been successful, primarily because the function of linear dunes has been misunderstood. A chronological framework for the past 40,000 years has been established, based on a corpus of 239 radiocarbon dates, of which nearly half, using calcrete as the dating medium, require careful interpretation. One of the most important findings is that the entire Kalahari experienced a wetter episode from 16,000-13,000 B.P., followed by a period of lowering water tables and return to the present climatic mean. The Kalahari is thus out of phase with Africa north of the Equator, and with other major subtropical deserts. This has implications for global palaeo-climatic modelling, although the phenomenon could be explained by the orbital forcing hypothesis. The recent use of U/Th dating on cave sinters suggests this pattern extends back into earlier glacial/interglacial cycles.
Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts w... more Silcrete-calcrete intergrade duricrusts (surface or near-surface chemically precipitated crusts with a cement comprising a mixture of silica and CaCO3) have been widely identified in the geological, geomorphological and pedological literature, but have not, to date, been systematically described or classified. This paper presents a review of previous definitions of the end members of the silcrete-calcrete spectrum and subsequently identifies the major silica-carbonate relationships within intergrade types are identified on the the Kalahari of Botswana and Namibia. Three main intergrade types are identified on the basis of silica-carbonate associations: duricrusts where secondary silica occurs within a calcareous matrix; varieties where secondary carbonate occurs within a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in close succession. Within each of these three groups, sub-types are identified dependent upon whether secondary materials have replaced or been emplaced within a pre-existing duricrust. Finally, a practical procedure for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-section analyses.
Geografiska Annaler, Series A: Physical Geography, 1997
Recent studies of pan sediment stratigraphy in southern Africa have indicated the potential of ge... more Recent studies of pan sediment stratigraphy in southern Africa have indicated the potential of geophysical and geochemical analyses for elucidating the evolution of pans and the environmental conditions under which formation took place. We have investigated three cores, of 1.8 to 2.6 m in length, from Lebatse Pan in southeast Kalahari, Botswana, and compare the results with the few other similar studies available. Through the sedimentological characteristics of Lebatse we identify cycles of deposition, chemical precipitation and post-depositional alteration under shifting environmental conditions from c. 60,000 years ago to the present. Four periods are separated by depositional hiatuses, marking dry conditions during which deflation of the pan cannot be excluded. During the two oldest periods the sequence of finely laminated, magnesia-rich sediments suggests deposition in a permanent shallow waterbody under evaporative conditions, causing chemical precipitation of dolomite from groundwater. Less evaporative conditions are inferred for the third period of deposition between hiatuses H2 and H1. The youngest period, 14 C-dated to 29,000 years BP, is characterised by water-logged conditions and the development of reed beds. It is believed to have been laid down under fluctuating water levels. The cycles of deposition exhibited in the pan cores appear synchronous with wetter periods in the Southern Kalahari, indicated by the limited evidence currently available.
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