Origins of strandline duricrusts around the Makgadikgadi Pans (Botswana Kalahari) as deduced from their chemical and isotope composition

Ringrose, Susan; Harris, Christina; Huntsman-Mapila, P.; Vink, B.W.; Diskins, S.; Vanderpost, Cornelis; Matheson, Wilma

doi:10.1016/j.sedgeo.2009.05.021

Cited by 37 publications

(39 citation statements)

References 61 publications

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“…This explains the strong correlation between Sr and Ca in the studied profiles. Ringrose et al (2009) suggest that such enrichments may be the product of Sr-enriched plant uptake within the calcrete level. The shape of calcrete REE patterns does not differ greatly from those of the siliciclastic parent material, but the concentration of REE elements is different, indicating a common source for the REE.…”

Section: Calcretesmentioning

confidence: 98%

“…Calcrete and silcrete duricrusts associate in semiarid, closed continental basins (Watts 1980;Armenteros et al 1995;Ringrose et al 2009;Pérez-Jiménez 2010). Such associations are indicators of the prevailing sedimentary regimes, vegetation, and climate, but they also reveal major changes in the position and chemistry of the water table (Watts 1980;Bustillo and Alonso-Zarza 2007).…”

Section: Introductionmentioning

confidence: 99%

“…Such associations are indicators of the prevailing sedimentary regimes, vegetation, and climate, but they also reveal major changes in the position and chemistry of the water table (Watts 1980;Bustillo and Alonso-Zarza 2007). Most studies on calcretes and silcretes have been petrological or have examined their stable isotopes (carbon and oxygen); few have determined their major-element, minor-element, rareearth-element (REE), and uranium concentrations (Ramakrishnan and Tiwari 1998;Ringrose et al 2009) despite their possible hosting of uranium and gold deposits (McQueen 2006;Liu and Jaireth 2011). Differences in the concentrations of certain elements in the parent material, calcretes, and silcretes can provide clues to the mechanisms and environments of formation of these duricrusts (Kampunzu et al 2007).…”

Section: Introductionmentioning

confidence: 99%

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Root Calcretes and Uranium-Bearing Silcretes At Sedimentary Discontinuities In the Miocene of the Madrid Basin (Toledo, Spain)

Bustillo

Plet

Alonso-Zarza

2014

Journal of Sedimentary Research

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This paper reports a detailed study of the calcrete and silcrete profiles in the Miocene detrital deposits in the western area of Madrid, at the boundary of two main sedimentary units. The aims of this work were to better understand the pedogenic and diagenetic environments in which these profiles formed and to determine the cause(s) of their enrichment in uranium. Calcrete and silcrete duricrusts are characteristic features of closed continental basins in semiarid climates; this paper discusses the significance of duricrusts as indicators of important change in such basins.The detailed macromorphological, micromorphological, and geochemical study of three duricrust profiles revealed the sequence of pedogenic, vadose, and groundwater processes responsible for their formation. During the first stage of their development, carbonate laminae formed a white ''grill-like'' structure within the detrital parent materials. The microstructure and macrostructure of the carbonate, which includes alveolar septal structures and needle-fiber calcite, indicates the important role of roots and their associated microorganisms in calcrete formation. Early silicification occurred in the pedogenic-vadose environment affecting the detrital parent material, roots, and calcretes, forming an early silcrete defined by opaline glaebules and silica rhizoliths. The detailed preservation of the cells in the silicified roots denotes the early replacement of root organic matter. The green or green-yellowish fluorescence of the silicified root structures under short-wavelength UV shows their preferential enrichment in uranium. Calcitization and silicification coexisted in the pedogenic vadose environment, leading to several reversible replacements of calcite and silica. Later, the rise of the water table promoted silicification under phreatic conditions, as indicated by the good preservation of the texture of the detrital host rocks and calcretes. Other silcrete textures, such as ovoidal opaline accumulations, intraclasts produced by autobrecchification, and vadose silica cements, indicate later vadose environments, and consequently variations in the water table.The geochemical features of the calcretes and silcretes (major, minor, and rare earth elements) were inherited from their parent materials. The rare-earth-element patterns of some silcretes show them to have a positive Ce anomaly, suggesting that oxidizing conditions reigned during their formation. The good correlation between silica and uranium suggests that the silica phases acquired uranium through the direct silicification of roots that had fixed uranium from organic matter.This study shows that calcrete-silcrete duricrusts provide detailed information regarding the processes occurring in semiarid continental basins. In the studied basin, roots played a key role in both the development of the duricrust profiles and their enrichment in uranium. These duricrusts provide important information for understanding the overall stratigraphy of the studied basin and its large-scale ...

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Section: Calcretesmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Root Calcretes and Uranium-Bearing Silcretes At Sedimentary Discontinuities In the Miocene of the Madrid Basin (Toledo, Spain)

Bustillo

Plet

Alonso-Zarza

2014

Journal of Sedimentary Research

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“…CaO is also positively correlated at the 99% confidence level with MgO and Sr (r 2 = 0.70 and 0.95). The results are similar to those found elsewhere in intergrade duricrusts derived from Kalahari sands (Nash et al ., ; Ringrose et al ., , ) and Cape Coastal silcretes (Nash et al ., 1994(b)). The associations suggest that Boteti estuary samples form two geochemical groups; one being SiO 2 /Al 2 O 3 dominated, while the second group comprises CaCO 3 dominated associations.…”

Section: Resultsmentioning

confidence: 99%

“…() and Ringrose et al . (, ). Silcrete–calcrete intergrade deposits are prevalent throughout Botswana and other semi‐arid regions within North and South America, and Australia (Nash et al ., and references therein).…”

Section: Introductionmentioning

confidence: 99%

Diagenetic transformations and silcrete–calcrete intergrade duricrust formation in palaeo‐estuary sediments

Ringrose

Cassidy²,

Diskin

et al. 2014

Earth Surf Processes Landf

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The Boteti palaeo‐estuary in northern Botswana is located where the endoreic Boteti river, an overflow from the regional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonate dominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete–calcrete intergrade deposits while developing insight into pene‐contemporaneous silica and calcite matrix formation. General precursor conditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough to sustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater. Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH). The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptor saline pan. The source of most silica and localized CaCO3 is derived from the dissolution and precipitation of micro‐fossils, while more general CaCO3 enrichment stems from saline pan based carbonate fractionation. Diagenetic change leads to colloidal then more consolidated bSiO2/CaO aggregate formation (amorphous silica) followed by transformations into opaline silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregular occurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits. The distribution of calcareous micro‐fossils may have a similar converse effect. Diagenetic evidence from an intergrade deposit with a low SiO2/CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit with a high SiO2/CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene‐contemporaneous silcrete–calcrete intergrade formation under the above conditions may take place where dissolved silica crystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO2/CaO aggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO3 may act as a catalyst leading to pene‐contemporaneous bSiO2/CaO aggregate formation. However the processes involved require further work. Copyright © 2013 John Wiley & Sons, Ltd.

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Reassessing southern African silcrete geochemistry: implications for silcrete origin and sourcing of silcrete artefacts

Webb

Nash

2020

Earth Surf Processes Landf

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A synthesis of the geochemistry of silcretes and their host sediments in the Kalahari Desert and Cape coastal zone, using isocon comparisons, shows that silcretes in the two regions are very different. Kalahari Desert silcretes outcrop along drainage-lines and within pans, and formed by groundwater silicification of near-surface Kalahari Group sands. Silicification was approximately isovolumetric. Few elements were lost; silicon (Si) and potassium (K) were gained as microquartz precipitated in the sediment porosity and glauconite formed in the sub-oxic groundwater conditions. The low titanium (Ti) content reflects the composition of the host sands. Additional elements in the Kalahari Desert silcretes were supplied in river water and derived from weathering of silicates in basement rocks. Evaporation under an arid climate produced high-pH groundwater that mobilized and precipitated Si; this process is still occurring. In the Cape coastal zone, pedogenic silcretes cap hills and plateaus, overlying deeply weathered argillaceous bedrock. Silicification resulted from intensive weathering that destroyed the bedrock silicates, almost completely removing most elements and causing a substantial volume decrease. Some of the silica released formed a microcrystalline quartz matrix, and most Ti precipitated as anatase, so the Cape silcretes contain relatively high Ti levels. The intense weathering that formed the Cape silcretes could have occurred in the Eocene, during and after the Palaeocene-Eocene Thermal Maximum, when more acidic rainfall and high temperatures resulted in intensified silicate weathering worldwide. This could have been responsible for widespread formation of pedogenic silcretes elsewhere in Africa and around the globe. Trace element sourcing of silcrete artefacts to particular outcrops has most potential in the Cape, where differences between separate bedrock areas are reflected in the silcrete composition. In the Kalahari Desert, gains of some elements can override compositional differences of the parent material, and sourcing should be based on elements that show the least change during silicification.

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Origins of strandline duricrusts around the Makgadikgadi Pans (Botswana Kalahari) as deduced from their chemical and isotope composition

Cited by 37 publications

References 61 publications

Root Calcretes and Uranium-Bearing Silcretes At Sedimentary Discontinuities In the Miocene of the Madrid Basin (Toledo, Spain)

Root Calcretes and Uranium-Bearing Silcretes At Sedimentary Discontinuities In the Miocene of the Madrid Basin (Toledo, Spain)

Diagenetic transformations and silcrete–calcrete intergrade duricrust formation in palaeo‐estuary sediments

Reassessing southern African silcrete geochemistry: implications for silcrete origin and sourcing of silcrete artefacts

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