Silcretes
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Recent papers in Silcretes
Cap structures within silcretes have long been used as a diagnostic indicator of pedogenic silicification. However, a growing number of studies of the micromorphology of non-pedogenic silcretes indicate that this may no longer be... more
Cap structures within silcretes have long been used as a diagnostic indicator of pedogenic silicification. However, a growing number of studies of the micromorphology of non-pedogenic silcretes indicate that this may no longer be appropriate. This paper presents the first systematic investigation of the micro-fabric, geochemistry and mineralogy of cap structures in groundwater silcretes, through an analysis of conglomeratic varieties (puddingstones) from the southern UK. Our results suggest that cap structures in groundwater silcretes fall within a spectrum of types, related to the degree of sorting in the inter-gravel host sediment. At one end of this spectrum are well-defined caps within otherwise well-sorted, overgrowth-dominated silcretes. These caps exhibit a grain-supported fabric, are cemented by micro-and/or cryptocrystalline silica, and contain floating silt-sized quartz and Ti-oxide grains. We propose that these structures developed mainly as a result of in-washing of fine sediments that were subsequently silicified. At the other end of the spectrum are silcretes with caps defined by concentrations of Ti-oxide grains, as opposed to cement type and grain size. These formed mainly as a result of the remobilisation and precipitation of Ti during the silicification of gravels containing interstitial clay-rich sandy sediment. Between these end-members are silcretes with cap structures formed by a combination of in-washing and redistribution of fines plus some local remobilisation of Ti. Overall, the cap structures in this study exhibit a simple micromorphology, lacking the alternating Ti-and silica-rich lamellae typical of pedogenic silcrete. We conclude that the presence of cap structures alone should not be considered diagnostic of pedogenic silicification unless accompanied by other indicators such as a differentiated profile and abundant, complex, way-up structures within the micro-fabric.
- by Stewart Ullyott and +1
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- Geochemistry, Geomorphology, Sedimentology, Silcrete
This study utilises geochemical provenancing of silcrete raw materials, in combination with chaîne op eratoire analyses, to explore lithic procurement and behavioural patterns in the northern Kalahari Desert during the Middle Stone Age... more
This study utilises geochemical provenancing of silcrete raw materials, in combination with chaîne op eratoire analyses, to explore lithic procurement and behavioural patterns in the northern Kalahari Desert during the Middle Stone Age (MSA). New data from the sites of Rhino Cave, Corner Cave, and sGi in northwest Botswana, combined with earlier results from White Paintings Shelter, reveal that the long distance transport of silcrete for stone tool manufacture was a repeated and extensively used behaviour in this region. Silcrete was imported over distances of up to 295 km to all four sites, from locations along the Boteti River and around Lake Ngami. Significantly, closer known sources of silcrete of equivalent quality were largely bypassed. Silcrete artefacts were transported at various stages of production (as partially and fully prepared cores, blanks, and finished tools) and, with the exception of sGi, in large volumes. The import occurred despite the abundance of locally available raw materials, which were also used to manufacture the same tool types. On the basis of regional palaeoenvironmental data, the timing of the majority of silcrete import from the Boteti River and Lake Ngami is constrained to regionally drier periods of the MSA. The results of our investigation challenge key assumptions underlying predictive models of human mobility that use distanceedecay curves and drop-off rates. Middle Stone Age peoples in the Kalahari appear to have been more mobile than anticipated, and repeatedly made costly choices with regard to both raw material selection and items to be transported. We conclude that (i) base transport cost has been overemphasised as a restrictive factor in predictive models, and (ii) factors such as source availability and preference, raw material quality, and potential sociocultural influences significantly shaped prehistoric landscape use choices.
Silcretes are clearly observed and abundant as components of paleolandscapes on several continents. Mechanisms for the formation of several varieties of silcrete, with specific relationships to paleolandscapes, are described. Each type of... more
Silcretes are clearly observed and abundant as components of paleolandscapes on several continents. Mechanisms for the formation of several varieties of silcrete, with specific relationships to paleolandscapes, are described. Each type of silcrete displays particular morphological features in its profile in the paleo-regolith, and these features provide pointers to its origin via mechanisms of absolute or relative accumulation of silica in specific environments relating to groundwater or soil-water hydrology. The characters of silcrete varieties that may have triggered the interest of prehistoric peoples to exploit them for manufacturing stone tools, and which control knappability, include granulometry and the specific nature of silica cements. The successions of silica precipitation and recrystallisation events are clearly evident as a complex of micromorphological features that provide clues to the hydrological environment and its geochemistry at the time or times of silicification. Examples are given of the distribution of different silcrete facies, which could have had differing values for exploitation for stone tool production, in modern-day landscapes in France and Australia.
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... more
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.
- by David Nash and +1
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- Geochemistry, Geomorphology, Groundwater, Southern Africa
Special Issue: Silcrete as a lithic raw material in global context: Geology, sourcing and prehistoric techno-economics
Cap structures within silcretes have long been used as a diagnostic indicator of pedogenic silicification. However, a growing number of studies of the micromorphology of nonpedogenic silcretes indicate that this may no longer be... more
Cap structures within silcretes have long been used as a diagnostic indicator of pedogenic silicification. However, a growing number of studies of the micromorphology of nonpedogenic silcretes indicate that this may no longer be appropriate. This paper presents the first systematic investigation of the micro-fabric, geochemistry and mineralogy of cap structures in groundwater silcretes, through an analysis of conglomeratic varieties (puddingstones) from the southern UK. Our results suggest that cap structures in groundwater silcretes fall within a spectrum of types, related to the degree of sorting in the inter-gravel host sediment. At one end of this spectrum are well-defined caps within otherwise well-sorted, overgrowth-dominated silcretes. These caps exhibit a grainsupported fabric, are cemented by micro-and/or cryptocrystalline silica, and contain floating silt-sized quartz and Ti-oxide grains. We propose that these structures developed mainly as a result of in-washing of fine sediments that were subsequently silicified. At the other end of the spectrum are silcretes with caps defined by concentrations of Ti-oxide grains, as opposed to cement type and grain size. These formed mainly as a result of the remobilisation and precipitation of Ti during the silicification of gravels containing interstitial clay-rich sandy sediment. Between these end-members are silcretes with cap structures formed by a combination of in-washing and redistribution of fines plus some local remobilisation of Ti. Overall, the cap structures in this study exhibit a simple micromorphology, lacking the alternating Ti-and silica-rich lamellae typical of pedogenic silcrete. We conclude that the presence of cap structures alone should not be considered diagnostic of pedogenic silicification unless accompanied by other indicators such as a differentiated profile and abundant, complex, way-up structures within the micro-fabric.
En este trabajo se presenta un estudio tecno-morfológico sobre conjuntos líticos de diversas materias primas, entre ellas silcretes, rocas volcánicas/subvolcánicas, cuarzos y metacuarcitas, procedentes de siete cuadrículas de recolección... more
En este trabajo se presenta un estudio tecno-morfológico sobre conjuntos líticos de diversas materias primas, entre ellas silcretes, rocas volcánicas/subvolcánicas, cuarzos y metacuarcitas, procedentes de siete cuadrículas de recolección de los sitios canteras taller de silcrete El Ranchito 1 y 2 (Dpto. Ischilín, Córdoba, Argentina). Los resultados demuestran la existencia de distintas trayectorias para estas materias primas, en las que se realizaron tareas de obtención, manufactura y uso mayormente de silcretes. La forma geométrica de los nódulos de esta roca silícea, a saber, tabular, laminar y esférica, ha sido una opción para la selección de formas base para su posterior manufactura final y uso en otros lugares, como así también para su manufactura y uso en las canteras. Palabras claves: análisis tecno-morfológico; canteras taller; norte de Córdoba
In this work we report a techno-morphological study on the lithic assemblage of diverse raw materials, including silcretes, volcanic/subvolcanic, quartz and metaquartz rocks, all coming from seven collection grids belonging to silcrete quarry workshop sites El Ranchito 1 and 2 (Ischilín, Córdoba, Argentina). Results show different trajectories for these raw materials, in which tasks consisting of collection, manufacture and use of mostly silcrete were carried out. The geometrical shape of the nodules of this siliceous rock, i.e., tabular, laminar and spherical, was an option for selecting base forms for later final manufacture and use in other places, as well as for manufacture and use in the quarries.
In this work we report a techno-morphological study on the lithic assemblage of diverse raw materials, including silcretes, volcanic/subvolcanic, quartz and metaquartz rocks, all coming from seven collection grids belonging to silcrete quarry workshop sites El Ranchito 1 and 2 (Ischilín, Córdoba, Argentina). Results show different trajectories for these raw materials, in which tasks consisting of collection, manufacture and use of mostly silcrete were carried out. The geometrical shape of the nodules of this siliceous rock, i.e., tabular, laminar and spherical, was an option for selecting base forms for later final manufacture and use in other places, as well as for manufacture and use in the quarries.
A range of geochemical and mineralogical factors trigger the weathering and influence the stability of various silica phases under near-surface temperatures and pressures. Based on the current knowledge of the crystallochemical basis of... more
A range of geochemical and mineralogical factors trigger the weathering and influence the stability of various silica phases under near-surface temperatures and pressures. Based on the current knowledge of the crystallochemical basis of silica behaviour in near-surface environments, the mineralogical and petrographical transformations of silica under weathering conditions in the regolith are reviewed using data from wide-ranging and detailed investigations of siliceous materials. Some of the investigations are of siliceous materials that evolved in near-surface environments over geological time and thus the weathering and alteration features are relatively clear and well expressed. Examples include weathering of tightly cemented sandstones, the formation of silica duricrusts (pedogenetic silcretes), and the weathering of flints. The various micromorphological and mineralogical forms of silica as determined by high resolution petrographic and electron-optical techniques can be related to geochemical and hydrological conditions in particular microenvironments. However, new studies have demonstrated that silica alteration and transformation mechanisms can also occur within prehistoric and historic time intervals. These are also described but are more difficult to observe, principally because of the microcrystalline character of the alteration phases and the infra-millimetric scale on which they develop. Of many potential applications of knowledge about the near-surface alteration of silica in regolith materials and the environmental conditions under which it occurs, our studies can be used as a basis determining the provenance and post-discard history of prehistoric flint tools and artefacts, in part to determine the source of the primary raw material. We suggest that the mineralogical and petrographical characteristics of the weathering rind (cortex) of the flint (specifically the recrystallization, dissolution and deposition of the silica phases, and the spatial arrangement of surfaces that have undergone successive transformations) are related to specific and identifiable weathering environments.