Driving forces for the weathering and alteration of silica in the regolith: Implications for studies of prehistoric flint tools
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Regolith
Most studies of sandstone provenance involve modal analysis of framework grains using techniques that exclude the fine-grained breakdown products of labile mineral grains and rock fragments, usually termed secondary matrix or pseudomatrix. However, the data presented here demonstrate that, when the proportion of pseudomatrix in a sandstone exceeds 10%, standard petrographic analysis can lead to incorrect provenance interpretation. Petrographic schemes for provenance analysis such as QFL and QFR should not therefore be applied to sandstones containing more than 10% secondary matrix. Pseudomatrix is commonly abundant in sandstones, and this is therefore a problem for provenance analysis. The difficulty can be alleviated by the use of whole-rock chemistry in addition to petrographic analysis. Combination of chemical and point-count data permits the construction of normative compositions that approximate original framework grain compositions. Provenance analysis is also complicated in many cases by fundamental compositional alteration during weathering and transport. Many sandstones, particularly shallow marine deposits, have undergone vigorous reworking, which may destroy unstable mineral grains and rock fragments. In such cases it may not be possible to retrieve provenance information by either petrographic or chemical means. Because of this, pseudomatrix-rich sandstones should be routinely included in chemical-petrological provenance analysis. Because of the many factors, both pre- and post-depositional, that operate to increase the compositional maturity of sandstones, petrologic studies must include a complete inventory of matrix proportions, grain size and sorting parameters, and an assessment of depositional setting.
Matrix (chemical analysis)
Lithic fragment
Arenite
Siliciclastic
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Regolith
Soil production function
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[1] Large areas distributed on the Earth's surface are covered by regolith, an unconsolidated heterogeneous material overlying bedrock. In high-latitude areas, most of the land surface has been reworked and eroded by both glacial and fluvial processes, leaving only remnants of formerly extensive regolith covers. In an effort to further the understanding of weathering patterns and processes in old regolith covers, a comprehensive study of localities spread across Norway was carried out. On the basis of the distribution of minerals and elements within regolith, as well as its internal structure and geomorphologic setting, we ascertained that it was formed in situ and originated in pre-Quaternary times. There are similarities between the study sites with respect to regolith thickness, zonation, and composition. The Chemical Index of Alteration (CIA) and the Weathering Index of Parker (WIP) suggests that the degree of chemical weathering in the regolith is advanced compared to the parental bedrock with a maximum change of over 80%, which indicates a substantial increase in the proportion of secondary versus primary minerals. Mineral analysis identified kaolinite and gibbsite, which are considered indicative of advanced weathering and therefore support this observation. On the basis of statistical relationships between different grain size fractions (<125 μm), we observed a consistent pattern, which revealed that physical weathering becomes progressively less important in the production of grains smaller than 32 μm. On the basis of this finding, we infer that chemical weathering progressively dominates the production of fine silt, very fine silt, and clay, whereas physical weathering primarily controls the production of grain size fractions larger than 32 μm. This particular pattern is suggested to be an intrinsic feature in the formation of weathered high-latitude regolith.
Regolith
Bedrock
Silt
Soil production function
Parent material
Saprolite
Gibbsite
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Regolith
Soil production function
Subaerial
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