Silcretes

Milnes, A. R.; Thiry, Médard

doi:10.1016/b978-0-444-89198-3.50019-2

Cited by 54 publications

(57 citation statements)

References 21 publications

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“…Cap features in groundwater silcretes appear to be 499 characterised by a much simpler structure and lack the alternating Ti-and silica-rich 500 lamellae which occur within pedogenic silcrete caps. This echoes the distinction already 501 drawn between the simple fabrics of non-pedogenic silcretes and complex fabrics of 502 pedogenic varieties (Milnes and Thiry, 1992). Some groundwater silcretes in this study 503…”

Section: Conclusion 479supporting

confidence: 67%

“…Cap structures within pedogenic silcretes are a direct product of the silicification 368 process, whereby silica and other components percolate downwards through the profile in 369 intermittent but repetitive cycles of flushing and precipitation (Milnes and Thiry, 1992). This 370 produces the characteristic lamellar structures present within caps, as well as the many 371 other way-up orientated features found in pedogenic silcrete fabrics (see section 1).…”

Section: Origins Of Cap Structures 357mentioning

confidence: 99%

“…Silcrete is an indurated geochemical sediment produced by the near-surface accumulation 41 of secondary silica within a soil, sediment, rock or weathered material (Summerfield, 1983; 42 Milnes and Thiry, 1992). It has been described on every continent except Antarctica 43 (Summerfield, 1983), and is recognised as a major terrestrial silicon sink (Basile-Doelsch 44 et al, 2005).…”

Section: Introduction 40mentioning

confidence: 99%

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Cap structures as diagnostic indicators of silcrete origin

Ullyott

Nash

Huggett³

2015

Sedimentary Geology

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pedogenic silcretes indicate that this may no longer be appropriate. This paper presents 17 the first systematic investigation of the micro-fabric, geochemistry and mineralogy of cap 18 structures in groundwater silcretes, through an analysis of conglomeratic varieties 19 (puddingstones) from the southern UK. Our results suggest that cap structures in 20 groundwater silcretes fall within a spectrum of types, related to the degree of sorting in the 21 inter-gravel host sediment. At one end of this spectrum are well-defined caps within 22 otherwise well-sorted, overgrowth-dominated silcretes. These caps exhibit a grain-23 supported fabric, are cemented by micro-and/or cryptocrystalline silica, and contain 24 floating silt-sized quartz and Ti-oxide grains. We propose that these structures developed 25 mainly as a result of in-washing of fine sediments that were subsequently silicified. At the 26 other end of the spectrum are silcretes with caps defined by concentrations of Ti-oxide 27 grains, as opposed to cement type and grain size. These formed mainly as a result of the 28 remobilisation and precipitation of Ti during the silicification of gravels containing interstitial 29 clay-rich sandy sediment. Between these end-members are silcretes with cap structures 30 formed by a combination of in-washing and redistribution of fines plus some local 31 remobilisation of Ti. Overall, the cap structures in this study exhibit a simple 32 micromorphology, lacking the alternating Ti-and silica-rich lamellae typical of pedogenic 33 silcrete. We conclude that the presence of cap structures alone should not be considered 34 diagnostic of pedogenic silicification unless accompanied by other indicators such as a 35 differentiated profile and abundant, complex, way-up structures within the micro-fabric. 36 37

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Section: Conclusion 479supporting

confidence: 67%

Section: Origins Of Cap Structures 357mentioning

confidence: 99%

Section: Introduction 40mentioning

confidence: 99%

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Cap structures as diagnostic indicators of silcrete origin

Ullyott

Nash

Huggett³

2015

Sedimentary Geology

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“…Milnes and Thiry 1992;Nash et al 1998;Ullyott and Nash 2006). The silicification proceeded in a vadose zone unfavourable for deep groundwater circulation (Głazek 1989;Głazek et al 1992;cf.…”

Section: Discussionmentioning

confidence: 99%

Epigenetic silicification of the Upper Oxfordian limestones in the Sokole Hills (Kraków-Częstochowa Upland): relationship to facies development and tectonics

Matyszkiewicz¹,

Kochman²,

Rzepa³

et al. 2015

Acta Geologica Polonica

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ABSTRACT:Matyszkiewicz, J., Kochman, A., Rzepa, G., Gołębiowska, B., Krajewski, M., Gajdzik, K. and Żaba, J. 2015. Epigenetic silicification of the Upper Oxfordian limestones in the Sokole Hills (Kraków-Częstochowa Upland): relationship to facies development and tectonics. Acta Geologica Polonica, 65 (2), 181-203. Warszawa.A spectacular epigenetic silicification was encountered in the Oxfordian bedded limestones exposed in the Sokole Hills situated in the Kraków-Częstochowa Upland. The main epigenetic mineral is microcrystalline quartz accompanied by minor goethite, hematite, barite, galena and sphalerite. Locally, the mineralized limestones reveal Pb and Cu contents exceeding over 150 times the background values of these metals in unmineralized limestones.The epigenetic mineralization of the bedded limestones was probably a two-stage process. During the first, Early Cretaceous stage, silicified limestones formed at the erosional surface of a denuded carbonate complex. Such silicification greatly limited the progress of the first karstification phase of the Upper Jurassic carbonates initiated in the Hauterivian. The sources of silica accumulated in the limestones were descending solutions enriched in silica derived from the weathering zone. This silicification affected the topmost part of the Upper Jurassic massive limestones and the deeper portions of the bedded limestones along the fracture systems and stylolites.Early Cretaceous tectonic activity generated new dislocations and re-opened the existing faults, which were subsequently filled with permeable Albian quartz sands. These openings became the migration pathways for ascending, warm, relict, sulphide-carrying hydrothermal solutions at the second formation stage of the epigenetic mineralization. The newly supplied silica from the Albian sands precipitated on the silicified limestones and, as concentric rims, on brecciated, early diagenetic cherts. The second-stage mineralization proceeded under phreatic conditions, presumably close to a fluctuating mixing zone of ascending, warm hydrothermal solutions and descending cold groundwaters. The brecciated cherts acting as silica crystallization nuclei indicate that the last mineralization stage probably followed the final phase of Cenozoic faulting.

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“…In general, the morphology of an individual silcrete profile reflects its mode of formation and the nature of its host material (Milnes and Thiry, 1992;Summerfield, 1983a and b). However, silcrete appearances vary even between profiles developed by comparable processes, such that it is can be difficult to distinguish silcrete types on the basis of macromorphology alone.…”

Section: General Characteristics Of Silcretesmentioning

confidence: 99%

Widespread Development of Silcrete in the Cretaceous and Evolution of the Poaceae Family of Grass Plants

Bata¹

2016

ESR

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The Cretaceous period, which is considered one of the most remarkable periods in Earth's history, saw episodes of abrupt greenhouse warming and cooling. The Cretaceous was also exceptional in that it was associated with the widespread occurrence of silcrete. To demonstrate this, the present study collated records of silcrete occurrences from the Jurassic to the present and compared them with records of variation in palaeotemperature and atmospheric CO 2 levels. Quartz solubility, which is one of the key factors that controls the rate of silcrete development, was also calculated over the same period. The results demonstrate that a marked increase (approximately 100%) in quartz solubility occurred during the Cretaceous. This was found to be a direct consequence of the extreme global warmth witnessed at that time. Furthermore, this study shows that silcrete occurrences are consistent with records of palaeotemperature and that silcretes have formed mostly in regions that have experienced warm climatic conditions, with no instances found in polar regions. The Poaceae family of grass plants are known to have evolved and diversified during the Cretaceous, which coincide with the period when silica was readily available. X-ray spectra and backscattered images from SEM examination of the internal structure of a modern wild grass (which belongs to the Poaceae family of grass plants) reveals that silica is an important constituent of the grass. This suggest a possible link between evolution of the Poaceae family in the Cretaceous and the high availability of silica during the Cretaceous.

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Silcretes

Cited by 54 publications

References 21 publications

Cap structures as diagnostic indicators of silcrete origin

Cap structures as diagnostic indicators of silcrete origin

Epigenetic silicification of the Upper Oxfordian limestones in the Sokole Hills (Kraków-Częstochowa Upland): relationship to facies development and tectonics

Widespread Development of Silcrete in the Cretaceous and Evolution of the Poaceae Family of Grass Plants

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