Continental Silicifications: A Review

Thiry, Médard

doi:10.1007/978-3-642-60525-3_10

Cited by 13 publications

(7 citation statements)

References 42 publications

(33 reference statements)

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“…) where it commonly occurs as isolated nodules up to a metre in diameter and as laterally limited chert horizons up to 4 m thick within sepiolite mudstone. According to Bustillo & Alonso‐Zarza (), the chert horizons can be interpreted as phreatic silcretes ( sensu Thiry, ).…”

Section: Results and Interpretationsmentioning

confidence: 99%

Pattern of sedimentary infilling of fossil mammal traps formed in pseudokarst at Cerro de los Batallones, Madrid Basin, central Spain

et al. 2013

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Fossil mammal sites of late Miocene age (ca 9 Ma) occur in hourglassshaped, non-interconnected cavities up to 15 m deep, hosted in mudstone (mostly sepiolite), chert and carbonate bedrock in Cerro de los Batallones. This paper provides a model for the sedimentary infilling of the cavities, which functioned as traps for vertebrate faunas and contain one of the richest and best preserved Neogene mammal assemblages of the Iberian Peninsula. Generation of the mammal-bearing cavities started with the solution of underlying evaporites, which resulted in fissures that were subsequently enlarged by subsurface piping, a process rarely preserved in the ancient sedimentary record. The system of subterranean cavities evolved into a pseudokarst landscape, resulting in doline-like shafts reaching the ancient land surface. The sedimentary infilling of the cavities comprises both clastic and carbonate lithofacies that were investigated by outcrop observation, standard and scanning electron microscope petrography, mineralogical analysis, and stable isotope geochemistry. Gravel and breccia talus deposits, clast and mud-supported gravel, pebbly sandstone and mudstone are common detrital infill deposits mostly derived by overflow erosion of bedrock. The deposits containing the mammal bones are marls, and occur both in subsurface cavities and doline-like depressions. In the underground cavities, marlstone was mainly of clastic origin and accumulated in ponds scattered over the floor of the cavity. In contrast, marlstone deposits in the surface dolines formed mostly as a result of biochemical carbonate deposition in small shallow lakes subjected to fluctuation of the water level. The d 18 O and d 13 C carbonate values indicate different origins for the two kinds of marls. During the final phases of pipe infill the doline marlstone sealed the mammal sites, usually off-lapping the adjacent bedrock.

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Section: Results and Interpretationsmentioning

confidence: 99%

Pattern of sedimentary infilling of fossil mammal traps formed in pseudokarst at Cerro de los Batallones, Madrid Basin, central Spain

et al. 2013

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“…Pedogenic silcretes may form in a vadose environment through intermittent phases of leaching, infiltration, and illuviation alternating with evaporation (Thiry and Milnes1991). In contrast, the formation of groundwater silcretes is related to silica transport driven by fluctuating water tables or lateral groundwater flows (Thiry et al 1988;Thiry 1997). Far fewer studies have been published on silcretes than on calcretes, and even fewer on duricrusts, in which calcrete and silcrete interfinger.…”

Section: Introductionmentioning

confidence: 99%

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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“…2, episode I) and clays (Fig. 2, episodes II–IV) and reproduced the textural and structural characteristics of the host rocks, indicating them to be the result of a replacement process that took place under the level of the water table (Thiry 1997). Later transformations in a vadose environment (above the water table) explain the generalized process of ageing seen, with transformation from opal to quartz.…”

Section: Geology and Petrology Of The Flint Levels In The Casa Montermentioning

confidence: 74%

Is the Macroscopic Classification of Flint Useful? A Petroarchaeological Analysis and Characterization of Flint Raw Materials From the Iberian Neolithic Mine of Casa Montero*

Bustillo

Castañeda²,

Capote³

et al. 2009

Archaeometry

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Casa Montero is a mining complex located outside Madrid (Spain), dated from the Early Neolithic (c. 5400–5000 cal bc). An area of some 4 ha has been investigated and some 4000 shafts recorded, of which 324 have been excavated. The characterization of its raw flint materials and the establishment of its diagnostic features are indispensable in the reconstruction of the distribution of the mine's products beyond the immediate site. This work reports the geological study of the mine's Miocene flint layers and their petrological characterization. Archaeological samples from the mine's shafts were classified according to macroscopic features and petrological characteristics.

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Continental Silicifications: A Review

Cited by 13 publications

References 42 publications

Pattern of sedimentary infilling of fossil mammal traps formed in pseudokarst at Cerro de los Batallones, Madrid Basin, central Spain

Pattern of sedimentary infilling of fossil mammal traps formed in pseudokarst at Cerro de los Batallones, Madrid Basin, central Spain

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

Is the Macroscopic Classification of Flint Useful? A Petroarchaeological Analysis and Characterization of Flint Raw Materials From the Iberian Neolithic Mine of Casa Montero*

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