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Titolo:
Development of tightly cemented sandstone lenses in uncemented sand: Example of the Fontainebleau Sand (Oligocene) in the Paris Basin
Autore:
Thiry, M; Marechal, B;
Titolo Testata:
JOURNAL OF SEDIMENTARY RESEARCH
fascicolo: 3, volume: 71, anno: 2001,
parte:, A
pagine: 473 - 483
SICI:
1527-1404(200105)71:3<473:DOTCSL>2.0.ZU;2-S
Fonte:
ISI
Lingua:
ENG
Soggetto:
QUARTZ CEMENTATION; PRECIPITATION; CONCRETIONS; INHIBITION; KINETICS; CALCITE; 25-DEGREES-C; SOLUBILITY; ARAGONITE; SEAWATER;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
51
Recensione:
Indirizzi per estratti:
Indirizzo: Thiry, M Ecole Mines Paris, Ctr Informat Geol, 35 Rue St Honore, F-77305 Fontainebleau, France Ecole Mines Paris 35 Rue St Honore Fontainebleau France F-77305
Citazione:
M. Thiry e B. Marechal, "Development of tightly cemented sandstone lenses in uncemented sand: Example of the Fontainebleau Sand (Oligocene) in the Paris Basin", J SED RES, 71(3), 2001, pp. 473-483

Abstract

In the Fontainebleau Sand (Oligucene), superposed lenses of sandstone result from a silicification process controlled by the watertable during the recent geomorphologic evolution of the landscape. The most outstanding feature of these silicified bodies is the contrast they show between the very hard, tightly cemented sandstone and the Loose and permeable embedding sand, This pattern raises the question of the growth mechanism of the lenses. The lenses are composed of tightly quartz-cemented sandstone with well-developed quartz overgrowths. Cathodoluminescence of the sandstones shows detrital grains with subeuhedral quartz overgrowths and isopachous quartz rims surrounding the detrital grains or the overgrowths, The isopachous rims suggest amorphous or poorly ordered silica deposits that later recrystallized into quartz, The syntaxial over-growths and the silica deposits alternate in a sequential way on a centimeter scale and reflect variations in the physicochemical characteristics of the feeding groundwater, The mechanism of the silica deposition is probably complex and can only be hypothesized. Whatever the mechanism, it appears from the arrangement of the sandstone lavers that silica precipitation occurred near the watertable and at the interfacebetween regional groundwater and local recharge water. Silica precipitation along an interface may explain the sharp boundary between cemented sandstone and Loose sand. The cementation was modeled with the coupled reaction-transport code METIS. The model was constrained with permeability values, hydraulic gradient, and dissolved silica contents measured in the Fontainebleau Sand, The simulation reproduced the characteristics and morphologies shown by the sandstonelenses in the field, It shows the importance of a high groundwater how rate to provide the silica necessary for cementation of the sandstones in a time span compatible with the geological constraints. The conventionally accepted kinetics of quartz precipitation did not result in simulating cementation of the sandstone lenses in a geologically reasonable time frame. To overcome this constraint, it was necessary to increase the kinetic reaction rate about 1,000 times, which agrees with the amorphous silica deposits observed in the Fontainebleau Sandstones.

ASDD Area Sistemi Dipartimentali e Documentali, Università di Bologna, Catalogo delle riviste ed altri periodici
Documento generato il 21/09/20 alle ore 12:17:41