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Article Dans Une Revue Sedimentary Geology Année : 2013

Intracontinental Miocene: Reconstruction of hydrology and paleoclimate change in the Forez Basin, France (Part II)

Résumé

Sedimentary successions of palustrine deposits from the Forez Graben recorded changes in depositional and hydrological conditions throughout the Middle to Late Miocene of France. Paleontological, mineralogical and geochemical data combined with mass-balance calculation allowed the identification of modern, diagenetic and eogenetic stages of authigenic precipitation within the sedimentary pile. Stratigraphic log contains six clay- and sand-rich sequences, some associated with freshwater ostracods. delta C-13, delta O-18 values and Sr isotopic ratios of whole-rock samples and ostracods indicate that the Montrond-les-Bains upper sequence (8 to 175 mbs) is a continental lake system that recorded continental weather condition (15 to 25 degrees C) during the upper Miocene time. Geochemical changes permit the estimation of variations of evaporation-inflow changes (E/I: 8 to 75% and delta O-18(rainfall) -6 to -8 parts per thousand) associated with two sequences. delta O-18 and E/I progressive changes were related to runoff to colder climate conditions. large E/I values in below 100 mbs are associated with the appearance of authigenic calcite and dolomite precipitation. Euhedral dolomite seems to be related to desiccation period that induced capillary pumping of water undersaturated with respect to calcite but saturated for dolomite. The lower part of the Middle Miocene sedimentary series (175 to 300 mbs) hosts continental deposits with scarce fossil, a larger proportion of dolomite and massive pyrite levels related to mixed eogenetic and diagenetic precipitation. Scarce preserved "eogenetic levels" with unrealistic E/I ratios suggest either delta O-18 values similar to the upper section but with temperature lower than 15 C, or a similar temperature but very low delta O-18(rainfall). Calculated delta O-18(H2O) values have to be as low as -15 parts per thousand. delta O-18(H2O) values are similar to present-day melted snow. This stable isotope suggests episodes of regional warming melting surrounding snow, and able to mobilize as well as deposit thick sand levels. Eogenetic to diagenetic calcite, dolomite and pyrite precipitation involves Sand Mg-fluid influxes associated with methanogenic process (eogenesis) mixed with recrystallized pyrite and dolomite with diagenetic fluid circulation.

Dates et versions

hal-00829931 , version 1 (04-06-2013)

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Christophe Renac, Anne-Marie Bodergat, Marie-Christine Gerbe, Frédérick Gal. Intracontinental Miocene: Reconstruction of hydrology and paleoclimate change in the Forez Basin, France (Part II). Sedimentary Geology, 2013, 288, pp.16-39. ⟨10.1016/j.sedgeo.2012.09.004⟩. ⟨hal-00829931⟩
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