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Gwénael Caravaca : «Integrated approach to resolve past paleoenvironments: example of the Early Triassic Sonoma Foreland Basin (western USA)»

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Vendredi 20 Avril 2018, 14:30 - 15:30
par Mickaël BONNIN
Looking to the aftermath of the end-Permian mass extinction (~252 Ma), the Sonoma Foreland Basin (SFB) provides excellent sedimentary and fossil records to describe and understand the Early Triassic Earth and associated biotic recovery. Nevertheless, despite its pivotal importance, this basin was still poorly constrained and knowledge of the paleoenvironments was scarce with most controlling factors rather unclear. To approach these parameters, we set up an original integrative multi-proxy and multi-scale approach based on ~50 localized sections and regional to basin-scale data and propose a 4D evolution of the SFB.

These data, assembled from extensive field-work, literature and remote sensing, encompass various domains that proved complementary in reconstructing the history of the SFB and its paleoenvironmental conditions. We notably used chemio- and biostratigraphy to constrain a highly- resolved timeframe (with a temporal resolution as low as 40 ka), paleontological and biogeographical data, geochemical data (carbon isotopes and trace & major elements concentration), sedimentary data (structures, macro- and microfacies), geological mapping and imagery, 2D rheological modeling, and basin-scale geophysical data (Bouguer and aeromagnetic anomalies). All of these were synthetized inside a GIS to obtain a comprehensive and basin-scale spatialized database.

During this study, we investigated the apparent partition between northern and southern parts of the SFB observed on the field, with a noticeable difference between time equivalent sedimentary series of the Moenkopi Group in the South and the Dinwoody/Woodside Fm. in the North. From a geochemical point of view, it appears that water column geochemistry reflects the globally recognized signal for this time (characterized by a couple of excursions during the Early Triassic), but that local paleoenvironments exert a strong control over this signal. Within the sedimentary record, we looked upon the Moenkopi Fm., characterized by massive siliciclastic “red beds” deposits from continental to marine transitional settings and associated carbonated microbial beds. We showed

that not only the latter do not sign any peculiar deleterious environments as shown in previous literature, but that on the contrary they are characteristic of very precise depositional position, helping us to finely describe the local paleoenvironments and their evolution through time using facies belts migration. Sources of siliciclastic material found in concurrent levels of the Moenkopi red beds are currently processed to refine the possible origin of the chemical changes and their relationship with the distribution of the microbial organisms. Finally, we showed that North/South differences within the Sonoma Foreland Basin found their roots in the heritage due to the ~2 Gyr- long tectono-sedimentary history of the region. We therefore were able to propose a model explaining the basin-scale behavior and geodynamical evolution of the SFB, marked by a differential subsidence, concluding that age and nature of the basement were responsible for a variable response to tectonic constrains provoked by the now-obliterated Golconda Allochthon relief with very scarce remnants available on the field.

To sum up, the integrative approach was of paramount importance in reconstructing the 4D evolution of this ancient basin and its controlling factors.

Lieu : Salle de réunion du bât. 9

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