Abstract
The Limagne Basin (Massif Central, France) originated during a major, European-scale, extensive event (European Cenozoic Rift System), which led to the formation of several rift systems in the foreland of the Alps between the Upper Eocene and Pliocene. A fluvio-lacustrine system emplaced in the basin and resulted in a mixed carbonate-siliciclastic sedimentation in which microbial and metazoan buildups occupy an important place. However, microbial deposits are not exclusive to the Cenozoic history of the Limagne Basin; nowadays, in the basin, they still form in association with thermal spring systems. A fieldtrip was carried out in the Limagne Basin as part of the Microbialites: formation, evolution and diagenesis (M-Fed) meeting (October 2019). The objective of this excursion was to assess the diversity of modern and fossil (Chattian to Aquitanian) microbial sediments and structures in three prime locations (the Jussat and Chadrat outcrops and the Grand Gandaillat quarry). A detailed description of the morphologies and fabrics of the buildups and their associated biotic components can be used to discuss the spatio-temporal distribution pattern. Different basin margin models are proposed based on the changes in the distribution, morphology and size of the microbial and metazoan-rich deposits through time. The Jussat outcrop offers novel perspectives to unravel the evolution of the lacustrine/palustrine cycles over time and to establish a long-term paleoenvironmental history of the western margin of the basin during the Aquitanian. These cycles are composed of (i) lacustrine sedimentation comprising microbial and metazoan buildups and organic matter-rich marls reflecting a period of high accommodation, and (ii) palustrine deposits made of mudstones and clayey paleosoils, indicative of a period of low accommodation. It is suggested that climatic, tectonic, volcanic and local parameters (physiography, substrate) control the deposition of the buildups in each of the different cycles. In addition, the modern microbial mats of the Sainte-Marguerite and La Poix outcrops offer an opportunity to constrain the controlling processes at the origin of the mineralization involved in the formation of the microbialites and their preservation in the fossil record.
Highlights
Is that extensive microbial mats still currently form associated with the thermal activity of the region, which in turn is directly linked to the tertiary geodynamic framework? Despite showing great similarities with the world-famous Eocene buildups of the Green River Formation, the buildups of the Limagne Basin have attracted less interest, even though their existence has been reported for a long time (D’Halloy, 1812)
We focus on the diversity of modern and fossil (Oligo-Miocene) microbial sediments and structures emplaced in the Limagne Basin
This publication results from a fieldtrip organized in the Limagne Basin as part of the Microbialites: formation, evolution and diagenesis (M-Fed) meeting (October 2019)
Summary
The accumulation and preservation of lacustrine microbial carbonates in both recent systems (e.g. Lake Tanganyika, Kenya; Casanova and Hillaire-Marcel, 1992; Cohen et al, 1997; Great Salt Lake, USA, e.g. Carozzi, 1962; Bouton et al, 2016; Vanden Berg, 2019; Vennin et al, 2019) and ancient systems (e.g. Green River Formation, USA; Seard et al, 2013; Chidsey et al, 2015) have been the focus of numerous studies with the aim to better constrain their petrology and isotopic composition with regards to the sedimentary and geodynamic context and to understand the processes leading to their formation. Based on the detailed investigations carried out on microbial and metazoan buildups preserved in the southern and central Limagne basins, Roche et al (2018) showed that the sedimentary cycles were mostly driven by a superimposition of regional (climatic, tectonovolcanism) and local factors, controlling the chemical and physical changes of the waterbody, the vegetation covers and the physiography of the lake margin. Climate and tectonics directly affect the morphology of lakes and their watersheds and are considered as first-order factors controlling sedimentation. When coupled, these two major factors may strongly and quickly impact base level variations and accommodation in lacustrine environments (Bohacs et al, 2013). It remains difficult to disentangle the role of both biotic and abiotic processes as they are closely coupled and carbonate precipitation results from a balance between both processes
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