Oligo-Miocene lacustrine microbial and metazoan buildups from the Limagne Basin (French Massif Central)

Abstract

The Limagne Basin (French Massif Central) is an extensive continental lacustrine system accommodating microbial and metazoan buildups from Chattian to Aquitanian age. A description of these buildups and their associated biotic components in Grand Gandaillat and Crechy quarries provides insights into their spatio-temporal distribution patterns. Flats, cauliflowers, domes, cones and coalescent columnar morphologies have been identified with a main laminated mesofabric and laminated, columnar, filamentous and caddisfly-coated microfabrics. Two low-gradient margin models emerged based on the changes in the distribution, morphology and size of the microbial and metazoan-rich deposits through time: the first model applies to the Chattian deposits in the Grand Gandaillat quarry and the second to the Aquitanian deposits in the Crechy quarry.This study emphasizes the heterogeneity of marginal lacustrine carbonate-siliciclastic cycles. The lacustrine/palustrine cycles of the Limagne Basin record (i) lacustrine deposits composed of microbial and metazoan buildups and organic matter-rich marls indicating periods of high accommodation, and (ii) palustrine deposits composed of mudstone and clayey paleosoils, indicative of periods of low accommodation. The cycles differ with thin and symmetrical deposits that dominate the Chattian cycles whereas thicker and asymmetrical deposits mark the Aquitanian cycles. In addition, the Chattian buildups are exclusively microbial and only a few centimeters thick whereas the Aquitanian ones are multiple meters in height and are composed of both microbes and metazoans. Climatic, tectonism, volcanic and local parameters are involved in the deposition of buildups and their organization in each different cycle. Climate may control the long-term arid/humid sedimentary succession, tectonic may explain the difference in the deposit thickness between two cycles of different ages and localizations as well as the specific distribution of the buildups throughout the basin, volcanism may control the chemical lacustrine conditions and local physiography may impact the soil removal ability of the lake margin.