Les grottes d'Azé (Saône-et-Loire, France) : de la roche altérée aux sédiments

Abstract

The Azé cave (south Burgundy), from alteration to ghost-rock. The goal of this study was to gain a better understanding of the formation of the Azé Caves and the relationships which exist at this site between the surrounding rock and the sediments. A detailed study of the limestones from the Late Aalenian to the Early Bathonian which make up the surrounding rock shows that they contain considerable quantities of quartz and hundreds of ppm of manganese. Certain levels contain numerous siliceous cherts. From the Jurassic, the region underwent several phases of immersion. This period was also marked by a tectonic phase of weak amplitude. The faulting which occurred at this time allowed the circulation of meteoric water through the limestone leading to the dissolving of calcium carbonates and to phases of dolomitisation and dédolomitisation. As a result, "ghost rock" was created in the different stratigraphie units. However the hydrodynamism of these flows was weak and the alteration of the rock was isovolumic without evacuation of the alterite. During the Oligocene, the Méconnais region underwent a more severe tectonic phase. The structure of leaning blocks that we see today was put in place and the "ghost rock" was fractured by larger faults. As a result a hydrodynamic potential was created between the different sides of the massifs and the alterite was removed by water thus creating the caves. From that time on bacterial activity was able to develop on the walls of the cavities and in the residual alterite. The bacteria oxidised the manganese and iron contained in the limestone leading to the accumulation of manganese and iron oxide deposits in the limestone and on the walls, often in the form of structured encrustings. These bacteria were still active during certain periods of the Quaternary as shown by the discovery of deposits on bear bones. Under the effect of gravity, the least soluble limestone residues and the conglomerations of manganese formed by bacteria fell to the bottom of the caves or were carried away by flowing water. Depending on the hydrodynamic regime, these residues (clays, sands, gravels, pebbles) were to varying extents transported by water and were removed from or left behind in the caves. As a result, the sandy fractions of sediments deposited at the time of flows with the weakest hydrodynamism are mostly made up of these residues.