Upper Cretaceous feldspars in the Cenozoic Limagne Basin : a key argument in reconstructing the paleocover of the Massif Central (France)

The northern part of the N–S-trending basin of the Limagne graben, located in the northern part of the Massif Central (France), contains Oligocene and Miocene calcareous lacustrine deposits. The area is known for its stromatolite reefs surrounded by marl, clay and uncemented carbonate sand with oncolites. Because the origin of the carbonate accumulated by cyanobacteria has long been debated, the clayey deposits associated with the stromatolites were analysed using XRD, SEM and K–Ar dating methods. The results show a monotonous assemblage of carbonates, dioctahedral smectite, illite–smectite (Reichweite) R=0, glauconitic illite and abundant orthoclase. Detrital minerals such as quartz and detrital illite are found near the basin's borders but are very scarce in its central part, indicating a lack of detrital input from the regional basement. Hence, the abundant orthoclase found in all the clayey deposits of the Limagne Basin cannot be linked to erosion of the surrounding Hercynian basement. The SEM analysis showed the orthoclase crystals to have a tabular morphology with sharp boundaries and to be embedded within a clay matrix, whilst the K–Ar dating of the orthoclase and glauconitic illite yielded ages ranging from 90 to 66±2 Ma, i.e. Turonian to Maastrichtian. These two results clearly indicate that the fill of the Limagne Basin, composed mainly of carbonate, clay and orthoclase, must have been partly due to the erosion of an Upper Cretaceous sedimentary blanket once covering the Massif Central basement. This cover was probably constituted of chalk, dioctahedral smectite, I/S and glauconitic illite, associated with authigenic orthoclase and flint. Flints are found reworked in the alluvial formations of the Late Pliocene Lower Bourbonnais sands and clays (‘Sables et argiles du Bourbonnais') and in the Pleistocene terraces. Moreover, relicts of the Upper Cretaceous cover were preserved during the sedimentary filling of the Cenozoic Limagne Basin. Authigenic orthoclase of similar shape has been described in the Chalk Group of the Paris Basin and from localities in northern France and Belgium. The mineral is rare in the Turonian chalk but increases rapidly at the base of the Coniacian to Campanian chalk; it has also been noted that the number of orthoclase crystals increases as the quantity of clastic components decreases. Similar orthoclase crystals were found associated with dioctahedral smectite in the decarbonated residues of the Coniacian to Santonian ch