Micromorphological and geochemical characterization of Tertiary ‘freshwater carbonates’ locally preserved north of the edge of the Miocene Molasse Basin (SW Germany)

Abstract

Miocene carbonates intercalated in a deposit of Upper Freshwater Molasse (Middle Miocene), exposed in a limestone quarry at Heidenheim-Mergelstetten (Germany), were analysed geochemically and micromorphologically. The heavy minerals (fine sand) and clay minerals confirm the sedimentary interpretation of the formation as a sequence of Paleogene soil residues overlain by younger Neogene soil horizons developed in sediments of the Upper Freshwater Molasse (OSM). The supposed continental origin of the massive, bedded, pisolitic limestone and white, chalky, columnar limestone indicated by terrestrial gastropods, is confirmed by analysis of stable carbon and oxygen isotopes. The microfacies show the characteristics of terrestrial carbonates: pisoids, nodules and pedodes. Nevertheless, the occurrence of intraclasts, reworking features, grey/black concretions, indications of swampy conditions, and carbonate enrichment restricted to the depression area cannot be explained by terrestrial pedogenetic processes. Cathodoluminescence suggests the meteoric character of the sparitic cements, which could be differentiated into three phases of cementation corresponding to conditions during lake regression and subsequent transgression. X-ray diffraction, differential thermal and transmission electron microscope analysis of the ammonium acetate insoluble residue of the carbonates indicate neoformation of palygorskite within the pisolitic limestone. Si and Al for formation of palygorskite were probably supplied by degradation of clay minerals (illite) under alkaline conditions. Mg was externally supplied to the system. The results suggest formation of the Miocene carbonates in a marsh or shallow ephemeral lake environment with repeated superimpositions of pedogenetic processes.