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

Miocene carbonates intercalated in a deposit of Upper Freshwater Molasse (M iddle Miocene), exposed in a limestone quarry at Heidenheim-Mergelstetten ( Germany), were analysed geochemically and micromorphologically. The heavy m inerals (fine sand) and clay minerals confirm the sedimentary interpretatio n of the formation as a sequence of Paleogene soil residues overlain by you nger Neogene sail horizons developed in sediments of the Upper Freshwater M olasse (OSM). The supposed continental origin of the massive, bedded, pisol itic limestone and white, chalky, columnar limestone indicated by terrestri al gastropods, is confirmed by analysis of stable carbon and oxygen isotope s. The microfacies show the characteristics of terrestrial carbonates: piso ids, nodules and pedodes. Nevertheless, the occurrence of intraclasts, rewo rking features, grey/black concretions, indications of swampy conditions, a nd carbonate enrichment restricted to the depression area cannot he explain ed by terrestrial pedogenetic processes. Cathodoluminescence suggests the m eteoric character of the sparitic cements, which could be differentiated in to three phases of cementation corresponding to conditions during lake regr ession and subsequent transgression. X-ray diffraction. differential therma l and transmission electron microscope analysis of the ammonium acetate ins oluble residue of the carbonates indicate neoformation of palygorskite with in the pisolitic limestone. Si and Al for formation of palygorskite were pr obably supplied by degradation of clay minerals (illite) under alkaline con ditions. ME was externally supplied to the system. The results suggest form ation of the Miocene carbonates in a marsh or shallow ephemeral lake enviro nment with repeated superimpositions of pedogenetic processes. (C) 2000 Els evier Science B.V. All rights reserved.