Lacustrine bioherms, spring mounds, and marginal carbonates of the Ries-impact-crater (Miocene, Southern Germany)

Abstract

The petrographic investigation of the Miocene Rieslake-carbonates gave rise to a modifiedDunham-scheme for classifying non-marine carbonates. If the fabric is not exclusively the result of hydromechanical or biogenic effects,Dunham-terms were extended by interpretative attributes describing processes responsible for the secondary fabric development (pedogenesis, early meteoric diagenesis). The lowermost investigated section reveals a distinct zonation of the lake shore during humid stages (carbonates of the infra-, eu-, and supralittoral), interrupted by playa-like interstages of arid conditions (desiccated mudflat). Dolomitic successions of sinter-veneered bioherms, built by green algae and cyanobacteria, form an incomplete ‘reef belt’ at the northern crater rim. Bioherm sequences were controlled by a seasonally oscillating lake level and fluctuations of higher order, which correspond to small-scale climatic fluctuations in the range of several hundreds to thousands of years. A superposed facies trend is recognized, which is the result of the decreasing eutrophy, alkalinity, and carbonate supersaturation. This reflects the climatic change to generally humid conditions at the end of Miocene. Originally thrombolitic sublacustrine spring mounds were caused by upwelling groundwater from permeable bedrock. Cement framestones of the mound core are considered to result from fabric alteration within the upwelling ground water and by emersions. Marginal carbonates of the Ries basin, inclusively bioherms, probably were originally Mg-calcitic. Aragonite was restricted to gastropod shells and sublacustrine spring mounds. Dolomitization is interpreted as essentially due to a fluctuating phreatic mixing-zone caused by meteoric groundwater, which underflowed saline water at the soda lake margin. The algal flora comprises cyanobacteria, possible cyanobacteria, green algae, few charophytes, endoliths, and problematic forms. A Recent contamination by endolithic fungi and lichens is evident. Occasionally preserved insect larval tubes, mass accumulations of pupal cases of flies, and arthropod eggs demonstrate that these groups were once a prominent part of the fossil soda lake ecosystem of the Nordlinger Ries.