Integrated stratigraphy and 40Ar/39Ar chronology of the Early to Middle Miocene Upper Freshwater Molasse in eastern Bavaria (Germany)

A detailed integrated stratigraphic study was carried out on middle Miocene fluvial successions of the Upper Freshwater Molasse (OSM) from the North Alpine Foreland Basin, in eastern Bavaria, Germany. The biostratigraphic investigations yielded six new localities thereby refining the OSM biostratigraphy for units C to E (sensu; Heissig, Actes du Congres BiochroM'1997) and further improving biostratigraphic correlations between the different sections throughout eastern Bavaria. Radioisotopic ages of 14.55 ± 0.19 and 14.88 ± 0.11 Ma have been obtained for glass shards from the main bentonite horizon and the Ries impactite: two important stratigraphic marker beds used for confirming our magnetostratigraphic calibration to the Astronomical Tuned Neogene Time Scale (ATNTS04; Lourens et al. in Geologic Time Scale 2004, Cambridge University Press, 2004). Paleomagnetic analysis was performed using alternating field (AF) and thermal (TH) demagnetization methods. The AF method revealed both normal and reverse polarities but proofs to yield unreliable ChRM directions for the Puttenhausen section. Using the biostratigraphic information and radioisotopic ages, the magnetostratigraphic records of the different sections are tentatively correlated to the Astronomical Tuned Neogene Time Scale (ATNTS04; Lourens et al. in Geologic Time Scale 2004, Cambridge University Press, 2004). This correlation implies that the main bentonite horizon coincides to chron C5ADn, which is corroborated by its radioisotopic age of 14.55 Ma, whereas the new fossil locality Furth 460, belonging to OSM unit E, probably correlates to chron C5Bn.1r. The latter correlation agrees well with the Swiss Molasse locality Frohberg. Correlations of the older sections are not straightforward. The Brock horizon, which comprises limestone ejecta from the Ries impact, possibly correlates to C5ADr (14.581 ± 14.784 Ma), implying that, although within error, the radioisotopic age of 14.88 ± 0.11 Ma is somewhat too old. The fossil localities in Puttenhausen, belonging to the older part of OSM unit C, probably coincide with chron C5Cn.2n or older, which is older than the correlations established for the Swiss Molasse. © Springer-Verlag 2007

Identification of remagnetization processes in Paleozoic sedimentary rocks of the northeast Rhenish Massif in Germany by K-Ar dating and REE tracing of authigenic illite and Fe oxides

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Remagnetization in bituminous limestones of the Neoproterozoic Araras Group (Amazon craton): Hydrocarbon maturation, burial diagenesis, or both?

International audienceNeoproterozoic carbonates of the Araras Group exhibit two distinct magnetic components across the same carbonate succession in a cross-section between the Amazon craton and the Paraguay fold belt. Pink dolostones of the Mirassol d'Oeste Formation carry a dual polarity, primary component, whereas black bituminous limestones of the Guia Formation yield a secondary postfolding component. Magnetic signatures of the Guia limestones, such as high anhysteretic remanence magnetization/saturation isothermal remanence magnetization ratios, high-frequency-dependent magnetic susceptibility and contradictory Lowrie-Fuller and Cisowski tests, are typical of remagnetized carbonates. Unblocking temperatures suggest that the stable high-temperature remanence is carried by both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning electron microscope observations. The different magnetic properties noted between dolostones with or without bitumen and between dolostones and limestones in the same metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well as the lithology influence nucleation of authigenic magnetic minerals in these rocks. Presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in fracture and voids are in favor of a chemical remanence (CRM). The presence of pyrrhotite as one of the main carriers of CRM in these rocks, and its association with bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage. However, hydrocarbon maturation solely could not explain the differences of the magnetic mineralogy observed in the craton and the fold belt. Enhanced magnetite formation in the thrust and fold belt is interpreted to be the result of higher temperatures leading to stronger diagenesis of clay minerals