The Mid-Cenomanian Event in southeastern France: Evidence from palaeontological and clay mineralogical data |
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| Institution: | 1. Department of Earth Sciences, Geological Institute, ETH Zurich, Sonneggstr. 5, 8092 Zürich, Switzerland;2. Palaeontological Institute, University of Zurich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland;3. Dipartimento di Scienze della Terra, della Vita e dell''Ambiente, Università di Urbino, Campus Scientifico località Crocicchia, 61209 Urbino, Italy;4. Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Mangiagalli 34, 20133 Milano, Italy;5. Geological-Paleontological Department, Museum of Natural History, Burgring 7, 1010 Vienna, Austria;2. Department of Earth & Environmental Sciences, University of Mary Washington, 432 Jepson Science Center, 1301 College Avenue, Fredericksburg, VA 2240, USA |
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| Abstract: | Reconstruction of main palaeoenvironmental conditions across the Mid-Cenomanian Event (MCE I) in the hemipelagic Tethyan section of Blieux (Southeast France, Vocontian Basin) is proposed. Quantitative analyses of calcareous nannofossil, ammonoid and clay mineral assemblages have been made and compared with respect to sea level changes and the carbon cycle perturbations. The nannofossil primary productivity, as recorded by nannofossil fluxes and relative abundances of meso-eutrophic taxa, is low just below and during the MCE Ia, then slightly increases in the interval including the MCE Ib. The clay assemblages mainly consist of illite/smectite mixed-layers with a smaller proportion of kaolinite. The percentage of kaolinite strongly decreases in the interval including the MCE Ia and slightly increases in the interval including the MCE Ib. The clay assemblages are mainly detrital in origin and reflect environmental changes including differential settling processes, climate, intensity of runoff and detrital sources. The ammonoid assemblages are characterised by a significant change during the MCE I: planispirals (mainly Schloenbachia) are dominant until the MCE Ia, whereas heteromorphs (mainly Sciponoceras) become dominant from the MCE Ib onwards. Strongly oligotrophic levels in sea surfaces are recorded during the MCE Ia and are related both to arid climatic conditions and major sea level fall (both 3rd order and medium scale lowstand deposits). A decrease in bathymetry could partly explain the decrease in the relative abundance of Schloenbachia. The first occurrence of Sciponoceras took place during the MCE Ib; this second positive increase in δ13C is not associated with enhanced nannofossil primary productivity. No clear relations can be established between the occurrence of Sciponoceras and trophic resources. |
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| Keywords: | Calcareous nannofossils Ammonoids Primary productivity Climatic conditions Middle Cenomanian Vocontian Basin |
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