Clay Mineralogy through the Low boundary of the Miocene Climatic Optimum in lacustrine sequences in the Alcubierre Sierra (Ebro Basin, NE Spain)

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Joaquín Bastida*, M. Cinta Osacar, Javier Pérez-Rivarés, Andrés Gil, Luis Auqué, M. José Gimeno, Concha Arenas

The evolution of the mineralogical composition of lutites and marls of lacustrine sequences through the Low boundary of the Miocene is studied in the Alcubierre Sierra (Central Ebro Basin, Spain). The mineral assemblage of sheet silicates of the studied materials is mainly of detrital origin and consisting of: +illite, ± chlorite, ± smectite, ± kaolinite, and with minor contents ± pyrophillite, ± palygorskite. Two distinct intervals are observed, below and above 17 Ma. Above this age there is a significant increase of 001 illite “crystallinity” index (increase of chemical weathering), and a significant increase of smectite/kaolinite ratio (increase of seasonality). A poorly significant increase of (illite + chlorite)/(kaolinite + smectite) ratio (no significant increase of physical weathering) is also observed. The evolution of the δ18O values from calcite of limestones and marls of the studied section — towards less negative values above 17 Ma — can be related to a rising evaporation/precipitation ratio, which is compatible with the aforementioned evolution of the clay-mineral based proxies.

Keywords: Clay minerals, weathering, illite “crystallinity”, Miocene Climatic Optimum, Ebro Basin

Proscience vol. 9

Pp 8-12

DOI: 10.14644/EC2023.002