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Environmental Conditions of Deposition, Origin, and Relative Maturity of Organic Matter in Uppermost Barremian - Lowest Aptian Limestones of the Eastern Prada Quarry Section, Organyà Basin, South-Central Pyrenees, Spain

Abstract

Well-exposed, dark nodular limestones and marlstones in a 29 m-section of the eastern Prada Mountain quarry were analyzed for petrographic and geochemical characteristics. The sequence shows low bioturbation index (2 – 3), microfacies of dark micrite changes into an isotropic coarser grain (8-13 microns) starting at 4.15 m (sample CC-15-21), with allochems of minute echinoderm, rare ostracod, ammonite, gastropod and roveacrinid fragments. Few benthic and planktic foraminifera, and serpulidae tubes are also present. Globigerinelloides blowi indicates a chronological position comprising the late Barremian–early Aptian. Smear slides revealed abundance of mainly disaggregated nannoconid fragments and calcareous nannofossils as the main micro-carb. Qualitative EDS analyses indicate CaCO3 as predominant in the rock matrix. Inter- and intraparticle pores and vugs are filled with apparent bitumen throughout the section. δ13Corg fluctuates from a low of -26.24‰ at ~0.7 m to a high of -23.78‰, (~ 2.46 ‰ change) at ~18.04 m with higher amplitudes towards the top of the section at 27 m. δ15N values range from 1.82 ‰ to 5.95 ‰ positively correlated with the δ13Corg profile specifically between 18 m and 27 m. Biomarker results show a predominance of short chain n-alkanes (