--> Geochemical Characteristics of Environmental Conditions Favorable to OM Preservation During the Barremian-Aptian in the Cabo Formation, Organya Basin, NE Spain

AAPG ACE 2018

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Geochemical Characteristics of Environmental Conditions Favorable to OM Preservation During the Barremian-Aptian in the Cabo Formation, Organya Basin, NE Spain

Abstract

The Organya Basin in the south-central Pyrenees, Spain, exposes several large outcrops of Mesozoic black to dark gray marlstones and limestones containing variable amount of organic matter. A multi-proxy study of the lower 36.5 m of the Cabo Formation (Barremian – Aptian) [1, 2, 3], north of the town of Organya, allows us to assess the lithostratigraphic and geochemical characteristics of the basin with respect to environmental conditions favorable to OM preservation at that time. The measured total organic carbon (TOC wt%) varies between 1 and 5.8 %, with the highest values in the black limestones (0.43 m, 4.38 m, 14.85 m, 29.95 m, and 35.6 m, respectively). These TOC values average about 2.0 %, except at a height of 0.43 m, where the TOC has a strong peak (5.78 %) and remains around 2.07 % above 4.38 m. Total inorganic carbon (TIC wt%) fluctuates between 86.7 and 96.7 % and shows a negative correlation with TOC (r=-0.78). Corg isotope (δ13Corg) ranges from -24.41‰ to -22.15‰ with a positive correlation (r = 0.58) with the TOC. Lipid biomarkers (n-alkanes) at 0 m, 0.43 m, 3.93 m, 10.75 m, 14.85 m, and 29.95 m, show chain lengths that vary between C14 to a highest of C32, with predominance below C21 indicative of a source in the open marine areas of the basin. Nonetheless, the highest chain length (C32) recorded at 0 m implies some terrestrial input at that level. Since the Pristane/Phytane (Pr/Ph) ratios vary between 0.65 and 1.27 and is indicative of an undermature organic matter (2.5 indicates overmature) the preserved n-alkanes most likely characterize the original composition of the OM. Redox sensitive trace element (V, Ni, Cu, and Mo) values peak simultaneously at levels where TOC is high (r > 0.6), thus compatible with fluctuating more pronounced dysoxic conditions associated with higher preservation of OM. Biolimiting trace elements (Fe, P) also show their highest concentrations at 14.85 m, consistent with high TOC (2.93 %) and high RSTE values, suggesting high primary productivity at that level. Thus, the base of the Cabo Formation shows a record of the interacting effects of nutrient supplies on high productivity that influenced redox conditions in the preservation of OM in the sediments.

References:

[1] García-Senz, J., 2002, PhD Thesis, University of Barcelona, 310 p; [2] Bernaus, J.M., et al., 2003. Sedimentary Geology 159 (3-4), 177-201; [3] Sanchez-Hernandez, Y., Maurrasse, F.J-M.R. 2014. Chem.Geology 372, 12-31.