Organic geochemical evidences of early-diagenetic oxidation of the terrestrial organic matter during the Triassic arid and semi arid climatic conditions

The molecular character of organic matter (OM) present in Triassic clays of the Upper Silesia Basin and NW border of the Holy Cross Mountains was determined using GC–MS analysis. Oxidation processes were the major cause of the main changes of extractable OM molecular composition during sedimentation and early diagenesis of the Triassic clays. They resulted in a very significant decrease in the OM content of the clays and transformation of n-alkanes, triterpanes and steranes. Despite the changes in biomarker composition resulting from the disappearance of unsaturated and ββ hopanes and dominance of αβ and βα hopanes as well as preponderance of αββ over ααα steranes, the aromatic ketones were identified. Such PACs as benzophenone, fluorenone, cyclopenta(def)phenanthrenone, antracenone and benzanthrone were among the compounds identified in the red clays. These commonly originate during oxidation of sedimentary organic matter. Moreover, the aromatic fraction is characterized by the presence of phenyl derivatives (phenylnaphthalenes, terphenyls, phenyldibenzofurans and phenylphenanthrenes) that are also products of the abiotic oxidation of organic matter. Periods of terrestrial sedimentation have been interrupted by long phases of OM weathering and oxidation during arid conditions, and OM might only have survived without significant oxidation changes when the sedimentation was fast and/or long-lasting.