A high resolution biomarker study of different lithofacies of organic sulfur-rich carbonate rocks of a Kimmeridgian lagoon (French southern Jura)

Abstract

The Upper Jurassic Calcaires en plaquettes Formation (southern Jura, France), which was deposited in a lagoonal setting, reveals three lithofacies: limestones with fine-scale parallel laminations, limestones with undulated (stromatolite-type) laminations and massive limestones. It was examined, at the millimetre scale, if the lithological differences are accompanied by differences in the composition of organic matter (OM), and if microbial mats played a role in the preservation of OM, as suggested previously. Four types of laminae (light/parallel, dark/parallel, light/undulated, dark/undulated), and a massive limestone sample were analysed. The OM in all facies is characterised by a high hydrogen index (870-970) and a variable 13C content (−23.5 or to −28.1 ). The molecular composition of OM appeared similar in all samples, e.g.: (i) Extractable biomarkers are dominated by n-alkane and hopanoid carbon skeletons, predominantly in a sulfur-bound form. (ii) Isorenieratene derivatives, indicating anoxic conditions in the photic zone, are present in relatively large amounts. (iii) Kerogen pyrolysates are dominated by organic sulfur compounds. Notwithstanding the overall similarity in OM composition, some significant differences were observed, e.g.: (i) Low-molecular-weight sulfur-bound hopanoids and steroids are relatively more abundant in TOC-lean, light-coloured samples than in TOC-rich, dark-coloured samples. This is probably related to differences in the degree of intermolecular sulfur cross-linking of the OM. (ii) The concentrations and distributions of several biomarkers in the light-coloured, undulated sample differ from those in the other samples. These differences are attributed to the periodic occurrence of microbial mats. Our data indicate that OM in the Calcaires en plaquettes Formation is predominantly composed of sulfur-bound lipids formed at times that bottom waters were euxinic. A periodic deepening of the oxic/anoxic interface, and of the light penetration depth, to below the sediment surface, allowed microbial mats to develop. Organic remains of the mats, however, are swamped by sulfur-bound planktonic lipids.