Abstract

An integrated optical (palynofacies) and bulk organic geochemical study has been undertaken in order to determine the controls on the organic facies of paralic Middle Jurassic Hebridean sediments. The total organic carbon (TOC) values are generally between 0.5 and 2.5%, but reach up to 3-4% in dark-coloured non-bioturbated shales within the Bearreraig Sandstone Formation and Lealt Shales Formation, and 6-8% in the Staffin Bay Formation. Hydrogen indices (HI) are mostly below 300, increasing to 400-500 in the aforementioned organic-rich intervals, and reaching 650-800 in the Kilmaluag Formation. The best overall source potential probably occurs in the brackish Lealt Shales Formation. Correlations between HI, TOC and the content of the freshwater alga Botryococcus (< or =9% of the kerogen), suggest that freshwater run-off may have led to stratification of the Middle Jurassic lagoons, and thus dysoxic-anoxic bottomwaters and better quantitative and qualitative preservation of the plankton-derived amorphous organic matter.

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