Palynofacies analysis of middle Jurassic sediments from the Inner Hebrides

Abstract

A quantitative palynofacies study of thermally immature Middle Jurassic sediments of Skye, Raasay, and Eigg has been undertaken in order to examine kerogen and palynomorph distributions in relation to various potential controlling factors (lithology, lithofacies, depositional environment, sequence stratigraphy). This has involved performing rigorous kerogen and palynomorph counts on 440 samples from 12 localities (average sampling density 5 per metre in sampled sections), supported by 250 TOC and 57'Rock-Eval' pyrolysis analyses. The facies of the Middle Jurassic sediments of the Inner Hebrides range from subtidal marine to freshwater alluvial mudflats. The succession is divided into three major units: the Bearreraig Sandstone Formation (Aalenian-Bajocian) which is 400m thick, and represents the deposition of tidalsubtidal sand sheets, the ? Bathonian Great Estuarine Group (250m thick) which represents a range of mostly lagoonal-deltaic freshwater to hypersaline facies, and finally the Staffin Bay Formation (lowermiddle Callovian; thickness 18m) which represents a transgressive lagoon-barrier bar complex. ... The detailed subdivision of the phytoclast group has permitted the calculation of a phytoclast preservation index (PPI) which summarises the overall degradation state of the phytoclast assemblage for each sample. The highest PPI values (= most refractory phytoclast assemblagesa) re found in the mudflat-alluvial facies of the Skudiburgh Formation, lowest PPI values, reflecting 'fresh' phytoclast input, are found in the transgressive Staffin Bay Formation. The TOC values are generally <2.5%, but reach up to 3-4% in parts of the Dun Caan Shales Member (Bearreraig Sandstone Formation) and the Lonfearn Member (Lealt Shales Formation), and 6-8% in the Upper Ostrea Member (Staffin Bay Formation). Hydrogen Index (HI) values mostly fall below 300 (organic facies C or CD), but are increased in parts of the Lonfearn and Upper Ostrea members (400- 500, organic facies B), and particularly in the Kilmaluag Formation where values of over 800 are found (= organic facies AB). The correlation between the optical and geochemical data varies from very poor to very good. In some cases TOC is correlated with %AOM, and also %Botryococcus. Multiple regression on the HI suggests that in most cases %AOM is the variable best correlated with the HI; this procedure has allowed the calculation of a predicted HI value for all 440 samples.