Abstract
Despite their dark colour, laminated fabric, and dysoxic character, the thermally immature shales of the Rhaetic Westbury Formation of the Bristol Channel area typically have total organic carbon (TOC) values of only 1.0–1.5%, and hydrogen indices (HI) >200. Variations in TOC and HI are correlated with the amorphous organic matter (AOM) fluorescence, but not the % AOM, suggesting that redox-related variations in AOM preservation control the organic facies. The diameter of equidimensional black “wood” phytoclasts decreases in intervals with higher TOC and HI, suggesting an association with lower energy depositional conditions. Such intervals also show a relative increase in light brown “wood”, probably a reflection of more humid onshore conditions and lower phytoclast oxidation at source. PhytOC values suggest the input of terrestrial OM was relatively low and stable during Westbury deposition, indicating a distal setting removed from fluviodeltaic sediment sources.
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