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Coring done on the Exmouth Plateau of the northwest Australian margin during Ocean Drilling Program Leg 122 encountered a sequence of organic-carbon-rich Lower Cretaceous siltstones and shales. The types of organic matter contained in these stratigraphic units have been assessed by Rock-Eval pyrolysis, extractable saturated hydrocarbon and biomarker analyses, pyrolysis GC/MS, and carbon isotope analysis. Rock-Eval, pyrolysis GC/MS, and carbon isotope results indicate predominance of continental plant organic matter throughout the sequence. Distributions of the saturated hydrocarbons and the biomarkers, however, reveal a progressively decreasing dominance of land-plant contribution in younger strata. Comparison of stratigraphic sequences, sediment provenances, and organic matter types shows that these undersea strata are thermally immature equivalents of the onshore Neocomian Barrow Formation and Aptian Muderong Shale and represent the distal portions of a subsiding wedge of fluviodeltaic sediments which accumulated on this passive margin during the Early Cretaceous.

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