Organic Matter Types in Early Cretaceous Strata Offshore Northwest Australia

Abstract

Coring done on the Exmouth Plateau of the northwest Australian margin during Ocean Drilling Program Leg 122 encountered organic-carbon-rich siltstones and shales deposited during the Early Cretaceous. The types of organic matter contained in these stratigraphic units have been assessed using information provided from pyrolysis GC/MS, by extractable biomarker analysis, by Rock-Eval pyrolysis, and be carbon isotope analysis. The indicators of total organic matter type - pyrolysis GC/MS, Rock-Eval, and carbon isotopes - reflect predominantly continental plant sources of this material throughout the Early Cretaceous. The extractable indicators, however, show a decreasing dominance of land-plant biomarkers in younger strata. Comparison of stratigraphic sequences, sediment provenance, and organic matter types indicates that these offshore deposits are equivalent to the onshore occurrences of the Neocomian Barrow Formation and the Aptian Muderong Shale. The strata on the Exmouth Plateau are thermally immature, unlike their equivalents on the Australian continent. Tectonic isolation of the Exmouth Plateau from the Mainland evidently terminated the deltaic mode of rapid sedimentation at the end of the Aptian. Subsequent marine environments produced little organic matter offshore of northwest Australia. Burial and concomitant thermal maturation of the land-derived organic matter were therefore limited.