Petrological and maturation characteristics of organic matter from the Middle Proterozoic McArthur Basin, Australia

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Abstract
Middle Proterozoic organic matter in the McArthur Basin occurs mostly within fine‐grained clastic sedimentary rocks and is made up mainly of lamalginite, migrabitumen and thucholite. With increasing maturation, fluorescence in lamalginite diminishes from bright yellow to dull brownish‐orange and eventually ceases. Reflectances of fluorescent lamalginite are about half that of non‐fluorescent lamalginite and migrabitumen occurring in the same specimen. Reflectances of non‐fluorescent lamalginite and migrabitumen are generally much the same where both types occur in the same specimen. Both non‐fluorescent lamalginite and migrabitumen have optical characteristics similar to those of vitrinite. Maturation indices (Tmax and reflectance) generally show a systematic increase with increasing depth. Contact metamorphism in the Roper Group and hydrothermal activity associated with base metal mineralization in the McArthur Group have locally increased maturation levels. Maturation levels in the Roper Group that are unaffected by contact metamorphism display a pattern consistent with the concept that the Roper Group formed during the sag phase of the basin's development and that there has been subsequently a relatively uniform geological history. Maturation levels in the McArthur Group that are unaffected by hydrothermal activity, show a different pattern due to variable subsidence rates and geothermal gradients during the final phase of extension in the basin which preceded the sag phase.