Compositional variation and palaeoenvironment of the volcanolithic Fort Cooper Coal Measures, Bowen Basin, Australia

Abstract

An integrated analysis of megascopic lithotype, microscopic maceral and mineral composition, stable carbon isotope and interburden sedimentology from a single well was used to interpret the response of the Late Permian Fort Cooper Coal Measures (FCCM) to regional and global environmental influences. The FCCM are differentiated from underlying, relatively high vitrinite Moranbah Coal Measures, and overlying higher inertinite Rangal Coal Measures in the Bowen Basin by their intercalation with abundant tuff and siliciclastic partings and interbeds. Besides this, there is little described about the variation in the organic composition of the FCCM and its causes. The FCCM can be subdivided into a lower aggradational Fair Hill Formation, transgressed by the shallow marine-derived Black Alley Shale that interfinger with/is overlain by the progradational Middle Main Seams and Burngrove Formation coal measures. The coals are dominantly dull with minor bright bands that are more abundant in the Burngrove Formation representing a change in plant composition. The maceral analysis shows that the coals in the Fair Hill Formation and Middle Main Seams are vitrinite-rich (80–90% mmf) albeit with high mineral matter suggesting the formation of precursory peat under rising water levels and with high sediment (tuff) influx and preservation. The coals in the Burngrove Formation have an increased inertinite content (30% mmf) but are also high in mineral matter suggesting a shift to increased decomposition arising from a fluctuating water table, possibly increased aridity and/or microbial activity. Tuffs occur throughout, and although their frequency is higher in the lower Fair Hill Formation, the preservation of thicker tuffs in the upper Burngrove Formation indicates increased intensity of volcanism that could have modified the environment. Variation in carbon isotope compositions show a parabolic trend, from around −24.1‰ in the Fair Hill Formation to more variable values in the Middle Main Seams with an overall 13C-enrichment upwards in the Burngrove Formation, prior to the δ13C values becoming negative (depleted between −1 to −4% from the average −24.1‰) in the top seams and into the overlying Rangal Coal Measures. The 13C-depletion trend in the upper part of the section is unexpected in view of corresponding increase in inertinite content of the coals indicating that the δ13C values/plant composition and inertinite content are decoupled, unless the inertinite origin is from intense microbial decay. Similar stable carbon isotope depletion trends have been observed elsewhere in an equivalent stratigraphic interval of the Bowen Basin suggesting that the carbon isotope values are responding to global conditions that cause negative excursions in carbon isotopes before the P-T boundary. Overall, the basin was continuously subsiding and peats accumulated with constant interruptions from excessive sedimentation and volcanic eruptions. The low proportion of visible thick bright bands coupled with high telovitrinite (mmf) content suggests a marsh to fen environment, with an open canopy and ponding able to preserve volcanic ash falls.