FLUVIAL RESPONSE TO FORELAND BASIN OVERFILLING - THE LATE PERMIAN RANGAL COAL MEASURES IN THE BOWEN BASIN, QUEENSLAND, AUSTRALIA

The latest Permian Rangal Coal Measures and equivalents accumulated during a phase of declining volcanism and active thrust loading in the complex retroarc foreland Bowen Basin in eastern Queensland, Australia. The unit was formed in predominantly alluvial environments which covered the entire Bowen Basin and it represents the final phase of coal formation in the basin. Large opencut mine exposures, mainly in the Blackwater area, have allowed the establishment of a facies scheme for the Rangal Coal Measures. Seven facies have been recognised on the basis of lithology, geometrical criteria and palaeocurrent relationships. (1) Sheet-like Sandstone Channel Bodies; (2) Laterally Accreted, Heterolithic Channel Bodies; (3) Levee (Proximal Overbank) Deposits; (4) Minor Crevasse Channel Fill; (5) Floodbasin; (6) Stagnant Lake Floor; and (7) Mire. Together, these constitute an array typical of alluvial, coal-bearing systems. The depositional style of the Rangal Coal Measure channels is unusual, and considered to be related to the periodic oversupply of coarse sediment to the system. Heterolithic channel fills (Facies 2), which comprise alternations of thinly interbedded sandstone/siltstone and sharp-bounded sandstone, formed under conditions of dramatically variable sediment supply. Such channel deposits, while formed dominantly by lateral accretion, were the product of only slightly sinuous streams (less than 1.5). Facies 1 sheet sandstones, however, are interpreted to have formed during times of sand oversupply to the basin, and were the product of low-sinuosity, probably braided streams. The overall character of the Rangals is considered a response to overfilling of the basin by immature, volcanic sediment released by the uplift of thrust sheets in the adjacent orogen.