Age and significance of the platypus tuff bed, a regional reference horizon in the upper permian moranbah coal measures, north Bowen Basin

The Platypus Tuff Bed in the Permian Moranbah Coal Measures provides a basin-wide marker horizon traceable for over 300 km along strike. The bed is a tephra event unit, the product of a large-scale volcanic eruptive episode involving a pyroclastic volume >10 km³. The relatively even thickness (∼ 1-1.5 m) of the tuff across the entire northern Bowen Basin (∼ 10 000 km²) implies a distant source. The tuff is ash-rich and its original geochemistry has been compromised by diagenetic alteration. Crystal content (10-15%) is dominated by quartz, suggesting a rhyolitic association, SHRIMP U-Pb analysis of zircons indicates an age of 258.9 ± 2.7 Ma for the Platypus Tuff Bed, confirming theLate Permian age that has generally been assigned to the Blackwater Group. The age framework now apparent for the coal-bearing Blackwater Group suggests an average depositional rate ranging from ∼ 133 m/10⁶ years for its eastern depocentre in the northern Bowen Basin to ∼ 70 m/10⁶ years in more marginal settings to the west.