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 basi n-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 i nvolving a pyroclastic volume > 10 km(3). The relatively even thickness (si milar to1-1.5 m) of the tuff across the entire northern Bowen Basin (simila r to 10 000 km(2)) implies a distant source. The tuff is ash-rich and its o riginal geochemistry has been compromised by diagenetic alteration. Crystal content (10-15%) is dominated by quartz, suggesting a rhyolitic associatio n. SHRIMP U-Pb analysis of zircons indicates an age of 258.9 +/- 2.7 Ma for the Platypus Tuff Bed, confirming the Late Permian age that has generally been assigned to the Blackwater Group. The age framework now apparent for t he coal-bearing Blackwater Group suggests an average depositional rate rang ing from similar to 133 m/10(6) years for its eastern depocentre in the nor thern Bowen Basin to similar to 70 m/10(6) years in more marginal settings to the west.