Age and significance of the Platypus Tuff Bed, a regional reference horizon in the Upper Permian Moranbah Coal Measures, north Bowen Basin

Abstract

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 the Late 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.