Triassic coal measures, Tasmania: new U–Pb CA-TIMS ash bed dates and numerical calibration of palynostratigraphy

Abstract

Four new high-precision U–Pb ash bed dates constrain the timing and rate of coal measures deposition in Tasmania and, combined with new palynological data, allow improved numerical age calibration of eastern Australian Late Triassic spore-pollen biozones. Felsic ashfall tuff beds are numerous in the middle and upper parts of the 400 m-thick Mount Nicholas Coal Measures of northeast Tasmania. Our four U–Pb CA-TIMS dates on zircon span 99 m of section and range from 222.52 to 217.84 Ma. All four dates are closely associated with palynomorph assemblages belonging to the Craterisporites rotundus Assemblage Zone. Our data indicate an age of at least 223 Ma, and likely several million years older, for our earliest observed C. rotundus Zone assemblages, whereas the base of the Polycingulatisporites crenulatus Zone may be ca 217 Ma (and no older than 217.84 Ma).

KEY POINTS

1. Felsic ash beds are common in the middle and upper parts of the Upper Triassic Mount Nicholas Coal Measures.

2. The ash beds have yielded four new U–Pb CA-TIMS dates on zircon that span 99 m of section and range from 222.52 to 217.84 Ma.

3. The new dates all lie within the upper part of the Craterisporites rotundus Assemblage Zone.

Keywords:

• Triassic
• palynostratigraphy
• CA-TIMS
• Tasmania Basin
• Parmeener Supergroup
• U–Pb zircon dating

Acknowledgements

Philip Sansom guided CRC to the Denison Rivulet outcrops and commented on an early draft. Carol Bacon generously provided information on tuff occurrences. We thank Mark Schmitz for assistance with geochronology data reduction. Simon Bodorkos kindly recalculated the old Denison Rivulet K–Ar date. Staff at the Mineral Resources Tasmania Core Store were helpful in facilitating core sampling. The paper was greatly improved by the reviews of Clinton Foster, John Laurie and Simon Bodorkos. CRC publishes with the permission of the Director, Mineral Resources Tasmania.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and online materials, which can be found at https://www.mrt.tas.gov.au/mrtdoc/dominfo/download/CC_001/.