Solving a tuff problem: Defining a chronostratigraphic framework for Middle to Upper Jurassic nonmarine strata in eastern Australia using uranium–lead chemical abrasion–thermal ionization mass spectrometry zircon dates

Abstract

To better predict the architecture of reservoirs and the location of undiscovered resources in fluvial-dominated strata, a sound chronostratigraphic framework is needed. This study reassesses the stratigraphic framework of petroleum-bearing Jurassic fluviolacustrine successions in the Eromanga, Surat, and Clarence–Moreton Basins of eastern Australia. Correlation of the strata is challenging because of the heterolithic facies, the absence of conventional stratigraphic marker beds, and the longevity of palynostratigraphic zones. The abundance of laterally discontinuous volcanic air–fall tuffs and volcanogenic sandstones across the Jurassic of eastern Australia allows for the construction of a new, regional chronostratigraphic framework. High-precision U-Pb zircon chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates ranging from 168.07 ± 0.07 Ma to 149.08 ± 0.06 Ma were obtained from 31 samples from 13 wells across 3 basins. Five chronostratigraphic datums were defined and extrapolated to 677 wells within a time interval of 420 ka or less over hundreds of kilometers across eastern Australia. The new chronostratigraphic framework reveals inaccuracies in picking lithostratigraphic units based on lithology and wire-line log characteristics and shows coal-bearing facies of the Walloon Coal Measures to be two episodes of coal (peat) accumulation separated by an unconformity. The study also demonstrates the feasibility of extending chronostratigraphic datums to neighboring basins without tuff beds by dating the youngest zircon in volcanogenic sandstones by U-Pb CA-TIMS following laser ablation–inductively coupled plasma mass spectrometry analysis. The dates provide a substantial revision to the Middle to Upper Jurassic stratigraphy of eastern Australia. The use of precise U-Pb CA-TIMS dates should help elucidate the lithofacies architecture of nonmarine successions in other basins and assist in petroleum development.