Stratigraphic constraints on the Lower Cretaceous Orallo Formation, southeastern Queensland: U-Pb dating of bentonite and palynostratigraphy of associated strata

This study presents two chemical abrasion-thermal ionisation mass spectrometry (CA-TIMS) U-Pb zircon ages from tuffs as well as palynostratigraphic results from the embracing sedimentary succession of the Orallo Formation, Surat Basin, Queensland. Samples were taken from two Geological Survey of Queensland (GSQ) stratigraphic holes, GSQ DRD 26 and GSQ Roma 2. Palynological results assign the upper part of the Orallo Formation to the lower part of theRuffordiaspora australiensisInterval Zone rather than the upper part of this zone as previously indicated. A late Valanginian isotopic date of 134.38 +/- 0.04 Ma from GSQ DRD 26 and an early Hauterivian date of <= 132.46 +/- 0.36 Ma from GSQ Roma 2 give the Orallo Formation a Berriasian?/Valanginian-Hautevarian age, rather than the previously inferred Berriasian-Valanginian age determined without any geochronological control. Additionally, when combined with previously published geochronological dates from the Westbourne Formation, these ages suggest that, in the Surat Basin succession, the Jurassic-Cretaceous boundary most likely lies within the Gubberamunda Sandstone.

Automated summary

This study presents U-Pb zircon ages and palynostratigraphic results from the Orallo Formation in the Surat Basin, Queensland. The results suggest a Berriasian?/Valanginian-Hautevarian age for the Orallo Formation. Additionally, combining these ages with previously published geochronological dates indicates the Jurassic-Cretaceous boundary likely lies within the Gubberamunda Sandstone in the Surat Basin succession.