Palynology of the Jurassic–Cretaceous transition, northern Surat Basin, Queensland, Australia

Abstract

The terrestrially deposited Gubberamunda Sandstone and Orallo Formation of the Surat Basin represent the most complete Late Jurassic – Early Cretaceous sedimentary sequence in Queensland. This thesis presents the results of palynological and geochronological studies of these formations, and the underlying upper Westbourne Formation and overlying lower Mooga Sandstone. An improved understanding of the palynostratigraphy of this interval, presented here, will enable future workers to more accurately correlate and date strata both within and beyond the Surat Basin. Samples were taken for biostratigraphy and geochronology from three Geological Survey of Queensland (GSQ) stratigraphic boreholes located in the northern Surat Basin. These were GSQ DRD 26 in the west, GSQ Roma 2 in the center of the basin and GSQ Dalby 1 in the east. A systematic taxonomic survey of 89 biostratigraphic samples from the three boreholes revealed a diverse palynoflora from which 218 taxa were identified. These taxa included both in situ and reworked (latest Carboniferous – Early Jurassic) spores and pollen, as well as palynomorphs of algal, fungal, acritarch and unknown affinities. Of these, six are new: Dictyotosporites esterleae, Retitriletes thomsonii, Rugulatisporites johniorum, Camarozonosporites dorsus, Contignisporites confractus and Aratrisporites woodii. One new combination of Dictyotosporites rugulatus (Sajjadi & Playford) is proposed herein. Palynological samples from the upper Westbourne Formation and the lower Gubberamunda Sandstone yielded palynofloras here assigned to the Retitriletes watherooensis Association Zone, while those from the upper Gubberamunda Sandstone to the lower Mooga Sandstone were assigned to the Ruffordiaspora australiensis Interval Zone. Furthermore, Subzones APJ6.2.1, APJ6.2.2, APK1.1 and APK1.2.1 of Price (1997) are also herein formally described and recognized in the Surat Basin respectively as the Neoraistrickia equalis Interval Subzone, Foraminisporis dailyi Interval Subzone, Lower Ruffordiaspora australiensis Interval Subzone and Upper Ruffordiaspora australiensis Interval Subzone. Based on taxon ranges from this study and from Burger (1974), Subzone APK1.2.2 (Price, 1997) does not appear to be useful in the Surat Basin succession. The geochronological samples yielded two U-Pb Chemical Abrasion-Thermal Ionizations Mass Spectrometry (CA-TIMS) ages from the upper Orallo Formation of 134.38 ± 0.04 Ma (late Valanginian) from GSQ DRD 26 and of ≤132.46 ± 0.36 Ma (early Hauterivian) from 3 GSQ Roma 2. On this basis, the Gubberamunda Sandstone is determined to be of Tithonian–Berriasian age and the Orallo Formation to be of (?)Berriasian/Valanginian– Hauterivian age. Accordingly, deposition of the Bungil Formation, which overlies the Mooga Sandstone, cannot have begun prior to the Hauterivian. Analysis of the sampled palynofloras shows that, during the Tithonian–Hauterivian, the Surat Basin flora remained relatively unchanging with no major floristic turnovers. This flora consisted of diverse ferns, conifers, lycopods, bryophytes, seed ferns and some variety of ginkgoes, cycads and/or gnetales. This flora is interpreted to have grown under humid, warm-temperate conditions and to have been deposited in floodplain or backswamp facies.