John Backhouse

Carboniferous-Permian palynostratigraphy of west Australian marine rift basins: resolving tectonic and eustatic controls during Gondwanan glaciations

Abstract The longest interval of cold climate in Earth’s Phanerozoic history spanned some 55 million years from the Namurian (late Carboniferous) to the Kungurian (early Permian) when glaciation affected much of high-latitude Gondwana. Uncertainty surrounds the timing of glaciation(s) and the frequency of interglacial episodes, ice volumes and the location of ice centres. This is because the ‘glacial’ record is preserved predominantly within marine strata deposited in intracontinental rift basins marginal to onshore ice centres on uplifted basement highs. The direct stratigraphic record of glaciation is meagre on adjoining basement highs, and extremely difficult to date. Many basins contain broadly similar threefold lithostratigraphic successions of lowermost glaciomarine diamictite facies, middle shales and uppermost carbonate and sandstone-rich, commonly coal-bearing, deltaic strata. In the absence of absolute age control and infrequent presence of macrofossils, such ‘deglaciation sequences’ are widely assumed to be correlative from basin to basin recording Gondwanan deglaciation(s) and a world-wide glacioeustatic sea level rise. A newly compiled biostratigraphic database for west Australia, using first appearance datums of spore-pollen types, allows correlation of Late Palaeozoic siliciclastic successions between seven basins (Bonaparte, Canning, Carnarvon, Collie, Gunbarrel, northern and southern Perth basins). These extend some 3800 km along Australia’s western continental margin adjacent to the west Australian shield. All but the Gunbarrel Basin contain tripartite glacially influenced successions (diamictite/shale/sandstone). Biostratigraphic data reveal sharp differences in ages and stratigraphic distributions, and marked variations in the thickness of coeval units. Diamictite/shale/sandstone successions accumulated diachronously from basin to basin recording a dominant tectonic control on subsidence and relative sea level. Middle shale units are not exact correlatives between basins and thus are unlikely to simply represent widespread climatically controlled glacioeustatic events. Instead, they are argued to record high relative sea levels during times of maximum synrift subsidence. This tectonostratigraphic model can be applied to other intracontinental rift-basin successions across Gondwana.

Calibrating the middle and late Permian palynostratigraphy of Australia to the geologic time-scale via U–Pb zircon CA-IDTIMS dating

ABSTRACT The advent of chemical abrasion-isotope dilution thermal ionisation mass spectrometry (CA-IDTIMS) has revolutionised U–Pb dating of zircon, and the enhanced precision of eruption ages determined on volcanic layers within basin successions permits an improved calibration of biostratigraphic schemes to the numerical time-scale. The Guadalupian and Lopingian (Permian) successions in the Sydney, Gunnedah, Bowen and Canning basins are mostly non-marine and include numerous airfall tuff units, many of which contain zircon. The eastern Australian palynostratigraphic scheme provides the basis for much of the local correlation, but the present calibration of this scheme against the numerical time-scale depends on a correlation to Western Australia, using rare ammonoids and conodonts in that succession to link to the standard global marine biostratigraphic scheme. High-precision U–Pb zircon dating of tuff layers via CA-IDTIMS allows this tenuous correlation to be circumvented—the resulting direct calibration of the palynostratigraphy to the numerical time-scale highlights significant inaccuracies in the previous indirect correlation. The new data show: the top of the Praecolpatites sinuosus Zone (APP3.2) lies in the early Roadian, not the middle Kungurian; the top of the Microbaculispora villosa Zone (APP3.3) lies in the middle Roadian, not the early Roadian; the top of the Dulhuntyispora granulata Zone (APP4.1) lies in the Wordian, not in the latest Roadian; the top of the Didecitriletes ericianus Zone (APP4.2) lies in the first half of the Wuchiapingian, not the latest Wordian; the Dulhuntyispora dulhuntyi Zone (APP4.3) is exceptionally short and lies within the Wuchiapingian, not the early Capitanian; and the top of the Dulhuntyispora parvithola Zone (APP5) lies at or near the Permo-Triassic boundary, not in the latest Wuchiapingian.

ALBIAN (LOWER CRETACEOUS) DINOFLAGELLATE CYST BIOSTRATIGRAPHY OF THE LOWER GEARLE SILTSTONE, SOUTHERN CARNARVON BASIN, WESTERN AUSTRALIA

Abstract Rich and diverse assemblages of dinoflagellate cysts were recovered from the Gearle Siltstone in the Boologooro-1 stratigraphic well, Southern Carnarvon Basin, Western Australia. The cored interval through the Winning Group is proposed as a reference interval for the Canninginopsis denticulata, Endoceratium ludbrookiae, Dioxya armata and Xenascus asperatus dinoflagellate cyst zones of the Australian dinoflagellate cyst zonal scheme. The ranges of sixty-three dinoflagellate cyst species are documented in detail over this interval. Apart from the bioevents that define the widely-used dinoflagellate cyst subzones, the highest occurrences of Endoceratium turneri, Craspedodinium indistinctum and Litosphaeridium arundum, and the lowest occurrences of Nematosphaeropsis densiradiata, Canninginopsis denticulata, Leberidocysta chlamydata, Aptea? sp. cf. Aptea polymorpha, Litosphaeridium siphoniphorum, Balcattia cirribarbata and Stephodinium australicum are recognized as potential biostratigraphic datums in the Southern Carnarvon Basin. Species diversity increased in the highest Canninginopsis denticulata Zone at approximately the horizon where Diconodinium spp. become dominant. Foraminiferal data indicate the interval from 361.15 m to 312.12 m is marginally inner neritic and corresponds with an interval of abundant Diconodinium spp., and fewer Spiniferites spp. Diversity increased in the Xenascus asperatus Zone with an increase in Spiniferites spp., and a shift to middle neritic paleobathymetry. The age of the top of the Xenascus asperatus Zone is constrained by calcareous nannofossils and dinoflagellate cyst correlations to the latest Albian, or possibly the early Cenomanian. The age of the base of the Canninginopsis denticulata Zone is constrained by dinoflagellate cyst and foraminiferal data to the early Albian, above the earliest Albian. One new species, Chlamydophorella haigii, is erected and aspects of several other dinoflagellate cyst taxa are reviewed.