Abstract The stratigraphic overview presented in this chapter substantially updates and revises the last major review of the Ordovician rocks of Australia and New Zealand published 40 years ago. In the western two-thirds of the present-day continent of Australia, Ordovician sedimentary rocks are restricted to intracratonic basins. The Canning Basin (Western Australia) and Amadeus Basin (central Australia) contain the best known Lower and Middle Ordovician shallow marine successions. The eastern third of the continent, known as the Tasmanides, comprises multiple orogens (i.e. Delamerian, Lachlan, New England, Thomson, Mossman) that formed along the convergent East Gondwana Margin. As a result, volcanic and intrusive rocks are much more common in these orogens than in the intracratonic basins. Their deep-water depositional environments span 31 graptolite biozones. Slope and basinal siliceous sedimentary rocks are constrained by a newly defined set of 12 conodont biozones, complementing the conodont biostratigraphic scheme refined for shallow-water environments from the basal boundary of the Ordovician to the latest Katian. In some places, these conodont biozones are integrated with radiometric ages from tuff interbeds (e.g. Canning Basin). Ordovician graptolitic strata in the Buller Terrane of New Zealand share palaeogeographic links with those in the Bendigo Zone of the western Lachlan Orogen.

ABSTRACT Making reliable correlations and sequence stratigraphic interpretations can be challenging in depositionally complex settings due to depositional heterogeneity and data-set limitations. To address these issues, the Canning Basin Chronostratigraphy Project documented the development of a high-resolution, chronostratigraphic correlation framework across different depositional environments in the Upper Devonian (Frasnian–Famennian) of the Lennard Shelf, Canning Basin, by integrating stable isotope chemostratigraphy, biostratigraphy, magnetostratigraphy, and sequence stratigraphy. This integrated data set allows for a rare, detailed look at the carbon isotope record, and specifically its potential as a sequence stratigraphic interpretation tool and its application to improve correlation capabilities, both of which have implications for better understanding of the depositional history of the Lennard Shelf. For platform-top settings, a sequence stratigraphic framework was constructed using stacking pattern analysis constrained by the paleomagnetic reversal record. In slope settings, where depositional variability and a lack of platform-top control have historically hindered our ability to recognize and correlate systems tracts, carbon isotope chemostratigraphy (in conjunction with conodont biostratigraphy and magnetostratigraphy) proved to be a useful chronostratigraphic tool because primary marine δ 13 C values were well preserved. Using the paleomagnetic reversal record, with additional age control from walkout correlations to key outcrop sections, we were able to confidently correlate from the platform-top into the slope. Evaluation of the slope isotope record, within the projected sequence stratigraphic framework from the platform-top, revealed that variations in δ 13 C values corresponded to changes in sea level. Using this relationship, isotopic trends were used as a proxy for delineating systems tracts in slope sections without direct platform-top control. In turn, this improved correlations through heterogeneous slope facies and also allowed for a refined sequence stratigraphic interpretation of Famennian strata in the Canning Basin. Results from this work also allowed us to develop a model that attempts to explain the observed relationships among global carbon cycling, sea-level fluctuations, and paleoceanographic conditions during the Late Devonian.

Abstract: High-resolution chronostratigraphic correlation using elemental chemostratigraphy in platform carbonates is typically difficult to achieve. Here, elemental chemostratigraphy is used to correlate between two platform-top, carbonate-dominated field sections from the narrow Lennard Shelf that existed on the NE margin of the Canning Basin, Western Australia, during the mid-late Frasnian. The correlation, constrained by magnetic polarity reversals and physical ground truthing, is based on recognition of distinctive cyclical “stacking patterns” defined by changes in concentrations of the trace element zirconium (Zr). Zr concentrations are controlled by the amount of the heavy mineral zircon in the sediments, which is derived from a terrigenous source and is diagenetically very stable. The stacking patterns in the lower part of the study sections display gradually upward-increasing values of Zr to a maximum, followed by an almost immediate fall to a minimum. In the upper part of the study interval, the cycles are more symmetrical, with both gradually increasing and decreasing portions. The point at which the change in Zr stacking pattern occurs in the two sections is synchronous and occurs in association with a supersequence maximum flooding surface. The correlation based on maximum and minimum Zr values throughout the two sections is demonstrated to be chronostratigraphic by comparison with correlations based upon paleomagnetism and physical ground truthing. When element ratios commonly used as provenance and paleoclimate proxies are plotted, the variations between closely spaced samples are greater than any systematic variations throughout the study intervals. Therefore, no isochemical chemozones can be defined, implying that during deposition of the study intervals, there were no long-lived changes in sediment provenance or paleoclimate that the elemental chemistry can detect. The work presented here shows that the standard approach of defining isochemical chemozones for chemostratigraphic correlation is not always appropriate. However, an approach using cyclical changes in elemental variables for chemostratigraphic correlation between two closely spaced sections is chronostratigraphically valid. The greater challenge is in application of the same approach to more widely spaced sections, potentially in different facies of a carbonate setting.

Abstract The Frasnian–Famennian Virgin Hills Formation represents fore-reef facies deposited as part of the extensive Late Devonian reef system that fringed the SW Kimberley Block in Western Australia. It contains a rich trilobite fauna dominated primarily by proetids and, to a lesser extent, harpetids, phacopids, scutelluids and odontopleurids. To date, 49 taxa have been described, 40 of these being restricted to the Frasnian. Herein five Frasnian taxa are described, three in open nomenclature, and two the new species Telopeltis intermedia and Otarion fugitivum . Evolutionary trends in the Virgin Hills trilobites are dominated by a reduction in body size and eye size and, to a lesser extent, a reduction in exoskeletal vaulting. Although recording no sedimentological signature, the fauna was strongly affected by the two globally recognized Kellwasser extinction events. The first, at the end of conodont Zone 12, affected taxa at the species and genus level. The second, within Zone 13b, had a much greater impact on the fauna, causing extinctions at the familial and ordinal levels. Evidence is presented to suggest that evolutionary trends in the trilobites during the late Frasnian reflect selection for forms adapted to low nutrient conditions. The two intensive Kellwasser extinction episodes may reflect periodic massive inputs of nutrients from the terrestrial into the shallow-marine environment.