Elsevier

Gondwana Research

Volume 29, Issue 1, January 2016, Pages 153-167
Gondwana Research

Developing a radiometrically-dated chronologic sequence for Neogene biotic change in Australia, from the Riversleigh World Heritage Area of Queensland

https://doi.org/10.1016/j.gr.2014.10.004Get rights and content

Highlights

  • First application of the U/Pb speleothem chronometer to studies of biotic change

  • First radiometric framework for correlating Australasian Miocene vertebrates

  • Allows reliable correlation of changes in Miocene vertebrates with global climates

Abstract

Radiometric U–Pb ages are presented for the Riversleigh World Heritage fossil mammal site in northwestern Queensland, Australia. The ages are determined on speleothems which are generally found in intimate or well-documented association with fossil remains and thus can be assumed to record the age of the latter with a high degree of confidence. The new ages encompass the early (18.2–16.5 Ma) and middle Miocene (15.1–13.5 Ma) deposits at Riversleigh in addition to the younger Rackham's Roost Site which returns early Pleistocene ages. Together, these provide a robust chronological framework for the interpretation of Neogene biotic change in Australia that has, until now, relied almost entirely upon biocorrelative techniques. In particular they permit closer investigation of links between other regions/faunas and allow comparison with other records of climatic and environmental change. This is the first documented example of a methodology that has widespread potential application across many continents and throughout much of Earth history.

Introduction

Australia is one of the last continents to have a securely dated framework for the evolution of its Cenozoic terrestrial biotas. Until now, the vast majority of Australia's mammal-bearing deposits have been dated by biocorrelation, anchored by little more than half a dozen radiometric dates for the entire continent. Some regions have superpositional biotas but most of these are relatively limited in taxic biodiversity making biocorrelation much more difficult.

The oldest Australian Cenozoic terrestrial mammal-bearing assemblage is the Tingamarra Local Fauna (LF)1 from southeastern Queensland, radiometrically dated at (minimally) 54.6 Ma (Godthelp et al., 1992). This is the only terrestrial mammal-bearing assemblage for the whole continent in the gap between the early Cretaceous and the late Oligocene (Black et al., 2012a). The only late Oligocene assemblages that have been radiometrically dated are those of the upper Etadunna Formation in South Australia, to which a single reported date has been tentatively applied (Woodburne et al., 1994; see also below).

Australia's many Miocene assemblages include the most diverse mammal faunas known for the pre-Quaternary Cenozoic. Only one relatively impoverished assemblage has been radiometrically dated: the sparse early Miocene Geilston Bay LF of Tasmania (Tedford and Kemp, 1998). However, the early Miocene Wynyard LF (one taxon) of Tasmania, the middle Miocene Batesford Quarry LF of Victoria with one mammal taxon, and the mammal-poor late Miocene Beaumaris LF of Victoria (Black et al., 2012a) have been dated on the basis of marine biocorrelation. Among Pliocene assemblages, the early Pliocene Hamilton LF from northwestern Victoria (Turnbull and Lundelius, 1970, Turnbull et al., 2003), the early Pliocene Sunlands LF of South Australia (Pledge, 1987), the middle Pliocene Bluff Downs LF from northeastern Queensland (Rich et al., 1991, Mackness et al., 2000, Mackness and Archer, 2001) and the late Pliocene Awe LF of New Guinea (Plane, 1967, Hoch and Holm, 1986) have been radiometrically dated. In distinct contrast, there are hundreds of radiometrically dated Pleistocene assemblages.

Developing an accurate and precise chronology for Australia's Miocene assemblages is critical for many reasons. First, the majority of Australia's pre-Quaternary mammals and mammal assemblages are now known from this epoch. Second, these assemblages document an overlap and gradual transition between relatively archaic groups (e.g., ilariids and wynyardiids) that were common in the late Oligocene and relatively more derived groups (e.g., macropodids and vombatids) that dominated the Pliocene and Quaternary. Third, the history of climate change impacting the continent during this period included a particularly profound transition from the globally warm climates of the early and middle Miocene to late Miocene drier conditions with corresponding impacts on Australia's continental biotas (Kershaw et al., 1994, McGowran and Li, 1994, McGowran et al., 2000). Investigation of any causal links with climate change requires accurate anchoring of the faunal record with those of various climate proxies that are themselves often well-dated. Fourth, it was primarily during the Miocene that the continent's globally unique megafauna began to develop. Finally, most of Australia's modern families of marsupials appear to have developed or at least first appeared during the Miocene. As a result of all these considerations, radiometric dating of Miocene sequences from anywhere on the continent would provide a greatly improved and far more reliable basis for developing an understanding of the timing of these regionally and globally significant evolutionary and environmental processes.

Only two regions of the continent have a reasonably rich as well as demonstrably superpositional sequence of mammal assemblages: northern South Australia (the Lake Eyre and Lake Frome Basins); and the Riversleigh World Heritage Area of northwestern Queensland (Fig. 1). The former has thus far been represented by just one radiometric age determination, in this case a reported Rb–Sr age of 25 Ma for an illite from sediments unrelated to any actual fauna but tentatively correlated with members of the Etadunna Formation that contain LFs of Mammal Zones A–E (Norrish and Pickering, 1983, Woodburne et al., 1994). By calibrating magnetostratigraphic data to a biocorrelated foraminifera fauna and this date, Woodburne et al. (1994) assigned these sediments of the upper Etadunna Formation to the period spanned by palaeomagnetic chrons 6Cr to 7ar.

Although a sustained research programme since 1976 has suggested that Riversleigh's palaeoassemblages span the late Oligocene to early Miocene, middle Miocene, possibly late Miocene, Pliocene and Pleistocene, until now all of these age assessments have been based mainly on biocorrelation, supported in some cases by lithostratigraphic relationships (Arena, 2004, Black et al., 2012a, Arena et al., 2014). Hypothesised biostratigraphic relationships of Riversleigh's more than 200 species-rich assemblages (e.g., Creaser, 1997, Arena, 2004, Travouillon et al., 2006) to each other and to those from other areas of Australia (e.g., Black, 1997a, Black, 1997b, Myers and Archer, 1997, Travouillon et al., 2006, Black, 2010, Black et al., 2013) as well as intercontinentally (e.g., Sigé et al., 1982, Hand et al., 1997, Hand et al., 2005) are still to be tested using fauna-independent techniques.

The Riversleigh region encompasses two limestone units of very distinctive character: an extensive Cambrian (~ 500 Ma) marine dolostone, the Thorntonia Limestone, overlain, down-cut and/or framed by inliers of much younger freshwater limestones containing Cenozoic vertebrate fossils (Archer et al., 1989). These two carbonate types have very distinct chemistries with the Tertiary limestones characterised by low Sr and Mg in contrast to the dolomitised Cambrian limestones with slightly higher Sr but substantially greater Mg, and greatly reduced Ca/Mg ratios (Table 1).

Although providing direct radiometric ages for fossil deposits worldwide is often difficult, many of the sites at Riversleigh are intimately associated with speleothems (stalagmites, stalactites, flowstones, and other secondary cave carbonates) developed in this karst landscape (e.g. Arena et al., 2014). These provide an excellent opportunity for establishing a radiometric chronology. Recent years have seen the rapid development of the U–Pb chronometer for speleothems (e.g. Woodhead et al., 2006, Woodhead et al., 2012) which extends the range of the previously employed carbonate U–Th chronometer back beyond its ~ 600 ka age limit to samples many hundreds of millions of years in age (e.g., Woodhead et al., 2010). This new technique has widespread application across diverse fields of research, from human evolution, palaeontology and ecosystem development, through studies of weathering and erosion, to the influence of tectonics on landscape evolution (Woodhead and Pickering, 2012). This is the first documented application of the method to studies of Neogene biotic change.

Application of the U–Pb chronometer to Riversleigh speleothems, however, is particularly challenging. The majority of the Riversleigh fossil assemblages are hosted in the Cenozoic limestones that are characterised by relatively low U (typically ~ 200 ppb) and high Pb content (often ppm levels) — see Table 1. While the exact controls on Pb incorporation into speleothems remain to be determined (e.g., Woodhead et al., 2012), it is generally accepted that U contents broadly reflect the nature of the host karst, with superimposed climatic controls. It is therefore not surprising that these same low U concentrations are also observed in many speleothems formed in the Riversleigh karst: these typically have U contents significantly less than 1 ppm, especially in older materials (see later section). Pb contents are also often quite high (many tens of ppb) and these two factors result in ratios of radiogenic to ‘common’ Pb2 which are rather demanding for geochronological purposes.

Section snippets

Materials and methods

The basal unit of the Cenozoic Riversleigh limestone sequence has been interpreted to be late Oligocene in age (Archer et al., 1989, Archer et al., 1994, Archer et al., 1995, Archer et al., 1997). It consists largely of calcarenites with interspersed micritic muds, suggestive of a fluvial–lacustrine freshwater environment (Depositional Phase 1) containing faunas of Faunal Zone A (Archer et al., 1989, Arena, 2004, Travouillon et al., 2006). Slightly younger deposits, biocorrelated to early

Results

A wide variety of Riversleigh speleothem samples were assessed for dating potential including stalagmites, flowstones, cave pearls and thin calcite rafts. Ultimately, only stalagmites and flowstones contained a sufficiently high proportion of radiogenic to common Pb for geochronological purposes and, of these, only a small proportion displayed the necessary range in U/Pb ratios to allow for successful isochron construction; these are reported below in order of decreasing age.

Temporal and spatial variation in the Riversleigh deposits

All of the successfully dated Riversleigh samples are speleothems from undisputed cave deposits. Efforts to date Depositional Phase 1 deposits (e.g., D Site and Hiatus Site), biocorrelated to be late Oligocene in age, have so far been unsuccessful. Further, sites with more diverse depositional facies such as Neville's Garden were far better suited to geochronology because they contained well-developed speleothems. Of the Miocene speleothem types trialed for dating – stalagmites, flowstones,

Conclusions

For the first time, it is possible to independently place Australia's early to middle Miocene fossil faunas into a secure geochronological framework. Temporal correlations between faunal composition, evolutionary change and ecological adaptations in response to climatic and environmental drivers during the mid-Cenozoic have been evident for a considerable time (e.g., Archer et al., 1989) but, in the absence of independent and precise radiometric dating, have largely been confined to

Acknowledgements

We thank Alan Greig for assistance with the ICPMS trace element analyses. Development of the U–Pb speleothem chronometer at Melbourne University and continued investigations of the Riversleigh World Heritage Sites have been funded by a variety of grants from the Australian Research Council including LE0989067, LP0989969, LP100200486, DP0985214, DP0664621, DP1094569, DP130100197, and DE130100476, and support from the XSTRATA Community Partnership Program (North Queensland); the University of New

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