Palaeogeographic, climatic and tectonic change in southeastern Australia: the Late Neogene evolution of the Murray Basin

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Abstract

The Murray Basin is a low-lying but extensive intracratonic depocentre in southeastern Australia, preserving an extraordinary record of Late Neogene sedimentation. New stratigraphic and sedimentologic data allow the long-term evolution of the basin to be re-evaluated and suggest a significant role for: (1) tectonism in controlling basin evolution, and (2) progressive and step-wise climatic change beginning in the early Pleistocene. Tectonic change is associated with regional uplift, occurring at approximately the same rate from the early Pliocene until the present day, and possibly associated with changing mantle circulation patterns or plate boundary processes. This uplift led to the defeat and re-routing of the Murray River, Australia’s major continental drainage system. Key to our interpretation is recognition of timing relationships between four prominent palaeogeographic features – the Loxton-Parilla Sands strandplain, the Gambier coastal plain, palaeo megalake Bungunnia and the Kanawinka Escarpment. Geomorphic and stratigraphic evidence suggest that during the Early Pliocene the ancestral Murray River was located in western Victoria, flowing south along the Douglas Depression. Relatively small amounts of regional uplift (<200 m) defeated this drainage system, dramatically changing the palaeogeography of southeastern Australia and forming Plio-Pleistocene megalake Bungunnia. At its maximum extent Lake Bungunnia covered more than 50,000 km2, making it one of the largest known palaeo- or modern-lakes in an intracontinental setting. Magnetostratigraphic constraints suggest lake formation c. 2.4 Ma. The formation of Lake Bungunnia influenced the Pliocene coastal dynamics, depriving the coastline of a sediment source and changing the coastal system from a prograding strandline system to an erosional one. Erosion during this period formed the Kanawinka Escarpment, a palaeo sea-cliff and one of the most prominent and laterally extensive geomorphic features in southeastern Australia. Marine sediments c. 800 ka to c. 1.16 Ma represent the time of re-establishment of depositional coastal dynamics and of a permanent outlet for the Murray River. This age range is consistent with our best estimate of the age of the youngest Lake Bungunnia sediments and points towards an early Pleistocene age for the demise of the lake system. The youngest Lake Bungunnia sediment, present on a number of distinct terraces, suggests that progressive, step-wise climatic change played a role in the demise of the lake. However, in order for the ancestral Murray River system to have been able to breach the pre-existing tectonic dam, it is likely that tectonic change and/or temporarily enhanced discharge was also significant. This scenario indicates that the modern Murray River has only been in existence for at most 700 ka.

Highlights

► Intraplate tectonism played a key role in the evolution of the Neogene Murray Basin. ► The Murray Basin records evidence for dramatic palaeogeographic and climatic change. ► Australia's main drainage, the Murray River, is a very young geomorphic feature. ► Arid climatic conditions began in southeastern Australia in the Plio-Pleistocene. ► The onset of arid conditions was step-wise and gradual.

Introduction

The period of Earth history from the Neogene to the present has seen extraordinary environmental and climatic change across the globe. Climatic oscillations and eustatic sea-level changes during this period have been reconstructed from stratigraphic records and variations in the isotopic composition of marine sediments (e.g., de Menocal, 1995, Lisiecki and Raymo, 2005, Raymo et al., 2006). There is general agreement that the late Miocene to Pleistocene was characterised by a global high-stand sea-level (Shackleton et al., 1995) on which a number of c. 100 ka glacio-eustatic cycles are overlain (e.g., Idnurm and Cook, 1980). But in detail the nature of change in this interval is highly variable. In Australia, climate changed from wet conditions in the Late Neogene to the arid conditions that characterise much of the continent today; understanding the nature and dynamics of this change remains an outstanding research question.

The Murray Basin in southeastern Australia is a low-lying but extensive depocentre (Fig. 1). It hosts Australia’s largest modern river system, the Murray-Darling, and its extensive groundwater and heavy mineral resources mean it plays a key role in the agricultural and economic sectors. The basin is thought to contain a near continuous record of sedimentation from the Neogene to the present, making it an excellent archive of climatic and environmental change (e.g., Brown and Stephenson, 1986). Key components of the Neogene basin system are: (1) the palaeo-shorelines of the Late Miocene to early Pliocene Loxton-Parilla Sands; (2) the palaeo-shorelines of the Pleistocene Bridgewater Formation; (3) Plio-Pleistocene palaeo megalake Lake Bungunnia; and (4) the prominent and laterally extensive Kanawinka escarpment. These striking remnant geomorphic features, at the scale of the basin, must reflect significant and profound changes in palaeogeography during the Late Neogene. Interpretation of this record of change is important for understanding environmental, tectonic and/or climatic processes in southeastern Australia in this interval.

This paper is concerned with understanding the Late Neogene evolution of the Murray Basin, and attempts the first synthesis of the geology of the basin system since the seminal work of Brown and Stephenson (1991). We present new stratigraphic and age constraints on the Miocene-Pleistocene evolution of the basin and use these data to understand the environmental, geographic, tectonic and climatic signature of change over the basins history. We then present a new integrated model for basin evolution involving: (1) tectonic change, leading to major continental-scale palaeogeographic reorganisation, and (2) progressive and step-wise climatic change, beginning in the early Pleistocene and accompanied by dramatic palaeoenvironmental change. The time scale of Gradstein et al. (2004) and the magnetic field reversal data of Ogg and Smith (2004) are used throughout this paper.

Section snippets

Geological framework

The Murray Basin is a large intracratonic basin in southeastern Australia, covering more than 300,000 km2 (Fig. 1). It is the most aerially extensive of a number of Tertiary basins, including the Gippsland, Bass, Westernport, Port Phillip, Torquay and Otway Basins, along the southern seaboard of the Australian continent (Fig. 1). These basins are all extensional continental margin basins formed as a result of the rifting of southern Australia from Antarctica beginning in the Mesozoic. The

Methods

Stratigraphic data were collected using measured sections. A Jacob’s Staff was used to obtain accurate unit thicknesses. Elevation data for the Murray Basin were sourced from the release of 3 arc-second radar data acquired by NASA’s Shuttle Radar Topography Mission (SRTM; Farr and Kobrick, 2000, Farr et al., 2007). A list of locations where stratigraphic sections were measured, and a list of samples analysed, is provided in Table 1. In order to relate stratigraphy to basin geometry accurate

Key geomorphic and palaeogeographic features

Notwithstanding its low average topography, the Murray Basin is characterised by a number of prominent basin-scale geomorphic features. Identification of these geomorphic features, and our understanding of basin geometry, have been greatly aided by the availability of high resolution elevation data from NASA’s Shuttle Radar Topography Mission (SRTM; Farr and Kobrick, 2000, Farr et al., 2007; Fig. 2).

Stratigraphy

This section provides a summary of the main sedimentologic and stratigraphic characteristics of the Late Neogene sedimentary sequences, including new age data. Stratigraphic relationships between and across the South Australia–Victoria state border are important in understanding the geomorphic evolution of southeastern Australia and thus this section includes a description of relevant units from both the Murray Basin sensu stricto, and also from the western Otway Basin (Fig 1, Fig. 6).

Palaeogeography

Stratigraphic observations and new age constraints from the Murray Basin and adjacent Otway Basin presented here, together with recently published work of Miranda et al., 2008, Miranda et al., 2009, McLaren et al., 2009 and McLaren and Wallace (2010), enable a reconstruction of palaeogeography of the Murray Basin from the Middle Miocene through to the Pleistocene.

Discussion

The sedimentary and stratigraphic record of the Murray Basin through the Neogene preserves a remarkable record of basin dynamics, highlighting a key role for regional tectonic change in controlling sedimentation and landform development. Previously, the extraordinary strandplain systems of the Loxton-Parilla Sands and the Bridgewater Formation have been assumed to represent “a more-or-less complete record of palaeo-shorelines deposited over the last 6 Ma” (Bowler and Sandiford, 2007) and

Acknowledgements

This contribution is the culmination of Australian Research Council funded project DP0558705. Roger Powell and Richard Howarth provided assistance and advice regarding the statistical treatment of strontium isotopic data and Roger Powell is thanked for the calculations and figure shown here. We are grateful to Russell Drysdale of the University of Newcastle, Australia, for oxygen isotope analysis. Strontium isotope analysis was undertaken at the School of Earth and Environmental Sciences at the

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