The Late Pleistocene evolution of palaeo megalake Bungunnia, southeastern Australia: A sedimentary record of fluctuating lake dynamics, climate change and the formation of the modern Murray River

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Abstract

The ephemeral lacustrine carbonates of the Bungunnia Limestone, deposited in palaeo megalake Bungunnia, preserve a detailed record of Pleistocene palaeoenvironmental and palaeoclimatic change in southeastern Australia. Five distinct lake shorelines (Lake Levels 0–4) are visible on digital elevation models, ranging in elevation from around 70 m to 30 m above sea level. The Bungunnia Limestone is preserved on the three lowest terraces ranging over 20 m in elevation (Lake Levels 2–4). On the two higher of these terraces Bungunnia Limestone contains ooids and stromatolites and is calcite- and aragonite-dominated whereas on the lower terrace level Bungunnia Limestone is dolomite-, gypsum- and magnesite-dominated and preserves tepee structures and mud-cracks. Thus, lacustrine conditions are interpreted to have become increasingly saline and evaporitic over time reflecting an overall trend to more arid climatic regimes. The highest lake level (Level 0 at around 70 m above sea level) appears to relate to a period of significantly increased rainfall that allowed the lake to overflow for the first time, marking the initiation of the modern Murray River. Subsequently, the lake fluctuated between: (1) a steady-state open lake system where water was overflowing across the Padthaway High spillpoint and active gorge cutting was proceeding, and (2) a closed lake system with no overflow that resulted in carbonate-precipitating conditions. During (1), shoreline erosion was occurring within the basin providing the terraces on which the Bungunnia Limestone carbonates were deposited during (2). In this, (1) is likely to correspond with significantly wetter Late Pleistocene interglacial and (2) with arid glacial conditions. The formation of the modern Murray River and the demise of palaeo megalake Bungunnia were therefore consequences of a combination of gorge cutting and increasing aridity resulting from interglacial–glacial climate oscillation superimposed on significant overall climatic change.

Highlights

Murray Basin sediments record dramatic palaeogeographic and climatic change. The onset of aridity in southeastern Australia was stepwise and gradual. The demise of Lake Bungunnia was due to both gorge cutting and increasing aridity. Australia's main drainage, the Murray River, is a very young geomorphic feature.

Introduction

Globally, dramatic climate change in the Late Neogene is well known. In Australia, this period marks a change from dominantly wet climatic conditions throughout the Miocene to the arid conditions that characterize much of the continent today. Although warm and wet climatic conditions throughout the Miocene (e.g., Firman, 1973, Kershaw et al., 1994, Macphail et al., 1994, Li et al., 1996, Gallagher and Gourley, 2007) and arid climatic conditions in the Pleistocene (e.g., Chen et al., 1991, Herczeg and Chapman, 1991, Kershaw et al., 1991, Nanson et al., 1992, Wagstaff et al., 2001, Hesse et al., 2004, Sniderman et al., 2007, Fitzsimmons et al., 2009) are both well documented, the interval of change is less well constrained and the timing and mechanism of climate change in this interval has remained an outstanding research question. In part this is because there are few continuous sequences that provide a window into the key period between the Late Miocene and the Early Pleistocene. Nonetheless, based principally on the apparent timing of contraction of large lakes as well as documented faunal changes, Bowler (1976), Martin (1978) and others have suggested that the first steps toward aridification began sometime in the Late Miocene, and increased into the Plio-Pleistocene. Bowler (1982) subsequently suggested that the demise of Lake Bungunnia, an intracontinental lake within the Murray Basin of southeastern Australia (Fig. 1), heralded the onset of fully arid climatic regimes in Australia. The timing of this change was dated to be around the Brunhes–Matuyama transition (c. 700–800 kA) by An et al. (1986).

But recent work by McLaren and Wallace (2010) and McLaren et al. (2011)—also based on the sedimentary record of Lake Bungunnia—suggests that the onset of aridity in southern Australia was progressive and stepwise and began significantly earlier than the Brunhes–Matuyama transition. McLaren and Wallace (2010) base their interpretation on the identification of regionally extensive and correlatable quartz silts, the Nampoo Member of the Blanchetown Clay (McLaren et al., 2009), that were deposited within the lake just prior to the middle Pleistocene transition at around 1.4 to 1.5 Ma. Based on grain-size distribution and grain morphology, McLaren and Wallace (2010) interpreted the silts to represent aeolo-lacustrine deposition and thus the Nampoo Member to represent a major step in the aridification of the continent that occurred prior to the demise of the megalake. McLaren and Wallace (2010) also showed that the youngest sediment of Lake Bungunnia, the Bungunnia Limestone (Firman, 1965), is diachronous and preserved on a number of distinct terraces ranging over more than 20 m in elevation. They suggest that the Bungunnia Limestone sediments provide a unique record of the increasing amplitude of arid climatic cycles into the Pleistocene in southeastern Australia.

Indeed, it is well known that lacustrine carbonates may preserve evidence for changing lake levels, lake geometry and lake chemistry, that may be related to changing tectonic and/or climatic conditions (e.g., Carroll and Bohacs, 1999, Bohacs et al., 2000, Freytet and Verrechhia, 2002, Sáez and Cabrera, 2002, Alonso-Zarza, 2003, Gierlowski-Kordesch, 2010, Nelson et al., 2010, Tanner, 2010). But the hypothesis of McLaren and Wallace (2010) is yet to be fully evaluated. To this end, in this paper we explore the variations in lithofacies, mineralogy and isotopic composition within the Bungunnia Limestone. We use these data to understand the evolution of Lake Bungunnia during the Pleistocene and to help evaluate the stepwise aridity hypothesis of McLaren and Wallace (2010). The time scale of Ogg and Smith (2004) is used throughout this paper.

Section snippets

Palaeo Megalake Bungunnia

The Murray Basin (Fig. 1) is a thin but extensive intracratonic depocentre in southeastern Australia that preserves an extraordinary record of marine and terrestrial sedimentation from the Late Neogene to the present. Key features of the basin sedimentary record include packages of: (1) Palaeocene–Eocene–Early Oligocene-aged sediments dominated by marginal marine, fluvial and lacustrine sediments (e.g., Lawrence, 1975, Brown and Stephenson, 1991); (2) Oligocene–Middle Miocene-aged dominantly

Methods

Stratigraphic data were collected using measured sections; section locations are shown in Fig. 2. A Jacob's Staff was used to obtain accurate unit thicknesses. Locations of stratigraphic sections of Bungunnia Limestone, and other sample sites, are provided in Table 1. In order to relate stratigraphy to basin geometry, accurate elevation data at outcrop localities were obtained using high-precision global positioning system (GPS) methods as outlined by McLaren and Wallace (2010). Sample heights

Bungunnia Limestone

The Bungunnia Limestone (Firman, 1965) is a thin (1–3 m) lacustrine carbonate unit, interpreted by Stephenson (1986) to represent an ephemeral hypersaline lake facies. The unit is confined to the Morgan sub-basin and western Loxton sub-basin (Fig. 1) where it caps the Blanchetown Clay (if present). Basin bedrock is an important control on the mineralogy of lake sediments (e.g., Gierlowski-Kordesch, 1998, Gierlowski-Kordesch, 2010) and at least in part this distribution reflects the presence of

The Pleistocene evolution of palaeo megalake Bungunnia

The occurrence of Bungunnia Limestone sediment at progressively lower elevations records the time-integrated evolution of Lake Bungunnia through the Pleistocene. Observations outlined in the previous sections imply significant changes in lake and environmental conditions during this period. Lake Levels 2 and 3 are characterised by the presence of abundant stromatolites and ooids. Ooids indicate regular agitation and a high-energy environment, suggesting littoral to sub-littoral conditions.

Formation of Megalake Bungunnia

McLaren et al. (2011) suggest that the ancestral Murray River flowed through western Victoria, along what is now the Douglas Depression (location shown in Fig. 1). As uplift on the Padthaway High (Fig. 1) commenced, it is likely that the ancestral Murray River was initially able to begin to cut a gorge through the uplifted region. But the river's reduced hydrologic capacity, as a result of gorge cutting, meant that palaeo megalake Bungunnia began to form behind the Padthaway High dam wall.

Acknowledgements

This contribution was funded by the Australian Research Council project DP0558705. We are grateful to Russell Drysdale (now at the University of Melbourne) for stable isotope analyses and thank Peter Kershaw, Paul Hesse and an anonymous reviewer for comments that improved the manuscript.

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