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

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.