Palaeomagnetic stratigraphy of Lake Bungunnia: Plio-pleistocene precursor of aridity in the murray basin, Southeastern Australia

The Murray-Darling river system in southeastern Australia formerly flowed into a large lake, Lake Bungunnia, before reaching the sea. Fine-grained clastics of this system, the Blanchetown Clay Formation, extend over an area of about 68,000 km² underlying the Mallee region with its surface array of dunes and salt lakes. Palaeomagnetic analyses are used to date the major hydrologic transition from the expansive freshwater system to the saline deflation playas and dunes that characterize this landscape today. Two sites were selected for sampling; one a cliff section on the left bank of the Murray River at Chowilla, the other from within sediments of Lake Tyrrell, a saline playa cut into sediments of the older freshwater system. Two sequences were sampled at Tyrrell; one by coring to 30 m in the basin centre, the other a composite cliff and 5-m core on the western margin of the playa. Post-Bungunnia sediments representing modern landforms were sampled in both surficial dune-palaeosol units of the Woorinen Formation and in saline sediments, the Tyrrell Beds, within the modern saline playa. The sequence of multiple aeolian units of the Woorinen Formation and gypseous sediments of the Tyrrell Beds are normally magnetized indicating deposition within the past 0.7 million years. By contrast, the uppermost horizons of bedded Blanchetown Clay below a surface zone of pedogenic alteration are reversed. The Brunhes-Matuyama (B-M) boundary, key to the age of draining of Lake Bungunnia by deepening the overflow channel to the sea, is obscured within the zone of pedogenesis separating unaltered Blanchetown Clays from overlying aeolian deposits. The onset of aeolian and saline gypseous deposits characteristic of the arid-zone facies is placed soon after the B-M boundary, about 500,000 years ago. Sequences through the Blanchetown Clay pass down through the Matuyama reversed chron into the Gauss normal chron with the lowermost section on the Tyrrell west bank recording a basal reversed zone of possible Gilbert age. Thus the origins of Lake Bungunnia extend back beyond basal Pleistocene into the late Pliocene. The lake's duration covers more than 2 million years spanning the Plio-Pleistocene boundary in this important region of southeastern Australia.