Revised stratigraphy of the Blanchetown Clay, Murray Basin: age constraints on the evolution of paleo Lake Bungunnia

Abstract

Paleo Lake Bungunnia covered more than 40 000 km² of southern Australia during the Plio-Pleistocene, although the age and origin of the lake remain controversial. The Blanchetown Clay is the main depositional unit and outcrop at Nampoo Station in far-western New South Wales provides the most continuous lacustrine section preserved in the basin. Here the Blanchetown Clay represents the maximum lake fill and comprises: (i) a basal well-sorted sand with interbedded clay (Chowilla Sand), representing initial flooding at the time of lake formation; (ii) a thick sequence of green-grey clay comprised dominantly of kaolinite and illite, with the apparently cyclic occurrence of illite interpreted to represent cool and dry glacial climatic intervals; and (iii) a 2.6 m-thick sequence of finely laminated silt and silty clay, here defined as the Nampoo Member of the Blanchetown Clay. New magnetostratigraphic data constrain the age of the oldest lake sediments to be younger than 2.581 Ma (Matuyama–Gauss boundary) and probably as young as 2.4 Ma. This age is significantly younger than the age of 3.2 Ma previously suggested for lake formation. The youngest Blanchetown Clay is older than 0.781 Ma (Brunhes–Matuyama boundary) and probably as old as 1.2 Ma. The Nampoo Station section provides a framework for the construction of a regional Plio-Pleistocene stratigraphy in the Murray Basin.

Key Words:

• Blanchetown Clay
• climate
• lacustrine sediments
• Lake Bungunnia
• magnetostratigraphy
• Nampoo Member
• Plio-Pleistocene

Acknowledgements

We are grateful to Paul Cohrs for his hospitality in allowing access to Nampoo Station. Jonathan Giddings and Barbara Wagstaff are thanked for assistance in the field and Redmond Lloyd is thanked for his assistance with XRD analysis. SM acknowledges support of a University of Melbourne Faculty of Science Centenary Research Fellowship and MWW and SJG acknowledge the support of Australian Research Council Discovery Grant DP0558705. We thank Brian McGowran and Tony Stephenson for their reviews of the manuscript.