Downloaded By: [sjgall@unimelb.edu.au] At: 04:50 16 July 2007 Revised Oligo-Miocene stratigraphy of the Murray Basin, southeast Australia S. J. GALLAGHER* AND T. L. GOURLEY School of Earth Sciences, University of Melbourne, Vic 3010, Australia. This paper reconciles the outcrop and subsurface Oligo-Miocene marine stratigraphy of the Murray Basin to arrive at a basin-wide correlation. The initial open-marine transgressive marl and clay facies of the basin belong to the Ettrick Formation, a unit that is also laterally equivalent to the calcisiltite and calcarenite that are present on the surface and in the subsurface in the basin. Since this unit is lithologically indistinguishable from the previously identified Victorian Oligocene to Middle Miocene Winnambool Formation, we suggest that the latter unit be replaced as a stratigraphic unit in the basin. The majority of outcropping units in South Australia can be recognised lithostratigraphically in the subsurface of Victoria. The calcisiltite, calcarenite, marl and clay of the lower and upper Mannum, the Finniss, Glenforslan, Bryant Creek and Pata Formations can be distinguished in the Victorian Lower to Middle Miocene Duddo Limestone. Our work suggests that the lithostratigraphic unit the Duddo Limestone can also be replaced as a stratigraphic unit in the region. Direct correlation to the outcropping detailed stratigraphy in South Australia has been achieved across into Victoria for the first time. The correlations will allow a better understanding of the geometry of the Oligo-Miocene strata in the Murray Basin allowing improved hydrostratigraphy. It will also form the basis for ongoing palaeoenvironmental and palaeoceanographic studies in the area. KEY WORDS: biostratigraphy, foraminifers, Miocene, Murray Basin, Murray Supergroup, Oligocene, South Australia, stratigraphy, Victoria. INTRODUCTION The Murray Basin is a large intracratonic basin on the southeastern Australian margin that extends over 300 000 km 2 in the three states: South Australia, New South Wales and Victoria (Figure 1). In the published literature, there is a clear resolution difference between the stratigraphic schemes of the subsurface Victorian and the combined outcrop and subsurface stratigraphies of South Australia and Victoria (Figure 2). For example, Ludbrook (1961) and Lukasik and James (1998) were able to subdivide in detail the stratigraphy of the outcrop and subsurface strata in South Australia (Figure 2). How- ever, in Victoria, constrained by the lack of outcrop, Lawrence (1966) and Lawrence and Abele (1976) erected the stratigraphy shown in Figure 2. Evans and Kellett (1989) and Brown and Stephenson (1991) subsequently summarised and simplified the stratigraphy of the Murray Basin, and this is incorporated into Figure 2. Even with the attempts to reconcile the stratigraphy by Brown and Stephenson (1991), stratigraphic correla- tion of the Oligo-Miocene strata across the South Australian and Victorian border remains problematic. Part of the problem is the nature of the stratigraphic record as alluded to above. In addition, few workers have tried to integrate these datasets. In our work, we use facies, wireline log and foraminiferal analyses of several subsurface sections of Oligo-Miocene strata of the Victorian Murray Basin with additional strati- graphic data from South Australia to: (i) erect a detailed regional lithostratigraphy; and (ii) provide a biostrati- graphic framework for this stratigraphy. GEOLOGICAL SETTING The Murray Basin is one of several Cenozoic basins along the southern Australian margin that formed after the breakup of Gondwana during northwards drift of Australia from Antarctica. Despite its lateral extent, a maximum of 600 m of Cenozoic carbonate and siliciclast- ic strata is present in the basin. The main depocentre of the basin is in the central-west region near Renmark and Wentworth (Figure 1). Marine sedimentation commenced in the Murray Basin in the Late Oligocene when a wide, semi-protected and shallow sea formed behind the Padthaway Ridge (Lukasik et al. 2000). Dark grey-greenish, glauconitic, calcareous clay and marl of the Ettrick Formation were deposited from Late Oligocene to lower Early Miocene time. In Victoria, the Ettrick Formation reaches a thickness of up to 40 m and becomes more calcareous and less clay-rich towards the northwest (Lawrence & Abele 1976). Towards the north and east, the Ettrick *Corresponding author: sjgall@unimelb.edu.au Australian Journal of Earth Sciences (2007) 54, (837 – 849) ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2007 Geological Society of Australia DOI: 10.1080/08120090701392705