Revised Oligo-Miocene stratigraphy of the Murray Basin, southeast Australia

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1 Australian Journal of Earth Sciences (2007) 54, ( ) 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 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). However, 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 correlation 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 stratigraphic data from South Australia to: (i) erect a detailed regional lithostratigraphy; and (ii) provide a biostratigraphic 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 siliciclastic 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 ISSN print/issn online Ó 2007 Geological Society of Australia DOI: /

2 838 S. J. Gallagher and T. L. Gourley Figure 1 Location of the outcrop sections (Lukasik & James 1998) and boreholes used for the stratigraphic analyses in this work. The isopachs of the Murray Supergroup and structural features are adapted from Brown and Radke (1989). The inset shows the palaeogeographic and oceanographic setting of the Miocene Murray Basin (adapted from Gallagher & Holdgate 2000; Gallagher et al. 2001). Formation passes laterally into shallow to marginal marine strata of the Winnambool Formation and Geera Clay (Lawrence 1966; Brown & Radke 1989; Brown & Stephenson 1986). The Late Oligocene (Janjukian) to Middle Miocene (Bairnsdalian) Geera Clay is a bioturbated, dark-grey to black, slightly calcareous and poorly fossiliferous clay age equivalent to the Winnambool and Ettrick Formations (Lawrence & Abele 1976). Stratigraphic interpretation is difficult near the eastern margin of the basin where interbedded marginal and non-marine units are present (Macumber 1991; Macphail & Kellett 1993). Murray Supergroup carbonates were deposited from Late Oligocene to Middle Miocene time in open marine conditions. Lukasik et al. (2000) suggested that the Murray Basin was an epeiric ramp allowing cool-water carbonate facies to be deposited in an inland sea. The carbonates are light-grey to yellow, bryozoan-rich calcisiltite and calcarenite with dark-grey, cherty fossiliferous units near the base (Lawrence 1975). In Victoria, the Oligo-Miocene carbonate facies are assigned to the Duddo Limestone (Figure 2). In South Australia, the carbonates have been subdivided into several formally identified members, formations and groups by Ludbrook (1961) and Lukasik and James (1998) (Figure 2). The Duddo Limestone reaches a maximum thickness of 130 m in the northwestern corner of Victoria in the deeper part of the basin (Holdgate & Gallagher 2003). Lawrence and Abele (1976) assigned a Late Oligocene Janjukian to early Middle Miocene Bairnsdalian (P21/22 to M3 planktonic zones) age to this unit. A regression during the Middle Miocene caused the more marginal marine units of the Winnambool Formation and Geera Clay to prograde seawards over the Murray Supergroup carbonate. Regional uplift and erosion from Late Miocene to Pliocene time created an unconformity surface known as the Mologa weathering surface (Brown 1985) in the Murray Basin associated with the Miocene Pliocene unconformity in southeast Australia (Dickinson et al. 2001, 2002). METHODS The outcrops studied by Lukasik and James (1998) in South Australia were sampled (and their carbonate percentage determined) and compared with the subsurface stratigraphy of North Renmark 1 (Figure 3)

3 Murray Basin Oligo-Miocene stratigraphy 839 Figure 2 (a) Oligo-Miocene planktonic foraminiferal biozonal scheme used in this study. This is modified from McGowran et al. (1997), Gallagher et al. (2001), and Holdgate and Gallagher (2003) tied to the chronology of the Gradstein et al. (2004) timescale. The SAN (Southern Australian Zones) are shown (Li et al. 2003, 2004). The Australian regional stages and large foraminiferal ingressions are adapted from Li et al. (1996) and McGowran et al. (2004). (b) Stratigraphy of the Oligo-Miocene outcrops in South Australia (Ludbrook 1961; Lukasik & James 1998) tied with the bio- and chronostratigraphic interpretation of Li and McGowran (1999). The stratigraphy of the Victorian side is shown adapted from Lawrence and Abele (1976). The basin-wide stratigraphic correlation of this work is shown as an adaptation of the Brown and Stephenson (1991) stratigraphy.

4 840 S. J. Gallagher and T. L. Gourley Figure 3 Correlation of the outcrop sections in South Australia (adapted from Lukasik & James 1998) with the subsurface section of Renmark North 1 (age and facies modified from Ludbrook in BMR 1964). The zonal ages of the outcrop are from Li and McGowran (1999). The correlation is aided by carbonate percentage profiles of four outcrop sections. The key to facies on Figures 4 8 is indicated.

5 (BMR 1964) and two bores near Waikerie (Figure 4) (Lindsay & Bonnett 1973), which allows direct correlation with the Victorian stratigraphy. Four boreholes were analysed in the Victorian subsurface (Figures 5 8). Carbonate, facies and quantitative foraminiferal analyses were carried out on the Olney 1 (24 core samples), Mildura West 1 (18 ditch cutting samples), Duddo 1 (24 ditch cuttings) and Mournpoul 1 (16 core samples) boreholes. The correlation of each of these boreholes is shown on Figures 5 8. Over foraminifers were studied, including 300 benthonic species and 40 planktonic species (Gourley 2005); the detailed biofacies framework will be published in a later paper. These data are used to erect a detailed biostratigraphic framework and were integrated with wireline log data to refine correlation. The wireline log data were kindly donated by Wiltshire Geoservices. BIOSTRATIGRAPHY Planktonic foraminiferal biostratigraphy provides a chronological framework for correlations in this paper. Nearly 40 planktonic species were identified, comprising up to 52% of the total foraminiferal assemblage with an average yield of 11%. They include planktonic species of the Globigerinoides Praeorbulina Orbulina bioseries. The biostratigraphic zonation used in this study is shown in Figure 2a. The biostratigraphy of Olney 1, Mildura West 1, Duddo 1 and Mournpoul 1 is shown in Figure 4. The strata in these sections range in age from the lower Late Oligocene (equivalent to the P21 Zone and the I2 Taylor Zonule) to the lower Middle Miocene (M6 Zone/E1 Zonule). An unconformity in Olney 1, Duddo 1 and possibly in Mournpoul 1, separates lower Middle Miocene strata from overlying Upper Miocene to Lower Pliocene strata. Benthonic foraminifers are the most common microfossils present. The presence of benthonic taxa of limited stratigraphic range allows significant benthonic taxa and associations (bioevents) to be identified. These are also shown on Figure 4 and detailed in Gourley (2005). Bolivinopsis cubensis is present in the Ettrick Marl of the Olney 1 (Figure 5). Associated plankton include Turborotalia euapertura, Guembelitria triseriata and Chiloguembelina cubensis, suggesting a Late Oligocene (P21 Zone) age. Lindsay and Bonnett (1973) and Lindsay (1985) also recorded C. cubensis in the Ettrick Marl at Waikerie in South Australia. Ludbrook (1961) first described Crespinella spp. from Early Miocene carbonates of the Mannum Formation in South Australia. This taxon is rare and appears to be endemic to the Murray Basin region. Lindsay (1965) found this taxon in Batesfordian strata of the Morgan Limestone in the Blanchetown 6, Sunlands 2, Waikerie 1, Ramco Heights 1 boreholes and the Waikerie Pumping Station cliff section, where it is associated with the larger benthonic foraminifers Operculina victoriensis, Amphistegina lessonii, Gypsina howchini and Lepidocyclina howchini. In this study, Crespinella spp. are Murray Basin Oligo-Miocene stratigraphy 841 present in coarse-grained calcarenite facies of Mildura West 1 and Duddo 1 (Figure 5) with plankton of latest Early Miocene age (M3 M5a Zone) and the larger foraminifers Operculina victoriensis and Amphistegina lessonii. Ehrenbergina marwicki is a semi-endemic taxon (sensu Li et al. 1996) present in southern Australia in strata of Early Miocene (M3) to Late Miocene (M13B M14/PL1) age. Acmes of this taxon are present in Olney 1 and Duddo 1 in strata of the M5a Zone (Figure 4), and in Batesford Quarry (Port Phillip Basin, Victoria) in the M6 Zone (Gourley & Gallagher 2004). Peaks of this species are indicators of upwelling events in strata of similar age from Gippsland (Li & McGowran 1994; Holdgate & Gallagher 1997; Gallagher et al. 2001). Abundant Amphistegina lessonii are present in strata of the M5a Zone in the Murray Basin (Figure 5). This correlates to the upper Amphistegina acme of Lindsay (1976, 1985) and Amphistegina maxima in Gippsland (Li & McGowran 1994; Holdgate & Gallagher 1997; Gallagher et al. 2001). Amphistegina lessonii is often associated with other large migratory taxa (sensu Li et al. 1996) such as Operculina victoriensis. Benthonic foraminifer associations can also be used as isochronous bioevents for local and regional biostratigraphic correlation. For example, Austrotrillina howchini, Heterolepa victoriensis, Marginopora vertebralis and Crespinella are present in most sections at a similar stratigraphic level (Figure 5). In the South Australian Murray Basin, Ludbrook (in BMR 1964) recorded an A. howchini, O. victoriensis and H. victoriensis association from the Pata Limestone in North Renmark 1, m. Lindsay (1965) identified O. victoriensis, C. umbonifera and M. vertebralis from the Pata Limestone in Ramco Heights 1, m in South Australia. The associated plankton include Orbulina suturalis, Praeorbulina glomerosa or Orbulina universa, indicating that this association has an early Middle Miocene, M5b/M6 Zone, age. Figure 5 shows the biostratigraphic and correlation of the four boreholes studied, using planktonic M Zones with key benthonic and planktonic foraminiferal bioevents. OLIGO-MIOCENE STRATIGRAPHY OF THE MURRAY BASIN The marker bed and biofacies events used by Ludbrook (1961), Lindsay and Bonnett (1973), Lukasik and James (1998) and Lukasik et al. (2000) in South Australian Murray Basin stratigraphy are incorporated into our work to facilitate subsurface correlation to the Victorian stratigraphy. Our stratigraphic analyses suggest that subsurface Duddo Limestone in Victoria can be subdivided into the formations and members recognised in South Australia by Ludbrook (1961), Lukasik and James (1998) and Lukasik et al. (2000) (Figures 5 8). The revised stratigraphy of the Murray Basin is summarised in Figure 2 and outlined below.

6 842 S. J. Gallagher and T. L. Gourley Figure 4 Correlation of the outcrop sections at Waikerie, South Australia (adapted from Lukasik & James 1998) with the subsurface section of the 27W and 28W bores 2 km south (age and facies modified from Lindsay & Bonnett 1973). The zonal ages of the outcrop are from Li and McGowran (1999). Ettrick and Winnambool Formations The Ettrick and Winnambool Formations are so lithologically similar that Brown and Stephenson (1991 p. 122) suggested that these two units are part of one and the same time-transgressive unit. However, Brown and Stephenson (1991) also suggested that the Oligocene part of this marine marl/clay unit be assigned to the Ettrick Formation and the Miocene part to the Winnambool Formation. We believe this lithostratigraphic distinction is difficult to apply in a practical sense when logging core or cuttings based on lithofacies. The problem is that a formation should be the

7 Murray Basin Oligo-Miocene stratigraphy 843 Figure 5 Biostratigraphy of four boreholes in the Victorian Murray Basin. The locations of these bores are shown on Figure 1.

8 844 S. J. Gallagher and T. L. Gourley Figure 6 Correlation of the Olney 1 with North Renmark 1. smallest mappable unit (Rawson et al p. 3), and if this unit relies on microfossil definition for its identification, it is a bio-lithostratigraphic unit. We suggest the two marl units be assigned to one lithostratigraphic unit, the Ettrick Formation, and that the Winnambool Formation be replaced as a stratigraphic unit. The base of the Ettrick Formation is characterised by a transgressive high gamma-ray foraminiferal clay, marl and limey marl overlying the fluvial Renmark Group. To the east, this unit grades laterally into the marginal marine

9 Murray Basin Oligo-Miocene stratigraphy 845 Figure 7 Northwest southeast correlation diagram of the Victorian Oligo-Miocene strata.

10 846 S. J. Gallagher and T. L. Gourley Figure 8 North south correlation of the Oligo-Miocene strata of the Victorian Murray Basin. The location of Duddo 1 is shown on Figure 1.

11 Geera Clay. The unit is overlain and equivalent to the limestone and marl of Mannum, Finniss, Glenforslan, Cadell, Pata and Bryant Creek Formations. The Ettrick Formation has a minimum thickness of *13 m thickening to over 45 m. Murray Supergroup carbonates Several carbonate units are recognised in the Murray Basin. Lawrence and Abele (1976) distinguished the Duddo Limestone with an average thickness 4100 m in Victoria. This unit is the direct lateral equivalent to Mannum, Finniss, Glenforslan, Cadell, Pata and Bryant Creek Formation carbonates recognised by Ludbrook (1961) and Lukasik and James (1998) in South Australia (Figure 2b). Our analyses shows that the South Australian units can be mapped in the Victorian subsurface, although some modification of their definitions is required to reconcile the stratigraphy. We also suggest that the Duddo Limestone should be abandoned as a lithostratigraphic unit. The reconciled stratigraphy of the carbonate units of the Murray Basin is described below. Mannum Formation The base of the Mannum Formation is identified as the first calcarenite/calcisiltite conformably overlying the marl/clay of the Ettrick Formation. This limestone unit is laterally equivalent to the Ettrick Formation marl in the eastern part of the basin. The unit is conformably overlain by the clay, marl and calcarenite units of the Finniss Formation of the Morgan Group in outcrop (Lukasik & James 1998) and by marl and marly limestone in the subsurface. The age of this formation ranges from Late Oligocene to Early Miocene. Two units, which can be mapped in the subsurface, were distinguished by Lukasik and James (1998) in this formation: the lower and upper Mannum Formation. The lower Mannum Formation has variable facies in outcrop ranging from cross-bedded calcarenite to marl. In the subsurface, marl or marly limestone dominate with minor calcisiltite and calcarenite units (defining its base). Gamma-ray values reduce markedly upsection in this unit associated with a spiky variable neutron log character reflecting the presence of strongly cemented horizons. The average thickness of lower Mannum Formation is *26 m. Its maximum thickness is 42 m at Olney 1; elsewhere, it thins near the edge of the basin. The presence of Operculina victoriensis in outcrop and in Renmark North 1 in South Australia is used to identify the lower/upper Mannum Formation boundary approximating the Olio-Miocene boundary (Ludbrook 1961, in BMR 1964; Lukasik & James 1998). This foraminifer is readily identifiable in cuttings and, if present, could possibly be used as a lithostratigraphic marker. However, this taxon is present near the top of the Mannum Formation in the Miocene (M2 Zone) in Olney 1; in the Late Oligocene Ettrick Formation in Mildura West 1 (Figure 7); and in the Late Oligocene lower Mannum Formation in Duddo 1. Ingressions of larger foraminifers have been used for lithostratigraphic correlation in South Australia, when the age Murray Basin Oligo-Miocene stratigraphy 847 of the surrounding strata is known from further fossil investigations. However, this represents a bio-lithostratigraphic approach to lithostratigraphy. We suggest that ingressions of this taxon are facies-controlled and not stratigraphic, so they are not used here to identify our lithostratigraphy in the subsurface. The boundary between the upper and lower Mannum Formation is here redefined as the first thick (20 60 m) cemented calcarenite/calcisiltite units with discontinuous dolomite units overlying the predominantly marly and fine-grained lower Mannum Formation. At this boundary, Lukasik and James (1998) reported one to three strongly cemented horizons interpreted as hardgrounds as one of their boundary criteria (in addition to the ingression of O. victoriensis). This lithostratigraphic boundary is close to, but does not necessarily coincide with, the Oligo-Miocene boundary in the subsurface. The lower part of the upper Mannum Formation has high carbonate and low gamma values with a spiky variable neutron log character reflecting strongly cemented horizons and the dolomitisation (Figures 5, 6). The upper part shows low gamma-ray and neutron variability due to the relative lack of cemented horizons. The upper contact of the Mannum Formation is defined below. Morgan Group Lukasik and James (1998) assigned the Finniss, Glenforslan, Cadell, Bryant Creek and Pata Formations to the Early to Middle Miocene Morgan Group in outcrop (Figures 2b, 3): several of these units can be distinguished in Victoria (Figures 6 8). The Finniss Formation is a marl-dominated unit that defines the base of the Morgan Group. This unit is laterally continuous and relatively thick (50 m) in the subsurface thinning to 4 m in outcrop in South Australia where local clay and calcarenite units are present (the Cowirra Clay and Potree Carbonate Members). The Finniss Formation merges laterally with Ettrick Formation in Mournpoul 1 (Figure 7). The unit yields high gamma values compared with the underlying Mannum Formation. Its neutron response is quite variable, reflecting interbedded cemented horizons. Li and McGowran (1999) interpreted an latest Early Miocene M4 age for this unit in outcrop. In the subsurface, the formation ranges from M4 to M5a in the uppermost Lower Miocene, although in Duddo 1 it is slightly older and thinner (M2 M3: Figure 8). The base of the Glenforslan Formation is identified by the first foraminiferal and bryozoan limestone overlying the marl-dominated Finniss Formation. The Lepidocyclina ingression of Lukasik and James (1998), identified in outcrop, is present in the subsurface in North Renmark 1 and Olney 1 (Figures 3, 7), although the taxon is absent in other sections. The calcarenite and calcisiltite of this unit are interbedded with marl in outcrop and in Olney 1, and in other sections it is represented by a well-identified calcarenite/calcisiltite unit that varies in thickness from 5 m (Mildura West 1: Figure 7) to 40 m (Duddo 1: Figure 8). The gamma-ray and neutron character of the calcarenite/calcisiltite have a bell-shape where relatively low gamma-ray

12 848 S. J. Gallagher and T. L. Gourley values correspond with high neutron values. Lukasik and James (1998) suggested a Middle Miocene age for this unit based on the occurrence of Lepidocyclina and other larger benthonic foraminifers (using the foraminiferal dates of Lindsay & Bonnett 1973). However, Li and McGowran (1999) interpreted an M5a M5b Zone age, indicating that the unit outcrops across the Early/ Middle Miocene boundary. Our lithostratigraphic correlation from the outcrop to the Waikerie Bores and North Renmark 1 (Figures 3, 4), based on the dates given by Ludbrook (in BMR 1964) and Lindsay and Bonnett (1973), suggests that the unit has a latest Early Miocene age. In addition, our foraminiferal data suggest that this formation has a latest Early Miocene (M4 M5a) age. It is possible therefore that there is a slightly younger part of this formation outcropping in South Australia compared with the subsurface. The base of the laterally continuous Cadell Formation is the first clay/marl overlying the uppermost cemented calcarenite/calcisiltite of the Glenforslan Formation. A strong gamma peak associated with a low carbonate percentage 5 m thick clay can readily be used to identify the base of this unit in the subsurface (Figures 3, Figures 6 8). Some m of marl overlies the lower clay unit of the Cadell Formation in most subsurface sections. Li and McGowran (1999) interpreted an early Middle Miocene (M5b Zone) age for this unit in outcrop. Our data suggest that this unit may extend into slightly older strata of M5a age and younger strata of M6 age. Improved biostratigraphic analyses may prove this laterally continuous unit to be timeequivalent in all sections. In outcrop, Lukasik and James (1998) identified two Middle Miocene limestone-rich units (the Bryant Creek and the Pata Formation) overlying the marl and clay of the Cadell Formation. Distinguishing these units in the subsurface is difficult, and we have assigned uppermost calcarenite/calcisiltite units overlying the Cadell Formation and underlying the Upper Miocene to Pliocene Bookpurnong beds to undifferentiated Pata/Bryant Creek Formation. These units vary in thickness (10 15 m) and are not present in all sections in the subsurface. The wireline log character of these units is similar to the limestone of the underlying Glenforslan Formation. Li and McGowran (1999) interpreted a Middle Miocene M5b M6 age for the Bryant Creek Formation and an M6 M7 age to the Pata Formation in outcrop. In the subsurface, these carbonates have a similar age, although no section studied extends to the M7 Zone. The top of the carbonate succession is unconformably overlain by the clay-rich Bookpurnong beds that were deposited from the Late Miocene to Pliocene. SUMMARY manifest in figure 28 of Brown and Stephenson (1991) and figure 1 of Lukasik and James (1998). The improved correlation will contribute to a better understanding of the stratal geometry of important groundwater aquifers in the Murray Basin. In addition, the stratigraphic framework will form the basis for ongoing biofacies and palaeoceanographic research (Gourley 2005). CONCLUSIONS Our detailed integrated stratigraphic analysis of the subsurface Oligo-Miocene strata of the Victorian Murray Basin has resulted in the following conclusions. (1) The strata range in age from the Late Oligocene (P21 Zone) to the Middle Miocene (M6 Zone). (2) The initial transgressive facies of the basin belong to the Ettrick Formation, a unit that is also laterally equivalent to the calcisiltite and calcarenite that outcrops and occurs in the subsurface of the basin. Since this unit is lithostratigraphically indistinguishable from the previously identified Winnambool Formation, we suggest that the latter unit be abandoned as a stratigraphic unit. (3) The majority of outcropping units in South Australia can be recognised in the subsurface Victoria. For example, the lower and upper Mannum, the Finniss, Glenforslan, Bryant Creek and Pata Formations can readily be distinguished in the Duddo Limestone. We suggest that the Duddo Limestone be replaced in the subsurface by these regionally mappable units. (4) Direct correlation to the outcropping stratigraphy in South Australia has been achieved for the first time. (5) The correlations will allow a better understanding of the geometry of the Oligo-Miocene strata in the Murray Basin allowing improved hydrostratigraphic correlations. ACKNOWLEDGEMENTS Thanks go to Wiltshire Geological Services for providing wireline data and Geoscience Victoria, for providing access to core and cuttings at the Werribee Core Store. Terry Smith (Geoscience Victoria) and Elinor Alexander (PIRSA) are thanked for providing wellcompletion reports for the sections studied. Guy Holdgate and Charles Lawrence provided useful and detailed comments on an earlier draft of this manuscript. The reviews of Tony Stephenson and Brian McGowran improved the text. We acknowledge the financial assistance from ARC Discovery Grant No. DP (Murray Basin) and ARC Discovery Grant No. DP (Southern Gateways). In this paper, we have used facies, wireline log, and microfossil data to subdivide and correlate the Oligo- Miocene marine strata in the Victorian Murray Basin. The correlations are extended from the outcropping sections studied by Ludbrook (1961) and Lukasik and James (1998) in South Australia. This work has reconciled the lack of stratigraphic resolution that was REFERENCES BMR Drilling operations in the Murray Basin, New South Wales and South Australia Petroleum Search Subsidies Act Publication 52. BROWN C. M Murray Basin, southeastern Australia: stratigraphy and resource potential a synopsis. Bureau of Mineral Resources Report 264.

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