Overview

Location of the Oaklands Basin in NSW

The Oaklands Basin covers approximately 3800 km2 in the Riverina district of southern New South Wales, between the towns of Mulwala, Jerilderie, Griffith, Coleambly, Oaklands and Albury. This Permo-Triassic basin trends north-northwest to south-southeast and is concealed beneath the south-eastern portion of the Murray Basin. It is approximately 25 km wide at Oaklands and approximately 15 km wide at its northern extent. The Oaklands Basin is hosts up to 2000 m of sediment (Davidson & Fahey 2014). The basin overlies the Ovens Graben (Ovens Valley Graben in Victoria) which extends from the Murrumbidgee River, west of Darlington Point, in New South Wales to Wangaratta in Victoria.

The basin was first discovered in 1916 when water drillers discovered coal near Oaklands. Extensive coal exploration drilling was undertaken, mostly in the Oaklands area, including two coal drilling programs by the Department, designed to help define the limits of the basin.

Petroleum exploration within the basin has been minimal, consisting of a small number of geophysical surveys and two stratigraphic wells. In 2009, four lines of high quality 2D seismic were acquired by the Department. This survey further delineated the boundaries of the basin, established the thickness of the sediment fill and imaged the structures of the Ovens Valley Graben along the margin and central portion of the basin.

Stratigraphy

The stratigraphy of the Oaklands Basin is based on exploration and waterbore drilling data, since there are no surface outcrops. The stratigraphy of the deepest portions of the basin is unknown and has been inferred based on seismic data interpretation (Hawley 2003).

Basement to the Oaklands Basin consists of Ordovician metasediments of the Lachlan Orogen, which crop out to the southeast of the basin, and Siluro-Devonian granites, which are exposed in various locations outside the basin. Pre-Permian (Devonian?) sedimentary rocks appear to be thin or absent in the Oaklands Basin (Pitt Research, 1995). It has been suggested by Earth Satellite Corporation (1981) that a thick sequence of Late Devonian and Carboniferous sedimentary rocks may exist in the deeper portions of the Ovens Graben.

The oldest sedimentary unit encountered in the Oaklands Basin, in the Jerilderie 1 well, is the Early Permian Urana Formation. Outside the Oaklands Basin the Urana Formation consists of early-Early Permian and Late Carboniferous sequences. The Urana Formation consists of marine sediments deposited in glaciogene environment (O'Brien 1981) changing to deltaic towards the upper part of the formation. The top of the formation is weathered and unconformably overlain by the Narrow Plain Formation/Coorabin Coal Measures in the Ovens Graben and to the south-east, and by the Jerilderie Formation in the west. The Coorabin Coal Measures were interpreted as meandering stream deposits formed by cyclic fluvial point bar sequences (Morgan 1977).

The Triassic Jerilderie Formation unconformably overlies the Coorabin Coal Measures where it is preserved, the Urana Formation in the west, and basement elsewhere. The Jerilderie Formation was deposited in a fluvial channel and floodplain environment. The formation is very poorly consolidated, friable (Yoo 1997), and lithologically similar to the overlying Tertiary Olney Formation (the oldest unit of the Murray Basin sequence).

There is a disconformity between the mid-Triassic Jerilderie Formation and the oldest Murray Basin Eocene units. The Oaklands Basin is located on the eastern margin of the Murray Basin and the Cenozoic sequence here is non-marine and the marine units that occur elsewhere in the Murray Basin are absent from the Oaklands Basin.

Structure and tectonic settings

East-west cross-section through the Oaklands Basin

The Oaklands Basin is one of a small number of troughs with similar structural trends that underlie the Murray Basin in southern New South Wales (Netherby Trough, Numurkah Trough). These structural trends appear to reflect the structural fabric of the underlying Lachlan Orogen. Other troughs also occur below the Murray Basin, further to the north and northwest, overlying the Kanmantoo Orogen (Hawley 2003).

The dominant structural feature of the Oaklands Basin is the northwest trending Ovens Graben. In New South Wales it is approximately 190 km long by 20 km wide. The Ovens Graben was originally regarded as a simple graben with a continuous western bounding normal fault and an eastern bounding fault that has been offset near Jerilderie (Yoo 1981). However, later interpretations suggest that it is more complex than previously thought. It was suggested by Gunn (2003) that much of the gravity anomaly associated with the Oaklands Basin/Ovens Graben is due to granites within the basement.

The unconformities in the Oaklands Basin may be due to movement on the bounding faults, from the latest Carboniferous to the Early Tertiary (Earth Satellite Corporation, 1981). The poorly consolidated nature of the Late Permian to Cenozoic section of Oaklands Basin suggests that the basin has been isolated from major compressive deformational events, and/or the graben bounding faults have taken up almost all of the compressive deformation that the basin was subjected to. Folding within the basin is gentle and units, particularly in the upper portion of the sedimentary pile, are relatively flat-lying.

Exploration data for the Oaklands Basin

Exploration data related to the Oaklands Basin can be obtained through online services.

For further information
Strategic Resource Assessment and Advice, Geological Survey of NSW
Phone:
+61 (0)2 4063 6726
geoscience.petroleum@geoscience.nsw.gov.au
Postal:
Strategic Resource Assessment and Advice, Geological Survey of NSW, PO Box 344, Hunter Region Mail Centre NSW 2310
Office:
516 High Street Maitland NSW 2320 Map