The Murray Basin extends over 300 000 km of inland southeastern Australia, is flanked by subdued mountain ranges, and forms a low-lying saucer-shaped basin with thin flat-lying Cainozoic sediments. Over the past 100 years, the Murray Basin has become one of the most important agricultural regions in Australia. Unfortunately it is also a closed groundwater basin, which consists of a thin sequence of sediments containing a number of aquifer systems, with little capacity to absorb additional recharge. Irrigation and clearing of natural vegetation have increased recharge to these aquifer systems. Resultant rising groundwater levels and discharge of saline water into the landscapes and river systems of the basin, have created salinity problems that threaten to have an increasingly adverse impact on both the regional economy and natural environments. Many of the reasons for salinisation lie in the subsurface geology, and can be related to the development of the structural and stratigraphic framework of the basin over the past 60 Ma. Knowledge of these is a prerequisite to understanding hydrogeological systems and processes contributing to the salinity problem. This document summarises the geology of the Murray Basin.
Beneath the Murray Basin, geophysical and borehole evidence indicates that folded and partly metamorphosed Proterozoic and Lower Palaeozoic basement is block-faulted, and that the Cainozoic sequence is locally underlain by poorly defined infrabasins preserved in graben-like troughs. These contain thick sequences of Devonian to Lower Carboniferous sedimentary rock and discontinuous, erosional remnants of Upper Carboniferous, Permian, Triassic and Cretaceous platform-cover sediments.
The Cainozoic succession of the Murray Basin forms an extensive blanket of sediment, with a maximum thickness of about 600 m preserved in the deeper, central-western parts of the basin. A subsidiary depocentre with over 400 m of sediment underlies the central-west Riverine Plain, but in most northern, eastern, and southern parts of the basin the sediment succession is generally less than 200-300 m thick, and could be more accurately described as forming a thin platform-cover succession rather than a true basinal sequence. Within the Tertiary succession at least three major depositional sequences (Paleocene Eocene to Lower Oligocene, Oligocene Middle Miocene, and Upper Miocene Pliocene) have been identified. Each sequence consists of a package of genetically related formations separated by disconformities. Poorly consolidated, non-marine sand, silt, clay, and carbonaceous sedimentary rocks predominate in the east and north, but each of the depositional sequences includes weakly lithified marine sedimentary rocks in central and southwestern areas. The stratigraphy translates into a number of regional aquifer systems, confining layers and permeability barriers to groundwater flow, each with distinctive characteristics.
In the Mallee region of the west, the Tertiary sediments of the Murray Basin are almost entirely concealed beneath a mainly fossil' arid and semi-arid landscape of Quaternary aeolian dunefields, with minor fluvial and lacustrine morphostratigraphic units. Farther east, where the basin and adjacent highlands are drained by the Murray, Murrumbidgee and Lachlan Rivers, the Tertiary sequence underlies flat-lying fluvio-lacustrine and minor aeolian sediments of the semi-arid landscape of the Riverine Plain. Within the Mallee and Riverine Plain landscapes, active and fossiV (currently inactive) groundwater discharge lake complexes can be identified by characteristic assemblages of Upper Quaternary sediments forming stranded lake floors, gypsum flats, salinas, gypsum and clay pellet dunes and lunettes. These have developed within low-lying areas during the past 0.5 Ma. Their extent indicates the presence of widespread salinisation under 'natural9 conditions at times in the recent geologic past.
The main emphasis of the study is on improving our understanding of the geological context of groundwater and surface discharge in the Murray Basin, but at an early stage the scope of the study was expanded to include reference to other mineral resources. These include Cainozoic limestone, alluvial gold, kaolin, heavy minerals, gypsum and halite deposits. The Tertiary succession contains extensive deposits of currently sub-economic brown coal, underlain by Upper Permian coal in the Oaklands Infrabasin in New South Wales. The Cainozoic Murray Basin is not prospective for hydrocarbons, but several concealed pre-Cainozoic infrabasins remain poorly investigated.
