E. B. Joyce

School of Earth Sciences

The University of Melbourne

VIC 3010

ebj@unimelb.edu.au

The Western Plains sub-province is a part of the Newer Volcanic Province of Southeastern Australia, which includes the Mt Gambier sub-province in southeastern South Australia, and the Western Uplands sub-province in Victoria. In the late Tertiary, a shallow sea reached to the southern margin of the Western Uplands, the east-west spine of Western Victoria. The retreat of this sea exposed for the first time marine deposits of sand, clay and marl, with many parallel ridges marking successive shorelines. Most of these shorelines ridges have only just been recognised on new radiometric, magnetic and digital terrain imagery. As the sea retreated, rivers in the Uplands gradually extended southwards, reworking and partly covering the marine surface with thin alluvial deposits.

About the same time both the Uplands and the Plains gave birth to a new volcanic province, and over the last 5 Ma nearly 400 small, monogenetic, Strombolian/Hawaiian scoria cones, maars and lava shields have been built up, with fluid basalt flows spreading laterally around vents, and often southwards for many kilometres down stream valleys. Where the lava has blocked drainage, lakes and swamps have formed, and on the plateau-like flow surfaces collapse depressions have produced further lakes and swamps.

Some lava flows have been dated by K/Ar and radiocarbon, and changes in landforms, drainage, and soil and regolith development, using new airborne geophysical imagery, can be used to build up a detailed chronosequence of lava flows through the Quaternary. The youngest dated eruption is that of Mt Gambier in nearby southeastern South Australia, at 4000-4300 B.P. by radiocarbon, and perhaps a dozen volcanoes may eventually be found to have erupted within the last 20,000 to 30,000 years. Along a line from Port Fairy to Colac, near the southern limit of volcanic activity, groundwater has interacted with rising magma to cause phreatomagmatic explosions, reaming out some 40 or so deep maar craters and building up rims of ash or tuff. Rain and groundwater have filled many of these craters, and sediment, pollen and microfauna accumulated in the lakes record changing climate during the latter part of the Quaternary.

Many hundreds of small lakes and swamps can also be found on the widespread flat to undulating clay plains, with duplex soils and gilgai, which have developed on lava flows erupted between 3 and 1 Ma ago. On areas of exposed Tertiary marine sediments between the flows larger lakes have built up lunette complexes by deflation; in what is possibly a tectonic depression, Lake Corangamite and nearby lakes make up a major lake-lunette complex. Megafaunal remains were first discovered last century in a number of maar craters and lunette-bounded swamps. Further evidence of neotectonics is found in faults and monoclines affecting lava flows, and the warping of sandy Tertiary shoreline ridges; broad uplifted blocks of Tertiary sediments are associated with several volcanic complexes as at Staughtons Hill.

Regolith-landform mapping of flows, studies of the changing morphology of cones, craters and flows though time, and most recently the recognition and dating of neotectonic activity are providing further details of the history of the development of this striking landscape.