Bet Bet Geology
|Geological mapping shown here comprises a combination of four 1:100 000 geological mapsheets, with various authors. It has been generalised and reproduced here from the Geological Survey of Victoria (GSV) geoscientific data set. Relevant references for the four studies (together with available links to information about them on DPIs website) are shown below:|
|The Bet Bet catchment is dominated by Cambrian-Ordovician deep marine metasediments that form the basement rock. These are marine sediments (including shales and sandstones) that were laid down in deep marine troughs and subsequently extensively folded, faulted and uplifted.|
In the Devonian period large masses (plutons) of granite and granodiorite were intruded through areas of the Ordovician sediments. Such plutons occur in the vicinity of Mt Hooghly, Dunluce, Ben More, Ben Major and Mt Beckworth. Contact metamorphism (caused by intense heat) occurred to some of the Ordovican rocks adjacent to the granite intrusions, resulting in the sedimentary rock becoming significantly altered and hardened (e.g. mudstones or shales can be altered into slates, schists or gneiss depending on the intensity of metamorphism). Subsequent erosion has left some of the more resistant metamorphic rock as raised ridges around some of the granite. In the Timor West area, for example, a metamorphic aureole of muscovite, schist, hornfels, slates and quartzites surrounds the granodiorite pluton (the 'Natte Yallock Pluton') - forming Mt Hooghly and the Black Ranges. These ridges rise sharply from the surrounding terrain with slopes exceeding 20%. Skirting these on all sides are colluvial aprons which grade gently (3-8 % slopes) to the alluvial plains (Day 1985).
Ordovician rocks exposed in
a road cutting near Bung Bong.
Exposed granodiorite on western slopes of Mt Hooghly
|Between the late Devonian and Tertiary periods, extensive erosion occurred and thousands of metres of sediments were worn away and re-deposited. This would have exposed more of the granite plutons. In the Tertiary period, deep weathering of the older marine sediments and granites occurred. This resulted in lateritic soil profiles developing (although in this area somewhat poorly developed). These lateritic profiles have since been extensively eroded. From the late Tertiary to early Quaternary period, further uplift occurred in the highlands which initiated further significant erosion and the development of large fast flowing streams – originating in the highlands and flowed northwards.|
|Extensive areas of basalt were extruded in the late Tertiary and early Quaternary periods. These are often referred to as the ‘Newer Volcanics’ on geology maps. These volcanic deposits covered large areas of ancestral stream-bed gravels as the lava flowed down former river valleys. Also, areas of older marine sediments and lower areas of granite outcrop were covered in some areas. The volcanic deposits are normally less than 60 metres deep (but can be up to 150 metres deep in some areas) and made up of a series of separate flows. Deposits can be quite extensive from south of Timor West through to Maryborough.|
Basalt flow in foreground – with Bet Bet Creek valley in background. Note gilgai microrelief.
Lower terrace adjacent to present day Bet Bet Creek.
|The most recent deposits have occurred in the late Tertiary to Quaternary period. These include more recent stream activity. Some of this activity was initiated as volcanic lava flows blocked existing streams. Extensive plains were traversed by leveed streams and the pattern of soil formation was controlled by climatic cycles. Commonly a lower terrace exists besides the present day floodplain and an older terrace occurs higher in the landscape. Different soils are present on the different terrace levels (reflecting age of development). Some areas have been covered by Quaternary deposits of both alluvial and windblown (aeolian) sediments (including calcareous clays). Soils can be quite variable in these areas. For example, the sandy clays developed on the alluvial plains adjacent to granitic country differ from the shrinking-swelling clays derived from basaltic sources (Day 1985).|