Discussion Paper - Part A - Victorian Environmental Assessment ...

outcropping and subsurface sequences of volcanogenicand fluvial sandstones and siltstones (Otway, Bass andGippsland basins) and minor occurrences within theMurray Basin at depth. During the Late Cretaceous(95–65 Ma), the mountains of the Great Dividing Rangerose as the rift between Australian and Antarcticaspread. Downwarping on the northwestern side of therange enhanced the broad depression of the MurrayBasin which rests upon basement rocks with a hillytopography much like the adjoining landscape. Streamdeposits on both sides of the Great Dividing Rangecontain rich alluvial gold deposits accumulated duringthe prolonged erosion following the Permian glaciation.Subsequent basalt eruption, especially south of theDivide during the Cainozoic, buried the river sedimentsand gold-rich gravels that are now known as ‘deepleads’.CainozoicSedimentation in the Murray Basin and the riversdraining into it was largely regulated by sea levelchanges with fault movements throughout theCainozoic influencing deposition patterns and hydrologymore generally (Figure 2.3). For example, movement onthe Tawonga Fault influenced the development of theOvens and King Rivers in the eastern part of the studyarea. Across much of the study area only small tectonicmovement has occurred in the Cainozoic, although evensmall vertical offsets have resulted in major deflections ofdrainage lines on the flat land of northern Victoria. Themost significant of these is the Cadell Fault near Echucawhere the fault block diverted both the Murray andGoulburn rivers prior to 35,000 years ago (see chapter 3Geomorphology and Land Systems).Cainozoic sediments, of both marine and terrestrialorigin, blanket and infill the basement surface acrossnearly the entire study area (Map 2.1). Conditions foraccumulation of sediments were favourable during threemajor episodes of the Cainozoic era (65–0 Ma) reflectedin three layers comprising the Renmark, Murray andWunghnu groups. These sediments consist of fluviallacustrinesandstones, claystones and minor coals of theRenmark Group (Palaeocene–Early Oligocene), thepredominantly marine carbonate Murray Group(Oligocene–Middle Miocene), overlain unconformably bythe Late Miocene–Pliocene marine to fluvial WunghnuGroup. In the eastern part of the basin the marineMurray Group is replaced by non-marine sediments.In general, Palaeocene to Pliocene sediments havelimited outcrop in the study area and are largelyblanketed by the younger Quaternary sediments of theriverine plains, and the aeolian (wind-blown) sequencesof the dunefields and sand plains (Map 2.1).Renmark GroupThe Palaeocene to Oligocene Renmark Group restsunconformably on older basement rocks, infilling thepre-existing topography. The sequence has variablethicknesses up to a maximum of 300 m in the Milduraarea, and up to 60 m in the palaeo-drainage systems ofthe Loddon, Campaspe and Goulburn rivers but less overthe palaeo-topographic highs. Renmark Group sandunits are important groundwater recharge sources foroverlying aquifers. Brown coal seams occur at the top ofthe Renmark Group, especially in the Kerang,Torrumbarry and Echuca areas (see chapter 16 EarthResources). The Renmark Group is overlain by the marineOligocene–Miocene Murray Group or its non-marineequivalents and does not outcrop in the study area.Murray GroupThe Murray Group refers to all marine carbonatesediments between the top of the Renmark Group andthe base of the Late Miocene or Early Pliocene WunghnuGroup. The limited Murray Group outcrop consistspredominantly of exposures along river incisions andover the Gredgwin Ridge south of Kerang. It is notpresent in the Kerang–Cohuna and Goulburn areas ofthe Murray Basin and non-marine sediment equivalentsoccur to the east of this region. A complex mix ofmarine and non-marine sediments occurs across a poorlydefined boundary between these areas. The MurrayGroup comprises a mixture of marine muds, clays andlimestones (Ettrick Marl, Duddo Limestone, Geera Clay)and their non-marine equivalents such as the CalivilFormation. The Geera Clay is an important barrier togroundwater and provides a salt source to underlyingRenmark Group aquifers, as well as restricting upwardsmovinggroundwater.Wunghnu GroupThe deposition of the Late Miocene to PlioceneWunghnu Group commenced with a short-lived marinetransgression followed by a slower regression of the sea.Sediments deposited during this time include marineunits such as the green micaceous glauconitic marls ofthe Bookpurnong Beds, marginal marine units such asthe Parilla Sand and non-marine equivalents such as theShepparton Formation. These units are allunconformable on the underlying rocks. Theunconformity is likely to represent a LateMiocene–Pliocene tectonic event similar to thatidentified in other areas of Victoria. This sequencecomprises the majority of outcropping rocks throughoutthe study area and is therefore described in detail below.The Wunghnu Group is highly variable with both marineand non-marine units from changing conditions of theMurray Basin including increasing climate variability(which affected both drainage and sediment derivedfrom the Eastern Highlands) and changing sea levels.Sandstone aquifers and claystone units within thissequence have varying degrees of groundwaterinterconnection, both vertically and horizontally,providing significant groundwater resources in theeastern part of the Murray Basin.The Pliocene Parilla Sand forms a series of sub-parallelridges separated by swales across the western half of theMurray Basin in Victoria. It extends into the subsurfaceforming widespread sheet sandstones. These variablyferruginous quartz sands are cross-bedded, medium tofine-grained, and contain some bands of heavy minerals.Individual ridges can be up to 50 m high and severalkilometres wide and extend for several hundredkilometres in a north-northwest direction. Beneath theLoddon Plains the formation is subdivided into theKerang Sand (which laterally replaces the BookpurnongBeds), an intermediate Tragowel Clay Member and theupper Wandella Sandstone, which is the mainoutcropping unit.14 River Red Gum Forests Investigation 2006