Introduction
River systems
geology
- general overview of Victoria's geology
- north-west basaltic lava plains

geology and geography of Melbourne

see also:

Introduction

Melbourne sits on the distal reaches of the Yarra River before it flows into the northern shores of Port Phillip Bay
Port Phillip Bay and the surrounding lands are surrounded by hills
the north-western regions are dominated by basaltic rock from relatively recent volcanic laval flows - see Victoria's volcanoes
north-east and east is dominated by the Kinglake and Mt Dandenong hills
the south-eastern regions - the Mornington Peninsula - is dominated by sandy soils - hence called the “sand belt”
the You Yangs to the west are the eroded remnants of a granite landscape

River systems

Melbourne sits on two main rivers which meet at the north-west apex of Port Phillip Bay:
- Yarra River which flows from the Yarra Valley in the north-east of Melbourne
  - receives flows from:
    - Moonee Ponds Creek which:
      - passes east of Tullamarine Airport through Woodlands Estate
      - receives Yuroke Creek in Broadmeadows
      - joins the Yarra River at Docklands
    - Merri Creek which arises near Heathcote Junction and runs southwards, passing east of Wallan and Criagieburn, receiving Aitken Creek at Craigieburn, passing through Coburg, Northcote before joining the Yarra at Yarra Bend in Fairfield
    - Plenty River drains the southern slopes of the western parts of Kinglake NP flows southwards, passing through Whittlesea and joins the Yarra at Rosanna Golf Course region between Templestowe and Heidelberg
    - Diamond Creek arises at Kinglake and drains the southern slopes of the eastern parts of Kinglake NP flows south-westwards, passing through St Andrews and Hurstbridge, joins the Yarra at Lower Eltham
    - Olinda, Lilypond and Steels Creeks near Yering
    - Paul Creek, Watts River
    - Don River and Little Yarra River near Launching Place
    - a number of small tributaries such as Don Creek, Platts Creek, Frenchmans Creek after flows through Warburton
- Maribyrnong River which flows from the north-west
  - the river is formed by the union of Jackson Creek and Deep Creek near Keilor
  - Jacksons Creek:
    - arises from the Bullengarook region draining the southern parts of the central highlands (the northern parts drain into the Campaspe River which flows to the Murray River)
    - it has been dammed to form Rosslyn Reservoir north-west of Gisborne
    - passes through Gisborne and Sunbury townships and through the gorge of the Organ Pipes NP (laval volcanic features line the creek)
    - receives flows from:
      - Riddels Creek which originates on the south-west aspect of Mt Macedon and flows through Macedon
  - Deep Creek:
    - arises on the north-east aspect of the Macedon Ranges to flow to Lancefield, then southwards
further west, there are various streams which flow into Port Phillip Bay:
- Werribee River
  - arises south of Bullarto, passes south-west to Ballan then flow south-easterly and receives flows from Lerderderg River, Coimadai Creek (dammed to form Merrimu Reservoir), Pykes Creek (dammed at Pykes Creek Reservoir)
- Little River
  - arises south of Beremboke, draining the eastern slopes of the northern parts of the Brisbane Ranges

geology

general overview of Victoria's geology

1324-1150mya Musgravian Orogeny in SW Australia in the Grenville era (formed gneiss and granitoids) ¹⁾
1100-750mya, a single global supercontinent Rodinia
c750mya, the Terra Australis Orogen, from the Neoproterozoic to late Palaeozoic, records the beginning of rifting when the Pacific Ocean opened and the initiation of subduction associated with ocean closure. Prior to dispersal the orogen was about 18,000 km long and about 1600 km wide, and incorporated in it were the Tasman Orogen of Australia, the Ross Orogen of Antarctica, the Tuhua Orogen of New Zealand and the Andean Cordillera of South America.
600mya, southern part of the Australian craton is related to the East Antarctic craton
570-530mya, SW parts of the Australian craton collide with the Indian craton to form the Gondana super continent and causing orogeny along the SW coast of Australia, and later, c515mya, the Ross Delamerian Belt orogeny and the Adelaide Fold Belt that created the mountain ranges (including the Flinders Ranges) running north in the Adelaide region which once uplifted were rapidly eroded due to high rainfalls in this period and would form the sandstones and limestones of the Glenelg river basin as well as sandstones of Uluru in the NT, and the shales in submarine Victoria, and in places serpentinite, marble and zircons.
- the Ross Orogen in Antarctica commenced ~25mya before the Delamerian orogeny in Australia, and over only 8 million years created the 7km thick Kanmantoo Group from sediments and magmatism.²⁾
- Victoria and most of NSW as well as NZ were still submarine at this time - the Murray Basin ³⁾
up until the Silurian period around 400 mya, although major submarine volcanic activity was occurring, Victoria had been submerged under ancient oceans and sediment from primeval mountain ranges near the current Flinders Ranges was to form Ordovician shale deposits embedded with graptolite fossils - best seen in the Bullengarook shale quarry
in the Silurian and Devonian periods (446-359mya), the oceans were retreating and half of Victoria became to be above sea level due to the Lachlan Fold Belt orogeny ⁴⁾ which formed much of eastern Victoria and NSW as the Great Dividing Range (in Victoria, the western limit of the LFB is defined by the Stawell-Ararat Fault and to the west of this is the Moornambool Metamorphic Complex which formedc440mya), and this period produced Victoria's most explosive volcanoes - in some parts of central Victoria, the volcanic layers are 1km thick. The remains of this peak vulcanic era now form Mount Macedon, the Dandenong Ranges, the mountainous region between Healesville, Warburton and Eildon (including Mount Donna Buang and the Cerberean Ranges) and near Violet Town, in the northern part of the Strathbogie Ranges.
During the late Devonian period, at 380mya, there was extensive plate movement (the Tabberabberan orogeny) which buckled and folded the Ordovician sediments in a north-south direction as evidenced in Werribee Gorge, while molten magma melted through them to form granite which formed the Ingliston Granites (at the west end of Werribee Gorge), the You Yangs, Anakies and Dog Rocks near Geelong. Over subsequent millenia, further ocean sediments covered these granite formations until sea levels fell around 200mya and they were eventually eroded again to reveal the granite.
300-250 Ma the Terra Australis Orogen ended, the termination being associated the assembly of Pangaea
in the Triassic and Jurassic periods (251-145mya), Victoria was above sea level but covered by glaciers in an ice age (eg. Werribee Gorge). Eruptions were mostly in western and north-east areas. The far majority of dinosaur fossils laid down in this period has long since been eroded.
in the Cretaceous period (145-65mya), earth had a mainly tropical climate due to high CO₂ levels, flowering plants, primitive birds and mammals including marsupials evolve, and Australia started to tear away from Antarctica causing depression between Victoria and Tasmania where lakes and swamps formed
the Tasman Sea's mid-ocean ridge developed between 85-55mya as Australia and Zealandia broke apart during the breakup of supercontinent Gondwana
in the Cenozoic Era (65mya to now), earth's climate began to cool allowing Antarctica's ice caps to again form (~40-50mya), but as the Australian continent drifted further north and became warmer, the formation of the Tasman Sea allowed magma from the mantle to reach the surface starting a new period of vulcanism and the formation of the basaltic lava plains outlined below and in many places the lava flowed down ancient valleys (forming the deep leads in Victoria's gold mining regions which resulted from several generations of fluvial placer deposits of sedimented mineralisation). In many cases, when the softer walls of these valleys became eroded over millenia, the former laval valley deposits became the spurs of the local hills creating an inverted topography.
the cool climate resulted in thick forests of the ancestors of Antarctic Beech and Myrtle Beech Nothofagus cunninghamii but in lower lying regions, these became flooded with the Tasman Sea waters 15-50mya, and resulting compression from layers of sedimentation over the peat resulted in the extensive brown coal deposits in Victoria which in some places is up to 330m thick (Australia has 19% of the world's recoverable brown coal)
around 6.25mya, stiff magma pouring from a vent and congealing in place formed the mamelons of solvsbergite, a form of trachyte, and what we currently know as Hanging Rock, Camel's Hump and Brock's Monument around the Mount Macedon region.
around 2.5mya-4mya, there were further major plate movements and the Rowsley, Hanover and Meredith Faults uplifted the Brisbane Ranges above the “sunk-land” plains to the east (Port Phillip), while vulcanism from Mt Anakie covered the Werribee plains with lava ⁵⁾
during the Ice Ages, Victoria and Tasmania were connected by land
some 10,000yrs ago, at the end of the last Ice Age (which peaked 30,000yrs ago), the ocean waters rose and Tasmania became cut off from Victoria with the formation of Bass Strait, and the sunk-lands of Port Phillip around the Yarra River became inundated to form Port Philip Bay

north-west basaltic lava plains

the south-west regions of Victoria are dominated by relatively recent volcanic activity over the past 6 million years, up until around 8,000 yrs ago (the most recent at 8,000yrs ago is the Red Rock Volcanic Complex at Lake Corangamite, west of Melbourne)
the laval structures in the Organ Pipes NP in Keilor on Melbourne's outskirts are from early in this period of volcanism and are around 2.5-2.8 million years old, dating from the Pleistocene and these were exposed by 1-2 million years of erosion from the waters of the Jacksons Creek, the base of which also exposes a prehistoric buried creek valley, which is cut into 400 million year-old (Silurian) mudstones and sandstones which contain marine fossils indicating they were formed under an ancient ocean
basalt is a fine grained volcanic rock which accounts for 90% of all volcanic rock on earth and is formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava). Mafic magma - as lava is called before erupting on the surface - forms by partial melting of peridotite (a magnesium-iron-silicate-rich rock) found in Earth's upper mantle.

¹⁾

http://austhrutime.com/rodinia.htm

²⁾

http://austhrutime.com/delamerian_orogeny.htm

³⁾

https://digital.library.adelaide.edu.au/dspace/bitstream/2440/23647/1/hdl_23647.pdf

⁴⁾

https://www.gsavic.org/vic-geology.html

⁵⁾

https://astherivergoesby.wordpress.com/2016/06/22/the-epic-geologic-saga-of-bacchus-marsh/