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Neoproterozoic to Ordovician

Neoproterozoic and Cambrian rocks and the Delamerian and Tyennan Orogenies (520-490 Ma)

Pale band is Bear Gully Chert Bed disrupted by extensional Tabberabberan faults and overlain by dark Digger Island Marlstone. Waratah Bay
Pale band is Bear Gully Chert Bed disrupted by extensional Tabberabberan faults and overlain by dark Digger Island Marlstone. Waratah Bay
The oldest rocks of mainland southeastern Australia are Late Proterozoic or Cambrian volcanics, volcaniclastics and turbidites. Only the igneous rocks are exposed, in isolated belts at Waratah Bay, Philip Island and the Barrabool Hills and in a series of structural windows in the east of  the Melbourne Zone. The volcanics, along with the Delamerian Fold Belt rocks of the Glenelg and Grampians–Stavely zones, have been affected by an early deformation that is not present in any of the Lachlan Fold Belt rocks.


Images of total magnetic intensity data clearly show that these are northern extensions of the Tasmanian Neoproterozoic crust. The structural windows in the Melbourne Zone imply that this older crust occurs extensively below the Palaeozoic rocks.


The Glenelg Zone extends from an undefined margin west of the South Australian border, east to the Yarramyljup Fault. It contains Cambrian volcanics with ultramafic and within-plate basalt and mid-oceanic-ridge basalt (MORB) associations, and turbidites of the Moralana Supergroup. The Grampians–Stavely Zone extends east from the Yarramyljup Fault  to the Moyston Fault.


Transposition schistosity with folded (left) and stretched (right) pegmatite veins in calc-silicate rocks, Killicrankie Gorge, Corea Creek
Transposition schistosity with folded (left) and stretched (right) pegmatite veins in calc-silicate rocks, Killicrankie Gorge, Corea Creek
It consists of the largely buried Cambrian tholeiite–boninite rocks, and poorly outcropping Cambrian calc-alkaline volcanics and Glenthompson Sandstone. Both zones represent a poorly understood collage of volcanic and sedimentary rocks accreted to the older craton as a series of NW-trending  fault-bounded strips. Folding, faulting and regional metamorphism of the rocks of the Glenelg and Grampians–Stavely zones and the Selwyn Block occurred in the Cambrian. Although probably the same event, this is termed the Delamerian Orogeny in mainland Australia and the Tyennan Orogeny in Tasmania (and its northern extension into central Victoria).


East of the Moyston Fault, Cambrian volcanics not affected by Cambrian deformation are exposed in the Stawell, Bendigo and Tabberabbera zones along the largest faults. These faults penetrate to an inferred major mid-crustal décollement lying within the Cambrian volcanics.


The rocks are mainly tholeiitic basalts and less commonly boninites and ultramafics that are interpreted to be the basement to much of the Lachlan Fold Belt. Cambrian sedimentary rocks form the bulk of the exposure in the Stawell Zone, where they consist of thick turbidites (St Arnaud Group). Farther east, however, they overlie the Cambrian volcanics in long narrow belts and are a relatively condensed sequence of mudstone and chert.

Ordovician Sedimentation (490-440 Ma)

Slab of Yapeenian graptolites from Willey's Quarry, Woodend.
Slab of Yapeenian graptolites from Willey's Quarry, Woodend.
During the Late Cambrian and Ordovician, the Cambrian tholeiitic volcanics were covered by an extensive sheet of quartz-mica turbidites deposited into a deep marine environment along the eastern edge of the Australian craton. In the central west, the St Arnaud and Castlemaine groups comprise the bulk of these rocks in the Stawell and Bendigo zones respectively.


Quartz-mica turbidites also dominate the Lower and Middle Ordovician Adaminaby Group which extends over much of the eastern Lachlan Fold Belt.The only area with restricted Lower and Middle Ordovician sedimentation was in the Melbourne Zone, especially in the east where sediments are extremely condensed and include hale, sedimentary phosphate and, on the Waratah Bay Platform, a marginal marine marlstone. The Waratah Bay Platform contains no record of sedimentation between earliest Ordovician and mid-Early Devonian time.


Upper Ordovician sedimentary rocks show a very clear eastward-fining trend with the turbiditic Sunbury Group conformably overlying the very similar Castlemaine group now restricted to a relatively small area straddling the Bendigo and Melbourne zones. Farther east in central Victoria, and across much of the eastern Lachlan Fold Belt, the relatively condensed Bendoc Group (and Mount Easton Shale) dominated by siltstone and black shale accumulated on a deep anoxic seafloor.


Adaminaby Group: Pinnak Sandstone.Thick-bedded sandstone interbedded with thin-bedded sandstone and mudstone. Genoa River
Adaminaby Group: Pinnak Sandstone.Thick-bedded sandstone interbedded with thin-bedded sandstone and mudstone. Genoa River.
Contribution of volcanic turbidites from the Molong Volcanic Arc farther north was limited to the  Kiandra Group, which outcrops in a small area in eastern Victoria.