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Early Silurian (440-~425 Ma)

Bendigo Zone: Benambran structures. Small chevron folds with axial planar S1 cleavage. Wedderburn.

Bendigo Zone: Benambran structures. Small chevron folds with axial planar S1 cleavage. Wedderburn.

The interval beginning in the Early Silurian and lasting to the end of the Early Devonian was one of great complexity in the Lachlan Fold  Belt. Much of the protracted Benambran Orogeny occurred early in the Silurian, and in the Benambra Terrane it was a period of extension and basin formation alternating with strong deformation during the Bindian and Tabberabberan orogenies. In strong contrast with the regions both to the west and east, the Melbourne Zone remained unaffected by the Benambran and Bindian orogenies, a  factor which is very important in reconstructing the Palaeozoic history of the Lachlan Fold Belt.

In the Grampians–Stavely Zone, the Grampians Group platform sequence was deposited on low-relief Delamerian crust. In eastern Victoria, the Ordovician–Silurian boundary coincides with widespread facies changes from euxinic Bendoc Group, dominated by black shale, to the sandier Cobbannah and Yalmy groups deposited in aerated sea-water. The eastern Victorian units were deformed at the end of  the Early Silurian and underlie the widespread Benambran Orogeny unconformity.

During the Benambran Orogeny three metamorphic complexes were generated in Victoria:
  • the Moornambool Metamorphic Complex in the Stawell Zone;
  • the Omeo Metamorphic Complex in the Omeo and Deddick zones;  and
  • the Kuark Metamorphic Complex in the Kuark Zone.
 Reverse fault with associated extension veins. Wattle Gully mine, Castlemaine.

Reverse fault with associated extension veins. Wattle Gully mine, Castlemaine.

Shortening across the Stawell and Bendigo zones during the Benambran Orogeny was accommodated by closely spaced tight meridional folds with near-vertical axial planes, and by high-angle thrusts spaced at regular intervals, with a consistent west-over-east displacement. Despite the tight folding, the overall enveloping surface is gently dipping so that there is only minor variation in the metamorphic grade and the stratigraphic levels exposed. In the Bendigo Zone  the rocks range from zeolite to lower greenschist facies, and through most of the Stawell Zone the outcrop is typically lower greenschist facies. Metamorphic grade is much higher in the western 15m of the Stawell Zone, rising to amphibolite facies in the hanging wall of the Moyston Fault, where there are abundant interleaved fault slices of mafic rock.

In the Melbourne Zone, marine deposition continued without  interruption from the Cambrian to the Early Devonian, in the latter stages as a  foreland basin. In eastern Victoria, tectonism in the Benambran Orogeny began in the Late Ordovician in the Narooma  Accretionary Complex and was more strongly expressed by a regional facies change, uplift and local deformation, at about the Ordovician-Silurian boundary at 440 Ma. During continuing deformation Llandovery rocks were folded and faulted together with Ordovician rocks at about 430 Ma (end of the Llandovery) and the Omeo Metamorphic Complex was generated. It was followed by the intrusion of I and S type granites with ages of about 425 to 420 Ma.

Late Silurian (425-420 Ma)       

 The Grampians Group near Halls Gap. View from Boroka lookout looking SE along the Fyans Creek valley

The Grampians Group near Halls Gap.View from Boroka lookout looking SE along the Fyans Creek valley.

In the Late Silurian, deformation of the Grampians Group to form a thrust-and-fold belt commenced, and, together with widespread reactivation of western Lachlan Fold Belt faults, may represent the culmination of the effects of the Benambran Orogeny in western Victoria.

The evolution of eastern Victoria at this time is quite different to western Victoria, and must be seen in the context of prolonged horizontal displacement of eastern Victoria, travelling south with respect to the Gondwana landmass. During and following the Benambran Orogeny, large-scale faults subdivided eastern Victoria into a series of dislocated blocks (structural zones and subzones) that show a prolonged history of differential horizontal displacement during the remainder of Silurian and Early Devonian time. Crustal extension and horizontal displacement in the Late Silurian  produced the Cowombat Rift, consisting of the Limestone Creek and Wombat Creek graben. These grabens contain mainly shallow marine rhyolitic lava and  high-level intrusions overlain by marine to subaerial volcaniclastic siltstone, sandstone and conglomerate below fine-grained turbidites with stratiform lenses of altered andesitic lava and limestone.

In the Melbourne Zone, shallow to deep marine sedimentation (Yarra Supergroup) continued without interruption  through the Silurian and Early Devonian.