Variable hangingwall palaeotransport during Silurian and Devonian thrusting in the western Lachlan Fold Belt: Missing gold lodes,synchronous Melbourne Trough sedimentation and Grampians Group fold interference

The western margin of the Lachlan Fold Belt contains early ductile and brittle structures that formed during northeast‐southwest and east‐west compression, followed by reactivation related to sinistral wrenching. At Stawell all of these structural features (and the associated gold lodes) are dismembered by a complex array of later northwest‐, north‐ and northeast‐dipping faults. Detailed underground structural analysis has identified northwest‐trending mid‐Devonian thrusts (Tabberabberan) that post‐date Early Devonian plutonism and have a top‐to‐the‐southwest transport. Deformation associated with the initial stages of dismemberment occurred along an earlier array of faults that trend southwest‐northeast (or east‐west) and dip to the northwest (or north). The initial transport of the units in the hangingwall of these fault structures was top‐to‐the‐southeast. ‘Missing’ gold lodes were discovered beneath the Magdala orebody by reconstructing a displacement history that involved a combination of transport vectors (top‐to‐the‐southeast and top‐to‐the‐southwest). Fold interference structures in the adjacent Silurian Grampians Group provide further evidence for at least two almost orthogonal shortening regimes, post the mid‐Silurian. Overprinting relationships, and correlation with synchronous sedimentation in the Melbourne Trough, indicates that the early fault structures are mid‐ to late‐Silurian in age (Ludlow: ca 420–414 Ma). These atypical southeast‐vergent structures have regional extent and separate significant northeast‐southwest shortening that occurred in the mid‐Devonian (‘Tabberabberan orogeny’) and Late Ordovician (‘Benambran orogeny’).     

Fluviodeltaic sedimentology and ichnology of part of the Silurian Grampians Group,western Victoria

The mid‐Silurian Major Mitchell Sandstone of the Grampians Group outcrops at Mt Bepcha, western Victoria, represent a prograding fluviodeltaic sequence comprising four lithofacies and five ichnofacies. The stratigraphically lowest Interbedded Sandstone/Siltstone Facies is characterised by thin sandstone and siltstone beds with soft‐sediment deformation and scours with gravelly lag deposits. This lithofacies contains Thalassinoides, Palaeophycus, Rhizocorallium and intrastratal burrows, together indicative of the Cruziana Ichnofacies, and is interpreted as a shallow‐marine depositional environment on a low‐energy delta front with minor tidal influences. The overlying Massive Sandstone Facies lacks silt, and consists of predominantly massive and some plane‐laminated sandstone, abundant Skolithos linearis , rare Palaeophycus and a single small Cruziana problematica ; the trace‐fossil assemblage is assigned to the Skolithos Ichnofacies. This facies is believed to have been deposited in a marine high‐energy shoreface environment with continuously shifting sands, affected by periodic flooding events from the mouth of a nearby river. Above this is the Trough Cross‐bedded Facies, which contains trough cross‐bedding with gravelly lag deposits, a northwest palaeocurrent direction and large Taenidium barretti burrows (Burrowed Ichnofacies). This facies also contains abundant plane‐laminated sandstone with a northeast‐southwest palaeocurrent direction and ichnofossils of Scoyenia and Daedalus , representing the Scoyenia Ichnofacies. The Trough Cross‐bedded Facies is interpreted to have been deposited in shallow low‐sinuosity channels by overbank‐flooding events, most likely on a delta plain. The uppermost facies, the Plane‐laminated Facies, contains thin beds of current‐lineated, plane‐laminated graded coarse to fine sandstone that preserve arthropod trackways (Arthropod Ichnofacies). This facies was deposited on a periodically sheet‐flooded, subaerially exposed delta plain.     

Geological note: New field evidence resolving the relationship between the Grampians group and the Rocklands Rhyolite,western Victoria

The Rocklands Rhyolite is a latest Silurian to Early Devonian sequence of silicic ignimbrite, lava, volcanic sedimentary rocks and dykes in western Victoria. These volcanic rocks lie west of the Grampians Ranges, which consist of a thick succession of quartz sandstone of presumed Silurian age called the Grampians Group. The previously unresolved stratigraphic relationship between these two sequences of rocks is clarified by an exposed contact between steeply dipping Grampians Group cut by quartz veins, and overlying undeformed rhyolite. The implications of this relationship are that the Grampians Group is older than the Rocklands Rhyolite and that parts of the sandstone succession were locally deformed prior to volcanism. Furthermore, other outlying areas of sandstone and rhyolite, previously correlated with the Grampians Group and Rocklands Rhyolite, respectively, display different timing relationships and are proposed to be significantly younger.