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Kanmantoo Trough

Age

Sedimentation — Early Cambrian (526–514 Ma)

Deformation — Delamerian Orogeny, Middle Cambrian to Early Ordovician (514–485 Ma)

Prospective commodities

Ag–Pb–Zn, Cu–Au, Au, Ni–Cu–PGE, Mo, diamonds, construction materials, slate and dimension stone.

Major exploration models

The following exploration models have the potential to be applied to the Kanmantoo Trough.

  • SEDEX — Sediment-hosted hydrothermal volcanogenic exhalative Ag–Pb– Zn±Au deposits within greywacke.
  • VHMS — Volcanic-hosted Ag–Pb–Zn massive sulphide deposits in Truro Volcanics equivalents from the Padthaway Ridge to the Nackara Arc.
  • Cu–Au feeder systems to SEDEX and VHMS mineralisation.
  • Cu–Au mineralisation associated with Delamerian Orogeny granitoids such as the Palmer Anabama and Bendigo Granites.
  • Fault-related gold mineralisation — potential along crustal-scale faults such as the Teal Flat and Kangaroo Island Shear Zones.
  • Ni–Cu–PGE and PGE mineralisation within layered and zoned ultramafic–mafic intrusives.
  • Molybdenum mineralisation related to Delamerian granitoids.
  • Diamond potential of lamprophyre and kimberlite plugs and dykes.

Summary

The Kanmantoo Trough forms the eastern part of the Stansbury Basin and is the youngest succession in the southern part of the Adelaide Geosyncline. Predominantly immature clastic sediments were deposited in the Kanmantoo Trough during the Early Cambrian as a result of the last of several phases of rifting in the Adelaide Geosyncline.

View the Tectonic Setting of the Kanmantoo Trough Map.

Deposition closely followed mafic volcanism of the Truro Volcanics and equivalents in the eastern Mount Lofty Ranges and Murray Basin basement, particularly in the Padthaway Ridge.

Renewed extensional tectonics in the Early Cambrian allowed predominantly siliclastic sediments of the Kanmantoo Group to be rapidly deposited into a tectonically active fault-controlled basin. Outcrop extends in a 300km arcuate zone along the eastern Mt Lofty Ranges with Glenelg River Complex (Victoria) metasediments tentatively interpreted as distal equivalents.

The Kanmantoo Group is partly in fault contact with older rocks and partly in disconformable contact with the underlying Heatherdale Shale of the Normanville Group. Uranium/Lead zircon dating of a tuff within the Heatherdale Shale indicates a minimum age of deposition for the upper Normanville Group and lower Kanmantoo Group at 526±4 Ma.

Deposition

Sedimentation of the Kanmantoo Group was cyclic with at least three major transgressive-regressive sequences.

The Carrickalinga Head Formation at the base comprises sandstone and shale couplets of the Madigan Inlet Member, which were deposited in water depths close to storm wave-base.

The Blowhole Creek Siltstone Member reflects transgression and a deeper water environment with the Milendella Limestone Member recording maximum flooding. Overlying conformably, are upward-shallowing sandstone and siltstone of the Campana Creek Member.

Conformably overlying the Carrickalinga Head Formation is the Backstairs Passage Formation. This formation consists of cross-bedded, tidally influenced, arkosic, laminated, predominantly medium-grained sandstone. A widespread, thin siltstone marks the boundary with the underlying Campana Creek Member.

Above the Backstairs Passage Formation, a tectonically influenced fall in relative sea level produced erosional valley-fill deposits of the Cooalinga and Malabena Members, before high-stand sedimentation of the Talisker Calc-siltstone (Dyson et al., 1994), including the carbonaceous and pyritic Nairne Pyrite Member deposited under anoxic conditions with restricted sediment supply. The Talisker Calc-siltstone passes conformably up into rapidly deposited fluxo-turbidites of the Tapanappa Formation, which contains several shaly pyritic lenses toward the base, representing several periods of anoxic, deep-water, quiescent sedimentation.

Another erosional surface with local conglomerate lenses marks the base of the next sequence, commencing with carbonaceous shale of the Tunkalilla Formation. The conformably overlying Balquhidder Formation marks a return to turbiditic deposition, which also contains deeper water pyritic shale lenses toward the base.

The finer grained and thinner bedded Petrel Cove Formation possibly represents another transgression. The relationship of the overlying shallow water, tidally influenced, laminated and large-scale cross-bedded Middleton Sandstone to the Petrel Cove Formation is uncertain because the contact is a major shear zone; there is a possibility that it represents a tectonic repetition of the Backstairs Passage Formation.

Deformation

Cessation of sedimentation in the southern part of the Adelaide Geosyncline, during the Middle Cambrian, is considered to have been caused by compressive deformation of the Cambro-Ordovician Delamerian Orogeny.

The orogeny was accompanied by the intrusion of synkinematic granitoids at ~515-500 Ma (Preiss, 1995) and low pressure/high temperature "Buchan Style" metamorphism (Sandiford et al., 1995). Metamorphism was dominantly of biotite grade in the north and south, but reached amphibolite facies in the eastern Mount Lofty Ranges, with sillimanite and migmatite indicating temperatures near 550-600° C and pressures of 3-4Kb (Sandiford et al., 1995) in the high grade core.

In the Kanmantoo Group, the initial stages of deformation (D1) produced northwest-directed thrusts, partly as a reactivation of syn-sedimentary faults and, in the high-grade zones of the eastern Mount Lofty Ranges, a schistosity sub-parallel to bedding.

This was followed by a major asymmetric but more upright en-echelon F2 folding event with generally north-south axes, associated with an axial plane crenulation cleavage. Northwest-trending F3 folds are prominent only in the high-grade zone, but late kinks were widely developed along with block faulting in the waning stages of the Delamerian Orogeny.