Torlesse Greywacke

Torlesse Greywacke is a type of sedimentary rock. It is a hard and rather drab grey sandstone that is predominantly found in New Zealand though it is found elsewhere in the world as well.

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Location

Torlesse Greywacke is found east of the Alpine Fault in the Southern Alps of New Zealand. It lies between the western edge of the Haast Schists and the Canterbury Plains.

Creation

Torlesse Geywacke was deposited on the eastern side of New Zealand from the Upper Carboniferous through to the Middle Cretaceous. It is created on giant undersea fans that extend around submarine canyons.

A fan starts with a submarine canyon in the continental shelf. Then Turbidity currents rush down the canyon like giant undersea avalanches. As it does this it collects all sorts of sediments from the seafloor. At the edge of the canyon the turbidity current spreads out and creates giant fans that blanket the old seafloor. This fan then turns into Torlesse sediment which in turn turns into Torlesse Greywacke.

Some geologists think that Torlesse Greywacke was derived from the granitic rocks of northwestern Australia due to the large amount of quartz and feldspar that is in it.

Metamorphis

Torlesse Greywacke metamorphises into Haast Schist when it is exposed to immense pressure and the heat of the mantle. This pressure then crushes the rock into Haast Schist. In the Haast Schists minerals that make up Torlesse Greywacke sometimes become visible. These minerals include quartz, feldspar and biotite.

References

• The Rise and Fall of the Southern Alps, G. Coates published 2002