The 600 Million Year Mystery

The world of rocks and minerals has many puzzling and interesting areas for exploration but only a few of them have caused as much bewilderment as a mysterious rock type found in the Kununurra district of Western Australia. The Rock is known as Zebra Rock.

Zebra Rock is a very attractive fine-grained siliceous argillite (indurate siltstone or clay stone) with rhythmic patterns of red bands or spots contrasting sharply with a white background.

As yet, it is not known how the rhythmic and regular patterns were formed, but it is known that the red portions are coloured by ferric (iron) oxide.

Geologists have investigated Zebra Rock for the past 40 years without producing any valid explanation for its origin. It is realised that the rock poses an interesting problem of genesis.

The origins of the rock may be based on one or more of the following explanations:

• Ripple patterns imposed on the rock during the deposition of sediment.
• Slump structure formed while the rock was still in a plastic state prior to solidification.
• Selective leaching and reconcentration of iron oxides and silica.

The age of Zebra Rock has been placed at 600 million years in the Upper Proterozoic era or Pre-Cambrian period.

The only known deposits of Zebra Rock in the world have been found near Kununurra in the East Kimberley region of Western Australia. The deposits are found in small reef outcrops of stratified claystone or shale in the Ranford Formation.

Digging is difficult because of the shale surrounding of the reef. The best implements to use for the extraction of the rock are crowbars and picks.

The Zebra Rock is not found throughout the whole reef but in small individual reefs, which are spasmodically placed within the major reef. Individual pieces of Zebra Rock may appear as brown stone without any apparent patterns but when they are cut with a diamond saw they may show one of the many Zebra patterns.

Most of the Zebra Rock can be cut with a hacksaw but a diamond saw does a better job and is more economical from a commercial point of view.

Zebra Stone varies in hardness and quality, depending on the site from which it is taken. Some types are very porous and chalky with visible pitting faults and in different patterns whilst other specimens are dense with sharply defined rhythmical patterns.

The best sites found are on a pastoral property near Kununurra. Stone from these sites is used in the local manufacture of jewellery and gift lines.

Zebra Rock Gallery, Kimberley Ornamental Stonecraft and Rockalong Stonework in Kununurra and Zebra Rock Mine on the Duncan Road NT supply hand crafted goods made from Zebra Rock as well as supplying slabbed or bulk pieces.

The Rock is a fine collector's item and has many beautiful rhythmic patterns, which vary considerably, but the rock is too soft to polish. The rock is simply rubbed back with sandpaper, then left or sprayed with a clear lacquer to give a "wet look".

If you ever visit Kununurra, then the opportunity should be taken to see the Zebra Rock at Zebra Rock Gallery, Kimberley Ornamental Stonecraft and Rockalong Stonework. You may even solve the mystery of six million years standing.

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