Australia: The Land Where Time Began

A biography of the Australian continent 

Hamersley Range 

The Hamersley Range stretches north-west south-east.

Between 3.5 billion years ago, the period of the North Pole Stromatolites, and the 2.8 billion years ago period of the stromatolites on what is now the bank of the Nullagine River about 80 km south-east of North Pole, there had been a huge increase in the area of continental crust. Erosion would have begun as soon as the rocks were exposed to the atmosphere and one of the consequences was that huge amounts of soluble iron had been leached from the emerging rocks and was accumulating in the oceans. Prior to large-scale production of oxygen by photosynthetic organisms using processes which produced oxygen as a by-product, the iron remained in solution, but as soon as dissolved oxygen became available the iron began to rust to iron oxide. So much of it was produced that at least parts the oceans probably turned red. The rust began to settle to the sea floor and the result was that by 2.5 billion years ago, at places like the present-day Hamersley Range, huge deposits of iron oxide began to accumulate about 300 million years after the proliferation of stromatolites around 2.8 Ga, and accumulated to the point where they are now mined as iron ore. The amount of iron deposited here is so great that after the sediments were compressed to rock, then raised above the sea surface, and after many millions of years of erosion, the ore deposits are still 2.5 km thick.

In the Karijini National Park, originally called the Hamersley Ranges National Park, there are iron deposits in the 2.5 billion-year-old bedrock.

Most of the rocks of the range are conglomerates, shales and quartzites, but with thick layers of blue asbestos and iron. These rocks formed from marine sediments deposited about 2.4 - 1.8 billion years ago, and were subsequently uplifted and warped. A mature plateau formed during a long period of erosion, after which further uplift rejuvenated the drainage, the water courses then carved gorges along faults and joints.

The spectacular red landscape of the Hamersley Range contrasts with the white trunks of the eucalypt trees that are common in the area. The range is an iron province that has developed on strata of Proterozoic age, and also includes some Banded Iron Formations. The mostly undisturbed strata dip steeply locally. Where the strata are undisturbed, the Banded Iron Formations form plateaux and benches, and where folding has taken place they form ridges. The landscape is dominated by a very old rolling high plain that was formed by groundwater in the Cretaceous, about 170-100 Ma. When the Cretaceous seas withdrew from the areas to the north and west, riverbeds were incised into the soil or regolith, that was rich in iron, that had formed during the earlier period of relative stability, removing it down to the bedrock on which it had formed, to form the Hamersley Surface, the high plain of the present. The iron-rich soil was deposited in the valleys of rivers that radiated from the uplands, the iron being re-precipitated as the Robe River Pisolite. The pisolite and the various Banded Iron Formations are mined. The rivers cut more rapidly into the slopes of adjacent hills than those draining the pisolite, that was hard and resistant. As a result of landscape inversion the river valleys that were capped by pisolite eventually became higher than the slopes of the same valleys. The result of the inversion is the form of the spectacular elongate hills, mesas, that are winding in some places.

The landscape has been dated to the Middle Eocene by pollen spores preserved beneath the Robe River Pisolite. During the Cretaceous the Hamersley Surface was developed as a weathering front, the resulting soil and regolith being eroded and transported to the river valley floors where the pisolite was deposited during the Middle Eocene.

Sources & Further reading

  1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
  2. Hellen Grasswill & Reg Morrison, Australia, a Timeless Grandeur, Lansdowne, 1981
  3. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd
Author: M. H. Monroe
Last updated 05/11/2008 


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