Semi-retired geologist Peter Alcock is fascinated by the influence of natural earth processes over long periods of time on landscapes, human history and behaviour. His presentation, illustrated with stunning photos and appropriate musical interludes (of “Rock” music of our youth) took us through a geological journey spanning 4500 Ma 1.
Peter used the recognised geological eons (large time spans) to show how our continent essentially grew from west to east (left, Wikipedia commons, Geology of Queensland). Commencing with the Archean (4560 to 2500 Ma), we learnt that our original landmass consisted of three cratons, Pilbara, Yilgarn and Gawler.
In the Pilbara Craton, the Stromatolite fossils at Strelley Pool (right, Abigail Allwood) are about 3430 Ma, about the same age as the Banded Chert at Marble Bar (3400 Ma).
The northern end of the Yilgarn Craton includes the Jack Hills, a range of hills containing what, until very recently, was considered to be the oldest material of terrestrial origin found on Earth. Zircons in these rocks formed about 4390 Ma. The part of the Yilgarn Craton more familiar to people living in the southwest is the area stretching from the Darling Scarp to well beyond the eastern goldfields. The rocks become progressively younger eastward, reflecting the periodic accretion of new terranes (fragments of crustal material formed on, or broken off, from another tectonic plate) onto older granites. Well known landscapes include Wave Rock (the granites are greater than 2700 Ma) and the Superpit at Kalgoorlie (where the gold was formed about 2600 Ma).
The Proterozoic Eon spans the period between 2500 to 545 Ma. Rocks of this age cover or underlie the rest of Western Australia and most of the Northern Territory, South Australia and northern Queensland. As oxygen levels increased with the spread of organisms capable of photosynthesis, large amounts of iron settled out of the sea water and between 2500 and 1600 Ma formed the massive iron deposits found around the world including Western Australia’s Hamersley Basin (often erroneously considered to be part of the Pilbara Craton). In the Northern Territory the massive Kombolgie Sandstones that now line the spectacular Katherine Gorge were laid down around 1800 to 1400 Ma. During this same period, rocks at Mt Isa were impregnated with some of the world’s richest veins of copper, lead, zinc and silver. Nearer to home, the granites of the Albany-Fraser fold belt were formed during the collision with another continent about 1300 Ma.
Earth has gone through many ice ages, one of the most significant being “Snowball Earth” that may have lasted as much as 200 Ma, ending 650 Ma. Some of the best evidence for glaciation can be seen in the tillites (an aggregate of rock types formed from parent rocks gouged and crushed by glacial action) found in the Flinders Ranges, South Australia.
Australia has the world’s best evidence of earliest complex life that began soon after the end of “Snowball Earth”. The Ediacaran fauna also found in the Flinders Ranges, has been dated as living between 630 and 542 Ma.
The Phanerozoic Eon (541 Ma to Present) commenced with the Palaeozoic Era (541 – 252 Ma), often considered to be the age of ancient life. Australia’s ancient equivalent to the Great Barrier Reef is the Devonian Reef through which the Fitzroy River has cut to form Geikie Gorge. The reef consists mainly of fossil-rich carbonate rocks (limestone) deposited 385-370 Ma. The unusual banded rocks of the Bungle Bungle Ranges are slightly younger at 350 Ma. The banding reflects the clay content—clay-bearing sandstone remains damp longer than the clay-poor sandstone which dries too quickly to support the growth of cyanobacteria which die in the dry season to produce a dark stain on the surface of the clay rich layers.
On the other side of the landmass (still joined to India and Antarctica) a major period of mountain building was taking place and new glaciers (from 280 Ma) were once again leaving their marks in South Australia. Later, cool climate forests decayed to give rise to the world’s largest coal deposit, now at un-mineable depths within the Cooper Basin in northeast South Australia and southwest Queensland.
This basin was soon to be covered in the Cretaceous period by the world’s largest and deepest artesian basin covering 1.7 million square kilometres. But not before herds of dinosaurs left their footprints in the soft mud around Winton, Queensland, 102 to 98 Ma.
The geological time periods referred to as the Tertiary Period 2 (66 to 2.58 Ma) and Quaternary Period (2.58 Ma to Present) are best known for the completion of the separation of Australia and Antarctica (about 30 Ma) and the large amount volcanism in eastern and south-eastern Australia, which become younger from north to south.
The Warrumbungles Ranges are the remnants of volcanic eruptions which occurred 17 to 13 Ma while the youngest volcanism is regarded as that which occurred around Mt Gambier, South Australia less than ten thousand years ago.
- Numerical Ages are quoted using the International Commission of Stratigraphy’s (ICS) Geologic Time Scale and convention of millions of years (Ma). Current time scales charts and terminology can be found at the ICS website.
- The Tertiary Period is no longer recognised by the ICS. The Cenozoic Era which used to consist of the Tertiary and Quaternary Periods is now divided into the Paleogene (66.0 – 23.03 Ma), Neogene (23.03-2.58 Ma) and Quaternary (2.58Ma to Present) Periods.