Geological History of the Perth region, Western Australia, within a biblical framework

The geology around Perth is quite different from the eastern states in that there is little variety in the types of rocks exposed. All the same there are some very interesting and special features to observe.

Floodwaters rising

The oldest rocks in the Perth area are exposed in quarries in the Darling Range to the east. These rocks are part of the vast Yilgarn Craton, which is regarded as an old and stable portion of continental crust.

The Yilgarn Craton occupies a lot of Western Australia (figure 1), from Perth to beyond Kalgoorlie and from Peak Hill in the north to the south coast. The craton includes most of the gold-bearing greenstone belts of Western Australia. It is thought that the craton is made from a multitude of huge, broken ‘blocks’ of continental crust that were sheared, pushed around and metamorphosed. These blocks, or terranes as they are called, were intruded by huge volumes of granitic magma that welded the area into a stable craton. Now exposed in the quarries, these rocks would have formed kilometres below the earth’s surface.

I lean to the view that the Yilgarn Craton formed early during the global Flood (Genesis 6–8), which began with the breaking of the earth’s crust and continued with volcanic eruptions and crustal movements. (Some biblical geologists would interpret these rocks as forming during Creation week but at present I think the huge volumes of volcanic magma seem inconsistent with a “very good” creation.)

After the craton had stabilized (over a period of weeks) ongoing large-scale movements in the earth’s crust opened enormous cracks in the craton allowing magma, probably derived from the earth’s mantle, to flow to the surface. These have been called dyke swarms (figure 2). The exact timing of these is very hard to determine.

As the Flood continued, more sediment was deposited in the area on the craton but this sediment has been subsequently eroded away. Only isolated remnants of these early sediments are preserved along the Darling Range in places such as Armidale Quarry. The floodwaters continued to rise and ongoing sedimentation deposited layers of sediment kilometres thick on the continent. Today these are preserved in the Perth and Carnarvon Basins (figure 1), which are confined by the Darling Fault to the east. However, the thick layers of sediment are not likely to have stopped abruptly at the faults but covered a much larger area—possibly the whole Yilgarn Craton. Remnants of this sedimentary blanked are preserved in the Collie area.

These sediments are not visible around the Perth area but have been identified in drill cores and seismic sections and are shown on cross sections (figure 3). They extend well out into the ocean. The sediments depositing at this time are exposed in the Kalbarri area to the north. They provide evidence of fast flowing water covering a wide area carrying abundant sediment.

Floodwaters receding

Eventually the Floodwaters reached their peak and tectonic movements slowly lifted the continent and lowered the ocean basins. This caused immense faulting on the edge of the continent around Perth (figure 3) with blocks of continental crust west of the Darling Range dropping by as much as 12 km. As the ocean basins lowered, the floodwaters started to flow off the continent into the oceans.
The floodwaters receded first in continent-wide sheets that eroded the flat plateaus that can be seen in inland Australia, including the Darling Plateau east of the Darling Range on the Yilgarn Craton. Kilometres of sediment were eroded from the top of the Yilgarn Craton but the strata west of the Darling Range were preserved from erosion because they had dropped down.

However a few isolated remnants of the sedimentary strata on the plateau were preserved from erosion, some that contain coal deposits around Collie.
As the flow reduced, the receding water was confined to wide flowing channels. We see evidences of these receding waters in wind and water gaps as well as the huge drainage patterns upon the Darling Plateau atop the Yilgarn Craton.

During this time there was a period when the land surface was “deeply weathered” producing thick regolith (also called laterite or “soil”). This could have been due to characteristics of the receding waters such as their containing aggressive corrosive agents or fine sediments being deposited at this time.

After the Flood

After the Flood, sea temperatures were warmer than today causing thick ice sheets to build up on a few of the continents of the globe, lowering sea level by some 100 m. After several hundred years, the oceans cooled and the ice melted back and sea levels rose. Sea levels could also have been affected by ongoing adjustment of the continental crust. Higher sea levels in the Perth area allowed deposition of relatively thin limestone deposits as well as sand dunes to build up on the flat coastal strip around Perth, such as the Tamala Limestone and the Spearwood and Quindalup dune systems.³
Also, after the floodwaters had receded the continent was vegetated by seeds and plants left on the surface after the Flood. These plants have adapted to the changing climates that have occurred since that time, producing some distinctive plants in Western Australia.

The oceans and waterways were colonized by marine animals that were left in the waters on and around the continent after the Flood. However, the air-breathing land animals that now live in Eastern Australia migrated from Mt Ararat in the Middle East, probably using land bridges through the Indonesian Islands.

Humans also came to Western Australia after migrating from the Middle East through India and Indonesia. Humans have probably been responsible for much of the animal migration.

Landscape erosion, sedimentation and volcanic eruptions have occurred in the 4,300 years since the Flood, but these were minor compared with what happened in the catastrophic year of the Flood itself.

References and Notes

1. Pigeon, R.T. and Cook, T.J.F., 1214 ± 5 Ma dyke from the Darling Range, southwestern Yilgarn Craton, Western Australia, Australian Journal of Earth Sciences 50:769–773, 2003.

2. Future potential and deeper resources, Geothermal Centre of Excellence, University of Western Australia, http://www.geothermal.uwa.edu.au/programs/future-potential, accessed 5 September 2012.

3. These relatively thin limestone deposits and associated sand dunes have been classified as Pleistocene, which would equate to the period around the post-Flood Ice Age some 4,000 years ago. In parts the limestone deposits display very large cross beds and have consequently been interpreted as deposited as wind-blown sand dunes that subsequently were cemented into limestone rock. That is quite feasible, which is why I have interpreted them above as post-Flood deposits. However, it is also possible that the sediments were deposited very late in the Flood as the Floodwaters were receding and that the large crossbeds are due to flowing water. It could be considered that the deposits cover too large an area, have too consistent a thickness of strata, and are too pure to be explained as air blown dunes.