Perth, Western Australia, geological cross section

posted in: Big Picture | 4
Geological Cross Section Perth Western Australia
Geological Cross Section, Perth, Western Australia (Click for larger image)
The figure here shows an interpreted geological cross-section near Perth, Western Australia. It’s taken from the 1:250,000 scale geological map SH-50-14 publised in 1978 by the Geological Survey of Western Australia.

As I have mentioned before, I look at sections like these and interpret them using biblical history.

Section A-B is approximately 80 km long and shows a depth of 17 km (remember it’s an interpreted section). The vertical and horizontal scales are the same. The left hand (west) side of the section begins in the ocean some 43 km west of Perth and runs to the north-north-east for 80 km, ending about 37 km inland. As we look at the section we are looking to the north. The coastline is approximately half-way along the section (marked by an arrow visible on larger image).

The section is dominated by thick sediments (mauve, blue, green, etc) that are almost horizontal but have been broken and moved up and down by many near-vertical faults. The faults extend into the ‘basement’ (red).

The lowest horizontal layer has been labelled S? D? meaning it could be Silurian or Devonian. The next layers as we move up are labelled P, TR, Jl, Jmu, K, and T, meaning Permian, Triassic, Lower Jurassic and Middle to Upper Jurassic, Cretaceosu and Tertiary. The sediments sit on a basement (red) labelled as Precambrian. (The labels are visible in the larger image.)

Notice that these horizontal sediments stop abruptly at the east (right) side of the section against the Darling Fault (labelled Darling Fault Zone on the section, visible in the larger image). The rocks to the east (right) of the fault are similar to the red basement but the detailed geology of the basement beneath the ocean is not known so it is simply shown as red. From the section it seems that the rocks to the west (left) of the Darling Fault have dropped down some 15 km.

Because the sediments stop abruptly at the fault it makes sense, according to the principle of lateral continuity, that they once extended to the east for a long distance. However none are present now in the cross section shown.

So, here is my take on the story, the geological history from a biblical Flood perspective.

  1. The Devonian-to-Jurassic sediments (mauve, blue, green, etc.) were deposited during Noah’s Flood as the floodwaters were rising. These sediments covered most of the Australian continent (including far east of the Darling Fault).
  2. The floodwaters eventually covered the whole earth. They peaked somewhere around the time the Cretaceous sediments were being deposited in Western Australia.
  3. The ocean basins of the earth began to sink relative to the continents. This caused faulting along the west coast of Western Australia as the crust of the earth to the west of the continent sank. The Devonian-to-Jurassic sediments were faulted and sank by as much as 15 km.
  4. The floodwaters flowed from the Australian continent into the ocean. In the section shown the waters would have flowed in a westerly direction. This eroded kilometres of sediment from the top of the continent but the sediments to the west of the Darling Fault were preserved because they had dropped down.
  5. The upper Cretaceous and Tertiary sediments (above an unconformity, which is shown as a wiggly line, visible in the larger image) were deposited after the faulting had occurred, which was when the floodwaters were receding.

We see that it is reasonably simple to interpret the geological cross section using the geological model based on biblical history.

4 Responses

  1. “Because the sediments stop abruptly at the fault it makes sense, according to the principle of lateral continuity, that they once extended to the east for a long distance. However none are present now in the cross section shown. ”

    What about syntectonic deposition? Is there any thickening in the sediments closer to the Darling fault? Any curvature of the sediments up to the fault plane? Are there any similar sediments at the top of planation surfaces to the east?

    Tas Walker responds:
    Hi J.S., That is a good point. Remarkably, there are similar sediments on top of the planation surfaces to the east and I hope to write an item on one such example—the sediments that comprise the Collie basin—soon.

  2. […] 1. Location of Collie, Wilga and Boyup basins, Western AustraliaIn my discussion of the Perth geological cross-section I concluded that the thick sedimentary deposits filling the Perth Basin likely extended across the […]

  3. […] happened later in the Flood, especially as the floodwaters receded from the land (see discussion on Perth geology and Collie basin[…]

  4. I’ve been hiking the west australian hills of roleystone region, i am wondering if anyone here can tell me what the surface ‘rubble’ would be, there are white crystal-like stones and various coloured stone, red clay covered stone etc. I would love to know more, I’ve found it hard to find answers anywhere on the internet. Insights appreciated! thanks

    Hi Lavida:
    The Roleystone region is east of the Darling Fault and sits on the Yilgarn Craton. The surface rubble has been described as ‘weathering’. Typically it forms hard crusts and gravels of silica (silcrete) and iron (fericrete). It is recognized to have formed late in geological history and I suspect it was as the floodwaters were flowing off the continent.