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.
- 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).
- The floodwaters eventually covered the whole earth. They peaked somewhere around the time the Cretaceous sediments were being deposited in Western Australia.
- 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.
- 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.
- 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.