The Geological History of the Sydney area, Australia

© Tas Walker  October 09

The Basement Rocks—Early Flood—The Ascending Phase

The oldest rocks in the Sydney area occur at considerable depth and are only known from regional mapping and geophysical investigations. It is likely that the deepest rocks are related to the exposed fold belts of eastern Australia. These were deposited rapidly early during the global Flood (Genesis 6–8) deep under the ocean. There was a large volume of a variety of materials deposited including fine silt, poorly sorted sand, beds of chemically deposited silica, and black volcanic lava. The basement areas are large and have been given different names depending on where they are exposed (figure at right). Tectonic movements in the earth’s crust compressed and uplifted these deposits above sea level. Some rocks were folded by the compression while others were severely deformed and disrupted. The exposed rocks were severely eroded as the water uplifted with the sediment flowed off the continent.

Eastern Australia fold belts

Volcanic Eruptions

Violent volcanic eruptions took place soon after (or while) the basement rocks were compressed and uplifted (possibly the catastrophic compression heated and melted the sediment locally). Some of the molten rock erupted over the landscape while other molten rock pooled under the earth forming granite plutons. These features are not exposed in the Sydney area, but there are a number of plugs called diatremes that are likely the result of these eruptions.

Local Sedimentary Basin

Because of this rapid uplift the water flowed off the continent depositing sediment and animals in the Galilee, Cooper and Bowen Basins. It is likely that these basins were connected to the Sydney Basin where the waters flowed into the sea (figure at right).

Galilee, Cooper, Bowen, and Sydney Basins

Last Basin Deposited in Rising Floodwaters—The Zenithic Phase

Further tectonic movements gently folded these rocks and changed the levels of the surface, causing the positions of the sedimentary basins to change. This, coupled with rising floodwaters meant that sediment, vegetation and animals were deposited over a much larger series of basins covering a large part of Eastern Australia. These sediments now contain the water reservoir known as the Great Artesian Basin (figure at right). The sediment buried the vegetation so quickly that the plants are generally very well preserved. The sediments alos contain abundant fossils of dinosaurs including their trackways as well as their remains as they sought to escape the rising water. Gas liberated from the heated vegetation is still contained under pressure in the coal seams, a danger for underground mining but a potential source of energy. The sediments also contain abundant underground artesian water, water that was captured in the sediment as it was deposited.

The Sediments Comprising the Great Artesian Basin

Cross Section of Artesian Basin

This cross section above of the sediments comprising the Great Artesian Basin is greatly exagerated vertically. The depth of sediments is immense, reaching nearly 3 km in places. The strata can be traced across the continent and contain valuable coal and gas deposits.

The fact that the strata are so continuous and so flat (similar to the flat horizontal layers in Grand Canyon in the USA) is evidence that the entire depth of sediment was deposited quickly and continuously without long periods of time where surfaces were exposed to weathering and erosion. If sedimentation had taken place slowly there would have been irregular eroded surfaces.

Cross Section of Great Artesian Basin Sediments

The Recessive Stage of the Flood—Erosion of the Landscape

Eventually the Floodwaters reached their peak. Then further large scale tectonic movements slowly lifted up the continent and lowered the ocean basins. The Floodwaters then started to flow off the continent into the oceans.

The receding floodwaters (first in huge sheets and then in wide channels) severely eroded the surface of the continent of what is now Australia and carved the landscape into pretty much the form we see it today. The sediment was deposited at the continental margins.

You can see from the dotted lines in the cross section above that the whole basin was gently folded after it was deposited. This folding was probably associated with the ocean basins sinking and the continents rising. Not only that, but the top of the sediments were shaved off—in some places more than a kilometer of sediment shaved from a huge area. This was shaved flat, either by the receding Floodwaters, or by circulating water currents on the land while the whole continent was under water in the middle of the Flood. Note on the section how much of the sediment has been removed at the eastern edge of the basin near Brisbane. This was eroded during the channelized flow phase of the Flood (the Dispersive phase) after the sheet flow reduced and there were huge channels of water flowing.

While the Floodwaters were receding volcanic eruptions occurred (caused by earth movements that produced heating, pressure and cracks in the crust). These eruptions now form basalt caps along eastern Australia such as the well-known plateaus near Brisbane such as Lamington Plateau and Maleny Plateau. Eruptions during this time also formed volcanic plugs such as the Glasshouse Mountains and Mt Warning. The land around these plateaus and mountains was eroded away by the final stages of the receding floodwaters, leaving them exposed as spectacular landmarks.

In the Sydney area remnants would include Mt Wilson, Mt Irvine and Mt Tomah. The receding floodwaters eroded the areas around the Blue Mountains, including the steep escarpments and gorges, such as Jamison Valley and Grose Gorge.

The receding floodwaters also left sediment in some small, local sedimentary basins including the flat areas around Richmond.

The Post-Flood Era

After the Floodwaters receded the continent was vegetated by seeds and plants left on the surface after the Flood. The oceans and waterways were colonised 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 landbridges through the Indonesian Islands. 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.

Comparison of Biblical Interpretation with Usual Evolutionary One

The following diagram compares the sequence of events in the geological development of the Sydney area.

Comparison of Geological History of the Sydney Area from the evolutionary and biblical interpretations. Click for larger image.

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