The Geological History of the Brisbane area, Australia

© Tas Walker  August 05

The Brisbane area is a very interesting area because there is a large variety of different rocks exposed and these can be inspected without a lot of travel.

The Basement Rocks—Early Flood—The Ascending Phase

Early during the global Flood (Genesis 6–8) the basement rocks of the Brisbane area were deposited 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. Tectonic movements in the earth’s crust compressed and uplifted these deposits above sea level. The compression and uplift deformed the rocks and mixed them up. The basement complex is large and has been called the New England Fold Belt. Different parts of the basement are given different names (eg Neranleigh-Fernvale Beds, Bunya Phyllite). The exposed rocks were severely eroded as the water that was uplifted with the sediments flowed off the continent.

The New England Fold Belt

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 (eg Kangaroo Point Cliffs) while other molten rock pooled under the earth forming granite plutons (eg Enoggera Granite).

Local Sedimentary Basin

The rapid uplift of the continent also lifted up enormous volumes of water, which needed to flow off the continent back into the ocean. It was a bit like the water flowing off the deck of a surfacing submarine. This water flowed down the middle of Queensland, down the western edge of the New England Fold Belt, and rushed into the ocean near Sydney. This water eroded the newly-deposited land surface and deposited sediment, vegetation and the remains of animal life in huge basins. I have not shown these sediments on the map because they are not exposed near Brisbane. (They were deposited to the west of the New England Fold belt, forming the Bowen Basin in the north, and the Sydney Basin in the south.) However, as this water drained off the continent, and the Flood continued, some sediment, vegetation and animals were deposited in a local sedimentary basin called the Ipswich Basin.

The Ipswich Basin

Last Basin Deposited in Rising Floodwaters—The Zenithic Phase

Conditions changed (possibly due to tectonic movements) and sediment, vegetation and animals were deposited in 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. Just a tiny portion of the Ipswich Basin has been left exposed and is the source of valuable coal deposits. The sediment buried the vegetation so quickly that the plants are generally very well preserved. 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 of the sediments comprising the Great Artesian Basin is greatly exagerated vertically. Yet 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 and 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 sheets and then in huge 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.

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. In technical terms this is called a peneplane. 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 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. To the east the water carved the wide Brisbane Valley.

While the Floodwaters were receding volcanic eruptions occurred (caused by earth movements that produced heating, pressure and cracks). These eruptions now form plateaus such as Lamington Plateau and Maleny Plateau. Eruptions during this time also formed 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.

The receding floodwaters also left sediment in some small, local sedimentary basins.

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 in the decades and centuries following the Flood, probably using landbridges through the Indonesian Islands. Humans have probably been responsible for moving many different kinds of animals to different parts of the globe.

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 Brisbane area.

Schematic Geological History of the Brisbane Area. Click for larger image.

---

Share this article with your social network