The purple area represents the Neranleigh-Fernvale Beds, which cover a large part of the map and are visible in rock outcrops (figure 2) and road cuts on the Gold Coast. A number of different kinds of rocks make up the Neranleigh-Fernvale Beds but geologists have lumped them together under the same name because they are pushed around, tipped up, mixed up, and not clear how they all connect together. The different kinds of rocks include: sandstone, mudstone, altered basalt, and beds of crystallized silica called chert.On the cross-section, the Neranleigh-Fernvale Beds sit at the bottom forming the ‘basement’. There are wiggly lines drawn on the beds indicating that the beds have been folded and pushed around. They form part of a huge area of folded rocks called the New England Fold Belt, which extends from around Newcastle in New South Wales to Townsville in North Queensland (see map here).
These rocks were deposited early in the Flood, likely the Ascending phase as the waters were rising. They were folded soon afterward likely due to movements in the earth’s crust as the Flood progressed (see Basement rocks of the Brisbane area).
The Chillingham volcanic were deposited on top of the folded beds. It would appear that the volcanic eruptions were related to the intense energy and movement of the folding itself. It is common for granitic intrusions and volcanic eruptions to be connected with large structural deformations of the earth’s crust, which geologists have dubbed orogenies.
After this major crustal movement, intense sedimentation resumed as the floodwaters continued to rise, moving back and forth across the earth, perhaps the way the water moves back and forth across the beach due to the waves, swells and currents as the tide comes in.
First the sediments of the Ipswich Basin were deposited, and then the sediments of the enormous basins connected with the Great Artesian Basin: the Woogaroo Subgroup, the Marburg Formation, and the Walloon Coal Measures. These sediments can be traced to the Northern Territory and South Australia. Eventually every high mountain under the entire heavens was covered, as the account in Genesis says.
After the floodwaters peaked the ocean basins began to sink (an event geologists recognize and describe as the breakup of Gondwana), and the floodwater drained from the continent into the ocean. This was a period of intense erosion on the continents. This erosion can be seen on the cross section, with the thick sedimentary deposits east of Canungra having been removed (see also figure 2).
As the energy of the receding floodwaters waned, sedimentation once again was possible on the continent, depositing in localized ‘lakes’—areas of water that temporarily ponded as the water drained. The Beauderset Beds on this geological map are small, localized basins that appear to have been deposited at this time, after the floodwaters were almost fully drained (see discussion on similar basins where fossils were found at Geebung, Brisbane).
As the ocean basins continued to sink and the floodwaters continued to recede, there were immense volcanic eruptions of basaltic magma from volcanoes along the coast, including the Tweed Volcano centred on Mount Warning. The lava that flowed from the vent covered a large area forming a shield volcano, and part of this shield is visible on the map, shown in red (figure 1). On the cross section it can be seen that the lava sits on top of the folded Neranleigh Fernvale Beds as well as on top of the sediments of the Ipswich and Great Artesian Basins. This gives an idea for the timing of the eruption in relation to the receding of the floodwaters. It can also be seen that the lava flows have been severely dissected leaving erosional remnants unconnected with the main flows, such as the remnant at Coolangatta and Tamborine. This erosion also gives a clue for the timing of the volcanic eruptions with respect to the receding floodwaters.
After the Flood, sea level changes during and after the post-Flood Ice Age allowed for localized deposition of alluvium and coastal sediments (figure 1) connected with post-Flood coastal processes.
Geological maps and cross sections such as these are a great tool to help us understand the geological history of the area. With the aid of the biblical geological model they allow us to connect these geological events with the history of the world as recorded in the Bible.