|Best of all lookout - Springbrook National Park|
Except for the hills on the horizon the rock in this photo is mainly
of the Neranleigh-Fernvale beds.
To get to Springbrook national park from the Gold Coast it is necessary to traverse the oldest rocks in the Tweed region. These are sediments of the Neranleigh-Fernvale beds. These are represented by the initially steep hilly terrain as you head westward up the range. Hinze Dam, for example, is located on this rock type. Time has weathered and eroded much of this rock away but still it remains as a significant landscape feature. These rocks and hills would probably be better known if the lavas associated with the Tweed Volcano had not erupted.
The Neranleigh-Fernvale beds are interesting rocks because of their mode of formation. They are essentially muds and debris flows that have been deposited in a trench during a period known as the Paleozoic. The trench was caused by the subduction of a continental plate under the then eastern Australian landmass. These sediments were then scrapped off and buckled into a large mountain range that has since been mostly eroded away. All of this occurred while Australia was part of the super-continent Pangaea which existed well before Gondwana.
Today, in the Northern Rivers the Neranleigh-Fernvale beds form the steep eroded terrain in the Tweed Valley (with the exception of some lavas and intrusions associated with the Tweed Volcano). They outcrop in a band at the very edge of the Alstonville Plateau to Byron Bay. They only occur as a band in the Ballina area because they are obscured by Jurassic sediments and the Cenozoic volcanic rocks. Like the Springbrook area, driving from Ballina to Alstonville or from Cabarita to Chillingham means traversing this formation. As soon as you get off the coastal plain and head up the hills you are passing the rocks of the Neranleigh-Fernvale beds. These beds are then obscured by the more recent sediments or volcanic rocks associated with the Tweed Volcano.
As for the Springbrook area, if you’d like to know more I recommend a book by Warwick Wilmott called Rocks and Landscapes of the Gold Coast Hinterland. The processes and timing of events in the Gold Coast area are very very similar to those processes that occurred in the Tweed valley area and so might be worth a read even if you don’t cross the border!
Warwicks book can be obtained from the Queensland Division of the Geological Society of Australia here.