The oozy beginning
The history of the ‘Bigscrub’ starts a very long time in the past, yet it may be a surprise that by Australian standards the geology of our region is comparatively young. The oldest parts of Australia are 4400Ma (4400 million) years old but our little part of the Australian continent did not exist back then. It was not until 363-320Ma that the foundation rocks of our region were formed. These rocks are called the Neranleigh-Fernvale Beds
|The Neranleigh-Fernvale Beds at Broken Head National Park near Byron Bay|
The process of plate tectonics means that oceanic plates move slowly under continental plates. Gradually, as this process continued during the Carboniferous our area came closer to the continental landmass as the oceanic plate was subducted (is pushed under) under the Australian continent. Tall active volcanic mountain ranges existed along the edge of the continent much like the mountain ranges of the Andes Mountains in South America today. As our area approached the continent it meant that the size of particles increased. Erosion of the mountain ranges and continuing submarine landslides created layers in the sediments called turbidite sequences. These are sequences where the bottom is coarse grained (usually sandy) and the top is fine grained (mud). This is because fine sediments take longer to fall out of the water. Subsequent nearby landslides would start a new layer with coarse grained sands followed by finer grained mud and so on.
|Neranleigh-Fernvale Beds are quarried at many locations this is an old face at Teven|
As mentioned before, the Neranleigh-Fernvale Beds are the foundation rocks of our region. Today they form the ‘basement’ of the coastal Australian landmass from Gympie in Queensland to Broadwater on the Richmond River. However, in our region they are mostly obscured by younger rocks (discussed below) though it is possible to view outcrops of turbidite in several areas. These include on the Wilsons River at Laverty’s Gap, at Tintenbar and on the escarpment at Uralba between Ballina and Alstonville. Quartzite (from the silica rich sediment) is present on a hill near Nashua.
At the earlier stages of the carboniferous period the collision of the oceanic and crustal plates in our region stopped. The Neranleigh-Fernvale bed formation had been completed. The continent with its new additions ‘relaxed’ as compression eased and eventually stopped. Many more millions of years passed and what happened during that time is lost to history. No rocks are preserved in our region from the next time period known as the Permian. It is probable that the ‘Bigscrub’ area remained stable for a long time with only erosion being the most significant geological process.