Geology of the Big Scrub Rainforest (Part 4)

Tasman Sea to the Alstonville Plateau

Following the Cretaceous during the Paleogene and possibly as a result of the action of the Tasman Sea formation, volcanic systems began to form throughout parts of eastern Australia. In the New England, the Hunter and Sydney areas swiftly flowing lavas erupted from long vents and covered large areas with basalt. The same thing happened in our region and until now it is one of the relatively unknown parts of our history. At  around 43Ma during the Eocene Epoch basalt lava flowed from vents somewhere in the area now known as the Alstonville Plateau (The area between Lismore to Lennox Head). The lava flow (called the Alstonville Basalt) tended to flow towards the North and West because the hills of the Blackhall Range formed a barrier to the South. The old Wilsons River seems to have been deflected from its southerly course and sent inland to join the Richmond River as lava flows dammed off the rivers original course. Small lakes were formed where the lava flows dammed streams and created little areas of sedimentary rock known as diatomite and even poor quality opal. It is interesting to note that during the 20^th century diatomite was even been mined for use as a filter medium from layers between basalt at Tintenbar and Wyrallah.

Layers of diatomite were subsequently covered by new layers of lava and during the time between lava flows rich red soils developed and were subsequently covered by new lava. The rate of soil formation was high during this time because the world climate was the warmest (more than 10 degrees C hotter than the average today) it had been for 400 million years and be when combined with atmospheric moisture this time was known as the Mid-Eocene Climate Optimum. The lava covered soils (known as paleosols) are important today because they are conduits for groundwater (aquifers) which create long lasting springs resulting continually running streams and ecosystems dependent upon them. The aquifers are also drilled into for groundwater for some of the livestock, irrigation and town water supply on the Alstonville Plateau today. Eventually, the eruptions stopped and lovely fertile deep red soils developed and continue to develop today.