A westward Wilsons River

The Wilsons River flows from east to west between Booyong and Lismore which is unusual for coastal rivers in the region. You’d expect a river to find the path of least resistance and head to the sea quite quickly, in the case of the Wilsons River the path of least resistance appears to have been away from that range of hills or mountains in the Alstonville area and away from the sea.

In his masters research, Cotter (1998) discovered that the landform of the tweed volcano was more complex than the simple shield volcano model proposed by earlier researchers. The shield volcano model essentially shows a radial drainage pattern from the centre of the shield a bit like the spokes on a bicycle wheel. While this does hold up well for the remaining skeleton of the Tweed Volcano of particular interest is the area to the south where a previously unidentified Cenozoic volcanic unit was discovered and shows that pre-existing structures explain the river drainage. Cotter (1998) suggested the name of Alstonville Basalt for the new Cenozoic (up to 41 Million Years old) unit as it appeared to pre-date the tweed volcano (23 Million Years). Additionally, it has been identified that the even older Mesozoic Chillingham Volcanics (but here consisting of basalts rather than the rhyolites that are seen further north) occur on what was once considered the southern flank of the volcano.

Brodie and Green (2002) observed that the dip of the Alstonville Basalt is to the north west which to me seems to indicate a volcanic centre further to the south east (in the opposite direction for lavas from the Tweed Volcano) assuming that not too much deformation has occurred since the rocks were erupted. Taken together this implies that during the Mesozoic hills existed to the south of the present day Alstonville Plateau and that during the early Cenozoic volcanic hills were emplaced and created a barrier for southerly or easterly discharge.

Cotter (1998) suggests that the Wilsons River has actually roughly followed its current path since the Late Mesozoic. The diatomite deposits located at Tintenbar and Wyrallah are of lacustrine origin and may be the result of lakes forming on the newly erupted Alstonville Basalt as the Wilsons River was intermittently impounded by the existing hills of the Chillingham Volcanics. It has been following a westerly course certainly before the Tweed Volcano (c. 23 Million Years) for the Chillingham Volcanics and Alstonville Basalt has stopped the Wilsons from flowing south or east. This continuity of flow direction implies that any lavas from the Tweed volcano would have cut through the Lamington Volcanics of the Tweed volcano unless the lava was of significant enough volume to change river direction. This volume of lava appears unlikely given the distance of this area from the centre of the volcano now represented by Mount Warning.

Putting all the background together shows that the section of the Wilsons River from Booyong to Lismore may have been flowing away from the sea for more than 40 Million Years, yet, additionally it is worth noting that the majority of the length of the Wilsons River, Richmond River and even Clarence River is north-south parallel to the coast. This implies some form of pre-existing structural control, probably associated with the deposition of the Clarence Morton Basin or even older Palaeozoic basement rocks, in turn; suggesting that the northern rivers have been following similar flow paths for a long, long time. However, this is a discussion for another post.

References/Bibliography:

*Brodie, R.S. & Green, R. 2002. A Hydrogeological Assessment of the Fractured Basalt Aquifers on the Alstonville Plateau, NSW. Australian Bureau of Rural Sciences, Australia
*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Ferrett, R. Australia's Volcanoes. New Holland Publishers 2005.