Softer sediments in the Wilsons River Valley

I’ve recently been observing an interesting environmental restoration programme on the Wilsons River upstream of Lismore. During this programme I started to think about the flood plain of the river and ‘recent’ geological history of the area. Cotter 1998 and in his earlier undergraduate work developed a concept of the geomorphology due to the lava flows associated with the Tweed Volcano and the earlier Alstonville Basalt. I did an earlier post on the flow direction of the Wilsons River as related to the volcanic history of the area but I’ve done just about nothing on the post volcanic sequences.

The best work done on the ‘recent’ sedimentary formations of the Wilsons River valley was a PhD thesis, Drury 1982. This was done as part of the then Water Resources Commission (now State Water) back when NSW government departments actually collected new information to guide future decision making (oops, there is a political comment in there). Drury 1982 was a huge thesis that provides a vast amount of information on the development of the Richmond Valley based mainly on the groundwater bores operating at the time supplemented by some (then) new drilling and geophysical techniques. To my knowledge no significant further published scientific assessment of the Quaternary sequences has occurred since Drury's thesis was written.

Cenozoic Stratigraphy of the Lismore area

It is probably hard to follow the stratigraphy very easily so hopefully my sketch to the left which is based on Drury’s work helps. Drury 1982 indicates that the upper most layer of sediment in the Wilsons River and Leycester Creek valleys upstream from Lismore was unsurprisingly, flood plain sands, silts and clays which continue to be deposited today following floods. Conformably underlying this flood plain sediment the material encountered is called the Green Ridge formation. This formation appears to be a delta system being built at the end of the Upper Pleistocene (~12,000 years ago). Often the top of the Green Ridge Formation is cut by the Wilsons River and its tributaries, for instance at Boat Harbour Nature Reserve the lower banks of the river seem to be quite deep maybe even cutting into the even older formations (e.g. the Gundurimba Clay). Drury (1982) demonstrated that the Green Ridge formation is both contemporary with and overlies the Gundurimba Clay, which is made from estuarine clays.

The Gundurimba Clay is a unit was formed during a period of relatively high sea level (higher than the present day) and warmer conditions. Shells were common but coral was found maybe indicating the idea that the area where the Gundarimba Clay was being deposited went through a warmer spell than we experience now.  Drury 1982 identified pollen spores indicating the surrounding area was dominated by rainforest with some eucalypt forest too, in my mind this creates a picture that it was possibly a proto-‘Big Scrub’ low-land rainforest with the ‘Big Scrub’ proper forming after the next cold period at the beginning of the Holocene. However worth noting that the Upper Pleistocene is recognised around the world as starting off in a warm period turning into a glacial period with the last glacial maximum occurring around 22,000 years ago.

Drury 1982 demonstrated that preceding the deposition of the Gundurimba Clay there was a period of erosion meaning that the Gundurimba Clay unconformably overlies the South Casino Gravel. The South Casino gravel in turn uncomformably overlies the Cenozoic volcanic rocks of the Lismore and/or Alstonville Basalt. The South Casino gravel is at least Middle Pleistocene in age and is derived from the erosion of the underlying volcanics. Given its coarse nature it is highly permeable and is considered a good source of groundwater in other parts of the Richmond Valley but to my knowledge is rarely used in the Wilsons River area.

I'm probably trying to combine a lot into this one post so I'll have to tease out the details a bit more in future posts, especially that relating to the Gundurimba Clay and palaeo-environmental conditions which I know at least some of my blog readers have a keen interest in. At least I hope that this provides a starting point.

References/bibliography:

*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales: In Two Volumes. PhD thesis, University of New South Wales.
*Cotter, S. 1997. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of north eastern New South Wales. MSc thesis, Southern Cross University.

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.