Northern Rivers Geology

Text books are wonderful. They always have excellent ‘text-book’ examples! These show how a scenario can be interpreted and what information is used in that interpretation. As you get to know the textbook you get a feel for most or all of the information you can obtain to give you an answer. However, in geology many of the techniques are rarely all applicable to every field situation; or if they are they are applicable, they are unreasonably difficult to use.I have recently experienced one such example in an area south-west of Byron Bay. There is very little information available to interpret and therefore the possibility of misinterpretation can be high.

Byron Shire Council recently did some road works along a section of road between the village of Newrybar and the coast. This work refreshed some small road cuttings (road cuttings are geological tourist attractions). I took a close look at one of the road cuttings on the very edge of the Alstonville Plateau. The rock in this cutting was clearly different from the overlying and dominant Cenozoic aged basaltic lavas that make up the plateau. The exposure was made up of conglomerate.

Conglomerate is a sedimentary rock most often associated with high energy river environments. In this case the conglomerate contained clasts made from other older rocks that occur elsewhere in the region. This included chert, quartzite and fine to medium grained sedimentary rocks such as sandstones and siltstones. The rock though had been quite weathered and the sedimentary clasts had become quite broken down even though they retained their shape insitu.

There is nothing particularly special about this conglomerate. Here the mapping indicates that I was at the very edge of the Clarence-Moreton basin and therefore the oldest rocks of the basin would be likely to outcrop. Indeed, the oldest rock in the basin is known as the Laytons Range Conglomerate. This outcrop looks very much like it. But… further to the east (for example on Broken Head road) are rocks of the Ripley Road Sandstone. These are younger rocks of the Clarence-Moreton basin than the Laytons Range Conglomerate. The Ripley Road Sandstone contains small layers of pebble conglomerate but nothing compared to that exposed in the road cutting. Weirdly this means that the current mapping of the basin indicates that the Ripley Road Sandstone should be older than the road cutting rocks. This is the opposite of the known sequence of the area. To make the road cutting conglomerate fit there is several hypotheses:

1. The conglomerate in the cutting is actually not part of the Clarence-moreton basin but was deposited more recently and then covered by basalt. Maybe it was a pre-volcanic river system,
2. The conglomerate in the cutting is actually part of a younger Clarence-Moreton basin unit that has needs to be redefined to include this particular type of conglomerate.
3. The depositional structure of the Clarence-Moreton Basin is different in this area to the current model e.g. the road cutting is on the western side of a small sub basin.
4. Faulting or folding has up-thrown the conglomerate in this area giving the impression that it is stratigraphically higher
5. Other reasons I cannot think of at the moment

The only trouble is there seems to be inadequate information and field exposure to narrow down the possibilities. I’d love to get a drill rig and core a 200m interval but who has a spare hundred thousand dollars to do that?!

For the time being all I can do is assume the conglomerate was deposited sometime during the formation of the Clarence-Moreton Basin maybe as long as 250million years ago or deposited sometime before the Cenozoic basalts of the Alstonville Plateau possibly 40million years ago.

Alas, there is not enough information available to interpret this situation. But this is normal! We rarely are lucky enough to get a text-book example. In science the examples we are most confronted with are incomplete and generally frustrating. We can’t lie to ourselves that we can answer every question and know everything.

To the lady that stopped, looked at me curiously, and then asked me if I was “alright?” when I was examining the road cutting: Yes, I’m alright. But I still want to know the answer.


Evans head is a popular vacation spot. It has some lovely beaches which are interrupted by a proud and attractive headland. During the summer it is impossible to find accommodation in the area and the town is crowded with families enjoying sunshine, boating, fishing, swimming and relaxing. I don't go there for holidays but I'm close enough to enjoy a day or two by the beach as sometimes.
The interesting feature of Evans Head is the obvious rocky headland standing quite proud at the mouth of the Evans River and along a coast line with huge sandy beaches. The reason for this feature is the more erosion resistant rocks that occur here. Click here to link to a basic geological map of the area. The hardest and oldest rock outcropping just south of Evans Head town at the Half Tide Rocks is the Triassic aged Chillingham Volcanics being the earliest part of the Ipswich basin. These rocks can be seen as the darker coloured rock at the two headlands in the photograph above. Here the Chillingham Volcanics are comprised of basalt and andesite (elsewhere in NSW such as at Chillingham the Chillingham Volcanics are mainly rhyolitic in composition) and here at Evans show an uncommon rock called hayloclastite. The formation of hayloclastite in this area was the result of eruption of basalt into a coastal sea. Something you might see in modern day Hawaii or Iceland where lava flows directly into the sea. Unfortunately it is very hard to recognize because of weathering of the rock in this area.

Overlying the Chillingham are the Evans Head Coal Measures. These are located on the southern bank of the Evans River and extend around to the south of the Half Tide Rocks. Despite their name coal is a little hard to find and is only present in occasional thin bands. The Evans Head Coal Measures are therefore mainly comprised of sandstone (a type called arenite with the sand grains mainly composed of quartz sand and occasional small fragments of rock), siltstone, mudstone and some coal. The arenite frequently shows a feature called cross-bedding which is common in rocks that have formed in medium velocity rivers. The Evans Head Coal Measures are equivalent to the Ipswich Coal Measures in southern Queensland and the Redcliff Coal Measures which occurs south of McLean and is exposed on the coast near Brooms Head.

Ripley Road Sandstone is the youngest exposed rock unit at the headland. This is actually part of the Clarence Moreton Basin which overlies the Ipswich Basin. If you go to the lookout you can see boulders of a pale grey colour. This is the Ripley Road Sandstone. It is mainly comprised of quartz sand lightly cemented together with a grey clay (known as a clay matrix) but occasionally some bands of conglomerate are present such as in the picture opposite.

On the geological map you will notice that the areas around the headland are comprised of different types of sediments these are all very recent which geologically places them at Quaternary (or more specifically Pleistocene to Holocene aged). This pretty much means that these sediments are actively changing and being deposited. Mainly sands in the beach and dune systems and silts and clays around the river estuary. Many of the Holocene aged sediments contain potential acid sulfate soils, which are common in the region but present several environmental management issues when disturbed. In the beach sands there are also commonly found heavy minerals which have from time to time being mined. But more about these heavy minerals some other time.

References/bibliography:

*McElroy, C.T. 1969. The Clarence-Moreton Basin in New South Wales. In Packham, G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45
*Stephenson, A.E. , Burch, G.J. 2004. Preliminary Evaluation of the Petroleum Potential of Australia's Central East Margin. Geoscience Australia. Record 2004/06.