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.
|Conglomerate near Newrybar on the road to Broken Head and Byron Bay|
note the different clast types and sizes - typical of the Laytons Range Conglomerate
- 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,
- 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.
- 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.
- Faulting or folding has up-thrown the conglomerate in this area giving the impression that it is stratigraphically higher
- 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.