Saturday, 23 November 2013

A non textbook example

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.

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
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.

Friday, 1 November 2013

Hills of old sea floor muck

There has obviously been a bit of a lull in my blogging of late. I’ve been busy with family medical trips to Queensland and I’ve had less free time too. But some interesting things have happened with one formal presentation on coal seam gas and water and another presentation to be given in a couple of weeks. But on the aspects that interest me most (non-CSG geology), I’ve also been contacted by academics from a couple of different universities. It is nice to know that they feel I can help them with some research projects. I'll post more about that at a future date.

Best of all lookout - Springbrook National Park
Except for the hills on the horizon the rock in this photo is mainly
of the Neranleigh-Fernvale beds.
During the trip to Queensland I met up with family on the Gold Coast. We decided to have a day up in the popular Springbrook National Park area. In particular the views in this country are astonishing. The Best Of All Lookout certainly lives up to its name with incredible views of the valleys of the Tweed region. Mount Warning looks stunning and the rugged terrain of the volcanic shield remnants beautiful. And this was on a hazy day!

To get to Springbrook national park from the Gold Coast it is necessary to traverse the oldest rocks in the Tweed region. These are sediments of the Neranleigh-Fernvale beds. These are represented by the initially steep hilly terrain as you head westward up the range. Hinze Dam, for example, is located on this rock type. Time has weathered and eroded much of this rock away but still it remains as a significant landscape feature. These rocks and hills would probably be better known if the lavas associated with the Tweed Volcano had not erupted.

The Neranleigh-Fernvale beds are interesting rocks because of their mode of formation. They are essentially muds and debris flows that have been deposited in a trench during a period known as the Paleozoic. The trench was caused by the subduction of a continental plate under the then eastern Australian landmass. These sediments were then scrapped off and buckled into a large mountain range that has since been mostly eroded away. All of this occurred while Australia was part of the super-continent Pangaea which existed well before Gondwana.

Today, in the Northern Rivers the Neranleigh-Fernvale beds form the steep eroded terrain in the Tweed Valley (with the exception of some lavas and intrusions associated with the Tweed Volcano). They outcrop in a band at the very edge of the Alstonville Plateau to Byron Bay. They only occur as a band in the Ballina area because they are obscured by Jurassic sediments and the Cenozoic volcanic rocks. Like the Springbrook area, driving from Ballina to Alstonville or from Cabarita to Chillingham means traversing this formation. As soon as you get off the coastal plain and head up the hills you are passing the rocks of the Neranleigh-Fernvale beds. These beds are then obscured by the more recent sediments or volcanic rocks associated with the Tweed Volcano.

As for the Springbrook area, if you’d like to know more I recommend a book by Warwick Wilmott called Rocks and Landscapes of the Gold Coast Hinterland. The processes and timing of events in the Gold Coast area are very very similar to those processes that occurred in the Tweed valley area and so might be worth a read even if you don’t cross the border!

Warwicks book can be obtained from the Queensland Division of the Geological Society of Australia here.