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.


  1. We all like our questions answered, don't we. Rod, I find your posts to be very interesting, and I wish you could come and see a few places near where I live. I am sure you would be interested in those too.

    1. I am very interested in the area where you are (Lake Macquarie). I do hope that I get the chance to get down that way in the near future. Some of the most interesting places I enjoyed last time I was there was around Catherine Hill Bay. Great rock exposures, very interesting.

    2. Rod,

      I think the pebble conglomerate is younger than the Clarence-morton. It lacks deformation or signs of stress that the basin rocks typically exhibit. I've always thought that rock (which is quarried in Broken Head Quarry as a sand) is a just-pre-volcanism age rock - your option 1. I've also wondered if it was not quite fully lithified i.e. a sediment on the way to being a rock, and not a rock weathering back to being a sediment - but haven't taken time to stop and look at it in detail. It quite frankly presents a nice project for a budding sedimentologist for an honours thesis I think.


    3. Thanks Tom,

      I'm increasingly thinking it is a Cenozoic gravel deposit of some time. The advice of a sedimentologist would be quite handy!


    4. Hi Rod,

      Sorry for going MIA. Just to let you guys know that there is deformation within BHQ and also a large weathered basaltic dyke cutting across the eastern pit; also if you look in the base of the pit it seems to grade into the Layton's Range conglomerate before it hits the basement terrain. So this suggests to me that is highly weathered but in situ. However, it highly possible that option 1 may be applicable to the upper layers of the quarry and the ridge line behind the Bundaleer Retreat. I also think faulting is involved as there are certain units missing (i.e. the Nymboida C.M. etc) but these can be found in drill reports near Ewingsdale, however as you said Rod it is hard to get rocks to conform to your expectations.
      Hope you and the family are well and had a merry Christmas, also I'm in Lennox until just after NYE maybe we can catch up?


  2. G'day Rod,

    I wonder if you might be able to offer some advice. I'm a Bundjalung man, but I've been based/stuck in Melbourne for a few years. However I'm heading up to the Northern Rivers through January for camping and showing my sons my country, all over the region, but especially around Baryulgil. Whilst up that way I'm looking at collecting a few materials for making traditional tools and weapons (both a hobby and part of my job). I carve clubs and boomerangs, weave etc, but I also make ground edged axes and knapped blades. Would you know of some quality sources of rocks with properties that lend themselves to fracturing as glass would, along the lines of chert, potch, siltstone, quartzite etc?

    I also wonder if there is a Northern Rivers equivalent of metamorphosed basalt, per the greenstone found here in Victoria?

    I know you had at one point mentioned obsidian at Mt Warning, but it sounds like it wouldn't be in pieces large enough or of a quality that I might shape it into a spear point or blade? Any advice would be greatly appreciated.

    Cheers and Merry Xmas,