Wednesday, 1 August 2012

A magma chamber under Cabarita Beach

Again and again, I am amazed at how little we know about what is under our feet. It often takes an unexpected source of information to reveal some incredible knowledge of our region. The lastest information that has recently come to hand has been the preliminary geophysical survey results for the Grafton to Tenterfield survey. There are many results that may indicate some strange goings on, from some inconsistent features in the Mount Warning area (possibly indicating that the Tweed Shield Volcano might actually be a myth! More of this in a future post or two), to strange lineaments and responses showing hidden intrusions. This post is about just such a possible hidden intrusion in the Cabarita area.

Smith (1999), curiously reported that within the Neranleigh-Fernvale Beds at Norries Head, Cabarita (located on the coast midway between Tweed Heads and Mullumbimby) there appeared to be evidence of thermal metamorphism in the rocks there, but no evidence of what caused the heating. Metamorphism is a characteristic of the Neranleigh-Fernvale Beds, but the style of metamorphism is pressure related due to the formation being accreted (squashed) onto the Australian continent during a period of subduction during the Palaeozoic period. Not much heat was generated in this formation and based on the minerals identified in the rocks it is possible to estimate the pressure and temperature when these rocks were squashed. The feature that Smith (1999) identified was biotite crystallisation (a variety of the mica mineral group). This mineral is indicative of heating of rocks to a medium to high grade but the lack of a preferred orientation of this platy shaped mineral shows us that the metamorphism postdates the accretion period. ie. the heating of the rock has occurred some time after the pressure, meaning at least two periods of metamorphism.

As discussed in a previous post, the New South Wales Geological Survey has been collecting geophysical data over the region. One measurement has been the intensity of magnetism (related to the iron content of rocks). Magnetic results can display what is happening under the earths surface, not just on top. It is known to show a characteristic feature where intrusions are known, either a strong negative or strong positive anomaly, depending on the rock type. The picture to the left shows the total magnetic intensity map (courtesy of the 2012 preliminary data package from the geological survey) for the area around Cabarita. I’m sure you can pick out the obvious red and blue anomaly. the pattern is consistent with intrusions, indeed exactly the same feature can be seen in the Mount Warning area (and others that I will discuss in future). As such, I suggest that this anomaly is actually good evidence of an intrusion hidden below the heat affected surface rocks. Smith (1999) thinks that the biotite grade metamorphism occurred during the Mesozoic period (well before the Cenozoic aged Lamington Volcanics) and that there was once a body of molten rock below the ground in this area.

I’m so pleased to be able to see the preliminary dataset, it is obvious that there are many features that can be better understood.


*Smith, J.V. 1999. Structure of the Beenleigh Block, northeastern New South Wales. New England Orogen: Regional Geology, Tectonics and Metallogenesis. Papers presented at a conference at the University of New England.
*Geological Survey of New South Wales. 2012. Grafton Tenterfield Airborne Geophysical Survey: Gridded and imagery data. Preliminary package from the Department of Trade and Investment: Resources and Energy.


  1. Hi Rodney, this should have been your most popular post (not CSG!). I wonder what age this intrusive is and if it ties in with Eocene or Miocene events. I have seen this dipole anomaly on earlier, primitive surveys, but thought it was a bit suspect.

    1. Good questions Dylan. I can't recall why but John Smith suggested older (Triassic) intrusives like those in south eastern Queensland. I can't help think that these could be related to the Cenozoic volcanism though I have no evidence of this. John Smith must have had good reason to suspect Triassic - I'll have to ask him why.

      You can probably see two other areas of interest on the southern edge of geophysics image in the too. I wonder what is going on around the Nightcap National Park and at Byron Bay... more questions!

  2. Hi Rod,

    John Smith probably has a very good reason for the early age and I too would like to hear his thoughts and I’m very excited that you are able to contact him.
    It is interesting that you mention the Byron Bay anomaly. I have noticed an increase in hydrothermal deposits in the Paleozoic sediments of Cape Byron and Broken Head, maybe this is related in some way. Yes, more questions.

  3. Hi Rod,
    I'd very much like to see some coordinates/scale or perhaps the coastline overlaid to the Cabarita Beach plutons. Can I achieve this or access the primary information for this specific instance without having to pay? Grateful for any reference or additional information you may be able to supply

    1. Hi Anonymous,

      I will be doing another blog post on the Cabarita area in a month or so. I will add the coastline to the magnetic data provided above.

      Obtaining the geophysical data is easy (other than the cost ~$120). It can be obtained from the NSW Department of Resources web page:

      There are no published papers etc which specifically deal with the Cabarita Beach hidden intrusions. However, John Smith did allude to the likely presence of intrusions in his excursion guide 7 Excursion Guide: Geology of the southern Beenleigh Block. I think this can be obtained from the Geological Society of Australia Bookshop for less than $20.

      You can also email me and I'll see what I can do. My email address can be found on the "about this blog" page.