Cooking the rocks at Emerald Beach

I have always been interested in the little things in life. The things that don’t get the attention that everything else seems to get. This even applies to rocks and rock outcrops. It applies to a little headland that I visited on a trip to Coffs Harbour earlier this year. The headland has no name but lies on the northern side of Emerald Beach and the village of the same name. It is made from a granite-like rock of a poorly understood suite of intrusions in north eastern NSW.

Boulder on Emerald Beach. Note the xenolith at the bottom

The rock is formally called the Emerald Beach Monzogranite. It is the eastern most granite on the Australian continent is also one of the youngest rocks in the New England area. The Emerald Beach Monzogranite has been dated at 228.5Ma and part of an informally super suite of granites called the Coastal Supersuite (Chisholm et al 2014). Originally the unit was formerly defined as the Emerald Beach Adamellite (Korsch 1978) but has been renamed to reflect the most up-to-date nomenclature. However, the name Monzonite (and hence Adamellite) is misleading. The composition of the rock is consistent with the definition of Granodiorite (Plagioclase Feldspar abundance greater than that of Potassium Feldspar (Korsch 1971, Chisholm et al 2014). No reference to Monzogranite (or Adamellite) have been made and the samples I’ve seen were plagioclase feldspar dominant so the present classification appears erroneous. Maybe the name Emerald Beach Granodiorite might be more correct.

The dating of the Emerald Beach Monzogranite was only conducted in the last couple of years. It is an example of using multiple techniques together to get an answer. The mineral Zircon is formed in magma chambers of granite and granite-like composition. This is a very stable mineral. Zircon locks up uranium in small amounts and this uranium undergoes radioactive decay to lead. By measuring the proportions of uranium to lead it is possible to determine how long ago the zircon had formed. By this method Chisholm et al 2014 narrowed the age down to about 228.5 million years old. This is the Upper Triassic era which was the time of the best known dinosaurs.


Xenoliths of country rock are present in the rock (you can see an example in the picture above). These darker coloured xenoliths are inclusions of country rock which has been caught up in the magma chamber and have not quite been completely melted into the rest of the liquid rock. In the case of the Emerald Beach Monzonite the xenoliths are slightly elongated and display a preferred orientation. This orientation is probably caused by following the direction of intrusion of the molten rock (Korsch 1971).

The intrusion of the magma heated up the surrounding rock into which it had been emplaced. This heating up forms what is termed a contact metamorphic aureole (a metamorphic zone of effect). The Emerald Creek Monzonite had heated the muds in the surrounding deep sea Coramba Bed rocks to such an extent that new minerals were formed including very small but abundant crystals of biotite mica. Biotite mica forms at approximately 500 degrees Celsius (but varies by pressure) and disintegrates when hotter than about 800 degrees. Therefore the temperature of the molten rock was probably at least this. This type of contact metamorphic rock is referred to as hornfels.

It is an interesting example how little aspects again can tell a lot about how rock forms. Preferred orientation of xenolith inclusions and the formation of biotite in the surrounding rock show both the direction that the magma was moving and its temperature at the time. Have a look if you are in the area and see if you can spot some of the xenoliths. Those that are really in the know can say that the Emerald Creek Monzonite seems to have been incorrectly named.

References/Bibliography:

*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia
*Korsch, R.J. 1971. Palaeozoic Sedimentology and Igneous Geology of the Woolgoolga District, North Coast, New South Wales. Journal and Proceedings of the Royal Society of New South Wales. Vol. 104.
*Korsch, R.J. 1978. Stratigraphic and Igneous Units in the Rockvale-Coffs Harbour Region, Northern New South Wales. Journal and Proceedings of the Royal Society of New South Wales. Vol. 111.

Baseline CSG methane in groundwater

A friend recently let me know that a paper that one of his students wrote for the Journal of Hydrology had been published. I had a very minor involvement in the formative stages of the paper which came about indirectly as a result of the protests of many local people about potential coal seam gas (CSG) and other natural gas types in the region. The paper (Atkins et al 2015) is essentially the results of a data collection exercise but has some interesting techniques and findings about the baseline concentrations of gas in groundwater bores in the Richmond Valley area.

Methane concentration for different geological environments
(after Atkins et al 2015)

91 water samples were collected from government and private bores in geological units overlying the target CSG geological formations in the Clarence-Moreton Basin (e.g. the Walloon Coal Measures). These units were quite diverse and ranged from sedimentary rocks of the Piora Member of Grafton Formation and the Kangaroo Creek Sandstone (recently reclassified as the Orara Formation), basalt lava flows of the Lismore, Astonville and Kyogle Basalts and Quaternary aged alluvium including coastal sands and riverine sedimentary environments.

Special glass water sample containers were used to collect the samples. These were then injected with a carbon dioxide and methane free gas to create a clean “air bubble”. The methane and carbon dioxide naturally dissolved in the water will then come into equilibrium with the “air bubble”. The resulting gas from the bubble can then be extracted and the concentration and isotopic composition of the carbon in the two compounds determined by an electronic analyser. The isotopic signature can then be assigned to recent biological formation (biogenic) or geologically derived (thermogenic) origin.

The end result was annoyingly quite not straight forward. The concentration of methane showed no obvious relationship to the chemistry of the groundwater. However there was a relationship between geological units. Methane concentration was very low in the basalt aquifers and relatively higher than the Clarence-Moreton basin sedimentary rocks and much higher in the Quaternary alluvium of the Richmond River valley floodplain and coastal sands systems. So there was more methane in some of the aquifers that were the less likely to be connected to any CSG formations! Quite counter-intuitive.

The isotopic signatures did not really help clear up this confusion very much. There appeared to be a large thermogenic component to the coastal sands and flood plain aquifer systems sometimes at concentrations greater than the formations that should be the thermogenic CSG source. Why? It was noted by some CSIRO scientists working in the Great Artesian Basin that sometimes biogenic gas can be oxidised and then be chemically reduced back to methane and this process favours the thermogenic isotopes (Day et al. 2015). So, It gives the impression of thermogenic gas.

This means that the methane gas concentration is related to the biological activity in and around the aquifer. The shallowest groundwater systems are the most connected with surface water and biological processes and therefore these have the highest concentrations of methane. The Clarence-Moreton Basin sediments are not connected with the CSG and natural gas rich formations.

This means that if companies like Metgasco do commence gas operations in the area there is a statistical background that can be used to compare if anyone becomes concerned about methane in their water bores. Interestingly, it also shows that methane in groundwater is probably not a good method to search for natural gas in the region. It might apply to other areas like the Great Artesian basin but apparently there are good barriers between CSG and non-aquifers in the Northern Rivers. This is good news since if something does go wrong it is now more easy to identify if it has impacted upon any groundwater.

References/bibliography:

Atkins, M.L., Santos, I.R. & Maher, D.T. 2015. Groundwater methane in a potential goal seam gas extraction region. Journal of Hydrology: Regional Studies. V4.
Day, S., Ong, C., et al. 2015. Characterisation of regional fluxes of methane in the Surat Basin, Queensland. CSIRO report EP15369