Northern Rivers Geology

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.

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.

How do you find out about something you can’t visit? From time to time I’ve wanted to visit sites that were on private land but I was unable to contact the landholder. More recently I find that the landholders do not want me on their land because of fears that I’m something to do with a gas company exploring for coal seam gas reserves (which I’m not). However, there is one place that is nearly impossible to get to because of its remoteness and the level of control that a government department have (for good reasons). I’d dearly like to visit this place because of the history, biology and of course the geology. The place is South Solitary Island off the coast of Coffs Harbour and Woolgoolga.

South Solitary Island has a lighthouse and an old lighthouse keepers residence which is disused and slowly deteriorating. It is perched on a rock that just sticks straight out of the sea. A few small islands are part of the island group but they are all really just rocks sticking out of the ocean. I understand that the National Parks and Wildlife Service licence visits by tourists to the island lighthouse once a year by helicopter. I’d love to go but unfortunately I don’t think I could afford such a trip.
The Solitary Islands (and the South Solitary Island in particular) is known to be rock comprised of turbidites (marine mass wasting derived sediments) derived from volcanic parent rock and ash-fall tuff (Korsch 1993). This same assemblage is present on the mainland throughout the area called the Coffs Harbour Block or Coffs Harbour Association and is considered Carboniferous in age. The stratigraphic unit is probably the Coramba beds which mean there is also the possibility that chert, jasper and metabasalt are present as they are elsewhere on the mainland.

I had no idea about the geology of South Solitary Island until I read Korsch (1993) in which he was permitted to visit all of the solitary islands to determine whether the concept of a giant fold (called an orocline) was present off the coast. If the orientation of the rock strata was right it would demonstrate that the area between Brooms Head and Coffs Harbour and then inland up through the Orara region and eventually looping back up into Queensland was a giant fold in the earth. Korsch (1993) did observe just such features and this has resulted in much further interest and research (including papers published in the last 12 months) about the tectonic history of the New England and Northern Rivers. I will go into more details about the extraordinary folding and tectonic history in future posts as there is an incredible amount of detail and unknowns when it comes to our area.

Oddly, Weber et al (1978) mentioned that a report from 1945 that there is an area of molybdenum mineral deposit on the South Solitary Island. The size of the island (and being a national park) is such that it could never be mined but it is such an unknown curiosity. Webber et al (1978) describes the deposits:

Worthy of passing mention is an occurrence of molybdenite at the eastern extremity of the Demon Block. Narrow, molybdenite-bearing quartz veins have been reported from South Solitary Island, 16.5km northeast of Coffs Harbour, by Fisher (1945, p10). The host rock is unknown.

The reason this is a little odd in my mind is because molybdenite is not very common in the Coffs Harbour Block. Some molybdenum formed in areas related to specific types of intrusions to the south in the nearby Nambucca Block (e.g. see my earlier post on the Valla Monzogranite) but to my knowledge this has not occurred to any significant extent in the Coffs Harbour Block. Just another slightly out of place geological feature in our region.

References/bibliography:

Korsch, R.J. (1993) Reconnaissance geology of the Solitary Islands: constraints on the geometry of the Coffs Harbour Orocline. New England Orogen Conference 1993, University of New England.

Weber, C.R., Paterson, I.B.L & Townsend, D.J. (1978) Molybdenum in New South Wales. Geological Survey of New South Wales 43.

The Valla Adamellite now termed the Valla Monzogranite to reflect modern naming conventions is an interesting small to medium sized intrusion about 10km north east of Nambucca Heads. It is one of the suites of coastal granites which are mostly I-Types (melted igneous material), this means that the coastal granites show abundances of ore minerals within the granite or in the surrounding metamorphosed country rocks. A monzogranite is a granite with roughly equal proportions of (alkali-feldspar (potassium and sodium rich) and plagioclase feldspar (calcium rich)). The monzogranite is thought to have formed during the Triassic period.

The metamorphic aureole for the Valla Monzogranite is actually quite interesting as it shows a classic zonation of metamorphism (high grade at the contact grading to low grade further away) and also excellent examples of mineral zonation associated with metasomatism (hot-water or fluid alteration of rock). The Valla Monzogranite has been shown to be associated with gold, silver, arsenic and molybdenum mineralisation (as well as others). The rock that the monzogranite has been intruded into is called the Nambucca Beds which are part of the Nambucca Block. The Nambucca Beds are Permian to Carboniferous in age and are mainly comprised of the regionally metamorphic rock type called phyllite which was originally deposited on the sea floor. The Nambucca Block was accreted onto the Australian continent in the New England Orogen and this caused the regional metamorphism of the beds.

The Valla Monzogranite seems to be a Climax Molybdenum Deposit named after the Climax Mine in North America. This means that when the Monzogranite was cooling the upper portion of the pluton became residually enriched with fluids, metals and silica. These fluids cause alteration of the upper portion of the pluton forming what is called greisen and also are injected into the surrounding rock through veins and sometimes aggressively through breccia pipes. One of the first minerals to form in these veins is silica, quartz with metal sulphide such as molybdenite (molybdenum ore) and wolframite (tungsten ore). Further away from the intrusion the degree of alteration becomes less grading through potassic through to argillic which are defined alteration zones based on the changes in the rock forming minerals. As the degree of alteration becomes less so the types of metal ores change with increasing amounts or arsenic, gold and silver. Further out in the alteration zone minerals such as galena form (lead ore) and finally stibnite (antimony ore). These ore deposits seem to be fairly common in the New England area with Glen Eden being the most studied (Somarin 2001, Somarin & Ashley 2004) and have in some areas been extensively explored such as Kingsgate east of Glen Innes.

Some attempts of mining have occurred in the Valla Monzogranite in the past, the most significant being the Valla Gold mine which was located just to the north of Valla Beach. The mine was abandoned with very little rehabilitation and therefore has become an environmental problem for the local creek. However, rehabilitation efforts have recently been undertaken, though these will need another post to discuss in more detail.

References/bibliography:

*Somarin, A.K. 2011. Petrography, Geochemistry, and Petrogenesis of Late-Stage Granites: An Example from the Glen Eden Area, New South Wales, Australia. Earth and Environmental Sciences.
*Somarin, A.K. & Ashley, P.M. 2004. Hydrothermal Alteration and Mineralisation of the Glen Eden Mo-W-Sn deposit: A Leucogranite related hydrothermal system, southern New England Orogen, NSW, Australia. Mineralium Deposita.

One of the imposing landscape features on the north side of Armidale is the 1400m high Mount Duval. Some of my secondary education was in Armidale and I remember that the logo of my school actually had Mount Duval in it. Mount Duval is part of granite-like pluton called the Mount Duval Monzogranite. It was previously called the Mount Duval Adamellite; however the term Adamellite is no longer formally recognised. The intrusion actually extends in a crescent shape further to the west and includes Little Mount Duval which is roughly were the watershed for the Great Dividing Range sits, draining to the east all the way to the Macleay River. The monzogranite is considered to be middle Permian in age and intrudes several different complex rock units, one of these is a relatively small unit called the Dummy Creek Conglomerate.

The Dummy Creek Conglomerate is situated to the north of Mount Duval and extends to the east to the area of Puddledock, the northern side is intruded by the Highlands Igneous Complex. The Dummy Creek Conglomerate is comprised mainly of conglomerate but not exclusively. Lithic sandstone is a major component and it is actually what is in these sandstones that allow us to determine when the unit was formed, but more of that later. The abundance of conglomerate as well as sandstone and rarity of fine grained sediments like mudstones shows us that the sediments, gravels, etc that made up the Dummy Creek Conglomerate have not travelled far from their source. The clasts in the conglomerate show that the source rock was the underlying Carboniferous aged Sandon Beds (part of the Texas-Woolomin Block).

Korsch (1982) concludes that the original Sandon Beds was domed and uplifted by the intrusion of granite bodies of the New England Batholith such as the Mount Duval Monzogranite and the Highlands Igneous Complex. The hills formed from the deformation of the Sandon Beds began shedding rock, eroding and the sediments were deposited a short distance from these new hills. The intrusions continued to intrude shortly after the sediments were deposited which according to Holland (2001) created a complex system of overlapping zones of contact metamorphism. The intrusions were therefore emplaced in a very shallow crustal situation and volcanism was abundant and the Dummy Creek Conglomerate was quickly covered and preserved by a volcanic unit that is called the Annalee Pyroclastics which includes lavas, pyroclastic deposits and the like. It is worth noting that other models of formation by various other authors were summarized by Holland (2001) for instance some authors suggest that rock fabric studies may show a source only from the south.

A lot was happening in the Mount Duval-Tilbuster-Puddledock area during a relatively short period of geological time, indeed even during this time of change a substantial forest must have been growing in the area. The sandstone layers in the Dummy Creek Conglomerate preserve fairly common plant fossils. Most of the fossil remnants are fragments but there is enough to identify many plants with certainty. The most common fossil identified was the deciduous plant Gangopteris, a relative of the more commonly known Glossopteris, the main plant that formed the coal of the Sydney Basin. This plant existed abundantly in the middle of the Permian and so given that many of the rocks appeared to be forming at the same time these can be assumed to be close to this age too.

References/bibliography:

Holland, R. 2001. South western Margin and Contact Rocks of the Highlands Igneous Complex near Orana Falls, North of Armidale, NSW. Unpublished undergraduate research thesis, University of New England.
Korsch, R.J. 1982. The Dummy Creek Association: Rim Syncline Deposits. Journal and Proceedings of the Royal Society of New South Wales. V115.

I finally found them, photos of some of the strange metamorphic rock at Wongwibinda. I recently moved house and in the process I’ve lost many things but also found some things. Early this year I did a post on what were the broader conditions that lead to the geology of this area between Guyra and Ebor, namely thinning of the continental crust leading to increased heat flow and corresponding thermal metamorphism. I mentioned a rock type called migmatite and since I found my photos of the Wongwibinda migmatite, I thought I should go into a little more detail on this curious metamorphic feature.

The migmatites are strongly metamorphosed rocks of the Girrakool beds. The Girrakool beds are Carboniferous in age and were deposited in a marine environment. These beds were then accreted onto the edge of the Australian continent as part of the New England Orogen, much deformation occurred during this time. During or following this stage of tectonic forces that affected the New England region the Girrakool beds were subjected to a period of intense metamorphism. This affected one end of the beds more than the other. The western most part of the Girrakool beds in the Rockvale area remained relatively ‘uncooked’ but further to the east the effects of thermal metamorphism became greater creating schists known as the Ramspeck Schist and finally the zone of migmatites. The migmatites are faulted off by the Wongwibinda fault on the eastern side or are intruded by the Abroi Granodiorite which itself has been later metamorphosed into Gneiss.

The odd thing about the Wongwibinda migmatites generally is that they are actually three rocks in one: metamorphic sedimentary rocks becoming igneous at the same time. Usually rocks fit into the igneous and sedimentary categories neatly and then metamorphism can affect these rocks. In the case of migmatite the metamorphism is so great that the rock actually begins to melt, that is, it becomes an igneous rock with some of the sedimentary rock remaining unmelted. A characteristic of migmatite is ptygmatic folding, which is intense small scale folding with alternating light and dark bands. The dark bands are called the palaeosome which is the remains of the sedimentary rock and the lighter bands is insitu accumulation of melted rock called the Leucosome,. The leucosome is here comprised mainly of the minerals quartz, feldspar, mica and some garnet. Sometimes the leucosome can ‘break free’ from the ptygmatic folds and create dyke like structures. All of these features are visible in the picture opposite.

What can be seen at Wongwibinda is essentially the formation of a granite, specifically a S-type (sedimentary derived), frozen in time. Craven et al 2012 demonstrated that this time was very close to the Carboniferous-Permian age boundary, probably just in the Permian, that is around 297 million years ago. There are some fancy geological features in the New England highlands and in my mind this is one of them. If you travel up that way and see some rocks by the side of the road be sure to stop and look closely, there are so many unusual things to find.

References/bibliography:
*Danis, C.R., Daczko, N.R., Lackie, M.A. and Craven, S.J. 2010. Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences V57.
*Craven, S.J. Daczko, N.R. and Halpin, J.A., 2012. Thermal gradient and timing of high-T-low-P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia. Journal of Metamorphic Geology V30.
*Wilkinson, J.F.G. 1969 The New England Batholith - introduction. IN Packham G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.

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.


References/bibliography:

*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.

In an earlier post, Booyong not in the Clarence Moreton Basin? I indicated that I had been informed that there was evidence of Palaeozoic sedimentary rocks near Booyong, and that if this is the case there is significant implications on the morphology of the Mesozoic aged Clarence Moreton Basin in the area between Byron Bay and Lismore. It therefore has implications for the exploration for coal seam methane too.

Well, I can confirm that after much searching, I came across an ephemeral creek north of Booyong near the Casino to Murwillimbah Railway line at Nashua that clearly contained much chert. Additionally, a railway cutting appeared to have significant folding, but was very weathered so identification of the parent rock and indeed even whether it was folding was not clear.

Chert is a rock that is essentially absent from the Clarence Moreton Basin, however, it is common in the underlying rock of the Beenleigh Block, known as the Naranleigh-Fernvale Group. This group of rocks formed in a deep marine environment. It consists of turbidite sediments that occur when parts of the undersea continental slope erode and some slates and chert. The Chert is formed on the ocean floor where chemical and biological material settles over a long period of time. All of this rock was then transported and accreted onto the edge of the Australian Continent before the Devonian or Carboniferous period which was around 333-343 million years ago during the formation of the New England Orogen. This process of accretion has caused folding and low grade regional metamorphism to be common place in this geological unit. By comparison, the sediments of the Clarence-Moreton Basin are dominately continental derived and are un-metamorphosed. They were mainly deposited in river systems, lakes or at best shallow marine environments. Chert does not form very well in these environments.

But, what is so exciting about this outcrop? The Neranleigh Fernvale Group occur along the edge of the hills above the coastal plain south of Ballina and are exposed just about everywhere from the Gold Coast to Byron Bay. If you follow the line of outcrop from near Ballina you will find it is overlain between Byron Bay by the Clarence Moreton Basin which extends all the way to the Tabulum in the west. But because the outcrop occurs at Nashua this shows us that the area to the east is probably a different basin from the main part of the Clarence Moreton to the west. This has significant implications for understanding the structure of the earth here and also affects where resources such as gas can be explored for. For those that were worried about coal seam gas, you won't find any at Nashua.

In the headwaters of the Aberfoyle and Guy Fawkes Rivers, tributaries of the Clarence River lies a rare zone (for Australia) of rocks that have experienced high temperatures but surprisingly low pressure. This is another one of those “we don’t have a clear answer” posts, in particular what has actually caused the metamorphism of the rock in this area, but research just published in January (Craven et al. 2012) has shed a lot of light on the matter.

Originally it was thought that the metamorphism at Wongwibinda (The Wongwibinda Metamorphic Complex) was directly associated with the emplacement of the Granites since the most intensely metamorphosed rocks are close to the Permian aged Abroi Granodiorite and other Permian granites with a decrease in intensity of metamorphism further away from these intrusions (Wilkinson 1969). The depth of metamorphism was never considered very deep because the minerals that are present in the metamorphic rocks are not formed where intense compression is found. However, it has since been observed that contacts with some of the granites shows no or little metamorphic effects, notably along the contact with the southern part of the Abroi Granodiorite. Additionally, the Abroi Granodiorite itself displays some metamorphic textures making the picture relatively unclear.

Like many parts of the New England, the geology can be quite complex with many aspects and relationships not fully understood and this holds for the Wongwibinda area which is a area of metamorphism, abundant faulting, granite intrusions, deep sea sedimentary and volcaniclasitic rocks and basalt lavas. Describing the generalgeology may be easiest from east to west. Before coming to the properties of Abroi and Springfield is the Palaeozoic aged Dyambarin Beds which is neatly faulted off to the east. The eastern side of the Wongwibinda Fault lies rock of the Abroi Grandiorite (a type of granite), sometimes referred to as the Abroi Gneiss (in this case metamorphosed granite which appears to have been affected by the Wongwibinda Fault). Then just to the west of this we enter what is significantly altered Girrakool Beds and this is were it gets even more interesting.

The eastern most part of the Girrakool beds has been significantly affected by heat maybe up to about 700 degrees Celsius, but has experienced very low pressures and this has created an unusual texture called migmatite. Migmatite is a type of rock were the parent (in this case sandstone and mudstone sedimentary rocks of the Girrakool Beds) has been heated so much that it just starts to become liquid like, here it also shows Ptygmatic folding. The liquid usually accumulates or is formed in some individual layers creating essentially layers of molten rock between sediments. Sometimes the hot liquid rock follows cracks in the rocks creating little dyke like structures too.

Moving further to the west we enter a zone of schist (called the Rampsbeck Schist), which is a medium grade metamorphic rock that has had some of the crystals in the rock reform into layers, this schist extends further west showing less and less metamorphic effects until it is indistinguishable from the rest of the Girrakool Beds. There are also some areas of quartzite and amphibolite (other metamorphic rocks) in the schist zone. I’ve also seen a pegmatite dyke a bit further to the west, which I have no idea where it fits into the picture.

Over the top of all this are remnants of comparatively recent Basalt referred to as Tertiary (or Mid-Cenozoic aged) Alkali Basalt. Given its location this basalt is probably Doughboy Basalt, part of the Cenozoic aged (~40Ma) Doughboy Volcanic Province.

But what caused the formation of the metamorphism? Many mechanisms have been proposed by different authors such as Wilkinson (1969), Danis et al (2010) and Craven et al (2012) and other authors. Craven (2012) has carried much work, including dating to try and gain an understanding:

• Was it the Wongwibinda fault? No – otherwise the rock would pressure related textures.
• Was it the intrusion of the Abroi Granodiorite (or other granites in the area)? No – otherwise we’d see metamorphism around all the Abroi Granodiorite that we don’t see, and the age of the Abroi Granodiorite is older than the date of metamorphism.
• Was it an intrusion that we can’t see because it fairly deep underground? No – gravity surveys have been conducted and these don’t show any deep granites other than the Abroi Granodiorite in the area of maximum metamorphism.
• Was it the eruption of the Cenozoic Basalts? No, the age of metamorphism vastly predates the Cenozoic period.

Wow, so what options are left? Craven et al (2012) have come up with a theory: following the tectonic events that formed the granites in the area there was a period of stretching of the earth, this thinned out the crust and allowed for heat to be more easily transferred from the mantle. All of the other options are more common elsewhere in Australia and around the world, but each option has been refuted by different evidence until the only reasonable explanation left at this stage is the extension of the crust to allow convective heat transfer from the mantle to very shallow levels during the Permian.

I seem to have lost my old photographs of the migmatite but I have found a good website on Finnish migmatites that has some great pictures. It can be linked to from here.

A follow up post on the Wongwibinda migmatites can be found here.

References/bibliography:

*Danis, C.R., Daczko, N.R., Lackie, M.A. and Craven, S.J. 2010. Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences V57.
*Craven, S.J. Daczko, N.R. and Halpin, J.A., 2012. Thermal gradient and timing of high-T-low-P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia. Journal of Metamorphic Geology V30.
*Wilkinson, J.F.G. 1969 The New England Batholith - introduction. IN Packham G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.

Deep within the earth below the seas (so deep in fact we begin to enter the Earths upper mantle) we find material that is solid but so hot that it is viscous. This material is very low in quartz and when we see this rock on the surface it is unusual. The only way for such rock to come to the surface is through great wedges being thrust on to the edges of continents as the great oceanic plates move on the mantle. The upper units of rock from oceanic plates is greywacke from turbidites from collapsing continental shelves or the pelagic sediment accumulated over vast periods of time. But also you will find volcanic rocks erupted under the water at mid-ocean ridges and below these great thicknesses of basalt cooled into columns and even further below these great plutons of the mafic rock called gabbro which is the source of the basalt on the surface. Yet even deeper we start transitioning into the mantle and here we find rock that contains very little silica (ultramafic rocks) but is rich instead in iron and magnesium. These are called peridotites and dunites when found in rock form. From top to bottom the section is called an ophiolite sequence and these occur infrequently on the earths surface.

Given that the highlands of the New England region are derived from accretionary material scrapped off the sea floor during collision with the Australian Plate we have a good chance to find some. And we are in luck. I know of three significant areas in this region where ophiolite is preserved the two biggest are located north of Tamworth along the peel fault and at Port Macquarie. A smaller area can be found north-west of Grafton at the little village of Baryulgil, located midway between Tabulam and Copmanhurst.
The ophiolite at Baryulgil is unusual because only a portion of the ophiolite is preserved, this being the peridotite and dunite altered to a rock called serpentinite and a small area of gabbro. It is also worthy of note because of the damage such a rock has caused the local people. The serpentinite at Baryulgil is known as the Gordonbrook Serpentinite and includes such serpentine minerals as chrysotile – better known as a mineral of the asbestos group. Mining of this industrial mineral by Australian Asbestos and later by James Hardie occurred at Baryulgil for quite some time and it is this that has caused many problems.

Stepping slightly into the area of politics and aboriginal relations (and then quickly away again) the Baryulgil asbestos mine was often held as a wonderful example of how an indigenous population could be assimilated into the good things of western culture. Alas, as we know too well today that model of assimilation was flawed, in part in the case of Baryulgil because of the harm to its workers from such a carcinogenic material. Reportedly the mine and its processing plant had an appalling reputation for dust which is the main mechanism that causes the entry into the body and the subsequent long term damage including a massive increase in the risk of cancer. As an aside, it is worth noting that even the Nazi party in Germany before the Second World War (and greater than 40 years before the closure of the Baryulgil mine) introduced regulations to ensure that dust was minimised when working with asbestos because of the probable heath effects.

The Gordonbrook Serpentinite is a body approximately 25km long elongated unit right on the edge of the New England Fold Belt accretionary terrain. Geophysical surveys including gravity and magnetics indicate that the unit probably much larger than the area exposed as it appears to underlie the Clarence Morton basin just to the east of Baryulgil. The unit shows a gravity anomaly given its composition from heavy minerals and the magnetic signature shows up because of the richness of iron when compared to the more recent Jurassic aged sediments (Laytons Range Conglomerate and Gatton Sandstone) of the Clarence Moreton Basin and the accretionary complex meta-sediments to the west.

The gabbro unit of the ophiolite sequence is present as a small remnant unit on the north western most part of the serpentinite body on the northern side of the Clarence River. Interestingly the Clarence River pretty much runs straight though the middle of the serpentinite as it meanders from the mesozoic clarence moreton basin sediments into and out of the older accretionary terrain. This meandering has implications for indicating the history of the river development of the Clarence. But more about the Clarence River in another future post.

The minerals present in the serpentinite are mainly comprised of serpentine (a type called antigorite) but there is asbestos (chrysotile) occurring naturally in vein systems. Altered serpentinite also locally forms magnesite which is a white chalk like mineral formed through the affects of carbon dioxide rich ground water. The nature of the serpentinite and ground water alteration and reposition of secondary minerals is such that metals such as arsenic, and particularly nickel and cobalt are also quite rich in small patches. But these minerals are hard to come by unless intersected by cuttings or mine workings.

If you pass through that way to explore the more remote corners of our region take note of the roads. The councils that managed the area have previously maintained and unpgraded the roads with locally sourced rock. This means that the road base is often made from serpentinite. This has caused made road management problematic because the current Clarence Valley Council to minimise the risk of exposure to asbestos when staff or contractors are maintaining the roads!

Another feature of the Baryulgil Serpentinite is that it helps to demonstrate a theory about a major period of deformation in Eastern Australia. This formed tectonic features called the Coffs Harbour Orocline and the Texas Orocline, but there is too much to discuss about this now so I will have to dedicate a post about this in the future.

References/bibliography:

*Cornwell, J 2004 Hitlers Scientists: Science, War and the Devil's Pact. Penguin Books
*Henley, H.F. , Brown, R.E. , Brownlow, J.W. , Barnes, R.G. , Stroud, W.J. 2001 Grafton-Maclean 1:250 000 Metallogenic Map SH/56-6 and SH/56-7: Metallogenic Study and Mineral Deposit Data Sheets Geological Survey of New South Wales.
*Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.

A few months ago I was reading the 2011 NSW National Parks and Wildlife Service plan of management for the Julian Rocks Nature Reserve just offshore, near Byron Bay. The introduction said the Julian Rocks “are composed of Brisbane Metamorphics which date from the Carboniferous-Devonian period 345-405 million years ago and are the most resistant rock type in the region”. Sounds fine as a bit of background but why can’t I find recent geological work that refers to the Brisbane Metamorphics anywhere else?

Academics from Southern Cross University have used the term in published works as recently as 2007 (Specht and Specht 2007). But I can’t find it on any map or in any geological publication after 1990. Surely the rocks haven’t been eroded that quickly especially since it is “the most resistant rock type in the region”. I can, however, find reference to the Brisbane Metamorphics on the 1: 1000000 scale NSW geological map from 1962. But at such a scale it is hard to figure out exactly where it is. Broadly it appears to be located in some areas near Murwillumbah and some areas near the border with Queensland. The most specific paper I have is by Holcome (1977) which discussed the Brisbane Metamorphics in depth but doesn't say where it goes!

When in doubt try Google? But the result you get when typing in “northern rivers geology” is the website Big Volcano. It can be found here. Here too the geological history summary refers to the Brisbane Metamorphics but mistakenly links to a site that shows a small contact metamorphic area at Mount Coot-tha just to the west of Brisbane. This is a bit confusing because the metamorphic rock here is called the Bunya Phyllite which is a regional metamorphic rock which has undergone a second metamorphic even during the emplacement of the granite that makes up Mount Coot-tha. Interesting in itself, but it does not answer our question why the Brisbane Metamorphics were said to be at Byron Bay!

Well, The answer is simply a case of one of the most difficult aspects of geology, nomenclature. Geoscience Australia provides an excellent service in maintaining a database of all geological units named in Australia (past, present and proposed). It includes an entry on the Brisbane Metamorphics which can be found here. On the webpage you can see three fields that are important for knowing where this unit has gone. “Current: No”, “Status: Obsolete”. The comments field answers the question finally: “Name superseded by Rocksberg Greenstone, Bunya Phyllite, and Neranleigh-Fernvale Formation.”.

What this means is that the one description of Brisbane Metamorphics did not reflect the ages, genesis, and history of these three rock units. You will get more information about the structural history and rock composition if you deal with the new units individually. Indeed, Holcome (1977) discusses the constituents of the Brisbane Metamorphics at length and notes that the Rocksberg Greenstone, Bunya Phyllite and Neranleigh-Fernvale Group are the constituents of the Brisbane Metamorphics but these are substantially different in terms of formation, metamorphic history and exposures. In northern New South Wales some of these units are present as part of what is called the Beenleigh Block (Holcome (1997).

There are many cases where geological units have been renamed or reclassified after further research has been done. This is no different to any other area of science. The only challenge is keeping up with the change.

References/bibliography:

*Holcome, R.J. 1977. Structure and tectonic history of the Brisbane Metamorphics in the Brisbane Area. Journal of the Geological Society of Australia. V24.
*NSW National Parks and Wildlife Service. January 2011. Julian Rocks Nature Reserve: Plan of Management.
*Specht, R.L. , Specht, A. 2007. Pre-settlement tree density in the eucalypt open-forest on the Brisbane Tuff. Proceedings of the Royal Society of Queensland 113 p9-16