Northern Rivers Geology

Note that since this post was written the Towgon Grange Granodiorite has been renamed the Towgon Grange Tonalite.

Many people in the region know about “The Gorge”. It is a remote, yet popular area on the Clarence River. The road to The Gorge is interesting because of the change in geology that is experienced. The main route to The Gorge is via Grafton and Copmanhurst. By travelling west from Copmanhurst along the Clarence Way you move from the sedimentary rocks of the Clarence-Moreton Basin. First, the rugged cliffs made from the Kangaroo Creek Sandstone give way to the rolling hills of the Walloon Coal Measures then Koukandowie Formation. Some road cuttings show weathered examples of these rocks. Turning off the Clarence Way and passing over the camping ground, swimming hole and bridge at Lilydale leads you to The Gorge turn-off. The Lilydale and Newbold areas have some of the oldest rocks of the Clarence-Moreton particularly the Laytons Range Conglomerate. But on the day I was there, I was not so interested in those rocks… because I was getting into the New England Orogen.

It is rare opportunity for me to explore the foot hills of the New England region. I love the feeling of the place, the wonderful landscape, climate, history and even culture. The place just seems to have a feeling of connection with the people who live there. Luckily, I managed to visit the edges of the New England escarpment for a little while on the weekend. While there I managed to experience more of the rocks that are the foundations of the landscape of New England.

Driving along The Gorge road the rocks of the Silverwood Group are passed by. These are slightly enigmatic rocks of the New England Orogen, interpreted as subduction complex rocks (Van Noord 1999). Mainly outcropping in streams the rock of the Silverwood Group in this area are none the less quite hard and old metamorphosed marine sedimentary and volcanic rocks. The Silverwood Group is interesting because it also occurs near Texas in Southern Queensland and it is only partially understood in our region. But more about the Silverwood Group in a future post.

Round tors appear by the road side near Table Creek about 15km south of The Gorge. These tors are a classical shape formed by the weathering and erosion of granite type rocks. Here are rocks that make up part of the New England Batholith. The batholith is numerous masses of intrusive igneous rocks plutons that were molten well before Australia was separate from Gondwana. The ‘granite’ here is called the Towgon Grange Granodiorite. Like the Dumbudgery Creek Granodiorite that occurs about 20-30km further north the pluton is bisected by the path of the Clarence River. This helps to illustrate the unusual behaviour of the Clarence river as it travels backward and forward over soft and hard rocks. In fact the other side of the pluton can be easily found on the other side of the river just off the Clarence Way.

The Towgon Grange Granodiorite intrudes into the Silverwood Group meta-sediments. The rock sample at Table Creek (pictured) is actually not a granodiorite. It is notionally similar in appearance but contains much less potassium-feldspar. The main minerals are light coloured plagioclase feldspar, quartz and darker clinopyroxene and amphibole. The rock sample shows that much of the clinopyroxene is mantled (surrounded) by amphibole. The lack of potassium-feldspar means that this particular sample is probably a Tonalite according to the most popular rock classification (QAPF). In fact Bryant et al (1997) actually notes that the Towgon Grange Granodiorite only contains small amounts of Granodiorite, with most being Tonalite or Quartz Diorite. This is a good example how stratigraphic names may be misleading to first time geologists!

Bryant et al (1997) classifies the Towgon Grange Granodiorite as an I-type granite of the Clarence River Supersuite. This means that the Towgon Grange Granodiorite is derived from the melting of other igneous rocks. The Towgon Grange Granodiorite is also comparatively low on silica (quartz) in comparison to other Clarence-River suite intrusions. It still contains enough quartz that it is generally visible in hand specimens. The age of the Towgon Grange Granodiorite is about 248-249Ma old. The younger sedimentary rocks of the Clarence-Moreton Basin overlie parts of the Towgon Grange Granodiorite and Silverwood Group.

The Towgon Grange Granodiorite is one of those rocks that just about no one in the general public has heard of. But, it is a good example of rocks that illustrate many points about the landscape evolution of the New England Orogen and the Clarence River. It occurs in a scenic area and is also a very attractive rock in its own right.

References/bibliography:
*Bryan, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-Type Intrusions in Eastern Australia. Journal of Petrology V.38 No. 8.

*van Noord, K.A.A. 1999. Basin development, geological evolution and tectonic setting of the Silverwood Group IN Flood, P. G. (ed.) Regional Geology Tectonics and Metallogenesis: New England Orogen - NEO '99 Conference University of New England.

In a previous post, I discussed the metamorphism of limestone at an area north-west of Tabulam. I thought I’d take the opportunity to discuss the intrusion itself that caused the metamorphism (a rock unit called the Bruxner Monzogranite). Also briefly, put it in the context of the formation of the broader New England Batholith.

The Bruxner Monzogranite is a geological unit that is composed of a series of ‘granite’ plutons (intrusions of molten magma). These occur in a hourglass shape between Drake and Tabulam. The biggest areas occurring north and south of the Bruxner Highway and the central thin part of the ‘hourglass’ occurring where the Bruxner Highway crosses it.

The Bruxner Monzogranite is part of the Clarence River Super Suite of granites which is an I-type granite (Bryant et al 1997). I-type granites are derived from melted igneous rock. It contains two different varieties of ‘granite’ (Thomson 1976). One variety is the rock type monzogranite which contains roughly equal amounts of the two main feldspar groups (plagioclase feldspar and alkali feldspar). It also includes quartz, amphibole and biotite mica.

The slightly less common variety is the granodiorite which contains more alkali feldspar than plagioclase. Therefore, it is richer in the elements sodium and potassium . It is worth noting that the granodiorite is often more altered and is more quartz rich. The easiest way to distinguish between the two Bruxner Monzogranite varieties in the field is their colour: The granodiorite usually has a pink colour and the monzogranite grey. The relationship between the two varieties of granite is not very clear to me. The following questions immediately spring into my mind:

• Does one granite intrude the other?
• Were they both molten when they were emplaced? Or was one crystallised first?
• Was it fluids from the crystallising monzogranite that caused the alteration of the granodiorite?

Maybe, they are questions that someone knows about but has not published their work on, or maybe they are just one of the many geological questions unanswered.

Bryant et al (1997) gives the potassium-argon age of the Bruxner Monzogranite as 250Ma. This places it in the Triassic Era, the same age as the other nearby Clarence River Supersuite. Such as, the Jenny Lind Granite which occurs a few kilometres north of the Buxner Monzogranite. The Clarence River Supersuite ‘granites’ are a similar age to many other granites which occur throughout the New England. This was certainly a busy time for intrusions. Indeed, these granites probably represent the magma source for an eroded volcanic arc system. It was caused by a large west dipping subduction zone that was active during this time (Scheibner & Basden 1998).

The Bruxner Monzogranite was intruded into Emu Creek Formation which is Carboniferous to Permian aged (Bottomer 1986). It is comprised of mudstones, greywacke, siltstones, shale, sandstones, conglomerate and limestone. As mentioned in my earlier post on limestone in the area, metamorphism of these rocks has in places been quite pervasive with a distinct metamorphic aureole. This has created some interesting rocks and altered zones such as marble and iron rich skarn.

The Bruxner Monzogranite is overlain in some areas by sediments of the Clarence-Moreton Basin. In particular, the Woogaroo Subgroup of the Bundamba Group, mainly the Laytons Range Conglomerate. Weathered exposures of the Laytons Range Conglomerate can be seen in road cuttings on the Paddys Flat Road.

The Bruxner Monzogranite was once called the Bruxner Adamellite (the term adamellite is no longer recognised). It is named after the Bruxner Highway which passes right through the unit. Adjacent to the Bruxner Highway, approximately 2-3km west of Plumbago Creek, is one of the best places to see the outcrops of both the monzogranite and granodiorite. A good place to see the monzogranite is along the ridges along Sugarbag Road which is in the northern part of the unit, off Paddys Flat Road.


References/bibliography:

*Bottomer, L.R. (1986), Epithermal silver‐gold mineralization in the Drake area, northeastern New South Wales, Australian Journal of Earth Sciences. V33.
*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cirdilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. V38.
Scheibner, E. & Basden, H. 1998 Geology of New South Wales – Synthesis. Volume 2 – Geological Evolution. Geological Survey of New South Wales, Memoir Geology 13.
*Thomson, J. 1976 Geology of the Drake 1:100 000 sheet, 9340. Geological Survey of New South Wales 1v.

It has been some time since I spent a lot of time in the Clarence River area but occasionally I’ve got back there. Not too long ago I travelled along the Clarence Way. I took a quick detour down Lionsville Road when I came to the village of Baryulgil. Only a few kilometres down the road there is a quaint long low thin bridge over the Clarence. Just on the opposite side is a spot reguarly used as a swimming spot, there is also a tourist attraction for geologists (A road cutting).

The road cutting and the stream bank expose boulders of ‘granite’ rock which make up part of the New England Batholith. Right there at the bridge is a great spot to see one of rocks that make up what is called the Clarence Supersuite, a suite of ‘granites’ that have been derived from the melting of older igneous rocks. According to Bryant et al (1997) the Clarence River Supersuite for which this rock is a member is a type of ‘granite’ called an I-type. The ‘I’ stands for melted igneous in origin (as apposed to S-type for melted sedimentary). There is a lot to say about the New England batholith, its different granite types and its models of formation, such that I will do several blog posts in the future to cover this topic better.

The actual pluton in this area is called the Dumbudgery Granodiorite and it extends a few more kilometres to the north and for many kilometres to the south. Good outcrops can be seen in the hills on the southern side of Lionsville Road if you continue to the west a bit further. Indeed quartz veins in this area also contain small amounts of cinnabar (mercury ore), others contain some gold. If you want to take a sample or have a look at a fresh piece, a hammer (preferably a big one) with appropriate safety goggles is required. It is hard rock! But a fresh piece of granodiorite reveals a lovely white, pink and black speckled appearance. It can be so pretty that is is worth going on display.

The colours of the rock reflect the mineral composition. Normal granite has a large proportion of alkali feldspar (sodium and potassium rich) relative to the amount of plagioclase (calcium and sodium rich) feldspar. A granodiorite like the Dumbudgery Granodiorite contains more plagioclase than alkali feldspar but still enough to be common in the rock. In the specimens at the Clarence River the plagioclase is a cloudy grey colour, sometimes difficult to distinguish from the quartz (tends to clearer) but the other alkali feldspar is a lovely bright pink colour. The lighter colours are contrasted by the two black minerals which are hornblende and biotite. The hornblende is identified by its hardness relative to the biotite which is a form of mica and therefore very easy to scratch. The Dumbudgery Granodiorite has been previously dated at 249 million years (very early Triassic period which is part of the mesozoic era).

Oddly the mass of granodiorite is actually bisected by the Clarence River. This is surprising given the prominent hills (and very hard rock) that the Dumbudgery Granodiorite is made from compared with the relative softness of the Clarence-Moreton Basin sedimentary rocks a short distance to the east. I’ve discussed why it is likely and surprising that the river has created this route in a previous post. Additonally, I’ve quickly discussed in another post the nearby unusual rock called the Gordonbrook Serpentinite which was mined at Baryulgil for asbestos. The Gordonbrook Serpentinite forms the eastern contact with the Dumbugery Granodiorite in Baryulgil area.

The Clarence river here is quite wide with large sand and gravel deposits moving every time it floods and altering its course. Historically some gold was found in this sand and gravel and is thought to be mainly sourced from gold in mineralised granitic rocks further up the river and in its tributaries. Some of the little deposits in the hills and much of the river itself was mined by the old timers, around the end of the 19th Century.

The area around Baryulgil is off the beaten track and Baryulgil itself is a bit of a delapidated little community but the area is worth a visit for its wonderful scenery and geological significance given its location at the edge of the mountainous New England region and the edge of the Clarence-Moreton Basin. Apparently the swimming and fishing are lovely too.

References/bibliography:

*Bryant, C.J., Arculus, R.J., & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology V38.