The Great Dividing Ranges and Stonehenge

Granites occur throughout much of the north coast and New England region. I use the term granite here loosely, in reality the rocks I’m referring to have a range of compositions and ages. The things they have in common are their relatively high quartz content and they are igneous intrusive (plutonic) rocks. They have cooled slowly and therefore have allowed large crystals to form – giving them that typical granite appearance. I’ve covered a few granites in previous blog posts but in this post I’ll cover one New England “granite” called the Wards Mistake Monzogranite. I’ll continue to cover others in future posts.

Stratigraphically the Wards Mistake Monzogranite is part of the Wards Mistake Suite which in turn is part of the Uralla Supersuite. The Wards Mistake Monzogranite outcrops in a relatively extensive area between Glen Innes and Guyra. In places it straddles the Great Dividing Range but mainly occurs just on the eastern side within the upper reaches of many Clarence River tributaries. The unit was formed around 250million years ago, during the Lower Triassic to Lopingian (early Permian period).

The Wards Mistake Monzogranite consists of monzonite (a rock containing moderate quartz with equal parts potassium and sodium-calcium feldspar) with some granodiorite (abundant quartz and calcium-sodium feldspar). It has a typical equigranular black and white speckled appearance which is common of the Uralla Supersuite. It is like the other Uralla Supersuite granites as it is derived from the melting of other igneous rocks - I-Type Granites (Bryant et al 2003). However, it does contain some xenoliths (inclusions of other rock) which are sedimentary. It is possible that when the Wards Mistake Monzogranite was emplaced into the crust it incorporated bits of the surrounding sedimentary rock. This may have affected the chemistry of the magma and may be one of the reasons why there is both monzonite and granodiorite in the unit.

Typical tor outcrops of the Wards Mistake Monzogranite near Glen Innes

Many New England granites contain mineral deposits. Being an I-Type granite usually means a good chance of mineral deposit formation. However, the Wards Mistake Monzonite contains very sparse mineralisation with only a few small areas where there is some alteration zones that have more concentrated ore minerals. These include wolframite (tungsten), molybdenite (molybdenum) and cassiterite (tin) (Brown 1997). Other surrounding granites such as the Kingsgate Granite and Red Range Leucogranite have abundant mineralisation that was historically mined and is still under active mineral exploration permits.

A lovely feature of most New England granites is the interaction with the climate. This produces wonderful looking granite tors. This is a result of onion skin weathering (frost wedging) where water penetrates into the rock and freezes during the cold winters. This repeated action causes large flakes of rock to peel off. Some of these Tors are given their own names. In the Stonehenge area on the New England Highway you can stop and walk among these Tors and see the Balancing Rock which looks like it will topple over at any moment.

The landscape around Stonehenge between Guyra and Glen Innes is my favourite landscape in Australia. The high country agriculture, the cold weather and the geological conditions that form the rolling hills and special tors make it a special place. The picture above is of a portion of the Wards Mistake Monzogranite and partly shows the landscape I’m talking about. The accessibility of the granite is certainly worth a quick stop if you are travelling on the New England highway.

References/bibliography:

*Barnes, R.G , Willis, I.L. 1989. Preliminary geological plan of the 1:250 000 Grafton-Maclean sheet area - SH 56-6, SH 56-7. New South Wales Geological Survey Report

*Brown, R.E. 1997. Mineral deposits of the Glen Innes 1:100 000 map sheet area. Geological Survey of New South Wales. Quarterly Notes 103 p1-19

*Bryant, C.J. , Chappell, B.W. , Blevin, P.L. 2003. Granites of the southern New England orogeny. In Blevin, P. et al (eds) Magmas to Mineralisation: the Ishihara Symposium Geoscience Australia. Record 14 - extended abstracts.

How to emigrate from the Northern Rivers

Most people will be surprised that once, the Northern Rivers area was not east of the Great Dividing Range. It seems likely that there was a range which Ollier & Pain (1994) refer to as the Tasman Divide. This divide was originally speculated by Jones & Veevers (1983). This divide probably meant that rivers such as the modern day Clarence actually flowed to the west, indeed, if you had of stood on Cape Byron or looked out from the headlands of Port Macquarie you’d not see the wonderful blue ocean but land and possibly hills with the sea located possibly many hundreds of kilometres further away than today. I guess a good question to ask is where did all that land go?

Looking out to the Tasman Sea may have
been looking out to another mountain range

The short answer is the sea floor around one of Australia’s offshore territories, Lord Howe Island. Lord Howe Island was actually formed during the Cenozoic from volcanoes but these volcanoes were situated on what we call the Lord Howe Rise which despite being submerged in the ocean (where you’d expect to find oceanic crust) is actually made from continental crust, like the Australian landmass. This is a mostly huge submerged continent called Zealandia which extends from New Zealand to New Caledonia. Some of this old continental crust is visible in the North of the South Island of New Zealand for which the rocks are related to the Lachlan Fold Belt in Southern New South Wales and Victoria.

In short, the Australian continent was much bigger than it currently is with Zealandia being the eastern edge. Approximately greater than 80 million years ago (beginning in the Cretaceous period) something happened deep below the crust under the Tasman Divide, it seems that the convecting mantle was pulling the east coast of Australia in two different directions. The area of the future Australian Continent seemed to remain fairly stable but the west, the future Zealandia, was dragged to the east. This process split the continent in two and created a mid ocean spreading ridge. The Lord Howe Rise part of Zelandia was dragged and stretched, creating huge Horst and Graben fault systems and consequently many basins. The effect of stretching and faulting thinned the Lord Howe Rises Continental Crust which meant that it began to sink below sea level.

It is unknown whether the action of the convecting mantle may have been directly associated with the hotspot/hotspots associated with the Cenozoic volcanics such as those of the Tweed Volcano and Ebor Volcano as well as Lord Howe Island itself. One thing is clear though, is that as crust thinned it would increase the ability for molten rock to approach the surface and create volcanoes. Many of the areas in the Northern Rivers such as the Central Volcanic Province, Alstonville Basalt, Maybole Volcanics etc are seem to be in some way related to this episode of rifting instead of the later hotspot volcanoes. Indeed even the latest research such as Sutherland et al (2012) can't easily fit the ages of this volcanism into a traditional hotspot model.

If you look at a bathymetric map of our coastline you will see that the continental shelf is very thin in comparison to the rest of the world. It is also very abrupt, and this structure also points to the process of rifting that occurred during the late Cretaceous and early Cenozoic. The Tasman sea was the result of all this rifting and turmoil. It seems that the Zealandia just wanted to emigrate from the Australian continent. Sometimes I feel the same with our region including the wonderful New England region, I often think that we’d be better off if we were not part of New South Wales but a separate state but hopefully we don’t need to the extreme of rifting this part of the continent away to do it. A new state is, of course, politics and so I should probably end there.

References/Bibliography:

*Jones, J.G. & Veevers, J.J. 1983. Mesozoic origins & antecedents of Australias eastern highlands. Journal of the Geological Society of Australia V30.
*Ollier, C.D. & Pain, C.F. 1994. Landscape evolution and tectonics in southeastern Australia. AGSO Journal of Geology & Geophysics V15.
*Sutherland, F.L., Graham, I.T., Meffre, S., Zwingmann, H. & Pogson, R.E. 2012. Passive-margin prolonged volcanism, Eastern Australian Plate: Outbursts, progressions, plate controls and suggested causes. Australian Journal of Earth Sciences V59.

The New England tablelands seem to be upside down

The geomorphology of the Northern Rivers and New England region can be quite complex. There are many features around the region that have developed as a direct result of the underlying geology. Whether it be the great escarpment, the Ebor Volcano, the backward Clarence River or various other situations, there is always a geological reason for the landscape we see today. In a previous post on the Maybole Volcano near Guyra I quickly mentioned that there is an “inverted topography” which has been created following the deposition of the lava from this volcanic area. Maybole is not isolated in this situation, indeed according to Coenraads & Ollier (1992) much of the basalts in the New England region from Armidale, Walcha, Llangothlin and even places on the other side of the watershed and great dividing range of the Northern Rivers such as Nundle or Inverell show what is technically referred to as relief inversion.

The area around Armidale is actually a good example of the relief inversion, as most hills actually demonstrate the situation nicely. Take, for example, the hill that the University of New England is situated on. The Hill is capped with Cenozoic (Miocene) aged calc-alkaline olivine basalt (part of the Central Volcanic Province) just to the east of the hill (in the paddock below the university carparks) below the level of the lowest basalt flow is a fossil soil horizon, known as a palaeosol. This palaeosol has been affected by lava being deposited on it and has been turned into a material known as silcrete (soil which has been cemented with silica). The old soil was developed on rocks of the Carboniferous aged Sandon Beds. The Sandon Beds outcrop on the lower slopes and in the valleys in and around Armidale but once they were the hills themselves.

The basalts were erupted to the surface the chemical composition of the lava meant that they were quite low in viscosity, that is it was very liquid and consequently the lavas flowed down the valleys that existed at the time. The valleys tended to fill up to varying degrees, leaving only a thin layer of volcanic rock on the existing hill crests of the Sandon Beds or none at all. In the following millions of years the process of erosion would be more effective on the non-volcanic rock and the hills would eventually become incised, turning into gullies and eventually larger valleys. The basalt in the old valleys would remain relatively un-eroded and be become the modern hills.

Evidence of this process can be seen from historic mining of some of the gold around Armidale. The ‘old timers’ would dig under the basalt along ‘deep leads’ which were originally gravel and sand deposits associated with old creeks and rivers. These deep leads had been alluvial gold deposits preserved by the basalt flows. Many of these were mined in the 1800’s and early 1900’s in many areas of the New England district including one quite recently in the Tilbuster area (Ashley & Cook 1988). The silcrete deposits mentioned previously are also examples of the process.

References/bibliography:

*Ashley, P.M. & Cook, N.D.J. 1988. Geology of the Whybatong gold prospect and associated Tertiary deep lead, Puddledock, Armidale District. New England Orogen - Tectonics and Metallogenesis. Conference Papers presented at the University of New England.
*Coenraads, R.R. & Ollier, C.D. 1992. Tectonics and Landforms of the New England Region. 1992 Field Conference - New England District. Geological Society of Australia Queensland Division.

The Dummy you'll find north of Armidale

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.

Dummy Creek Conglomerate in the Sunnside area
metamorphosed by the Highlands Igneous Complex

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.

A special volcano on the edge of the Northern Rivers

I have previously mentioned several volcanoes that have existed during the Cenozoic period in and around the Northern Rivers region of the New England. But, it is worth noting that there was once a period of significant volcanism earlier in the Cenozoic which defines the landscape of the Great Dividing Range south of Glen Innes, near the villages of Glencoe (with its excellent pub: The Red Lion Inn) and Ben Lomond. This area is the headwaters of many wild rivers found flowing down the rugged New England escarpment that are tributaries of the Clarence River. On the other side of the divide eventually join the Darling and then Murray River. The Maybole volcano was apparently centred at the modern day and generally unheard of locality, Maybole. It erupted lavas over a large area in every direction including large areas to the west, east and south east.

Maybole lies just on or just outside of the headwaters of the Northern Rivers but none-the-less is worth mentioning because of the extent of volcanic rock that appears to have originated from it. The rocks that have come from the Maybole Volcano are mostly basalt type rocks which were once referred to as the Eastern division of the Central Volcanic Province (Coenraads & Ollier 1992), now referred to as the Maybole Volcanics but still part of the Central Volcanic Province according to Vickery et al (2007). The Maybole Volcanics are comprised of alkali olivine basalt to slightly less silica undersaturated basalt and andesite and reworked volcanic material (epiclastic and volcaniclastic sedimentary rocks) and was erupted around 36-39 million years ago.

Coenraads & Ollier (1992) identified that Maybole was a significant volcano by determining the thickness of basalt that occurred in the region and noticing that at Maybole the thickness was significant at several hundred metres. There are also apparently some dykes and vents that are present. Additionally, they had a close look at drainage patterns and realised that they radiated like the spokes on a bicycle, a classical indication of volcanic geomorphology.

Since Coenraads & Ollier (1992), Vickery et al (2007) has undertaken a major review of the Central Volcanic Province and delineated several constituents of the province. The most significant along this part of the Great Divide is now known as the Maybole Volcanics, obviously directly associated with the Maybole volcano. The age of the Central Volcanic Province including the Maybole Volcanics shows that these rocks are too old to be associated with the Eastern Australian hotspot which formed many of the other major volcanic centres in the region (such as the Focal Peak, Tweed and Ebor Volcanoes). Some time after the end of volcanism from the Maybole Volcano  other volcanoes between about 14-24Ma erupted their lavas over the top of the Maybole Volcanic suite rocks.

Interestingly, it appears that the Maybole Volcanics had affected exactly where the Great Divide was situated because the nature of the existing range was such that the lavas filled the valleys creating thick volcanic piles while the existing hills were only covered with thin layers. This meant redirection of streams and when the rock was eroded the more erodible hills were turned into valleys and the valleys became hills caped with basalt. This is termed an inverted topography. But more about this in another post.

Interestingly, Coenraads & Ollier (1992) have observed that the the great divide has moved over time with some of the old basalt filled valleys showing that they used to flow to the west but with the streams now flowing to the east. It actually appears that the Northern Rivers region is getting bigger!

Red Lion Inn (from Flickr)

PS. Like lots of geologists I like pubs with a good atmosphere and The Red Lion Inn at Glencoe is just such a beautiful place. It is an exceptional location to stop for a meal, especially during the middle of winter while snow is coming down. Alternatively, during autumn while the trees turn bright yellow and red, or during spring while the new leaves are coming out, or even summer! i.e. I recommend it!

References/bibliography:

*Coenraads, R. R., Ollier, C.D. 1992. Tectonics and Landforms of the New England Region in 1992 Field Conference - New England District. Geological Society of Australia Queensland Division.
*Vickery, N. M., Dawson, M.W., Sivell, W.J., Malloch, K.R., Dunlap, W.J. 2007. Cainozoic igneous rocks in the Bingara to Inverell area, northeastern New South Wales. Geological Survey of New South Wales Quarterly Notes v123.