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.

Are our volcanoes extinct?

Firstly, I've been a bit quiet on the blogging front for a couple of weeks. There has been a lot going on personally which has meant very little time for research or blog posts. I usually have a few scheduled posts up my sleeve for those times when I simply don't have the time... but as a measure of how busy I've been, even these have run out.

Having said all that, I must point out another interesting post by New England self-government advocate Jim Belshaw. It is interesting because it takes us back over a hundred years and shows us that we can sometimes have a little laugh about silly geological ideas from back then. But, it is important to know that miss-understandings of geology continue to this day, including a belief by some that Mount Warning (for example) might erupt again or that we are due for a magnitude 7.0 earthquake etc.

More climate clues on the Northern Tablelands

In January last year I did a post called How Cold Was It? Glaciers in New England? that showed evidence of peri-glacial features in the Northern Tablelands of New England, specifically in the area just to the east of Guyra. Bob H, gave me a tip-off for these interesting features which went unnoticed for a long time – including by me. I’d even taken a photograph of a solifluction lobe and not identified its true nature! It is important to know that Solifluction lobes and other peri-glacial features such as cirques are not glacial features per se. However, Bob did mention a probable moraine elsewhere in the New England, specifically, near Ebor in the vicinity of Duttons Trout Hatchery. A moraine IS a glacial feature. Because of these interesting features and because that part of the country is wonderfully beautiful I have wanted to do a road trip into the area but as yet have not been able to. The best I’ve been able to do is look at Google Maps but at least even consulting Google you can find some little gems.

A Google Earth image of the area to the North of Wollomombi

While looking at Google Maps I recognised more evidence of peri-glacial features in the Wollomombi area, which is about 20km to the south east of where the above-mentioned features were identified near Guyra. Here too was evidence of solifluction (movement of soil due to the partial thawing of summer permafrost). I’ve not been able to identify with certainty any other evidence of solifluction or related features even in the higher (and therefore colder) parts such as Ebor. Maybe, it was the case that during the last glacial maximum (about ten to twelve thousand years ago) only isolated areas formed permafrost - seemingly small areas of south facing hills.

However, when noticing the places where periglacial features are present such as east of Guyra at Malpas Dam and those I just noticed north of Wollomombi, I thought that they seemed only to be present on hills that looked like they had soils derived from basalt rock. Indeed, upon inspection of the geological maps it became apparent that the only places where I can see these peri-glacial features are mapped as being on Cenozoic aged basalts. The map shows the south facing hills that are derived from other rock types such as granites and meta-sediments do not show the same evidence of being affected by permafrost or related processes. This is interesting because there are two possible reasons for this:

1. There was only isolated areas that were cold enough to maintain permafrost during the last glacial maximum; or
2. The soils derived from granites and meta-sediments did not preserve evidence of permafrost

Given that the solifluction lobes evident at both Wollomombi and Guyra are about 20km from each other I would suggest that it is unlikely that the effects would only occur in these two areas and not in the area in between, so option 2 is the most likely. This may have the following implications:

• Zones of permafrost (peri-glacial environments) and maybe small glacial environments probably existed in frequent patches on south facing slopes all the way between Guyra and Wollomombi and maybe even further to Ebor an area 60km long;
• The soils in this area are derived from three major types consisting of Carboniferous aged Meta-sediments of the Girrakool Beds and Sandon Beds, Permian and Triassic aged New England Batholith ‘granites’ of the Abroi Granodiorite, Rockvale Monzogranite and Round Mountain Leucomonzogranite and finally Cenozoic aged ‘basalts’ including the Doughboy Volcanics and others which are unnamed;
• Only the soils derived from the basalts have properties available to behave in a manner which produces and or preserve the evidence of permafrost in features such as cirques and solifluction lobes.

A Google Earth image of a spot next to Malpas Dam near Guyra.
Here the solifluction lobes are comparatively big

So, what does this mean? Well, it means that it was very cold over a large area in the New England. So much, that during the last glacial maximum, water was permanently frozen in the soil in south facing topographic areas over a widespread region extending at least from Guyra to Ebor. But, evidence for this was only preserved in the soils derived from basalts (I need to consult a pedologist (soil scientist) to figure out exactly why this might be the case).

So, if you are shivering and experiencing snow flurries in the area during winter, know that you would have been shivering harder had you been there about 20 000 years ago. It makes me wonder if the indigenous people of the region experienced that cold or whether the land was too cold and marginal for them to live there at that time.

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.

How cold was it? Glaciers in New England?

I was very, very pleased to receive this comment by Bob (a physical geographer by ‘trade’) a bit more than a week ago. Bob posted the comment on my blog about Point Lookout and has been reproduced below:

"Rod,

You and your readers might be interested in a discovery made around Ebor and Guyra just last week, when I was working with a team of glacial buffs from UQ and UT. We found definite evidence of periglacial activity, presumably from the Last Glacial Maximum, ~20 000 years ago. The best examples were at Guyra on the slopes around Malpas Dam -- clear evidence of solifluction lobes, rock glaciers, snow hollows, and other freeze-thaw features, and what could only be described as incipient cirques -- ponds and bogs sapping back into the escarpment and probably still holding some snow in the frigid Guyra winters. The curious point is that they were between 1200 and 1300 metres, not at the highest points above 1500m where we were expecting them, but all on steep south facing escarpments.

One intriguing feature near Point Lookout, on the road up where it crosses the Little Styx River and at 1450m, was what looked very like a terminal moraine. This of course is a glacial, not a periglacial feature, so it is very hard to believe-but have a look at it, and see what you can make of it. There were at least 16 glacial cycles in the Pleistocene, so maybe one of them at least was really severe, and glaciated this far north. Otherwise, you have to go back to the Permian...

Bob H."


probable solifluction lobes and terraces on hill slope at Malpas Dam

This is exciting stuff to hear about because to my knowledge there is little or no evidence of cold climate landforms in the region. In fact I think the areas of the Tasmania are probably the only areas in Australia were these are frequent, though they have also been possibly identified in the Southern Alps and areas of Victoria. Certainly authors such as Petherick et al. (2011) and Hope (2005) did not identify such equivalent indicators of how far north such extreme cold could be detected. I understand that the cold climate landforms that Bob mentions have been found by researchers various universities such as from the University of Queensland, University of Technology, Sydney and the University of Tasmania. Acting on the tip off from Bob, I found an example of solifluction right on the northern side of Malpas Dam using Google Maps, this one is actually visible from the lookout on the southern side of the dam too (sorry about the quality of the photos, it was a long time since I took them and I didn’t realise what I was looking at that time). Get on Google maps and visit the dam yourself and have a look.


Same solifluction lobes visible south of the hill (same hill as picture above)

Solifluction is caused from the thawing of surface layers of permafrost during the summer leading to the thawed part of the soil profile slipping over the un-thawed permafrost and creating ‘lobes’ of soil. The cold climate structures that have been identified near Guyra are present on the southern side of the hill slopes where the sun was unable to melt much of the ice in the soil and therefore creates conditions of permafrost. Permafrost is not present anywhere on the Australian mainland today and demonstrates a significant change in climate has occurred (though those that know Guyra will still argue it is still uncomfortably cold there!).

What Bob appears to have found near point lookout is even more incredible, as a moraine is formed through the action of glaciers which are accumulations of ice on the surface that slowly moves through a landscape under the action of their own weight. Glaciers in Australia were thought to be limited to Tasmania and the Snowy Mountains. No doubt we will expect to see some published papers on the structure and context of these cold climate features in the region some time in the near future. I can't wait to read the published work that comes from this discovery. 

For further information on the individual cold climate features described above by Bob  visit the glossary, an online encyclopeadia or a good physical geography book such as Geosystems by Christopherson. If you are a little unclear about the locations of these sites and how they fit into a ‘Northern Rivers’ blog then it is worth mentioning this part of the New England, at Guyra and Malpas Dam are right at the head waters of the Gara River which is a tributary of the Macleay River that runs through Kempsey. The New England Highway in the Guyra area is pretty close to the actual crest of the catchments of the northern rivers, with the rivers to the west of it flowing into the Murray-Darling Basin and those to the east to the Pacific Ocean. Point Lookout is in the headwaters of the Bellinger River which runs trough Bellingen and is also part of the headwaters of the Macleay too.

References/Bibliography:

*Christopherson, R.W., 1997. Geosystems Wiley.
*Hope, P. 2005. The Weather and Climate of Australia During the Last Glacial Maximum. University of Melbourne, PhD Thesis, unpubl.
*Petherick, L.M., Moss, P.T & McGowan, H.A., 2011. Climatic and Environmental Variability During the Termination of the Last Glacial Stage in Coastal Eastern Australia: A Review. Australian Journal of Earth Science V.58.