A rock of Gibraltar Range National Park - Part 1.

A lookout on the Gwydir Highway

I was going to write a very long post on the Dandahra Creek Leucogranite but I think it lends itself to two posts. This post will focus on the amazing Gibraltar Range National Park and the second will focus on Australian ingenuity and dating of the Dandahra Creek Leucogranite.

A few months ago I travelled from Glen Innes to Grafton via the Gwydir Highway. The landscape in this area is wonderfully diverse and surprisingly contradictory. For example usually Sandy soils on the plateau give rise to swamps with peat. It is a special area because the link between the geology, vegetation and even bush fire patterns is quite obvious. I'd like to focus on one rock unit that makes up the balance of the Gibraltar Range National Park area, the Dandahra Creek Leucogranite.

The Dandahra Creek Leucogranite was often referred to as the Danhahra Granite (and still regularly called this in botanical circles). It is part of the New England Batholith and has recently been dated at at 237.6 Ma (Chisholm et al 2014). It is the youngest member of the Stanthorpe supersuite of granites. Outcrops are very frequent in the Mulligans Hut area and the Gwydir highway transverses the unit.

The spectacular tors which are major features of the landscape of Gibraltar Range National Park arise from weathering from the Dandahra Creek Leucogranite. These tors form through onion peel weathering (technically called exfoliation or spheroidal weathering). This weathering process is where water enters cracks in the rocks and then freezes over night. As water turns to ice it expands and sheets off rock just like an onion skin. This is usually a fairly slow process except with the last sloughing off of the onion peel occurring quite rapidly.

Tall open forest is a major feature of the landscape of the Dandahra Creek Leucogranite. These eucalyptus dominated forests can have an open, grassy understorey featuring grass-trees and/or tree-ferns. These landscapes are quite fire prone. Indeed their structure is dependent on multi-decadal scale fires.

There are also some more unusual vegetation communities on rock outcrops because the tor outcrops lend themselves to protecting some vegetation from fires. They are also very thin soils with low nutrient content so even carnivorous plants can be found.

Heathlands and grasslands occur around the rock outcrops and are particularly important as they contain the greatest concentration of rare, threatened or geographically restricted species, or species found at the limits of their distribution (NPWS 2005). The grass and heath land burns very frequently often with bush fires only every several years.

The shallow wide valleys that are formed on the sandy granitic derived soils result in common large peat swamps. The shape of the valleys slows down water and the underlying massive granite means that the water does not infiltrate. The swamps contain sedges and other water loving plants.

If you are interested in the bush or interested in rock the Gibraltar Range National Park is for you. If you are in to camping, bush walking, amazing views of rugged valleys the Gibraltar Range National Park is for you. If you are in to spectacular flowers, rainforests, exploring a rocky creek the Gibraltar Range National Park is for you. If you are in to staying in a lodge, want to see some snow, or bathe in a rock pool on a summers day the Gibraltar Range National Park is for you.

References/Bibliography:

*Chisholm, E.I., Blevin, P.L. and Simpson, C.J. 2014. New SHRIMP U–Pb zircon ages from the New England Orogen, New South Wales: July 2012–June 2014. Record 2014/52. Geoscience Australia

*Clarke, Peter J. & Myerscough, Peter J. 2006. Introduction to the Biology and Ecology of Gibraltar Range National Park and Adjacent areas: Patterns, Processes and Prospects. Proceedings of the Linnean Society of New South Wales

*New South Wales National Parks and Wildlife Service 2005. Gibraltar Range Group of National Parks (Incorporating Barool, Capoompeta, Gibraltar Range, Nymboida and Washpool National Parks and Nymboida and Washpool State Conservation Areas) Plan of Management. February 2005. ISBN 0 7313 6861 4

Guest Post - Dynamic beach sediments

Thank you to Dylan Gilliland for providing this guest post for us.

We all enjoy going to the beach but not every beach is the same. There are distinct differences between a north facing beach and a south facing one. An example of this is the Clarkes beach and Tallows Beach at Cape Byron. Most of the sand that makes up the beaches of the North Coast is derived from the granites of the Great Dividing Range. These granites are eroded and discharged into the coastal regime by flooding rivers. A smaller portion of the beach sediment is derived directly from the headlands and can sometimes form boulder beaches as seen at Lennox Head and Angourie near Yamba. This process has been in effect for at least 65 million years since the break-up of Gondwana and the opening of the Tasman Sea.

Once the sediment is incorporated onto the coastal fringe it is then subject to size sorting and further transportation. This is done through wind, wave and currents off the Tasman Sea which is predominantly from the south to the north and is due to anticlockwise flow of high pressure weather systems that dominate the Australian continent particularly during winter (Short and Woodroffe, 2009). This gives rise to the term that many earth scientists refer to as "the great river of sand". It has played an integral part in the formation of the Morton, Stradbroke and Fraser sand islands.

On a smaller scale, size sorting and northerly transportation affect a beaches shape and composition. This will ultimately dictate how we interact with it. An example would be to examine the location of where to launch a boat. This is usually done in southern beach corners as it is not only protected from waves but the beach has a very gentle slope and the sand is very compact allowing vehicle access without sinking in the sand. What causes this? Headlands form barriers to the dominant southerly swell and will deflect wave energy past the southern corners. This will leave the northern expanse of the beach exposed to the full force of generated wave energy. Therefore, many east coast beaches particularly long beaches develop a zeta-curve shape much like the curve inside a spiral shell.

The amount of energy to reach a beach has a profound effect on the mechanics of sand grains and where they are distributed. In the southern corners there is less energy directed toward the beach therefore smaller particles will be able to settle without being swept away. The smaller particles pack together tighter than large particles and this reduces the beach porosity. When waves wash up the beach it doesn’t soak into the sand dumping its load, instead any particles will recede with the wash resulting in a beach with a low incline and hard packed sand. The northern end of the beach will exhibit characteristics typical of a higher energy environment with coarser sand that has a higher permeability. This can result in a steeper, less compact beach. These can often have formations such as swales, berms and cusps. This is due to waves coming up the beach loaded with sand that gets dumped higher on the shore. The water percolates quickly into the beach and it doesn’t wash the sand back out into the surf zone. For these reasons, near-shore sand bars on the northern end of a beach can be hazardous to inexperienced swimmers due to steep drop-offs, currents and instability.

Beaches are highly dynamic systems that are constantly changing; they are constrained by local geology and dominated by regional weather systems. These dynamic systems give us the beaches that people enjoy so much and the coastal erosion many people fear.

This information is adapted from field notes taken from a coastal geomorphology course conducted by Dr Robert Baker at The University of New England.


References/bibliography:

*Short, A.D. and Woodroffe, C.D., 2009. The Coast of Australia. Cambridge University Press

A history of unstable North Coast sea levels?

Last summer much of the northern rivers area had been hit hard by summer storms. These storms often caused erosion on the fore-dune systems behind some beaches. For example, at Kingscliff this has become a major problem. At other locations this erosion has revealed some hidden features.

In the last few months I had a trip to Coffs Harbour where I was able to walk along some of the lovely beaches. On Diggers Beach I noticed a strange looking band through the exposed face of a dune system that had been recently been eroded away by stormy seas. Upon closer inspection the band was a layer of fine gravel and shell fragments. Underlying this layer of gravel and shell was sand with some isolated gravel which graded into the previous layer. The top of the layer was distinct and comprised of fine well-sorted sand, typical of a dune system. I noted another exposed gravel layer about 50 metres further south along the beach at roughly the same height.

Evidence of a palaeo-beach on present day Diggers Beach.

What struck me about the layer below the dune sand was the similarity of the materials when compared with the deposits of fine gravel and shells that exist on Diggers Beach. The gravel and shells have in places been deposited in the berm by the action of wave swash. I could not help think that what I was looking at was an old berm, some of the remnants of a palaeo-beach (an old preserved beach). The sand on beaches is dynamic. Sand moves inland or seaward because of storms and sediment supply (amongst other things). The difference between this old beach was approximately 1.1-1.2 metres above the present high tide mark.

The height of the palaeo-beach seems to indicate that maybe it was formed by a higher sea level, or a lower ground level. Tectonically eastern Australia has been very stable for millions of years so I think it unlikely that the earth has been uplifted. The most likely explanation in my mind is that the sea level was higher.

Thom & Roy (1983) suggested that Holocene sea levels have been very stable. However, sea levels varied in the time period before the Holocene. The Pleistocene sea levels were much higher and much lower than today. In the Pleistocene on north coast NSW sea level variations were first documented in detail by authors including Den Exter (1974) and Drury (1982). The apparent Holocene sea level low-fluctuation and high-stability of Thom & Roy (1983), if true, would be an aberration.

Baker et al (2001b) used fixed biological indicators to attempt to reconstruct Holocene sea levels. Baker et al (2001b) dated the remnants of tubeworms, barnacles and oysters that occurred above their natural ecological limit (i.e. above the intertidal zone). These indicators can be used to trace sea level changes. Baker et al (2001a & 2001b) undertook this work up and down eastern Australia and compared them with other sites including those in Brazil. The resulting information showed that Holocene sea levels have not been as stable as first thought. The sea level changes have been shown by earlier authors (e.g. Thom & Roy 1983) to occur during periods of known palaeo-climate change.

According to Baker et al (2001a & 2001b) the last time the sea level was 1 metre higher than present was around 2400-1800 years ago. Maybe, the layer is a preserved berm from a beach that existed at the time of the Roman Empire (sometimes referred to as the Roman Warm Period). I don’t know for sure, but to my thinking it seems quite plausible.

References/bibliography:

*Baker. R.G.V, Haworth, R.J. & Flood, P.G. 2001a. Inter-tidal fixed indicators of former Holocene sea levels in Australia: a summary of sites and a review of methods and models. Quaternary International v83-85 p247-273.
*Baker. R.G.V, Haworth, R.J. & Flood, P.G. 2001b. Warmer or cooler late Holocene marine palaeoenvironments? Interpreting southeast Australian and Brazilian sea-level changes using fixed biological indicators and their d18O composition. Palaeogeography, Palaeoclimatology, Palaeoecology v168 p249-272.
*Den Exter, P. 1974. The coastal morphology and Late Quaternary evolution of the Camden Haven district, NSW. Australia. PhD Thesis, University of New England, Armidale.
*Drury, L.W. 1982. Hydrogeology and Quaternary stratigraphy of the Richmond River valley, New South Wales. PhD Thesis. University of New South Wales. Kensington.
*Thom, B.G. & Roy, P.S. 1983. Sea Level Change in New South Wales over the past 15 000 years. In: Hopley, D. Australian Sea Levels in the Last 15,000 Years: a review. James Cook University, Townsville.

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 lonely delta

Mark's wonderful picture of  the delta on Watson Taylor's Lake

A couple of weeks ago I saw a wonderful picture on Mark Bellamy's Clarence Valley Today photo blog, a picture of Watson Taylors Lake. Watson Taylors Lake which is where the Camden Haven River ends up just before it meets the sea. What struck me most about this picture was the text-book development of a delta system into the lake. Marks blog can be found here.

A delta is formed when sediment suspended in flowing water settles out as it reaches a large water body. Probably the most well known deltas in the world are the Mississippi River Delta, the Ganges River and the Nile River. However, it also creates a question, why don’t we see deltas up and down the Northern Rivers and North Coast areas?

Several studies of off-shore sedimentation have been done along the coast, the earliest studies tended to be looking mainly for heavy mineral deposits such as ilmanite, rutile, zircon and even gold or for military/oceanographic purposes. However, both these studies and others specifically to understand the off-shore environment have demonstrated some interesting facts including why we don’t have river deltas.

The first part of understanding the off-shore sedimentary environment is to understand that currently the sea level is at a very high level in historic terms. It reflects the current warm interglacial period that has arisen. The lowest sea levels that most 'recently' occurred was following the beginning of the Pleistocene which was the period since the the last 130 000 years or so (Roy & Thom 1981 & Drury 1982). According to Drury (1983) and many other authors, sea levels much lower early in the Pleistocene including instances of maybe 100 metres or more (Den Dexter 1974 suggested around 200metres lower at the beginning of the Pleistocene . This caused erosion of most pre-existing soft sediments along what is now the submerged the continental shelf. But it was not a simple transition from glacial to interglacial with many cycles during the Pleistocene and corresponding to alternating periods of coastal sedimentary deposition followed by erosion of those new sediments, so it was a fairly complicated period.

Since the beginning of the Pleistocene Roy & Thom (1981) thought that it was likely that there were only two major causes of movement of sediments along the coast, the first was the effect of sea level fluctuations during interglacial and glacial periods and the second wave and wind action which had the effect of transporting sediment northward. These forces were probably enough to create sand barriers such as those preserved on the Northern Rivers inland from the active Holocene sand barriers and beach systems we enjoy today (more about the Pleistocene sand barriers in a future post). But, Roberts and Boyd (2004) indicated that Roy & Thom (1981) might not be totally correct in thinking there were only two major causes because in some areas the Eastern Australian current also seems to be a significant driver of sediment. In fact they noted that off the coast of Byron Bay in as little as 30metres of water the Eastern Australian Current was present and could scour away any sediments that might have been deposited or stopping sediments from being deposited.

This means that when the rivers, be they the Tweed, Clarence, Richmond, Bellinger, Nambucca, Macleay, Hastings or any others drop their sediment load, the presence of currents then sweeps the finest sediments away, mainly further out to sea, maybe to the edge of the continental shelf. The heavier sediments which drop closest to the coast are affected by waves and storms which drive the sandy sediments northward along the coast which contribute to the barrier beach systems we have in abundance.

This is probably a simplistic way of explaining and I've missed a few complicating factors such as continental shelf slopes but it seems that because of the combination wave, storm and sea current process we don’t get any river deltas in our region, unless they are protected by sand barriers such as the one protecting Watsons Talylors Lake on the Camden Haven River.

References/bibliography:

*Den Exter, P. 1974. The coastal morphology and late Quaternary evolution of the Camden Haven District. University of New England, PhD thesis.
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales. University of New South Wales, PhD thesis.
*Roberts, J.J. & Boyd, R. 2004. Late Quaternary core stratigraphy of the northern New South Wales continental shelf. Australian Journal of Earth Sciences v51.
*Roy, P.S. & Thom, B.G. 1981. Late Quaternary marine deposition in New South Wales and southern Queensland – an evolutionary model. Journal of the Geological Society of Australia v28.

Into the Parrots Nest

At least 3 lava flows are evident from the different 'steps'

I had the opportunity a few weeks back to visit a quarry near the locality called Canaiba situated mid way between Casino and Lismore. The quarry is an operating variable quality rock quarry probably excavating Miocene aged basalt lavas from the geological unit known as the Lismore Basalt or possibly the earlier Eocene aged Alstonville Basalt. It was a site I'd wanted to visit for quite a while because the quarry is located at the lower side of a long ridge with an old abandoned quarry located at the top of the ridge on the way to a locality called Parrots Nest. In my mind having two quarries could give an interesting perspective on the any variations in lava flows.  But even before I visited the old quarry, while I was driving along the road to visit the operating one I noticed an interesting feature in the shape of a spur from the main ridge. Visible were several 'steps' in the spur. These steps create what is referred to, unsurprisingly, a stepped topography.

The steps are caused by the erosion of different lava flows. The flows are up to 20 metres of so thick which according to Duggan and Mason (1978) is a bit uncharacteristic for the Lismore Basalt (thin 2-3 thick flows). Looking back along the ridge it is pretty evident that the flows are of consistent thickness through the whole area.  They are probably from the Lismore Basalt that are related to the formation of the Tweed Volcano which was centered around present day Mount Warning. I wonder if there were closer vents that could be the source of the lavas but there is little evidence of any in the immediate vicinity. Indeed authors such as Cotter (1998) feel that the pre-existing topography was such that the area through Blakebrook Quarry (another site north of the quarry I was visiting) through to places like Parrots Nest may have been a valley. The swift flowing basaltic lavas flowed down these valleys filling them and creating thick sequences of rock.

The red layer overlain by another basalt lava flow
indicates the presence of a fossil soil horizon

The operating quarry cuts several of the lava flows that make up the ridge, the boundaries of the lava flows were very easy to make out because of the weathered zones especially the presence of palaeosols, that is, fossil soil horizons. The palaeosol gives an idea of the nature of the eruptions of lava too. Obviously enough time needs to have passed for the formation of a soil profile to occur on the earlier lava flow before the next lava flows over the top of it. Depending on the climatic conditions this could be many decades between flows or even thousands of years.

Anyway, a good trip even if it was just for the palaeosol or the stepped topography alone. But I'd like to do another blog on some of the macro scale igneous textures that are present in the lava including dykes, vesicles, voids and veins and I've still not got to the top abandoned quarry but when time allows I'll get there. I took some samples at the operating quarry to examine under the microscope to see if there were any microscopic textures that are of interest too, but once again, time does not seem to be on my side... though I will get to these tasks sometime!

References/bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.

Brown Under and Green on Top

A few months ago I took this photograph at a site I was working on (located mid-way between The Channon and Dunoon). It was a cold spring day with strong cold winds and rain threatening. I love the cold weather, it seems to make you feel more alive! Anyway, I thought it would be a good photo to share since it shows several attributes of our landscape and how  it was formed.

The south side of the valley between The Channon and Dunoon

Firstly the background geology. Where the photo is taken from is on a hill made from rock of the Walloon Coal Measures within a larger steep sided valley. The sides of the Valley are two rock formations more resistant to erosion which is the Miocene aged Lismore Basalt (not visible in the picture) and the Kangaroo Creek Sandstone (Cliffs of which can be in the picture). Here the Lismore Basalt overlies the Late Jurassic aged Kangaroo Creek Sandstone which in turn overlies the Jurassic Walloon Coal Measures. I’ve done some earlier posts which describe the nature of the Kangaroo Creek Sandstone and Walloon Coal Measures, just click on the respective link for more.

Rocky Creek runs through the valley today and it is the action of that creek that formed the valley. The creek must have cut through the lavas of the Lismore Basalt and eventually cut through the Kangaroo Creek Sandstone. Once it was through these hard layers it had an easier task of cutting into the softer and finer grained sediments of the Walloon Coal Measures. It is also possible there is some underlying structural control such as folding or doming but I’m not confident of the extent of this.

The top of the ridge in the photo shows a wet sclerophyll vegetation type, an open forest which contains many Eucalypt species reflecting the Kangaroo Creek Sandstones poor nutrient soils and rapid drainage. Also the ridge is quite exposed to direct sunlight and desiccating winds. Below the cliffs the Walloon Coal Measures start and here is found dry rainforest type vegetation reflecting the better, more nutrient rich and finer grained soils that are developed on the Walloon Coal Measures. The nearby Basalts also have the same vegetation type and in places approach wet rainforest especially in gullys and protected places.

In addition the picture shows the indirect and direct effect of Australians on the environment. The indirect effect is weeds. Many of the bright green trees in the middle of the picture are Camphor Laurel (Cinnamomam camphora) loving the dry rainforest environment. Right in the foreground is Wild Tobacco Bush (Solanum mauritianum) overtaking some of the grazing country. But you will also see a line of dead trees which is part of a successful effort to reclaim the weedy forest into quality native vegetation. The dead trees are poisoned Camphor Laurel with many hectares of forest in this area regenerated by staff working for the local water authority in an ongoing rehabilitation project.

Note that the stratigraphy of the Kangaroo Creek Sandstone has been recently revised since this blog post. See the this post for details.

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.

Who has heard of the Belmore Volcano?

Most of us know about the two large remnants of volcanic provinces in the region, one the Tweed Volcano and the other the Ebor Volcano. Many too will know that the Tweed Volcano erupted first (23 million years old) and as the Earths crust moved over the mantle the probable hot-spot that caused this volcano migrated further south and formed the Ebor Volcano (19 million years old). Few people will have heard of the Belmore Volcano, this is a volcanic area that is located roughly midway between the Tweed and Ebor volcanic provinces and it also erupted in the interval between the other two (21 million years).

Before I go on I should point out that the term volcano is used very loosely here as it may also consist of many active cones and vents which erupted at a similar time period and are related to each other. Indeed the definition of what a volcano is defined as (such as the terms central volcano and volcanic province) has been an on-going argument for a long period of time anyway!



Trachyte makes up Dome Mountain in the Fineflower area

The area of the Belmore Volcano is away from the main travelled routes and for that reason it has probably been relatively unnoticed for a period of time. It lies to the east of the village of Baryulgil in the southern areas of the Belmore State Forest and Mount Neville Nature Reserve which is about halfway to Coaldale as you head towards Grafton. It is near the southern extents of the Richmond Range. The volcanics have produced some very interesting and rugged landforms such as Dobie Mountain, Mount Mookima, Mount Neville and Dome Mountain.

Most of the lavas have been eroded away but many eruptive sources for the volcano have been identified including plugs, pipes, dykes and possibly some sills. The lavas and intrusion preserved were erupted through the rocks of the Mesozoic aged Clarence-Moreton Basin which outcrop in the area as the Kangaroo Creek Sandstone and Walloon Coal Measures (probably including the MacLean Sandstone Member), but get older as you head west towards the edge of the Basin. The Mesozoic rocks in the area is actually quite deformed (as far as the Clarence-Moreton Basin goes) with large north-south trending folds and several faults nearby. The folds are visible as ridges and valleys (except those landforms associated with the more recent Belmore volcanics).

The Belmore Volcano is interesting because it shows the migration path of the hot-spot that formed the volcanoes that occur along the northern rivers area. There are actually four recognised volcanoes/volcanic provinces. These are all evenly spaced both in distance and time of eruption. From north to south these are the Tweed (23Ma), Belmore (21Ma), Ebor (19Ma) and Comboyne (16Ma) with the migratory trail of the hot-spot lost after this point. Sutherland et al. (2005) demonstrates that the Belmore Volcano is also curious because of the lava type erupted, whereas the other volcanoes erupted mainly more mafic volcanics (basalts and andesites) with later minor more felsic phase (rhyolite, dacite and trachyte ), the Belmore had very little basalt but lots of trachyte. But Isotope analysis by Sutherland et al (2005) has shown that the Belmore Volcanics were associated with the same mantle plume that generated the other volcanoes listed above.

Like most other recognised volcanoes in the region there is an earlier basalt type rock which occurs in the area which appears to have little to do with the most recent volcanic rocks. This is no exception in the Belmore area, where a basalt (dated at 31Ma) is present just to the north of the main eruptive area. Very little is known about this earlier volcanism and how it ties in with the geological history of the region.

References/Bibliography:

*Sutherland, F.L., Graham, I.T., Zwingmann, H., Pogson, R.E. & Barron, B.J. 2005. Belmore Volcanic Province, northeastern New South Wales, and some implications for plume variations along Cenozoic migratory trails. Australian Journal of Earth Sciences V52.

Disappearing sand from the North Coast

I was interested to read an article in my areas 'local rag' The Northern Star. It was a thoughtful piece by someone who loves the regions beaches. It was also a controversial one as it implied a man-made cause for the erosion of many of the regions beaches. You can read the article here: http://www.northernstar.com.au/story/2012/08/24/is-our-sand-on-goldy-beaches/. It actually, provides a good follow on from my last post on the matter.

Waves, wind, currents and a thin strip of sandy beach
One of the regions typical beaches near Ballina

In this article the author (Ben Bennick) suggests that although the mechanism of northward long-shore drift of sand is recognised as a significant driver for the erosion of many beaches, it raises the question of whether the Tweed River sand bypass scheme actually affects beaches further to the south. It is suggested that this is as far south at beaches such as Kingscliff or even those at Byron Bay. The Tweed River sand bypass scheme was introduced to stop the mouth of the Tweed river from being constantly dammed by sand deposited at the mouth. The closing of the mouth of the river would adversely affect water quality in the esturine reaches of the river. It has been operating for more than a decade now and Ben is worried that this might be affecting more than the Tweed River. The bypass scheme has been active since approximately 2001.

Ben suggests that during some times of the year sand would actually migrate to the south, contrary to the potentially simplistic concept of inexorable northward sand migration. As discussed in my previous post about long-shore sand drift, the action of the East Australia Current travelling south actually does not have the effect of causing sand to drift along the coast instead currents generated by the prevailing wind direction means that there are smaller coastal currents which tend to travel in a northward direction.

But Ben does raise an interesting question and rightfully this questions the absolute nature of the eastern Australian coastal currents. Maybe the situation does arise where sand can actually be transported from north to south from time to time. I wonder if such a phenomenon would be great enough to transport sand from the Tweed as far as Byron Bay and beyond? This would find a culprit in the Tweed River sand bypass scheme and would show us that the coastal strip is even more fragile than is already assumed.

In addition to the above comments I also suggest that local knowledge is very important to reconstruct the recent history of our area. Sometimes it is the bloke who has visited the holiday camp at Broken Head for the last 40 years who has some important observations to share. Local knowledge might be pointing to something we are missing. But, and a big but, there are also times where local knowledge is actually completely flawed! Tibby et al. (2007) demonstrated that the recollection of the behaviour of the sand bar at Lake Ainsworth near Ballina was often quite different to what was revealed in aerial photographs, indeed many anecdotal observations which were considered high reliability were in fact impossible when compared with historical photographs.

So, what does this mean? I think it requires someone with a good coastal management background to put us straight. Southern Cross University, despite its shortcomings has an excellent coastal management school. Maybe the answer is not known at the moment, in which case maybe this knowledge gap can be filled. It might just be that Frazer Island is indeed made from 100% Kingscliff and Byron Bay sand, and that is the way it always was. The sand dunes along the coast hide many a change to the coastline in the last 100 000 years, we can't claim to know what caused more than one or two of the many changes during this period and they are generally natural things like extended storm systems... but you never know.

References/Bibliography:

*Tibby, J., Lane, M.B. & Gell, P.A. 2007. Local knowledge and environmental management: a cautionary tale from Lake Ainsworth, New South Wales, Australia. Environmental Conservation V34.
*White, M. E., 2000. Running Down, Water in a Changing Land. Kangaroo Press.

The 'older' Rhyolite in the North East

In some of my earlier posts I mentioned that there are many areas in the mountains around the Tweed Valley that are comprised of rhyolite. I mentioned that this rhyolite was formed during eruptions associated with the Tweed Volcano during the Cenozoic era. This rhyolite is called the Nimbin Rhyolite or the Binna Burra Rhyolite (depending what side of the state border you are on). However, there is actually another large distribution of rhyolite not associated with the Tweed Volcano, erupting much earlier, during part of the Mesozoic known as the Triassic. These older mainly rhyolitic rocks are called the Chillingham Volcanics with the type location unsurprisingly located at Chillingham, a village west of Murwillimbah. Those of you who have seen my earlier posts will recognise that I have briefly mentioned the Chillingham Volcanics before, but in this post I intend to go into it further.

Layers of pyroclastics and volcaniclastic of the Chillingham Volcanics
(Murwillimbah - Kyogle Road)

The Chillingham Volcanics have been studied in a fair amount of detail by Roach (1997) in his thesis. This included all of the Triassic volcanic rocks from Brisbane to Uki. So, obviously there is a relationship with the rocks of the southern Queensland, Indeed Roach (1997) indicates that the Brisbane Tuff is a deposit of volcanic rock of rhyolitic composition. The Brisbane Tuff is most well known by the Kangaroo Point Cliffs opposite the Brisbane River in Brisbane City and was erupted during the same general period of time as the Chillingham Volcanics.

The Brisbane Tuff provides a miniature version of the Chillingham Volcanics and is well known because the volcanic centre can be identified in the northern suburbs of Brisbane and the tuff was laid down in the valleys that existed in the Palaeozoic aged basement. The situation which lead to the formation of the Brisbane Tuff also developed further west and south where a larger valley now known as the Ipswich Basin was forming. The eruptions occurred in and around the basin as the crust in this area was subsiding during thermal fluctuations and as the basin filled up with volcanic rocks subsidence continued leading to a very thick unit of mostly rhyolite and reworked volcanic rocks (actually a sedimentary rock known as a volcaniclastic rock). So the Chillingham Volcanics are actually the lower most stratigraphic unit in the Ipswich Basin.

The Chillingham volcanics are mainly comprised of rhyolite in the form of lavas, pyroclastic, ash and tuff deposits as well as the above mentioned volcaniclastics. Many volcanic vents are recognised from structural characteristics of the rocks, however, only one area really shows an obvious modern geomorphological character. This area is around Uki and Clarie Hall Dam where eruptions formed a large mass due to the slow moving nature of the lava. Interestingly the northern most parts of the Chillingham Volcanics in Queensland shows us that there was not just rhyolite but also some andesite and even basalt, but in the area between Chillingham and Uki it is pretty much all rhyolite.

Outcrops of the Chillingham volcanics occur over a long distance with the eastern most side of the Ipswich Basin exposed in New South Wales meaning that a band of the Chillingham Volcanics is visible within the eroded valleys of the Tweed Volcano. The band is actually interupted by the Mount Warning Complex which appears to have intruded right along the line of the pre-existing Chillingham Volcanics. Also the volcanics are covered by the Lamington Volcanics of the Tweed Volcano too, both along the Queensland Border and between Clarie Hall Dam and Evans Head. Indeed the Chillingham Volcanics appears to change composition through this area with authors such as Smith et al 1997 and Cotter 1998 identifying andestite and basalt at Evans Head and an area near Wardell.

The Chillingham Volcanics overlie palaeozoic aged rocks of the Beenleigh Block, mainly rocks of the Neranleigh-Fernvale Group. The overlying rocks are more components of the Ipswich Basin such as the Ipswich Coal Measures and its equivalent (such as the Evans Head Coal Measures).

Although I have said that the Chillingham Volcanics contain the older rhyolitic rock in this area, there are actually still older rhyolites in the region... But I'll talk about those rocks in a future post.

References/Bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Roach, A. 1998. Late Triassic Volcanism of the Ipswich Basin, Masters Thesis, Macquarie University.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45

Mythical geology at the mouth of the Tweed River

My knowledge of Gaelic mythology is a bit limited but it is interesting to see where geology, Gaelic mythology, Captain Cook and Tweed heads have something in common. I’ve not been to Ireland or Scotland but I’ve experienced a feature that is quite famous in these countries that is also present on the northern rivers.

Fingal Head, clearly showing the basalt columns

Just to the south of the Tweed River mouth lies Fingal Head and Cook Island. Cook Island, is of course named after then-Lieutenant Cook who sailed along the section of coast in 1770. Fingal Head, however, is named after Fingal, a mythological Gaelic hero from Scotland, who never came to this part of Australia! So why is it named so?
To understand the name of Fingal Head you need to know about the story of the Giants Causeway in Ireland and Fingal’s Cave in Scotland. I’m not a good story teller so here is a link (if this link is still not working try this one instead). My summing up of the story is that one of the two warring giants built a causeway to the other side of the Irish Sea so that he could fight the other. The other giant tore it down so that only each side of the causeway remains, one in Northern Ireland the other, Western Scotland. Local tourist information says that Fingal Head is named after the Irish hero. This is actually incorrect, the Irish hero is named Finn MacCool. The name Tweed River should hint that it is actually the Scottish hero that Fingal Head is named after. So, where does the geology come in?

The giants causeway is made from basalt. The volume and thickness of the basalt lava flows means that different parts of the lava flow usually cool at different rates (though, as pointed out by Goehring et al. 2006, the actual mechanism is completely unknown). However, the general idea is that in the case of Fingal Head the lava flow has cooled quite quickly, resulting in contraction of the rock and cooling joints being formed. The incredible thing about nature is that these cooling joints forms columns of rock that are of similar thickness and cross sectional shape, usually hexagons. This formation style is called columnar basalt. Indeed the rock that makes up the causeway has been shown to extend under the sea all the way from Ireland to Scotland. While the scale is not as great as in the British Isles, Cook Island just a short distance off the coast is part of the same lava flow at Fingal Head. This area, therefore has very similar features as the Giants Causeway and in my opinion the name Fingal Head is very appropriate.

The lava at Fingal Head is apparently derived from the Tweed Volcano (classified as Lismore or Beechmont Basalt, depending on what side you are of the Queensland border). Whether it is a lava flow erupted from the original central vent or vents on the northern flank of the volcano is not known. It is worth knowing that columnar volcanic rock is actually fairly common. Indeed, even better columnar formations can be seen elsewhere in the region. If you travel inland from Bellingen up to Ebor and visit the waterfall there (Ebor Falls) you will be able to see some spectacular formations. Columnar jointing is not restricted to basalt lavas either, some rhyolite cliffs around the Tweed Volcano also show this feature too.

References/Bibliography:

*Goehring, L, Morris, S.W. &  Lin, Z. 2006. Experimental investigation of the scaling of columnar joints. Physical Review. V64.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

Rocks and Landscapes of the Gold Coast Hinterland

Since rocks tend not to follow political boundaries but our understanding of them often does it is good to know about what is north of the Northern Rivers/New England Border in southern Queensland. Last year I was going to do a post on the Focal Peak Volcano but then I remembered that the Queensland Division of the Geological Society had produced some excellent publications on the subject and recommended one in particular, so the post was essentially a recommendation for the Book the Rocks and Landscapes of the National Parks of Southern Queensland. But I deliberately omitted from the post comments on another brilliant book that had recently been fully revised so that I could deal with is separately.

The other book is called Rocks and Landscapes of the Gold Coast Hinterland by Warwick Willmott. I enjoy this book very much because it is simple to understand but goes into a good amount of detail. It also shows you exactly where to go to see a feature of interest just like a self guided tour.

However, the detailed knowledge of the northern part of the Tweed Volcano may have skewed research and our understanding of the volcano in general. For instance, although the Tweed Volcano has been assumed to be centred around the site of present day Mount Warning in New South Wales most of our understanding including the detailed research of PhD and MSc level on the volcano actually comes from the University of Queensland. The University of Queensland has been the driving institution for decades in research on these northern flanks by exceptional researchers like Professor Anthony Ewart and Dr Jan Knutson.

As I have discussed in numerous other posts on the Tweed Volcano, the model of what the volcano looked like and how it was formed has recently been questioned by authors such as Cotter (1998). In my mind this raises some questions about elaborating the northern side of the volcano to the remainder in New South Wales. While I have nothing to question the good work on the northern side of the border, including the wonderful books produced by the Australian Geological Society's Queensland Division, it appears that the model of volcanism of the Tweed Volcano has been interpreted to fit into a Queensland model. This has occurred ever since authors like Duggan & Mason (1978) and continued to Stevens et al (1989) and most recently by Howden (2009). I do not question to model of rock formation to the north of the border (it works for what is there) but according to Cotter (1998) south of the border pre-volcanic geological conditions seemed to be different and this had a significant effect on the mode of volcanism in the area. This however, does not mean that the Rocks and Landscapes of the Gold Coast Hinterland is incorrect in any way on its description of Queensland geology, it is just important to note that interpreting the geology south of the border can sometimes be problematic even if a cursory look means that it appears reasonable.

But I have digressed a great deal. Back to the Book! The Rocks and Landscapes of the Gold Coast Hinterland is formatted in a way that makes it a geological tour. If you end up traveling through the Gold Coast area, do get a copy of this book. It is only about $12 including postage and is quite large and detailed for its price. In fact I'm surprised that the cost is so low, but I think that all the time that Warwick Willmott has put into writing it has been for free. As I have said in other posts, Warwick is one of the great science educators in Australia, and the book really helps understand the Gold Coast area a lot.

References/Bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Howden, S. 2009. An Evaluation of Mafic Extrusives Spatially Assoicated with the South-Western Aspect of the Tweed Shield Volcano, BSc(Hons.) thesis, University of New England, Armidale.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

A warning about Mount Warning

Here are some common quotes about Mount Warning:

"World Heritage listed Mount Warning (Wollumbin) is the remnant central plug of an ancient volcano." 

"The Mount Warning volcano was a huge shield volcano."

"Considered the central magma plug, Mt Warning and a system of ring dykes, being extremely hard rock, have resisted erosion, and dominate the valley landscape."

"Mt Warning, Wollumbin, the cloud catcher, is the basalt plug of the world's largest and oldest extinct volcano. "

"Now, Mt. Warning is the first place that that the sun hits at sunrise… the highest point in New South Wales….almost the highest in Australia!"

These are quotes typical of tourist and even educational resources. They are quite definite and the comments makes sense, mostly. There are also some points of view that I espoused for a long time... Except aspects of each of the quotes are technically wrong and in some cases completely wrong. Like my post on the "erosion caldera" something that is technically incorrect has become general knowledge. It is a little pedantic of me, but it is one of my hobby horses... so what is technically wrong with the quotes above?

Western face of Mount Warning (composed of syenite).
One of the ring dykes is visible in the foreground
and Mount Uki and the Pacific Ocean in the background

Interestingly, Stevens et al (1989) and earlier authors noted that the rock composition of the intrusions that make up Mount Warning (the Mount Warning Complex) is different from most of the lavas (The Lamington Volcanics) that exist in the region. It is also slightly older than most of the lavas. Geologically speaking the age difference is not huge at only about 2-3 million years, but still significant enough.

It is apparent from Smith & Houston (1995) and other authors that much of the rhyolite lavas that remain of the Lamington Volcanics were not erupted from the central area now the site of Mount Warning but from vents on the flanks. Given the coverage of the mafic components (the Lismore Basalt, for example) it is more difficult to identify any vents.  

An idea has been raised by Cotter (1998) which questions the volume of lava that was erupted from the Tweed Volcano. It is known that the Palaeozoic aged meta-sedimentary rocks of the Beenleigh Block, called the Neranleigh Fernvale Beds and the Mesozoic aged Chillingham Volcanics and Clarence Moreton Basin were not domed upwards by the underlying magma except a little around the Mount Warning Complex itself. However, other areas such as the nearby slightly older Focal Peak Volcano have been lifted by the Cenozoic aged volcanism. But in the case of Mount Warning, Cotter (1998) felt that lithology, the remnants of the rhyolitic lavas, the pre-existing Chillingham and Alstonville Volcanics was the main control on the geomorphology, not as suggested by others the volcanism that formed the shield volcano itself.

The idea suggested by Cotter (1998) has significant implications for the size of the Tweed Volcano. The volcano is considered the biggest by far of its age in eastern Australia. It appears likely that the extent of the shield volcano is not as great as originally thought. The underlying Chillingham Volcanics would have been an existing mountain range and therefore reduced the thickness of the Tweed volcanic pile and the Alstonville Basalts would have reduced the southerly extent. I think that when you add to this the idea that the rhyolite units have erupted away from Mount Warning, but instead from flanks on the volcano, the volume of lavas from the Tweed Volcano may actually be more in keeping with the other intra-plate volcanoes in Eastern Australia. It was also possible that before it was eroded into the present shape (which implies a central shield type volcano) it may have looked more irregular than we imagined...

But don't get me started on the comments about the biggest volcano in the world and the highest point in New South Wales!!! What were these people thinking?!

...but does any one want to talk down something that was presumed to be huge, just to something large? Emotionally, many (including myself) have an emotional attachment to the beauty and wonder of the Tweed Volcano, sometimes it is hard to take a step back and consider it is not quite as fabulous as originally thought, but what we see is still stunning... and it is still very, very big. To put that in perspective I think that even the small volcanoes in the region are stunning. We don't need to exaggerate something for it to inspire us.

Bibliography/References:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University. 
*Smith, J.V. , Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences V42(1) p69-74.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

Where the river joins the sea

In previous posts I've discussed a few peculiarities with the way some of our rivers flow, in particular the Clarence River which once ran backwards and the Wilsons River which flows away from the sea. This post is about another strange feature of the Northern Rivers which is the way many of them discharge into the sea.

Many people in the region will be aware of various issues with regard to erosion of sand our beaches or even deposition of sand choking river and creek mouths. Many people may be aware of Byron Shire Council having a policy of planned retreat from the areas along Belongil Beach at Byron Bay. Others may have heard of the silting up of Nambucca Harbour. But even less will realise that the biggest cause of these different problems is actually the same.

Richmond River mouth at Ballina. Note the white water of the Bar.
 Because of longshore drift the Ballina Bar is often treacheous.

But, let me back up for a moment. Have a look at Google maps or (even better) a paper map of the north coast of the New England / New South Wales area. Look at most of the major rivers. The Nambucca River, Clarence River, Richmond River, Tweed River. Look too at some of the smaller streams such as Tyagarah Creek, Cudgen Creek and others. What you might notice about all these streams is that they seem to flow north and roughly parallel to the coast only a short distance inland. They also join the sea on the southern side of headlands and on the northern side of long sandy beach systems. And therein lies the cause.

Along the coast of Eastern Australia are currents, the most well known is the Eastern Australian Current that flows south. However, the prevailing wind conditions which blow from the south to the north means that the direction of small currents and wave action is directed northward, these are called longshore currents. This has been the case during the Holocene (for many thousands of years) and has resulted in enormous amounts of sand being transported slowly up the coast line, where much of it ends up in southern Queensland forming Fraser Island.

Where the most direct route for the regions rivers would be to join the sea at right angles, longshore drift has caused sand dunes to build up sometimes even to the extent that it sometimes closes the mouths of the rivers. The movement of the sand has slowly pushed the river mouths further and further to the north until the come to an outcrop of rock which blocks the way. At this point the river mouth will cease to migrate along the coast and remain relatively stable until some storm, flood or man-made change occurs. A great example of a man-made change is Coffs Harbour, but more on that another time.

But why does the beach erode in many other places? Well, simply it is the impact of the headlands. On the northern side of the headlands along our coast there is only a little supply of sand (since the headland directs the sand away). Instead this is were sand is sourced to be transported north along the beaches. Places like Belongil Beach at Byron Bay are excellent examples where sand is naturally carried away northward along the edge Byron Marine Park, leaving houses built next to the sea at risk of being destroyed by the erosive processes.

As an aside, longshore currents are also partly responsible for the creation of some mineral deposits which have historically been mined. But more on that in a future post. 

Since I wrote the above, an anonymous comment raised an interesting point which quite reasonably raises questions my statements about the sand stability north of Byron Bay headland. I have reproduced the comment in red below:

Despite the position of rock headland anchor points and the change in coastal alignment along Northern NSW, any differential in longshore drift rates (sand losses from the sediment budget)should have equilbrated during the Holocene period, including sand losses into the deepwater sand lobe off Cape Byron. Erosion at Belongil Spit is more likely due to the interrupted supply caused by the Richmond River breakwaters at Ballina.

Bibliography/references:

White, M. E., 2000. Running Down, Water in a Changing Land. Kangaroo Press.

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.

Lindesay and the volcano

I recently went to Woodenbong via Kyogle. The trip along this section of the Summerland Way is very pretty as you climb into the McPherson Ranges. It also provides many opportunities for good views of imposing Mount Lindesay which is around 1180m high, located right on the state border and is a reminder about mistakes that people make when seeing mountains that are shaped the way they are.

Mount Lindesay from the south

Mount Lindesay is often referred to as a volcanic plug. I've heard this from different people several times. This is not surprising as the shape does imply this, but this is a trick of nature. The upper parts and 'peak' are flows of what is called the Binna Burra Rhyolite (or Mount Gillies Volcanics in Queensland) and some basalt, below this is a layer of obsidian (rhyolitic glass) overlying a layer of rhyolitic ash and agglomerates. The lower parts of the mountain is made from another volcanic rock, basalt (Kyogle Basalt). This basalt however overlies sediments of the Clarence Moreton Basin.

Mount Lindesay gets its shape by the rhyolite that forms the top most layer. The rhyolite is hard, resistant to weathering and therefore remains relatively difficult to erode. It is for this reason that the rhyolite has protected the underlying softer rock at Mount Lindsay and you can see the same process for ridges to the east and south of the mountain too. The actual vents for the rhyolite and underlying basalt lavas is actually a little tricky to definitely locate but we do know that the main volcanic centre for these rocks was at the Focal Peak Volcano located in the vicinity of present day Mount Barney a significant distance to the north. Additionally, there are some real volcanic plugs further to the west which I mention below.

Rhyolite from focal peak was thought by Duggan and Mason (1978) and other authors to extend as far Nimbin to the east. However, recent work by Cotter (1998) has shown that this is not the case but the Binna Burra Rhyolite still extends a long way to the east past places like Wiangaree.

There are however, some clearly identifiable volcanic plugs in the region. A good one is sometimes referred to as the Nightcap Peak and is located half way between Woodenbong and Urbenville just a little to the west of the road. It stands out from the rolling hills, is difficult to miss and is made from the rock granophyre (fine grained granite-like rock). At Urbenville the Northern Obelisk is another example of a plug, a bit one! Additionally, large dykes exist to the south west of Urbenville too.

References/bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.