In a previous post I discussed how tight gas is defined and how it behaves. I promised in that post I would expand on the theme of hydrocarbon geology and discuss other sources or techniques for obtaining natural gas. These include the aforementioned tight gas, but I will eventually cover all aspects including coal seam gas, basin gas and lastly “conventional” gas.
In this post I’ll quickly cover underground coal gasification (UCG) producing “syngas”. This is to make clear that it is different from coal seam gas. Presently, syngas is not produced in Australia and recent trials in Queensland and South Australia have ceased and moved off-shore to China. The companies cited a more conducive research and regulatory environment there than Australia. I only cover UCG because the geology of the southern Clarence-Moreton, Ipswich and maybe Lorne basins may be seen as sources for syngas in the distant future.
Syngas is produced through the process of underground coal gasification. This is a relatively new and novel way to turn coal into gas, though the concepts are in many ways similar to the older concepts of shale oil extraction and town gas production. these techniques having been used for more than a hundred years. Like most aspects of science, something new builds upon something old.
The first step in UCG is to find coal rich strata confined by a high pressure of natural water in the coal seam. A vertical drill hole is installed in one end of the coal seam and is terminated at the bottom of the target coal seam. A second drill hole is drilled at the other end of the gas field, possibly 2 or 3 kilometres away. This second hole is however, directionally drilled and follows the bottom of the target coal seam all the way until it intersects the first vertical drill hole. A well head is then set up at the first vertical drill hole and gasification infrastructure set up at the horizontal (directional drill hole). It is from the directionally drilled hole that all the interesting action takes place. The vertical one is just used for pumping the gas to the surface.
Gasification infrastructure is comprised of pumps for forcing air and guiding an ignition source into the ground. The the actual process of UCG occurs in-situ, that is, in the coal seam itself. The coal is first ignited underground at the point where the horizontal and vertical drill holes intersect. Air is pumped into the coal seam to displace some of the water which allows the process to continue. If air is not injected the water occurring in the rock extinguishes the gasification process. The coal is continually kept ‘burning’ underground and slowly moves along the directional drill hole as air and ignition is applied.
This process is essentially incomplete combustion. A process that was used to produce town gas in most major towns and cities in Australia up until the 1970’s. The incomplete combustion leads to production of CO and CH4. Adding too much air into the process simply produces more CO2 and so a balance of water pressure, air pressure and gas production is needed.
UCG differs from coal seam gas (CSG) in that water is only partially displaced from the coal seam. CSG requires as much water as possible to be removed to stimulate the natural flow of gas. Groundwater in CSG can be considered a waste product of the extraction process, a bit like overburden in a coal mine. UCG leaves the “overburden” water essentially intact.
UCG is an interesting, challenging and clever way to turn coal into a gas resource. It has been marketed as an alternative to digging a huge hole in the ground to extract the coal in a mine. The groundwater issues are regarded as less invasive than direct mining but there is added potential for incomplete burning residues to contaminate the groundwater. For example incomplete combustion can produce chemicals such as polycyclic aromatic hydrocarbons (PAH). Although generally poorly soluble, the presence of these chemicals is perceived as a concern by many people. Whether or not there is an avenue for these chemicals to become a risk to the environment is hotly debated. It is therefore now surrounded by a lot of controversy. Like underground coal mining there is also the possibility of ground subsidence. But regardless, it appears that in the short run this process will not be used in our region.
Sunday, 1 December 2013
Saturday, 23 November 2013
A non textbook example
Text books are wonderful. They always have excellent ‘text-book’ examples! These show how a scenario can be interpreted and what information is used in that interpretation. As you get to know the textbook you get a feel for most or all of the information you can obtain to give you an answer. However, in geology many of the techniques are rarely all applicable to every field situation; or if they are they are applicable, they are unreasonably difficult to use.I have recently experienced one such example in an area south-west of Byron Bay. There is very little information available to interpret and therefore the possibility of misinterpretation can be high.
Byron Shire Council recently did some road works along a section of road between the village of Newrybar and the coast. This work refreshed some small road cuttings (road cuttings are geological tourist attractions). I took a close look at one of the road cuttings on the very edge of the Alstonville Plateau. The rock in this cutting was clearly different from the overlying and dominant Cenozoic aged basaltic lavas that make up the plateau. The exposure was made up of conglomerate.
Conglomerate is a sedimentary rock most often associated with high energy river environments. In this case the conglomerate contained clasts made from other older rocks that occur elsewhere in the region. This included chert, quartzite and fine to medium grained sedimentary rocks such as sandstones and siltstones. The rock though had been quite weathered and the sedimentary clasts had become quite broken down even though they retained their shape insitu.
There is nothing particularly special about this conglomerate. Here the mapping indicates that I was at the very edge of the Clarence-Moreton basin and therefore the oldest rocks of the basin would be likely to outcrop. Indeed, the oldest rock in the basin is known as the Laytons Range Conglomerate. This outcrop looks very much like it. But… further to the east (for example on Broken Head road) are rocks of the Ripley Road Sandstone. These are younger rocks of the Clarence-Moreton basin than the Laytons Range Conglomerate. The Ripley Road Sandstone contains small layers of pebble conglomerate but nothing compared to that exposed in the road cutting. Weirdly this means that the current mapping of the basin indicates that the Ripley Road Sandstone should be older than the road cutting rocks. This is the opposite of the known sequence of the area. To make the road cutting conglomerate fit there is several hypotheses:
The only trouble is there seems to be inadequate information and field exposure to narrow down the possibilities. I’d love to get a drill rig and core a 200m interval but who has a spare hundred thousand dollars to do that?!
For the time being all I can do is assume the conglomerate was deposited sometime during the formation of the Clarence-Moreton Basin maybe as long as 250million years ago or deposited sometime before the Cenozoic basalts of the Alstonville Plateau possibly 40million years ago.
Alas, there is not enough information available to interpret this situation. But this is normal! We rarely are lucky enough to get a text-book example. In science the examples we are most confronted with are incomplete and generally frustrating. We can’t lie to ourselves that we can answer every question and know everything.
To the lady that stopped, looked at me curiously, and then asked me if I was “alright?” when I was examining the road cutting: Yes, I’m alright. But I still want to know the answer.
Byron Shire Council recently did some road works along a section of road between the village of Newrybar and the coast. This work refreshed some small road cuttings (road cuttings are geological tourist attractions). I took a close look at one of the road cuttings on the very edge of the Alstonville Plateau. The rock in this cutting was clearly different from the overlying and dominant Cenozoic aged basaltic lavas that make up the plateau. The exposure was made up of conglomerate.
Conglomerate is a sedimentary rock most often associated with high energy river environments. In this case the conglomerate contained clasts made from other older rocks that occur elsewhere in the region. This included chert, quartzite and fine to medium grained sedimentary rocks such as sandstones and siltstones. The rock though had been quite weathered and the sedimentary clasts had become quite broken down even though they retained their shape insitu.
.JPG) |
| Conglomerate near Newrybar on the road to Broken Head and Byron Bay note the different clast types and sizes - typical of the Laytons Range Conglomerate |
- The conglomerate in the cutting is actually not part of the Clarence-moreton basin but was deposited more recently and then covered by basalt. Maybe it was a pre-volcanic river system,
- The conglomerate in the cutting is actually part of a younger Clarence-Moreton basin unit that has needs to be redefined to include this particular type of conglomerate.
- The depositional structure of the Clarence-Moreton Basin is different in this area to the current model e.g. the road cutting is on the western side of a small sub basin.
- Faulting or folding has up-thrown the conglomerate in this area giving the impression that it is stratigraphically higher
- Other reasons I cannot think of at the moment
The only trouble is there seems to be inadequate information and field exposure to narrow down the possibilities. I’d love to get a drill rig and core a 200m interval but who has a spare hundred thousand dollars to do that?!
For the time being all I can do is assume the conglomerate was deposited sometime during the formation of the Clarence-Moreton Basin maybe as long as 250million years ago or deposited sometime before the Cenozoic basalts of the Alstonville Plateau possibly 40million years ago.
Alas, there is not enough information available to interpret this situation. But this is normal! We rarely are lucky enough to get a text-book example. In science the examples we are most confronted with are incomplete and generally frustrating. We can’t lie to ourselves that we can answer every question and know everything.
To the lady that stopped, looked at me curiously, and then asked me if I was “alright?” when I was examining the road cutting: Yes, I’m alright. But I still want to know the answer.
Friday, 1 November 2013
Hills of old sea floor muck
There has obviously been a bit of a lull in my blogging of late. I’ve been busy with family medical trips to Queensland and I’ve had less free time too. But some interesting things have happened with one formal presentation on coal seam gas and water and another presentation to be given in a couple of weeks. But on the aspects that interest me most (non-CSG geology), I’ve also been contacted by academics from a couple of different universities. It is nice to know that they feel I can help them with some research projects. I'll post more about that at a future date.
During the trip to Queensland I met up with family on the Gold Coast. We decided to have a day up in the popular Springbrook National Park area. In particular the views in this country are astonishing. The Best Of All Lookout certainly lives up to its name with incredible views of the valleys of the Tweed region. Mount Warning looks stunning and the rugged terrain of the volcanic shield remnants beautiful. And this was on a hazy day!
To get to Springbrook national park from the Gold Coast it is necessary to traverse the oldest rocks in the Tweed region. These are sediments of the Neranleigh-Fernvale beds. These are represented by the initially steep hilly terrain as you head westward up the range. Hinze Dam, for example, is located on this rock type. Time has weathered and eroded much of this rock away but still it remains as a significant landscape feature. These rocks and hills would probably be better known if the lavas associated with the Tweed Volcano had not erupted.
The Neranleigh-Fernvale beds are interesting rocks because of their mode of formation. They are essentially muds and debris flows that have been deposited in a trench during a period known as the Paleozoic. The trench was caused by the subduction of a continental plate under the then eastern Australian landmass. These sediments were then scrapped off and buckled into a large mountain range that has since been mostly eroded away. All of this occurred while Australia was part of the super-continent Pangaea which existed well before Gondwana.
Today, in the Northern Rivers the Neranleigh-Fernvale beds form the steep eroded terrain in the Tweed Valley (with the exception of some lavas and intrusions associated with the Tweed Volcano). They outcrop in a band at the very edge of the Alstonville Plateau to Byron Bay. They only occur as a band in the Ballina area because they are obscured by Jurassic sediments and the Cenozoic volcanic rocks. Like the Springbrook area, driving from Ballina to Alstonville or from Cabarita to Chillingham means traversing this formation. As soon as you get off the coastal plain and head up the hills you are passing the rocks of the Neranleigh-Fernvale beds. These beds are then obscured by the more recent sediments or volcanic rocks associated with the Tweed Volcano.
As for the Springbrook area, if you’d like to know more I recommend a book by Warwick Wilmott called Rocks and Landscapes of the Gold Coast Hinterland. The processes and timing of events in the Gold Coast area are very very similar to those processes that occurred in the Tweed valley area and so might be worth a read even if you don’t cross the border!
Warwicks book can be obtained from the Queensland Division of the Geological Society of Australia here.
.JPG) |
| Best of all lookout - Springbrook National Park Except for the hills on the horizon the rock in this photo is mainly of the Neranleigh-Fernvale beds. |
To get to Springbrook national park from the Gold Coast it is necessary to traverse the oldest rocks in the Tweed region. These are sediments of the Neranleigh-Fernvale beds. These are represented by the initially steep hilly terrain as you head westward up the range. Hinze Dam, for example, is located on this rock type. Time has weathered and eroded much of this rock away but still it remains as a significant landscape feature. These rocks and hills would probably be better known if the lavas associated with the Tweed Volcano had not erupted.
The Neranleigh-Fernvale beds are interesting rocks because of their mode of formation. They are essentially muds and debris flows that have been deposited in a trench during a period known as the Paleozoic. The trench was caused by the subduction of a continental plate under the then eastern Australian landmass. These sediments were then scrapped off and buckled into a large mountain range that has since been mostly eroded away. All of this occurred while Australia was part of the super-continent Pangaea which existed well before Gondwana.
Today, in the Northern Rivers the Neranleigh-Fernvale beds form the steep eroded terrain in the Tweed Valley (with the exception of some lavas and intrusions associated with the Tweed Volcano). They outcrop in a band at the very edge of the Alstonville Plateau to Byron Bay. They only occur as a band in the Ballina area because they are obscured by Jurassic sediments and the Cenozoic volcanic rocks. Like the Springbrook area, driving from Ballina to Alstonville or from Cabarita to Chillingham means traversing this formation. As soon as you get off the coastal plain and head up the hills you are passing the rocks of the Neranleigh-Fernvale beds. These beds are then obscured by the more recent sediments or volcanic rocks associated with the Tweed Volcano.
As for the Springbrook area, if you’d like to know more I recommend a book by Warwick Wilmott called Rocks and Landscapes of the Gold Coast Hinterland. The processes and timing of events in the Gold Coast area are very very similar to those processes that occurred in the Tweed valley area and so might be worth a read even if you don’t cross the border!
Warwicks book can be obtained from the Queensland Division of the Geological Society of Australia here.
Tuesday, 8 October 2013
Being tight with loose terminology?
There has been a lot of discussion recently about a local company resuming exploration for gas in our region. In particular the announcement by the company that they intend to drill a deep borehole next to the Lismore-Kyogle Road at Bentley has raised a great deal of heated debate. For example, this story in the Northern Star shows just how intense the feelings (one way or another) can be. One thing has been clear though is people are sometimes having trouble figuring out what gas companies are doing. The news release from Metgasco and their Review of Environmental Factors report state the proposed drill hole will be for "conventional gas". Critics of gas companies say since hydraulic fracturing (fraccing, fracking etc) may be carried out in the proposed drill hole the gas must be "unconventional" tight gas. Some people (including the local members of parliament) seem to think any drill hole in the area must involve coal seam gas. It is all a little confusing.
The first thing to note is gas should not be described as either "conventional" or "unconventional". There is essentially no difference in the gas (mainly comprised of methane). The difference is in how it is extracted.
The second thing to note is "tight gas" is only termed such by an arbitrary permeability value assigned by oil and gas engineers. In the real world there is a spectrum between traditionally sourced gas and tight gas. The tighter gas is gas occurring in a reservoir but does not flow as rapidly as in other locations. Tight gas is restricted from flowing by the fill in the cross connecting voids by a material formed after the gas migrated there (usually a natural cement such as calcite or quartz). The lack of cross connection between gas filled pore spaces is what reduces the permeability of the rock.
To clarify I use filter analogies. A new filter will let a substance flow through it easily but an old one is more clogged up and doesn't let the substance flow as rapidly. In the oil and gas industry an arbitrary permeability value is used as an indication of when it is called tight gas. It usually has a permeability of less than 0.1mD (millidarcy). It is also important to be clear that permeability is not the same as porosity because even tight gas reservoirs still have high porosity.
What is a millidarcy? I should do a blog post specifically on Darcy's Law but in the mean time it is good to visualise 1 millidarcy as the permeability of water in a fine sand filter. The way a millidarcy is calculated is through measurement of the velocity of fluid flow through the filter, viscosity of the fluid, cross sectional area of the filter and the pressure. In the case we are talking about the fluid is gas. The main difference being gas has a much lower viscosity and therefore can pass through a finer filter even easier. For 1 millidarcy our analogy for gas might be a fine paper filter instead of fine sand filter for water. It is interesting to note that the measurements needed to calculate the permeability of a gas reservoir are identical for hydrogeologists trying to understand groundwater flow or brewers filtering a favorite beer.
When the permeability of a gas reservoir decreases to 0.1mD the gas flows at a lower and lower rate. This means that it becomes less economical to let the gas migrate out of the geological formation on its own. Instead companies often look to reservoir stimulation which in its simplest form is introducing acid to dissolve minerals. Such a common mineral is calcite that may have clogged the formation, opening up the blockages between the pores. This is a very useful technique in natural calcium cement rich formations. Acidification has been used for hundreds (perhaps thousands) of years to increase the flow rate of groundwater sources for drinking, irrigation and other purposes. It is still commonly used in Australia today for groundwater purposes too. However, the most well known form of reservoir stimulation is the increasingly used hydraulic fracturing (fraccing/fracking). Fraccing involves the introduction of a fluid such as water (plus other ingredients) under pressure to propagate fractures through the formation. These fractures allow gases to escape much more easily. I don't want to go into details about fraccing here but I will suffice to say that the method is controversial.
As far as the terminology goes, tight gas is a very loose term. Tight gas is not an "unconventional" gas, it is a bog standard gas that sometimes require "unconventional" techniques to extract it. It is also important to note that reservoir stimulation is an "unconventional" method of extracting gas, but this does not in itself say much because the "conventional" method of extracting gas is just sticking a big hole in the ground.
I hope this blog post makes sense. While I was writing, it became obvious that several different posts are needed to explain the different areas of gas reservoirs. In the mean time I hope that this short post makes sense. I'll see what I can do over the coming months to further delve into the hidden world of petroleum geology (while steering as far away from controversy as possible).
The first thing to note is gas should not be described as either "conventional" or "unconventional". There is essentially no difference in the gas (mainly comprised of methane). The difference is in how it is extracted.
The second thing to note is "tight gas" is only termed such by an arbitrary permeability value assigned by oil and gas engineers. In the real world there is a spectrum between traditionally sourced gas and tight gas. The tighter gas is gas occurring in a reservoir but does not flow as rapidly as in other locations. Tight gas is restricted from flowing by the fill in the cross connecting voids by a material formed after the gas migrated there (usually a natural cement such as calcite or quartz). The lack of cross connection between gas filled pore spaces is what reduces the permeability of the rock.
To clarify I use filter analogies. A new filter will let a substance flow through it easily but an old one is more clogged up and doesn't let the substance flow as rapidly. In the oil and gas industry an arbitrary permeability value is used as an indication of when it is called tight gas. It usually has a permeability of less than 0.1mD (millidarcy). It is also important to be clear that permeability is not the same as porosity because even tight gas reservoirs still have high porosity.
What is a millidarcy? I should do a blog post specifically on Darcy's Law but in the mean time it is good to visualise 1 millidarcy as the permeability of water in a fine sand filter. The way a millidarcy is calculated is through measurement of the velocity of fluid flow through the filter, viscosity of the fluid, cross sectional area of the filter and the pressure. In the case we are talking about the fluid is gas. The main difference being gas has a much lower viscosity and therefore can pass through a finer filter even easier. For 1 millidarcy our analogy for gas might be a fine paper filter instead of fine sand filter for water. It is interesting to note that the measurements needed to calculate the permeability of a gas reservoir are identical for hydrogeologists trying to understand groundwater flow or brewers filtering a favorite beer.
When the permeability of a gas reservoir decreases to 0.1mD the gas flows at a lower and lower rate. This means that it becomes less economical to let the gas migrate out of the geological formation on its own. Instead companies often look to reservoir stimulation which in its simplest form is introducing acid to dissolve minerals. Such a common mineral is calcite that may have clogged the formation, opening up the blockages between the pores. This is a very useful technique in natural calcium cement rich formations. Acidification has been used for hundreds (perhaps thousands) of years to increase the flow rate of groundwater sources for drinking, irrigation and other purposes. It is still commonly used in Australia today for groundwater purposes too. However, the most well known form of reservoir stimulation is the increasingly used hydraulic fracturing (fraccing/fracking). Fraccing involves the introduction of a fluid such as water (plus other ingredients) under pressure to propagate fractures through the formation. These fractures allow gases to escape much more easily. I don't want to go into details about fraccing here but I will suffice to say that the method is controversial.
As far as the terminology goes, tight gas is a very loose term. Tight gas is not an "unconventional" gas, it is a bog standard gas that sometimes require "unconventional" techniques to extract it. It is also important to note that reservoir stimulation is an "unconventional" method of extracting gas, but this does not in itself say much because the "conventional" method of extracting gas is just sticking a big hole in the ground.
I hope this blog post makes sense. While I was writing, it became obvious that several different posts are needed to explain the different areas of gas reservoirs. In the mean time I hope that this short post makes sense. I'll see what I can do over the coming months to further delve into the hidden world of petroleum geology (while steering as far away from controversy as possible).
Tuesday, 1 October 2013
The Woodburn sands of time
I’ve been spending some time working on a project in the lower reaches of the Richmond River Valley. This project got thinking about the stratigraphy and depositional history of that area. Particularly about a unit of unconsolidated sand called the Woodburn Sands (Drury 1982). In some ways this post follows on from a couple of posts that touched on the subject of sea level changes during the Quaternary.
To begin to understand this unconsolidated sediments of the Richmond River Valley we turn to the most recent mapping of the area. Troedson et al. (2004) comprehensively mapped the coastal Quaternary sediments of the whole east coast of NSW. Troedson et al. (2004) demonstrated that over large areas of the lower Richmond Valley there are two units of coastal sand which formed in barrier environments. The most obvious coastal sands are active dune and beach systems formed from a barrier by the action of present day long-shore drift. These active barrier systems occur in many places along the coast. Troedson et al. (2004) also mapped extensive areas of what an earlier researcher Thom (1965) first identified as an inner coastal barrier. This inner barrier is comprised of an old beach system that is no longer active.
Drury (1982) undertook a comprehensive study of the Quaternary sediments of the Richmond River Valley. He confirmed the view by Thom (1965) that there was an old inner barrier system. This system was formed during a higher period of sea level than today and caused regional changes to coastal sedimentation (e.g. I previously posted on the estuarine sediments of the Lismore area). The high sea level eroded away the pre-existing beaches and formed new beach systems a significant distance inland (sometimes 15km or more). Then as the sea level retreated, the new beaches were no longer subject to erosion from the sea and were left intact. The beach systems continued to form on the sea-ward side of the old beaches and eventually built up a very large area of sand. These old beach systems are what made the Woodburn Sands.
The Woodburn Sands occur in a discontinuous zone from Broken Head National Park to the Evans River and the lowest reaches of the Richmond River (Swan Bay). The maximum thickness intersected is about 35 metres, so the sand layers can be very thick.
Like many places in eastern Australia, the action of coastal wave and wind processes can lead to concentrations of heavy mineral sand. These deposits are called mineral placers. The Woodburn Sands is another of these areas where placers are common. Indeed, a lot of sand mining took place on the north coast to exploit the high concentrations of zircon, ilmenite and even gold. Presently, the Woodburn Sands is not mined for minerals but is used as an important source of good quality groundwater, this includes the regional town water supply authority.
Drury (1982) also included an unusual feature within the Woodburn Sands. This feature was named the Broadwater Sandrock by Mcgarity (1956). McGarity (1956) demonstrated that the Broadwater Sandrock was formed by the cementation of sand by organic rich material probably formed by changes occurring in a peat swamp environment. This sandrock is a common feature up and down the east coast of Australia. Another common feature is the diversity of names given to this material which include ‘indurated sand’, ‘coffee rock’, ‘coastal sandrock’, ‘painted rock’, ‘beach rock’, ‘humate’ and ‘B-horizon of the humus podzol’ (Drury (1982), Mcgarity (1956), Thom (1965) and Den Exter (1974)). Take your pick! I follow the terminology proposed by Drury (1982) who included the Broadwater Sandrock as a member of the Woodburn Sands, i.e. the Broadwater Sandrock member.
Postscript:
Since doing the above post an anonymous commenter has rightly corrected and provided further information. You can see the full comment below, the comment much more accurately describes 'coffee rock' formation but I reproduce this section specifically:
...humicrete (coffee rock) forms as the B-horizon of a fossil soil on sand (a podsol). It is NOT a sedimentary layer itself ie NOT a stratigraphic unit, so should not have been referred to as a "member" ...
To begin to understand this unconsolidated sediments of the Richmond River Valley we turn to the most recent mapping of the area. Troedson et al. (2004) comprehensively mapped the coastal Quaternary sediments of the whole east coast of NSW. Troedson et al. (2004) demonstrated that over large areas of the lower Richmond Valley there are two units of coastal sand which formed in barrier environments. The most obvious coastal sands are active dune and beach systems formed from a barrier by the action of present day long-shore drift. These active barrier systems occur in many places along the coast. Troedson et al. (2004) also mapped extensive areas of what an earlier researcher Thom (1965) first identified as an inner coastal barrier. This inner barrier is comprised of an old beach system that is no longer active.
Drury (1982) undertook a comprehensive study of the Quaternary sediments of the Richmond River Valley. He confirmed the view by Thom (1965) that there was an old inner barrier system. This system was formed during a higher period of sea level than today and caused regional changes to coastal sedimentation (e.g. I previously posted on the estuarine sediments of the Lismore area). The high sea level eroded away the pre-existing beaches and formed new beach systems a significant distance inland (sometimes 15km or more). Then as the sea level retreated, the new beaches were no longer subject to erosion from the sea and were left intact. The beach systems continued to form on the sea-ward side of the old beaches and eventually built up a very large area of sand. These old beach systems are what made the Woodburn Sands.
The Woodburn Sands occur in a discontinuous zone from Broken Head National Park to the Evans River and the lowest reaches of the Richmond River (Swan Bay). The maximum thickness intersected is about 35 metres, so the sand layers can be very thick.
Like many places in eastern Australia, the action of coastal wave and wind processes can lead to concentrations of heavy mineral sand. These deposits are called mineral placers. The Woodburn Sands is another of these areas where placers are common. Indeed, a lot of sand mining took place on the north coast to exploit the high concentrations of zircon, ilmenite and even gold. Presently, the Woodburn Sands is not mined for minerals but is used as an important source of good quality groundwater, this includes the regional town water supply authority.
Drury (1982) also included an unusual feature within the Woodburn Sands. This feature was named the Broadwater Sandrock by Mcgarity (1956). McGarity (1956) demonstrated that the Broadwater Sandrock was formed by the cementation of sand by organic rich material probably formed by changes occurring in a peat swamp environment. This sandrock is a common feature up and down the east coast of Australia. Another common feature is the diversity of names given to this material which include ‘indurated sand’, ‘coffee rock’, ‘coastal sandrock’, ‘painted rock’, ‘beach rock’, ‘humate’ and ‘B-horizon of the humus podzol’ (Drury (1982), Mcgarity (1956), Thom (1965) and Den Exter (1974)). Take your pick! I follow the terminology proposed by Drury (1982) who included the Broadwater Sandrock as a member of the Woodburn Sands, i.e. the Broadwater Sandrock member.
Postscript:
Since doing the above post an anonymous commenter has rightly corrected and provided further information. You can see the full comment below, the comment much more accurately describes 'coffee rock' formation but I reproduce this section specifically:
...humicrete (coffee rock) forms as the B-horizon of a fossil soil on sand (a podsol). It is NOT a sedimentary layer itself ie NOT a stratigraphic unit, so should not have been referred to as a "member" ...
As such, I have now changed my mind! The Broadwater Sandrock member is not the best name after all. It seems that 'B-horizon of the humus podzol' is indeed one of the best ones. Humicrete is another good one. Well, it seems that the diversity of names will probably continue, but we can remove the one I thought the simplest (Broadwater Sandrock member) from the list!
References/bibliography:
*Den Exter, P.M. 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, NSW. PhD thesis. University of New South Wales, Kensington.
*McGarity, J.W. 1956. Coastal sandrock formation at Evans head, NSW. Proceedings of the Linnean Society of New South Wales. V81 p52-58.
*Thom, B.G. (1965). Late Quaternary morphology of the Port Stephens-Myall Lakes area, NSW. Journal of the Royal Society of New South Wales V98 p23-36.
*Troedson, A., Hashimoto, T.R., Jaworska, J., Malloch, K., Cain, L., 2004. New South Wales Coastal
Quaternary Geology. In NSW Coastal Quaternary Geology Data Package, Troedson, A., Hashimoto, T.R. (eds), New South Wales Department of Primary Industries, Mineral Resources, Geological Survey of New South Wales, Maitland.
References/bibliography:
*Den Exter, P.M. 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, NSW. PhD thesis. University of New South Wales, Kensington.
*McGarity, J.W. 1956. Coastal sandrock formation at Evans head, NSW. Proceedings of the Linnean Society of New South Wales. V81 p52-58.
*Thom, B.G. (1965). Late Quaternary morphology of the Port Stephens-Myall Lakes area, NSW. Journal of the Royal Society of New South Wales V98 p23-36.
*Troedson, A., Hashimoto, T.R., Jaworska, J., Malloch, K., Cain, L., 2004. New South Wales Coastal
Quaternary Geology. In NSW Coastal Quaternary Geology Data Package, Troedson, A., Hashimoto, T.R. (eds), New South Wales Department of Primary Industries, Mineral Resources, Geological Survey of New South Wales, Maitland.
Thursday, 12 September 2013
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.
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.
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. |
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.
Sunday, 1 September 2013
The right age for Mount Warning
In previous posts on the Tweed Volcano, especially those relating to the Mount Warning Central Complex I indicated that there were some strange anomalies to do with the dating of these intrusions. Graham (1990), in his natural history summary of the Tweed region illustrated how confusing the dates recorded for the rocks that made up the Mount Warning Central Complex could be.
Wellman and McDougall (1974) summarised existing and provided new evidence for the date of the Mount Warning Central Complex and the surrounding Lamington Volcanics. Wellman and McDougall (1974) and earlier researchers used a very good technique of dating called potassium-argon dating (K-Ar dating). This is a radiometric dating method based on measurement of the radioactive decay of an isotope of potassium (40K) into argon (40Ar). Note, that the numbers in front of the chemical symbol for each element refers to the number of neutrons in the atoms nucleus. The decay rate of 40K to 40Ar is known accurately because the time it takes for half of the 40K to turn into 40 Ar is about 1.25billion years (the half-life). Therefore, the ratio of the two can be used to determine just how old the rock is.
The accuracy of using the K-Ar dating method is very good, but has some provisos. The most important being that the rock sample must be very 'fresh'. There must be no weathering, alteration or metamorphism of the sample. Because potassium is more reactive than argon and it can be removed or added respectively during weathering and alteration. Additionally, the K-Ar dating 'clock' can be reset during any recrystallisation during metamorphism.
K-Ar dating by Wellman and McDougall (1974) and earlier authors showed that the intrusive complex at Mount Warning was emplaced between ~23.7Ma and 23.0Ma and the surrounding lavas erupted from ~22.3Ma to ~20.5Ma. This doesn't make a lot of sense because an intrusion of magma needs to intrude into something else (otherwise it is not an intrusion!). In the case of shield volcanoes this intrudes the earlier lava that was erupted before. The K-Ar dating shows this is apparently not the case.
What is going on? No one could suggest any reasonable ideas. Cotter (1998) suggested a possibility there may have been a large volume of pre-existing Palaeozoic and/or Mesozoic sedimentary rocks that have now been eroded away. However, Cotter (1998) did date a sample of basalt lava from the Terania Creek area at ~23.9Ma (using K-Ar). This suggested maybe the dating by Wellman and McDougall (1974) and earlier authors might have either missed later lavas or maybe there was something else wrong.
Cohen (2007) spent a lot of time resampling the K-Ar dated volcanic rocks of eastern Australia. This time instead of using K-Ar he used another technique called 40Ar-39Ar dating. This is similar to K-Ar dating in concept. It instead measures the abundance of two isotopes of argon and is much less affected by any effects of weathering and alteration (though not metamorphism). What did he find? He found some of the K-Ar dates were wrong.
Cohen (2007) found the actual date of the lavas was within the range ~24.3 to ~23.6 million years, about 2 million years older than first thought. Though the 40Ar-39Ar date of the Mount Warning Central Complex was quite close at ~23.1Ma it fell within the range of the K-Ar dating (23.7-23.0Ma). This reverses the idea the intrusion of the Mount Warning Central Complex was before the lavas. So, now we know that the final intrusions of the Mount Warning Central Complex does indeed fit the model for shield volcanoes. That is, the intrusions were likely to have been emplaced into already erupted volcanic rock. They were also erupted and emplaced over a period much quicker than first thought. The new dating shows volcanism possibly lasting 1 million years instead of the 3 million previously suggested.
References/bibliography
*Cohen, B.E. 2007. High-resolution 40Ar/39Ar Geochronology of Intraplate Volcanism in Eastern Australia. PhD Thesis, University of Queensland.
*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.
*Graham, B.W. 1990. A Natural History - Tweed Gold Coast Region. Tweed River High School Library.
*Wellman, P. & McDougall, I. 1974. Potassium-argon Dates on the Cainozoic Volcanic Rocks of New South Wales. Journal of the Geological Society of Australia v21.
 |
| Mount Warning Central Complex from the southern rim of eroded shield |
The accuracy of using the K-Ar dating method is very good, but has some provisos. The most important being that the rock sample must be very 'fresh'. There must be no weathering, alteration or metamorphism of the sample. Because potassium is more reactive than argon and it can be removed or added respectively during weathering and alteration. Additionally, the K-Ar dating 'clock' can be reset during any recrystallisation during metamorphism.
K-Ar dating by Wellman and McDougall (1974) and earlier authors showed that the intrusive complex at Mount Warning was emplaced between ~23.7Ma and 23.0Ma and the surrounding lavas erupted from ~22.3Ma to ~20.5Ma. This doesn't make a lot of sense because an intrusion of magma needs to intrude into something else (otherwise it is not an intrusion!). In the case of shield volcanoes this intrudes the earlier lava that was erupted before. The K-Ar dating shows this is apparently not the case.
What is going on? No one could suggest any reasonable ideas. Cotter (1998) suggested a possibility there may have been a large volume of pre-existing Palaeozoic and/or Mesozoic sedimentary rocks that have now been eroded away. However, Cotter (1998) did date a sample of basalt lava from the Terania Creek area at ~23.9Ma (using K-Ar). This suggested maybe the dating by Wellman and McDougall (1974) and earlier authors might have either missed later lavas or maybe there was something else wrong.
Cohen (2007) spent a lot of time resampling the K-Ar dated volcanic rocks of eastern Australia. This time instead of using K-Ar he used another technique called 40Ar-39Ar dating. This is similar to K-Ar dating in concept. It instead measures the abundance of two isotopes of argon and is much less affected by any effects of weathering and alteration (though not metamorphism). What did he find? He found some of the K-Ar dates were wrong.
Cohen (2007) found the actual date of the lavas was within the range ~24.3 to ~23.6 million years, about 2 million years older than first thought. Though the 40Ar-39Ar date of the Mount Warning Central Complex was quite close at ~23.1Ma it fell within the range of the K-Ar dating (23.7-23.0Ma). This reverses the idea the intrusion of the Mount Warning Central Complex was before the lavas. So, now we know that the final intrusions of the Mount Warning Central Complex does indeed fit the model for shield volcanoes. That is, the intrusions were likely to have been emplaced into already erupted volcanic rock. They were also erupted and emplaced over a period much quicker than first thought. The new dating shows volcanism possibly lasting 1 million years instead of the 3 million previously suggested.
References/bibliography
*Cohen, B.E. 2007. High-resolution 40Ar/39Ar Geochronology of Intraplate Volcanism in Eastern Australia. PhD Thesis, University of Queensland.
*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.
*Graham, B.W. 1990. A Natural History - Tweed Gold Coast Region. Tweed River High School Library.
*Wellman, P. & McDougall, I. 1974. Potassium-argon Dates on the Cainozoic Volcanic Rocks of New South Wales. Journal of the Geological Society of Australia v21.
Wednesday, 14 August 2013
Looking for the signs of Palaeontology
Over at the Highly Allochthonous blog, Chris draws us to some signs of experimentation with palaeontology in our children. A very interesting parental advice poster! Are you checking what your child is dabbling in?
Thursday, 1 August 2013
Bruxner Monzogranite on the Bruxner Highway
In a previous post, I discussed the metamorphism of limestone at an area north-west of Tabulam. I thought I’d take the opportunity to discuss the intrusion itself that caused the metamorphism (a rock unit called the Bruxner Monzogranite). Also briefly, put it in the context of the formation of the broader New England Batholith.
The Bruxner Monzogranite is a geological unit that is composed of a series of ‘granite’ plutons (intrusions of molten magma). These occur in a hourglass shape between Drake and Tabulam. The biggest areas occurring north and south of the Bruxner Highway and the central thin part of the ‘hourglass’ occurring where the Bruxner Highway crosses it.
The Bruxner Monzogranite is part of the Clarence River Super Suite of granites which is an I-type granite (Bryant et al 1997). I-type granites are derived from melted igneous rock. It contains two different varieties of ‘granite’ (Thomson 1976). One variety is the rock type monzogranite which contains roughly equal amounts of the two main feldspar groups (plagioclase feldspar and alkali feldspar). It also includes quartz, amphibole and biotite mica.
The slightly less common variety is the granodiorite which contains more alkali feldspar than plagioclase. Therefore, it is richer in the elements sodium and potassium . It is worth noting that the granodiorite is often more altered and is more quartz rich. The easiest way to distinguish between the two Bruxner Monzogranite varieties in the field is their colour: The granodiorite usually has a pink colour and the monzogranite grey. The relationship between the two varieties of granite is not very clear to me. The following questions immediately spring into my mind:
Bryant et al (1997) gives the potassium-argon age of the Bruxner Monzogranite as 250Ma. This places it in the Triassic Era, the same age as the other nearby Clarence River Supersuite. Such as, the Jenny Lind Granite which occurs a few kilometres north of the Buxner Monzogranite. The Clarence River Supersuite ‘granites’ are a similar age to many other granites which occur throughout the New England. This was certainly a busy time for intrusions. Indeed, these granites probably represent the magma source for an eroded volcanic arc system. It was caused by a large west dipping subduction zone that was active during this time (Scheibner & Basden 1998).
The Bruxner Monzogranite was intruded into Emu Creek Formation which is Carboniferous to Permian aged (Bottomer 1986). It is comprised of mudstones, greywacke, siltstones, shale, sandstones, conglomerate and limestone. As mentioned in my earlier post on limestone in the area, metamorphism of these rocks has in places been quite pervasive with a distinct metamorphic aureole. This has created some interesting rocks and altered zones such as marble and iron rich skarn.
The Bruxner Monzogranite is overlain in some areas by sediments of the Clarence-Moreton Basin. In particular, the Woogaroo Subgroup of the Bundamba Group, mainly the Laytons Range Conglomerate. Weathered exposures of the Laytons Range Conglomerate can be seen in road cuttings on the Paddys Flat Road.
The Bruxner Monzogranite was once called the Bruxner Adamellite (the term adamellite is no longer recognised). It is named after the Bruxner Highway which passes right through the unit. Adjacent to the Bruxner Highway, approximately 2-3km west of Plumbago Creek, is one of the best places to see the outcrops of both the monzogranite and granodiorite. A good place to see the monzogranite is along the ridges along Sugarbag Road which is in the northern part of the unit, off Paddys Flat Road.
References/bibliography:
*Bottomer, L.R. (1986), Epithermal silver‐gold mineralization in the Drake area, northeastern New South Wales, Australian Journal of Earth Sciences. V33.
*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cirdilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. V38.
Scheibner, E. & Basden, H. 1998 Geology of New South Wales – Synthesis. Volume 2 – Geological Evolution. Geological Survey of New South Wales, Memoir Geology 13.
*Thomson, J. 1976 Geology of the Drake 1:100 000 sheet, 9340. Geological Survey of New South Wales 1v.
.JPG) |
| Typical Bruxner Monzogranite monzogranite |
The Bruxner Monzogranite is part of the Clarence River Super Suite of granites which is an I-type granite (Bryant et al 1997). I-type granites are derived from melted igneous rock. It contains two different varieties of ‘granite’ (Thomson 1976). One variety is the rock type monzogranite which contains roughly equal amounts of the two main feldspar groups (plagioclase feldspar and alkali feldspar). It also includes quartz, amphibole and biotite mica.
The slightly less common variety is the granodiorite which contains more alkali feldspar than plagioclase. Therefore, it is richer in the elements sodium and potassium . It is worth noting that the granodiorite is often more altered and is more quartz rich. The easiest way to distinguish between the two Bruxner Monzogranite varieties in the field is their colour: The granodiorite usually has a pink colour and the monzogranite grey. The relationship between the two varieties of granite is not very clear to me. The following questions immediately spring into my mind:
- Does one granite intrude the other?
- Were they both molten when they were emplaced? Or was one crystallised first?
- Was it fluids from the crystallising monzogranite that caused the alteration of the granodiorite?
Bryant et al (1997) gives the potassium-argon age of the Bruxner Monzogranite as 250Ma. This places it in the Triassic Era, the same age as the other nearby Clarence River Supersuite. Such as, the Jenny Lind Granite which occurs a few kilometres north of the Buxner Monzogranite. The Clarence River Supersuite ‘granites’ are a similar age to many other granites which occur throughout the New England. This was certainly a busy time for intrusions. Indeed, these granites probably represent the magma source for an eroded volcanic arc system. It was caused by a large west dipping subduction zone that was active during this time (Scheibner & Basden 1998).
The Bruxner Monzogranite was intruded into Emu Creek Formation which is Carboniferous to Permian aged (Bottomer 1986). It is comprised of mudstones, greywacke, siltstones, shale, sandstones, conglomerate and limestone. As mentioned in my earlier post on limestone in the area, metamorphism of these rocks has in places been quite pervasive with a distinct metamorphic aureole. This has created some interesting rocks and altered zones such as marble and iron rich skarn.
The Bruxner Monzogranite is overlain in some areas by sediments of the Clarence-Moreton Basin. In particular, the Woogaroo Subgroup of the Bundamba Group, mainly the Laytons Range Conglomerate. Weathered exposures of the Laytons Range Conglomerate can be seen in road cuttings on the Paddys Flat Road.
The Bruxner Monzogranite was once called the Bruxner Adamellite (the term adamellite is no longer recognised). It is named after the Bruxner Highway which passes right through the unit. Adjacent to the Bruxner Highway, approximately 2-3km west of Plumbago Creek, is one of the best places to see the outcrops of both the monzogranite and granodiorite. A good place to see the monzogranite is along the ridges along Sugarbag Road which is in the northern part of the unit, off Paddys Flat Road.
References/bibliography:
*Bottomer, L.R. (1986), Epithermal silver‐gold mineralization in the Drake area, northeastern New South Wales, Australian Journal of Earth Sciences. V33.
*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cirdilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. V38.
Scheibner, E. & Basden, H. 1998 Geology of New South Wales – Synthesis. Volume 2 – Geological Evolution. Geological Survey of New South Wales, Memoir Geology 13.
*Thomson, J. 1976 Geology of the Drake 1:100 000 sheet, 9340. Geological Survey of New South Wales 1v.
Tuesday, 30 July 2013
Blog update #5
Wow! 50,000 page views earlier today. Admittedly, about 20% seem to be bots. I could never have thought that in less than two years, I should get so many visits to my blog. I certainly hope those visitors find what they are looking for or at least something of interest when reading my blog.
I've been very busy the last few weeks and will continue to be in the weeks coming. This means that I may be a little slow in getting new blog posts up. I normally aim to get one up a week but this time frame might double in the near future. There is no shortage of material to discuss, I'm not sure there ever will be, but simply having the time to discuss the material is my biggest limitation.
In the coming months, there is a few subjects that I'd like to post on. These are gold in the Orara River area, granites of the New England Batholith and touch on recent theories about the development of the Texas-Coffs Harbour Orocline/mega-fold.
In the mean time, thanks to all my readers, regular or just those that have stumbled by. I appreciate your comments and questions and I try to get decent answers to all of them, so please keep commenting. If you have a detailed question, a picture or information I might be interested in, please email me. My email address can be found by clicking on the "About This Blog" tab at the top of the page and scrolling down.
Thank you too to my wife Beck who has edited and proof read many of my latest posts. The clarity of language and ease of reading has certainly increased dramatically since I have received her help.
I've been very busy the last few weeks and will continue to be in the weeks coming. This means that I may be a little slow in getting new blog posts up. I normally aim to get one up a week but this time frame might double in the near future. There is no shortage of material to discuss, I'm not sure there ever will be, but simply having the time to discuss the material is my biggest limitation.
In the coming months, there is a few subjects that I'd like to post on. These are gold in the Orara River area, granites of the New England Batholith and touch on recent theories about the development of the Texas-Coffs Harbour Orocline/mega-fold.
In the mean time, thanks to all my readers, regular or just those that have stumbled by. I appreciate your comments and questions and I try to get decent answers to all of them, so please keep commenting. If you have a detailed question, a picture or information I might be interested in, please email me. My email address can be found by clicking on the "About This Blog" tab at the top of the page and scrolling down.
Thank you too to my wife Beck who has edited and proof read many of my latest posts. The clarity of language and ease of reading has certainly increased dramatically since I have received her help.
Saturday, 20 July 2013
It's a Demon of a Fault
Many people have requested that I do a post on the Demon Fault. I've struggled to put something together because structural geology is not one of my strong points and secondly because there was so little published information about it, except for some specific papers in the 1970’s. Thankfully, a few months ago Babaahmdi & Rosenbaum (2013) published a detailed paper summarising what was known in the 1970s, presenting how the fault appears, how it seems to have developed and how it may fit into the development of eastern Australia. It is worth noting that Gideon Rosenbaum from The University of Queensland has been the major researcher on New England structural geology for the last 5 years. If it was not for him and his student’s research we would be struggling to understand some of the basic features of the older rocks of the region including the Demon Fault.
Large faults are usually have quite distinctive landscape features. The Demon Fault is mainly a transverse type fault (movement on the fault horizontally rather than vertically) which displays very obvious topographical features. Transverse faults often form valleys, where the rock of the fault has been broken down into what is called gouge or rock-flour. Gouge is very weak material. It is easily eroded and rivers often preferentially follow the route of the fault carving out the gouge into deep valleys. The presence of deformational features in the surrounding rock can give an indication of how deep the fault was when it was active.
The Demon Fault is a prominent feature because it is evident from a series of valleys from near the Queensland Border to Dorrigo. At the Queensland end it is partly obscured by the Cenozoic Main Range Volcanics and in the Dorrigo area the end is obscured by the Cenozoic aged Ebor Volcanics. Geological maps of the area show a nice linear feature with obvious truncation of pre-existing geological units. Aerial photos also show the fault up nicely with streams preferentially flowing along the trace of the fault and contrasting with the rugged forested mountains surrounding it. I’ve never taken a photo of any part of the Demon Fault but a nice photo taken from an aeroplane can be found here: http://www.panoramio.com/photo/35890361
Korsch et al (1978) observed that the Demon Fault had displaced several geological units including intrusions of the Bungulla Monzogranite (now known as the Rocky River Monzogranite), Dumbudgery Granodiorite and Newton Boyd Granodiorite as well as the Drake Volcanics. The fault was interpreted as a dextral strike-slip fault (a fault where the eastern side had moved south relative to the western side). Korsch et al (1978) calculated that the fault had displaced these units 17km which is substantial in Eastern Australia. Dating of the displaced granite intrusions provides a possible maximum date of within Triassic period (249-232 million years). The nature of deformation features adjacent to the faulting indicates that the fault was shallow and/or was created in a brittle environment. Badaahmadi & Rosenbaum (2013) speculate that the timing of the faulting may actually be similar to that of faulting and extension in the earth’s crust that formed the Ipswich and Clarence-Moreton Basins (more about this in future posts).
There are many factors in understanding the Demon Fault. It is interesting to note that other authors have come up with different lengths of displacement including 30km in the northern part of the fault and 23Km in the central part. Badaahmadi & Rosenbaum (2013) have calculated that the northern part of the fault displaced 35km, in the centre by 25km and south by 19km. Some components of reverse faulting (where one side of the fault slid down and away from the other side) were observed. Additionally, it was noted that the Demon Fault did not appear to follow one big long line but instead had numerous splays (deviations, splitting, etc) especially in the south. Badaahmadi & Rosenbaum (2013) suspect that there may be two causes to the different lengths:
References/bibliography:
*Babaahmadi, A. & Rosenbaum, G. 2013. Kinematics of the Demon Fault: Implications for Mesozoic strike-slip faulting in eastern Australia. Australian Journal of Earth Sciences. V.60
*Korsch, R.J., Archer, R. & McConachy, G.W. 1978. The Demon Fault. Journal and Proceedings, Royal Society of New South Wales. V111.
Large faults are usually have quite distinctive landscape features. The Demon Fault is mainly a transverse type fault (movement on the fault horizontally rather than vertically) which displays very obvious topographical features. Transverse faults often form valleys, where the rock of the fault has been broken down into what is called gouge or rock-flour. Gouge is very weak material. It is easily eroded and rivers often preferentially follow the route of the fault carving out the gouge into deep valleys. The presence of deformational features in the surrounding rock can give an indication of how deep the fault was when it was active.
The Demon Fault is a prominent feature because it is evident from a series of valleys from near the Queensland Border to Dorrigo. At the Queensland end it is partly obscured by the Cenozoic Main Range Volcanics and in the Dorrigo area the end is obscured by the Cenozoic aged Ebor Volcanics. Geological maps of the area show a nice linear feature with obvious truncation of pre-existing geological units. Aerial photos also show the fault up nicely with streams preferentially flowing along the trace of the fault and contrasting with the rugged forested mountains surrounding it. I’ve never taken a photo of any part of the Demon Fault but a nice photo taken from an aeroplane can be found here: http://www.panoramio.com/photo/35890361
 |
| The Timbarra River has followed the Demon Fault creating linear valley http://www.panoramio.com/photo/35890361 (used with permission) |
Korsch et al (1978) observed that the Demon Fault had displaced several geological units including intrusions of the Bungulla Monzogranite (now known as the Rocky River Monzogranite), Dumbudgery Granodiorite and Newton Boyd Granodiorite as well as the Drake Volcanics. The fault was interpreted as a dextral strike-slip fault (a fault where the eastern side had moved south relative to the western side). Korsch et al (1978) calculated that the fault had displaced these units 17km which is substantial in Eastern Australia. Dating of the displaced granite intrusions provides a possible maximum date of within Triassic period (249-232 million years). The nature of deformation features adjacent to the faulting indicates that the fault was shallow and/or was created in a brittle environment. Badaahmadi & Rosenbaum (2013) speculate that the timing of the faulting may actually be similar to that of faulting and extension in the earth’s crust that formed the Ipswich and Clarence-Moreton Basins (more about this in future posts).
There are many factors in understanding the Demon Fault. It is interesting to note that other authors have come up with different lengths of displacement including 30km in the northern part of the fault and 23Km in the central part. Badaahmadi & Rosenbaum (2013) have calculated that the northern part of the fault displaced 35km, in the centre by 25km and south by 19km. Some components of reverse faulting (where one side of the fault slid down and away from the other side) were observed. Additionally, it was noted that the Demon Fault did not appear to follow one big long line but instead had numerous splays (deviations, splitting, etc) especially in the south. Badaahmadi & Rosenbaum (2013) suspect that there may be two causes to the different lengths:
- Splays may have created movement of the fault which had a vertical component as well as horizontal.
- There may have been some fault reactivation of the northern part of the fault as recently as the Cenozoic era.
References/bibliography:
*Babaahmadi, A. & Rosenbaum, G. 2013. Kinematics of the Demon Fault: Implications for Mesozoic strike-slip faulting in eastern Australia. Australian Journal of Earth Sciences. V.60
*Korsch, R.J., Archer, R. & McConachy, G.W. 1978. The Demon Fault. Journal and Proceedings, Royal Society of New South Wales. V111.
Sunday, 14 July 2013
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.
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.
Tuesday, 2 July 2013
Pacific Islands on holiday to the North Coast
I often find some stories in newspapers touch too lightly on the subject of geology. These articles are often quite limited in scope and generally indicate quite simplistic notions of natural processes. This morning when reading a local newspaper The Tweed Daily News, I came across one such article. A link can to the article can be found here. This article is interesting because it covers some surprising points, but as Dr Malcom Clark an Environmental Geochemist from Southern Cross University implies in the article, there is more to the story than just a once-off beaching of pumice on Kingscliff Beach.
Pumice is a highly vesicular (aerated) volcanic glass. It is created when super-hot, highly pressurized rock is violently ejected from a volcano, especially those found in volcanic island arcs which are near active subduction zones. The unusual foamy feel of pumice occurs because of simultaneous rapid cooling and rapid depressurization. During the eruption the air bubbles are frozen in the rock. The amount of air trapped means that pumice usually has the unusual property of a rock being able to float on water.
If we work backward in time from the Tweed Daily News article a story starts to emerge on how the pumice on the beach got there. The first thing to note is that there are no active volcanoes on the Australian mainland or close to the eastern Australian coast. So, the pumice must have been brought in from somewhere else. Pumice has been common on Byron Bay beaches for the last few weeks ever since winter storms gave a good battering the coast in June. But a large amount of Pumice was also observed on the Queensland sunshine coast in April following late summer storms and the tail ends of cyclones. The storms force floating materials like rubbish and pumice onshore. This gives a clue about movement. It has taken a month or two to travel down the east coast on prevailing currents such as the south moving Eastern Australian Current. But there are no active volcanoes in Queenland either.
Bryan et al (2004) published an interesting article in Earth and Planetary Science Letters on pumice that was washed ashore all down the east coast of Australia in 2002. Here lies more of the answer. Bryan et al (2004) demonstrated that the pumice rafts were transported a vast distance across the Coral Sea and South Pacific Ocean, taking about almost a year to complete its trip on the prevailing currents and winds (the pumice was even blown backwards at one stage by a tropical cyclone). Surprisingly the 2002 Pumice landfall came from the Tonga area (North of an island and seamount chain that stretches to New Zealand called the Kermadec Islands), which is a long way away! Between the Kermadec Islands and Australia lies the Solomon Islands, Vanuatu and Fiji which all have active volcanic systems. However, The pumice that washed ashore in 2002 was erupted in a submarine volcano (underwater) un-excitingly named Volcano 0403-091 from the Kermadec Islands and swept past all the other islands.
As for the current pumice landfall, in the last year there has been several eruptions of island arc volcanoes the Vanuatu islands, but significantly in July 2012 there was a major eruption of pumice from the vicinity of the Havre Seamount in the Karmadec Islands (Smithsonian Institute 2012). The time between eruption and East Australian landfall is interesting because it is similar to that for the 2001-2002 event discussed by Bryan et al (2004). More recently in 2012 an article was published (Bryan et al 2012) that demonstrated that rapid and long distance movement can be a frequent occurrence. So, maybe the pumice on our beach today this is just a little bit of history repeating – a bit of a pacific volcano on a holiday to the north coast of New South Wales.
Postscript:
Scott Bryan sent me this email yesterday. Being so informative I thought I should post it here.
References/bibliography:
*Bryan, Scott Edward, S., Cook, Alex, Evans, Jason, Hebden, Kerry, Hurrey, Lucy, Colls, Peter, Jell, John S., Weatherley, Dion, & Firn, Jennifer (2012) Rapid, long-distance dispersal by pumice rafting. PLoS ONE, V7.
*Byran, S.E., Cook, A., Evans, J.P., Colls, P.W., Wells, M.G., Lawrence, M.G., Jell, J.S., Greig, A. & Leslie, R. 2004. Pumice Rafting and faunal dispersion during 2001-2002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga. Earth and Planetary Science Letters V227.
*Smithsonian Institute 2012. Havre Seamount. Bulletin of the Global Volcanism Network. Smithsonian Institute. September 2012.
Pumice is a highly vesicular (aerated) volcanic glass. It is created when super-hot, highly pressurized rock is violently ejected from a volcano, especially those found in volcanic island arcs which are near active subduction zones. The unusual foamy feel of pumice occurs because of simultaneous rapid cooling and rapid depressurization. During the eruption the air bubbles are frozen in the rock. The amount of air trapped means that pumice usually has the unusual property of a rock being able to float on water.
If we work backward in time from the Tweed Daily News article a story starts to emerge on how the pumice on the beach got there. The first thing to note is that there are no active volcanoes on the Australian mainland or close to the eastern Australian coast. So, the pumice must have been brought in from somewhere else. Pumice has been common on Byron Bay beaches for the last few weeks ever since winter storms gave a good battering the coast in June. But a large amount of Pumice was also observed on the Queensland sunshine coast in April following late summer storms and the tail ends of cyclones. The storms force floating materials like rubbish and pumice onshore. This gives a clue about movement. It has taken a month or two to travel down the east coast on prevailing currents such as the south moving Eastern Australian Current. But there are no active volcanoes in Queenland either.
Bryan et al (2004) published an interesting article in Earth and Planetary Science Letters on pumice that was washed ashore all down the east coast of Australia in 2002. Here lies more of the answer. Bryan et al (2004) demonstrated that the pumice rafts were transported a vast distance across the Coral Sea and South Pacific Ocean, taking about almost a year to complete its trip on the prevailing currents and winds (the pumice was even blown backwards at one stage by a tropical cyclone). Surprisingly the 2002 Pumice landfall came from the Tonga area (North of an island and seamount chain that stretches to New Zealand called the Kermadec Islands), which is a long way away! Between the Kermadec Islands and Australia lies the Solomon Islands, Vanuatu and Fiji which all have active volcanic systems. However, The pumice that washed ashore in 2002 was erupted in a submarine volcano (underwater) un-excitingly named Volcano 0403-091 from the Kermadec Islands and swept past all the other islands.
As for the current pumice landfall, in the last year there has been several eruptions of island arc volcanoes the Vanuatu islands, but significantly in July 2012 there was a major eruption of pumice from the vicinity of the Havre Seamount in the Karmadec Islands (Smithsonian Institute 2012). The time between eruption and East Australian landfall is interesting because it is similar to that for the 2001-2002 event discussed by Bryan et al (2004). More recently in 2012 an article was published (Bryan et al 2012) that demonstrated that rapid and long distance movement can be a frequent occurrence. So, maybe the pumice on our beach today this is just a little bit of history repeating – a bit of a pacific volcano on a holiday to the north coast of New South Wales.
Postscript:
Scott Bryan sent me this email yesterday. Being so informative I thought I should post it here.
Hi Rodney,
...I was actually at point lookout (nth Stradbroke) today collecting the pumice. The pumice is indeed from the Havre submarine eruption in the Kermadecs last year. There is a good summary of the eruption and discovery of the pumice rafts at the global volcanism program of the smithsonian institution (USA) at www.volcano.si.edu.
This pumice is distinctive in being white when fresh; there is also a lot of grey/dark grey pumice at north stradbroke which is from tonga and the previous eruptions I have published on. It has been eroded out of the beach dunes.
The main influx along our shores began in mid-late march, continuing up to early May. There has been a bit of a break, but with the windy and wild weather this last weekend, some more pumice has come in, as well as probably reworked material (abraded and cleaned of attached biota) which seems to be what has washed up at Kingscliff. Newly washed up pumice will be covered in a black or dark green slime (Cyanobacteria) and be loaded with lots and large goose barnacles. You will also find on closer inspection, some molluscs, bristle worms (feeding on the barnacles), bryozoans, hydroids, anemones. Look up Denis Riek and his web page www.roboastra.com - he has taken some fantastic close ups of the pumice and biota found on it at Brunswick Heads.
This pumice has travelled about 3000 km in 8-12 months. We have observed it as far north as Heron Island.
Let me know if you need more info.
I would appreciate further reports of any new strandings as I have a Masters student beginning her research on this pumice and the attached biota. New strandings give us a temporal perspective as the biota mature and diversify with time and also begin recruiting species locally.
References/bibliography:
*Bryan, Scott Edward, S., Cook, Alex, Evans, Jason, Hebden, Kerry, Hurrey, Lucy, Colls, Peter, Jell, John S., Weatherley, Dion, & Firn, Jennifer (2012) Rapid, long-distance dispersal by pumice rafting. PLoS ONE, V7.
*Byran, S.E., Cook, A., Evans, J.P., Colls, P.W., Wells, M.G., Lawrence, M.G., Jell, J.S., Greig, A. & Leslie, R. 2004. Pumice Rafting and faunal dispersion during 2001-2002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga. Earth and Planetary Science Letters V227.
*Smithsonian Institute 2012. Havre Seamount. Bulletin of the Global Volcanism Network. Smithsonian Institute. September 2012.
Monday, 24 June 2013
Tharz gold in them hills!
Jim Belshaw, blogger and New England self government advocate has several interesting blogs. I thought I'd take the opportunity to share his latest New England History blog post on gold in the Timbarra/Rocky River area.
Jim has a very interesting writing style and I enjoy his blogs. He also seems to capture many parts of the New England landscape and history that go poorly documented. I understand this fascinating article was also published in the Armidale Express Extra which is not available online.
Jim has a very interesting writing style and I enjoy his blogs. He also seems to capture many parts of the New England landscape and history that go poorly documented. I understand this fascinating article was also published in the Armidale Express Extra which is not available online.
Sunday, 16 June 2013
Doctor John Lindsay
Recently I had a discussion with Gordon Smith who has a wonderful photoblog on the New England. Gordon was curious about some conglomerate that he and a well-known bushwalking writer, Bob Harden had come across in the Oxley Wild Rivers and Carrai National Parks. During the discussion, it became apparent that most of the understanding of the geology of the area was established in the early 1960's by a student named John Lindsay. Later, Bob informed me that John Lindsay was well-known for his later work for NASA.
 |
| Photo of John Lindsay obtained from the Luna and Planetary Institute |
On occasions I've posted short blogs on individual geologists and given John Lindsay's background I thought it worth doing the same for him.
John was born in the middle of World War 2 in January 1941. I don't know where he was born or what schools he attended but his tertiary education was in sedimenary geology with a solid background in chemistry, physics, mathematics, and statistics, earning his Bachelor of Science Degree (with Honours) in 1962 and Master of Science degree in 1964 from the University of New England.
During his time at UNE, John completed mapping and research projects which identified anomalous terrestrial rocks in a terrain that was mainly of marine origin. He worked in an area that was very difficult to understand because it was so broken up into different blocks by numerous faults. This is the area that is the gorge country between Kempsey and Walcha which includes the Oxley Wild Rivers and Carrai National Parks. To the best of my knowledge this day the geology of the area has not advanced much since John's work. This area was the subject of my discussion with Gordon Smith and Bob Harden.
Following his UNE study, John moved to the United States where he studied for his PhD at Ohio State University. Following his PhD John obtained a position with NASA in the Apollo Program where he was involved in Luna mission planning and the training of astronauts. John’s other professional background also included positions as Research Scientist at the Marine Science Institute of the University of Texas, Program Manager at Exxon Production Research, Adjunct Professor at Oxford University, and NRC Senior Research Associate at the Astrobiology Institute at NASA Johnson Space Center. While he was at NASA John studied extreme environments on Earth as analogues for extra-terrestrial environments. This lead him to become an authority on aspects of Antarctic geology.
John also maintained contact with Australia and held an academic position at La Trobe University in Victoria in the 1970's and joined the Bureau of Mineral Resources in Canberra in 1984. He spent much of his time developing a deep understanding of the inland Australian sedimentary basins and the ancient sedimentary rocks of the Pilbara. His works on the Pilbara rocks in particular helped us to learn about the very earliest life on earth.
John contributed not just to our present understanding of geology but also to life on earth and even the search for life in the universe. From what I have read he also was held in high regard with those that worked with him.
John contributed not just to our present understanding of geology but also to life on earth and even the search for life in the universe. From what I have read he also was held in high regard with those that worked with him.
John died in the United States from cancer in June 2008.
References/bibliography:
Much of the information I have about John Lindsay was obtained from his Luna and Planetary Institue Obituary and the dedication of the book Earliest Life on Earth: Habitats, Environments and Methods of Detection by Golding& Glickson (2011). However, I've used a few internet sources that I seem to have misplaced.
*Golding, S., Glickson, M. 2011. Earliest Life on Earth: Habitats, Environments and Methods of Detection. Springer.
A list of John's published papers, conference proceedings etc. can be found at this Luna and Planetary Institute webpage:
http://www.lpi.usra.edu/lpi/lindsay/papers/lp_papers/
A list of his most recent papers can be found here:
http://www.lpi.usra.edu/lpi/lindsay/papers/
A list of John's published papers, conference proceedings etc. can be found at this Luna and Planetary Institute webpage:
http://www.lpi.usra.edu/lpi/lindsay/papers/lp_papers/
A list of his most recent papers can be found here:
http://www.lpi.usra.edu/lpi/lindsay/papers/
Monday, 10 June 2013
How wonderfully marbleous!
There are some rock types that are very common around the country and around the world that just don’t seem to rate much of a mention in the Northern Rivers. One very common rock is limestone formed from corals in a shallow sea, just like the Great Barrier Reef. Limestone is made almost entirely of the mineral calcite. Some parts of the world have vast terrains dominated by limestone called karst landscapes and it is quite distinctive. Limestone terrains sometimes form amazing subterranean cave systems as the stone is dissolved by rainwater infiltration into the formation. These karst terrains include north-west Mexico and other parts of North America, a giant band through northern England and a wide area of South Australia along the Great Australian Bight. However, it is a landscape absent from the Northern Rivers.
Having said that vast areas of limestone don’t exist in the region it is worth noting that they do exist in small areas here and there within the older rocks of the New England Orogen. The reason for this is interesting. The New England Orogeny was a period of mountain building during periods of plate collision which included a period of subduction of an oceanic plate under the Australian continental landmass during the Silurian period. The material on the surface of the oceanic plate was often accreted, that is scraped off and squashed onto the Australian continent. Seamounts are old islands in the middle of the sea. Such as, those around modern day Hawaii or Fiji. The seamounts were accreted onto the continental mass where they created little pockets of limestone in midst of the jumbled, squashed mass of deep seafloor sediments.
This means that if you find limestone in the New England area you are actually finding the preserved remnants of a little tropical island reef or lagoon. An especially nice thought, when you find some limestone on a cold frosty New England winter morning. One relatively accessible place to see some limestone is an old quarry on the Pretty Gully Road just north-west of the town of Tabulam which sits on the Bruxner Highway crossing of the Clarence River. The stratigraphic unit that the limestone of the area is part is the Emu Creek Formation which also includes areas of interesting fossils (more about that in yet another post). However, the quarry is interesting for more reasons than just as an occurrence of limestone.
Following the period of subduction and accretion a period occurred where intrusions of molten magma pushed their way into the accretionary sedimentary rocks. It occurred a couple of times including during the Late Permian to Early Triassic and created one part of what is referred to as the New England Batholith. The batholith is an array of granitic rocks that stretches through the whole New England Tablelands. The intrusions of the Late
Permian to Early Triassic included the emplacement of the Bruxner Monzogranite, a type of granite pluton (more about this specific rock in a future post). This pluton heated up and metamorphosed the rocks around it and one of which was that body of limestone near Tabulam. Contact metamorphism of limestone creates the rock called marble and this has happened at Tabulam. Although, the quality of marble is questionable because of the amount of impurities.
Other things happened to the limestone during metamorphism too. The transfer of fluids into and out of the cooling magma created chemical reactions which concentrated elements such as iron. This process develops what is called a skarn, a body of altered limestone with sometimes economic amounts of minerals. The minerals in a skarn can be diverse and very, very valuable but the minerals are based on the chemistry of the granite pluton. In the case of the chemistry of the Bruxner Monzogranite, there was not much of value except lots of iron which formed abundant amounts of the minerals magnetite and haematite. This has been considered for mining in the past but the small size and low grade means it is not a viable iron mine.
There are other small limestone deposits all around the New England and all of them are interesting for one reason or another. Some north of Inverell have lovely caves, others near Tamworth are mined for lime on a large scale. While others, just have interesting little features that illustrate what happened during the formation of our region.
References/bibliography:
*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. v38.
*Lishmund, S.R., Dawood, A.D. & Langley, W.V. 1986. The Limestone Deposits of New South Wales. 2nd Ed. Geological Survey of New South Wales
 |
| Outcrop of limestone north west of Tabulam |
This means that if you find limestone in the New England area you are actually finding the preserved remnants of a little tropical island reef or lagoon. An especially nice thought, when you find some limestone on a cold frosty New England winter morning. One relatively accessible place to see some limestone is an old quarry on the Pretty Gully Road just north-west of the town of Tabulam which sits on the Bruxner Highway crossing of the Clarence River. The stratigraphic unit that the limestone of the area is part is the Emu Creek Formation which also includes areas of interesting fossils (more about that in yet another post). However, the quarry is interesting for more reasons than just as an occurrence of limestone.
Following the period of subduction and accretion a period occurred where intrusions of molten magma pushed their way into the accretionary sedimentary rocks. It occurred a couple of times including during the Late Permian to Early Triassic and created one part of what is referred to as the New England Batholith. The batholith is an array of granitic rocks that stretches through the whole New England Tablelands. The intrusions of the Late
 |
| fresh face of limestone - note the sparkles from the calcite crystals |
Other things happened to the limestone during metamorphism too. The transfer of fluids into and out of the cooling magma created chemical reactions which concentrated elements such as iron. This process develops what is called a skarn, a body of altered limestone with sometimes economic amounts of minerals. The minerals in a skarn can be diverse and very, very valuable but the minerals are based on the chemistry of the granite pluton. In the case of the chemistry of the Bruxner Monzogranite, there was not much of value except lots of iron which formed abundant amounts of the minerals magnetite and haematite. This has been considered for mining in the past but the small size and low grade means it is not a viable iron mine.
There are other small limestone deposits all around the New England and all of them are interesting for one reason or another. Some north of Inverell have lovely caves, others near Tamworth are mined for lime on a large scale. While others, just have interesting little features that illustrate what happened during the formation of our region.
References/bibliography:
*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. v38.
*Lishmund, S.R., Dawood, A.D. & Langley, W.V. 1986. The Limestone Deposits of New South Wales. 2nd Ed. Geological Survey of New South Wales
Saturday, 1 June 2013
The Koukandowie Formation has a cool Name
Previously I’ve completed blogs on the stratigraphy of the upper units of the Clarence-Moreton Basin. These upper units have been the Grafton Formation (youngest at Late Jurassic), Woodenbong Beds, Kangaroo Creek Sandstone and Walloon Coal Measures including the Maclean Sandstone Member of that unit (Middle Jurassic). Now, as we get towards the middle units of the basin we get into the Early Jurassic with much more complexity to the mode of formation of the geological units. Because of this the stratigraphic units have been divided into groups, subgroups, formations and members. The First one that I will tackle is the Koukandowie Formation, which is part of the Marburg Subgroup which in turn is part of the Bundamba Group.
The Koukandowie Formation actually is made up of an additional three members known as the Heifer Creek Sandstone Member, Ma Ma Creek Member and Towallum Basalt. But I’ll focus on these individually in future posts, for the time being it is worth noting that the Towallum Basalt is a very important unit for understanding the relative age relationships of all of the units in the Bundamba Group. For the time being I’ll focus on the Koukandowie Formation specifically.
Unfortunately I do not have a photo specifically of the Koukandowie Formation. But I have attached a photograph a similar type of rock as the Koukandowie of the an undifferentiated part of the Bundamba Group.
The Koukandowie Formation was deposited in a dominantly fluvial (riverine) environment. Essentially the unit is comprised of sets of channel lithic sandstone (sandstone made from fragments of older rock) with some finer grained rock such as siltstone and even shale. But the formation also thin layers of conglomerate or occasional woody fragments that have turned into coal. The way the lithic sandstone was deposited means that a feature known as cross-bedding is very common and this structure is further evidence of its fluvial origin. The exact nature of these particular cross-bedding structures it interpreted by Wells & O'Brien 1994 as meaning that the river system that created the Koukandowie Formation was not a meandering stream but fairly straight. A modern example might be the middle reaches of the modern day Clarence River.
The Koukandowie Formation was considered an important formation for gas and oil exploration in the region. The Koukandowie was thought to be a generally an impermeable unit, that is, stops the migration of fluids and gases such as oil and natural gas. This means that the underlying units of the Bundamba Group which are more conducive to forming and storing these gases and fluids may retain them in structural traps (such as folds in the earth or faults). How effective this unit has been seems to be a bit hit and miss. I understand that the NSW Government and some companies during the 1970’s and 1980’s had some success with this model but not enough to make it viable financially at the time. More recent work by exploration companies has shown this to be on its own not-viable. But when combined with similar modes of gas source rocks in the overlying Walloon Coal Measures, other more deeply buried organic rich units and other sources of gas directly from coal seams the economics seem to have looked good for some companies.
As for ground water sources, like the overlying Walloon Coal Measures the Koukandowie Formation does not contain much in the way of useful fresh groundwater. This is for two reasons:
These reasons also imply the nature of recharge of what aquifers to occur in the area. Essentially vertical percolation from surface water through fractures is the main driver of aquifer recharge. Though there are exceptions due to the location of the sub-units in the Formation.
As for the name the Koukandowie Formation takes its name from Mount Koukandowie which is located near Nymboida. The formation outcrops in a relatively thin band in from the margins of the Clarence-Moreton Basin. I don't think that the formation outcrops anywhere in the middle areas the Clarence-Moreton Basin but certainly occurs at within the basin depth. The formation tends to weather and erode easily and therefore most of the outcrop of the unit shows relatively subdued landforms of rolling hills.
References/bibliography:
*McMahon, G.A. & Cox, M.E. 1996. The relationship between groundwater chemical type and Jurassic sedimentary formations: The example of the Sandy Creek Catchment, Lockyer, southeast Queensland. Mesozoic 96 Conference at Brisbane - extended abstracts.
*O’Brien, P.E. & Wells, A.T. 1994. Sedimentology of the Bundamba Group. In Wells, A.T. & O’Brien, P.E. 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Bulletin 241. Australian Geological Survey Organisation
*Wells, A.T. & O’Brien, P.E. 1994. Lithostratigraphic Framework of the Clarence-Moreton Basin. In Wells, A.T. & O’Brien, P.E. 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Bulletin 241. Australian Geological Survey Organisation.
*Wells, A.T., O’Brien, P.E. Willis, I.L. & Cranfield, L.C. 1990. A new lithostratigraphic framework of the Early Jurassic units in the Bundamba Group, Clarence-Moreton Basin, Queensland and New South Wales. B.M.R. Journal of Australian Geology and Geophysics. V11.
The Koukandowie Formation actually is made up of an additional three members known as the Heifer Creek Sandstone Member, Ma Ma Creek Member and Towallum Basalt. But I’ll focus on these individually in future posts, for the time being it is worth noting that the Towallum Basalt is a very important unit for understanding the relative age relationships of all of the units in the Bundamba Group. For the time being I’ll focus on the Koukandowie Formation specifically.
 |
| Bundamba Group with conglomerate and abundant organic fragments - Tabulam |
The Koukandowie Formation was deposited in a dominantly fluvial (riverine) environment. Essentially the unit is comprised of sets of channel lithic sandstone (sandstone made from fragments of older rock) with some finer grained rock such as siltstone and even shale. But the formation also thin layers of conglomerate or occasional woody fragments that have turned into coal. The way the lithic sandstone was deposited means that a feature known as cross-bedding is very common and this structure is further evidence of its fluvial origin. The exact nature of these particular cross-bedding structures it interpreted by Wells & O'Brien 1994 as meaning that the river system that created the Koukandowie Formation was not a meandering stream but fairly straight. A modern example might be the middle reaches of the modern day Clarence River.
 |
| A rough stratigraphic guide to the Bundamba Group (Walloon Coal Measures are above and Gatton Sandstone under) |
As for ground water sources, like the overlying Walloon Coal Measures the Koukandowie Formation does not contain much in the way of useful fresh groundwater. This is for two reasons:
- the finer grained components of the formation tend to contain more salt due to the some of the sedimentary depositional environment; and
- the Koukandowie Formation tends to show very little lateral porosity. This means that the water is stored in smaller localised aquifers of low long term yield.
These reasons also imply the nature of recharge of what aquifers to occur in the area. Essentially vertical percolation from surface water through fractures is the main driver of aquifer recharge. Though there are exceptions due to the location of the sub-units in the Formation.
As for the name the Koukandowie Formation takes its name from Mount Koukandowie which is located near Nymboida. The formation outcrops in a relatively thin band in from the margins of the Clarence-Moreton Basin. I don't think that the formation outcrops anywhere in the middle areas the Clarence-Moreton Basin but certainly occurs at within the basin depth. The formation tends to weather and erode easily and therefore most of the outcrop of the unit shows relatively subdued landforms of rolling hills.
References/bibliography:
*McMahon, G.A. & Cox, M.E. 1996. The relationship between groundwater chemical type and Jurassic sedimentary formations: The example of the Sandy Creek Catchment, Lockyer, southeast Queensland. Mesozoic 96 Conference at Brisbane - extended abstracts.
*O’Brien, P.E. & Wells, A.T. 1994. Sedimentology of the Bundamba Group. In Wells, A.T. & O’Brien, P.E. 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Bulletin 241. Australian Geological Survey Organisation
*Wells, A.T. & O’Brien, P.E. 1994. Lithostratigraphic Framework of the Clarence-Moreton Basin. In Wells, A.T. & O’Brien, P.E. 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Bulletin 241. Australian Geological Survey Organisation.
*Wells, A.T., O’Brien, P.E. Willis, I.L. & Cranfield, L.C. 1990. A new lithostratigraphic framework of the Early Jurassic units in the Bundamba Group, Clarence-Moreton Basin, Queensland and New South Wales. B.M.R. Journal of Australian Geology and Geophysics. V11.
Sunday, 19 May 2013
Lismore GEMFEST 2013
Back from another trip to Hospital for my daughter meant that we were ready to go out and enjoy the late Autumn sunshine. It was good timing too because Lismore Gemfest had started for the weekend.
Gemfest is a lovely combination of professional and amateur sellers of gems, minerals, fossils and jewelry. I found Frank, the fellow from Sydney that I'd bought some good mineral specimens off last year so I added some more to my collection. Frank is an interesting seller because he sells what he finds and what he finds is in Australia, so you know you are getting local examples. Mind you my collection is really just a selection of boring looking minerals with very little value. For instance this time I got hold of some muscovite, tourmaline, wolframite and epidote - quite valueless but interesting to me. The value is in the fun of collecting.
Again, this year Gemfest was full of great diversity of sellers and some good food stalls. I enjoyed my steak sandwich for lunch. I managed to take a few snaps of the end of Saturday but I forgot to take pictures inside the professional dealers area. Oh, well. Something to do next year. In the mean time here are a few snaps:
Thursday, 9 May 2013
Ahh Ahh
Readers of this blog will probably notice I have an intense interest in volcanology. Volcanology has a wide variety of aspects some of which I’m comfortable, some less so. These aspects can be the chemistry of molten materials, the physics of earthquakes or the dynamic processes of pyroclastic flows. Volcanology and igneous rocks more generally seem to have their own weird language that can stop you and make you turn to a dictionary.
One of my favourite words in the ‘language’ of geology is the name of a large scale structure of lava flows. It is called aa. So, turning to a dictionary (this time the Omnificent English Dictionary in Limerick Form) you get the following possible definitions:
No consonants! Does this seem ominous?
It's with rough-surfaced lava synonymous.
Yet the thought it conveys
With two capital A's
Is, of course, Alcoholics Anonymous.
By Chris J. Strolin
I'm ascending a gentle volcano;
The climb's not the cause of my strain.
No, This lava is stressed,
Pretty jagged at best.
Cut my feet on sharp aa — the pain, Oh!
By Aliza
On Hawaii the lava's aflame
As observers, in awe, cry its name.
When that molten rock's oozing
Down paths of its choosing,
It's "A'a!" that tourists exclaim.
By David
Probably one of the more interesting dictionary definitions I’ve seen. I Hope that helps with understanding? If these are a little bit obscure you can always visit my Glossary.
Wednesday, 1 May 2013
Greeny Stuff at Port Macquarie
 |
| Serpentinite at Port Macquarie |
The picture to the right shows the nature of one of the rock types at Port Macquarie. If I recall correctly this photo was taken at the southern end of Flynns Beach. It is a characteristic rock and given the odd shapes preserved in it implies quite an interesting history. The rock is serpentinite in two forms. The first being the banded appearing one which is called serpentinite schist. The second is a block of serpentinite which has not had the schistose fabric developed in it. I’ve discussed serpentinite occurring elsewhere such as at Baryulgil in previous posts but as far as Serpentinite goes the Port Macquarie area has heaps of it.
Serpentinite is a rock mainly comprised of the mineral group Serpentine. This is a very silica poor rock formed by the regional metamorphism of Olivine rich rocks such as Dunite or Peridotite. These parent rocks are from deep below the oceanic crust in the deepest parts of a layered sequence called Ophiolite and because of this it is rarely preserved on land. The metamorphism of the serpentinite is actually at the same time as large blocks of the Dunite and Peridotite rich oceanic crust are thrust and rotated during tectonic plate collision. Because serpentinite tends to be ‘slippery’ it is mostly present around major regional fault systems where it is ‘squeezed’ into place. However, its relationship to other nearby tectonic blocks is detailed and requires a separate blog post on its own.
At Port Macquarie the parent rock appears to have been a calcium rich variety of Peridotite called Harzburgite. There are also other rocks mixed in with the Serpentite, so much so that the area is often referred to as a melange. These other rocks are sometimes (but not always) part of the Ophiolite. For example slightly shallower ones such as gabbro which has been metamorphosed to rocks called Blue Schists. Also occurring are non Ophiolite rocks such as marble and other types of schist. Because of the complexity some 'inclusions' in the melange are from a different source than the Ophiolite, that is a story for another post.
As for the age of the Serpentinite unit, direct dating is impossible due to metamorphism re-setting the dating clock of the rock. The best that can be achieved is the last date of metamorphism. Even then the ultramafic (silica poor) nature of the rock means that minerals that can be used for dating (such as zircons) are uncommon or simply absent. Therefore the age of the Port Macquarie Serpentinite is only estimated from the surrounding rocks. However recent work by Nutman et al (2013) has narrowed the age of metamorphism and probable emplacement of the serpentinite to 251-220Ma which is the late Permian to early Triassic. How they found the date is quite interesting with adopting multiple techniques physical, nuclear and chemical.
Bibliography/references:
*Aitchison, J.C. & Ireland, T.R. (1995). Age Profile of Ophiolitic Rocks across the Late Palaeozoic New England Orogen, New South Wales: Implications for tectonic models. Australian Journal of Earth Sciences. Vol.42.
*Nutman, A.P., Buckman, S., Hidaka, H., Kamiichi, T., Belousova, E., Aitchinson, J.C. 2013. Middle Carboniferous-Triassic eclogite-blueschist blocks within a serpentinite melange at Port Macquarie, eastern Australia: Implications for the evolution of Gondwana’s eastern margin. Gondwana Research.
*Och, D.J., Leitch, E.C. & Caprarelli, G. 2007. Geological Units of the Port Macquarie-Tacking Point tract, north-eastern Port Macquarie Block, Mid North Coast Region of New South Wales. Quarterly Notes of the Geological Survey of New South Wales. Vol.126.
Friday, 19 April 2013
100,000 Geotourism Maps
Geoz had an article this week titled New South Wales holidays all mapped out. This article refers to a new map which has been developed which should help those that would like to know more about the physical features of the areas they are touring. The Geoz article is reproduced below:
There are several areas which relate to our Northern Rivers:
The first state Geotourism Map in Australia has been released by Cartoscope Pty. Ltd. This NSW map features 96 sites and has an accompanying website so that users can get extensive geological detail in layman's terms and maps on each site. The map was supported by a TQUAL grant and sponsors helped lessen some of the costs. So far 15,000 of the 100,000 maps have been distributed mostly to visitor centres and many to secondary schools science departments. The map is receiving very favourable comments both from geoscientists and tourism information services. Accompanying website: http://bit.ly/XUsS9m
There are several areas which relate to our Northern Rivers:
I hope you find something for your area or something you’d like to look at while travelling through.
Saturday, 13 April 2013
An excellent outcome from atmospheric atomic bomb testing
Human ingenuity surprises me again and again, especially the efficiency in which we can annihilate each other. During the 1940’s and 1950’s the superpowers were focused on increasing the efficiency in the way they could destroy everyone on the planet. It was a very worthy goal (yes that was a joke) and to achieve maximum efficiency they needed to conduct atmospheric tests of their bombs. Sometimes, unforeseen obvious benefits other than the benefits of death and destruction of humanity can arise.
I have recently been thinking about groundwater in the Richmond River area for which I have been consulting sections of a PhD thesis written by Leonard Drury in 1982 (Drury 1982). Drury's comprehensive thesis included qualitative identification on the age of groundwater in aquifers in the Richmond River by using an unstable isotope of hydrogen called tritium. Hydrogen is an atomic component of water (the H in H2O) but hydrogen actually comes in three natural forms based on the number of neutrons are in the nucleus of the hydrogen atom. These different forms are called isotopes. Hydrogen naturally has one neutron or less commonly two neutrons (called deuterium) and very rarely three neutrons (called tritium). In nuclear explosions the third isotope tritium, is created at concentrations much higher than the background. The reason why tritium is rare naturally is that it is only formed in the upper atmosphere but is unstable and loses the extra neutrons to become a smaller isotope over a period of time.
Half of the tritium in a given amount of water (or whatever) decays over a period of 12.5 years (this is called a half-life). Which means that over 25 years there is only a quarter of the original tritium left, 37.5 years one eighth, 50 years one sixteenth etc. Since tritium is not naturally occurring there is no practical use to measure for tritium unless you can introduce it into a system as a tracer and then measure its behaviour. This means that a large ‘slug’ of tritium was created during the 1940’s and 1950’s during atmospheric nuclear testing. Therefore if you can look for tritium in groundwater and if it is not present you can assume that that groundwater has been in existence for more than 50 years, i.e. it was present in the ground before any nuclear tests. If you detect tritium in several locations in an aquifer the relative abundance of the tritium will give an indication of the age of the water and whether mixing is occurring between old groundwater and new groundwater. It won’t give you an exact date but it will let you know a lot about behaviour of an aquifer.
The trouble is time is running out. The half-life of tritium means that as time goes on the ability for us to accurately measure the smaller amount of the isotope means that one day we won’t be able to use this as a technique. I was aware that time was running out on using tritium as an effective groundwater tracer but I was not aware how soon. I have had a few chats with an academic at Southern Cross University one of which was about using tritium, he said we actually only have about 5 or 10 years left to which I jokingly suggested to him that we should reset the tritium clock with some more atmospheric nuclear explosions! To which he informed me that actually there appears to be some more tracers that can be used following the Fukushima Nuclear Accident.
Bibliography/References:
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales. University of New South Wales, PhD thesis.
*Moran, J.E. & Hudson, G.B. 2005. Using Groundwater Age and Other Isotopic Signatures to Delineate Groundwater Flow and Stratification. University of Illinois.
*U.S. Geological Survey (USGS), 2004, Stable Isotopes and Radiochemicals, in National Field Manual for the Collection of Water-Quality Data, Chapter A5 Processing of Water Samples. USGS Techniques of Water-Resources Investigation
I have recently been thinking about groundwater in the Richmond River area for which I have been consulting sections of a PhD thesis written by Leonard Drury in 1982 (Drury 1982). Drury's comprehensive thesis included qualitative identification on the age of groundwater in aquifers in the Richmond River by using an unstable isotope of hydrogen called tritium. Hydrogen is an atomic component of water (the H in H2O) but hydrogen actually comes in three natural forms based on the number of neutrons are in the nucleus of the hydrogen atom. These different forms are called isotopes. Hydrogen naturally has one neutron or less commonly two neutrons (called deuterium) and very rarely three neutrons (called tritium). In nuclear explosions the third isotope tritium, is created at concentrations much higher than the background. The reason why tritium is rare naturally is that it is only formed in the upper atmosphere but is unstable and loses the extra neutrons to become a smaller isotope over a period of time.
Half of the tritium in a given amount of water (or whatever) decays over a period of 12.5 years (this is called a half-life). Which means that over 25 years there is only a quarter of the original tritium left, 37.5 years one eighth, 50 years one sixteenth etc. Since tritium is not naturally occurring there is no practical use to measure for tritium unless you can introduce it into a system as a tracer and then measure its behaviour. This means that a large ‘slug’ of tritium was created during the 1940’s and 1950’s during atmospheric nuclear testing. Therefore if you can look for tritium in groundwater and if it is not present you can assume that that groundwater has been in existence for more than 50 years, i.e. it was present in the ground before any nuclear tests. If you detect tritium in several locations in an aquifer the relative abundance of the tritium will give an indication of the age of the water and whether mixing is occurring between old groundwater and new groundwater. It won’t give you an exact date but it will let you know a lot about behaviour of an aquifer.
The trouble is time is running out. The half-life of tritium means that as time goes on the ability for us to accurately measure the smaller amount of the isotope means that one day we won’t be able to use this as a technique. I was aware that time was running out on using tritium as an effective groundwater tracer but I was not aware how soon. I have had a few chats with an academic at Southern Cross University one of which was about using tritium, he said we actually only have about 5 or 10 years left to which I jokingly suggested to him that we should reset the tritium clock with some more atmospheric nuclear explosions! To which he informed me that actually there appears to be some more tracers that can be used following the Fukushima Nuclear Accident.
Bibliography/References:
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales. University of New South Wales, PhD thesis.
*Moran, J.E. & Hudson, G.B. 2005. Using Groundwater Age and Other Isotopic Signatures to Delineate Groundwater Flow and Stratification. University of Illinois.
*U.S. Geological Survey (USGS), 2004, Stable Isotopes and Radiochemicals, in National Field Manual for the Collection of Water-Quality Data, Chapter A5 Processing of Water Samples. USGS Techniques of Water-Resources Investigation
Saturday, 6 April 2013
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.
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:
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.
 |
| A Google Earth image of the area to the North of Wollomombi |
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:
- There was only isolated areas that were cold enough to maintain permafrost during the last glacial maximum; or
- The soils derived from granites and meta-sediments did not preserve evidence of permafrost
- 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, 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.
Subscribe to:
Posts (Atom)