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).
Tuesday, 8 October 2013
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.
Subscribe to:
Posts (Atom)