Why is coal rare at the beginning of the Triassic? and other questions.

I'm still around!

I came across this very good, but long video on the Permian-Triassic boundary (which is defined by a massive extinction event).

What jumped out to me was the lack of coal early in the Triassic. Interestingly, in the Northern Rivers the first major coal measures don't occur for many millions of years into the Middle Triassic. Those are the Nymboida Coal Measures.

But there is a lot interesting here so it is a recommended video.

The Great Dying with sound - YouTube

On the Rocks

Three of the four in my family has been hit by the Flu. The only one that thankfully doesn't have it is the newborn. I hope he doesn't get it. Now, I wish I had taken that free flu-shot that my work offered me this year! I've infected most of the family! Obviously this means that original blog posts are going to be a bit light on for a little while. So, in the mean time I can recommend some interesting bits and pieces on the internet.

On the Rocks with Wangiwriter - Catherine Hill Bay

Today Wangiwriter presented a very interesting blog post on some of the rocks of the Newcastle area, part of the Sydney Basin. The pictures are lovely and Wangiwriter tells an interesting story. I know this is a bit out of the usual subject area but the piece is worth a good look. I've been to Catherine Hill Bay and loved the scenery. The geology was typical of a section of the Permian aged Sydney Basin but well exposed and situated in a stunning historical landscape. The conglomerate shown so well in Wangiwriters photographs are part of the Newcastle Coal Measures. Specifically, they are either part of the Moon Island Beach Formation or the Boolarroo Formation. I don't know Newcastle geology well enough to know exactly which.

Thanks to everyone that has been supporting my blog. I've noticed an increase in my advertising revenue of late. Now I'm up to at least a cup of coffee a week! I'm amazed at the value of some advertising clicks. Some earn more than $1, others just a few cents. I got excited when I see that Google payed my almost $1.70 for one click. I wonder what that advertisement was! I'll never know because Google handles all the advertising in the background.

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.

Outcrop of limestone north west of Tabulam

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

fresh face of limestone - note the sparkles from the calcite crystals

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

Greeny Stuff at Port Macquarie

Serpentinite at Port Macquarie

A fascinating area of the northern rivers is right at Port Macquarie. This is a post that I’ve wanted to start for a long time but because the area is quite complex I’ve baulked at the prospect. I’ve just wanted to discuss too much, to dive into the deep end. I now realise that I should just start with an introduction to the formation of the fascinating rocks and come back for many more posts about more specific details in the near future.

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.

In the hills of Valla and Nambucca Heads

The Valla Adamellite now termed the Valla Monzogranite to reflect modern naming conventions is an interesting small to medium sized intrusion about 10km north east of Nambucca Heads. It is one of the suites of coastal granites which are mostly I-Types (melted igneous material), this means that the coastal granites show abundances of ore minerals within the granite or in the surrounding metamorphosed country rocks. A monzogranite is a granite with roughly equal proportions of (alkali-feldspar (potassium and sodium rich) and plagioclase feldspar (calcium rich)). The monzogranite is thought to have formed during the Triassic period.

The metamorphic aureole for the Valla Monzogranite is actually quite interesting as it shows a classic zonation of metamorphism (high grade at the contact grading to low grade further away) and also excellent examples of mineral zonation associated with metasomatism (hot-water or fluid alteration of rock). The Valla Monzogranite has been shown to be associated with gold, silver, arsenic and molybdenum mineralisation (as well as others). The rock that the monzogranite has been intruded into is called the Nambucca Beds which are part of the Nambucca Block. The Nambucca Beds are Permian to Carboniferous in age and are mainly comprised of the regionally metamorphic rock type called phyllite which was originally deposited on the sea floor. The Nambucca Block was accreted onto the Australian continent in the New England Orogen and this caused the regional metamorphism of the beds.

The Nambucca Beds are intruded by the Monzogranite. The Beds are extensive and
extend far into the rugged Nambucca Hinterland. This photo is west of Bowraville.

The Valla Monzogranite seems to be a Climax Molybdenum Deposit named after the Climax Mine in North America. This means that when the Monzogranite was cooling the upper portion of the pluton became residually enriched with fluids, metals and silica. These fluids cause alteration of the upper portion of the pluton forming what is called greisen and also are injected into the surrounding rock through veins and sometimes aggressively through breccia pipes. One of the first minerals to form in these veins is silica, quartz with metal sulphide such as molybdenite (molybdenum ore) and wolframite (tungsten ore). Further away from the intrusion the degree of alteration becomes less grading through potassic through to argillic which are defined alteration zones based on the changes in the rock forming minerals. As the degree of alteration becomes less so the types of metal ores change with increasing amounts or arsenic, gold and silver. Further out in the alteration zone minerals such as galena form (lead ore) and finally stibnite (antimony ore). These ore deposits seem to be fairly common in the New England area with Glen Eden being the most studied (Somarin 2001, Somarin & Ashley 2004) and have in some areas been extensively explored such as Kingsgate east of Glen Innes.

Some attempts of mining have occurred in the Valla Monzogranite in the past, the most significant being the Valla Gold mine which was located just to the north of Valla Beach. The mine was abandoned with very little rehabilitation and therefore has become an environmental problem for the local creek. However, rehabilitation efforts have recently been undertaken, though these will need another post to discuss in more detail.

References/bibliography:

*Somarin, A.K. 2011. Petrography, Geochemistry, and Petrogenesis of Late-Stage Granites: An Example from the Glen Eden Area, New South Wales, Australia. Earth and Environmental Sciences.
*Somarin, A.K. & Ashley, P.M. 2004. Hydrothermal Alteration and Mineralisation of the Glen Eden Mo-W-Sn deposit: A Leucogranite related hydrothermal system, southern New England Orogen, NSW, Australia. Mineralium Deposita.

The Dummy you'll find north of Armidale

One of the imposing landscape features on the north side of Armidale is the 1400m high Mount Duval. Some of my secondary education was in Armidale and I remember that the logo of my school actually had Mount Duval in it. Mount Duval is part of granite-like pluton called the Mount Duval Monzogranite. It was previously called the Mount Duval Adamellite; however the term Adamellite is no longer formally recognised. The intrusion actually extends in a crescent shape further to the west and includes Little Mount Duval which is roughly were the watershed for the Great Dividing Range sits, draining to the east all the way to the Macleay River. The monzogranite is considered to be middle Permian in age and intrudes several different complex rock units, one of these is a relatively small unit called the Dummy Creek Conglomerate.

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

The Dummy Creek Conglomerate is situated to the north of Mount Duval and extends to the east to the area of Puddledock, the northern side is intruded by the Highlands Igneous Complex. The Dummy Creek Conglomerate is comprised mainly of conglomerate but not exclusively. Lithic sandstone is a major component and it is actually what is in these sandstones that allow us to determine when the unit was formed, but more of that later. The abundance of conglomerate as well as sandstone and rarity of fine grained sediments like mudstones shows us that the sediments, gravels, etc that made up the Dummy Creek Conglomerate have not travelled far from their source. The clasts in the conglomerate show that the source rock was the underlying Carboniferous aged Sandon Beds (part of the Texas-Woolomin Block).

Korsch (1982) concludes that the original Sandon Beds was domed and uplifted by the intrusion of granite bodies of the New England Batholith such as the Mount Duval Monzogranite and the Highlands Igneous Complex. The hills formed from the deformation of the Sandon Beds began shedding rock, eroding and the sediments were deposited a short distance from these new hills. The intrusions continued to intrude shortly after the sediments were deposited which according to Holland (2001) created a complex system of overlapping zones of contact metamorphism. The intrusions were therefore emplaced in a very shallow crustal situation and volcanism was abundant and the Dummy Creek Conglomerate was quickly covered and preserved by a volcanic unit that is called the Annalee Pyroclastics which includes lavas, pyroclastic deposits and the like. It is worth noting that other models of formation by various other authors were summarized by Holland (2001) for instance some authors suggest that rock fabric studies may show a source only from the south.

A lot was happening in the Mount Duval-Tilbuster-Puddledock area during a relatively short period of geological time, indeed even during this time of change a substantial forest must have been growing in the area. The sandstone layers in the Dummy Creek Conglomerate preserve fairly common plant fossils. Most of the fossil remnants are fragments but there is enough to identify many plants with certainty. The most common fossil identified was the deciduous plant Gangopteris, a relative of the more commonly known Glossopteris, the main plant that formed the coal of the Sydney Basin. This plant existed abundantly in the middle of the Permian and so given that many of the rocks appeared to be forming at the same time these can be assumed to be close to this age too.

References/bibliography:

Holland, R. 2001. South western Margin and Contact Rocks of the Highlands Igneous Complex near Orana Falls, North of Armidale, NSW. Unpublished undergraduate research thesis, University of New England.
Korsch, R.J. 1982. The Dummy Creek Association: Rim Syncline Deposits. Journal and Proceedings of the Royal Society of New South Wales. V115.

Weirdly Wonderful Wongwibinda

I finally found them, photos of some of the strange metamorphic rock at Wongwibinda. I recently moved house and in the process I’ve lost many things but also found some things. Early this year I did a post on what were the broader conditions that lead to the geology of this area between Guyra and Ebor, namely thinning of the continental crust leading to increased heat flow and corresponding thermal metamorphism. I mentioned a rock type called migmatite and since I found my photos of the Wongwibinda migmatite, I thought I should go into a little more detail on this curious metamorphic feature.


Close angular folds in the Girakool Beds, Rockvale

The migmatites are strongly metamorphosed rocks of the Girrakool beds. The Girrakool beds are Carboniferous in age and were deposited in a marine environment. These beds were then accreted onto the edge of the Australian continent as part of the New England Orogen, much deformation occurred during this time. During or following this stage of tectonic forces that affected the New England region the Girrakool beds were subjected to a period of intense metamorphism. This affected one end of the beds more than the other. The western most part of the Girrakool beds in the Rockvale area remained relatively ‘uncooked’ but further to the east the effects of thermal metamorphism became greater creating schists known as the Ramspeck Schist and finally the zone of migmatites. The migmatites are faulted off by the Wongwibinda fault on the eastern side or are intruded by the Abroi Granodiorite which itself has been later metamorphosed into Gneiss.

Migmatite in the Aberfoyle-Wongwibinda area.
Note the ptygmatic folds and dyke on the left

The odd thing about the Wongwibinda migmatites generally is that they are actually three rocks in one: metamorphic sedimentary rocks becoming igneous at the same time. Usually rocks fit into the igneous and sedimentary categories neatly and then metamorphism can affect these rocks. In the case of migmatite the metamorphism is so great that the rock actually begins to melt, that is, it becomes an igneous rock with some of the sedimentary rock remaining unmelted. A characteristic of migmatite is ptygmatic folding, which is intense small scale folding with alternating light and dark bands. The dark bands are called the palaeosome which is the remains of the sedimentary rock and the lighter bands is insitu accumulation of melted rock called the Leucosome,. The leucosome is here comprised mainly of the minerals quartz, feldspar, mica and some garnet. Sometimes the leucosome can ‘break free’ from the ptygmatic folds and create dyke like structures. All of these features are visible in the picture opposite. 

What can be seen at Wongwibinda is essentially the formation of a granite, specifically a S-type (sedimentary derived), frozen in time. Craven et al 2012 demonstrated that this time was very close to the Carboniferous-Permian age boundary, probably just in the Permian, that is around 297 million years ago. There are some fancy geological features in the New England highlands and in my mind this is one of them. If you travel up that way and see some rocks by the side of the road be sure to stop and look closely, there are so many unusual things to find.

References/bibliography:
*Danis, C.R., Daczko, N.R., Lackie, M.A. and Craven, S.J. 2010. Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences V57.
*Craven, S.J. Daczko, N.R. and Halpin, J.A., 2012. Thermal gradient and timing of high-T-low-P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia. Journal of Metamorphic Geology V30.
*Wilkinson, J.F.G. 1969 The New England Batholith - introduction. IN Packham G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.

Lots of heat but low pressure between Ebor and Guyra

In the headwaters of the Aberfoyle and Guy Fawkes Rivers, tributaries of the Clarence River lies a rare zone (for Australia) of rocks that have experienced high temperatures but surprisingly low pressure. This is another one of those “we don’t have a clear answer” posts, in particular what has actually caused the metamorphism of the rock in this area, but research just published in January (Craven et al. 2012) has shed a lot of light on the matter.

Originally it was thought that the metamorphism at Wongwibinda (The Wongwibinda Metamorphic Complex) was directly associated with the emplacement of the Granites since the most intensely metamorphosed rocks are close to the Permian aged Abroi Granodiorite and other Permian granites with a decrease in intensity of metamorphism further away from these intrusions (Wilkinson 1969). The depth of metamorphism was never considered very deep because the minerals that are present in the metamorphic rocks are not formed where intense compression is found. However, it has since been observed that contacts with some of the granites shows no or little metamorphic effects, notably along the contact with the southern part of the Abroi Granodiorite. Additionally, the Abroi Granodiorite itself displays some metamorphic textures making the picture relatively unclear.

An old geological map of the area (NSW Geological Survey)-
Note that the Glen Bluff fault should not define the edge of the schist (gradational)
(Phag - Abroi, Plr - Ramspeck, Pl - Girrakool and Dyambarin, Tb - Basalt)

Like many parts of the New England, the geology can be quite complex with many aspects and relationships not fully understood and this holds for the Wongwibinda area which is a area of metamorphism, abundant faulting, granite intrusions, deep sea sedimentary and volcaniclasitic rocks and basalt lavas. Describing the generalgeology may be easiest from east to west. Before coming to the properties of Abroi and Springfield is the Palaeozoic aged Dyambarin Beds which is neatly faulted off to the east. The eastern side of the Wongwibinda Fault lies rock of the Abroi Grandiorite (a type of granite), sometimes referred to as the Abroi Gneiss (in this case metamorphosed granite which appears to have been affected by the Wongwibinda Fault). Then just to the west of this we enter what is significantly altered Girrakool Beds and this is were it gets even more interesting.

The eastern most part of the Girrakool beds has been significantly affected by heat maybe up to about 700 degrees Celsius, but has experienced very low pressures and this has created an unusual texture called migmatite. Migmatite is a type of rock were the parent (in this case sandstone and mudstone sedimentary rocks of the Girrakool Beds) has been heated so much that it just starts to become liquid like, here it also shows Ptygmatic folding. The liquid usually accumulates or is formed in some individual layers creating essentially layers of molten rock between sediments. Sometimes the hot liquid rock follows cracks in the rocks creating little dyke like structures too.

Moving further to the west we enter a zone of schist (called the Rampsbeck Schist), which is a medium grade metamorphic rock that has had some of the crystals in the rock reform into layers, this schist extends further west showing less and less metamorphic effects until it is indistinguishable from the rest of the Girrakool Beds. There are also some areas of quartzite and amphibolite (other metamorphic rocks) in the schist zone. I’ve also seen a pegmatite dyke a bit further to the west, which I have no idea where it fits into the picture.

Over the top of all this are remnants of comparatively recent Basalt referred to as Tertiary (or Mid-Cenozoic aged) Alkali Basalt. Given its location this basalt is probably Doughboy Basalt, part of the Cenozoic aged (~40Ma) Doughboy Volcanic Province.

But what caused the formation of the metamorphism? Many mechanisms have been proposed by different authors such as Wilkinson (1969), Danis et al (2010) and Craven et al (2012) and other authors. Craven (2012) has carried much work, including dating to try and gain an understanding:

• Was it the Wongwibinda fault? No – otherwise the rock would pressure related textures. 
• Was it the intrusion of the Abroi Granodiorite (or other granites in the area)? No – otherwise we’d see metamorphism around all the Abroi Granodiorite that we don’t see, and the age of the Abroi Granodiorite is older than the date of metamorphism. 
• Was it an intrusion that we can’t see because it fairly deep underground? No – gravity surveys have been conducted and these don’t show any deep granites other than the Abroi Granodiorite in the area of maximum metamorphism.
• Was it the eruption of the Cenozoic Basalts? No, the age of metamorphism vastly predates the Cenozoic period.

Wow, so what options are left? Craven et al (2012) have come up with a theory: following the tectonic events that formed the granites in the area there was a period of stretching of the earth, this thinned out the crust and allowed for heat to be more easily transferred from the mantle. All of the other options are more common elsewhere in Australia and around the world, but each option has been refuted by different evidence until the only reasonable explanation left at this stage is the extension of the crust to allow convective heat transfer from the mantle to very shallow levels during the Permian.

I seem to have lost my old photographs of the migmatite but I have found a good website on Finnish migmatites that has some great pictures. It can be linked to from here.

A follow up post on the Wongwibinda migmatites can be found here.

References/bibliography:

*Danis, C.R., Daczko, N.R., Lackie, M.A. and Craven, S.J. 2010. Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences V57.
*Craven, S.J. Daczko, N.R. and Halpin, J.A., 2012. Thermal gradient and timing of high-T-low-P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia. Journal of Metamorphic Geology V30.
*Wilkinson, J.F.G. 1969 The New England Batholith - introduction. IN Packham G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.