Thursday, 14 May 2015

New England Geological Tour 2015

Just a quick note to let people know that the Australian Institute of Geoscientists (AIG) and the Geological Society of Australia (Queensland Branch) will be jointly running a field trip to the New England area of New South Wales and southern Queensland over the June long weekend 6th to 8th June). The field trip follows a one day seminar by the AIG. 

Geoz reports thusly:
6 - 8 June 2015 GSAQ–AIG Field Conference: New England District Regional and Economic Geology
A joint GSA–AIG field trip to the New England Orogen, with a preceding one-day seminar.
As a prelude to the Field Conference, GSAQ and Queensland Branch of the AIG
are proposing to run a one day seminar “New England Orogen, Regional and Economic Geology - an update” to showcase recent advances in the understanding of the New England Orogen.
 The main focus sessions of the pre-field trip seminar will include:
  • The New England Orogen – geology, granites and tectonic setting
  • Mineralisation styles of the northern and central New England Orogen
  • Geochemistry applications in the New England Orogen
  • Intrusive related mineralisation styles of the southern New England Orogen
The Field trip will start from Brisbane and and include tours and presentations in the Stanthorpe, Texas, Tenterfield and Drake areas.

Accomodation and some meals are included in the cost of the field trip. For more information on the field trip contact the GSA or AIG, for more information on the Brisbane pre-trip seminar contact the AIG.

Friday, 3 April 2015

Jesus’ Easter: a geological tour

Limestone is not common in the Northern Rivers but there are several
small locations where it does occur (This picture is from near Tabulam).
Since this is the first day of Easter where Christians remember the death and resurrection of Jesus at Jerusalem, I thought I’d give some background by way of the geology of the city. Like all landscapes the landform that makes up the hills and valleys around the Holy City can be seen in the geology.

At the festival of Passover Jesus entered the city of Jerusalem from the western side from Bethany. Passing into the Kidron Valley and then up to the city. At Bethany the rock types are dominated by Cretaceous aged chert and chalk of the Mishash formation of the Mount Scopus Group. These rocks are typically marine deposited sediments made from the build-up of microscopic creatures called diatoms. Descending into the Kidron Valley the chert which caps the hills to the east of the city gives way to chalk and claystone which is much more erodible. This chalk and claystone is the Menusha Formation which is the earliest formation in the Mount Scopus Group.

Stratigraphy of the Jerusalem area
Image courtesy of  Dov Frimerman
Ascending into Jerusalem the geology changes to limestone of the Nezer and Shivta formations of the Judea Group. The limestones of the Judea Group dip at an angle of around 10-15 degrees. This means that any ground water travelling though the limestone flows to the west to the sacred springs along the top of the Kidron valley. The garden of Gethsemane where Jesus spent his last night praying is in the area of these springs.

The limestone is the rock that underlies all of the places where Jesus spent his last days. Jesus drove people from the Temple claiming that they were stopping people from reaching God. The foundations of the Temple are built on this limestone. Jesus was placed on trials for treason at the Roman governor Pontius Pilate’s palace and also at King Herod’s Palace around Mount Zion. Again, these places were built on the same Limestone.

The exact place of Jesus’ execution and burial is the subject of some debate. There appears to be a couple of alternative sites but all of which are in areas were limestone is dominant. This is particularly evident with the description of Jesus being crucified ‘near’ the city and the description of Jesus being buried in a cave. The old city of Jerusalem was built entirely on the Judea Group and limestone landscapes are very well known for having many cave systems. Caves are well known in the area around Jerusalem.

In the Northern Rivers of New South Wales there is a mountain called Mt Jerusalem which is part of the world heritage system of National Parks around the Tweed Valley. But the geology of Mount Jerusalem, NSW is a post for another day.

Want to see more? Here is the Israeli geological survey’s 1:25 000 scale maps of the country or here is a good website describing the geology of Israel in more detail. To find out more about Jesus during his last days the accounts of his apostles in the Bible is the most detailed description that remains. There are other references from other sources such as Josephus and Tacitus but nothing as comprehensive as the accounts of Matthew, Mark, Luke and John in the Bible.

Saturday, 21 March 2015

Bedding with crossbedding through it

Cross-bedding in sandstone of the Evans Head Coal Measures
Shark Bay - Evans Head area
Cross-bedding is a common feature in many of the Mesozoic aged sedimentary rocks in our region particularly in the Clarence-Moreton Basin. Cross-bedding is a structure that can be confusing. However, is it is often very useful for understanding how a sedimentary rock is laid down. Because it is common in our region I thought it might be interesting to describe what this feature is.

Cross bedding forms in sedimentary rocks that have undergone transport. It is most easy to find cross bedding in sandier sediments that have been deposited in Aeolian (windy) and fluvial (riverine) environments. However, it is a feature that may be found less commonly in shallow marine, and estuarine environments but these processes are a tiny bit different and more complicated to describe, so I’ll deal with them some other time.

Sketch showing how cross-bedding is formed
The feature can be confusing especially because the cross-bedding can be mistaken as actual bedding layers. However, technically speaking, cross-beds are always laid down within the same bed. The cross-beds in a riverine environment form when the water in a stream loses energy and its ability transport sediment. The sediment then drops out of the water and is deposited along a point bar. Over time the river may dry up or migrate away and the point bar (now one big bed with cross-bedding) can then be preserved.

Cross-bed sets in sandstone of the Orara Formation (Kangaroo Creek Sandstone)
Eden Creek - Kyogle Area
Often sets of cross-bedding are present where the river will deposit another point bar over the top of the original. When this occurs the new bed usually erodes the upper part of the original bed. This is a useful bit of information because in some areas the rocks have been so deformed that it can be hard to tell whether they are upside down or not. If you are able to find cross-bedding in these rock looking for the erosion surface will tell you whether the rocks are right way up or have been turned over. It may be surprising to note that over turned bedding is actually common in the metamorphosed sediments in the New England and Tweed region. Since deformation of the Clarence-Moreton Basin has been relatively small it is unlikely that you will come across in-situ rocks that have been turned over in this basin.

The two pictures show examples from some of the oldest rocks of the Clarence-Moreton and Ipswich Basins and the one of the youngest. Despite being laid down up to 100 million years apart the manner of deposition of these two separate units was a very similar riverine environment. Nearly any outcrop of Orara Formation will show cross-bedding. So keep a look out at road cuttings or sandstone quarries.

Monday, 2 March 2015

Do you trust a geological map? (Part 2)

In an earlier post I showed an example of how making the assumption that published geology maps are correct has some big problems. In that example it was the digging of test pits where coal was uncovered which clearly showed the mapping was wrong. Since that post I found another example and this one only needed a look out the car window to know that something was wrong!

My interpretation of the geology of the area in question
Note that the area is approximately 7km across
I had a 4WD day a few weeks ago in the Bungabee State Forest between Lismore and Kyogle. The state forest is located on the southern section of the Mackellar Rangers. It is a nice area but as usual for the Richmond River catchment is invaded by noxious and environmental weeds (In fact the worst variety of weeds I saw that day was at the National Parks managed Muckleewee Mountain Nature Reserve). But I digress... It was during this 4WD trip that I looked out of the window! Where there were cliffs or rocks exposed in streams I glanced out and saw that they were clearly Clarence-Moreton Basin sedimentary rock. I didn't think this was particularly unusual at the time but when a quick opportunity arose I had a closer look.

Present mapping (Brunker et al 1978)
Area is the same as my map above
At the base of the range (near Muckleewee Mountain Nature Reserve) I broke off a fresh bit of rock from a cliff face. The ‘fresh’ sedimentary rock was a rusty brown colour. It was composed of grains that were made from other rock fragments, from feldspar and from quartz. I'd call it a litharenite according to the classification of Pettijon et al (1987), however I might have underestimated the amount of clay particles in it. The appearance of the litharenite was quite dull. I consulted a geological map that night and noticed that the area was not even mapped as sedimentary rock. It was mapped as Lismore Basalt, old lava flows. There were however some areas on the map, a few kilometres away that did have some sedimentary rock (mapped as Kangaroo Creek Sandstone). However, the Kangaroo Creek Sandstone has a distinct saccaroidal texture, a sparkly sugar grained appearance. It was clearly quite different to the rock I was looking at. The only thing that looked consistent to me was larger scale features in the cliff faces showing cross-bedding. However, cross-bedding is a very common feature in most of the Clarence Moreton Basin.

So, with my field observations mind and while reviewing a new stratigraphic guide for the youngest members of the Clarenece-Moreton Basin, the rock I was looking at appeared to be consistent with the expected rock in the Bungawalbin Member of the Orara Formation as defined by Doig & Stanmore (2012). See this previous post for more details. When I found some spare time I did a quick remapping of the area based… As you can see from the pictures my mapping is quite different. This again just goes to show that a geological map might not give you an answer. It is best to look in the field and deeply consider what you find.

*Brunker R.L., Cameron R.G., Tweedale G. and Reiser R., 1972, Tweed Heads 1:250 000 Geological Sheet SH/56-03, 1st edition, Geological Survey of New South Wales, Sydney
*Doig, A., & Stanmore, P. 2012. The Clarence-Moreton Basin in New South Wales; geology, stratigraphy and coal seam gas characteristics. Eastern Australian Basins Symposium IV. Brisbane.
*Pettijohn, F.J., Potter, P.E., & Siever, R. 1987. Sand and Sandstone. Springer-Verlag, Berlin

Monday, 16 February 2015

Eidsvold Earthquake 2015

I woke this morning to the news that a town to the west of Bundaberg had experienced a substantial earthquake. Well, substantial by Australian standards anyway. Geoscience Australia gives the intensity of 5.2 on the Richter scale. The quake occurred at about 2am local time (3am for those of us in the other eastern states coping with daylight saving).

The preliminary report from Geoscience Australia can be found here.
Seismograph from Eidsvold Station
It is in an interesting area because the area of the earthquake is in the northern part of the New England Orogen. This belt of squashed rocks extends from the Bundaberg area in a big arc all the way to Port Macquarie in the South. There are many faults in this area and some are still active, although they are generally small. An earthquake between Gunnedah and Tamworth in 2013 springs to mind.

The scale of the earthquake is quite large for Australia. Indeed the Newcastle earthquake was measured at 5.6. I've not done the maths but the new Eidsvold quake of 5.2 is about half the size of the Newcastle one (The Richter Scale is NOT linear).

Historically, the area is prone to small to medium sized earthquakes with Bundaberg being hit by a size 6.0 in 1918. This is nearly ten times more powerful than the most recent one though the 1918 quake occurred just off the coast.

Oh... and humour starts quickly:

Monday, 12 January 2015

Guest Post - Dynamic beach sediments

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

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

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

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

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

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

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


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

Thursday, 1 January 2015

The name of Paddy's Emu? At last a good answer

Paddy's flat is an area that many consider the middle of nowhere. It is not a well known area but it probably should be. There is a nearby place called Pretty Gully and this name gives a better indication of the Paddy's Flat area. It is some of the headwaters for the mighty Clarence River and includes major tributaries such as the Cataract River and Emu Creek. Researchers have returned to the Paddy's Flat area numerous times for more than a hundred years to try and resolve the tricky geology. But agreement on the geological relationships of the area has been mainly unreachable. However, one of the latest papers in the Australian Journal of Earth Sciences may have resolved many of those issues.

Gideon Rosenbaum and his team from the University of Queensland has been responsible for huge advances in geological knowledge in the Northern Rivers headwaters. The latest paper from Gideon Rosenbaum's team (Hoy et al. 2014) is another for which we should be thankful. The level of research by local universities is sadly very close to non-existent and one of the preeminent research universities has thankfully filled some of the gaps. But, I digress. What is so great about the Hoy et al (2014) paper?

The many ideas about the Stratigraphy of the Emu Creek Block.
from Hoy et al (2014)
There are many great things in Hoy et al (2014) but to me the biggest is something I've struggled with for a few years. It is how and when the area formed. It demonstrated that some of the rocks of the area probably formed in the same geological environment and time as those to the west of Tamworth. Hoy et al (2014) has resolved the three stratigraphic units of the Emu Creek Block. In doing so has demonstrated that the block was formed during the late Carboniferous period. This was when a great unit of subducting crust was sliding from the west under the New England region to the east. According to Hoy et al (2014) the rocks seem to have been deposited in a shallow ocean basin (a fore-arc basin) formed at the front edge of a chain of volcanoes (a volcanic arc). A modern day active fore-arc basin is the area between Sumatra Island and its offshore islands in Indonesia. This means it was the same processes that occurred in the Tamworth area. At the same time it showed just how big the continental collision zones were that created the New England region.

In proposing a new stratigraphy for the Paddy's Flat area, Hoy et al (2014) has now come close ending more than 100 years of head-scratching. There has been more than eight different relationships proposed for the units in the Emu Creek Block starting from the first in 1906. The best one until now was probably the Geological Survey of Queensland (Murray et al 1981).  Hoy et al (2014) proposes that the youngest unit in the block is the Emu Creek Formation which is overlain by the Paddy's Flat Formation which was deposited after a haitus. The Paddy's Flat Formation is then overlain by the Razorback Creek Mudstone. Hoy et al (2014) dated zircons in the rocks using the uranium and lead composition and compared this with the age of the fossils found in the area. The results were inconsistent with the Paddy's Flat fossils. This lead to the conclusion that in-situ fossils are present in the Emu Creek Formation but probably not in the Paddy's Flat Formation. Any fossils that were found within the Paddy's Flat Formation were probably eroded out of the Emu Creek Formation. Coming to this conclusion brings to an end to so much confusion that was present.

Once the Emu Creek Block were formed along with its related rocks from Coffs Harbour to Texas through to the Tamworth area, there was large scale bending of the New England area. So much so that the western facing fore-arc basin at Paddy's Flat was bent around so that it seems to be facing the north-east. This is what Rosenbaum (2012) terms the Coffs Harbour and Texas Oroclines and is the biggest but largely unknown tectonic features of our part of Australia. But more about that in a future post.


*Hoy, D., Rosenbaum, G.,Wormald, R. & Shaanan, U. (2014) Geology and geochronology of the Emu Creek Block (northern New South Wales, Australia) and implications for oroclinal bending in the New England Orogen. Australian Journal of Earth Sciences. Vol8.
*Murray C., McClung G., Whitaker W. & Degeling P. (1981) Geology of late Palaeozoic sequences at Mount Barney, Queensland and Paddys Flat, New South Wales. Queensland. Government Mining Journal V82.