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.

Tuesday, 30 December 2014

Don't Panic!

Don't Panic! I've been very quiet on the blogging front for the last few months. But don't worry! A new post will go live 1st of January. It will be followed shortly after by a guest post on the wonders of our coast line and its beaches.

Hope everyone had a lovely Christmas. 2014 years is a blink of the geological eye but still a long time in human experience. It is nice to know that the message of Christmas is still nice and clear even after so many years of human dispute. In the account of Matthew in the Bible, Jesus was presented with three gifts at his birth. Two of the gifts were from plants and the third from the earth. The mineral is still sought after today just as it was many thousands of years ago. This is much like the sadness of human relations and the whole story of Jesus's subsequent life and death... we don't really change do we!

See you again in 2015.

Saturday, 13 December 2014

What do people think I do

What do people think that geologists do? Well, here is one answer from the Geosphere Blog.

Sorry all for the delay in posting. Once I get my act together I will post again more regularly.

Sunday, 2 November 2014

Unconventional Gas - Gaps in Baseline Data

The NSW Office of Water has been busy compiling a series of videos on YouTube explaining various aspects of hydrogeology. This recent video is about data gaps in understanding baseline hydrogeological conditions in areas of 'unconventional' gas.

Monday, 20 October 2014

Blog Update #6

Over 100 000 page views for this blog, 3 years and over 130 blog posts. I honestly did not think that there would be so much interest in the geology of the Northern Rivers. This milestone was topped off by heaps of interest during a short presentation on the weekend at the Big Scrub Rainforest Day.

I was a little worried about my presentation since it directly followed Robyn Williams from the ABC Science Show. His whole professional life revolves around talking to an audience. I on the other hand am not used to public speaking. I wish my talk was more structured and I did not try and condense so much into it. But I think it was received surprisingly well all the same! The questions were excellent and I wish I had more time to answer them. There were still a lot of hands up when I had to hand over to A/Prof Isaac Santos from SCU for his talk.

Following the talk the 100 information sheets disappeared and many people were asking if there was more. So thanks everyone for your interest and sorry that I didn’t have more copies. The information sheets are the short articles I have recently done for a couple of the Big Scrub Rainforest Newsletter. The specific articles can be downloaded from the links below. The full newsletters can be obtained by contacting the Big Scrub Rainforest Landcare Group.

January 2013 – Mt Warning and Reading the Rocks

September 2014 – The Importance of Basalt in the Big Scrub and One CSG Related Anomaly

Regular visitors to my blog may have noticed the slightly declining number of posts over the months. This is nothing to do with a lack of subject matter. I am finding the time a little hard to find at the moment. As such, I am happy to open up this blog to guest bloggers from time to time. If you have a story about Earth Science that has even the slightest New England/Northern Rivers/ North Coast bent, please feel free to contribute to this blog. I can be contacted on the email address listed on the page “about this blog”.

Monday, 6 October 2014

Rocks in the Rocky River

Rocky River Monzogranite (Bungulla Suite).
The Monzogranite here contains large crystals of twinned pink K-feldspar.
The Rocky River Road is a very quiet, scenic and out of the way route to travel. It is slow and windy, but a pretty alternative to the Bruxner Highway route between Drake and Tenterfield. I had the pleasure of a trip along Long Gully Road and Rocky River Road just last week. I enjoyed it very much for the scenery and the clear water of the Rocky River (also known as the Timbarra River). The area is also very interesting in a geological sense. The rock that is found along Rocky River Road (the Rocky River Monzogranite) is actually remnants of outer part of a very large batholith that makes up Timbarra Tableland.

Previously, understanding of the inner rocks of the Timbarra Tableland were incorrectly thought to be Moonbi Supersuite, while the outer rocks were correctly part of the Stanthorpe Supersuite. Having two parts of an intrusion being apparently related to different Suites was all quite confused. Mustard (2004) suggested an informal renaming of the Bungulla Monzogranite in the area of Rocky River to the Rocky River Monzogranite. The Rocky River Monzogranite would in turn be part of the Bungulla Suite. The Bungulla Suite being rocks that are I-type (derived from melted igneous rocks) of the Stanthorpe Supersuite.  Although the nomenclature by Mustard (2004) was suggested as informal it is quite reasonable to adopt the name of Rocky Creek Monzogranite as formal. The previous identification of some rocks in the Timbarra Tableland as Moonbi Supersuite has since been shown to be incorrect - they are all Stanthorpe Supersuite.

The Rocky River Monzogranite is in the extensive eastern edge of the Timbarra Tablelands. It is comprised mainly of the rock monzogranite. This rock is comprised of abundant quartz and roughly equal proportions of plagioclase feldspar (sodium and calcium feldspar) and potassium feldspar. There are also smaller amounts of dark biotite mica and amphibole in the rock. The Rocky River Monzogranite is quite a course grained and the crystals are very, very large. The monzonite is notable as it has many 'inclusions' called xenoliths. These are blobs of rock are of a less granitic composition. They are very, very common in some areas as the rock comprises of about 10% or more xenoliths. The xenoliths indicate that mixing of different composition magmas was occurring when the intrusion formed.

A monzogranite tor in the sandy bed of the Rocky River.
Note different sized irregular shaped xenoliths.
Along the very margin of the intrusion (I didn't get to see this) the crystals are smaller in size and the feldspars are even more potassium rich forming the rock syenite. The central area of the Timbarra tablelands is comprised of granitic rocks that were high in fluids when the rock was crystallizing. These fluids (formed by residual enrichment of the original magma chamber), has resulted in the concentration of metals, most notably gold (Mustard 2004). The Timbarra gold mine targeted this inner zone of the tablelands as the outer granite (Rocky Creek Monzogranite) do not contain nearly as much gold. The erosion of the gold has led to alluvial gold deposits in the Rocky River and Clarence Rivers but the gold is very fine grained so fossickers panning can be tricky.

The many components of the Timbarra tablelands intrusion were emplaced in the Triassic period. They intruded the Drake Volcanics. The size of the granite plutons has caused significant contact metamorphism, creating a large metamorphic aureole around the intrusion.

There is much more to say about the zones in the Timbarra tablelands intrusion described by Mustard (2004). This includes the neatness of the tablelands cross section, the way that the slightly different granites tapped different parts of a deeper magma chamber and the way that differentiation of granite types occurred are all worthy of a discussion. Though, this needs more than just a few paragraphs and so I will have to cover these matters in future posts. In the mean time I hope this post gives a taste for some of the 'granite'.

*Mustard, R. 2004. Textural, mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton, New South Wales. Australian Journal of Earth Sciences, 51.