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.