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.


References/bibliography:

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

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.

Evidence of a palaeo-beach on present day Diggers Beach.

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.

An excellent outcome from atmospheric atomic bomb testing

Human ingenuity surprises me again and again, especially the efficiency in which we can annihilate each other. During the 1940’s and 1950’s the superpowers were focused on increasing the efficiency in the way they could destroy everyone on the planet. It was a very worthy goal (yes that was a joke) and to achieve maximum efficiency they needed to conduct atmospheric tests of their bombs. Sometimes, unforeseen obvious benefits other than the benefits of death and destruction of humanity can arise.

I have recently been thinking about groundwater in the Richmond River area for which I have been consulting sections of a PhD thesis written by Leonard Drury in 1982 (Drury 1982). Drury's comprehensive thesis included qualitative identification on the age of groundwater in aquifers in the Richmond River by using an unstable isotope of hydrogen called tritium. Hydrogen is an atomic component of water (the H in H2O) but hydrogen actually comes in three natural forms based on the number of neutrons are in the nucleus of the hydrogen atom. These different forms are called isotopes. Hydrogen naturally has one neutron or less commonly two neutrons (called deuterium) and very rarely three neutrons (called tritium). In nuclear explosions the third isotope tritium, is created at concentrations much higher than the background. The reason why tritium is rare naturally is that it is only formed in the upper atmosphere but is unstable and loses the extra neutrons to become a smaller isotope over a period of time.

Half of the tritium in a given amount of water (or whatever) decays over a period of 12.5 years (this is called a half-life). Which means that over 25 years there is only a quarter of the original tritium left, 37.5 years one eighth, 50 years one sixteenth etc. Since tritium is not naturally occurring there is no practical use to measure for tritium unless you can introduce it into a system as a tracer and then measure its behaviour. This means that a large ‘slug’ of tritium was created during the 1940’s and 1950’s during atmospheric nuclear testing. Therefore if you can look for tritium in groundwater and if it is not present you can assume that that groundwater has been in existence for more than 50 years, i.e. it was present in the ground before any nuclear tests. If you detect tritium in several locations in an aquifer the relative abundance of the tritium will give an indication of the age of the water and whether mixing is occurring between old groundwater and new groundwater. It won’t give you an exact date but it will let you know a lot about behaviour of an aquifer.

The trouble is time is running out. The half-life of tritium means that as time goes on the ability for us to accurately measure the smaller amount of the isotope means that one day we won’t be able to use this as a technique. I was aware that time was running out on using tritium as an effective groundwater tracer but I was not aware how soon. I have had a few chats with an academic at Southern Cross University one of which was about using tritium, he said we actually only have about 5 or 10 years left to which I jokingly suggested to him that we should reset the tritium clock with some more atmospheric nuclear explosions! To which he informed me that actually there appears to be some more tracers that can be used following the Fukushima Nuclear Accident.

Bibliography/References:

*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales. University of New South Wales, PhD thesis.
*Moran, J.E. & Hudson, G.B. 2005. Using Groundwater Age and Other Isotopic Signatures to Delineate Groundwater Flow and Stratification. University of Illinois.
*U.S. Geological Survey (USGS), 2004, Stable Isotopes and Radiochemicals, in National Field Manual for the Collection of Water-Quality Data, Chapter A5 Processing of Water Samples. USGS Techniques of Water-Resources Investigation

More climate clues on the Northern Tablelands

In January last year I did a post called How Cold Was It? Glaciers in New England? that showed evidence of peri-glacial features in the Northern Tablelands of New England, specifically in the area just to the east of Guyra. Bob H, gave me a tip-off for these interesting features which went unnoticed for a long time – including by me. I’d even taken a photograph of a solifluction lobe and not identified its true nature! It is important to know that Solifluction lobes and other peri-glacial features such as cirques are not glacial features per se. However, Bob did mention a probable moraine elsewhere in the New England, specifically, near Ebor in the vicinity of Duttons Trout Hatchery. A moraine IS a glacial feature. Because of these interesting features and because that part of the country is wonderfully beautiful I have wanted to do a road trip into the area but as yet have not been able to. The best I’ve been able to do is look at Google Maps but at least even consulting Google you can find some little gems.

A Google Earth image of the area to the North of Wollomombi

While looking at Google Maps I recognised more evidence of peri-glacial features in the Wollomombi area, which is about 20km to the south east of where the above-mentioned features were identified near Guyra. Here too was evidence of solifluction (movement of soil due to the partial thawing of summer permafrost). I’ve not been able to identify with certainty any other evidence of solifluction or related features even in the higher (and therefore colder) parts such as Ebor. Maybe, it was the case that during the last glacial maximum (about ten to twelve thousand years ago) only isolated areas formed permafrost - seemingly small areas of south facing hills.

However, when noticing the places where periglacial features are present such as east of Guyra at Malpas Dam and those I just noticed north of Wollomombi, I thought that they seemed only to be present on hills that looked like they had soils derived from basalt rock. Indeed, upon inspection of the geological maps it became apparent that the only places where I can see these peri-glacial features are mapped as being on Cenozoic aged basalts. The map shows the south facing hills that are derived from other rock types such as granites and meta-sediments do not show the same evidence of being affected by permafrost or related processes. This is interesting because there are two possible reasons for this:

1. There was only isolated areas that were cold enough to maintain permafrost during the last glacial maximum; or
2. The soils derived from granites and meta-sediments did not preserve evidence of permafrost

Given that the solifluction lobes evident at both Wollomombi and Guyra are about 20km from each other I would suggest that it is unlikely that the effects would only occur in these two areas and not in the area in between, so option 2 is the most likely. This may have the following implications:

• Zones of permafrost (peri-glacial environments) and maybe small glacial environments probably existed in frequent patches on south facing slopes all the way between Guyra and Wollomombi and maybe even further to Ebor an area 60km long;
• The soils in this area are derived from three major types consisting of Carboniferous aged Meta-sediments of the Girrakool Beds and Sandon Beds, Permian and Triassic aged New England Batholith ‘granites’ of the Abroi Granodiorite, Rockvale Monzogranite and Round Mountain Leucomonzogranite and finally Cenozoic aged ‘basalts’ including the Doughboy Volcanics and others which are unnamed;
• Only the soils derived from the basalts have properties available to behave in a manner which produces and or preserve the evidence of permafrost in features such as cirques and solifluction lobes.

A Google Earth image of a spot next to Malpas Dam near Guyra.
Here the solifluction lobes are comparatively big

So, what does this mean? Well, it means that it was very cold over a large area in the New England. So much, that during the last glacial maximum, water was permanently frozen in the soil in south facing topographic areas over a widespread region extending at least from Guyra to Ebor. But, evidence for this was only preserved in the soils derived from basalts (I need to consult a pedologist (soil scientist) to figure out exactly why this might be the case).

So, if you are shivering and experiencing snow flurries in the area during winter, know that you would have been shivering harder had you been there about 20 000 years ago. It makes me wonder if the indigenous people of the region experienced that cold or whether the land was too cold and marginal for them to live there at that time.

The lonely delta

Mark's wonderful picture of  the delta on Watson Taylor's Lake

A couple of weeks ago I saw a wonderful picture on Mark Bellamy's Clarence Valley Today photo blog, a picture of Watson Taylors Lake. Watson Taylors Lake which is where the Camden Haven River ends up just before it meets the sea. What struck me most about this picture was the text-book development of a delta system into the lake. Marks blog can be found here.

A delta is formed when sediment suspended in flowing water settles out as it reaches a large water body. Probably the most well known deltas in the world are the Mississippi River Delta, the Ganges River and the Nile River. However, it also creates a question, why don’t we see deltas up and down the Northern Rivers and North Coast areas?

Several studies of off-shore sedimentation have been done along the coast, the earliest studies tended to be looking mainly for heavy mineral deposits such as ilmanite, rutile, zircon and even gold or for military/oceanographic purposes. However, both these studies and others specifically to understand the off-shore environment have demonstrated some interesting facts including why we don’t have river deltas.

The first part of understanding the off-shore sedimentary environment is to understand that currently the sea level is at a very high level in historic terms. It reflects the current warm interglacial period that has arisen. The lowest sea levels that most 'recently' occurred was following the beginning of the Pleistocene which was the period since the the last 130 000 years or so (Roy & Thom 1981 & Drury 1982). According to Drury (1983) and many other authors, sea levels much lower early in the Pleistocene including instances of maybe 100 metres or more (Den Dexter 1974 suggested around 200metres lower at the beginning of the Pleistocene . This caused erosion of most pre-existing soft sediments along what is now the submerged the continental shelf. But it was not a simple transition from glacial to interglacial with many cycles during the Pleistocene and corresponding to alternating periods of coastal sedimentary deposition followed by erosion of those new sediments, so it was a fairly complicated period.

Since the beginning of the Pleistocene Roy & Thom (1981) thought that it was likely that there were only two major causes of movement of sediments along the coast, the first was the effect of sea level fluctuations during interglacial and glacial periods and the second wave and wind action which had the effect of transporting sediment northward. These forces were probably enough to create sand barriers such as those preserved on the Northern Rivers inland from the active Holocene sand barriers and beach systems we enjoy today (more about the Pleistocene sand barriers in a future post). But, Roberts and Boyd (2004) indicated that Roy & Thom (1981) might not be totally correct in thinking there were only two major causes because in some areas the Eastern Australian current also seems to be a significant driver of sediment. In fact they noted that off the coast of Byron Bay in as little as 30metres of water the Eastern Australian Current was present and could scour away any sediments that might have been deposited or stopping sediments from being deposited.

This means that when the rivers, be they the Tweed, Clarence, Richmond, Bellinger, Nambucca, Macleay, Hastings or any others drop their sediment load, the presence of currents then sweeps the finest sediments away, mainly further out to sea, maybe to the edge of the continental shelf. The heavier sediments which drop closest to the coast are affected by waves and storms which drive the sandy sediments northward along the coast which contribute to the barrier beach systems we have in abundance.

This is probably a simplistic way of explaining and I've missed a few complicating factors such as continental shelf slopes but it seems that because of the combination wave, storm and sea current process we don’t get any river deltas in our region, unless they are protected by sand barriers such as the one protecting Watsons Talylors Lake on the Camden Haven River.

References/bibliography:

*Den Exter, P. 1974. The coastal morphology and late Quaternary evolution of the Camden Haven District. University of New England, PhD thesis.
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales. University of New South Wales, PhD thesis.
*Roberts, J.J. & Boyd, R. 2004. Late Quaternary core stratigraphy of the northern New South Wales continental shelf. Australian Journal of Earth Sciences v51.
*Roy, P.S. & Thom, B.G. 1981. Late Quaternary marine deposition in New South Wales and southern Queensland – an evolutionary model. Journal of the Geological Society of Australia v28.

Softer sediments in the Wilsons River Valley

I’ve recently been observing an interesting environmental restoration programme on the Wilsons River upstream of Lismore. During this programme I started to think about the flood plain of the river and ‘recent’ geological history of the area. Cotter 1998 and in his earlier undergraduate work developed a concept of the geomorphology due to the lava flows associated with the Tweed Volcano and the earlier Alstonville Basalt. I did an earlier post on the flow direction of the Wilsons River as related to the volcanic history of the area but I’ve done just about nothing on the post volcanic sequences.

The best work done on the ‘recent’ sedimentary formations of the Wilsons River valley was a PhD thesis, Drury 1982. This was done as part of the then Water Resources Commission (now State Water) back when NSW government departments actually collected new information to guide future decision making (oops, there is a political comment in there). Drury 1982 was a huge thesis that provides a vast amount of information on the development of the Richmond Valley based mainly on the groundwater bores operating at the time supplemented by some (then) new drilling and geophysical techniques. To my knowledge no significant further published scientific assessment of the Quaternary sequences has occurred since Drury's thesis was written.

Cenozoic Stratigraphy of the Lismore area

It is probably hard to follow the stratigraphy very easily so hopefully my sketch to the left which is based on Drury’s work helps. Drury 1982 indicates that the upper most layer of sediment in the Wilsons River and Leycester Creek valleys upstream from Lismore was unsurprisingly, flood plain sands, silts and clays which continue to be deposited today following floods. Conformably underlying this flood plain sediment the material encountered is called the Green Ridge formation. This formation appears to be a delta system being built at the end of the Upper Pleistocene (~12,000 years ago). Often the top of the Green Ridge Formation is cut by the Wilsons River and its tributaries, for instance at Boat Harbour Nature Reserve the lower banks of the river seem to be quite deep maybe even cutting into the even older formations (e.g. the Gundurimba Clay). Drury (1982) demonstrated that the Green Ridge formation is both contemporary with and overlies the Gundurimba Clay, which is made from estuarine clays.

The Gundurimba Clay is a unit was formed during a period of relatively high sea level (higher than the present day) and warmer conditions. Shells were common but coral was found maybe indicating the idea that the area where the Gundarimba Clay was being deposited went through a warmer spell than we experience now.  Drury 1982 identified pollen spores indicating the surrounding area was dominated by rainforest with some eucalypt forest too, in my mind this creates a picture that it was possibly a proto-‘Big Scrub’ low-land rainforest with the ‘Big Scrub’ proper forming after the next cold period at the beginning of the Holocene. However worth noting that the Upper Pleistocene is recognised around the world as starting off in a warm period turning into a glacial period with the last glacial maximum occurring around 22,000 years ago.

Drury 1982 demonstrated that preceding the deposition of the Gundurimba Clay there was a period of erosion meaning that the Gundurimba Clay unconformably overlies the South Casino Gravel. The South Casino gravel in turn uncomformably overlies the Cenozoic volcanic rocks of the Lismore and/or Alstonville Basalt. The South Casino gravel is at least Middle Pleistocene in age and is derived from the erosion of the underlying volcanics. Given its coarse nature it is highly permeable and is considered a good source of groundwater in other parts of the Richmond Valley but to my knowledge is rarely used in the Wilsons River area.

I'm probably trying to combine a lot into this one post so I'll have to tease out the details a bit more in future posts, especially that relating to the Gundurimba Clay and palaeo-environmental conditions which I know at least some of my blog readers have a keen interest in. At least I hope that this provides a starting point.

References/bibliography:

*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales: In Two Volumes. PhD thesis, University of New South Wales.
*Cotter, S. 1997. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of north eastern New South Wales. MSc thesis, Southern Cross University.

Disappearing sand from the North Coast

I was interested to read an article in my areas 'local rag' The Northern Star. It was a thoughtful piece by someone who loves the regions beaches. It was also a controversial one as it implied a man-made cause for the erosion of many of the regions beaches. You can read the article here: http://www.northernstar.com.au/story/2012/08/24/is-our-sand-on-goldy-beaches/. It actually, provides a good follow on from my last post on the matter.

Waves, wind, currents and a thin strip of sandy beach
One of the regions typical beaches near Ballina

In this article the author (Ben Bennick) suggests that although the mechanism of northward long-shore drift of sand is recognised as a significant driver for the erosion of many beaches, it raises the question of whether the Tweed River sand bypass scheme actually affects beaches further to the south. It is suggested that this is as far south at beaches such as Kingscliff or even those at Byron Bay. The Tweed River sand bypass scheme was introduced to stop the mouth of the Tweed river from being constantly dammed by sand deposited at the mouth. The closing of the mouth of the river would adversely affect water quality in the esturine reaches of the river. It has been operating for more than a decade now and Ben is worried that this might be affecting more than the Tweed River. The bypass scheme has been active since approximately 2001.

Ben suggests that during some times of the year sand would actually migrate to the south, contrary to the potentially simplistic concept of inexorable northward sand migration. As discussed in my previous post about long-shore sand drift, the action of the East Australia Current travelling south actually does not have the effect of causing sand to drift along the coast instead currents generated by the prevailing wind direction means that there are smaller coastal currents which tend to travel in a northward direction.

But Ben does raise an interesting question and rightfully this questions the absolute nature of the eastern Australian coastal currents. Maybe the situation does arise where sand can actually be transported from north to south from time to time. I wonder if such a phenomenon would be great enough to transport sand from the Tweed as far as Byron Bay and beyond? This would find a culprit in the Tweed River sand bypass scheme and would show us that the coastal strip is even more fragile than is already assumed.

In addition to the above comments I also suggest that local knowledge is very important to reconstruct the recent history of our area. Sometimes it is the bloke who has visited the holiday camp at Broken Head for the last 40 years who has some important observations to share. Local knowledge might be pointing to something we are missing. But, and a big but, there are also times where local knowledge is actually completely flawed! Tibby et al. (2007) demonstrated that the recollection of the behaviour of the sand bar at Lake Ainsworth near Ballina was often quite different to what was revealed in aerial photographs, indeed many anecdotal observations which were considered high reliability were in fact impossible when compared with historical photographs.

So, what does this mean? I think it requires someone with a good coastal management background to put us straight. Southern Cross University, despite its shortcomings has an excellent coastal management school. Maybe the answer is not known at the moment, in which case maybe this knowledge gap can be filled. It might just be that Frazer Island is indeed made from 100% Kingscliff and Byron Bay sand, and that is the way it always was. The sand dunes along the coast hide many a change to the coastline in the last 100 000 years, we can't claim to know what caused more than one or two of the many changes during this period and they are generally natural things like extended storm systems... but you never know.

References/Bibliography:

*Tibby, J., Lane, M.B. & Gell, P.A. 2007. Local knowledge and environmental management: a cautionary tale from Lake Ainsworth, New South Wales, Australia. Environmental Conservation V34.
*White, M. E., 2000. Running Down, Water in a Changing Land. Kangaroo Press.

Where the river joins the sea

In previous posts I've discussed a few peculiarities with the way some of our rivers flow, in particular the Clarence River which once ran backwards and the Wilsons River which flows away from the sea. This post is about another strange feature of the Northern Rivers which is the way many of them discharge into the sea.

Many people in the region will be aware of various issues with regard to erosion of sand our beaches or even deposition of sand choking river and creek mouths. Many people may be aware of Byron Shire Council having a policy of planned retreat from the areas along Belongil Beach at Byron Bay. Others may have heard of the silting up of Nambucca Harbour. But even less will realise that the biggest cause of these different problems is actually the same.

Richmond River mouth at Ballina. Note the white water of the Bar.
 Because of longshore drift the Ballina Bar is often treacheous.

But, let me back up for a moment. Have a look at Google maps or (even better) a paper map of the north coast of the New England / New South Wales area. Look at most of the major rivers. The Nambucca River, Clarence River, Richmond River, Tweed River. Look too at some of the smaller streams such as Tyagarah Creek, Cudgen Creek and others. What you might notice about all these streams is that they seem to flow north and roughly parallel to the coast only a short distance inland. They also join the sea on the southern side of headlands and on the northern side of long sandy beach systems. And therein lies the cause.

Along the coast of Eastern Australia are currents, the most well known is the Eastern Australian Current that flows south. However, the prevailing wind conditions which blow from the south to the north means that the direction of small currents and wave action is directed northward, these are called longshore currents. This has been the case during the Holocene (for many thousands of years) and has resulted in enormous amounts of sand being transported slowly up the coast line, where much of it ends up in southern Queensland forming Fraser Island.

Where the most direct route for the regions rivers would be to join the sea at right angles, longshore drift has caused sand dunes to build up sometimes even to the extent that it sometimes closes the mouths of the rivers. The movement of the sand has slowly pushed the river mouths further and further to the north until the come to an outcrop of rock which blocks the way. At this point the river mouth will cease to migrate along the coast and remain relatively stable until some storm, flood or man-made change occurs. A great example of a man-made change is Coffs Harbour, but more on that another time.

But why does the beach erode in many other places? Well, simply it is the impact of the headlands. On the northern side of the headlands along our coast there is only a little supply of sand (since the headland directs the sand away). Instead this is were sand is sourced to be transported north along the beaches. Places like Belongil Beach at Byron Bay are excellent examples where sand is naturally carried away northward along the edge Byron Marine Park, leaving houses built next to the sea at risk of being destroyed by the erosive processes.

As an aside, longshore currents are also partly responsible for the creation of some mineral deposits which have historically been mined. But more on that in a future post. 

Since I wrote the above, an anonymous comment raised an interesting point which quite reasonably raises questions my statements about the sand stability north of Byron Bay headland. I have reproduced the comment in red below:

Despite the position of rock headland anchor points and the change in coastal alignment along Northern NSW, any differential in longshore drift rates (sand losses from the sediment budget)should have equilbrated during the Holocene period, including sand losses into the deepwater sand lobe off Cape Byron. Erosion at Belongil Spit is more likely due to the interrupted supply caused by the Richmond River breakwaters at Ballina.

Bibliography/references:

White, M. E., 2000. Running Down, Water in a Changing Land. Kangaroo Press.

Coraki has its faults

Coraki is a nice little town on the Richmond River just near its confluence with the Wilsons River. The town is located on the flood plain and therefore many parts of it can be inundated in the case of major floods. The flood plain provides a relatively fertile plain that grows excellent pastures and much sugar cane, especially the further down stream on the Richmond you go. But Coraki has its hidden faults.

Being an active flood plain the area surrounding Coraki is dominated by recent alluvial deposits generally of Holocene age but with lots of slightly older Pleistocene alluvial and estuarine sedimentary deposits. Areas that are under permanent shallow unconfined ground water influence tends to retain pyrite which is produced by bacteria in an anaerobic (oxygen poor) environment (i.e. under stagnant water). When this pyrite is exposed to the atmosphere or more oxygenated water by the action of drainage for agricultural, construction or flood mitigation purposes the pyrite oxidises. Pyrite is Iron Sulphide (Fe₂S) which with water (H₂O) forms H₂SO₄ which is more well known as sulphuric acid. This acid can then be discharged causing degradation to aquatic life or degradation of land creating unproductive acid scalds.

Not all of the town is in the flood plain, in fact about half is located on some low hills that are comprised of Kangaroo Creek Sandstone. The Kangaroo Creek Sandstone is part of the Clarence Moreton Basin and its exposure here may be partly due to a fault called the Coraki Fault. In the area of Coraki and also at Tullymorgan and maybe even places like Clifden near Grafton the faulting of the Coraki Fault has created some unusual features within the Mesozoic Clarence Morton Basin and the underlying Palaeozoic basement rocks. These features cannot be seen on the Earths surface but can only be identified by geophysical techniques, in particular seismic surveys.

So, what are the features that can’t be seen? Well, there is the Coraki fault itself which is a dextral strike-slip fault meaning that the eastern side of the fault has moved northwards relative to the western side. But there is also a weird structure which is referred to as a “flower structure”. This occurs when another fault is present perpendicular to the main fault. This creates a central wedge shaped block which near Coraki has been squeezed by the faults upward and created here, slightly more elevation in the Kangaroo Creek Sandstone and possibly other units of the Clarence Morton Basin. This is probably hard to visualise, so maybe a diagram will help when I can get one to embed.

Blog Note: I like to provide photos for these sort of posts but recently where I store photos (skydrive and/or GoogleDocs) has changed its method for providing URLs to allow embedding of these files and Blogger doesn't like the new URLs. So, these next blogs might be a bit more bland looking until I figure out a better way to store and embed photos.

Note that the stratigraphy of the Kangaroo Creek Sandstone has been recently revised since this blog post. See the this post for details.

References/Bibliography:

*O’Brien, P.E., Korsch, R.J., Wells, A.T., Sexton, M.J. Wake-Dyster, K. Structure and Tectonics of the Clarence-Morton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.