The Woodburn sands of time

I’ve been spending some time working on a project in the lower reaches of the Richmond River Valley. This project got thinking about the stratigraphy and depositional history of that area. Particularly about a unit of unconsolidated sand called the Woodburn Sands (Drury 1982). In some ways this post follows on from a couple of posts that touched on the subject of sea level changes during the Quaternary.

To begin to understand this unconsolidated sediments of the Richmond River Valley we turn to the most recent mapping of the area. Troedson et al. (2004) comprehensively mapped the coastal Quaternary sediments of the whole east coast of NSW. Troedson et al. (2004) demonstrated that over large areas of the lower Richmond Valley there are two units of coastal sand which formed in barrier environments. The most obvious coastal sands are active dune and beach systems formed from a barrier by the action of present day long-shore drift. These active barrier systems occur in many places along the coast. Troedson et al. (2004) also mapped extensive areas of what an earlier researcher Thom (1965) first identified as an inner coastal barrier. This inner barrier is comprised of an old beach system that is no longer active.

Drury (1982) undertook a comprehensive study of the Quaternary sediments of the Richmond River Valley. He confirmed the view by Thom (1965) that there was an old inner barrier system. This system was formed during a higher period of sea level than today and caused regional changes to coastal sedimentation (e.g. I previously posted on the estuarine sediments of the Lismore area). The high sea level eroded away the pre-existing beaches and formed new beach systems a significant distance inland (sometimes 15km or more). Then as the sea level retreated, the new beaches were no longer subject to erosion from the sea and were left intact. The beach systems continued to form on the sea-ward side of the old beaches and eventually built up a very large area of sand. These old beach systems are what made the Woodburn Sands.

The Woodburn Sands occur in a discontinuous zone from Broken Head National Park to the Evans River and the lowest reaches of the Richmond River (Swan Bay). The maximum thickness intersected is about 35 metres, so the sand layers can be very thick.

Like many places in eastern Australia, the action of coastal wave and wind processes can lead to concentrations of heavy mineral sand.  These deposits are called mineral placers. The Woodburn Sands is another of these areas where placers are common. Indeed, a lot of sand mining took place on the north coast to exploit the high concentrations of zircon, ilmenite and even gold. Presently, the Woodburn Sands is not mined for minerals but is used as an important source of good quality groundwater, this includes the regional town water supply authority.

Drury (1982) also included an unusual feature within the Woodburn Sands. This feature was named the Broadwater Sandrock by Mcgarity (1956). McGarity (1956) demonstrated that the Broadwater Sandrock was formed by the cementation of sand by organic rich material probably formed by changes occurring in a peat swamp environment. This sandrock is a common feature up and down the east coast of Australia. Another common feature is the diversity of names given to this material which include ‘indurated sand’, ‘coffee rock’, ‘coastal sandrock’, ‘painted rock’, ‘beach rock’, ‘humate’ and ‘B-horizon of the humus podzol’ (Drury (1982), Mcgarity (1956), Thom (1965) and Den Exter (1974)). Take your pick! I follow the terminology proposed by Drury (1982) who included the Broadwater Sandrock as a member of the Woodburn Sands, i.e. the Broadwater Sandrock member.

Postscript:
Since doing the above post an anonymous commenter has rightly corrected and provided further information. You can see the full comment below, the comment much more accurately describes 'coffee rock' formation but I reproduce this section specifically:

...humicrete (coffee rock) forms as the B-horizon of a fossil soil on sand (a podsol). It is NOT a sedimentary layer itself ie NOT a stratigraphic unit, so should not have been referred to as a "member" ...

As such, I have now changed my mind! The Broadwater Sandrock member is not the best name after all. It seems that 'B-horizon of the humus podzol' is indeed one of the best ones. Humicrete is another good one. Well, it seems that the diversity of names will probably continue, but we can remove the one I thought the simplest (Broadwater Sandrock member) from the list!

References/bibliography:

*Den Exter, P.M. 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, NSW. PhD thesis. University of New South Wales, Kensington.
*McGarity, J.W. 1956. Coastal sandrock formation at Evans head, NSW. Proceedings of the Linnean Society of New South Wales. V81 p52-58.
*Thom, B.G. (1965). Late Quaternary morphology of the Port Stephens-Myall Lakes area, NSW. Journal of the Royal Society of New South Wales V98 p23-36.
*Troedson, A., Hashimoto, T.R., Jaworska, J., Malloch, K., Cain, L., 2004. New South Wales Coastal
Quaternary Geology. In NSW Coastal Quaternary Geology Data Package, Troedson, A., Hashimoto, T.R. (eds), New South Wales Department of Primary Industries, Mineral Resources, Geological Survey of New South Wales, Maitland.

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.

Pacific Islands on holiday to the North Coast

I often find some stories in newspapers touch too lightly on the subject of geology. These articles are often quite limited in scope and generally indicate quite simplistic notions of natural processes. This morning when reading a local newspaper The Tweed Daily News, I came across one such article. A link can to the article can be found here. This article is interesting because it covers some surprising points, but as Dr Malcom Clark an Environmental Geochemist from Southern Cross University implies in the article, there is more to the story than just a once-off beaching of pumice on Kingscliff Beach.

Pumice is a highly vesicular (aerated) volcanic glass. It is created when super-hot, highly pressurized rock is violently ejected from a volcano, especially those found in volcanic island arcs which are near active subduction zones. The unusual foamy feel of pumice occurs because of simultaneous rapid cooling and rapid depressurization. During the eruption the air bubbles are frozen in the rock. The amount of air trapped means that pumice usually has the unusual property of a rock being able to float on water.

If we work backward in time from the Tweed Daily News article a story starts to emerge on how the pumice on the beach got there. The first thing to note is that there are no active volcanoes on the Australian mainland or close to the eastern Australian coast. So, the pumice must have been brought in from somewhere else. Pumice has been common on Byron Bay beaches for the last few weeks ever since winter storms gave a good battering the coast in June. But a large amount of Pumice was also observed on the Queensland sunshine coast in April following late summer storms and the tail ends of cyclones. The storms force floating materials like rubbish and pumice onshore. This gives a clue about movement. It has taken a month or two to travel down the east coast on prevailing currents such as the south moving Eastern Australian Current. But there are no active volcanoes in Queenland either.

Bryan et al (2004) published an interesting article in Earth and Planetary Science Letters on pumice that was washed ashore all down the east coast of Australia in 2002. Here lies more of the answer. Bryan et al (2004) demonstrated that the pumice rafts were transported a vast distance across the Coral Sea and South Pacific Ocean, taking about almost a year to complete its trip on the prevailing currents and winds (the pumice was even blown backwards at one stage by a tropical cyclone). Surprisingly the 2002 Pumice landfall came from the Tonga area (North of an island and seamount chain that stretches to New Zealand called the Kermadec Islands), which is a long way away! Between the Kermadec Islands and Australia lies the Solomon Islands, Vanuatu and Fiji which all have active volcanic systems.  However, The pumice that washed ashore in 2002 was erupted in a submarine volcano (underwater) un-excitingly named Volcano 0403-091 from the Kermadec Islands and swept past all the other islands.

As for the current pumice landfall, in the last year there has been several eruptions of island arc volcanoes the Vanuatu islands, but significantly in July 2012 there was a major eruption of pumice from the vicinity of the Havre Seamount in the Karmadec Islands (Smithsonian Institute 2012). The time between eruption and East Australian landfall is interesting because it is similar to that for the 2001-2002 event discussed by Bryan et al (2004). More recently in 2012 an article was published (Bryan et al 2012) that demonstrated that rapid and long distance movement can be a frequent occurrence. So, maybe the pumice on our beach today this is just a little bit of history repeating – a bit of a pacific volcano on a holiday to the north coast of New South Wales.

Postscript:

Scott Bryan sent me this email yesterday. Being so informative I thought I should post it here.

Hi Rodney,

...I was actually at point lookout (nth Stradbroke) today collecting the pumice. The pumice is indeed from the Havre submarine eruption in the Kermadecs last year. There is a good summary of the eruption and discovery of the pumice rafts at the global volcanism program of the smithsonian institution (USA) at www.volcano.si.edu. 

This pumice is distinctive in being white when fresh; there is also a lot of grey/dark grey pumice at north stradbroke which is from tonga and the previous eruptions I have published on. It has been eroded out of the beach dunes.

The main influx along our shores began in mid-late march, continuing up to early May. There has been a bit of a break, but with the windy and wild weather this last weekend, some more pumice has come in, as well as probably reworked material (abraded and cleaned of attached biota) which seems to be what has washed up at Kingscliff. Newly washed up pumice will be covered in a black or dark green slime (Cyanobacteria) and be loaded with lots and large goose barnacles. You will also find on closer inspection, some molluscs, bristle worms (feeding on the barnacles), bryozoans, hydroids, anemones. Look up Denis Riek and his web page www.roboastra.com - he has taken some fantastic close ups of the pumice and biota found on it at Brunswick Heads.

This pumice has travelled about 3000 km in 8-12 months. We have observed it as far north as Heron Island.

Let me know if you need more info.

I would appreciate further reports of any new strandings as I have a Masters student beginning her research on this pumice and the attached biota. New strandings give us a temporal perspective as the biota mature and diversify with time and also begin recruiting species locally.

References/bibliography:

*Bryan, Scott Edward, S., Cook, Alex, Evans, Jason, Hebden, Kerry, Hurrey, Lucy, Colls, Peter, Jell, John S., Weatherley, Dion, & Firn, Jennifer (2012) Rapid, long-distance dispersal by pumice rafting. PLoS ONE, V7.
*Byran, S.E., Cook, A., Evans, J.P., Colls, P.W., Wells, M.G., Lawrence, M.G., Jell, J.S., Greig, A. & Leslie, R. 2004. Pumice Rafting and faunal dispersion during 2001-2002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga. Earth and Planetary Science Letters V227.
*Smithsonian Institute 2012. Havre Seamount. Bulletin of the Global Volcanism Network. Smithsonian Institute. September 2012.

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.

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.