This is what one aquifer looks like

In some amazing places you can immerse yourself in an aquifer. These places are rare and dominated by a rock type that does not occur in any substantial amounts in our region. However, people dive in the sub-terrainian waters of the limestone caves of the Nullabour in South Australia. The best aquifers in our region do not contain large caves compared with limestone areas. They are hosted in riverine alluvial sediments, fossil soil horizons in volcanic rocks, or fractures in hard metamorphic and volcanic environments. The main aquifers being on the coastal river flood plains, Alstonville Basalt and the New England areas respectively. However, volumetrically the sources that are very large are those in coastal sands.

Auger containing saturated sand from the Woodburn Sands

This post is an illustration of how one of those coastal sands aquifers looks. I've covered the Woodburn Sands in several previous posts but a quick summary is still needed. The Woodburn Sands are beach and dune sand that was laid down during the last significant interglacial. This was around 130 000 years ago during the Pleistocene period. The sea level was much higher than now and this meant that beach systems were often formed a significant way inland.

From the picture you can actually see what the medium that hosts an aquifer looks like. The Woodburn Sands are just that, sands. The sand grains are mostly quartz but there are also some grains made from volcanic and metamorphic rock fragments. Occasionally you can see grains of heavier minerals that were mined until the 1980's. The sand grains are very similar in size which is typical of wave and wind sorting. There is a very small fine fraction of clayey material.

Where the clay content is higher the ability of the water to flow through the aquifer is reduced. This is why some bores can only produce a small amount of water compared to the huge volume that is in the whole aquifer. This is an example of why aquifers tend not to behave as underground lakes. You can pump water out of one end and run out because the hydraulic conductivity (flow velocity) is not high enough to allow the water at the other end of the aquifer to flow in.

The Woodburn Sands is not the only important coastal sands aquifer in the region. Another very important water source include the Macleay sand coastal aquifers. These aquifers were formed in a similar way to the Woodburn Sands and are used for similar purposes. Usage includes irrigation, stock, domestic use and town water supply for places such as Kempsey and Evans Head. There are also some interesting arsenic contamination issues in one aquifer system (Stuarts Point) in the Macleay area which I will post on in the near future.

The similar characteristics of the coastal sands aquifer systems in the North Coast area has motivated the NSW state government to develop a Water Sharing Plan for these systems as a whole. The Water Sharing Plan is expected to be formally adopted this year (2014). Local governments regard groundwater from the coastal sands aquifers as very important. Rous Water has recently adopted its future water strategy which identifies coastal sands as the main source of additional information in the medium to long term and Mid-coast water have recently increased their production of groundwater for drinking too.

Available here is a presentation by the NSW Office of Water on the overall coastal sands systems in North East New South Wales

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.

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.

How cold was it? Glaciers in New England?

I was very, very pleased to receive this comment by Bob (a physical geographer by ‘trade’) a bit more than a week ago. Bob posted the comment on my blog about Point Lookout and has been reproduced below:

"Rod,

You and your readers might be interested in a discovery made around Ebor and Guyra just last week, when I was working with a team of glacial buffs from UQ and UT. We found definite evidence of periglacial activity, presumably from the Last Glacial Maximum, ~20 000 years ago. The best examples were at Guyra on the slopes around Malpas Dam -- clear evidence of solifluction lobes, rock glaciers, snow hollows, and other freeze-thaw features, and what could only be described as incipient cirques -- ponds and bogs sapping back into the escarpment and probably still holding some snow in the frigid Guyra winters. The curious point is that they were between 1200 and 1300 metres, not at the highest points above 1500m where we were expecting them, but all on steep south facing escarpments.

One intriguing feature near Point Lookout, on the road up where it crosses the Little Styx River and at 1450m, was what looked very like a terminal moraine. This of course is a glacial, not a periglacial feature, so it is very hard to believe-but have a look at it, and see what you can make of it. There were at least 16 glacial cycles in the Pleistocene, so maybe one of them at least was really severe, and glaciated this far north. Otherwise, you have to go back to the Permian...

Bob H."


probable solifluction lobes and terraces on hill slope at Malpas Dam

This is exciting stuff to hear about because to my knowledge there is little or no evidence of cold climate landforms in the region. In fact I think the areas of the Tasmania are probably the only areas in Australia were these are frequent, though they have also been possibly identified in the Southern Alps and areas of Victoria. Certainly authors such as Petherick et al. (2011) and Hope (2005) did not identify such equivalent indicators of how far north such extreme cold could be detected. I understand that the cold climate landforms that Bob mentions have been found by researchers various universities such as from the University of Queensland, University of Technology, Sydney and the University of Tasmania. Acting on the tip off from Bob, I found an example of solifluction right on the northern side of Malpas Dam using Google Maps, this one is actually visible from the lookout on the southern side of the dam too (sorry about the quality of the photos, it was a long time since I took them and I didn’t realise what I was looking at that time). Get on Google maps and visit the dam yourself and have a look.


Same solifluction lobes visible south of the hill (same hill as picture above)

Solifluction is caused from the thawing of surface layers of permafrost during the summer leading to the thawed part of the soil profile slipping over the un-thawed permafrost and creating ‘lobes’ of soil. The cold climate structures that have been identified near Guyra are present on the southern side of the hill slopes where the sun was unable to melt much of the ice in the soil and therefore creates conditions of permafrost. Permafrost is not present anywhere on the Australian mainland today and demonstrates a significant change in climate has occurred (though those that know Guyra will still argue it is still uncomfortably cold there!).

What Bob appears to have found near point lookout is even more incredible, as a moraine is formed through the action of glaciers which are accumulations of ice on the surface that slowly moves through a landscape under the action of their own weight. Glaciers in Australia were thought to be limited to Tasmania and the Snowy Mountains. No doubt we will expect to see some published papers on the structure and context of these cold climate features in the region some time in the near future. I can't wait to read the published work that comes from this discovery. 

For further information on the individual cold climate features described above by Bob  visit the glossary, an online encyclopeadia or a good physical geography book such as Geosystems by Christopherson. If you are a little unclear about the locations of these sites and how they fit into a ‘Northern Rivers’ blog then it is worth mentioning this part of the New England, at Guyra and Malpas Dam are right at the head waters of the Gara River which is a tributary of the Macleay River that runs through Kempsey. The New England Highway in the Guyra area is pretty close to the actual crest of the catchments of the northern rivers, with the rivers to the west of it flowing into the Murray-Darling Basin and those to the east to the Pacific Ocean. Point Lookout is in the headwaters of the Bellinger River which runs trough Bellingen and is also part of the headwaters of the Macleay too.

References/Bibliography:

*Christopherson, R.W., 1997. Geosystems Wiley.
*Hope, P. 2005. The Weather and Climate of Australia During the Last Glacial Maximum. University of Melbourne, PhD Thesis, unpubl.
*Petherick, L.M., Moss, P.T & McGowan, H.A., 2011. Climatic and Environmental Variability During the Termination of the Last Glacial Stage in Coastal Eastern Australia: A Review. Australian Journal of Earth Science V.58.