Friday 8 March 2013

Blog update #4

30,000 page views! I'm amazed how many page visits I've had, I'm very pleased and thankful for the support this blog has received since it started. Though admittedly about 10-15% of these views appear to be bots, the balance are real people some of which have become public followers and many comment. Thank you!

To try and make it easier for people to comment I removed the verification requirement for posting, alas, after about three weeks of a a huge amount of spam I decided to reintroduce the verification. During the time that verification was off the amount of visits per day almost doubled but the visitors during this time was about 50-60% bots. So, that also discouraged me from keeping the verification off. Sorry to those, which is just about everyone, who struggle with the verification codes, it is so hard to read them sometimes.

While I'm giving my blog update I should mention a GNS Science blog (from New Zealand). At about the time in our area that a company was drilling dull stratigraphic drill holes to estimate coal and gas reserves near Grafton and Kyogle, GNS Science was drilling something exciting and incredible. For the first time the Alpine Fault in New Zealand was being drilled to understand the nature of the fault and to install long term monitoring equipment. A GNS Science blog by Rupert Sutherland gave the day to day details. It is well worth a read as are the two other personal GNS Science blogs by Julian Thompson and Sara.

As for my blog the most popular posts to date are listed below. You'll probably notice that three of the five are directly or indirectly relating to the controversial coal seam gas topic. I find that a good because people are clearly wanting to know about the science relating to that topic but unfortunately the average time spent on the blog when the search terms include the letters CSG is only around 5 seconds so maybe I'm not providing visitors with the information that they want, but I avoid as much as possible the politically sensitive and I'll continue to do so - I'm not very bold!

1. Why You Wont Find CSG Here
2. Walloon Coal Measures of the Southern Clarence-Moreton Basin
3. Do You Trust a Geological Map?
4. Mythical Geology at the Mouth of the Tweed River
5. Some Musings on Coal Seam Methane

The 5 biggest referrers were:

1. Google
2. Wikipedia
3. Clarence Valley Today (blog)
4. isearch
5. Look and See New England (blog)

Please note that I probably won't get another blog post up this month as our family is away with our daughter in hospital in Brisbane. It looks like she may be in for a fair while so updates will not occur during this time.

Friday 1 March 2013

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.


*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.