Thursday 30 August 2012

What to do with a rock collection?

I am in the process of moving into a new house at the moment but because I have developed a bit of a collection of rocks in the last few years I have developed some peculiar problems. These are the dilemma of whether something that looks boring and would not be of any use to anyone else should be kept? What do you do with the garden once it has become overloaded with broken pieces of rock? How do you transport large quantities of rock without overloading boxes and earning the ire of removalists and friends?

I have a special collection of important rocks that I have labelled and wish to use further in future such as producing thin-sections or undertaking geochemical analysis. But there is so much that is just, well, miscellaneous. Rocks from overseas that look interesting but I can’t remember where they are from, or rocks from around Australia illustrating some salient point (that I can’t remember either), or just some vaguely pretty cobble picked up from a beach. Do I hold on to them hoping to remember what they were important for or just throw them out?... and how do you dispose of rocks anyway?

Some other problems arise with some broken up bits of metal ores, the minerals that make them up can soluble and when this is the case they can be toxic to plants. Speaking of toxic, how do I transport my samples of chrysotile (asbestos)? I’ve got a great piece with a ‘fibrous vein’ of chrysotile through other green serpentine minerals, it would be a shame to triple wrap it in plastic and notify the local council landfill that I intend to dispose of it. If it were not asbestos I’d put it on a shelf for display since it looks so cool.

Oh well, back to packing. I’ll figure it out somehow.

Friday 24 August 2012

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: 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.


*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.

Friday 17 August 2012

A basin in the hills

During the Triassic and into the Jurassic periods (Being part of the Mesozoic era) three major sedimentary basins formed in our region which are preserved today. The biggest, the one most people know about, and the youngest is the Clarence-Moreton Basin. This is a thick sequence of rocks which extends to Nymboida in the South up into southern Queensland. The Clarence-Moreton formed on top of, and with the Ipswich Basin. In southern Queensland it also begins grading into the Surat Basin.

The Ipswich Basin is smaller than the Clarence-Moreton as both basins are formally defined, but various sub-basins within the Clarence-Moreton actually formed at the same time as many of the parts of the Ipswich Basin and several units appear to conformably underlie (there is no time gap in deposition) or even inter-bed with the lower units of the Clarence-Moreton. The Ipswich Basin outcrops in a north-south line west of Murwillimbah and at Evans Head and Brooms Head. It is well known in southern Queensland for large actively mined coal deposits, it is not so well known south of the border and is often confused with being part of the Clarence-Moreton Basin.

The least known Triassic-Jurassic Sedimentary Basin is the Lorne Basin. This is further south than the Ipswich and Clarence-Moreton Basins and for the purposes of this blog will define the southern limit of the Northern Rivers. The Lorne Basin is the smallest of the three Basins. The middle of the basin is located at the village of Kew, it extends west almost to the village of Comboyne, south to Coppernook, almost to Wauchope in the North, and is present on the coast at Camden Haven and Diamond Head (Bob and Nancy have a tour of Diamond Head). The modern day Camden Haven River flows across the basin.

Unlike its contemporaries the Lorne Basin has rather poor pickings as far as coal deposits goes. This at first might be surprising given the thick units of coal formed further to the North and South at the roughly the same time as the Lorne basin was forming. In fact the coal seams found in the Lorne Basin are only of any significance in the units known as Camden Haven Group, and even then these are ‘thin coaly beds’ according to Pratt (2010) and earlier authors. What gives us a clue about the apparent absence of coal is the abundance of another rock type, conglomerate. According to Pratt (2010) there are several units of conglomerate which show that the sediments that were deposited in the basin traveled only a short distance and the river systems that transported these sediments was in a high energy environment (remember that for organic rich sediments to accumulate that will form coal the environment needs to be stable and swampy).

The clasts that make up the conglomerate in the Lorne Basin are derived from the Palaeozoic aged basement rock of the New England Orogen that surrounds the Basin. The clast composition reflects the slight variability in the New England Orogen Rock which is slightly different if the rock came from the north or the south side of the Basin. I will discuss the individual units of the Basin in future posts. But the whole picture of high energy deposition in the basin shows us that the Lorne Basin is a little unusual. It actually appears that the basin was elevated (not low lying like the Clarence-Moreton, Ipswich, Surat, Gunnedah, Sydney etc Basins) and situated in between large mountain ranges, this is known as an inter-montane basin. The well-known examples large active of inter-montane basins are in Asia in places such as Mongolia (these are much bigger than the Lorne Basin). Closer to home the McKenzie Basin near Mount Cook in New Zealand is a good example, although it is a bit smaller than the Lorne.

After the sediments had been consolidated there was a period of faulting through the Lorne Basin and during the Cenozoic era intrusions of granitic rock affected some parts of the basin and it was also partly obscured by lavas from the same era, though much of this lava has now been eroded away. The nearby Comboyne Volcano/volcanic centre was probably associated with these lavas and intrusions. Erosion of the lavas has caused a very attractive landscape including the Ellensborough Waterfall.

Since writing the above post Dylan reminded me that there is a theory that the Lorne Basin was initially formed during a meteorite impact (See comments below). I'll have to dig up some literature and discuss why this might be the case, however, for the time being it is worth noting that according to Tonkin (1998) the overall shape of the basin is very similar to other impact structures around the world. As Dylan points out: we have yet more unanswered questions!


Pratt, G.W. 2010. A Revised Stratigraphy for the Lorne Basin, NSW. NSW Geological Survey Quarterly Notes.
Tonkin, P.C. 1998. Lorne Basin, New South Wales: Evidence for a possible impact origin? Australian Journal of Earth Sciences. V45.

Monday 13 August 2012

Northern Rivers Geology Blog Update #1

I've been surprised about the success of this blog. I set out to put little facts that I knew about the geology of our region so that others who might be interested could pick these up. It has almost been a year of posting and in the last two days the blog had its 10 000th page view. Now, admittedly about 10% of the page views appear to be bots, but there is a greater than 90% chance that visitor number ten thousand was a person.

The most popular posts have been:
1. Why you wont find CSG here
2. Mythical Geology at the mouth of the Tweed River
3. The 'older' rhyolite in the north east
4. Mining and the Bible
5. Geology in the air

The biggest referrers have been:
1. Google
2. Wikipedia
3. Clarence Valley Today (blog)
4. New England Australia (blog)
5. Look and See New England (blog)

Thanks to all my followers for encouraging me with the blog. I hope that everyone continues to find something of interest here. I'll keep adding posts on about a weekly basis... I've got heaps of topics in mind so at this rate there should be another year (or two) worth of stuff before I start slowing down. Who would have thought how much geological diversity and how many natural features there are in the Northern Rivers.

Sorry to those of you from the highlands in the New England area and those from Coffs Harbour and further south, I've neglected the headwaters of the northern rivers and the southern areas but I'll look to fixing these oversights in the coming months. Feel free to leave your comments or ask questions if you wish.

Wednesday 1 August 2012

A magma chamber under Cabarita Beach

Again and again, I am amazed at how little we know about what is under our feet. It often takes an unexpected source of information to reveal some incredible knowledge of our region. The lastest information that has recently come to hand has been the preliminary geophysical survey results for the Grafton to Tenterfield survey. There are many results that may indicate some strange goings on, from some inconsistent features in the Mount Warning area (possibly indicating that the Tweed Shield Volcano might actually be a myth! More of this in a future post or two), to strange lineaments and responses showing hidden intrusions. This post is about just such a possible hidden intrusion in the Cabarita area.

Smith (1999), curiously reported that within the Neranleigh-Fernvale Beds at Norries Head, Cabarita (located on the coast midway between Tweed Heads and Mullumbimby) there appeared to be evidence of thermal metamorphism in the rocks there, but no evidence of what caused the heating. Metamorphism is a characteristic of the Neranleigh-Fernvale Beds, but the style of metamorphism is pressure related due to the formation being accreted (squashed) onto the Australian continent during a period of subduction during the Palaeozoic period. Not much heat was generated in this formation and based on the minerals identified in the rocks it is possible to estimate the pressure and temperature when these rocks were squashed. The feature that Smith (1999) identified was biotite crystallisation (a variety of the mica mineral group). This mineral is indicative of heating of rocks to a medium to high grade but the lack of a preferred orientation of this platy shaped mineral shows us that the metamorphism postdates the accretion period. ie. the heating of the rock has occurred some time after the pressure, meaning at least two periods of metamorphism.

As discussed in a previous post, the New South Wales Geological Survey has been collecting geophysical data over the region. One measurement has been the intensity of magnetism (related to the iron content of rocks). Magnetic results can display what is happening under the earths surface, not just on top. It is known to show a characteristic feature where intrusions are known, either a strong negative or strong positive anomaly, depending on the rock type. The picture to the left shows the total magnetic intensity map (courtesy of the 2012 preliminary data package from the geological survey) for the area around Cabarita. I’m sure you can pick out the obvious red and blue anomaly. the pattern is consistent with intrusions, indeed exactly the same feature can be seen in the Mount Warning area (and others that I will discuss in future). As such, I suggest that this anomaly is actually good evidence of an intrusion hidden below the heat affected surface rocks. Smith (1999) thinks that the biotite grade metamorphism occurred during the Mesozoic period (well before the Cenozoic aged Lamington Volcanics) and that there was once a body of molten rock below the ground in this area.

I’m so pleased to be able to see the preliminary dataset, it is obvious that there are many features that can be better understood.


*Smith, J.V. 1999. Structure of the Beenleigh Block, northeastern New South Wales. New England Orogen: Regional Geology, Tectonics and Metallogenesis. Papers presented at a conference at the University of New England.
*Geological Survey of New South Wales. 2012. Grafton Tenterfield Airborne Geophysical Survey: Gridded and imagery data. Preliminary package from the Department of Trade and Investment: Resources and Energy.