Monday, 21 May 2012

Lismore GEMFEST 2012

This year I was able to visit the gem festival GEMFEST in Lismore. I had never been before and I must say that I was very impressed! The number of exhibitors was incredible. I was told it is the biggest gem and lapidary show in New South Wales... or was it Australia? I can believe either. It is a lapidary delight!... even for someone like me who, surprisingly, is not really into gems.

The weather was wonderful, the people friendly, the curry stall smelled great. It was nice to see activities for the kids and exhibitions on using various equipment. The variety of exhibitors was large too, from hippie jewels and alternative uses for crystals to fossils, gems and obscure minerals. My interest was on the less shiny things, on the fossils and unpolished and uncut minerals.

I was struck by the number of specimens, especially fossils, that come from Morocco (in north Africa). But, if you are aware of the most famous paleontological sites in the world this will not be a surprise. Morocco has wondrous numbers and variety of exquisitely preserved Permian aged sea life especially Trilobites. However, my favourite was actually not super expensive but something I still could not afford. It was both sides of some Miocene? aged mudstone that revealed a wonderful small fish and some leaves in the same layer. The vendor had identified the fish and plant but I cannot remember what they were. But, it was a perfect example of identifying the environment in which the fish lived: a still water environment probably a late or maybe an estuary. And the specimen just looked stunning.

Getting late on day one.
Just one of the Vendor avenues.

Since there was so much I could have bought if I had a spare $50 000, I thought the best thing to do was focus on one theme (I had less than $100). Although, I would loved to have obtain some of the wonderful fossils I instead bought from several different vendors, some uncut mineral specimens. The balance of these being minerals with a metamorphic theme such as Kyanite, Staurolite, Forsterite and Almandine Garnet plus some others. I will do a future blog post on Kyanite and how it and other minerals can be used to estimate the temperature and pressure of metamorphism.

I know that if I get the chance I will go next year and get some more metamorphic minerals and maybe if I save enough look at other rock forming and ore minerals and of course, fossils. Anyone that is as boring as me will be delighted... as will others who like prettier things!

Saturday, 19 May 2012

Rocks named after a creek named after an Australian marsupial

Note that the stratigraphy of this formation has been revised since this blog post. See the this recent post for details.

One of the most widely outcropping rock units of the mesozoic aged Clarence Moreton Basin is the Kangaroo Creek Sandstone named after its type locality at Kangaroo Creek in the Nymboida area. It is also one of the most recognisable stratigraphic units in the basin.

McElroy (1963) showed that the Kangaroo Creek Sandstone consisted mainly of white to cream coloured quartz sand. The texture of the sandstone is saccharoidal, that is, it has a glistening sugar like appearance of the quartz sand grains. This sand glistens more than usual because while buried, fluids in the rock caused extra silica (quartz) to crystallise on the existing sand grains creating new tiny crystal faces that reflect light in a vivid way. The nature of the rock in this formation tends to weather less readily than other units and as a result tends to form prominent topographic features such as hills, cliffs, ridges and the like.

Crossbedding and typical saccharoidal texture in Kangaroo Creek Sandstone

The Kangaroo Creek Sandstone was deposited in a fluvial (river) setting and as a result cross bedding structures are very common in outcrops. Sorting of grains in the unit is very well developed, that is, the grain size is very similar at any particular outcrop. Additionally, the thickness of the beds is very consistent which together indicates that the tectonic setting was relatively unchanged through the period of deposition. Following burial of the sandstone fluids present in the rock caused extra dissolved silica to precipitate out onto the existing sand grains filling in voids and creating the characteristic texture.

The Kangaroo Creek Sandstone is considered by some authors (Wells and O'Brien 1998) to grade into the Woodenbong Beds in the north west of the NSW portion of the basin. However, it is noted that others (Willis 1998) consider the Woodenbong Beds the equivalent to the McLean Sandstone Member of the Walloon Coal Measures (but more about this in future post). The Kangaroo Creek Sandstone underlies the Grafton Formation but the contact with this formation is gradational. According to (Wells and O'Brien 1998) it also sometimes shows a conformable boundary with the underlying Walloon Coal Measures, however, in most areas the boundary is shown by an unconformity. It is easy to tell the difference however, because compositionally any sandstones in the Walloon Coal Measures are composed of feldspar and lithic grains rather than the quartz of the Kangaroo Creek Sandstone.

Outcrop of Kangaroo Creek Sandstone on the Clarence River near Grafton
It is interesting to note that the recrystalisation of quartz in the Kangaroo Creek Sandstone means that this unit is now essentially dry with respect to Ground Water. There is very few spaces left for the water to travel through. for example O'Brien et al (1998) shows that most other sandstones in other basins such as the Great Artesian Basin, is where most ground water is obtained. In fact, in the whole of the Clarence Moreton Basin the only unit to have useful ground water bores is the Grafton Formation which is recharged from rainfall. The Kangaroo Creek Sandstone does have some bores that produce a very little water in the upper most portion of the unit (probably rainwater recharging fractures in these locations (Kwantes 2011), like the overlying Grafton Formation) but it appears that no other bores obtain water from the Kangaroo Creek Sandstone because the formation actually behaves like an aquiclude or aquitard. Water is not obtained from aquifers below the Kangaroo Creek Sandstone because the water quality is generally poor.

It is interesting to note that according to some gas exploration results it is apparent that areas of the Kangaroo Creek Sandstone (assuming this is not mistakenly identified McLean Sandstone) that are directly overlying the Walloon Coal Measures contain substantial areas of conventional natural gas. This is gas that has migrated from the underlying Walloon Coal Measures and been trapped in either pore spaces or fracture zones. I understand that several companies in the area such as Metgasco and Red Sky Energy intend to exploit these reserves.

Pollen spores in drill holes give an age of middle to late Jurassic for the Kangaroo Creek Sandstone (Wells and O'Brien 1998).


*Kwantes, E. 2011. Future Water Strategy: Groundwater Options - Position Paper. Report for Rous Water by Parsons Brinkerhoff.
*McElroy, C.T. 1963 The geology of the Clarence-Moreton Basin. New South Wales Geological Survey, Memoir 9, 172 pp.
*Moran, C., Vink, S. 2010 Assessment of impacts of the proposed coal seam gas operations on surface and groundwater systems in the Murray-Darling Basin. The University of Queensland.
*New South Wales Government. 2010. State of the Catchment Report: Groundwater. Northern Rivers Region. Department of Environment, Climate Change and Water.
*Wells, A.T. , O'Brien, P.E. 1994 Lithostratigraphic framework of the Clarence-Moreton 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.
*Willis, I.L. 1994 Stratigraphic Implications of Regional Reconnaissance Observations in the Southern Clarence-Morton Basin, New South Wales 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.

Monday, 14 May 2012

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.

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

Thursday, 3 May 2012

Geology in the air

A few months ago the New South Wales Geological Survey released their preliminary data package for the Grafton-Tenterfield Airborne Geophysical Survey (link to info here). The survey was conducted as part of a program that has been going on for several years known as New Frontiers New South Wales. Nearly all of the state has now had high quality data obtained for it instead of the old, poor quality data for the whole state. But why has this been done?

Area of latest survey from NSW DPI
The regional survey has been conducted to better understand the geology of the region and as a result this information may guide mineral exploration as well as provide additional data that can help land management especially in relation to soil transport, erosion potential and soil chemistry. This is done through three primary measurements: radiometrics (measurement of natural radiation), magnetics (measurement of how magnetic the earth is), and DEM (measurement of the elevation of the earth). How these can be used to understand the earth through the aerial survey is outlined below:

Radiometric measurements show the relative abundance of three naturally occurring elements in the rock and soil these are Potassium, Thorium and Uranium. The different ratios of these elements can distinguish broadly between different rock types without the need to visit the site and the intensity can be used to do the same. If an area shows up as being rich in radioactive elements it may be underlain by a granite or similar rock as the radioactivity is directly related to the mineralogy of the parent rocks. If an area is eroded the visibility (or comparative lack of) of radioactive minerals may be seen in downstream. The different types of radiation also provides an indication of the chemistry of the soil and rock. This process can only show the radiation on the surface (e.g. in soils, sediment and exposed rock)

Magnetic measurements do a similar thing as far as identifying elements. But instead of radioactive elements it shows the abundance of iron especially in the form mineral magnetite. Again, the geophysical response will be based on the rock mineralogy. Rocks such as basalt will have a strong response (more iron) and so will wet areas. However, unlike radiometrics, magnetics can show deeper structures in the earth such as buried plutons, faults, dykes and others. This means that drilling is not necessarily required to have a good idea of what lies beneath the surface. It is interesting stuff and the maps produced are often very pretty colours! All of the measurements require “calibration” with a digital elevation model (DEM) which shows the shape of the earth.

Recently (2005?) obtained information in the New England and Tamworth areas have shown up major hidden fault systems, areas prospective for gold, deformation of sedimentary basins that may increase prospectivity for oil and gas, new granite plutons, hidden granites, a better understanding of volcanism including confirmation of new volcanic centres and its associated rock (such as the Maybole Volcano). One aspect that appears promising that the survey will aid in determining the viability for is geothermal energy. Geothermal energy may be obtained from related to deep buried granites and magnetics can be used to find likely locations.

It is interesting to note that I have seen reports in the media (and formal submissions to Coal Seam Gas parliamentary inquiries) that some people have been upset by being “buzzed” by aeroplanes from companies undertaking coal-seam gas exploration. Looking at the timing, this may be related to concern about Coal Seam Gas exploration by several companies in the area and I tend to think that it is likely that the aeroplane may have been part of the regional geophysical survey.

It is important to know just how useful this information is to understanding our planet, even if the information can be used in a way that some do not appreciate. Geology usually has two aspects, one is scientific (understanding the world we live) and the other is applied (using such knowledge for other purposes such as mining). The geophysical information obtained by New Frontiers New South Wales is purely scientific but this knowledge can be applied to Mining but it can also be applied for Environmental purposes. Unlike many others, I am cautious about stopping research because of what it may be potentially be able to be applied to.