Thursday, 21 August 2014

Facebook, night times and Iceland volcanoes

The Northern Rivers Geology blog has nearly a hundred thousand page views! I’m very excited about how much of a success this blog has been. I do want to continue to reach out to a large audience so I’ve taken a reluctant step into social media. I have set up a Facebook Page for the blog which contains links to the latest blog updates as well as other interesting geologically related matters. Let's see how it goes. If you are a Facebook user please follow me. Followers on Facebook can post their own interesting information or even ask questions. Of course readers can continue to comment as usual on this blog.

A picture of sandstone with rock hammer marks at night: doesn't really work.
On another matter, I was travelling into Queensland earlier this week via the Mount Lindesay Highway. It is a scenic but winding road (I was going to use the adjective ‘windy’ but since the weather that day was windy it could have got confusing - crazy English language). I noticed many interesting road cuttings including what appeared to be a coal seam. But, I could not stop because time was against me. I returned back the same way that night so I thought I’d try and do something I’ve never done night-time geology. It was not very successful. Trying to identify the larger scale features in a cutting by torch and car light is not an easy task. Even trying to look at sand grains was beyond me. My assessment of night time geology, don’t bother!

On yet another matter, I want to draw readers attention to this weeks volcanic activity in Iceland. Some interesting things are occurring under glacial cover. The only way that geologists can have an idea of what is going on is by measuring earthquakes. The number of earthquakes around two important concealed volcanoes has been in the many thousands this week. This has made processing the data time-consuming but presently the seismic records indicate some strange "goings on". In particular it appears that magma from the volcano Bardarbunga has made its way into fissures that are part of another volcano, Grimsvotn. This is quite unusual and with the added background these volcanoes have a very turbulent history and have taken many lives, all the more complicated. Volcano café has a good summary of the situation.

Friday, 8 August 2014

Crystals or No Crystals?

The landscapes of the mountains surrounding the Tweed Valley are very spectacular. I have discussed some of the facets of the Tweed Volcano and Mount Warning area in previous posts. However, I have not covered much on the main rock type that is mainly responsible for the rugged steep cliffs and valleys of the Nightcap National Park World Heritage Area. This rock is the Nimbin Rhyolite, a quartz rich lava that was dominant in the final phases of the Tweed Volcano. Because of its resistance to weathering it results in inspiring cliffs and rugged ranges.

Rhyolite is a volcanic rock that contains a high volume of silica (quartz) in it. Because of the silica content rhyolite lavas tend to be “sticky” and slow moving. This also causes gases to be trapped in the lava or magma chamber feeding the lava flows. The release of trapped gases can cause explosive eruptions. Therefore, accompanying the lava flows there are also deposits of volcanic ash and glass caused by the rapid cooling of lava during explosive eruptions. All of these features are present in the Nightcap Ranges and surrounding areas.

In a future post I will show a picture of a Nimbin Rhyolite lava which exhibits flow banding. There are many examples of flow banding in lava near Minyon Falls. It is a tricky lava to look at in hand specimen because it is very fine grained. You can only see occasional tiny specks that are crystals but most of the time it is just a grey mass. In outcrop you might see some flow structures like the one pictured, but generally it is a boring looking rock! The same rock is in the Mount Matheson area. Smith and Houston (1995) referred to this rhyolite as crystal-poor rhyolite. It compares very differently to the crystal-rich rhyolite identified elsewhere in the area.

As for the crystal rich rhyolite, I was lucky enough to go for a walk in a property that has just been purchased by the NSW National Parks and Wildlife Service. It is located in the valley between the Goonengerry and Nightcap National Parks. While inspecting the excellent work done to remove exotic weeds from this property and celebrate the inclusion of an important vegetative link between National Parks. I came across some good examples of the crystal-rich rhyolite. In these samples the rock contains large quartz crystals which are very evident (see the picture below). The more crystalline form of rhyolite occurs in about a third of the total area mapped as rhyolite. This includes the area from the Koonyum and Goonengerry ranges in the east to Whian Whian in the west.

Quartz crystals in Nimbin Rhyolite - upper Coopers Creek area
Smith and Houston (1995) observe the crystal abundance is related to the vent (or group of vents) from which the lava was erupted. Only occasionally do crystal rich and crystal poor varieties occur on top or under each other indicating a high degree of lava mixing. The relationship between specific vents and crystal richness shows the vents must have been tapping different magma sources (different magma chambers). Alternatively the vents may have erupted magma from a single, somewhat heterogeneous magma chamber.

However, it is worth noting there is a third major form of rhyolite in the area and is known as the volcanic glass, obsidian. This volcanic glass occurs around the bases of the major lava flows and is often referred to as perlite. The glass is rarely a massive unit but tends to appear brecciated and as an agglomerate. I will discuss this obsidian further in a future post as many interesting features and textures are preserved showing the way that rhyolite lavas move across the lands surface. In the mean time, it is worth remembering that lavas ain’t just lavas. There can be many differences which provide a window into how the landscape was formed.

Wednesday, 23 July 2014

The Great Dividing Ranges and Stonehenge

Granites occur throughout much of the north coast and New England region. I use the term granite here loosely, in reality the rocks I’m referring to have a range of compositions and ages. The things they have in common are their relatively high quartz content and they are igneous intrusive (plutonic) rocks. They have cooled slowly and therefore have allowed large crystals to form – giving them that typical granite appearance. I’ve covered a few granites in previous blog posts but in this post I’ll cover one New England “granite” called the Wards Mistake Monzogranite. I’ll continue to cover others in future posts.

Stratigraphically the Wards Mistake Monzogranite is part of the Wards Mistake Suite which in turn is part of the Uralla Supersuite. The Wards Mistake Monzogranite outcrops in a relatively extensive area between Glen Innes and Guyra. In places it straddles the Great Dividing Range but mainly occurs just on the eastern side within the upper reaches of many Clarence River tributaries. The unit was formed around 250million years ago, during the Lower Triassic to Lopingian (early Permian period).

The Wards Mistake Monzogranite consists of monzonite (a rock containing moderate quartz with equal parts potassium and sodium-calcium feldspar) with some granodiorite (abundant quartz and calcium-sodium feldspar). It has a typical equigranular black and white speckled appearance which is common of the Uralla Supersuite. It is like the other Uralla Supersuite granites as it is derived from the melting of other igneous rocks - I-Type Granites (Bryant et al 2003). However, it does contain some xenoliths (inclusions of other rock) which are sedimentary. It is possible that when the Wards Mistake Monzogranite was emplaced into the crust it incorporated bits of the surrounding sedimentary rock. This may have affected the chemistry of the magma and may be one of the reasons why there is both monzonite and granodiorite in the unit.

Typical tor outcrops of the Wards Mistake Monzogranite near Glen Innes
Many New England granites contain mineral deposits. Being an I-Type granite usually means a good chance of mineral deposit formation. However, the Wards Mistake Monzonite contains very sparse mineralisation with only a few small areas where there is some alteration zones that have more concentrated ore minerals. These include wolframite (tungsten), molybdenite (molybdenum) and cassiterite (tin) (Brown 1997). Other surrounding granites such as the Kingsgate Granite and Red Range Leucogranite have abundant mineralisation that was historically mined and is still under active mineral exploration permits.

A lovely feature of most New England granites is the interaction with the climate. This produces wonderful looking granite tors. This is a result of onion skin weathering (frost wedging) where water penetrates into the rock and freezes during the cold winters. This repeated action causes large flakes of rock to peel off. Some of these Tors are given their own names. In the Stonehenge area on the New England Highway you can stop and walk among these Tors and see the Balancing Rock which looks like it will topple over at any moment.

The landscape around Stonehenge between Guyra and Glen Innes is my favourite landscape in Australia. The high country agriculture, the cold weather and the geological conditions that form the rolling hills and special tors make it a special place. The picture above is of a portion of the Wards Mistake Monzogranite and partly shows the landscape I’m talking about. The accessibility of the granite is certainly worth a quick stop if you are travelling on the New England highway.

References/bibliography:

*Barnes, R.G , Willis, I.L. 1989. Preliminary geological plan of the 1:250 000 Grafton-Maclean sheet area - SH 56-6, SH 56-7. New South Wales Geological Survey Report

*Brown, R.E. 1997. Mineral deposits of the Glen Innes 1:100 000 map sheet area. Geological Survey of New South Wales. Quarterly Notes 103 p1-19

*Bryant, C.J. , Chappell, B.W. , Blevin, P.L. 2003. Granites of the southern New England orogeny. In Blevin, P. et al (eds) Magmas to Mineralisation: the Ishihara Symposium Geoscience Australia. Record 14 - extended abstracts.

Sunday, 6 July 2014

The Orara Formation and the reviewed Kangaroo Creek Sandstone

There has been an increase in our understanding of the Clarence-Moreton Basin in recent years. The central upper portions of the basin have had several cored and un-cored boreholes drilled during exploration for natural gas, especially Coal Seam Gas. In this post, I will cover the implications of this exploration on our understanding of the Kangaroo Creek Sandstone and the recognition of another previously unknown unit.

In a previous post I described the Kangaroo Creek Sandstone. This unit was identified as a formation directly overlying the Walloon Coal Measures (and the MacLean Sandstone Member of the Walloon Coal Measures) (McElroy 1963). However, a recent paper (Doig & Stanmore 2012) attempts to resolve newly identified characteristics which have resulted in the authors proposing the creation of a new formation called the Orara Formation. It is proposed by Doig & Stanmore (2012) that the Orara Formation is comprised of two member units. These are another new unit called the Bungawalbin Member and the now demoted Kangaroo Creek Sandstone Member.

Doig & Stanmore (2012) found that the proposed Bungawalbin Member differed substantially from the Kangaroo Creek Sandstone and overlying Grafton Formation. They describe the Bungawalbin as between 45m-115m thick and dominated by mudstone and carbonaceous mudstone that is thinly bedded with fine grained sandstone with increasing amounts of massive, medium-grained quartzose sandstone beds near the base of the member. The unit is interpreted as a persistent low energy riverine floodplain environment.

The Bungawalbin Members contact with the underlying Kangaroo Creek Sandstone Member is transitional. With the medium-grained quartz rich sandstone becoming dominant in the Kangaroo Creek Sandstone. The composition and formation history of the Kangaroo Creek Sandstone has not been questioned but the significant fine grained component of the Bungawalbin Member necessitates the distinction between these two members. Additionally, Doig & Stanmore (2012) clearly demonstrated that the geophysical response of the Bungawalbin Member was substantially different from the Kangaroo Creek Sandstone.

Hence, we are learning more about the land on which we live. The geology is slowly becoming better understood. It is interesting to observe that there has been no detailed work on the upper most formations of the Clarence-Moreton since the 1960’s. The drilling that has occurred has unlocked more hidden characteristics of the basin. It helps our understanding of basin history as well as the original intention of finding gas resources. There is more to cover in future posts including understanding of the basins youngest formation, the Grafton Formation, but that will come soon.

References/bibliography:

*Doig, A. & Stanmore, P. 2012. The Clarence-Moreton Basin in New South Wales: geology, stratigraphy and coal seam gas characteristics. Paper presented at the Eastern Australasian Basin Symposium IV. Brisbane.
*McElroy, C.T., 1963. The Geology of the Clarence-Moreton Basin. Memoirs of the Geological Survey of New South Wales, Geology. 9.

Tuesday, 1 July 2014

On the Rocks

Three of the four in my family has been hit by the Flu. The only one that thankfully doesn't have it is the newborn. I hope he doesn't get it. Now, I wish I had taken that free flu-shot that my work offered me this year! I've infected most of the family! Obviously this means that original blog posts are going to be a bit light on for a little while. So, in the mean time I can recommend some interesting bits and pieces on the internet.

On the Rocks with Wangiwriter - Catherine Hill Bay
Today Wangiwriter presented a very interesting blog post on some of the rocks of the Newcastle area, part of the Sydney Basin. The pictures are lovely and Wangiwriter tells an interesting story. I know this is a bit out of the usual subject area but the piece is worth a good look. I've been to Catherine Hill Bay and loved the scenery. The geology was typical of a section of the Permian aged Sydney Basin but well exposed and situated in a stunning historical landscape. The conglomerate shown so well in Wangiwriters photographs are part of the Newcastle Coal Measures. Specifically, they are either part of the Moon Island Beach Formation or the Boolarroo Formation. I don't know Newcastle geology well enough to know exactly which.

Thanks to everyone that has been supporting my blog. I've noticed an increase in my advertising revenue of late. Now I'm up to at least a cup of coffee a week! I'm amazed at the value of some advertising clicks. Some earn more than $1, others just a few cents. I got excited when I see that Google payed my almost $1.70 for one click. I wonder what that advertisement was! I'll never know because Google handles all the advertising in the background.

Tuesday, 24 June 2014

Earth Learning Idea

I have recently come across an interesting and easy to understand site that explains earth processes in fun and creative ways. This is a site called Earth Learning Idea. By way of examples here is a demonstration of sink holes that you can try yourself (just like those I mention in the addendum to this earlier blog post):
http://www.earthlearningidea.com/PDF/185_Sink_hole.pdf.

I'm pleased to see such an interesting site. It made me wonder whether I should be more pro-active in earth science education. Recently helping to sort out a rock collection for a local environmental centre is something that I enjoyed. I've also been honoured by an invitation to do a formal presentation at this years Big Scrub Rainforest Day. I'm happy to do these things because I enjoy geology and I enjoy talking with people.

Interestingly, as a member of the Geological Society of Australia I can be more formally aligned with geology education by joining the Specialist Group in Geological Education. I just don't want to stretch myself too far as I'm already a member of three specialist groups: the Environmental Engineering and Hydrogeology Specialist Group, Specialist Group in Geochemisty, Mineralogy and Petrology and the Specialist Group in Vocanology (Learned Australasian Volcanology Association). I have another 6 months to decide... I think it is good to take time in making decision... even if this is a little one.