Sunday, 23 August 2015

Hillgrove Monzogranite


Hillgrove is known for its mining history. The fortunes of the place have been directly related to gold and antimony mining for more than a hundred years. Armidale in comparison was tiny, a village in comparison with Hillgrove at its peak. Hillgrove still operates a mine for antimony and gold but is now quite a sleepy place with a handful of inhabitants. Most people working in the mine commute from Armidale. But the mine itself is not what I want to write about, it is about the attractive rock that is known as the Hillgrove Monzogranite. Despite its name the Hillgrove Monzogranite is not the extensive source of gold and antimony that is mined in the area. Most of the ore mineralisation is either directly or indirectly related to the nearby Bakers Creek Diorite.

Hillgrove Monzogranite on the Waterfall Way
According to the Australian Stratigraphic Names Database the Hillgrove Monzogranite was until recently known as the Hillgrove Adamellite (Adamellite being the outdated synonym for Monzogranite). It was previously classified as part of the Hillgrove suite which in turn is part of the Hillgrove Supersuite.  However, based on geochemical properties (and possibly just to confuse people) the Hillgrove Monzogranite is no longer considered part of the Hillgrove suite instead just being a member of the Hillgrove supersuite! However, it is clearly one of the plutonic rocks collectively known as the New England Batholith.

Monzonite is unsurprisingly the dominant rock type of the Hillgrove Monzonite. It is an S-Type granite (derived from melted sedimentary rock). It is comprised mainly of quartz and feldspars (roughly equal potassium feldspar and sodium-calcium Feldspar), quartz, biotite mica and hornblende. The biotite often shows a foliation, which is a preferred alignment in the rock. The age of the Hillgrove monzogranite is estimated at between around 270 to 290 million years. To my knowledge, the age has not been directly measured but instead is based on its relationship to the surrounding rocks with their either calculate or approximate ages.

The landscape formed by the Hillgrove Monzogranite is one of my favourites. It forms a relatively large plateau which contains low rolling hills and lovely boulder outcrops. These outcrops often form lovely torrs (see pictures) formed by “onion-skin” weathering. Water enters cracks in the rock and during winter this freezes and expands gradually wedging the layers off the boulder. This is correctly termed frost wedging.

The Bakers Creek gorge has cut into some of the unit near the Hillgrove area but overall the appearance of the country is quite gentle. The rock unit extends a long distance from the location of Argyle in the west almost to Chandler Gorge in the east. The Waterfall Way (Armidale-Dorrigo Road) crosses in and out of the Hillgrove Monzogranite and Girrakool Beds into which it has intruded. Therefore it is an easy stop on the road when travelling this route.

The soils are sandy and not very fertile leading to an area used for cattle and sheep grazing on native and improved sown pastures. The forest is an open dry sclerophyll snow-gum type bush which is one of the typical environments of the New England high country. I love the appearance of this country. It is the quintessential high-lean New England landscape.

Friday, 7 August 2015

Don't you hate it when...

Don't you just hate it when you have information to share but you are not permitted by contracts and commercial in confidence so share it? Our society is more and more being constrained by bureaucratic regulations set up by people who are career managers but have very little understanding of the real world. Little understanding that science and engineering knowledge benefit all people and that some perceived public image issue is more important than the wider good. This means that innovation can be stifled... at least in my personal opinion. The perceived public perception of scientific discoveries hinders the development of knowledge from Climate Change to Panadol!


By way of one a specific local example, I was helping an environmental centre have a ground water bore installed. The Department of Education which runs the environmental centre put absurd restrictions on access to the groundwater. One such limitation was that school children had to wear gloves when touching the groundwater in case it was contaminated. After running some tests it is apparent that the groundwater is actually better quality than the filtered tank water that they are currently drinking... but still the safety controls need to be in place! I'm happy to drink the water but the children must still wear gloves... go figure!


Another example is a cutting edge research project in the Woodburn-Evans area. The information gained from this research is very important for most coastal sand groundwater systems in eastern Australia. Alas, the words from a senior manager in a NSW government department are that no scientific information gained from the research is to be released to the public in the short term. What a shame. I understand that people are risk adverse today especially with regards to perceived public opinion but I don't think scientific knowledge should be hidden away.


On a slightly different note I have received a copy of an in-press paper on Coal Seam Gas monitoring in the Northern Rivers area. I provided some minor assistance in the paper and so an author kindly showed me before it was published. It is expected to be released in a few weeks and is likely to be in the newspapers too. Keep an eye out for those three letters C, S & G.
Sorry for the rant... just had to get that off my chest... hopefully a less opinionated post coming up shortly!

Tuesday, 14 July 2015

Northern Rivers Geology Immortalized by the National Library of Australia!


People will have noticed that I have been very quiet of late. Unfortunately there are many family matters which are taking all my spare time and therefore this blog is suffering in the short term. The local newspaper ran a story about my family that may help to illustrate where my efforts are presently focused. A big thanks to Lismore City Lions Club, the congregation at Cross Roads Presbyterian Church and many anonymous donors who have helped our family recently.

http://www.northernstar.com.au/news/confidence-boost-for-eleanor/2702047/

However, even though I've been unable to post further stories on this blog I was chuffed to be contacted by the National Library of Australia seeking permission to be added to their web archive called PANDORA. The National Library describes PANDORA thusly:
The National Library's PANDORA web archive has been building a collection of Australian websites since 1996. Many of the significant sites preserved by PANDORA, such as the Sydney 2000 Olympic Games website are no longer available on the web.
So, wow! A big honour to be asked and one that I will accept. I was wondering what would happen to all my posts if blogger hit the wall. 

Thursday, 14 May 2015

New England Geological Tour 2015

Just a quick note to let people know that the Australian Institute of Geoscientists (AIG) and the Geological Society of Australia (Queensland Branch) will be jointly running a field trip to the New England area of New South Wales and southern Queensland over the June long weekend 6th to 8th June). The field trip follows a one day seminar by the AIG. 

Geoz reports thusly:
6 - 8 June 2015 GSAQ–AIG Field Conference: New England District Regional and Economic Geology
A joint GSA–AIG field trip to the New England Orogen, with a preceding one-day seminar.
As a prelude to the Field Conference, GSAQ and Queensland Branch of the AIG
are proposing to run a one day seminar “New England Orogen, Regional and Economic Geology - an update” to showcase recent advances in the understanding of the New England Orogen.
 The main focus sessions of the pre-field trip seminar will include:
  • The New England Orogen – geology, granites and tectonic setting
  • Mineralisation styles of the northern and central New England Orogen
  • Geochemistry applications in the New England Orogen
  • Intrusive related mineralisation styles of the southern New England Orogen
The Field trip will start from Brisbane and and include tours and presentations in the Stanthorpe, Texas, Tenterfield and Drake areas.

Accomodation and some meals are included in the cost of the field trip. For more information on the field trip contact the GSA or AIG, for more information on the Brisbane pre-trip seminar contact the AIG.

Friday, 3 April 2015

Jesus’ Easter: a geological tour

Limestone is not common in the Northern Rivers but there are several
small locations where it does occur (This picture is from near Tabulam).
Since this is the first day of Easter where Christians remember the death and resurrection of Jesus at Jerusalem, I thought I’d give some background by way of the geology of the city. Like all landscapes the landform that makes up the hills and valleys around the Holy City can be seen in the geology.

At the festival of Passover Jesus entered the city of Jerusalem from the western side from Bethany. Passing into the Kidron Valley and then up to the city. At Bethany the rock types are dominated by Cretaceous aged chert and chalk of the Mishash formation of the Mount Scopus Group. These rocks are typically marine deposited sediments made from the build-up of microscopic creatures called diatoms. Descending into the Kidron Valley the chert which caps the hills to the east of the city gives way to chalk and claystone which is much more erodible. This chalk and claystone is the Menusha Formation which is the earliest formation in the Mount Scopus Group.

Stratigraphy of the Jerusalem area
Image courtesy of  Dov Frimerman
Ascending into Jerusalem the geology changes to limestone of the Nezer and Shivta formations of the Judea Group. The limestones of the Judea Group dip at an angle of around 10-15 degrees. This means that any ground water travelling though the limestone flows to the west to the sacred springs along the top of the Kidron valley. The garden of Gethsemane where Jesus spent his last night praying is in the area of these springs.

The limestone is the rock that underlies all of the places where Jesus spent his last days. Jesus drove people from the Temple claiming that they were stopping people from reaching God. The foundations of the Temple are built on this limestone. Jesus was placed on trials for treason at the Roman governor Pontius Pilate’s palace and also at King Herod’s Palace around Mount Zion. Again, these places were built on the same Limestone.

The exact place of Jesus’ execution and burial is the subject of some debate. There appears to be a couple of alternative sites but all of which are in areas were limestone is dominant. This is particularly evident with the description of Jesus being crucified ‘near’ the city and the description of Jesus being buried in a cave. The old city of Jerusalem was built entirely on the Judea Group and limestone landscapes are very well known for having many cave systems. Caves are well known in the area around Jerusalem.

In the Northern Rivers of New South Wales there is a mountain called Mt Jerusalem which is part of the world heritage system of National Parks around the Tweed Valley. But the geology of Mount Jerusalem, NSW is a post for another day.

Want to see more? Here is the Israeli geological survey’s 1:25 000 scale maps of the country or here is a good website describing the geology of Israel in more detail. To find out more about Jesus during his last days the accounts of his apostles in the Bible is the most detailed description that remains. There are other references from other sources such as Josephus and Tacitus but nothing as comprehensive as the accounts of Matthew, Mark, Luke and John in the Bible.

Saturday, 21 March 2015

Bedding with crossbedding through it

Cross-bedding in sandstone of the Evans Head Coal Measures
Shark Bay - Evans Head area
Cross-bedding is a common feature in many of the Mesozoic aged sedimentary rocks in our region particularly in the Clarence-Moreton Basin. Cross-bedding is a structure that can be confusing. However, is it is often very useful for understanding how a sedimentary rock is laid down. Because it is common in our region I thought it might be interesting to describe what this feature is.

Cross bedding forms in sedimentary rocks that have undergone transport. It is most easy to find cross bedding in sandier sediments that have been deposited in Aeolian (windy) and fluvial (riverine) environments. However, it is a feature that may be found less commonly in shallow marine, and estuarine environments but these processes are a tiny bit different and more complicated to describe, so I’ll deal with them some other time.

Sketch showing how cross-bedding is formed
The feature can be confusing especially because the cross-bedding can be mistaken as actual bedding layers. However, technically speaking, cross-beds are always laid down within the same bed. The cross-beds in a riverine environment form when the water in a stream loses energy and its ability transport sediment. The sediment then drops out of the water and is deposited along a point bar. Over time the river may dry up or migrate away and the point bar (now one big bed with cross-bedding) can then be preserved.

Cross-bed sets in sandstone of the Orara Formation (Kangaroo Creek Sandstone)
Eden Creek - Kyogle Area
Often sets of cross-bedding are present where the river will deposit another point bar over the top of the original. When this occurs the new bed usually erodes the upper part of the original bed. This is a useful bit of information because in some areas the rocks have been so deformed that it can be hard to tell whether they are upside down or not. If you are able to find cross-bedding in these rock looking for the erosion surface will tell you whether the rocks are right way up or have been turned over. It may be surprising to note that over turned bedding is actually common in the metamorphosed sediments in the New England and Tweed region. Since deformation of the Clarence-Moreton Basin has been relatively small it is unlikely that you will come across in-situ rocks that have been turned over in this basin.

The two pictures show examples from some of the oldest rocks of the Clarence-Moreton and Ipswich Basins and the one of the youngest. Despite being laid down up to 100 million years apart the manner of deposition of these two separate units was a very similar riverine environment. Nearly any outcrop of Orara Formation will show cross-bedding. So keep a look out at road cuttings or sandstone quarries.

Monday, 2 March 2015

Do you trust a geological map? (Part 2)

In an earlier post I showed an example of how making the assumption that published geology maps are correct has some big problems. In that example it was the digging of test pits where coal was uncovered which clearly showed the mapping was wrong. Since that post I found another example and this one only needed a look out the car window to know that something was wrong!

My interpretation of the geology of the area in question
Note that the area is approximately 7km across
I had a 4WD day a few weeks ago in the Bungabee State Forest between Lismore and Kyogle. The state forest is located on the southern section of the Mackellar Rangers. It is a nice area but as usual for the Richmond River catchment is invaded by noxious and environmental weeds (In fact the worst variety of weeds I saw that day was at the National Parks managed Muckleewee Mountain Nature Reserve). But I digress... It was during this 4WD trip that I looked out of the window! Where there were cliffs or rocks exposed in streams I glanced out and saw that they were clearly Clarence-Moreton Basin sedimentary rock. I didn't think this was particularly unusual at the time but when a quick opportunity arose I had a closer look.

Present mapping (Brunker et al 1978)
Area is the same as my map above
At the base of the range (near Muckleewee Mountain Nature Reserve) I broke off a fresh bit of rock from a cliff face. The ‘fresh’ sedimentary rock was a rusty brown colour. It was composed of grains that were made from other rock fragments, from feldspar and from quartz. I'd call it a litharenite according to the classification of Pettijon et al (1987), however I might have underestimated the amount of clay particles in it. The appearance of the litharenite was quite dull. I consulted a geological map that night and noticed that the area was not even mapped as sedimentary rock. It was mapped as Lismore Basalt, old lava flows. There were however some areas on the map, a few kilometres away that did have some sedimentary rock (mapped as Kangaroo Creek Sandstone). However, the Kangaroo Creek Sandstone has a distinct saccaroidal texture, a sparkly sugar grained appearance. It was clearly quite different to the rock I was looking at. The only thing that looked consistent to me was larger scale features in the cliff faces showing cross-bedding. However, cross-bedding is a very common feature in most of the Clarence Moreton Basin.

So, with my field observations mind and while reviewing a new stratigraphic guide for the youngest members of the Clarenece-Moreton Basin, the rock I was looking at appeared to be consistent with the expected rock in the Bungawalbin Member of the Orara Formation as defined by Doig & Stanmore (2012). See this previous post for more details. When I found some spare time I did a quick remapping of the area based… As you can see from the pictures my mapping is quite different. This again just goes to show that a geological map might not give you an answer. It is best to look in the field and deeply consider what you find.

References/bibliography:
*Brunker R.L., Cameron R.G., Tweedale G. and Reiser R., 1972, Tweed Heads 1:250 000 Geological Sheet SH/56-03, 1st edition, Geological Survey of New South Wales, Sydney
*Doig, A., & Stanmore, P. 2012. The Clarence-Moreton Basin in New South Wales; geology, stratigraphy and coal seam gas characteristics. Eastern Australian Basins Symposium IV. Brisbane.
*Pettijohn, F.J., Potter, P.E., & Siever, R. 1987. Sand and Sandstone. Springer-Verlag, Berlin