Saturday, 28 January 2017

Stunning in Red and White

A friend and colleague showed me his new lifestyle property on the edge of Armidale a couple of weeks ago. He is an observant soil scientist and noted that his land consisted of poor quality soils which grew only resilient grasses and some typical New England gum, stringy-bark and box woodland. He was curious about the rocks that were common on the surface of the dusty grey-brown soil. I was not surprised of the poor soils because the property is located on a geological unit called the Sandon Beds.

The Sandon Beds are common in the Armidale district, especially just to the north of the town. They were laid down sometime during the Devonian to Carboniferous periods. The rocks of the Sandon Beds are varied and include mudstones, conglomerates, volcanics and bio-chemical sedimentary rocks. The deposition of the unit was in the ocean debris flows from the continental shelf would form turbidites (coarse to fine grained repeating sequences), layers of fine mud would accumulate and occasional basalt volcanic rocks would occur. Sometimes, while a long distance from landmasses or spreading ridges very little would happen - only the gentle settling of dead primitive ocean organisms with silica skeletons.
Brecciated Jasper (chert) of the Sandon Beds

The settling of silica on the sea bed produces a rock called chert. It is common in the Sandon Beds with a red colour. The chert occurs in beds interspersed with dull mudstones, siltstones and the like. Possibly because of regional scale metamorphism or the effect of fluids in the rock the chert has been affected and displays its red colour. Because of the red colour it is often referred to as Jasper which is seen by some as a semi-precious stone.

Throughout my friend's property could be traced lines of chert running essentially north-south. This is because the beds have been tilted to a nearly vertical direction. There was nothing out of the ordinary with these beds but in one area some of the red chert caught my eye. I could not see the actual outcrop but scattered around one little area was red chert with bright white quartz veins. The chert had been broken apart and re-cemented together with the quartz rich fluid. The result was quite striking, a stunning red and white. In this one little area, at some time after the chert had formed and turned into hard rock it had been blasted apart apparently by hot fluids. A 'brecciated jasper' occurring in a little area that just happened to be on a friends new property just ready to be discovered.

Monday, 16 January 2017

Blog Update #11 - More rocks in our region

Not a lot to mention as far as the blog goes at this point, except that I've added photographs of three more stratigraphic units to the Rocks of the Region page. These are three of the many 'granites' in the Armidale district:

Gara Monzogranite --- Fickr Photos --- Stratigraphic Names Database
Glenburnie Leucomonzogranite --- Flickr Photos --- Stratigraphic Names Database; and
Rockvale Monzogranite --- Flickr Photos --- Stratigraphic Names Database
Typical landscape and outcrop characteristics of the Rockvale Monzogranite, Wollomombi area

On another note, while visiting a friends property near Armidale I observed a brecciated jasper in the Sandon Beds. I was aware of an abundance of jasper beds (red chert) in the region ever since my university days, however, I'd never seen a brecciated type and this was quite attractive. More to come in a week or so.

Saturday, 17 December 2016

Blog Update #10 - Pain and a paper

Eleanor Faith Holland
The blog has been on hold for a while. But I have always intended to keep on writing. There has been some major changes in where I live and work and most recently our lovely 6 year old daughter Eleanor passed away. There has been much pain but there is much celebration too for our special girl. My wife Becky and I are in feelings of loss. That said, we want to celebrate the life of our daughter who demonstrated so much strength in so many ways. A service will be held at St Peter’s Cathedral in Armidale at 2pm on Monday (19th December).
Coming to Christmas it is a subdued feeling at our home and my birthday was yesterday as well so I didn’t feel like celebrating. However, there was something I was excited to see. A confirmation that a paper that I was a co-author of has been accepted to the journal Science of the Total Environment (Santos et al 2016). I was only a minor co-author but there is something about having your name up there that caters for ones ego.

The highlights of the article are:
  • We assess groundwater recharge through a pervasive layer of floodplain muds.
  • Modelled groundwater flow paths were consistent with tritium dating.
  • The clay layer did not prevent recharge because of macropores and cracks.
  • Fine-grained floodplain soils do not necessarily protect underlying aquifers from pollution.
  • Combining multiple techniques gives more confidence in recharge estimates.
The article can be found here:


*Santos, I.R., Zhang, C., Maher, D.T., Atkins, M. L. Holland, R. Morgenstern, U & Li, Ling. 2016. Assessing the Recharge of a Coastal Aquifer using Physical Observations, Tritium, Groundwater Chemistry and Modelling, Science of The Total Environment, Available online 15 December 2016, ISSN 0048-9697,

Monday, 24 October 2016

Blog Update #9

Frequent readers may have observed that I have not updated this blog for some time. Due to changes in my personal arrangements I have run out of spare time! My computer has also died! I have therefore had to put this blog on hold until I can find some more time and purchase a new computer. I envisage this will probably be late December. Until then, there may be an isolated post or two but I can't promise anything. Sorry all.

I have had some opportunity to update the rocks of the region page.

Sunday, 21 August 2016

Cutting Through Mysterious Granite on a Country Highway

Australia is known for its remoteness. There are some quite remote areas in the Northern Rivers too. Along the escarpment there are rugged areas and visitors are rare. This means that sometimes rocks even though mapped broadly have geological units that have not been researched enough to relate them to surrounding units. It is a rare thing though, and rarely have rock units not been named, and categorised, even rarer is when a rock is found by the side of one of the national highways!

un-categorized granite on the New England Highway, Glencoe

The picture shows a granite that is currently mapped as "unassigned Permian intrusive - felsic". There may have been some investigations here in the past. I just can't believe some place so obvious like this one has not been investigated in detail.

Monday, 25 July 2016

Blog Update #8

The Rocks of the Region page has been a big success and will continue to be expanded on. Unfortunately I've had very little time to do some actual blog posts recently. This is not for want of material (there is a huge range of topics and places just asking to be covered) but due to a change in my work commitments. I have taken up a new job with much longer days and therefore my free time is very limited. I have also relocated away from Lismore to Armidale and taken my family with me. We are still very much settling in.

Thank you to all the readers out there. I hope that I can continue to build a blog that is a good resource for the community to use. I guess there are people still reading this blog since there are more than 150 000 page views!

Coming up soon... what is going on with this rock?
Coming up

Wednesday, 6 July 2016

Geology of the Big Scrub Rainforest (Part 4)

Tasman Sea to the Alstonville Plateau

Following the Cretaceous during the Paleogene and possibly as a result of the action of the Tasman Sea formation, volcanic systems began to form throughout parts of eastern Australia. In the New England, the Hunter and Sydney areas swiftly flowing lavas erupted from long vents and covered large areas with basalt. The same thing happened in our region and until now it is one of the relatively unknown parts of our history. At  around 43Ma during the Eocene Epoch basalt lava flowed from vents somewhere in the area now known as the Alstonville Plateau (The area between Lismore to Lennox Head). The lava flow (called the Alstonville Basalt) tended to flow towards the North and West because the hills of the Blackhall Range formed a barrier to the South. The old Wilsons River seems to have been deflected from its southerly course and sent inland to join the Richmond River as lava flows dammed off the rivers original course. Small lakes were formed where the lava flows dammed streams and created little areas of sedimentary rock known as diatomite and even poor quality opal. It is interesting to note that during the 20th century diatomite was even been mined for use as a filter medium from layers between basalt at Tintenbar and Wyrallah.

Layers of diatomite were subsequently covered by new layers of lava and during the time between lava flows rich red soils developed and were subsequently covered by new lava. The rate of soil formation was high during this time because the world climate was the warmest (more than 10 degrees C hotter than the average today) it had been for 400 million years and be when combined with atmospheric moisture this time was known as the Mid-Eocene Climate Optimum. The lava covered soils (known as paleosols) are important today because they are conduits for groundwater (aquifers) which create long lasting springs resulting continually running streams and ecosystems dependent upon them. The aquifers are also drilled into for groundwater for some of the livestock, irrigation and town water supply on the Alstonville Plateau today. Eventually, the eruptions stopped and lovely fertile deep red soils developed and continue to develop today.