Geology of the 'Big Scrub Rainforest' (Part 1)

Later this year it is intended that the Big Scrub Landcare group and many other contributors will release a book on people's connection with the 'Big Scrub'. It will be a multi-faceted book that introduces the emotional connection that people can have with a lovely part of the world. The book will even help picture the geological events that contributed to the formation of the amazing region. I understand that the book is intended to be launched in September at annual Big Scrub Rainforest Day. For those that would like some background information on the geology of the 'Big Scrub' I have provided a detailed outline of the rock types and events that went into building the foundations of this forest. There is a fair bit of information so I've broken this history into a series of blog posts. The book will have a different emphasis to this series of posts and will be a high quality visual feast. So I'll let everyone know more details about the book closer to the release date.

The oozy beginning

The history of the ‘Bigscrub’ starts a very long time in the past, yet it may be a surprise that by Australian standards the geology of our region is comparatively young. The oldest parts of Australia are 4400Ma (4400 million) years old but our little part of the Australian continent did not exist back then. It was not until 363-320Ma that the foundation rocks of our region were formed. These rocks are called the Neranleigh-Fernvale Beds

The Neranleigh-Fernvale Beds at Broken Head National Park near Byron Bay

The Neranleigh-Fernvale Beds are derived from the deep sea. In the Devonian Period sediments settled to the bottom of the deep sea crust. The sediments were at first very fine and mainly consisted of the microscopic silica based skeletons of algae called pelagic ooze. The amount of sediment was only slowly deposited but over a long time a great thickness accumulated. Occasionally thicker layers of mud would be deposited from submarine landslides which occurred at the edge of the far away continental land masses, rarely submarine volcanoes laid down lava that was quickly solidified by the deep, cold ocean water.

The process of plate tectonics means that oceanic plates move slowly under continental plates. Gradually, as this process continued during the Carboniferous our area came closer to the continental landmass as the oceanic plate was subducted (is pushed under) under the Australian continent. Tall active volcanic mountain ranges existed along the edge of the continent much like the mountain ranges of the Andes Mountains in South America today. As our area approached the continent it meant that the size of particles increased. Erosion of the mountain ranges and continuing submarine landslides created layers in the sediments called turbidite sequences. These are sequences where the bottom is coarse grained (usually sandy) and the top is fine grained (mud). This is because fine sediments take longer to fall out of the water. Subsequent nearby landslides would start a new layer with coarse grained sands followed by finer grained mud and so on.

Neranleigh-Fernvale Beds are quarried at many locations this is an old face at Teven

Eventually our part of the oceanic crust with its mass of deep sea muds and turbidite travelled to the zone of subduction where the collision with the Australian plate forced the oceanic plate under it. In this process the sediments that are on the oceanic plate are squashed and stuck onto the continent. Australia’s size grew as the process of accretion built up a thick wedge of submarine mud and turbidite. The pressure of the crustal plates sliding past each other squashed the sediments together and bent the layers into a complex arrangement of folds and faults. These are what we now know as the Neranleigh-Fernvale Beds.

As mentioned before, the Neranleigh-Fernvale Beds are the foundation rocks of our region. Today they form the ‘basement’ of the coastal Australian landmass from Gympie in Queensland to Broadwater on the Richmond River. However, in our region they are mostly obscured by younger rocks (discussed below) though it is possible to view outcrops of turbidite in several areas. These include on the Wilsons River at Laverty’s Gap, at Tintenbar and on the escarpment at Uralba between Ballina and Alstonville. Quartzite (from the silica rich sediment) is present on a hill near Nashua.

At the earlier stages of the carboniferous period the collision of the oceanic and crustal plates in our region stopped. The Neranleigh-Fernvale bed formation had been completed. The continent with its new additions ‘relaxed’ as compression eased and eventually stopped. Many more millions of years passed and what happened during that time is lost to history. No rocks are preserved in our region from the next time period known as the Permian. It is probable that the ‘Bigscrub’ area remained stable for a long time with only erosion being the most significant geological process.

Part 2.

Hills of old sea floor muck

There has obviously been a bit of a lull in my blogging of late. I’ve been busy with family medical trips to Queensland and I’ve had less free time too. But some interesting things have happened with one formal presentation on coal seam gas and water and another presentation to be given in a couple of weeks. But on the aspects that interest me most (non-CSG geology), I’ve also been contacted by academics from a couple of different universities. It is nice to know that they feel I can help them with some research projects. I'll post more about that at a future date.

Best of all lookout - Springbrook National Park
Except for the hills on the horizon the rock in this photo is mainly
of the Neranleigh-Fernvale beds.

During the trip to Queensland I met up with family on the Gold Coast. We decided to have a day up in the popular Springbrook National Park area. In particular the views in this country are astonishing. The Best Of All Lookout certainly lives up to its name with incredible views of the valleys of the Tweed region. Mount Warning looks stunning and the rugged terrain of the volcanic shield remnants beautiful. And this was on a hazy day!

To get to Springbrook national park from the Gold Coast it is necessary to traverse the oldest rocks in the Tweed region. These are sediments of the Neranleigh-Fernvale beds. These are represented by the initially steep hilly terrain as you head westward up the range. Hinze Dam, for example, is located on this rock type. Time has weathered and eroded much of this rock away but still it remains as a significant landscape feature. These rocks and hills would probably be better known if the lavas associated with the Tweed Volcano had not erupted.

The Neranleigh-Fernvale beds are interesting rocks because of their mode of formation. They are essentially muds and debris flows that have been deposited in a trench during a period known as the Paleozoic. The trench was caused by the subduction of a continental plate under the then eastern Australian landmass. These sediments were then scrapped off and buckled into a large mountain range that has since been mostly eroded away. All of this occurred while Australia was part of the super-continent Pangaea which existed well before Gondwana.

Today, in the Northern Rivers the Neranleigh-Fernvale beds form the steep eroded terrain in the Tweed Valley (with the exception of some lavas and intrusions associated with the Tweed Volcano). They outcrop in a band at the very edge of the Alstonville Plateau to Byron Bay. They only occur as a band in the Ballina area because they are obscured by Jurassic sediments and the Cenozoic volcanic rocks. Like the Springbrook area, driving from Ballina to Alstonville or from Cabarita to Chillingham means traversing this formation. As soon as you get off the coastal plain and head up the hills you are passing the rocks of the Neranleigh-Fernvale beds. These beds are then obscured by the more recent sediments or volcanic rocks associated with the Tweed Volcano.

As for the Springbrook area, if you’d like to know more I recommend a book by Warwick Wilmott called Rocks and Landscapes of the Gold Coast Hinterland. The processes and timing of events in the Gold Coast area are very very similar to those processes that occurred in the Tweed valley area and so might be worth a read even if you don’t cross the border!

Warwicks book can be obtained from the Queensland Division of the Geological Society of Australia here.

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.


References/bibliography:

*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.

At Nashua where chert should not be

In an earlier post, Booyong not in the Clarence Moreton Basin? I indicated that I had been informed that there was evidence of Palaeozoic sedimentary rocks near Booyong, and that if this is the case there is significant implications on the morphology of the Mesozoic aged Clarence Moreton Basin in the area between Byron Bay and Lismore. It therefore has implications for the exploration for coal seam methane too.

Well, I can confirm that after much searching, I came across an ephemeral creek north of Booyong near the Casino to Murwillimbah Railway line at Nashua that clearly contained much chert. Additionally, a railway cutting appeared to have significant folding, but was very weathered so identification of the parent rock and indeed even whether it was folding was not clear.

Chert at a location between Nashua and Booyong

Chert is a rock that is essentially absent from the Clarence Moreton Basin, however, it is common in the underlying rock of the Beenleigh Block, known as the Naranleigh-Fernvale Group. This group of rocks formed in a deep marine environment. It consists of turbidite sediments that occur when parts of the undersea continental slope erode and some slates and chert. The Chert is formed on the ocean floor where chemical and biological material settles over a long period of time. All of this rock was then transported and accreted onto the edge of the Australian Continent before the Devonian or Carboniferous period which was around 333-343 million years ago during the formation of the New England Orogen. This process of accretion has caused folding and low grade regional metamorphism to be common place in this geological unit. By comparison, the sediments of the Clarence-Moreton Basin are dominately continental derived and are un-metamorphosed. They were mainly deposited in river systems, lakes or at best shallow marine environments. Chert does not form very well in these environments.

But, what is so exciting about this outcrop? The Neranleigh Fernvale Group occur along the edge of the hills above the coastal plain south of Ballina and are exposed just about everywhere from the Gold Coast to Byron Bay. If you follow the line of outcrop from near Ballina you will find it is overlain between Byron Bay by the Clarence Moreton Basin which extends all the way to the Tabulum in the west. But because the outcrop occurs at Nashua this shows us that the area to the east is probably a different basin from the main part of the Clarence Moreton to the west. This has significant implications for understanding the structure of the earth here and also affects where resources such as gas can be explored for. For those that were worried about coal seam gas, you won't find any at Nashua.

A warning about Mount Warning

Here are some common quotes about Mount Warning:

"World Heritage listed Mount Warning (Wollumbin) is the remnant central plug of an ancient volcano." 

"The Mount Warning volcano was a huge shield volcano."

"Considered the central magma plug, Mt Warning and a system of ring dykes, being extremely hard rock, have resisted erosion, and dominate the valley landscape."

"Mt Warning, Wollumbin, the cloud catcher, is the basalt plug of the world's largest and oldest extinct volcano. "

"Now, Mt. Warning is the first place that that the sun hits at sunrise… the highest point in New South Wales….almost the highest in Australia!"

These are quotes typical of tourist and even educational resources. They are quite definite and the comments makes sense, mostly. There are also some points of view that I espoused for a long time... Except aspects of each of the quotes are technically wrong and in some cases completely wrong. Like my post on the "erosion caldera" something that is technically incorrect has become general knowledge. It is a little pedantic of me, but it is one of my hobby horses... so what is technically wrong with the quotes above?

Western face of Mount Warning (composed of syenite).
One of the ring dykes is visible in the foreground
and Mount Uki and the Pacific Ocean in the background

Interestingly, Stevens et al (1989) and earlier authors noted that the rock composition of the intrusions that make up Mount Warning (the Mount Warning Complex) is different from most of the lavas (The Lamington Volcanics) that exist in the region. It is also slightly older than most of the lavas. Geologically speaking the age difference is not huge at only about 2-3 million years, but still significant enough.

It is apparent from Smith & Houston (1995) and other authors that much of the rhyolite lavas that remain of the Lamington Volcanics were not erupted from the central area now the site of Mount Warning but from vents on the flanks. Given the coverage of the mafic components (the Lismore Basalt, for example) it is more difficult to identify any vents.  

An idea has been raised by Cotter (1998) which questions the volume of lava that was erupted from the Tweed Volcano. It is known that the Palaeozoic aged meta-sedimentary rocks of the Beenleigh Block, called the Neranleigh Fernvale Beds and the Mesozoic aged Chillingham Volcanics and Clarence Moreton Basin were not domed upwards by the underlying magma except a little around the Mount Warning Complex itself. However, other areas such as the nearby slightly older Focal Peak Volcano have been lifted by the Cenozoic aged volcanism. But in the case of Mount Warning, Cotter (1998) felt that lithology, the remnants of the rhyolitic lavas, the pre-existing Chillingham and Alstonville Volcanics was the main control on the geomorphology, not as suggested by others the volcanism that formed the shield volcano itself.

The idea suggested by Cotter (1998) has significant implications for the size of the Tweed Volcano. The volcano is considered the biggest by far of its age in eastern Australia. It appears likely that the extent of the shield volcano is not as great as originally thought. The underlying Chillingham Volcanics would have been an existing mountain range and therefore reduced the thickness of the Tweed volcanic pile and the Alstonville Basalts would have reduced the southerly extent. I think that when you add to this the idea that the rhyolite units have erupted away from Mount Warning, but instead from flanks on the volcano, the volume of lavas from the Tweed Volcano may actually be more in keeping with the other intra-plate volcanoes in Eastern Australia. It was also possible that before it was eroded into the present shape (which implies a central shield type volcano) it may have looked more irregular than we imagined...

But don't get me started on the comments about the biggest volcano in the world and the highest point in New South Wales!!! What were these people thinking?!

...but does any one want to talk down something that was presumed to be huge, just to something large? Emotionally, many (including myself) have an emotional attachment to the beauty and wonder of the Tweed Volcano, sometimes it is hard to take a step back and consider it is not quite as fabulous as originally thought, but what we see is still stunning... and it is still very, very big. To put that in perspective I think that even the small volcanoes in the region are stunning. We don't need to exaggerate something for it to inspire us.

Bibliography/References:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University. 
*Smith, J.V. , Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales. Australian Journal of Earth Sciences V42(1) p69-74.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

Where have the Brisbane Metamorphics gone?!

A few months ago I was reading the 2011 NSW National Parks and Wildlife Service plan of management for the Julian Rocks Nature Reserve just offshore, near Byron Bay. The introduction said the Julian Rocks “are composed of Brisbane Metamorphics which date from the Carboniferous-Devonian period 345-405 million years ago and are the most resistant rock type in the region”. Sounds fine as a bit of background but why can’t I find recent geological work that refers to the Brisbane Metamorphics anywhere else?

Academics from Southern Cross University have used the term in published works as recently as 2007 (Specht and Specht 2007). But I can’t find it on any map or in any geological publication after 1990. Surely the rocks haven’t been eroded that quickly especially since it is “the most resistant rock type in the region”. I can, however, find reference to the Brisbane Metamorphics on the 1: 1000000 scale NSW geological map from 1962. But at such a scale it is hard to figure out exactly where it is. Broadly it appears to be located in some areas near Murwillumbah and some areas near the border with Queensland. The most specific paper I have is by Holcome (1977) which discussed the Brisbane Metamorphics in depth but doesn't say where it goes!

When in doubt try Google? But the result you get when typing in “northern rivers geology” is the website Big Volcano. It can be found here. Here too the geological history summary refers to the Brisbane Metamorphics but mistakenly links to a site that shows a small contact metamorphic area at Mount Coot-tha just to the west of Brisbane. This is a bit confusing because the metamorphic rock here is called the Bunya Phyllite which is a regional metamorphic rock which has undergone a second metamorphic even during the emplacement of the granite that makes up Mount Coot-tha. Interesting in itself, but it does not answer our question why the Brisbane Metamorphics were said to be at Byron Bay!

Well, The answer is simply a case of one of the most difficult aspects of geology, nomenclature. Geoscience Australia provides an excellent service in maintaining a database of all geological units named in Australia (past, present and proposed). It includes an entry on the Brisbane Metamorphics which can be found here. On the webpage you can see three fields that are important for knowing where this unit has gone. “Current: No”, “Status: Obsolete”. The comments field answers the question finally: “Name superseded by Rocksberg Greenstone, Bunya Phyllite, and Neranleigh-Fernvale Formation.”.

What this means is that the one description of Brisbane Metamorphics did not reflect the ages, genesis, and history of these three rock units.  You will get more information about the structural history and rock composition if you deal with the new units individually. Indeed, Holcome (1977) discusses the constituents of the Brisbane Metamorphics at length and notes that the Rocksberg Greenstone, Bunya Phyllite and Neranleigh-Fernvale Group are the constituents of the Brisbane Metamorphics but these are substantially different in terms of formation, metamorphic history and exposures. In northern New South Wales some of these units are present as part of what is called the Beenleigh Block (Holcome (1997).

There are many cases where geological units have been renamed or reclassified after further research has been done. This is no different to any other area of science. The only challenge is keeping up with the change.

References/bibliography:

*Holcome, R.J. 1977. Structure and tectonic history of the Brisbane Metamorphics in the Brisbane Area. Journal of the Geological Society of Australia. V24.
*NSW National Parks and Wildlife Service. January 2011. Julian Rocks Nature Reserve: Plan of Management.
*Specht, R.L. , Specht, A. 2007. Pre-settlement tree density in the eucalypt open-forest on the Brisbane Tuff. Proceedings of the Royal Society of Queensland 113 p9-16