Geology of the 'Big Scrub Rainforest' (Part 2)

The story of the 'Big Scrub' s preserved in the rock unit known as the Neranleigh-Fernvale Beds. The first post in this series is about this rock, the foundations of the region. This post deals with the period of big sedimentary basins which corresponds with the age of the dinosaurs. Some 50 or 60 million years elapse from the Neranleigh-Fernvale Beds until we come to our next period of rock formation.

The Big Basins

Walloon Coal Measures overlain by Orara Formation at Bexhill

This period begins in the age of the dinosaurs during the end of the Triassic Period. During this time tectonic forces became extensional, that is, the east coast of Australia was pulled and twisted apart, very little compression occurred. The crust became thinner as the once colliding continental plates began to relax and lowland basin-shaped regions formed. The thin crust allowed more volcanism to occur and the first geological units of the Ipswich and Clarence-Moreton Basin were formed. The Chillingham Volcanics consisting of lavas and volcanic ash was laid down in after 229Ma and this was subsequently overlain by units of lake and river deposits including the Evans Head Coal Measures, Laytons Range Conglomerate, Walloon Coal Measures and many other layers. By the height of the age of the dinosaurs, during the Jurassic great river valleys spread out from the mountains of the New England over our region. These rivers laid down great expanses of alluvial sand which were further overlain by other units. The great expanses of river sand are called the Orara Formation.

Today, like the Neranleigh-Fernvale Beds the sediments of the Clarence-Moreton Basin in the ‘Bigscrub’ area are mostly obscured by younger rocks. However, some of the Chillingham Volcanics seem to present in the Blackhall Range behind Wardell though this is difficult to identify. Rocks of the Orara Formation are present at the edge of Meerschumvale but are most obvious at Bexhill, indeed at Bexhill the Walloon Coal Measures are evident in the old brickpit. The Walloon Coal Measures are overlain by a sub-unit of the Orara Formation called the Kangaroo Creek Sandstone. This Sandstone forms nutrient very well-draining but poor soils. Lovely examples of the Kangaroo Creek Sandstone can be seen at Bexhill Open Air Cathedral or in the creeks near The Channon. This means that the vegetation on these areas consists of different plants to that of the rest of the ‘Bigscrub’. The soils in these areas cannot support the lowland subtropical rainforest that is the biggest component of the ‘Bigscrub’ unless they are well sheltered in a gorge.

Even though the units of the Ipswich and Clarence-Moreton Basins are dated from the age of the dinosaurs (the Triassic, Jurassic and Cretaceous Periods) no one has yet found fossils of dinosaurs preserved in any of these units in our ‘Bigscrub’ area. However, abundant fossils of plants and fish do exist in many units of the Clarence-Moreton Basin and dinosaur footprints have been seen in rocks of the Walloon Coal Measures in the Queensland part of the basin. Some fossil fish have been observed in the creeks near Nimbin. Along with the abundant coal during the Jurassic shows there was a very large quantity of organic matter and plants growing at the time. This was a time rich in life.

A Volcanic Sedimentary Rock

My Wife and I have been in South Brisbane for a few weeks while my daughter has received treatment in a hospital there so I have not compiled any posts on the geology of the Northern Rivers during this time. However, I thought it might be worthwhile to tell you about an interesting rock I found in Brisbane that is of a type that can occasionally be found in the Northern Rivers especially in the New England Tablelands.

One morning while walking to the hospital, down the driveway of the apartment I was staying at I caught a glimpse of a rock fragment that was different to what I had previously seen in this area. The driveway was cut into weathered old Paleozoic aged rock called the Bunya Phyllite. But the rock fragment that I saw of interest because it was quite different from the phyllite as it had a large quartz cobble in it. Later when walking back to the apartment I had a closer glimpse. It appeared that this rock had fallen down the slope and there were other rocks inconsistent with the phyllite. I picked the piece up that first got my attention and washed it clean. It was a conglomerate, with large rounded clasts of quartzite and basalt and an angular clast of the aforementioned phyllite. The clasts were cemented together with a grey material with small angular crystal fragments. All of this was a surprise until I remembered that I was close to Kangaroo Point which is a cliff line made from a volcanic rock called the Brisbane Tuff part of a Triassic aged volcanic terrain.

It was apparent that what I had was conglomerate formed in the throws of the volcanic eruptions that created the Brisbane Tuff. The Roach (1997) and earlier authors interpreted the Brisbane Tuff as series of pyroclastic flows, surges and air falls that were deposited in pre-existing valleys formed during the Triassic. The valleys probably have had rocky streams evident from the rounded nature of the clasts in the conglomerate. After or during an eruption of the volcano combined with a lots of rain or the failure of a natural dam or lake a mud flow probably ran down the valley mixing all the rock, debris, mud and what ever got in its way stopping after the energy had been spent. The conglomerate would then have been covered and preserved by material from subsequent eruptions.

The sort of volcanic related mud flow described above is called a lahar. They are actually quite common in modern volcanic terrains but are often quickly eroded away so tend to be a little less common than would be expected in older volcanic terrains. Lahars are part of a larger group of volcanic-sedimentary rocks called volcaniclastic rocks. Volcaniclastic rocks are found in the Northern Rivers areas, particularly in the areas of the escarpment and tablelands where the Permian (pre-Brisbane Tuff) Wandsworth Vocanic Group is present (Barnes et at 1991), (The Wandsworth Volcanic Group includes such diverse units as the Annalee Pyroclasics near Armidale to the Drake Volcanics near Drake). The group is very extensive and deserves to be considered in several future posts. It is also worth noting that the Brisbane Tuff was deposited at the same time in a similar way as the Chillingham Volcanics which filled the bottom of the Ipswich Basin and now outcrops in the Tweed and lower Richmond River Valleys.

References/bibliography:

*Barnes, R., Brown, R.E., Brownlow, J.W. & Stroud, W.J. 1991. Late Permian Volcanics in the New England - The Wandsworth Volcanic Group. Quarterly Notes of the New South Wales Geological Survey.
NSW geosurvey quarterly notes, 84.
*Roach, A. 1997. Late Triassic Volcanism of the Ipswich Basin. Macquarie University, PhD Thesis.

The 'older' Rhyolite in the North East

In some of my earlier posts I mentioned that there are many areas in the mountains around the Tweed Valley that are comprised of rhyolite. I mentioned that this rhyolite was formed during eruptions associated with the Tweed Volcano during the Cenozoic era. This rhyolite is called the Nimbin Rhyolite or the Binna Burra Rhyolite (depending what side of the state border you are on). However, there is actually another large distribution of rhyolite not associated with the Tweed Volcano, erupting much earlier, during part of the Mesozoic known as the Triassic. These older mainly rhyolitic rocks are called the Chillingham Volcanics with the type location unsurprisingly located at Chillingham, a village west of Murwillimbah. Those of you who have seen my earlier posts will recognise that I have briefly mentioned the Chillingham Volcanics before, but in this post I intend to go into it further.

Layers of pyroclastics and volcaniclastic of the Chillingham Volcanics
(Murwillimbah - Kyogle Road)

The Chillingham Volcanics have been studied in a fair amount of detail by Roach (1997) in his thesis. This included all of the Triassic volcanic rocks from Brisbane to Uki. So, obviously there is a relationship with the rocks of the southern Queensland, Indeed Roach (1997) indicates that the Brisbane Tuff is a deposit of volcanic rock of rhyolitic composition. The Brisbane Tuff is most well known by the Kangaroo Point Cliffs opposite the Brisbane River in Brisbane City and was erupted during the same general period of time as the Chillingham Volcanics.

The Brisbane Tuff provides a miniature version of the Chillingham Volcanics and is well known because the volcanic centre can be identified in the northern suburbs of Brisbane and the tuff was laid down in the valleys that existed in the Palaeozoic aged basement. The situation which lead to the formation of the Brisbane Tuff also developed further west and south where a larger valley now known as the Ipswich Basin was forming. The eruptions occurred in and around the basin as the crust in this area was subsiding during thermal fluctuations and as the basin filled up with volcanic rocks subsidence continued leading to a very thick unit of mostly rhyolite and reworked volcanic rocks (actually a sedimentary rock known as a volcaniclastic rock). So the Chillingham Volcanics are actually the lower most stratigraphic unit in the Ipswich Basin.

The Chillingham volcanics are mainly comprised of rhyolite in the form of lavas, pyroclastic, ash and tuff deposits as well as the above mentioned volcaniclastics. Many volcanic vents are recognised from structural characteristics of the rocks, however, only one area really shows an obvious modern geomorphological character. This area is around Uki and Clarie Hall Dam where eruptions formed a large mass due to the slow moving nature of the lava. Interestingly the northern most parts of the Chillingham Volcanics in Queensland shows us that there was not just rhyolite but also some andesite and even basalt, but in the area between Chillingham and Uki it is pretty much all rhyolite.

Outcrops of the Chillingham volcanics occur over a long distance with the eastern most side of the Ipswich Basin exposed in New South Wales meaning that a band of the Chillingham Volcanics is visible within the eroded valleys of the Tweed Volcano. The band is actually interupted by the Mount Warning Complex which appears to have intruded right along the line of the pre-existing Chillingham Volcanics. Also the volcanics are covered by the Lamington Volcanics of the Tweed Volcano too, both along the Queensland Border and between Clarie Hall Dam and Evans Head. Indeed the Chillingham Volcanics appears to change composition through this area with authors such as Smith et al 1997 and Cotter 1998 identifying andestite and basalt at Evans Head and an area near Wardell.

The Chillingham Volcanics overlie palaeozoic aged rocks of the Beenleigh Block, mainly rocks of the Neranleigh-Fernvale Group. The overlying rocks are more components of the Ipswich Basin such as the Ipswich Coal Measures and its equivalent (such as the Evans Head Coal Measures).

Although I have said that the Chillingham Volcanics contain the older rhyolitic rock in this area, there are actually still older rhyolites in the region... But I'll talk about those rocks in a future post.

References/Bibliography:

*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.
*Roach, A. 1998. Late Triassic Volcanism of the Ipswich Basin, Masters Thesis, Macquarie University.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45

A brief geological tour of Evans Head



Half Tide Rocks: Made from Chillingham Volcanics (dark rock)

Evans head is a popular vacation spot. It has some lovely beaches which are interrupted by a proud and attractive headland. During the summer it is impossible to find accommodation in the area and the town is crowded with families enjoying sunshine, boating, fishing, swimming and relaxing. I don't go there for holidays but I'm close enough to enjoy a day or two by the beach as sometimes.
The interesting feature of Evans Head is the obvious rocky headland standing quite proud at the mouth of the Evans River and along a coast line with huge sandy beaches. The reason for this feature is the more erosion resistant rocks that occur here. Click here to link to a basic geological map of the area. The hardest and oldest rock outcropping just south of Evans Head town at the Half Tide Rocks is the Triassic aged Chillingham Volcanics being the earliest part of the Ipswich basin. These rocks can be seen as the darker coloured rock at the two headlands in the photograph above. Here the Chillingham Volcanics are comprised of basalt and andesite (elsewhere in NSW such as at Chillingham the Chillingham Volcanics are mainly rhyolitic in composition) and here at Evans show an uncommon rock called hayloclastite. The formation of hayloclastite in this area was the result of eruption of basalt into a coastal sea. Something you might see in modern day Hawaii or Iceland where lava flows directly into the sea. Unfortunately it is very hard to recognize because of weathering of the rock in this area.

Overlying the Chillingham are the Evans Head Coal Measures. These are located on the southern bank of the Evans River and extend around to the south of the Half Tide Rocks. Despite their name coal is a little hard to find and is only present in occasional thin bands. The Evans Head Coal Measures are therefore mainly comprised of sandstone (a type called arenite with the sand grains mainly composed of quartz sand and occasional small fragments of rock), siltstone, mudstone and some coal. The arenite frequently shows a feature called cross-bedding which is common in rocks that have formed in medium velocity rivers. The Evans Head Coal Measures are equivalent to the Ipswich Coal Measures in southern Queensland and the Redcliff Coal Measures which occurs south of McLean and is exposed on the coast near Brooms Head.

Ripley Road Sandstone with a small conglomerate layer

Ripley Road Sandstone is the youngest exposed rock unit at the headland. This is actually part of the Clarence Moreton Basin which overlies the Ipswich Basin. If you go to the lookout you can see boulders of a pale grey colour. This is the Ripley Road Sandstone. It is mainly comprised of quartz sand lightly cemented together with a grey clay (known as a clay matrix) but occasionally some bands of conglomerate are present such as in the picture opposite.

On the geological map you will notice that the areas around the headland are comprised of different types of sediments these are all very recent which geologically places them at Quaternary (or more specifically Pleistocene to Holocene aged). This pretty much means that these sediments are actively changing and being deposited. Mainly sands in the beach and dune systems and silts and clays around the river estuary. Many of the Holocene aged sediments contain potential acid sulfate soils, which are common in the region but present several environmental management issues when disturbed. In the beach sands there are also commonly found heavy minerals which have from time to time being mined. But more about these heavy minerals some other time.

References/bibliography:

*McElroy, C.T. 1969. The Clarence-Moreton Basin in New South Wales. In Packham, G.H.(ed) - The geology of New South Wales. Geological Society of Australia. Journal V16.
*Smith, J.V., Miyake, J., Houston, E.C. 1998. Mesozoic age for volcanic rocks at Evans Head, Northeastern New South Wales. Australian Journal of Earth Sciences V45
*Stephenson, A.E. , Burch, G.J. 2004. Preliminary Evaluation of the Petroleum Potential of Australia's Central East Margin. Geoscience Australia. Record 2004/06.