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.

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 westward Wilsons River

The Wilsons River flows from east to west between Booyong and Lismore which is unusual for coastal rivers in the region. You’d expect a river to find the path of least resistance and head to the sea quite quickly, in the case of the Wilsons River the path of least resistance appears to have been away from that range of hills or mountains in the Alstonville area and away from the sea.

In his masters research, Cotter (1998) discovered that the landform of the tweed volcano was more complex than the simple shield volcano model proposed by earlier researchers. The shield volcano model essentially shows a radial drainage pattern from the centre of the shield a bit like the spokes on a bicycle wheel. While this does hold up well for the remaining skeleton of the Tweed Volcano of particular interest is the area to the south where a previously unidentified Cenozoic volcanic unit was discovered and shows that pre-existing structures explain the river drainage. Cotter (1998) suggested the name of Alstonville Basalt for the new Cenozoic (up to 41 Million Years old) unit as it appeared to pre-date the tweed volcano (23 Million Years). Additionally, it has been identified that the even older Mesozoic Chillingham Volcanics (but here consisting of basalts rather than the rhyolites that are seen further north) occur on what was once considered the southern flank of the volcano.

Brodie and Green (2002) observed that the dip of the Alstonville Basalt is to the north west which to me seems to indicate a volcanic centre further to the south east (in the opposite direction for lavas from the Tweed Volcano) assuming that not too much deformation has occurred since the rocks were erupted. Taken together this implies that during the Mesozoic hills existed to the south of the present day Alstonville Plateau and that during the early Cenozoic volcanic hills were emplaced and created a barrier for southerly or easterly discharge.

Cotter (1998) suggests that the Wilsons River has actually roughly followed its current path since the Late Mesozoic. The diatomite deposits located at Tintenbar and Wyrallah are of lacustrine origin and may be the result of lakes forming on the newly erupted Alstonville Basalt as the Wilsons River was intermittently impounded by the existing hills of the Chillingham Volcanics. It has been following a westerly course certainly before the Tweed Volcano (c. 23 Million Years) for the Chillingham Volcanics and Alstonville Basalt has stopped the Wilsons from flowing south or east. This continuity of flow direction implies that any lavas from the Tweed volcano would have cut through the Lamington Volcanics of the Tweed volcano unless the lava was of significant enough volume to change river direction. This volume of lava appears unlikely given the distance of this area from the centre of the volcano now represented by Mount Warning.

Putting all the background together shows that the section of the Wilsons River from Booyong to Lismore may have been flowing away from the sea for more than 40 Million Years, yet, additionally it is worth noting that the majority of the length of the Wilsons River, Richmond River and even Clarence River is north-south parallel to the coast. This implies some form of pre-existing structural control, probably associated with the deposition of the Clarence Morton Basin or even older Palaeozoic basement rocks, in turn; suggesting that the northern rivers have been following similar flow paths for a long, long time. However, this is a discussion for another post.

References/Bibliography:

*Brodie, R.S. & Green, R. 2002. A Hydrogeological Assessment of the Fractured Basalt Aquifers on the Alstonville Plateau, NSW. Australian Bureau of Rural Sciences, Australia
*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.
*Ferrett, R. Australia's Volcanoes. New Holland Publishers 2005.

Radioactive paradise (slightly)

The areas of the Tweed Valley, Nightcap National Park and Byron Bay are often seen as fresh clean and natural. Well, I can argue that especially Byron Bay may be a little unnatural but certainly there is a feeling of 'freshness' with the rainforests and the beaches. Given this, few people would think that you'd get a bigger dose of radiation from living in these areas than you would in Brisbane or Sydney (even living near the Lucas Heights Reactor).

Few people realise that radiation occurs naturally in the environments in which we live. Yes, most of you would know that the Sun is a thermonuclear power station bombarding Earth with gamma radiation on a daily basis. But it is also a natural part of the earth and actions either natural or man made can result in these areas being elevated in radiation. In the cases below the sources are formed through different ways but all provide an increase in radiation sometimes thousands of times higher or more than what would be considered background.

Let us look at the little village of Uki first. This little place is located in the Tweed River valley and is known for its rainforest surroundings and rugged, scenic landscapes. Geologically some of the area around Uki is situated on mesozoic aged rhyolite of the Chillingham Volcanics and this rock type provides an added level of radiation due to the minerals that exist naturally in it. But even more interesting is that a mineral exploration company discovered a very tiny sized but significant anomaly in the radiation levels just south of the village. The source was not clear but sampling showed that a five square metre anomaly existed in the already slightly elevated rhyolite terrain background radiation. Analysis showed a nearly 0.05% concentration in uranium which is quite high. This is many thousands of times higher than the normal level expected. The reason for this anomaly remains unknown.

Byron Bay is located on the southern side of expansive active and historic beach systems. Much of the Byron Bay area (and much of the north coast itself) was subjected to heavy mineral mining up until the 1980's but this has ceased now. The heavy minerals sought after were mainly titanium rich ilmanite and rutile and there are other heavy minerals too such as zircon and monazite. These minerals were naturally enriched through the processes of wave and tidal action which created zones amongst the dunes that were targeted for mining. But many of the left over heavy mineral sands were not needed once the rutile and ilmanite were removed. So the left over mineral sand was discarded in some cases used as fill for future building sites. Little did people realise monazite rich left over sand would cause issues which may be unsafe for building homes on. This is because monazite is a radioactive mineral and when the residually enriched sands were dumped this increased the concentration of thorium and uranium and the associated radiation. In fact this situation didn't just occur at Byron Bay but all along the north coast.

More broadly, but less significantly many areas where rhyolite or granite is the underlying rock also have higher than normal background radiation. This too is because of radioactive minerals being enriched naturally when these sorts of magmas are being formed. So this would apply to areas in or close to the national parks of the nightcap ranges and many areas inland in the headwaters of the northern rivers such as the Clarence or Bellinger Rivers and large expanses of the New England tablelands.

References/bibliography:

 *Pechiney Resources (1970). Report on air and ground prospection, Clarence-Moreton Basin, EL 278, Nimbin - Murwillumbah area. Unpubl. Exploration Progress Report.

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.