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.

Clarence-Moreton Basin Geology Video

The NSW Office of Water have posted an excellent video summary of the geology of the Clarence-Moreton Basin. It provides an excellent overview.

Brown Under and Green on Top

A few months ago I took this photograph at a site I was working on (located mid-way between The Channon and Dunoon). It was a cold spring day with strong cold winds and rain threatening. I love the cold weather, it seems to make you feel more alive! Anyway, I thought it would be a good photo to share since it shows several attributes of our landscape and how  it was formed.

The south side of the valley between The Channon and Dunoon

Firstly the background geology. Where the photo is taken from is on a hill made from rock of the Walloon Coal Measures within a larger steep sided valley. The sides of the Valley are two rock formations more resistant to erosion which is the Miocene aged Lismore Basalt (not visible in the picture) and the Kangaroo Creek Sandstone (Cliffs of which can be in the picture). Here the Lismore Basalt overlies the Late Jurassic aged Kangaroo Creek Sandstone which in turn overlies the Jurassic Walloon Coal Measures. I’ve done some earlier posts which describe the nature of the Kangaroo Creek Sandstone and Walloon Coal Measures, just click on the respective link for more.

Rocky Creek runs through the valley today and it is the action of that creek that formed the valley. The creek must have cut through the lavas of the Lismore Basalt and eventually cut through the Kangaroo Creek Sandstone. Once it was through these hard layers it had an easier task of cutting into the softer and finer grained sediments of the Walloon Coal Measures. It is also possible there is some underlying structural control such as folding or doming but I’m not confident of the extent of this.

The top of the ridge in the photo shows a wet sclerophyll vegetation type, an open forest which contains many Eucalypt species reflecting the Kangaroo Creek Sandstones poor nutrient soils and rapid drainage. Also the ridge is quite exposed to direct sunlight and desiccating winds. Below the cliffs the Walloon Coal Measures start and here is found dry rainforest type vegetation reflecting the better, more nutrient rich and finer grained soils that are developed on the Walloon Coal Measures. The nearby Basalts also have the same vegetation type and in places approach wet rainforest especially in gullys and protected places.

In addition the picture shows the indirect and direct effect of Australians on the environment. The indirect effect is weeds. Many of the bright green trees in the middle of the picture are Camphor Laurel (Cinnamomam camphora) loving the dry rainforest environment. Right in the foreground is Wild Tobacco Bush (Solanum mauritianum) overtaking some of the grazing country. But you will also see a line of dead trees which is part of a successful effort to reclaim the weedy forest into quality native vegetation. The dead trees are poisoned Camphor Laurel with many hectares of forest in this area regenerated by staff working for the local water authority in an ongoing rehabilitation project.

Note that the stratigraphy of the Kangaroo Creek Sandstone has been recently revised since this blog post. See the this post for details.

Boring fossils, but fossils none-the-less

Note that the stratigraphy of the Kangaroo Creek Sandstone has been recently revised since this blog post. See the this post for details.

In a previous post I discussed some interesting finds in the Dunoon and The Channon area. I discussed how the geological maps of the area are in places incorrect because coal was found in areas that were not expected to contain any. There are two old (Jurassic aged) sedimentary formations present in the area the younger one is called the Kangaroo Creek Sandstone. The Kangaroo Creek Sandstone is mainly comprised of quartz rich sands cemented together and altered by a later period where silica was precipitated on the sand grains creating what is called a saccharoidal texture. 
The slightly older formation is called the Walloon Coal Measures and although It was known to occur in the area was not expected to be as widely found. The upper most part of Walloon Coal Measures in this area is a lithic sandstone, a sandstone where the sand grains are actually pieces of rock with different minerals including quartz and feldspar. This sandstone is much duller and weathers to form more silts and clays rather than sandy soils as you’d expect to be found around weathered Kangaroo Creek Sandstone.



Woody fossil in lithic sandstone

The lithic sandstone also contains a fair amount of woody fossil fragments. I was surprised how easy it was to find some. The photo to the right shows some of the poor quality fossil wood (technical problems mean that I cant upload the photo - will sort this out soon hopefully). These fossils were located within several metres below the boundary between the Kangaroo Creek Sandstone and the Walloon Coal Measures. It was interesting though, that I could not recognise the type of boundary between these two units even though I could recognise them a short distance apart. That is, I don’t know whether the boundary was gradational or an unconformity.



A moderately thick layer of weathered coal

Of course as far as fossils go, there are thick units of coal in the Walloon Coal Measures and there is no exception in this formation in the Dunoon area. Coal is accumulations of organic material such as leaves and wood and algae that has not been exposed to the oxidising environment before it is compressed by the overlying strata to turn it into a rock. When I was able to be involved in digging some excavator holes in the ground looking for some clay earlier this year I was surprised to see how much coal was present. The coal found was of course very weathered and degraded by the process of recent erosion and natural soil formation, but it burns a bit if you let it dry out (you are still better off getting fire wood).
I think what was most interesting for me was the way in which fossil material could be found wherever the Walloon Coal Measures outcrop. This means that they could be just about anywhere in the valleys in the Nimbin area or near Tabulam, on the way to Wooli, Coaldale Valley, all over the place. If you are interested in finding poor quality fossils grab a geological map and look for the boundary between the Kangaroo Creek Sandstone and Walloon Coal Measures.

Do you trust a geological map?

The NSW Geological Survey have produced the maps that we use today. They have recently placed all of them online for people to view which is... well... excellent! They can be found here. Some areas have excellent, up-to-date 1:100 000 scale maps which are exceptional. However, it is worth mentioning that the best scale you will find for most of the state is 1:250 000 older 1970's maps. These maps are good but they were mostly done through looking at aerial photographs with limited field checking and since nearly 40 years have passed our understanding of the rocks has changed and this means that geological maps can be misleading if you are not careful.

Geology according to the current published maps
(after Brunker et al 1972)- scale approximate

I recently had the opportunity to be involved in a project looking for clay deposits in The Channon / Dunoon area. During the project it became obvious that the geology was not what was mapped (to the left is the geology that was mapped in 1972). The investigation that I was lucky enought to be involved with was pretty simple it just involved an excavator digging a few holes in the ground (testpits). Importantly I had an experienced Engineering Geologist to show me what was going on.

Rock weathers to form soils but there is rarely a distinct boundary between soil and rock, a transition occurs. This transition zone is called the regolith which lies below the soil proper at the surface, it is the  transition into saprolite (weathered rock) and then to unweathered rock. If the weathered rock was derived from shales, mudstones and other fine grained sediments then often these layers will become clay. It was this clay that was looked for.

A better interpretation following the testpit investigation
scale approximate

What was done was to dig into the regolith and depending on the characteristics of the saprolite it would be possible to tell what the original rock would have been. From the mapping it was assumed that what we would find would be related to the Lismore Basalt or Kangaroo Creek Sandstone. What was found was neithers. Instead layers of clayey and silty material and bands of weathered coal were visible as well as lithic sandstone. Coal would certainly not occur in abundance in volcanic rocks like the Lismore Basalt and nor does it occur in the Kangaroo Creek Sandstone. Lithic sandstone is also absent from these units. What must have found was lots more of the Walloon Coal Measures.

From my understanding of the area around Dunoon and on the basis of what was found during the hole digging exercise I put together a  rough new map of the area (the second map above). As you can see there is actually a fair amount of difference. So, don't take it for granted that when you look at a geological map it is exactly right. It should be used as a guide and your knowledge should be applied to check it. The amount of coal we found was so abundant that a discussion about this is probably worth another post in the future.

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

Walloon Coal Measures of the Southern Clarence-Morton Basin

In previous posts I’ve briefly discussed the upper most layers of the Clarence-Moreton Basin. The Grafton Formation which overlies the Kangaroo Creek Sandstone which in turn overlies the Woodenbong Beds/MacLean Sandstone Member. The MacLean Sandstone Member is a member of a larger unit called the Walloon Coal Measures and it is this unit that I will briefly comment on now.

I’ve often heard people mistakenly say that the Walloon Coal Measures is a coal seam. This is not correct because the balance of the unit is actually made up of mixed rocks. According to Wells & O’Brien (1994) the coal measures include sandstones (made from volcanic rock fragments), carbonaceous siltstone, shale, mudstone, coal and clayey siltstones. Also clayey ironstone and infrequently oil shale and limestone can be found. Apparently tree stumps remaining in their growth position have also been found, though these have become carbonised (coal). The coal layers themselves are thin (millimetre scale) to occasionally thick (30-40cm) in the Southern Basin but the whole unit of all the different rock types that make up the Walloon Coal Measures totals at least 200 metres of thickness and is variable from location to location.

The coal in the measures is formed from peat that grew in a moist but temperate environment during the early to middle Jurassic in this area (smack in the middle of the age of the dinosaurs). The depositional environment appears to have been mainly flood-plain and meandering stream environments. Boggy mires forming the peat were common, but layers of volcanic ash from occasional volcanic eruptions from close by are preserved. This makes some of the coal seams high in ash content which reduces the quality of the coal. The environment was thought to be reflective of a period of high sea level.

The Walloon Coal Measures in Bexhill Brick Pit at Bexhill

Interestingly, the Walloon Coal Measures are some of the most extensive and continuous sedimentary rock formations in eastern Australia. They are correlated with almost identical units in the Surat Basin and the Maryborough Basin making the potential spatial extent of the unit huge. The outcrop of the Walloon Coal Measures is fairly limited with much obscured by the Grafton Formation, Kangaroo Creek Sandstone and Woodenbong Beds as well as Cenozoic aged volcanic rock especially associated with the Focal Peak and Tweed Volcanic areas. In our region the best exposures are in the Nimbin area and further north but also at Coaldale where the Clarence-Moreton Basin has been deformed creating a bulge which has been eroded exposing the Walloon Coal Measures. Areas to the south of MacLean show some outcrop and on the other side of the Basin, the Kangaroo Creek and areas near Tabulam show good exposures. Other places have exposures of the Walloon Coal Measures because of local faulting and folding that has occurred in places like the Richmond Range.

I understand that coal mining was historically carried out near Tabulam, Kangaroo Creek and Nimbin but the size of the deposits was such that these were only small and fairly short lived enterprises, though Murwillimbah did have a power station earlier last century which was fueled on local coal transported from the area around Tyalgum. Of course now the Walloon Coal Measures has been frequently under discussion regarding its gas potential especially in the form of coal seam gas (CSG) also known as coal bed methane.

The presence of gas in the coal measures is a natural function of coal and the formation of coal when it was formed. As the rock is gently ‘cooked’ following its deposition as peat gases are given off. Peat is made from decayed plant and animal matter which when broken down into its elemental constituents is mainly hydrogen (H) and carbon (C) atoms. The hydrogen is bonded to the carbon in oxygen poor environments and forms methane (CH4) and sometimes more slightly moe complex organic molecules such as C2H6, C3H10 etc, or if conditions are right the molecules are big enough and complex enough to form oils. In the case of the Southern Clarence-Moreton Basin Walloon Coal Measures the conditions were too hot for oil to be stable so the smaller gas molecules are formed. Gas may be trapped in the layers of coal within voids and cracks (called cleats) or they may sometimes migrate to other layers where they can be trapped. This is actually the difference between ‘conventional’ gas and coal seam gas, i.e. all conventional gas was once coal seam gas. Oil shale and shale gas are also present in some areas of the Walloon Coal Measures but these are very rare and are small deposits (I might do a post on these in the future but given their insignificance I might not get there). Russell 1994 noted that the best quality gas, mature or 'dry gas' was likely to be found abundantly in the eastern portion of the basin, whereas wetter gas and oils were likely to be more prevalent in the west. Interestnigly it is thought that the maturity is a response to the thermal changes in the Earths crust during the formation of the Tasman Sea.

The Walloon Coal Measures contains both conventional and coal seam gas and very little oil. Indeed, I understand that substantial amounts of conventional gas was first discovered in the Hogarth Ranges about 40 years ago and that more recently Metgasco have discovered significant amounts at Kingfisher which I think is to the south of Casino. As far as coal seam gas goes, if Walloon Coal Measures are present there is coal and so there is also a chance that gas may also be present.

References/bibliography:

*O’Brien, P.E., Korsch, R.J., Wells, A.T., Sexton, M.J. Wake-Dyster, K. (1994) Structure and Tectonics of the Clarence-Morton Basin. In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*O'Brien, P.E., Powell, T.G. & Wells, A.T. (1994). Petroleum Potential of the Clarence-Moreton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Russell, N.J. 1994. A Palaeogeothermal study of the Southern Clarence Moreton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Wells, A.T. and O'Brien, P.E. 1994. Lithostratigraphic framework of the Clarence-Moreton Basin. In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.

Who has the answer? Woodenbong, MacLean, Grafton and a Kangaroo Creek

How well do we understand how the Clarence-Moreton Basin was formed? We know a little but not much about areas have never been drilled to understand the stratigraphy. A good example of our lack of geological understanding is the areas to the north and west of Kyogle, Bonalbo, Urbenville, etc. This area on the most recently published geological maps includes the area referred to as the Woodenbong Beds. We know very little about this particular formation.

It was actually Queensland based geologists (Exon et al. 1974) that first named the Woodenbong Beds. Exon et al (1974), (although according to the stratigraphic names database Coote (1986) is considered the first reference) described the lower portion of the Woodenbong Beds as massive to medium bedded pale-grey, fine to coarse grained, cross-bedded, labile (easily decomposed) sandstone. The upper portions were described as fine-medium grained feldspathic sandstone with siltstone, mudstone and minor coal. Stratigraphically, Exon et al (1974) also suggested that the Woodenbong Beds were lateral equivalents of the Kangaroo Creek Sandstone and/or the Grafton Formation. The boundary between the underlying Walloon Coal Measures was also described as conformable (that is, no significant time gap between deposition of the units).

Woodenbong beds possible stratigraphic relationships

Subsequent authors such as Wells & O'Brien (1994) have followed on with the definition provided by Exon et al (1974), who extrapolated the interpretation of the Woodenbong Beds to suggest that they may actually be equivalents of the Injune Creek Group (Springbok Sandstone and Westborne Formation) in the Surat Basin.

However, in the very same volume of work as Wells & O'Brien (1994) a different author, Willis (1994) proposed that the Woodenbong Beds actually underlie the Kangaroo Creek Sandstone (and therefore Grafton Formation), suggesting that the MacLean Sandstone Member of the Walloon Coal Measures was equivalent to the Woodenbong Beds. Willis (1994) also cited other authors such as (McElroy 1963, Ellice-Flint 1973 and Scott 1982 (note I have not seen these three publications)). These authors contradicted Exon et al 1974, and Wells & O'Brien 1994 by indicating that the boundary between the underlying Kangaroo Creek Sandstone is in places disconformable/unconformable (meaning there is a hiatus of deposition or a period of erosion preceding the formation of the Kangaroo Creek Sandstone).

The only thing all of the above authors agree on is that the composition of the Woodenbong Beds is very different from the Kangaroo Creek Sandstone and Grafton Formation. I'm sure you would agree that we obviously need more information to figure this one out!

Note that the stratigraphy of these formations have been recently revised since this blog post. See the this post for details.

References/Bibliography:

*Wells, A.T. and O'Brien, P.E. 1994. Lithostratigraphic framework of the Clarence-Moreton Basin. In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Wells, A.T. and O'Brien, P.E. (eds.) 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Willis, I.L. 1994 Stratigraphic Implications of Regional Reconnaissance Observations in the Southern Clarence-Morton Basin, New South Wales In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.

see *Wells, A.T. and O'Brien, P.E. (eds.) 1994. Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.  for other cited authors.