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.

Do you trust a geological map? (Part 2)

In an earlier post I showed an example of how making the assumption that published geology maps are correct has some big problems. In that example it was the digging of test pits where coal was uncovered which clearly showed the mapping was wrong. Since that post I found another example and this one only needed a look out the car window to know that something was wrong!

My interpretation of the geology of the area in question
Note that the area is approximately 7km across

I had a 4WD day a few weeks ago in the Bungabee State Forest between Lismore and Kyogle. The state forest is located on the southern section of the Mackellar Rangers. It is a nice area but as usual for the Richmond River catchment is invaded by noxious and environmental weeds (In fact the worst variety of weeds I saw that day was at the National Parks managed Muckleewee Mountain Nature Reserve). But I digress... It was during this 4WD trip that I looked out of the window! Where there were cliffs or rocks exposed in streams I glanced out and saw that they were clearly Clarence-Moreton Basin sedimentary rock. I didn't think this was particularly unusual at the time but when a quick opportunity arose I had a closer look.

Present mapping (Brunker et al 1978)
Area is the same as my map above

At the base of the range (near Muckleewee Mountain Nature Reserve) I broke off a fresh bit of rock from a cliff face. The ‘fresh’ sedimentary rock was a rusty brown colour. It was composed of grains that were made from other rock fragments, from feldspar and from quartz. I'd call it a litharenite according to the classification of Pettijon et al (1987), however I might have underestimated the amount of clay particles in it. The appearance of the litharenite was quite dull. I consulted a geological map that night and noticed that the area was not even mapped as sedimentary rock. It was mapped as Lismore Basalt, old lava flows. There were however some areas on the map, a few kilometres away that did have some sedimentary rock (mapped as Kangaroo Creek Sandstone). However, the Kangaroo Creek Sandstone has a distinct saccaroidal texture, a sparkly sugar grained appearance. It was clearly quite different to the rock I was looking at. The only thing that looked consistent to me was larger scale features in the cliff faces showing cross-bedding. However, cross-bedding is a very common feature in most of the Clarence Moreton Basin.

So, with my field observations mind and while reviewing a new stratigraphic guide for the youngest members of the Clarenece-Moreton Basin, the rock I was looking at appeared to be consistent with the expected rock in the Bungawalbin Member of the Orara Formation as defined by Doig & Stanmore (2012). See this previous post for more details. When I found some spare time I did a quick remapping of the area based… As you can see from the pictures my mapping is quite different. This again just goes to show that a geological map might not give you an answer. It is best to look in the field and deeply consider what you find.

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

*Doig, A., & Stanmore, P. 2012. The Clarence-Moreton Basin in New South Wales; geology, stratigraphy and coal seam gas characteristics. Eastern Australian Basins Symposium IV. Brisbane.

*Pettijohn, F.J., Potter, P.E., & Siever, R. 1987. Sand and Sandstone. Springer-Verlag, Berlin

The Orara Formation and the reviewed Kangaroo Creek Sandstone

There has been an increase in our understanding of the Clarence-Moreton Basin in recent years. The central upper portions of the basin have had several cored and un-cored boreholes drilled during exploration for natural gas, especially Coal Seam Gas. In this post, I will cover the implications of this exploration on our understanding of the Kangaroo Creek Sandstone and the recognition of another previously unknown unit.

In a previous post I described the Kangaroo Creek Sandstone. This unit was identified as a formation directly overlying the Walloon Coal Measures (and the MacLean Sandstone Member of the Walloon Coal Measures) (McElroy 1963). However, a recent paper (Doig & Stanmore 2012) attempts to resolve newly identified characteristics which have resulted in the authors proposing the creation of a new formation called the Orara Formation. It is proposed by Doig & Stanmore (2012) that the Orara Formation is comprised of two member units. These are another new unit called the Bungawalbin Member and the now demoted Kangaroo Creek Sandstone Member.

Doig & Stanmore (2012) found that the proposed Bungawalbin Member differed substantially from the Kangaroo Creek Sandstone and overlying Grafton Formation. They describe the Bungawalbin as between 45m-115m thick and dominated by mudstone and carbonaceous mudstone that is thinly bedded with fine grained sandstone with increasing amounts of massive, medium-grained quartzose sandstone beds near the base of the member. The unit is interpreted as a persistent low energy riverine floodplain environment.

The Bungawalbin Members contact with the underlying Kangaroo Creek Sandstone Member is transitional. With the medium-grained quartz rich sandstone becoming dominant in the Kangaroo Creek Sandstone. The composition and formation history of the Kangaroo Creek Sandstone has not been questioned but the significant fine grained component of the Bungawalbin Member necessitates the distinction between these two members. Additionally, Doig & Stanmore (2012) clearly demonstrated that the geophysical response of the Bungawalbin Member was substantially different from the Kangaroo Creek Sandstone.

Hence, we are learning more about the land on which we live. The geology is slowly becoming better understood. It is interesting to observe that there has been no detailed work on the upper most formations of the Clarence-Moreton since the 1960’s. The drilling that has occurred has unlocked more hidden characteristics of the basin. It helps our understanding of basin history as well as the original intention of finding gas resources. There is more to cover in future posts including understanding of the basins youngest formation, the Grafton Formation, but that will come soon.

References/bibliography:

*Doig, A. & Stanmore, P. 2012. The Clarence-Moreton Basin in New South Wales: geology, stratigraphy and coal seam gas characteristics. Paper presented at the Eastern Australasian Basin Symposium IV. Brisbane.
*McElroy, C.T., 1963. The Geology of the Clarence-Moreton Basin. Memoirs of the Geological Survey of New South Wales, Geology. 9.

Geology, stratigraphy, water and CSG a bit more understood

A CSIRO researcher recently provided me with a copy of a conference paper on the Clarence-Moreton Basin that I have been searching for (Doig & Stanmore 2012). I was looking for this information for quite some time as I thought there was much to be learned from it. This is because the research was based upon coal seam gas (CSG) exploration results. It did not disappoint me at all. I have previously blogged on the stratigraphy of the basin but frequent visitors will be aware that there has been a hiatus on this topic. This is because I knew more information had been compiled as a result of gas exploration in the region. In particular this was to do with the Grafton Formation and Kangaroo Creek Sandstone. You can read my previous posts but note that Doig & Stanmore (2012) propose to reclassify these units (see figures 1 and 2 on this post). The information my previous posts were based upon Wells and O’Brien (1994). This is still the most comprehensive guide to the basin but now there is potentially some significant refinements.

Interpretation of the stratigraphy of the upper sequences of the Clarence-Moreton Basin
after Wells & O'Brien (1994) and Willis (1994)

Doig & Stanmore (2012) noted that CSG exploration drilling has provided important clues to the layers that make up the Clarence-Moreton basin that were inferred only through limited field exposure. Drilling provides a nice continuous profile which can be compared to other drill holes and to outcropping information. In the case of Doig & Stanmore (2012) this has completely redrawn the stratigraphy of the upper Clarence-Moreton Basin.

Reviewed Clarence-Moreton Basin stratigraphy after Doig & Stanmore (2012)

I will go into more detail in future posts but I note that Doig & Stanmore (2012) have made some major changes to the Grafton Formation. In particular, they have identified two new distinct members of the formation. The Piora Member and the Rappville Member. As for the underlying Kangaroo Creek Sandstone, this spatially and significant unit has been demoted simply to a member of a newly proposed formation called the Orara Formation. The Orara Formation itself has two distinct formations the demoted Kangaroo Creek Sandstone Member and the new Bungawalbin Member. (See figure 2).

The Woodenbong beds don’t get a guernsey at all in this paper. I suspect that this is because it would better fit into either the Bungawalbin Member of the Kangaroo Creek Sandstone or the Piora Member of the Grafton formation. More work needs to be carried out to make it more certain.

Clearly we do not understand much about the Clarence-Moreton Basin. Even the shallowest geological components! Knowledge keeps improving the more people investigate. The paper provides further interest because identification of the stratigraphic units and geochemical data obtained provide an indication of the risk associated with groundwater resources and CSG production. So as usual, further blog posts are required.

References/bibliography:
*Doig, A. & Stanmore, P. (2012). The Clarence-Moreton Basin in New South Wales; geology, stratigraphy and coal seam gas characteristics. Paper presented at the Eastern Australian Basins Symposium IV, Brisbane.
*Wells, A.T. & O’Brien (1994). Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland, Australian Geological Survey Organisation, Bulletin 241.