Saturday, 21 March 2015

Bedding with crossbedding through it

Cross-bedding in sandstone of the Evans Head Coal Measures
Shark Bay - Evans Head area
Cross-bedding is a common feature in many of the Mesozoic aged sedimentary rocks in our region particularly in the Clarence-Moreton Basin. Cross-bedding is a structure that can be confusing. However, is it is often very useful for understanding how a sedimentary rock is laid down. Because it is common in our region I thought it might be interesting to describe what this feature is.

Cross bedding forms in sedimentary rocks that have undergone transport. It is most easy to find cross bedding in sandier sediments that have been deposited in Aeolian (windy) and fluvial (riverine) environments. However, it is a feature that may be found less commonly in shallow marine, and estuarine environments but these processes are a tiny bit different and more complicated to describe, so I’ll deal with them some other time.

Sketch showing how cross-bedding is formed
The feature can be confusing especially because the cross-bedding can be mistaken as actual bedding layers. However, technically speaking, cross-beds are always laid down within the same bed. The cross-beds in a riverine environment form when the water in a stream loses energy and its ability transport sediment. The sediment then drops out of the water and is deposited along a point bar. Over time the river may dry up or migrate away and the point bar (now one big bed with cross-bedding) can then be preserved.

Cross-bed sets in sandstone of the Orara Formation (Kangaroo Creek Sandstone)
Eden Creek - Kyogle Area
Often sets of cross-bedding are present where the river will deposit another point bar over the top of the original. When this occurs the new bed usually erodes the upper part of the original bed. This is a useful bit of information because in some areas the rocks have been so deformed that it can be hard to tell whether they are upside down or not. If you are able to find cross-bedding in these rock looking for the erosion surface will tell you whether the rocks are right way up or have been turned over. It may be surprising to note that over turned bedding is actually common in the metamorphosed sediments in the New England and Tweed region. Since deformation of the Clarence-Moreton Basin has been relatively small it is unlikely that you will come across in-situ rocks that have been turned over in this basin.

The two pictures show examples from some of the oldest rocks of the Clarence-Moreton and Ipswich Basins and the one of the youngest. Despite being laid down up to 100 million years apart the manner of deposition of these two separate units was a very similar riverine environment. Nearly any outcrop of Orara Formation will show cross-bedding. So keep a look out at road cuttings or sandstone quarries.

Monday, 2 March 2015

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.

*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