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. Crossbedding is a structure that can be confusing but is often very useful for understanding how a sedimentary rock was laid down. Given how common it is 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 in shallow marine, and estuarine environments. However, 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 confused as actual bedding layers. However - technically, cross beds are always laid down within the same bed. The cross beds in a riverine environment 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 crossbedding 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 rocks simply looking for the erosional 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. However, 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.
