The oldest rocks (Mesozoic aged Clarence-Moreton Basin) exposed in the area are actually exposed downstream from the dam itself. Several hundred metres downstream are poor exposures of what appears to be rocks of the Jurassic Walloon Coal Measures, immediately downstream (and all around the dam) is the Kangaroo Creek Sandstone which is obvious to identify up close. The rocks which are apparently of the Walloon Coal Measures are a little harder to distinguish. It is possible that they are members of the MacLean Sandstone (which are considered part of the Walloon Coal Measures) or maybe Woodenbong Beds or even the underlying Bundamba Group but they are certainly younger than the Kangaroo Creek Sandstone.
|Inclined bedding in Kangaroo Creek Sandstone |
In Iron Pot Creek below the dam. Cross-bedding is also evident
The large rugged hill and ridge about 5km north of the dam is made from basalt lava, I'm not sure of the exact composition of this rock but it is likely to be part of the Kyogle Basalt which is associated with the Focal Peak Volcano. Interestingly, I think that the basalt is likely not to have been sourced from the actual peak of the volcano but from a distant vent on the side. This is because a few kilometres to the north west just on the north side of the lake is actually one of at least two intrusions of gabbro (the intrusive equivalent of basalt) near Toonumbar, one of these is crossed by Murrays Scrub Road. It is possible that these intrusions were the feeder systems for vents which erupted the Kyogle basalt in this area. This probably demonstrates the nature of volcanism in the area during the Cenozoic period. It seems apparent that the central volcano models of the Focal Peak and even the Tweed Volcanoes appears to be a bit too simplistic.
But, whether you are interested in geology or just enjoy the forests of the Northern Rivers, a trip to Toonumbar Dam is worth while.
Note that the stratigraphy of the Kangaroo Creek Sandstone has been revised since this blog post. See the this post for details.
*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.
*Bell, A.D.M. (1968). Report on the geology of Toonumbar Dam and Appurtenant Works. Water Conservation and Irrigation Commission.