Geological diversity of the Toonumbar Dam area

Toonumbar Dam is a lovely area that, like so many other places, wish I could visit often. It would be lovely to relax around the dam, maybe stay the night camping or in a cabin. When I last visited, I was rather pathetic... I was looking at the rip-rap on the dam wall and trying to figure out where it was likely to have been quarried! I later found out and visited the quarry to obtain samples and look for structures. But that is a story for another day. As I was saying, the dam is a lovely place and like many beautiful places owes itself to the geological conditions of the area.

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

It is worth noting the bedding plains in the sedimentary rocks if you are downstream of the dam. The plains are actually inclined to the west in this area and the further you go down stream the flatter the beds become, then they tilt back the other way (eastward) for a short distance. This is actually a large basin structure called the Toonumbar Anticline (the top of a fold in the rock layers). Another structure, much bigger and of regional significance is located only another couple of kilometres to the east. This is the East Richmond Fault which extends into southern Queensland and down almost to Grafton. I have actually never seen evidence of this fault in the field, but there is geophysical evidence for it and I'm assured it is there. Apparently the fault is much more evident further south between the villages of Mummelgum and Mallanganee.

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.

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.
*Bell, A.D.M. (1968). Report on the geology of Toonumbar Dam and Appurtenant Works. Water Conservation and Irrigation Commission.

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.

Rocks named after a creek named after an Australian marsupial

Note that the stratigraphy of this formation has been revised since this blog post. See the this recent post for details.

One of the most widely outcropping rock units of the mesozoic aged Clarence Moreton Basin is the Kangaroo Creek Sandstone named after its type locality at Kangaroo Creek in the Nymboida area. It is also one of the most recognisable stratigraphic units in the basin.

McElroy (1963) showed that the Kangaroo Creek Sandstone consisted mainly of white to cream coloured quartz sand. The texture of the sandstone is saccharoidal, that is, it has a glistening sugar like appearance of the quartz sand grains. This sand glistens more than usual because while buried, fluids in the rock caused extra silica (quartz) to crystallise on the existing sand grains creating new tiny crystal faces that reflect light in a vivid way. The nature of the rock in this formation tends to weather less readily than other units and as a result tends to form prominent topographic features such as hills, cliffs, ridges and the like.

Crossbedding and typical saccharoidal texture in Kangaroo Creek Sandstone

The Kangaroo Creek Sandstone was deposited in a fluvial (river) setting and as a result cross bedding structures are very common in outcrops. Sorting of grains in the unit is very well developed, that is, the grain size is very similar at any particular outcrop. Additionally, the thickness of the beds is very consistent which together indicates that the tectonic setting was relatively unchanged through the period of deposition. Following burial of the sandstone fluids present in the rock caused extra dissolved silica to precipitate out onto the existing sand grains filling in voids and creating the characteristic texture.

The Kangaroo Creek Sandstone is considered by some authors (Wells and O'Brien 1998) to grade into the Woodenbong Beds in the north west of the NSW portion of the basin. However, it is noted that others (Willis 1998) consider the Woodenbong Beds the equivalent to the McLean Sandstone Member of the Walloon Coal Measures (but more about this in future post). The Kangaroo Creek Sandstone underlies the Grafton Formation but the contact with this formation is gradational. According to (Wells and O'Brien 1998) it also sometimes shows a conformable boundary with the underlying Walloon Coal Measures, however, in most areas the boundary is shown by an unconformity. It is easy to tell the difference however, because compositionally any sandstones in the Walloon Coal Measures are composed of feldspar and lithic grains rather than the quartz of the Kangaroo Creek Sandstone.

Outcrop of Kangaroo Creek Sandstone on the Clarence River near Grafton

It is interesting to note that the recrystalisation of quartz in the Kangaroo Creek Sandstone means that this unit is now essentially dry with respect to Ground Water. There is very few spaces left for the water to travel through. for example O'Brien et al (1998) shows that most other sandstones in other basins such as the Great Artesian Basin, is where most ground water is obtained. In fact, in the whole of the Clarence Moreton Basin the only unit to have useful ground water bores is the Grafton Formation which is recharged from rainfall. The Kangaroo Creek Sandstone does have some bores that produce a very little water in the upper most portion of the unit (probably rainwater recharging fractures in these locations (Kwantes 2011), like the overlying Grafton Formation) but it appears that no other bores obtain water from the Kangaroo Creek Sandstone because the formation actually behaves like an aquiclude or aquitard. Water is not obtained from aquifers below the Kangaroo Creek Sandstone because the water quality is generally poor.

It is interesting to note that according to some gas exploration results it is apparent that areas of the Kangaroo Creek Sandstone (assuming this is not mistakenly identified McLean Sandstone) that are directly overlying the Walloon Coal Measures contain substantial areas of conventional natural gas. This is gas that has migrated from the underlying Walloon Coal Measures and been trapped in either pore spaces or fracture zones. I understand that several companies in the area such as Metgasco and Red Sky Energy intend to exploit these reserves.

Pollen spores in drill holes give an age of middle to late Jurassic for the Kangaroo Creek Sandstone (Wells and O'Brien 1998).

References/Bibliography:

*Kwantes, E. 2011. Future Water Strategy: Groundwater Options - Position Paper. Report for Rous Water by Parsons Brinkerhoff.
*McElroy, C.T. 1963 The geology of the Clarence-Moreton Basin. New South Wales Geological Survey, Memoir 9, 172 pp.
*Moran, C., Vink, S. 2010 Assessment of impacts of the proposed coal seam gas operations on surface and groundwater systems in the Murray-Darling Basin. The University of Queensland.
*New South Wales Government. 2010. State of the Catchment Report: Groundwater. Northern Rivers Region. Department of Environment, Climate Change and Water.
*Wells, A.T. , 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.
*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.