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.

Lindesay and the volcano

I recently went to Woodenbong via Kyogle. The trip along this section of the Summerland Way is very pretty as you climb into the McPherson Ranges. It also provides many opportunities for good views of imposing Mount Lindesay which is around 1180m high, located right on the state border and is a reminder about mistakes that people make when seeing mountains that are shaped the way they are.

Mount Lindesay from the south

Mount Lindesay is often referred to as a volcanic plug. I've heard this from different people several times. This is not surprising as the shape does imply this, but this is a trick of nature. The upper parts and 'peak' are flows of what is called the Binna Burra Rhyolite (or Mount Gillies Volcanics in Queensland) and some basalt, below this is a layer of obsidian (rhyolitic glass) overlying a layer of rhyolitic ash and agglomerates. The lower parts of the mountain is made from another volcanic rock, basalt (Kyogle Basalt). This basalt however overlies sediments of the Clarence Moreton Basin.

Mount Lindesay gets its shape by the rhyolite that forms the top most layer. The rhyolite is hard, resistant to weathering and therefore remains relatively difficult to erode. It is for this reason that the rhyolite has protected the underlying softer rock at Mount Lindsay and you can see the same process for ridges to the east and south of the mountain too. The actual vents for the rhyolite and underlying basalt lavas is actually a little tricky to definitely locate but we do know that the main volcanic centre for these rocks was at the Focal Peak Volcano located in the vicinity of present day Mount Barney a significant distance to the north. Additionally, there are some real volcanic plugs further to the west which I mention below.

Rhyolite from focal peak was thought by Duggan and Mason (1978) and other authors to extend as far Nimbin to the east. However, recent work by Cotter (1998) has shown that this is not the case but the Binna Burra Rhyolite still extends a long way to the east past places like Wiangaree.

There are however, some clearly identifiable volcanic plugs in the region. A good one is sometimes referred to as the Nightcap Peak and is located half way between Woodenbong and Urbenville just a little to the west of the road. It stands out from the rolling hills, is difficult to miss and is made from the rock granophyre (fine grained granite-like rock). At Urbenville the Northern Obelisk is another example of a plug, a bit one! Additionally, large dykes exist to the south west of Urbenville too.

References/bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.