Sunday, 15 July 2012

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.

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