Friday, 9 December 2011

Top of the Basin: The Grafton Formation

The Clarence Moreton Basin covers a large proportion of the catchment areas of the present day Clarence and Richmond Rivers in northern New South Wales and extends a significant distance more into south east Queensland. The portion of the basin which is most well known is the Queensland section but slowly we are learning more about the southern areas. The basin consists of many individual stratigraphic units which were deposited in slightly different environments at different times. The youngest unit is called the Grafton Formation and is thought to have been deposited during the Mesozoic era called the Cretaceous period which could be as young as 65Ma but it may be as old as late Jurassic.

The extent of the Grafton formation is small by Clarence Morton Basin standards because the majority of the unit appears to have been removed by erosion. Exposures can be found as far as 30km south of Grafton to about 10km north of Casino. The full remaining thickness of the formation has been estimated at up to 442m but is probably less with the best estimate of 267m obtained from a drill hole at Grafton.

Grafton Formation lithic sandstone near Casino
The formation is comprised of interbedded lithic to quartz arenites (sandstones), clayey siltstone, claystone and minor coal, sometimes 2metre thick conglomerate layers are present too. The lithic fragments frequently include the volcanic rock andesite implying active volcanism upstream at the same time as the sediments were being deposited. The bedding can be thin to thick and commonly a ferruginous (iron rich) lateritic weathering profile is present creating a very red coloured soil. This is particularly evident in the hills just to the north of Grafton such as Junction Hill. The sandstones are fairly characteristic in that they are usually tough and green-grey in colour.

One author (Wells and O'Brien 1994) suggests that the Grafton formation (and the Kangaroo Creek Sandstone) may also be equivalent to the Woodenbing beds (located between Urbenville/Woodenbong and Kyogle) and even though they are lithologically (rock composition) different this is still possible. An alternative by Willis 1994 is that it is the equivalent of the McLean Sandstone Member of the Walloon Coal Measures. But this will be discussed in detail in a future post.

The formation overlies the Kangaroo Creek Sandstone and is gradational meaning that the Kangaroo Creek Sandstone grades into the Grafton formation. Thankfully, recognising the difference is not hard on the basis of lithology (rock type) because the Kangaroo Creek Sandstone is very consistent in appearance (saccharoidal texture and abundant cross bedding) and consistent rock composition (quartz sandstone). The top Grafton formation has been eroded and is overlain by the more recent Cenozoic volcanics.

The Grafton formation was deposited in a mainly fluvial (riverine) environment with the more common siltstones and mudstones in the south probably being deposited in a lacustrine (lake) environment. This led to an idea that the source of the rivers and lakes that laid down the sediments in Grafton Formation was from the north but recent revisions of the probable mountain chains that existed at the time means that this many not necessarily be the case. Wells and O'Brien (1994) give the maximum age of the Grafton Formation as late Jurassic.

Interestingly, Grafton Formation is the only rock unit in the Clarence-Moreton Basin that has any significant or active ground water sources. The basin has proven to be a very poor source for water because of the lack of volume. In fact the only volume of water obtained from the Grafton Formation is really only unconfined aquifers recharged from surface water and overlying alluvium.

Note: Since writing this post it has been suggested in a new paper that the Grafton Formation appears to be made up of two members. The new paper by Doig & Stanmore (2012) significantly increases our knowledge of the Grafton Formation. I will endeavour to do a new blog post with the updated details.


*McElroy, C.T. 1969 The Clarence-Moreton Basin in New South Wales. In Packham G.H.(ed) The geology of New South Wales. Geological Society of Australia. Journal 16.
*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.


  1. Hi Rod, I've been recently trawling around the southern part of the Richmond Catchment photographing the flora, and have come across many of the rock and soil types you mention. For instance,Red Hill on the Busby's Flat Road is probably so named because of that iron-rich clay amongst the outcropping sandstone.

    I've also discovered old sand deposits/sand hills in the broad,very low-gradient Myrtle Creek area between Rappville and Main Camp,and wonder whether you know of any work that discusses them. I guess the creeks emerging from the Richmond Range carrying a high sand load have dumped extensive deposits [there is an old sand extraction point at Six Mile Swamp,as you may know],and past dry periods [Pleistocene?] may have seen wind-blown dunes forming here and there. The deposits seem to lie on the edge of the broad depositional plain,and are obviously elderly because they are forested [or now pastured] and weathered to a very low profile.

    These 'dunes',where not utterly cleared, often carry carry some plant species not found often or at all on the neighbouring claystones.

    Could Pleistocene aridity have caused drying to the point that small dune deposits were blown out of the dessicated wetlands? Is there any evidence of lakes in the Myrtle Creek past?

    cheers NF

    1. Hi Nick, quite a few interesting points you've mentioned. I envy you spending time out in the bush doing flora photography, I only occasionally get out and then time constraints usually mean that what I see is close to roads.

      The Bushby's Flat Road is a lovely drive. I don't specifically know Red Hill though. In that area if you travel from east to west you are crossing slightly deformed and faulted sediments of the Clarence Moreton Basin. Grafton Formation to the west moving into the Kangaroo Creek Sandstone which often forms rugged cliffs, rolling hills of the Walloon Coal Measures and then Bundamba Group. Having said that, I'm not sure if any faulting or folding complicates this simplistic cross-section. The folding and faulting in part forms the Richmond Range.

      As for possible sand dunes, you have potentially identified a very interesting feature for palaeoclimate reconstruction. I've never heard about any dune systems in the Richmond Range area. However, dune systems near several Lagoons on the Northern Tablelands (especially the Llangothlin Lagoons) have aeolian sand deposits which have formed in a more arid setting during the Pleistocene. This sounds very similar to the concept you've come up with.

      The area is worth looking at in more detail!

    2. Thanks for the reply,Rod.

      Red Hill is the short steepish pinch up from Hoult's Bridge over Busby's Creek when you are heading east back to Casino via Mongogarie Creek valley. It's not much of a hill,but I guess back in the horse and cart days it certainly was,particularly when wet. Soil is yellow/orange,and sandstone outcrops can be seen. Almost at the crest,the road bed exposes sandstone bedrock 'blocks' which appears ochre/orange with whitish margins along the fine joints. Very nice Spotted Gum/Ironbark forest along there.

      Re the dunes,I don't know whether they rise to 'system' status,but I've crossed isolated individuals. If you're travelling to Grafton via the Summerland Way, a little way south of the bridge at Myrtle Creek Elliott's Road joins from the left and travels down the creek at the edge of its floodplain. A few hundred metres along it the road rises gently over a low broad hill with some cattleyards on the left. At the bottom of the hill is an old creek channel/lagoon arcing in from the current creek bed location well north. It's all pretty degraded grazing land,but there are Red Gums,Bitter Bark trees,Coast Banksia and Large Nectar Heath shrubs persisting despite the trampling. The latter three are sand lovers,though not absolutely restricted to such substrate. Eroded ground south of the road shows a pale yellow/brown sand that seems quite deep...I think this low broad hill a few hundreds of metres across is a 'dune' accumulated on the toe of the bedrock slope where it joins the depositional plain of Myrtle Creek,it lies sort-of NW-SE on the SW edge of the Quaternary alluviums that occupy so many km2 of that valley....perhaps if you're travelling that way some time you could run your eye over it!

      I don't think this is the most convincing dune example but certainly it is the most easily accessed.

      Cheers and as ever thanks for the blog,it is always fascinating!