Boring fossils, but fossils none-the-less

Note that the stratigraphy of the Kangaroo Creek Sandstone has been recently revised since this blog post. See the this post for details.

In a previous post I discussed some interesting finds in the Dunoon and The Channon area. I discussed how the geological maps of the area are in places incorrect because coal was found in areas that were not expected to contain any. There are two old (Jurassic aged) sedimentary formations present in the area the younger one is called the Kangaroo Creek Sandstone. The Kangaroo Creek Sandstone is mainly comprised of quartz rich sands cemented together and altered by a later period where silica was precipitated on the sand grains creating what is called a saccharoidal texture. 
The slightly older formation is called the Walloon Coal Measures and although It was known to occur in the area was not expected to be as widely found. The upper most part of Walloon Coal Measures in this area is a lithic sandstone, a sandstone where the sand grains are actually pieces of rock with different minerals including quartz and feldspar. This sandstone is much duller and weathers to form more silts and clays rather than sandy soils as you’d expect to be found around weathered Kangaroo Creek Sandstone.



Woody fossil in lithic sandstone

The lithic sandstone also contains a fair amount of woody fossil fragments. I was surprised how easy it was to find some. The photo to the right shows some of the poor quality fossil wood (technical problems mean that I cant upload the photo - will sort this out soon hopefully). These fossils were located within several metres below the boundary between the Kangaroo Creek Sandstone and the Walloon Coal Measures. It was interesting though, that I could not recognise the type of boundary between these two units even though I could recognise them a short distance apart. That is, I don’t know whether the boundary was gradational or an unconformity.



A moderately thick layer of weathered coal

Of course as far as fossils go, there are thick units of coal in the Walloon Coal Measures and there is no exception in this formation in the Dunoon area. Coal is accumulations of organic material such as leaves and wood and algae that has not been exposed to the oxidising environment before it is compressed by the overlying strata to turn it into a rock. When I was able to be involved in digging some excavator holes in the ground looking for some clay earlier this year I was surprised to see how much coal was present. The coal found was of course very weathered and degraded by the process of recent erosion and natural soil formation, but it burns a bit if you let it dry out (you are still better off getting fire wood).
I think what was most interesting for me was the way in which fossil material could be found wherever the Walloon Coal Measures outcrop. This means that they could be just about anywhere in the valleys in the Nimbin area or near Tabulam, on the way to Wooli, Coaldale Valley, all over the place. If you are interested in finding poor quality fossils grab a geological map and look for the boundary between the Kangaroo Creek Sandstone and Walloon Coal Measures.

Do you trust a geological map?

The NSW Geological Survey have produced the maps that we use today. They have recently placed all of them online for people to view which is... well... excellent! They can be found here. Some areas have excellent, up-to-date 1:100 000 scale maps which are exceptional. However, it is worth mentioning that the best scale you will find for most of the state is 1:250 000 older 1970's maps. These maps are good but they were mostly done through looking at aerial photographs with limited field checking and since nearly 40 years have passed our understanding of the rocks has changed and this means that geological maps can be misleading if you are not careful.

Geology according to the current published maps
(after Brunker et al 1972)- scale approximate

I recently had the opportunity to be involved in a project looking for clay deposits in The Channon / Dunoon area. During the project it became obvious that the geology was not what was mapped (to the left is the geology that was mapped in 1972). The investigation that I was lucky enought to be involved with was pretty simple it just involved an excavator digging a few holes in the ground (testpits). Importantly I had an experienced Engineering Geologist to show me what was going on.

Rock weathers to form soils but there is rarely a distinct boundary between soil and rock, a transition occurs. This transition zone is called the regolith which lies below the soil proper at the surface, it is the  transition into saprolite (weathered rock) and then to unweathered rock. If the weathered rock was derived from shales, mudstones and other fine grained sediments then often these layers will become clay. It was this clay that was looked for.

A better interpretation following the testpit investigation
scale approximate

What was done was to dig into the regolith and depending on the characteristics of the saprolite it would be possible to tell what the original rock would have been. From the mapping it was assumed that what we would find would be related to the Lismore Basalt or Kangaroo Creek Sandstone. What was found was neithers. Instead layers of clayey and silty material and bands of weathered coal were visible as well as lithic sandstone. Coal would certainly not occur in abundance in volcanic rocks like the Lismore Basalt and nor does it occur in the Kangaroo Creek Sandstone. Lithic sandstone is also absent from these units. What must have found was lots more of the Walloon Coal Measures.

From my understanding of the area around Dunoon and on the basis of what was found during the hole digging exercise I put together a  rough new map of the area (the second map above). As you can see there is actually a fair amount of difference. So, don't take it for granted that when you look at a geological map it is exactly right. It should be used as a guide and your knowledge should be applied to check it. The amount of coal we found was so abundant that a discussion about this is probably worth another post in the future.

References/bibliography:

Brunker R.L., Cameron R.G., Tweedale G. and Reiser R., 1972, Tweed Heads 1:250 000 Geological Sheet SH/56-03, 1st edition, Geological Survey of New South Wales, Sydney

Walloon Coal Measures of the Southern Clarence-Morton Basin

In previous posts I’ve briefly discussed the upper most layers of the Clarence-Moreton Basin. The Grafton Formation which overlies the Kangaroo Creek Sandstone which in turn overlies the Woodenbong Beds/MacLean Sandstone Member. The MacLean Sandstone Member is a member of a larger unit called the Walloon Coal Measures and it is this unit that I will briefly comment on now.

I’ve often heard people mistakenly say that the Walloon Coal Measures is a coal seam. This is not correct because the balance of the unit is actually made up of mixed rocks. According to Wells & O’Brien (1994) the coal measures include sandstones (made from volcanic rock fragments), carbonaceous siltstone, shale, mudstone, coal and clayey siltstones. Also clayey ironstone and infrequently oil shale and limestone can be found. Apparently tree stumps remaining in their growth position have also been found, though these have become carbonised (coal). The coal layers themselves are thin (millimetre scale) to occasionally thick (30-40cm) in the Southern Basin but the whole unit of all the different rock types that make up the Walloon Coal Measures totals at least 200 metres of thickness and is variable from location to location.

The coal in the measures is formed from peat that grew in a moist but temperate environment during the early to middle Jurassic in this area (smack in the middle of the age of the dinosaurs). The depositional environment appears to have been mainly flood-plain and meandering stream environments. Boggy mires forming the peat were common, but layers of volcanic ash from occasional volcanic eruptions from close by are preserved. This makes some of the coal seams high in ash content which reduces the quality of the coal. The environment was thought to be reflective of a period of high sea level.

The Walloon Coal Measures in Bexhill Brick Pit at Bexhill

Interestingly, the Walloon Coal Measures are some of the most extensive and continuous sedimentary rock formations in eastern Australia. They are correlated with almost identical units in the Surat Basin and the Maryborough Basin making the potential spatial extent of the unit huge. The outcrop of the Walloon Coal Measures is fairly limited with much obscured by the Grafton Formation, Kangaroo Creek Sandstone and Woodenbong Beds as well as Cenozoic aged volcanic rock especially associated with the Focal Peak and Tweed Volcanic areas. In our region the best exposures are in the Nimbin area and further north but also at Coaldale where the Clarence-Moreton Basin has been deformed creating a bulge which has been eroded exposing the Walloon Coal Measures. Areas to the south of MacLean show some outcrop and on the other side of the Basin, the Kangaroo Creek and areas near Tabulam show good exposures. Other places have exposures of the Walloon Coal Measures because of local faulting and folding that has occurred in places like the Richmond Range.

I understand that coal mining was historically carried out near Tabulam, Kangaroo Creek and Nimbin but the size of the deposits was such that these were only small and fairly short lived enterprises, though Murwillimbah did have a power station earlier last century which was fueled on local coal transported from the area around Tyalgum. Of course now the Walloon Coal Measures has been frequently under discussion regarding its gas potential especially in the form of coal seam gas (CSG) also known as coal bed methane.

The presence of gas in the coal measures is a natural function of coal and the formation of coal when it was formed. As the rock is gently ‘cooked’ following its deposition as peat gases are given off. Peat is made from decayed plant and animal matter which when broken down into its elemental constituents is mainly hydrogen (H) and carbon (C) atoms. The hydrogen is bonded to the carbon in oxygen poor environments and forms methane (CH4) and sometimes more slightly moe complex organic molecules such as C2H6, C3H10 etc, or if conditions are right the molecules are big enough and complex enough to form oils. In the case of the Southern Clarence-Moreton Basin Walloon Coal Measures the conditions were too hot for oil to be stable so the smaller gas molecules are formed. Gas may be trapped in the layers of coal within voids and cracks (called cleats) or they may sometimes migrate to other layers where they can be trapped. This is actually the difference between ‘conventional’ gas and coal seam gas, i.e. all conventional gas was once coal seam gas. Oil shale and shale gas are also present in some areas of the Walloon Coal Measures but these are very rare and are small deposits (I might do a post on these in the future but given their insignificance I might not get there). Russell 1994 noted that the best quality gas, mature or 'dry gas' was likely to be found abundantly in the eastern portion of the basin, whereas wetter gas and oils were likely to be more prevalent in the west. Interestnigly it is thought that the maturity is a response to the thermal changes in the Earths crust during the formation of the Tasman Sea.

The Walloon Coal Measures contains both conventional and coal seam gas and very little oil. Indeed, I understand that substantial amounts of conventional gas was first discovered in the Hogarth Ranges about 40 years ago and that more recently Metgasco have discovered significant amounts at Kingfisher which I think is to the south of Casino. As far as coal seam gas goes, if Walloon Coal Measures are present there is coal and so there is also a chance that gas may also be present.

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.
*O'Brien, P.E., Powell, T.G. & Wells, A.T. (1994). Petroleum Potential 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.
*Russell, N.J. 1994. A Palaeogeothermal study of the Southern 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. 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.

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.

Coraki has its faults

Coraki is a nice little town on the Richmond River just near its confluence with the Wilsons River. The town is located on the flood plain and therefore many parts of it can be inundated in the case of major floods. The flood plain provides a relatively fertile plain that grows excellent pastures and much sugar cane, especially the further down stream on the Richmond you go. But Coraki has its hidden faults.

Being an active flood plain the area surrounding Coraki is dominated by recent alluvial deposits generally of Holocene age but with lots of slightly older Pleistocene alluvial and estuarine sedimentary deposits. Areas that are under permanent shallow unconfined ground water influence tends to retain pyrite which is produced by bacteria in an anaerobic (oxygen poor) environment (i.e. under stagnant water). When this pyrite is exposed to the atmosphere or more oxygenated water by the action of drainage for agricultural, construction or flood mitigation purposes the pyrite oxidises. Pyrite is Iron Sulphide (Fe₂S) which with water (H₂O) forms H₂SO₄ which is more well known as sulphuric acid. This acid can then be discharged causing degradation to aquatic life or degradation of land creating unproductive acid scalds.

Not all of the town is in the flood plain, in fact about half is located on some low hills that are comprised of Kangaroo Creek Sandstone. The Kangaroo Creek Sandstone is part of the Clarence Moreton Basin and its exposure here may be partly due to a fault called the Coraki Fault. In the area of Coraki and also at Tullymorgan and maybe even places like Clifden near Grafton the faulting of the Coraki Fault has created some unusual features within the Mesozoic Clarence Morton Basin and the underlying Palaeozoic basement rocks. These features cannot be seen on the Earths surface but can only be identified by geophysical techniques, in particular seismic surveys.

So, what are the features that can’t be seen? Well, there is the Coraki fault itself which is a dextral strike-slip fault meaning that the eastern side of the fault has moved northwards relative to the western side. But there is also a weird structure which is referred to as a “flower structure”. This occurs when another fault is present perpendicular to the main fault. This creates a central wedge shaped block which near Coraki has been squeezed by the faults upward and created here, slightly more elevation in the Kangaroo Creek Sandstone and possibly other units of the Clarence Morton Basin. This is probably hard to visualise, so maybe a diagram will help when I can get one to embed.

Blog Note: I like to provide photos for these sort of posts but recently where I store photos (skydrive and/or GoogleDocs) has changed its method for providing URLs to allow embedding of these files and Blogger doesn't like the new URLs. So, these next blogs might be a bit more bland looking until I figure out a better way to store and embed photos.

Note that the stratigraphy of the Kangaroo Creek Sandstone has been recently 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. 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. 

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.




References/bibliography:

*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.