Thursday, 22 November 2012

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.

Thursday, 15 November 2012

Some musings on coal seam methane

I thought I’d get myself in trouble again on the coal seam gas subject by having some musings on the public presentation by two academics at Southern Cross University last night. Of course the topic relates to the potential impact to the environment by the Coal Seam Gas Industry in the Northern Rivers. Research has been conducted by Dr Isaac Santos and Dr Damien Maher from the School of Environment, Science and Engineering. The subject of the presentation was a preliminary report on the status of water and air chemistry research at Tara in the southern Queensland Darling Downs which are being conducted Dr Santos and Dr Maher. This is up my alley and so I couldn’t help have some of my own thoughts of the matter. It is important to note that the research has not yet been published in a scientific journal as yet, but when or if it is, I will return to this topic. It is even more important to note that this post has no references as it is just some personal musings on the matter.

To summarise very, very broadly what the research appears to have found is evidence of methane directly attributable to geological derived methane gas stores (called thermogenic methane) as opposed to biogenic methane sources such as animals or organic matter decay in stagnant water, etc. This methane was measured in both surface water and in the air in an active coal seam gas extraction area in southern Queensland. The isotopes (isotopes are elements with slightly different numbers of neutrons in the atom, e.g. Carbon usually has either 13 or 14 neutrons) of the carbon that makes up methane (methane is a combination of carbon and hydrogen (CH4)) were measured during this process. The slightly different masses in these isotopes means that they have different stability in different environments (e.g. heating of coal would preferentially release one isotope over another). The isotopic composition can therefore be used to give an indication of whether the methane is biogenic or thermogenic. 

Where the interesting bit comes in is where Dr Santos and Dr Maher then draw on some comparisons. They had a look at the Richmond River valley and took observations. The observations were different, the Richmond River valley showed lower concentrations of thermogenic methane relative to Tara. Their suggestion then arises that it is the active gas field in Queensland has higher levels of fugitive emissions as a direct result of the coal seam gas industry operating in there. This suggestion can then be extended to presume that should the industry develop in the Richmond River Valley the levels of methane in the air and water would also increase. Now, this is a reasonable chain of assumptions but there are some things missed if the data is not interpreted properly. From my cursory understanding of the data sets gathered by Dr Maher and Dr Santos there are two (potentially contradictory) variables that could significantly affect their findings:
  1. The actual amount of thermogenic methane may be underestimated because CSIRO researcher John Smith has shown that during or following catagenesis (generally geological formation of gas), some methane may be converted into carbon dioxide depending on the abundance of dissolved oxygen in any formation waters, this carbon dioxide can then be naturally reconverted into methane. The process during reconversion from carbon dioxide into methane preferentially uses the biogenic indicator isotope. This can then give the false impression that the methane is biogenic and therefore actually underestimating the effect of the coal seam gas industry on gas emissions.
  2. The assumption that the natural levels of gas dispersal though normal processes of gas dispersal is the same in south Queensland (Surat Basin and Gunnedah Basins) is the same as that in the Northern Rivers (southern Clarence-Moreton Basin) may be erroneous. The geological units are different both in the depositional, structural, deformational, and erosional history for these basins. This is important because sources of thermogenic gas can be close to the surface in the Surat and Gunnedah Basins but in the Richmond River Valley the gas bearing layers are mostly trapped by a thick succession impermeable rocks such as the Kangaroo Creek Sandstone and Grafton Formation. This may mean a comparison is not a reasonable thing to do and therefore that the gas in Queensland is naturally occurring and not as a result of anthropogenic impacts. The low level of thermogenic methane in the Richmond River valley may simply be a reflection of the geology and any development of the industry in the northern rivers may actually not increase the amount of methane in the local water and air.
During the survey carbon dioxide levels were also measured. The weird thing is that the levels of carbon dioxide in the Tara area was higher than that in the Richmond Valley. When coal seam gas is targeted the presence of carbon dioxide sometimes means that the gas has been affected by some process that has degraded the quality of the gas and therefore carbon dioxide rich resevoirs are not as good economically as low carbon dioxide ones. As such these high CO2 reservoirs tend not to be the tapped to the same degree. The abundance of CO2 is therefore confusing as it does not seem to fit cleanly into the picture of an athropogenically affected local atmosphere.

Dr Maher and Dr Santos have both demonstrated many useful and interesting environmental studies over the years. I have no question of their ability to collect good quality data. It is important to note though that Dr Maher did caution about jumping to the immediate conclusion that the results they observed were totally due to the gas industry. Both of the researches did say that it was important that further data was required to give a more definitive answer by determining whether the geological assumptions were correct.

My view at this stage is that we should not jump to conclusions that it is actually the gas industry that is causing the measurable difference in methane from one location to another… though it may well be. When people say the science demonstrates this or that it is important to note that there are assumptions made in interpreting data and those assumptions could actually understate impacts or even the opposite, overstate them. One thing I think is important to note is that we really don’t understand much of the world around us and therefore we don’t really know if what we see is due to us or not. Slowly we learn more and this helps, but there is so still far to go.

Sunday, 11 November 2012

In the hills of Valla and Nambucca Heads

The Valla Adamellite now termed the Valla Monzogranite to reflect modern naming conventions is an interesting small to medium sized intrusion about 10km north east of Nambucca Heads. It is one of the suites of coastal granites which are mostly I-Types (melted igneous material), this means that the coastal granites show abundances of ore minerals within the granite or in the surrounding metamorphosed country rocks. A monzogranite is a granite with roughly equal proportions of (alkali-feldspar (potassium and sodium rich) and plagioclase feldspar (calcium rich)). The monzogranite is thought to have formed during the Triassic period.

The metamorphic aureole for the Valla Monzogranite is actually quite interesting as it shows a classic zonation of metamorphism (high grade at the contact grading to low grade further away) and also excellent examples of mineral zonation associated with metasomatism (hot-water or fluid alteration of rock). The Valla Monzogranite has been shown to be associated with gold, silver, arsenic and molybdenum mineralisation (as well as others). The rock that the monzogranite has been intruded into is called the Nambucca Beds which are part of the Nambucca Block. The Nambucca Beds are Permian to Carboniferous in age and are mainly comprised of the regionally metamorphic rock type called phyllite which was originally deposited on the sea floor. The Nambucca Block was accreted onto the Australian continent in the New England Orogen and this caused the regional metamorphism of the beds.

The Nambucca Beds are intruded by the Monzogranite. The Beds are extensive and
extend far into the rugged Nambucca Hinterland. This photo is west of Bowraville.
The Valla Monzogranite seems to be a Climax Molybdenum Deposit named after the Climax Mine in North America. This means that when the Monzogranite was cooling the upper portion of the pluton became residually enriched with fluids, metals and silica. These fluids cause alteration of the upper portion of the pluton forming what is called greisen and also are injected into the surrounding rock through veins and sometimes aggressively through breccia pipes. One of the first minerals to form in these veins is silica, quartz with metal sulphide such as molybdenite (molybdenum ore) and wolframite (tungsten ore). Further away from the intrusion the degree of alteration becomes less grading through potassic through to argillic which are defined alteration zones based on the changes in the rock forming minerals. As the degree of alteration becomes less so the types of metal ores change with increasing amounts or arsenic, gold and silver. Further out in the alteration zone minerals such as galena form (lead ore) and finally stibnite (antimony ore). These ore deposits seem to be fairly common in the New England area with Glen Eden being the most studied (Somarin 2001, Somarin & Ashley 2004) and have in some areas been extensively explored such as Kingsgate east of Glen Innes.

Some attempts of mining have occurred in the Valla Monzogranite in the past, the most significant being the Valla Gold mine which was located just to the north of Valla Beach. The mine was abandoned with very little rehabilitation and therefore has become an environmental problem for the local creek. However, rehabilitation efforts have recently been undertaken, though these will need another post to discuss in more detail.


*Somarin, A.K. 2011. Petrography, Geochemistry, and Petrogenesis of Late-Stage Granites: An Example from the Glen Eden Area, New South Wales, Australia. Earth and Environmental Sciences.
*Somarin, A.K. & Ashley, P.M. 2004. Hydrothermal Alteration and Mineralisation of the Glen Eden Mo-W-Sn deposit: A Leucogranite related hydrothermal system, southern New England Orogen, NSW, Australia. Mineralium Deposita.

Thursday, 1 November 2012

Softer sediments in the Wilsons River Valley

I’ve recently been observing an interesting environmental restoration programme on the Wilsons River upstream of Lismore. During this programme I started to think about the flood plain of the river and ‘recent’ geological history of the area. Cotter 1998 and in his earlier undergraduate work developed a concept of the geomorphology due to the lava flows associated with the Tweed Volcano and the earlier Alstonville Basalt. I did an earlier post on the flow direction of the Wilsons River as related to the volcanic history of the area but I’ve done just about nothing on the post volcanic sequences.

The best work done on the ‘recent’ sedimentary formations of the Wilsons River valley was a PhD thesis, Drury 1982. This was done as part of the then Water Resources Commission (now State Water) back when NSW government departments actually collected new information to guide future decision making (oops, there is a political comment in there). Drury 1982 was a huge thesis that provides a vast amount of information on the development of the Richmond Valley based mainly on the groundwater bores operating at the time supplemented by some (then) new drilling and geophysical techniques. To my knowledge no significant further published scientific assessment of the Quaternary sequences has occurred since Drury's thesis was written.

Cenozoic Stratigraphy of the Lismore area
It is probably hard to follow the stratigraphy very easily so hopefully my sketch to the left which is based on Drury’s work helps. Drury 1982 indicates that the upper most layer of sediment in the Wilsons River and Leycester Creek valleys upstream from Lismore was unsurprisingly, flood plain sands, silts and clays which continue to be deposited today following floods. Conformably underlying this flood plain sediment the material encountered is called the Green Ridge formation. This formation appears to be a delta system being built at the end of the Upper Pleistocene (~12,000 years ago). Often the top of the Green Ridge Formation is cut by the Wilsons River and its tributaries, for instance at Boat Harbour Nature Reserve the lower banks of the river seem to be quite deep maybe even cutting into the even older formations (e.g. the Gundurimba Clay). Drury (1982) demonstrated that the Green Ridge formation is both contemporary with and overlies the Gundurimba Clay, which is made from estuarine clays.

The Gundurimba Clay is a unit was formed during a period of relatively high sea level (higher than the present day) and warmer conditions. Shells were common but coral was found maybe indicating the idea that the area where the Gundarimba Clay was being deposited went through a warmer spell than we experience now.  Drury 1982 identified pollen spores indicating the surrounding area was dominated by rainforest with some eucalypt forest too, in my mind this creates a picture that it was possibly a proto-‘Big Scrub’ low-land rainforest with the ‘Big Scrub’ proper forming after the next cold period at the beginning of the Holocene. However worth noting that the Upper Pleistocene is recognised around the world as starting off in a warm period turning into a glacial period with the last glacial maximum occurring around 22,000 years ago.

Drury 1982 demonstrated that preceding the deposition of the Gundurimba Clay there was a period of erosion meaning that the Gundurimba Clay unconformably overlies the South Casino Gravel. The South Casino gravel in turn uncomformably overlies the Cenozoic volcanic rocks of the Lismore and/or Alstonville Basalt. The South Casino gravel is at least Middle Pleistocene in age and is derived from the erosion of the underlying volcanics. Given its coarse nature it is highly permeable and is considered a good source of groundwater in other parts of the Richmond Valley but to my knowledge is rarely used in the Wilsons River area.

I'm probably trying to combine a lot into this one post so I'll have to tease out the details a bit more in future posts, especially that relating to the Gundurimba Clay and palaeo-environmental conditions which I know at least some of my blog readers have a keen interest in. At least I hope that this provides a starting point.


*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales: In Two Volumes. PhD thesis, University of New South Wales.
*Cotter, S. 1997. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of north eastern New South Wales. MSc thesis, Southern Cross University.