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SocietyUpdate Stratigraphic COLUMN Magnetosomes: a new kind of rock unit At Magnetostratigraphy exploits variations in the magnetic properties of rocks as a basis for recognising or correlating units. The most familiar type of magnetostratigraphy is the use of the direction of the primary remnant magnetism, which records the polarity of the Earth’s magnetic field at the time of formation of the rock. A geomagnetic polarity time scale (GPTS) has been constructed, consisting of intervals of normal and reversed magnetism. Because polarity reversals happen simultaneously over the whole world, they can be used for global correlation. If you can measure magnetic reversals in a sedimentary section, you can use them to correlate it with other sections, and if you have fossil information as well, you can correlate the contained polarity reversals with the appropriate part of the radiometrically calibrated GPTS to give numerical ages. The intervals of the GPTS are magnetic polarity units, called chrons, but there is also a different kind of magnetostratigraphic unit that uses magnetic susceptibility or some other distinctive magnetic property of the rock to delineate the rock body. Magnetic susceptibility is essentially controlled by the composition, concentration and grain size of magnetic minerals in the rock. Examples of such bodies are granite plutons, which often have a distinctive magnetic signature. In a suite of plutons, completely subsurface ones can be mapped by their magnetic patterns. This has been done in Victoria, where the buried plutons have been named. As the word pluton is a form term rather than a stratigraphic one, these names have traditionally fallen outside the ambit of stratigraphy. Recently the Victorians fielded us something that was more problematic. Metamorphic units, such as the Adaminaby Group, can also be extrapolated below cover by using magnetics. The problem was that a unit, believed to be metamorphic, was being interpreted and a formal lithostratigraphic name was being proposed for it, despite the fact that it had no surface outcrop and had not been drilled into. No-one had seen a sample of the rock, and its lithology was being inferred from the magnetic character alone. Oh, and by the way, the unit was not even in Victoria: it was across the border in New South Wales – the Victorians publish complete maps, not just the bits on their side of the border. While one geophysicist was adamant that such interpretation is pretty reliable, others considered that relying totally on magnetic data to interpret a particular lithology or formation is very iffy. There is, for example, no way to be absolutely sure that the magnetic material is contained within a lithological formation: alteration and metamorphism can create magnetic mineral distributions that cut across lithological and structural boundaries. The observed pattern may be caused by more than one subsurface unit, at different levels. Associated non-magnetic lithologies may be present but not detected. In any case, formally defining a unit as a lithostratigraphic unit, requires that a type section be specified and described, which is not possible in these cases. It must be dealt with in some informal way, such as using only a symbol (eg, Omx) or an informal name that is clearly different from a formal lithostratigraphic one. But what informal term can be used There is no convenient formal term for sediments and metamorphics analogous to pluton for intrusive rocks. The word beds comes to mind, but this is already used for formal names despite being uncapitalized. Some other suggestions were magnetostratigraphic unit, magneto-beds, magnetodeme and magnetosome, among others. The Australian Stratigraphy Commission at its meeting last July decided on magnetosome. The –some affix comes from soma, Greek for body. The same root is used in the words lithosome and metasomatism. To emphasise its informal nature, the word must always remain uncapitalised. It will be used in conjunction with a geographic name, eg Tueloga magnetosome. The use of geographic names was a bit contentious, given the shortage of place names in some areas, but the expectation is that most of these units will eventually be converted into formal lithostratigraphic units when the required data becomes available, and it would make life easier if they don’t have to be renamed when that happens. In the meantime, the names will be recorded in the Australian Stratigraphic Units Database to ensure no duplications arise, so it will, as usual, be essential to reserve these names. As for any name, other workers have to know what exactly is meant by it, hence the publication of a new name should be accompanied by a description of what is known about the unit, and how it can be recognised. We also decided that because seismic properties can also be used to delineate subsurface units, there may be some cases in the future when informal names might be needed for these units as well. For them, the term seismosome is permitted. But don’t feel that there is any imperative to rush around naming new magnetosomes and seismosome: the terms are available if needed, but their use is not obligatory, and in many cases it will be quite sufficient to use only map symbols instead, as has already been done in most of the country. ALBERT BRAKEL National Convener Australian Stratigraphy Commission TAG December 2006 | 11
SocietyUpdate Heritage Matters Challenges Ahead There are lots of challenges ahead for geology, amongst which are challenges for geological heritage. The standing Committee for Geological Heritage has, over the years, regularly published material in TAG. This will now appear in each issue as a regular column ‘Heritage Matters’. This title has been chosen to encompass the wide range of heritage and conservation matters across Australia, while reinforcing the concept that geological heritage does matter. Sites and features have both been used in Australia to describe respectively areas of small or large extent, or aspects of geology and geomorphology such as a fossil locality, a type section, a landform or other geological features that may have no particular extent. The term geological monument has been used for sites and features that have been recognised as significant. GSA uses the following definition: Significant geological features are those features of special scientific or educational value which form the essential basis of geological education, research and reference. These features are considered by the geological community to be worthy of protection and preservation. The GSA's Heritage Policy details these principles further and is available on the main GSA website (www.gsa.org.au) Many people may think that geological heritage is a relatively new area of interest for GSA. Bernie Joyce has outlined how GSA has been involved in this for over thirty years in this issue (see ‘Letters’). Perhaps it was a rather special group (as distinct from a specialist group!) but it has been important in many areas. Bernie’s letter covers the issue comprehensively and does not need me to reiterate it. There are lots of challenges ahead for geology, amongst which are challenges for geological heritage. Issues of inappropriate development, poor management, and inaccurate signage, coupled with ignorance and bureaucratic intransigence all need to be countered. All of this takes time and energy. I hope this regular column will help and look forward to many members participating. SUSAN WHITE National Convener GSA Heritage Standing Committee 12 | TAG December 2006
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