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The Australian Geologist - Geological Society of Australia

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<strong>Society</strong>Update<br />

Stratigraphic COLUMN<br />

Magnetosomes: a new kind <strong>of</strong> rock unit<br />

At Magnetostratigraphy exploits variations in the magnetic<br />

properties <strong>of</strong> rocks as a basis for recognising<br />

or correlating units. <strong>The</strong> most familiar type <strong>of</strong><br />

magnetostratigraphy is the use <strong>of</strong> the direction <strong>of</strong> the primary<br />

remnant magnetism, which records the polarity <strong>of</strong> the Earth’s<br />

magnetic field at the time <strong>of</strong> formation <strong>of</strong> the rock. A geomagnetic<br />

polarity time scale (GPTS) has been constructed, consisting<br />

<strong>of</strong> intervals <strong>of</strong> normal and reversed magnetism. Because<br />

polarity reversals happen simultaneously over the whole world,<br />

they can be used for global correlation. If you can measure<br />

magnetic reversals in a sedimentary section, you can use them<br />

to correlate it with other sections, and if you have fossil<br />

information as well, you can correlate the contained polarity<br />

reversals with the appropriate part <strong>of</strong> the radiometrically<br />

calibrated GPTS to give numerical ages.<br />

<strong>The</strong> intervals <strong>of</strong> the GPTS are magnetic polarity units, called<br />

chrons, but there is also a different kind <strong>of</strong> magnetostratigraphic<br />

unit that uses magnetic susceptibility or some other<br />

distinctive magnetic property <strong>of</strong> the rock to delineate the rock<br />

body. Magnetic susceptibility is essentially controlled by the<br />

composition, concentration and grain size <strong>of</strong> magnetic minerals<br />

in the rock. Examples <strong>of</strong> such bodies are granite plutons,<br />

which <strong>of</strong>ten have a distinctive magnetic signature. In a suite <strong>of</strong><br />

plutons, completely subsurface ones can be mapped by their<br />

magnetic patterns. This has been done in Victoria, where the<br />

buried plutons have been named. As the word pluton is a form<br />

term rather than a stratigraphic one, these names have traditionally<br />

fallen outside the ambit <strong>of</strong> stratigraphy.<br />

Recently the Victorians fielded us something that was more<br />

problematic. Metamorphic units, such as the Adaminaby<br />

Group, can also be extrapolated below cover by using magnetics.<br />

<strong>The</strong> problem was that a unit, believed to be metamorphic,<br />

was being interpreted and a formal lithostratigraphic name<br />

was being proposed for it, despite the fact that it had no surface<br />

outcrop and had not been drilled into. No-one had seen a<br />

sample <strong>of</strong> the rock, and its lithology was being inferred from<br />

the magnetic character alone. Oh, and by the way, the unit was<br />

not even in Victoria: it was across the border in New South<br />

Wales – the Victorians publish complete maps, not just the bits<br />

on their side <strong>of</strong> the border. While one geophysicist was<br />

adamant that such interpretation is pretty reliable, others considered<br />

that relying totally on magnetic data to interpret a particular<br />

lithology or formation is very iffy. <strong>The</strong>re is, for example,<br />

no way to be absolutely sure that the magnetic material is contained<br />

within a lithological formation: alteration and metamorphism<br />

can create magnetic mineral distributions that cut<br />

across lithological and structural boundaries. <strong>The</strong> observed pattern<br />

may be caused by more than one subsurface unit, at<br />

different levels. Associated non-magnetic lithologies may be<br />

present but not detected.<br />

In any case, formally defining a unit as a lithostratigraphic<br />

unit, requires that a type section be specified and described,<br />

which is not possible in these cases. It must be dealt with in<br />

some informal way, such as using only a symbol (eg, Omx) or<br />

an informal name that is clearly different from a formal lithostratigraphic<br />

one. But what informal term can be used <strong>The</strong>re is<br />

no convenient formal term for sediments and metamorphics<br />

analogous to pluton for intrusive rocks. <strong>The</strong> word beds comes<br />

to mind, but this is already used for formal names despite being<br />

uncapitalized. Some other suggestions were magnetostratigraphic<br />

unit, magneto-beds, magnetodeme and magnetosome,<br />

among others.<br />

<strong>The</strong> <strong><strong>Australia</strong>n</strong> Stratigraphy Commission at its meeting last<br />

July decided on magnetosome. <strong>The</strong> –some affix comes from<br />

soma, Greek for body. <strong>The</strong> same root is used in the words lithosome<br />

and metasomatism. To emphasise its informal nature, the<br />

word must always remain uncapitalised. It will be used in conjunction<br />

with a geographic name, eg Tueloga magnetosome.<br />

<strong>The</strong> use <strong>of</strong> geographic names was a bit contentious, given the<br />

shortage <strong>of</strong> place names in some areas, but the expectation is<br />

that most <strong>of</strong> these units will eventually be converted into formal<br />

lithostratigraphic units when the required data becomes<br />

available, and it would make life easier if they don’t have to be<br />

renamed when that happens. In the meantime, the names will<br />

be recorded in the <strong><strong>Australia</strong>n</strong> Stratigraphic Units Database to<br />

ensure no duplications arise, so it will, as usual, be essential to<br />

reserve these names. As for any name, other workers have to<br />

know what exactly is meant by it, hence the publication <strong>of</strong> a<br />

new name should be accompanied by a description <strong>of</strong> what is<br />

known about the unit, and how it can be recognised.<br />

We also decided that because seismic properties can also<br />

be used to delineate subsurface units, there may be some cases<br />

in the future when informal names might be needed for these<br />

units as well. For them, the term seismosome is permitted.<br />

But don’t feel that there is any imperative to rush around<br />

naming new magnetosomes and seismosome: the terms are<br />

available if needed, but their use is not obligatory, and in many<br />

cases it will be quite sufficient to use only map symbols<br />

instead, as has already been done in most <strong>of</strong> the country.<br />

ALBERT BRAKEL<br />

National Convener<br />

<strong><strong>Australia</strong>n</strong> Stratigraphy Commission<br />

TAG December 2006 | 11

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