The Australian Geologist - Geological Society of Australia
The Australian Geologist - Geological Society of Australia
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