Jl

High stability sediments much older in age

Low stability sediments or igneous rock much older in age

Stable igneous rocks much older in age

Sediments of similar age

Igneous rocks of similar age

Fig. 3.8. The baked contact test. The variation in direction of magnetization (arrows), magnetization (M), and scatter (a) with distance from an igneous intrusion in five possible situations is illustrated schematically. After Irving (1964), with permission from John Wiley & Sons.

warmed zone is the unheated country rock. Five possible situations are illustrated in Fig. 3.8.

In the first case (Fig. 3.8A), the country rock is much older than the intrusion and consists of sedimentary material of high stability. The occurrence of high magnetization (M) at the contact falling to low values at a distance, coupled with agreement between the directions of magnetization of the intrusion and baked contact rock, indicates that both are stable and that the magnetization was acquired at the time the igneous intrusion cooled. The increase in the intensity of M may be up to a factor of 10, since the magnetization of the sediment is usually due to CRM, whereas the baked rock now has a TRM. The scatter (a = 1/k) of the directions may or may not change, except in the warmed zone due to the addition of two differing vector directions. Consistent directions in the unheated sedimentary rock, which differ from both that of the present Earth's field and that of the intrusion, provide further evidence for the stability of the unheated sedimentary rock (other than evidence from consistency or laboratory tests, etc.). The baked contact test now provides evidence that the magnetization of the unheated sediment has been stable at least since the time of the intrusion. This is referred to as the inverse contact test as defined in §3.3.1.

In Fig. 3.8B, the country rock is still much older than the intrusion but consists of sedimentary or igneous material of low magnetic stability. M decreases from the contact into the unheated zone, whereas a increases and scattered directions are observed. In Fig. 3.8C, the country rock is again much older than the intrusion but consists of stable igneous material. This situation is similar to that in Fig. 3.8A but in this case no change in the intensity of M need be observed between the baked rock and the unheated zone. The magnetization of the unheated country igneous rock is thus shown to have been stable since the time of the intrusion representing another example of an inverse contact test.

When the country rock is only a little older or of comparable age to that of the intrusion, the situation is less favorable. If the country rock is sedimentary material (Fig. 3.8D) the main variation will be a decrease in the intensity of M between the baked zone and the unheated zone, a change that could be up to a factor of 10 if the original magnetization in the sedimentary rocks was a CRM. The most unfavorable situation (Fig. 3.8E) is when the country rock is stable igneous rock of the same age as the intrusion. No variations will be seen from the baked zone to the unheated zone, a situation that could also occur from general heating due to a period of regional metamorphism.

It should be noted that the inverse is also true. If stable magnetizations are observed in unbaked rocks that provide positive evidence for a baked contact test such as illustrated in Figs 3.8A and Fig. 3.8C, then this also provides evidence that the unbaked sediments have retained their magnetization at least since the time of the baking. This is referred to as the inverse contact test.

3.3.5 Unconformity Test

The unconformity test can be applied in the special case when successive zones of normal and reverse magnetization are truncated by an unconformity in the sequence, as proposed by Kirschvink (1978). In the example shown in Fig. 3.9, the lower part of a rock sequence has recorded zones of normal and reverse magnetization. Sedimentation ceased and erosion took place to produce the unconformity prior to deposition of the upper part of the sequence. If the polarity zones in the upper younger sequence do not match those in the lower older

Fig. 3.9. The unconformity test. Zones of normal (shaded) and reverse (white) magnetizations in the lower sequence are truncated by an unconformity. If the polarity zonation in the upper sequence differs from that of the lower sequence, then the magnetization of the lower beds predates the unconformity. After Kirschvink (1978), with permission from Elsevier Science.

Fig. 3.9. The unconformity test. Zones of normal (shaded) and reverse (white) magnetizations in the lower sequence are truncated by an unconformity. If the polarity zonation in the upper sequence differs from that of the lower sequence, then the magnetization of the lower beds predates the unconformity. After Kirschvink (1978), with permission from Elsevier Science.

sequence, then the magnetization in the lower beds is older than the episode of erosion that created the unconformity. On the other hand, if the polarity zones show continuity across the unconformity, then the sediments must carry a CRM that postdates the younger sedimentation.

3.3.6 Consistency and Reversals Tests

If a single geological unit or formation can be sampled over a wide area and through a considerable thickness, in which are represented a variety of rock types with differing mineralogy, and if consistent directions of magnetization are observed, then there is good reason to believe that the magnetization has been stable since the time of formation. This is generally referred to as the consistency test. However, it is important that the directions observed should also differ from that of the present Earth's field or of any other field direction that has been observed in rocks of younger age from the same craton or block. Agreement with known directions of a younger age can be a strong indication that remagnetization has taken place.

The presence of reversals of magnetization represented by two groups of directions that are 180° apart is a much stronger consistency test than simple consistency of directions without reversals. If, subsequent to formation, both groups acquire a secondary component of magnetization, they will both change in the same direction towards the secondary magnetic field direction. The two resultant groups of directions will then not be 180° apart (Fig. 3.10). It is thus necessary to be able to test whether these opposing directions of magnetization differ discernibly from being 180° apart. Such a test is called the reversals test. Originally the simple procedure was to invert one of the directions by 180° and then test if the resulting two directions of magnetization were discernibly

Normal

Reverse

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