Post - Depositional Diagenesis and Chemical Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 75

a b c

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No
Tilt Correction

60%
Tilt Correction

Full
Tilt Correction

w

E

Bed to SE

Bed to NW
Average Bedding Strike

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Fig. 6.3 Stepwise fold test for the remagnetized Helderberg Formation carbonate rocks. The site means cluster best when the
fold limbs are 'unfolded' by 60% of their bedding tilt, indicating that the rocks were remagnetized during folding. C McCabe
and RD Elmore, The occurrence and origin of Late Paleozoic remagnetization in the sedimentary rocks of North America,
Review of Geophysics , 27, 4, 471-494, 1989. Copyright 1989 American Geophysical Union. Reproduced by permission from
American Geophysical Union.

magnetization. The hysteresis plots were described as
' wasp - waisted ' , i.e. their middle sections were narrower
than their upper and lower sections (Fig. 6.5).
Jackson interpreted the wasp-waisted plots as due to
a mixture of large (multi-domain) and small (single
domain) magnetic grains (magnetite in the case of the
remagnetized carbonates). The wasp-waisted hystere-
sis signature was subsequently used by workers to help
identify remagnetized carbonate rocks. Jackson also
showed that a high ARM/SIRM ratio (> 0.10) could
also be used as evidence for remagnetization.
Lu & McCabe (1993) and Sun et al . (1993) both
attacked the problem of primary appearing AMS
fabrics in a remagnetized carbonate, the Onondaga
Limestone, fi rst reported by Kent (1979). Lu and
McCabe measured remanence anisotropies, both AAR
and AIR, for remagnetized carbonates from the south-
ern Appalachians and found primary appearing fabrics
for the AAR in low-coercivity (large) magnetic grains
and secondary tectonic fabrics for the AIR in high-
coercivity (small) magnetic grains.
Sun et al . (1993) approached the problem slightly
differently, examining the AAR of many different

Fig. 6.4 Magnetite framboids assumed to be diagenetic
separated from the remagnetized Bonneterre Dolomite
(McCabe et al . 1987). Scale bar is 10 μm. C McCabe, R
Sassen and B Saffer, Occurrence of secondary magnetite
within biodegraded oil, Geology , 15, 7-10, 1987. Copyright
1987 American Geophysical Union. Reproduced by
permission from American Geophysical Union.