Diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments, South Island, New Zealand

Detailed magnetostratigraphic studies of Late Neogene siliciclastic sediments of the Awatere Group, South Island, New Zealand (41°45'S, 174°05'E) have revealed a wide range of palaeomagnetic behaviour. Examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic, X-ray diffraction and electron microprobe analyses. These analyses indicate that the ferrimagnetic iron sulphide minerals, greigite and pyrrhotite, are responsible for a stable and intense magnetic remanence in fine-grained sediments, whereas titanomagnetite is the only remanence-bearing mineral identified in coarser-grained sediments, which are less strongly and less stably magnetised than the fine-grained sediments. Detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation, which occurs commonly in rapidly deposited, anoxic sediments that support active sulphate reduction and H2S formation. Preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process, probably due to the low permeability of the fine-grained sediments and consumption of available H2S before full reaction to pyrite occurred. Relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution.