Low palaeolatitude of Late Proterozoic glaciation: early timing of remanence in haematite of the Elatina Formation, South Australia

Previous palaeomagnetic study of a ∼ 10 m thick rhythmite member of the Elatina Formation, part of the Late Proterozoic (∼ 650 Ma) Marinoan glacial succession in South Australia, argued strongly for a low palaeolatitude (∼ 5°) of deposition. However, a fold test was thwarted by the approximate alignment of the remanence with the axis of tectonic folding. Here we report results of a fold test on soft-sediment slump folds with a wavelength of 14–22 cm in the Elatina rhythmites, the axes of which are approximately perpendicular to the direction of remanence. Fifty-one standard palaeomagnetic specimens sampled across the slump structure gave α95 = 3.7° prior to unfolding, α95 = 1.8° with 67% unfolding, and α95 = 2.4° with 100% unfolding. The results indicate that the remanence was acquired very soon after deposition; apparently it was acquired prior to soft-sediment slumping and then slightly sheared by the disturbance. The Elatina pole must be considered a virtual geomagnetic pole because the tidal rhythmite member studied represents only 60–70 years of deposition. However, the mean pole position is similar to other Late Proterozoic poles for South Australia, which implies that the very low inclination (< 10°) for the Elatina rhythmites does not record a geomagnetic excursion or reversal but does indeed indicate deposition in low palaeolatitudes. The similarity of many of the Late Proterozoic pole positions is evidence also that inclination error is not significant. The low palaeolatitude of Late Proterozoic glaciation is one of the major enigmas in contemporary Earth science, raising questions concerning the nature of the geomagnetic field, climatic zonation, and the Earth's rotational parameters in Late Proterozoic time.