Palaeomagnetism of the ejecta-bearing Bunyeroo Formation, late Neoproterozoic, Adelaide fold belt, and the age of the Acraman impact

Abstract

A new palaeomagnetic study has been conducted on haematitic shales and siltstones of the late Neoproterozoic Bunyeroo Formation in the Adelaide fold belt (Geosyncline), South Australia, which host an extensive horizon of shock-deformed rock fragments and microtektite-like material of probable impact origin. Thermal step demagnetisation of 116 samples of red shale and siltstone from six sections (sites) revealed a high-temperature component with a bedding-corrected site-mean direction of remanence of D = 56.6°, I = 29.3° (α95 = 10.7°) that gives a pole at 18.1°S, 16.3°E (dp = 6.5°, dm = 11.8°). The high-temperature component provides a positive tectonic-fold test (99% level of confidence). The Bunyeroo high-temperature remanence direction is near the remanence direction (D = 50°, I = 40°) indicated by modelling the subsurface magnetic source of the central high-amplitude anomaly at Acraman, Australia's largest confirmed meteorite impact structure 220-350 km west of the Adelaide fold belt, and also is close to the mean direction (D = 48.3°, I = 54.7°, α95 = 5.2°) determined for surface melt rock from Acraman. Statistical tests show that the virtual geomagnetic poles indicated by the directions for the subsurface central magnetic source and surface melt rock at Acraman may be regarded as subsets of the Bunyeroo palaeomagnetic pole position, indicating that the three pole positions are statistically indistinguishable. The results imply that the subsurface magnetic source and surface melt rock acquired their remanence in the ambient geomagnetic field during cooling, after the impact when structural disturbance had ceased, while the Bunyeroo Formation was accumulating. The agreement among the various remanence directions argues strongly that the ejecta horizon in the Bunyeroo Formation was derived from Acraman. The present findings confirm that the Acraman impact occurred in the late Neoproterozoic, about 590 Ma, which is the age of the Bunyeroo Formation provided by Rb-Sr whole-rock shale dating of equivalent and contiguous strata in the Adelaide fold belt region. The Bunyeroo palaeomagnetic data give a palaeolatitude of ∼ 15°, indicating a low palaeolatitude for the Acraman impact and supporting other findings that the Adelaide fold belt occupied low to equatorial palaeolatitudes during the late Neoproterozoic.