PALEOMAGNETISM OF THE EJECTA-BEARING BUNYEROO FORMATION, LATE NEOPROTEROZOIC, ADELAIDE FOLD BELT, AND THE AGE OF THE ACRAMAN IMPACT

A new palaeomagnetic study has been conducted on haematitic shales and siltstones of the late Neoproterozoic Bunyeroo Formation in the Adela ide fold belt (Geosyncline), South Australia, which host an extensive horizon of shock-deformed rock fragments and microtektite-like materia l of probable impact origin. Thermal step demagnetisation of 116 sampl es of red shale and siltstone from six sections (sites) revealed a hig h-temperature component with a bedding-corrected site-mean direction o f remanence of D = 56.6 degrees, I = 29.3 degrees (alpha(95) = 10.7 de grees) that gives a pole at 18.1 degrees S, 16,3 degrees E (dp = 6.5 d egrees, dm = 11.8 degrees), The high-temperature component provides a positive tectonic-fold test (99% level of confidence). The Bunyeroo hi gh-temperature remanence direction is near the remanence direction (D = 50 degrees, I = 40 degrees) indicated by modelling the subsurface ma gnetic source of the central high-amplitude anomaly at Acraman, Austra lia's largest confirmed meteorite impact structure 220-350 km west of the Adelaide fold belt, and also is close to the mean direction (D = 4 8.3 degrees, I = 54.7 degrees, alpha(95) = 5.2 degrees) determined for surface melt rock from Acraman, Statistical tests show that the virtu al geomagnetic poles indicated by the directions for the subsurface ce ntral magnetic source and surface melt rock at Acraman may be regarded as subsets of the Bunyeroo palaeomagnetic pole position, indicating t hat the three pole positions are statistically indistinguishable. The results imply that the subsurface magnetic source and surface melt roc k acquired their remanence in the ambient geomagnetic field during coo ling, after the impact when structural disturbance had ceased, while t he Bunyeroo Formation was accumulating, The agreement among the variou s remanence directions argues strongly that the ejecta horizon in the Bunyeroo Formation was derived from Acraman. The present findings conf irm 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 Ade laide fold belt region, The Bunyeroo palaeomagnetic data give a palaeo latitude of similar to 15 degrees, indicating a low palaeolatitude for the Acraman impact and supporting other findings that the Adelaide fo ld belt occupied low to equatorial palaeolatitudes during the late Neo proterozoic.