Pw. Schmidt et Ge. Williams, PALEOMAGNETISM OF THE EJECTA-BEARING BUNYEROO FORMATION, LATE NEOPROTEROZOIC, ADELAIDE FOLD BELT, AND THE AGE OF THE ACRAMAN IMPACT, Earth and planetary science letters, 144(3-4), 1996, pp. 347-357
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.