Low paleolatitude for the late Cryogenian interglacial succession, South Australia: paleomagnetism of the Angepena Formation, Adelaide Geosyncline

Abstract

New paleomagnetic data for red beds (dolomitic siltstone and fine-grained sandstone) from the Angepena Formation of the late Cryogenian interglacial succession between deposits of the Sturt glaciation (ca 660 Ma) and Elatina glaciation (ca 635 Ma) in the Adelaide Geosyncline, South Australia, together with previous paleomagnetic data, indicate that the region lay in low paleolatitudes during the ca 25 Ma-long interglacial interval. Thermal demagnetisation of samples from the Angepena Formation at Depot Flat in the western Flinders Ranges and Nepouie Creek and Balcanoona Creek in the northeastern Flinders Ranges revealed a high-temperature component that decayed to the origin at ∼680°C, consistent with hematite being the only magnetic mineral present. Restoring the strata to the paleohorizontal yielded a positive fold test at the 99% confidence level. The concordance of results for widely separated sites and the positive fold test argue for the early acquisition of remanence before folding of the strata during the Cambrian Delamerian Orogeny. The mean direction after unfolding (n = 71 specimens) is declination D = 25.3°, inclination I = 8.7° (α95 = 4.0°), indicating a pole position at latitude λp = 46.7°S, longitude ϕp = 357.3°E (A95 = 3.2°) and a paleolatitude λ = 4.4 ± 2.0°. Correction for possible inclination shallowing with compaction could increase the inclination to between 12° and 20° depending on the flattening factor used, which would increase the paleolatitude to 6.1–10.3°. A reversal test was rated B, as the critical angle for the normal and reverse distributions is 8.6°, i.e. between 5° and 10°. The new data, and previous paleomagnetic data for Neoproterozoic strata from the Adelaide Geosyncline and the Officer Basin in Western Australia, together indicate that southern Australia remained in low paleolatitudes throughout the Cryogenian.