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 λ_p = 46.7°S, longitude ϕ_p = 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.

南澳阿德莱德地槽Sturt冰川作用沉积物（约660Ma）和Elatina冰川作用沉积物（约635Ma）之间的晚Cryogenian阶间冰期地层Angepena组的红层（白云质粉砂岩和细砂岩）中的新古地磁数据，连同以前的古地磁数据，表明该区域在约25Ma之久的间冰期期间位于低古纬度。从弗林德斯山脉西部的DepotFlat的Angepena组及弗林德斯山脉东北部的Nepouie Creek和Balcanoona Creek的Angepena组中取得的热退磁样品，显示高温组分衰减到原点〜680℃，与唯一的磁性矿物赤铁矿的存在相一致。恢复地层的古水平面得到置信度为99％的正褶皱检验。远隔地点的一致性结果和正褶皱检验支持这样的论点：寒武纪Delamerian造山运动期间，在地层褶皱之前就获得了剩磁。褶皱伸展（N = 71个标本）后的平均方向是磁偏角D = 25.3°，倾角I = 8.7°（α95= 4.0°），表明极位在南纬λp= 46.7°，东经φD357.3°（A95 = 3.2°）和古纬度λ= 4.4±2.0°之处。对随压实变浅的可能倾斜的校正可能将倾斜增大至12°和20°之间，根据所使用的平坦化因子，可能将古纬度增加到6.1-10.3°。逆转试验被评为B级，正常和扭转分布的临界角为8.6°，即在5°和10°之间。新的数据和以前的阿德莱德地槽及西澳Officer盆地的新元古代地层的古地磁资料共同表明，澳大利亚南部留在整个Cryogenian期间位于古纬度区。

KEY WORDS:

• Adelaide Geosyncline
• Cryogenian
• Sturt glaciation
• Elatina glaciation
• interglacial succession
• paleomagnetism

ACKNOWLEDGEMENTS

We thank Wolfgang Preiss (Geological Survey of South Australia) for guidance in the field, Dave Clark (CSIRO, Sydney) for commenting on the manuscript, and Sergei Pisarevsky and Jim Jago for helpful reviews.