The Neoproterozoic climatic paradox: Equatorial palaeolatitude for Marinoan glaciation near sea level in South Australia

Abstract

New palaeomagnetic analyses have been carried out for the Neoproterozoic (650-600 Ma) Elatina Formation, an important redbed unit of the Marinoan glaciogenic sequence in the Adelaide Geosyncline, South Australia, and flat-lying equivalent facies on the adjacent cratonic Stuart Shelf and Torrens Hinge Zone. The Marinoan rocks display strong evidence of marine glacial deposition, and coeval periglacial sand wedges in permafrost regolith on the Stuart Shelf indicate in-situ cold climate near sea level and marked seasonality. The palaeomagnetic data define a palaeopole for the formation and indicate that Marinoan glaciation, including permafrost, grounded glaciers and marine glacial deposition, occurred near the palaeoequator. Rocks analysed include 97 oriented outcrop samples from 15 sites at three widely separated sections (∼ 65-115 m thick) of gently folded and unmetamorphosed sandstone, siltstone and tillite spanning the Elatina Formation in the Central Flinders Zone of the Adelaide Geosyncline, soft-sediment folds from tidal rhythmites, and 60 specimens from 54 core samples from six deep drillholes on the Stuart Shelf and Torrens Hinge Zone. The most stable remanence components were only completely demagnetised by 685°C, indicating that haematite is the likely carrier of the remanent magnetisation. This conclusion is supported by the presence of ultrafine haematitic pigment coating clastic grains and filling interstices in the rocks. The observation of mixed polarities within some sandstone samples suggests that such lithologies acquired their remamence as chemical remanent magnetisation (CRM). A positive fold test on syndepositional soft-sediment folds in tidal rhythmites confirms that the rhythmites acquired a detrital remanent magnetisation (DRM) by the settling of haematite grains from suspension in quiet waters. Concordant palaeomagnetic directions determined for the rhythmites and other facies of the Elatina Formation show that the formation acquired its CRM close to the time of deposition. The existence of polarity reversals within a stratigraphic section and within some samples therefore argues strongly for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the formation. The palaeopole derived from the oriented sample results (structurally corrected) is located at 52.4°S, 347.1°E (dp = 3.7 °, dm = 7.4°); this pole position is consistent with the Neoproterozoic apparent polar wander path for Australia. The overall formation mean direction determined from the sample results (structurally corrected) has a declination of 197.3° and an inclination of -5.3° (α95 = 7.4°), and indicates a mean palaeolatitude of deposition of 2.7° ± 3.7°N. These results accord with the virtual geomagnetic pole previously determined for tidal rhythmites of the Elatina Formation and provide the strongest evidence yet for the equatorial palaeolatitude of Neoproterozoic glaciation. The palaeomagnetic results, together with geological observations for Marinoan glaciogenic rocks, therefore confirm the Neoproterozoic (pre-Ediacaran, ≥ 590 Ma) climatic paradox in South Australia: frigid strongly seasonal climate, permafrost, and grounded glaciers near sea level in equatorial palaeolatitudes. Resolution of this paradox may illuminate Precambrian planetary dynamics and the change in global state during the Ediacaran. © 1995 Elsevier Science B.V. All rights reserved.