THE NEOPROTEROZOIC CLIMATIC PARADOX - EQUATORIAL PALEOLATITUDE FOR MARINOAN GLACIATION NEAR SEA-LEVEL IN SOUTH AUSTRALIA

New palaeomagnetic analyses have been carried out for the Neoproterozo ic (650-600 Ma) Elatina Formation, an important redbed unit of the Mar inoan glaciogenic sequence in the Adelaide Geosyncline, South Australi a, and flat-lying equivalent facies on the adjacent cratonic Stuart Sh elf and Torrens Hinge Zone. The Marinoan rocks display strong evidence of marine glacial deposition, and coeval periglacial sand wedges in p ermafrost regolith on the Stuart Shelf indicate in-situ cold climate n ear sea level and marked seasonality. The palaeomagnetic data define a palaeopole for the formation and indicate that Marinoan glaciation, i ncluding permafrost, grounded glaciers and marine glacial deposition, occurred near the palaeoequator. Rocks analysed include 97 oriented ou tcrop samples from 15 sites at three widely separated sections(similar to 65-115 m thick) of gently folded and unmetamorphosed sandstone, si ltstone and tillite spanning the Elatina Formation in the Central Flin ders Zone of the Adelaide Geosyncline, soft-sediment folds from tidal rhythmites, and 60 specimens from 54 core samples from six deep drillh oles on the Stuart Shelf and Torrens Hinge Zone. The most stable reman ence components were only completely demagnetised by 685 degrees C, in dicating that haematite is the likely carrier of the remanent magnetis ation. This conclusion is supported by the presence of ultrafine haema titic pigment coating elastic grains and filling interstices in the ro cks. The observation of mixed polarities within some sandstone samples suggests that such lithologies acquired their remamence as chemical r emanent magnetisation (CRM). A positive fold test on syndepositional s oft-sediment folds in tidal rhythmites confirms that the rhythmites ac quired a detrital remanent magnetisation (DRM) by the settling of haem atite grains from suspension in quiet waters. Concordant palaeomagneti c directions determined for the rhythmites and other facies of the Ela tina Formation show that the formation acquired its CRM close to the t ime of deposition. The existence of polarity reversals within a strati graphic 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 pa laeopole derived from the oriented sample results (structurally correc ted) is located at 52.4 degrees S, 347.1 degrees E (d(p) = 3.7 degrees , d(m) = 7.4 degrees); this pole position is consistent with the Neopr oterozoic apparent polar wander path for Australia. The overall format ion mean direction determined from the sample results (structurally co rrected) has a declination of 197.3 degrees and an inclination of -5.3 degrees (alpha(95) = 7.4 degrees), and indicates a mean palaeolatitud e of deposition of 2.7 degrees +/- 3.7 degrees N. These results accord with the virtual geomagnetic pole previously determined for tidal rhy thmites of the Elatina Formation and provide the strongest evidence ye t for the equatorial palaeolatitude of Neoproterozoic glaciation. The palaeomagnetic results, together with geological observations for Mari noan glaciogenic rocks, therefore confirm the Neoproterozoic (pre-Edia caran, greater than or equal to 590 Ma) climatic paradox in South Aust ralia: frigid strongly seasonal climate, permafrost, and grounded glac iers near sea level in equatorial palaeolatitudes. Resolution of this paradox may illuminate Precambrian planetary dynamics and the change i n global state during the Ediacaran.