SOFT-SEDIMENT DEFORMATION STRUCTURES FROM THE MARINOAN GLACIAL SUCCESSION, ADELAIDE FOLDBELT - IMPLICATIONS FOR THE PALEOLATITUDE OF LATE NEOPROTEROZOIC GLACIATION

A suite of soft-sediment deformation structures occurs in tidal rhythm ites of fine sand to silt grade in the late Neoproterozoic (similar to 600 Ma) Elatina Formation, which is part of the Marinoan glaciogenic succession in the Adelaide foldbelt, South Australia. The structures i nclude: (1) sets of asymmetrical cuspate ridges (lambda = 13-50 cm, h = 3-5 cm) formed on bed surfaces and underlain by folds affecting as m uch as 60 cm thickness of strata; (2) symmetrical and interference rip ple forms (lambda = 3-15 cm, h = less than or equal to 1.5 cm) mostly confined to the troughs between the cuspate ridges and which are under lain by folds, involving up to 20 cm thickness of strata, that commonl y parallel the undulations of the bed surface but in places have steep ened limbs; (3) rill marks on the flanks of cuspate ridges and some ri pple forms. The crests of the cuspate ridges and ripple forms commonly were draped and locally eroded and truncated during overall vertical accretion. The cuspate structures are interpreted as gravity slide dep osits that formed after transformation of surficial sediment to a hydr oplastic state, possibly by the cyclic stresses generated by storm wav es, and its sliding on tidal-delta slopes. The ripple forms resulted f rom continuing wave activity and were maintained by draping and vertic al accretion from unidirectional currents and locally by deposition of supercritical cross-lamination. The further deformation of the cuspat e folds, as revealed by palaeomagnetic analyses of the structures, imp lies additional sliding and/or the differential loading of hydroplasti c sediment in the troughs between the cuspate ridges. This study confi rms that positive palaeomagnetic fold-tests on several cuspate folds i ndicate a primary origin for the shallow palaeomagnetic inclination (- 5.3 degrees) of the Elatina Formation and hence the equatorial palaeol atitude of late Neoproterozoic glaciation in South Australia.