Soft-sediment deformation structures from the Marinoan glacial succession, Adelaide foldbelt: implications for the palaeoaltitude of late Neoproterozoic glaciation

Abstract

A suite of soft-sediment deformation structures occurs in tidal rhythmites of fine sand to silt grade in the late Neoproterozoic (∼600 Ma) Elatina Formation, which is part of the Marinoan glaciogenic succession in the Adelaide foldbelt, South Australia. The structures include: (1) sets of asymmetrical cuspate ridges (γ = 13–50 cm, h = 3–5 cm) formed on bed surfaces and underlain by folds affecting as much as 60 cm thickness of strata; (2) symmetrical and interference ripple forms (γ = 3–15 cm, h = ≤1.5 cm) mostly confined to the troughs between the cuspate ridges and which are underlain by folds, involving up to 20 cm thickness of strata, that commonly parallel the undulations of the bed surface but in places have steepened limbs; (3) rill marks on the flanks of cuspate ridges and some ripple 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 deposits that formed after transformation of surficial sediment to a hydroplastic state, possibly by the cyclic stresses generated by storm waves, and its sliding on tidal-delta slopes. The ripple forms resulted from continuing wave activity and were maintained by draping and vertical accretion from unidirectional currents and locally by deposition of supercritical cross-lamination. The further deformation of the cuspate folds, as revealed by palaeomagnetic analyses of the structures, implies additional sliding and/or the differential loading of hydroplastic sediment in the troughs between the cuspate ridges.This study confirms that positive palaeomagnetic fold-tests on several cuspate folds indicate a primary origin for the shallow palaeomagnetic inclination (−5.3°) of the Elatina Formation and hence the equatorial palaeolatitude of late Neoproterozoic glaciation in South Australia.