Abstract The basal Ediacaran global boundary stratotype section and point (GSSP) horizon beneath Nuccaleena Formation cap dolostone in South Australia's central Flinders Ranges South Australia coincides with an interpreted unconformity preceding deglacial transgression. Detailed lithostratigraphy of three sections across the base of the Ediacaran System at its type area reveals contrasting character of the Elatina Formation—Nuccaleena Formation transition across c . 9 km of exposure, changing the interpretive context of the GSSP. We suggest that a locally pervasive, incisive flaser-bedded sandstone exposed between Elatina diamictites and Nuccaleena cap dolostone lies above an unconformity that correlates with the defined base of Wilpena Group, reflecting onset of terminal Elatina ‘Snowball Earth’ deglaciation and dynamic interplay between eustatic sea level change and isostatic rebound. Nuccaleena cap dolostone is sedimentologically mixed and conformable with underlying siliciclastics at Elatina Creek; hence the recently defined Ediacaran GSSP horizon, at the base of solid cap carbonate at Enorama Creek, lies in continuous section and not at an unconformity. Nuccaleena Formation cap dolostone contains pervasive terrigenous debris, including apparently detrital hematite. While magnetite and/or maghemite is produced in abundance upon heating of the cap carbonate above c . 400°C, and we cannot exclude secondary origin of any Nuccaleena magnetite, Nuccaleena Formation cap dolostone should preserve primary magnetization.

Abstract The Neoproterozoic Seacliff Sandstone occurs as a series of incised-valley fill deposits in the Adelaide Geosyncline of South Australia. It is the basal formation of an unconformity-bounded depositional sequence, formally referred to as the Sandison Subgroup, and is composed of a succession of interbedded sandstone, siltstone, and dolostone. A number of well exposed sections were measured to interpret the sedimentology, geometry, and cyclicity of the Seacliff Sandstone and to present a sequence-stratigraphic model of the incised-valley fill deposits. An unconformity at the base of the incised valley is interpreted as a sequence boundary formed by subaerial exposure and fluvial incision during a major fall of relative sea level. The incised-valley fill is composed of lowstand and transgressive facies that are separated by a transgressive surface. The lowstand facies consists of a basal-fluvial lag overlain by estuarine and shallow marine sandstone. Above the transgressive surface, a ragged transgressive blanket consisting of several backstepping parasequences was deposited as relative sea level continued to rise. Each parasequence of the ragged transgressive blanket consists of resedimented facies that commonly overlie a basal, decimeter-thick dolostone and passes upwards into mature sandstone deposited above storm wave base. The resedimented facies represent possible deposition of sandstone beds, derived from longshore drift, down the side of a progressively drowning paleovalley. Sandstone at the top of the uppermost parasequence is normally capped by a several metre-thick dolostone, deposited on a maximum flooding surface. The top of this dolostone marks the change from transgressive to regressive sedimentation. All dolostone beds within the incised-valley fill are assigned to the regionally occurring Nuccaleena Formation. They are interpreted to overlie marine flooding surfaces and represent background carbonate deposition below storm-wave base on sediment-starved hiatal surfaces. The interpretation of incised-valley fills within the Seacliff Sandstone may lead to a better understanding of the geometry and reservoir characteristics of similar sandstone bodies in hydrocarbon-prospective basins. Potential exists to find reservoir-quality sandstone within the ragged transgressive blanket and is dependent on differentiating between the transgressive surface and maximum flooding surface, and mapping the distribution of each parasequence to understand differences in reservoir quality.