Interglacial carbonates of the Cryogenian Umberatana Group, northern Flinders Ranges, South Australia

A detailed sedimentological and chronostratigraphic analysis of the Umberatana Group in the northern Adelaide Geosyncline has uncovered a depositional history involving the rapid progradation (at least 20km) of a giant reef complex (up to 1.1km relief) during mid-Cryogenian interglacial times. The reef complex, which occurs in the Balcanoona Formation, displays facies similar to Phanerozoic reefs. These include a basal forereef (slope) facies, overlain by a reef-margin facies (consisting of both stromatolitic and non-stromatolitic frameworks), and an upper backreef (platform) facies consisting of shallow-water peloidal and oolitic carbonate. The thickening of the reef complex in a basinward direction, and the distribution of the key facies are consistent with the progradation of the platform into deep water. Progradation was contemporaneous with deposition of the upper Tapley Hill Formation and had largely ceased after a major margin failure event. Following this event, reef growth continued for a short time before becoming extinct, possibly as a result of global climatic cooling and/or eustatic sea-level fall.