The Implications of Cryogenian NeoProterozoic Reefs

The Cryogenian Balcanoona reef complexes of the Northern Flinders Ranges: Implications for Neoproterozoic ocean chemistry

Authors:

Wallace et al

Abstract:

The Cryogenian Balcanoona reef complexes of South Australia consist of several dolomite platforms with well-defined backreef, reef margin and slope/basinal facies. The mature platforms have prograded large distances into the basin (greater than 15 km) and have high relief margins (over 1 km of relief at the platform margin). Backreef facies are dominantly ooidal–peloidal grainstones with abundant tepees and marine cemented sheet cavities. The reef margin facies consists of upper stromatolitic boundstones and, where present, deeper water non-stromatolitic chambered and clotted boundstones. The deepwater chambered and clotted boundstones grew on near vertical escarpments below 150 m paleo-water depths and consist of a complex array of marine cemented cavernous and calcimicrobial material. The strongly prograding platforms range from accretionary low-angle stromatolitic boundstone margins, through to more mature escarpment margins. The mature escarpment margins (greater than 200 m high escarpment) are characterized by the development of both stromatolitic and non-stromatolitic boundstones. The slope/basin facies of the mature escarpment margins consists of coarse debris deposits (talus, debrites and allochthonous blocks) proximal to the platforms and calcareous/dolomitic mudrocks distal to the platforms. The development of an accretionary escarpment margin, together with the large proportion of deepwater boundstone suggests that these Cryogenian reefs were fundamentally different from Phanerozoic reefal systems. The enormous volume of deepwater boundstone in these reefs may be due to a severely anoxic water column that prevailed during this period of the Cryogenian. Anoxia led to greater degrees of carbonate saturation at depth that enabled the development of deepwater boundstones with escarpment margins. A community of anoxic-adapted non-photosynthetic organisms that were perhaps similar in their ecology to Phanerozoic coelobiontic reefal communities may have constructed the enigmatic deepwater boundstone.

Just prior to the final termination of the reef, the system was subject to large-scale erosion and margin collapse, producing a major regional unconformity. Following this, the reefs briefly re-grew, before final and complete cessation of the reef complexes. The large-scale margin collapse event may have been triggered by sea level fluctuation, and/or changes in ocean chemistry associated with Late Cryogenian oxygenation.