Stromatolite framework builders: ecosystems in a Cryogenian interglacial reef

Abstract

A convergence of climate, paleolatitude, basin evolution, water depth and seawater chemistry conditions, favourable for carbonate precipitation and preservation, facilitated initiation and sustained growth of the ∼350 m-thick Arkaroola carbonate reef, part of the Balcanoona Formation, during the Cryogenian interglacial in South Australia’s northern Flinders region. The reef is distinguished by its diverse stromatolitic framework; its internal architecture as exposed on the gorge walls of Kingsmill Creek, reveals spatial and temporal variability of the stromatolite assemblage. Growth of planar microbial biostromes initiated development of a stable carbonate substrate on underlying thick mudstone-to-sandstone. Once stabilised, a robust reef framework developed, first through vertical growth of large domical thrombolitic stromatolites forming stacked bioherms of metre-scale relief. Biological modification of the local environment through substrate stabilisation, wave-energy baffling and aggradationally driven shallowing then enabled an ecological succession of stromatolite forms from microbial biostrome colonisation through to a climax community of domal, columnar and branching stromatolites. The spatial and stratigraphic distribution of stromatolite forms along with allochthonous facies, including intraclastic, peloidal and oolitic grainstone and packstone, characterises development of a windward forereef, reef crest, reef flat and leeward back-reef lagoon. Variability of stromatolite forms across the reef structure indicates ecological adaptation of function and form to niche conditions. Microbial (stromatolite) growth in a range of forms overwhelmingly controlled carbonate production in the Arkaroola reef. The range of stromatolite forms and their facies distribution within the reef is consistent with biological control on ecosystem development and stabilisation analogous to those of Phanerozoic and modern reef ecosystems. The Arkaroola reef exhibits a comparable ecological succession within the evolutionary realm of cyanobacteria in the late Proterozoic. Furthermore, the Arkaroola reef records relative climatic and sea-level stability within the Cryogenian Period, notable for severe climatic upheaval.