A Neoproterozoic fringing stromatolite reef complex, Flinders Ranges, South Australia

Abstract

A series of 14 microbial buildups outcrop in stark topographic relief along the western edge of the Enorama Diapir in the Central Flinders Zone of the Adelaide Geosyncline, South Australia. The enclosing stratigraphic unit, the Enorama Shale, is in the locally-named interglacial succession, between the Sturtian and Marinoan (Varanger equivalent) glacigenic units, thereby placing the reefs within the upper Cryogenian. Locally-derived conglomerates showing progressive cannibalisation indicate that this diapir was active during reef and mound growth. Apparent gradual onlap of the Enorama Shale against the diapir, controlled in part by the present structural disposition, has allowed good resolution of the individual buildups. Movement of the diapir during and after reef growth has turned the succession to vertical so that the features are seen in section view. Provision of a clean, stable substrate controlled the establishment of the buildups. A core facies of cumulate stromatolite domes first formed mounds, which interfingered with the surrounding sediments, then reefs, which stood well above the sea floor. Not all mounds progressed to reefs as the edge of the active diapir was unstable at times. The maximum size of a mound plus reef is ca. 50 m high and 200 m across the base. At a microscopic scale, the core facies shows distinctive clotted textures, reminiscent of the calcimicrobes Epiphyton and Renalcis, although a contribution to the stromatolitic domes from microbial activity is also possible. The entire reef complex has been completely dolomitised, probably by solutions emanating from the diapir. Early dolomite cement has allowed the preservation of the sedimentary textures and some porosity. The reefs grew during a time of sea level rise and show a close relationship with the Enorama Diapir. The diapir provided an area of shallow water around an island at a time when deeper water shale was being deposited in the rest of the basin. The mound to reef transition shows ‘catch up’ and ‘keep up’ growth habit but the sudden demise of individual reefs appears related to pulses of movement on the diapir.