Relationships between cyclicity and stromatolite form in the Late Proterozoic Bitter Springs Formation, Australia



    1. Division of Continental Geology, Bureau of Mineral Resources, PO Box 378, Canberra ACT 2601 Australia
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      Formerly Baas Becking Geobiological Laboratory.


Stromatolite biostromes and bioherms in the lower two units of the Late Proterozoic Loves Creek Member of the Bitter Springs Formation represent shallowing upward and deepening upward sequences. In the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence. Ooid and/or peloid-intraclast grainstones and small, irregular bulbous and columnar stromatolites characterize the basal, transgressive portion of cycles. Domal, columnar and stratiform stromatolites comprise the bulk of the cycle. These forms accreted in a gradually shallowing epeiric sea. Domal stromatolites predominate in the deeper parts of cycles. Here synoptic relief gradually increases upwards. Columnar and stratiform stromatolites predominate in the shallower parts of cycles, where synoptic relief rapidly diminishes upwards. In thin-bedded dolo-mudstones at the tops of cycles the co-occurrence of desiccation cracks, tepee structures, scalloped dissolution surfaces, gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure. In contrast, stromatolites in a unit at the base of the Loves Creek Member accreted during a gradual rise in sealevel. Stratiform, columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence, but only the lower half of the shallowing upward cycle is represented. Synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit. Recognition of a deepening upward stromatolite sequence at the base of the Loves Creek Member, and a disconformity surface between this sequence and the underlying Gillen Member, permits palaeoenvironmental re-interpretation of the Loves Creek Member as a single ‘large scale’ sea-level cycle.