Variations in microbialite morphology with depositional environment in a Precambrian ramp deposit, Hamersley Group, western Australia

The Neoarchean Carawine Formation was deposited before the oxidation of the atmosphere and preserves a carbonate ramp with a range of microbially produced structures. These structures reflect the interplay of physical and chemical conditions, microbial growth, and cement precipitation. Their distribution implies that specific structures might be used as paleoenvironmental indicators. Such microbial facies are a large part of the rock record, and their identification and interpretation are important for understanding the development of many carbonate deposits.; In the Carawine, shallow, subtidal facies contain biohermal and bedded stromatolites, centimeter-scale ridge-shaped microbialites, wavy-laminated microbialites, and microbial tube structures. Tube structures are products of dewatering, microbial growth, and cementation; these fluid escape structures previously have not been recognized in the rock record but may occur in other microbial facies (e.g. Famennian, Canning Basin). Some shallow, subtidal microbial facies, such as biohermal and bedded stromatolites, are characteristic of younger carbonates that were deposited after the transition from an anoxic to oxic atmosphere. Therefore, the morphology of shallow water microbial structures may be controlled more by physical conditions than by chemical conditions.; Deep subtidal facies are composed of fenestrate microbialites, planar laminated dolomite, and rolled up microbially laminated dolomite. Fenestrate microbialites are delicate organic structures that grew in quiet, sediment-free, deep subtidal environments. They composed of vertically oriented supports and sub-horizontal drapes. Some vertical supports were created when rising gas bubbles disrupted planar laminated mat, creating vertical flaps that were substrates for further microbial growth. Drapes of mat developed between two adjacent supports in a sequence of microbial growth, cell dispersal, and cementation. The demise of fenestrate microbialites seems to coincide with the development of a sulphidic deep ocean ∼2.3 Ga, during the stepwise oxidation of the Earth's surface. This suggests that in deep subtidal environments, the morphology of microbial structures may be controlled more by chemical processes than by physical ones.