Microtextures of modern tufa stromatolites from the peritidal zone near Schoenmakerskop, Eastern Cape, South Africa: investigating the relative contributions of microbes, algae and chemical precipitation to microbialite growth

At Schoenmakerskop on the coast near Gqeberha (Port Elizabeth), modern microbialite deposits are forming where CaCO₃-supersaturated groundwater emerges at the contact between Pleistocene aeolianites and the underlying Cape Supergroup bedrock (Edwards et al. 2017). Two different systems were investigated: well-laminated spring line tufa deposits above intertidal beachrocks, and a series of barrage pools with active and remnant rimstone deposits showing thrombolitic and laminated mesofabrics. Petrographic light-microscopy and scanning electron microscopy (SEM) were used to investigate the relative contributions of biotic and abiotic processes in the formation of these microbialites.

Light-microscopy identified isopachous light-brown sparry crusts consisting of tabular or platy carbonate, interpreted to have formed predominantly by chemical precipitation. Hybrid crusts were also found comprising alternating organic-rich layers and sparry crusts, along with colloform and micritic microtextures. Fossilised algal filaments were identified and SEM observations revealed both densely and loosely packed layers of hollow, unbranched filaments encrusted by microcrystalline carbonate. Exceptionally well-preserved draping biofilms were also found in some samples. Evidence for trapping and binding of clastic material was very limited, with only occasional diatom fragments. Taken together our observations point to a system dominated by chemical precipitation of carbonate with rapid precipitation leading to exceptional preservation of biofilms in submerged samples, and entombment of algal and sometimes microbial filaments. Evidence for biologically induced extracellular mineralisation of the algal filaments and microbial biofilms is recorded in these microbialites. The potential of these deposits to serve as analogues for chemical stromatolites in the fossil record is explored.

Edwards et al. (2017). Macro- and Meso-fabric structures of peritidal tufa stromatolites along the Eastern Cape coast of South Africa. Sedimentary Geology 359: 62-75.