Sedimentology and geomorphology of tufa barrages on rock coast shore platforms

Despite tufa deposits being widely distributed and increasingly recognised globally on rock coasts, their distribution in the tidal frame and geomorphology is poorly understood. The distribution, sedimentology and geomorphology of tufa barrages on supratidal rock coasts are assessed and their controls established. Tufa barrages are comprised of multiple facies including previously recognised tufa stromatolite, infaunal metazoan-bioturbated tufa, bryophyte phytoherm tufa and Ulva tufa, the latter of which has not previously been reported. These facies form an association, firmly placed in the supratidal zone, limited landward by the highest elevation of the shore platform (ca. 9.5 m AMSL); and seaward by the high tide level (1.625 m AMSL). A clear ecological zonation of facies is defined by formative primary mediators (i.e., microbial communities, bryophytes and Ulva intestinalis) each of which influences barrage morphology. Lacking emergent and riparian vegetation, these biological controls differ from tufa in terrestrial settings. Barrages increase in size with decreasing elevation AMSL at a 0.01 significance level (n=216); associated with the ecological zonation. Significant physical controls on barrage morphology include bedrock slope and topography (e.g., channelisation and bedrock steps). Tufa cascades occur across the shore platform in areas of high bedrock slope (ca. 20-90o), while barrages occupy shallower slopes (ca. 0-20o), and lacustrine and fluvial crusts develop across the full range of slopes. Barrage size decreases with bedrock slope. Greater understanding of the controls on tufa facies distribution and associated geomorphology enhances their utility as palaeoenvironment and sea-level indicators and archives.