Cryptic intertidal microbialites in beachrock, Heron Island, Great Barrier Reef: implications for the origin of microcrystalline beachrock cement

Abstract

Microbialites with microcolumnar growth morphology occur on the surfaces of solution-enlarged crypts in modern beachrock an Heron Island, Great Barrier Reef, Australia. The microbialites consist predominantly of felted aragonite, but blocky Mg-calcite and fringes of acicular aragonite also occur. A variety of microbial bodies and filaments occur within, and on the surface of, the microbialites. Although most Heron beachrock is cemented by isopachous fringes of acicular aragonite, irregular micritic cement is common adjacent to the microbialites. The micritic cement consists predominantly of felted aragonite and contains microbial filaments and organic matter similar to those in the microbialites. Both the microbialite and the micritic cement are interpreted to be biologically induced. Although physico-chemical models, including CO2 degassing and evaporation, are commonly held to be sufficient to promote the formation of beachrock, the similarity of Heron cryptic beachrock microbialite with micritic beachrock cements supports the contention that some beachrock cements are biologically induced. Additionally, the primary difference between fringes of acicular aragonite and the 'felts' of aragonite in microbialites appears to be that the former nucleated on a well defined surface of organic matter, whereas the latter nucleated within a framework of organic matter that contained irregularly disposed nucleation sites. Hence aragonite fringe cements may also be localized and/or induced by films of organic matter. (C) 1999 Elsevier Science B.V. All rights reserved.