Palaeoecology of a Late Devonian back reef: Canning Basin, Western Australia

Abstract

Back-reef ecologies within the celebrated mixed carbonate-siliciclastic Late Devonian (late Frasnian) Pillara Limestone of Windjana Gorge, in the Canning Basin, Western Australia, are re-interpreted as being dominated by microbial communities. Proposed microbialites are expressed as weakly-laminated, fenestral micrite, that show unsupported primary voids, peloidal textures, disseminated bioclastic debris, and traces of microfilaments. These grew as either extensive free-standing mounds or columns, often intergrown with encrusting metazoans, or thick post-mortem encrustations upon skeletal benthos. In some cases, microbial encrustations are inferred to have developed in protected cavities formed by progressive burial of the reef. The calcimicrobe Shuguria also shows a preferentially cryptic habit, encrusting either primary cavities formed by skeletal benthos, microbialite, or the ceilings of mm-sized fenestrae within microbialite. A further calcimicrobe, Rothpletzella, formed columns up to 0.3 m high in areas enriched by very coarse siliciclastic sediment. Stromatoporoid sponges with a diverse range of morphologies also formed in situ growth fabrics. Monospecific thickets of closely-aggregating dendroid stromatoporoid sponges (Stachyodes costulata), and platy-laminar forms (?Hermatostroma spp.) were common, as were remarkably large stromatoporoids (Actinostroma spp.) that grew as isolated individuals up to 5 m in diameter. Such sponges showed impressive powers of regeneration from partial mortality, and individual clones may have been capable of substantial longevities of up to 500 years. Actinostroma spp. showed highly complex growth forms including platy-multicolumnar (A. windjanicum), and a hitherto undescribed inferred whorl-forming foliaceous morphology (Actinostroma sp.) reminiscent of the modern photosymbiotic coral Acropora palmata. These complex morphologies formed abundant primary cavities, previously thought to be only rarely developed in association with stromatoporoids.