PETROGENESIS AND PALAEOENVIRONMENT OF DEVONIAN ALGAL LIMESTONES OF NEW SOUTH WALES

Abstract

SUMMARYA Lower Devonian algal reef complex, with a basinal algal turbidite facies, offers excellently preserved autochthonous and allochthonous algal textures and structures of the following types: algal calcilutite biolithites of dense, grumous, cellular, tubular, “pelletoid” and “granuloid” textures, algal calcilutite crusts and algal‐bound detritus (e.g., stromatolites), algal pisolites, oolites and circumcrusts, algal pellets and grains (in contrast to faecal and bahamite types), colonial and stem fragments, algal filaments and cells, stromatactis and birdseyes. Encrusting Foraminifera and stromatoporoid colonies were of very subordinate importance in forming the turbulent‐water reefs. Protected pools, crevices, and local lagoons offered a favourable niche for calm‐water Codiaceae, crinoids, polyzoans, Amphipora and some molluscs. Corals occur rarely.The wave‐resistant calcilutite reefs as well as the high percentage of subsequently introduced fine “internal sediments” within the limestone‐frameworks, indicate that grain‐matrix‐cement ratios do not necessarily reflect degree of turbulence and winnowing.The algal reefs have no effective primary porosity. Channels and cavities, predominantly horizontally oriented, were filled with internal sediments and early‐formed sparite cement.Fibrous calcite cement is confined to limestones that seem to have formed in a littoral environment whereas granular sparite appears to be an intra‐stratal product of a later stage.