Devonian carbonate buildup facies in an intra-oceanic island arc (Tamworth Belt, New South-Wales, Australia)

Biohermal and biostromal buildups were investigated in late Early and Middle Devonian carbonate complexes of the Tamworth Belt. The buildup types and subtypes were studied in three regions (Yarramanbully, Sulcor and, Wyaralong') to clarify their paleo-environmental position. Two stages of development are recognized: Incipient bioherms and bioherms. Incipient bioherms are carbonate buildups with organisms which commonly form true bioherms. They dominate the sediment with small growth forms but are not prolific enough to build large bio-frameworks. Small nodular and globular stromatoporoids characterize the incipient bioherms and are interpreted as stunted growth forms. In one location ('Wyaralong') the coarse stromatoporoid calcarenite represents a fore-reef facies, at Sulcor a shallow subtidal setting with moderate water energy can be deduced. The bioherms can be sub-divided into stromatoporoid-, stromatoporoid-Stachyodes-, and stromatoporoid-rugose coral bioherms. Their variable composition probably reflects growth and deposition in different zones of a reef complex and/or different proximity to areas of denundation indicated by high siliciclastic input. In the Tamworth region true bioherms occur only in the Moore Creek Limestone Member (Middle Devonian), and not in older carbonate successions. Biostromes are sub-divided into (1) incipient biostromes with stromatoporoid-heliolitid biostromes and alveolitid biostromes; (2) aggregate biostromes withAmphipora andStachyodes biostromes; (3) stratified biostromes; (4) mixed aggregate/stratified biostromes. The different types of biostromes are not limited to specific time-intervals, but rather to environmental conditions. (1) Incipient biostromes are characterized by laminar stromatoporoids and tabulate corals. Their forms are interpreted as initial layers of skeletons which were hampered by adverse conditions in growth. The stromatoporoid-heliolitid incipient biostrome (Eifelian Moore Creek Ls. Mbr., Yarramanbully) is characterized by abundance of dislodged laminar, ragged and tabular colonies associated with small globular and nodular heliolitids. An unstable substrate may have caused the growth disruptions. Decreasing grain-size of skeletal debris and increasing mud-content suggests deposition on a bathymetric gradient with deepening to the south. The alveolitid incipient biostrome (Eifelian Moore Creek Ls. Mbr., 'Wyaralong') is composed of nodular limestone with laminar alveolitids, stromatoporoids andSphae