Depositional Environments of the Pleistocene Reef Limestones at Huon Peninsul a, Papua New Guinea on the Basis of Hermatypic Coral Assemblages

Abstract

Community structures of Pleistocene coral reef assemblages are recorded in the limestone terraces along the coast of Huon Peninsula. We made topographic profiles at Kanzarua and Sialum areas, and described the litho-and biofacies as columnar sections along them. Depositional environments of the limestones are estimated on the basis of the results from the modern coral reef assemblages.
Six biofacies of the Quaternaryl imestone were recognized as follows. Hermatypic Coral Facies : This facies is characterized by the autochthonous hermatypic corals that are contained, and is accompanied with algal crusts of coralline algae. The lithofacies is represented by boundstone composed of bindstone, framestone and bafflestone. Its depositional environment is estimated to be the lagoon or forereef slope shallower than 30 m.
Rhodolith Facies : This facies is characterized by abundant rhodoliths and occurred on the cliffs of terraces V a, V b and IV. In this facies, tests of the larger foraminifer Cycloclypeus carpenteri are also scattered. This lithofacies is represented by packstone to wackestone. The rhodoliths are characterized by the foraminifer Acervulina inhaerens and coralline algal LS assemblage. This assemblage is identical with that of rhodoliths distribute in deep forereef and insular shelf environments at depth of 50-150 maround Okinawa, in the Ryukyu Islands.
Cycloclypeus Facies : Grains are mostly composed of tests of the larger foraminifer Cycloclypeus carpenteri. This facies occurs on the cliff ofterrace V b and in one horizon around VIb. The lithofacies is represented by packstone to wackestone. Around Okinawa Island, Ryukyu Islands, Cycloclypeus is distributed from 50 to 150 m in depth. This facies is inferred to have been deposited in a deep environment, deeper than the lower limit of hermatypic corals.
Solitary Coral Facies : Solitary and ahermatypic corals, such as Flabellum spp., Dendrophyllia spp., Caryophylliidae species and Diaseris sp., characterize this facies. Their matrix consists of detritus of the skeletons on reef organisms and fine micritic carbonate. The lithology is mainly packstone to wackestone. Based on the depth distribution of these coral species in the Indo-Pacific, the facies is inferred to have been distributed on the forereef slope deeper than 30 m.
Halimeda Facies : Grains are mostly composed of segments of Halimeda This facies widely spreads over the lagoon of terrace VIIb. Litho-facies is represented by packstone to wackestone. Halimeda thrives from low tide level to the edge of insular shelves in tropical to subtropical region. Sediments rich in Halimeda segments are known not only from shallow lagoonal environments but also from deep reef associated shelves and banks.
Detrital Facies : This facies is characterized by an absence of the mega-fossils mentioned above and consists of fine sand and mud. This facies is considered to have been deposited on the reef slope deeper than 30 m, because it is accompanied by bryozoans and solitary corals in places.
Fossil corals in this region can be divided into four assemblages on the basis of their species composition and growth forms.
Assemblage A : This assemblage is represented by dendritic Acropora spp., such as A. formosa. Matrix is fine sand and mud with segments of Halimeda spp. and mollusks. Lithology of the limestone is mostly bafflestone and mudstone. The assemblage is a counterpart of recent Lagoon Assemblage and indicates that the limestone was deposited in a lagoon.
Assemblage B : Encrusting Montipora spp. and massive Faviidae corals are useful for supplemental diagnostics. Framestone and rudstone are the dominant lithology. The assemblage is correlated to the recent Deep Reef Slope Assemblage, and is estimated to have inhabited the reef slope from 5 to 10 m in depth.