無節サンゴモ (紅藻綱) から見たパプアニューギニア, ヒュオン半島の第四紀段丘を構成する石灰岩

抄録

The extensive flight of terraces occurs on the northeast seaboard of Huon Peninsula, Papua New Guinea. The terraces consist of many Quaternary reef-complex limestone bodies of different ages.
Fossil nonarticulated coralline algal florae were examined in the limestones forming the Pleistocene terraces : II, IIc-lower, IIIc-upper, IIIb, IIIa-lower, IIIa-middle, IIIa-upper and IIIa-upper⁺ from the coast to the inland area at Kanzarua ; and IV, Vb, Va, Vib, VIa, Mic and VIIb from the coast to the inland area at Kwambu. Depositional environments of these limestones are determined on the basis of comparison with the distribution of coralline algae in the present-day reef complex around Huon Peninsula, the Ryukyu Islands and other Indo-Pacific regions.
Nineteen species coming under ten genera of fossil nonarticulated coralline algae have been identified. Two coralline algal assemblages are discriminated. One is characterized by the presence of Porolithon onkodes, and thought to be indicative of shallow forereef environment. This assemblage occurs on II, IIIc-lower, IIIc-upper, IIIb, IIIa-lower, upper and IIIa-upper⁺ of Kanzarua, and Va, VIIc, VIIb of Kwambu, implying that these terraces correspond to ancient lagoon to reef-core facies. The other is delineated by the presence of Lithothamnion australe associated with Lithothamnion, Sporolithon and Lithophyllum species. This assemblage occurs in rhodoliths of rhodolith floatstone and packstone-wackestone exposed at the cliffs of terrace IV, Vb, Va, VIb and VIa at Kwambu. It can be thought that these fossil rhodoliths were deposited in the deep forereef environment because their constituents are very similar to those of deep water rhodoliths around the Ryukyu Islands. Therefore, each rhodolith-bearing facies of terrace cliff is the deeper counterpart of reef-core facies forming some higher terraces and the antecedent sediments to the formation of its consisting terrace. At Kwambu, terraces IV, Vb, VIb and VIa are composed of the rhodolith-bearing limestones and, these terraces thought to be abrasion platforms on the older, uplifted, deep forereef deposits.