マレーシア・ランカウィ諸島における下部‐中部オルドビス系生層序の確立

Abstract

Eighteen species of Early and Middle Ordovician conodonts belonging to sixteen genera were discovered in the Kaki Bukit Formation in the Langkawi Islands, northern peninsular Malaysia. These conodonts are classified into faunas A and B, and are contained in the lower and uppermost parts of the study section, respectively. Fauna A including Cooperignathus nyinti, Reutterodus sp. cf. R. andius, and Acodus deltatus indicates the late Floian age, the latest Early Ordovician. Jumodontus gananda and Tropodus laevis in fauna B show that the upper part of the section is of the earliest Middle Ordovician age. Thus, the Lower and Middle Ordovician boundary exists in the middle of the section. It is inferred from lithological analyses that limestones of the Kaki Bukit Formation were deposited on a carbonate platform on a shelf during the Early and Middle Ordovician age. Biogeographical evidence suggests Early Ordovician conodonts from Southeast Asia belong to the Australian province, which was characterized by a tropical climate and a shallow-water environment. Further research on Lower and lower Middle Ordovician limestones in the Langkawi Islands would clarify the conodont biostratigraphy and transitions of paleobiogeography and depositional environments in detail.