Discovery O F Triassic Conodonts from Uson Islands O F the Calamian

Home , Busuanga Island, Cypridodella, Mindoro

No. 2] Proc. Japan Acad., 56, Ser. B (1980) 69

14. Discovery of Triassic Conodonts from Malajon and Uson Islands of the Calamian Island Group, Palawan Province, the Philippines, and Its Geological Significance

By wataru HASHIM0T0,*) Shigeru TAKIZAWA,**) Guillermo R. BALCE, * * * ) Ernesto A. ESPIRITU, * * * and Crisanto A. BAURA* * * >

(Communicated by Teiichi KoBAYASxi, M. J. A., Feb. 12, 1980)

This short article deals with the discovery of Epigondolella abneptis (Huckriede) , a Lower Norian index conodont of Japan, in the limestone at the southeastern coast of Malajon Island, the Calamian Island Group, and its geological significance. The general geology of the Calamian Island Group had not been surveyed since Smith (1924) reported the manganese-bearing chert formation from Busuanga Island and limestone from Coron Island, until 1978, when H. Fontaine reported the geological and palaeontological results of his reconnaissance works conducted in this island group (Fontaine, 1978, 1979; Fontaine et al., 1979). The Calamian Island Group is geographically situated between Mindoro and Palawan Islands, where the Mindoro and the Palawan Metamorphics are exposed respectively. Concerning the origin of these schists in the Recent Palawan Arc trending from Palawan to Mindoro, Gervasio (1971) and Hashimoto and Sato (1973) are of different opinions. Gervasio (1971) referred to the schist in the Palawan Arc as the basement of the Philippines and the non-metamorphosed Palaeozoic sediments including limestone in northern Palawan as well as that in the Calamian Island Group and the Permian limestone on Carabao Island as Palaeozoic formation deposited in a miogeosyncline, occur- ring in an east-west direction upon the basement schist terrain. Hashimoto and Sato (1973) subdivided the non-metamorphosed sediments of Gervasio (1971) into the Middle Permian Conodont-bearing Bacuit Formation, the Middle Permian Fusulinid-bearing Minilog Formation, the Middle Triassic Conodont-bearing Liminangcong For- mation and the non-f ossilif erous Guinlo Formation and designated them the Malampaya Sound Group. Further, Hashimoto and Sato

*) Emeritus Professor of Tokyo University of Education. Institute of Geosciences, The University of Tsukuba. Philippine Bureau of Hines. 70 W. HASHIMOTO et al. [Vol. 56(B),

Figs. 1-5. 1-4: Epigondolella abneptis (Huckriede). 1-3: Loc. BS-09 A, x140. 4: Loc. BS-09 B, x100. 5: Enantiognathus ziegleri (Diebel). Loc. BS-10, ><100. No. 2] Triassic Conodonts from Philippines 71

(1973) recognized a slight regional metamorphism in the Malampaya Sound Group and a similarity between the structure of the group and the Palawan Schist. Accordingly they thought the Malampaya Sound Group and the Palawan Schist to represent the sediments regionally metamorphosed at a shallower and a deeper tectonic level, respectively, in the same geosyncline which is thought to occur unconf ormably on the meta-arkose basement found by de los Santos (1959) below the semi-schist in northern Central Palawan. Further, Hashimoto and Sato (1973) compared the Palawan and the Mindoro Metamorphics to the Sangun Metamorphics in the Inner Zone of the southwestern tectonic region of Japan and the Jurassic Mansalay Formation to the Tetori Group in the same tectonic region, as post-orogenic molasses. Based on such geological knowledge of the Calamian Island Group, Hashimoto (1977, ms) inferred the thick chert of Busuanga Island to be a correlative formation to the Middle Triassic Conodont-bearing Liminangcong Formation of the Malampaya Sound Group ; the thick limestone in Coron Island may be an Eocene limestone following the geological map of the Philippines (Bureau of Mines, 1963) which shows a Palaeogene age for the limestone. Fontaine (1979) palaeontologically inferred the sandstone and shale formation and the limestone in Busuanga Island and some annex islands to be of heteropic origin and thought the radiolarite formation to unconf ormably underlie the former two formations. Fontaine et al. (1979) micro-palaeontologically studied the rock specimens collected by Fontaine from the Calamian Island Group and assigned Rhaetian to Lower Jurassic age to the limestone based on the following micro-fossils : Mesoendothyra ? sp., Involtina sp., Tria- sina cf. hantokeni (Majzon), Duostomina sp., Thaumatoporella par- vovesiculi f era (Pia), etc. This was the first discovery of a new stratigraphic unit between the Malampaya Sound Group and the Mansalay Formation. These fossils are all new to the Philippine palaeontology. Fontaine et al. (1979) palynologically assigned Upper Triassic age to the sandstone and shale formation due to the occurrence of Classopolites that ranges from Upper Triassic to Lower Cretaceous. The problem of the stratigraphic position of the sandstone and shale formation, like that of the structure of the formation, seems to be a matter for disagreement among previous workers (Fernandez, 1952; Momongan et al., 1979; Fontaine, 1979). The present authors conducted a palaeontological and structural geological reconnaissance in Busuang Island and some of the annex islands. A preliminary study on conodonts has just been completed 72 W. HASHIMOTOet al. [Vol. 56(B),

by one of the authors, Takizawa. Neohidiodella triassica (Muller), N. sp. and Cypridodella sp. were found from the radiolarite formation at Station BS-10, northwestern coast of Uson Island, west of Coron town. The range of the first mentioned species is limited to the Triassic. Epigondolella abneptis (Huckriede), a lower Norian index conodont of Japan (Koike, 1979), was found at Station BS-09 southeastern coast of Malaj on Island, together with Enantiognathus ziegleri (Diebel), Cornudina sp. and Neohindeodella sp. Enantio+gnathus ziegleri is known to range from Upper Anisian to Lower Norian in the Mediterranian region and from the beginning of Triassic to the end of Upper Carnian in North America. Accordingly, the limestone formation of the Calamian Group is safely be said to have a Lower Norian part at least in Malajon Island, and the unconformably under- lying radiolarite formation is highly probable to be correlatable to the Liminangcong Formation. The unconformity below the limestone indicates the time of first erosion of the Palawan Chain which originated from the Palawan Geosyncline. In consideration of the geological and palaeontological data stated above, it can safely be said that the area of the Calamian Island Group is quite well comparable in geo-historical development to the Gif u- Inuyama area geotectonically belonging to the Inner Zone of South- west Japan.

References

Fernandez, N. (1952) : Preliminary report on the geology of the manganese de- posits of Busuanga Island, Palawan. Bureau of Mines, no. 108 (unpublished report). Fontaine, H. (1978) : Preliminary notes on a Pre-Tertiary geological study of the Philippines (12 April-15 June, 1978). United Nat., ECAFE, CCOP News- letter, 5(1/2), 22-23. (1979) : Note on the geology of the Calamian Islands, North Palawan, Philippines. Ibid., 6(2), 40-47. Fontaine, H., Beauvais, L., Poumont, C., and Vachard, D. (1979) : Donnees nou- velles sur le Mesozoique de 1'Ouest des Philippines. Decouverte de Rhetien marin. C. R. somm. Soc. geol. Fr., 1979, fasc. 3, pp. 117-121. Gervasio, F. C. (1971) : Geotectonic development of the Philippines. J. Geol. Soc. Phil., 25(1), 18-38. Hashimoto, W. (1977, ms) : Geological development of the Philippines (The paper will be appeared on Geol. Palaeont. SE Asia, vol. 22, 1981). Hashimoto, W., and Sato, T. (1973) : Geological structure of North Palawan, and its bearing on the geological history of the Philippines. Ibid., 13, 145-161. Hashimoto, W., Takizawa, S., Balce, G. R., Espiritu, E. A., and Baura, C. A. (1979, ms) : Note on the geology of Busuanga Island and annex islands. Koike, T. (1979) : Biostratigraphy of the Triassic Conodonts. Mem. Retirement of Prof. M. Kanuma, pp. 21-77 (in Japanese). Momongan, A., Rollan, R., and Octaviano, D. (1978) : Progress report on the No. 2] Triassic Conodonts from Philippines 73

geological survey of Calamian Island Group. Bureau of Mines, no. 137 (unpublished report). Santos, V. de los (1959) : Preliminary report on the geology and mineral resources of central Palawan. Phil. Geologist, 13(4), 104-141. Smith, W. D. (1924) : Geology and mineral resources of the Philippine Islands. Dept. Agr. Nat. Res., Bureau of Sci., Publ., no. 19, 559 pp.