Miocene sandstone of ‘continental’ origin on Iriomote Island, southwest Ryukyu Arc, Eastern Asia

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Abstract

We evaluated the major mineral composition of 100 sandstone samples from the Iriomote Formation of the Yaeyama Group located at the southwestern end of the Ryukyu Islands, eastern Asia. These sandstones are feldspathic arenites. Paleocurrents inferred from these sandstones are generally from north to south. Compared with other Tertiary sandstones of the Japanese Islands arc, the Iriomote sandstones are enriched in quartz and contain less lithic fragments. The roundness and sorting of the sand grains are high, suggesting long transport distances. We interpret these compositional and textural data as indicating that the provenance of the Iriomote Formation was the Eurasian Continent.

Introduction

Iriomote Island, Okinawa Prefecture, is located in the East China Sea, and is a member of the Yaeyama Islands group, along with Ishigaki, Taketomi, Kohama, and Kuro islands, at the western end of the Ryukyu Islands arc. The area of Iriomote Island is 284 km2 and the circumference is 130 km. The Miocene Iriomote Formation of the Yaeyama Group crops out on the island, and another part of the Group is distributed on Yonaguni Island. Nakagawa et al. (1982) reported the stratigraphy and fossils of the island. Ogasawara and Masuda (1983) determined the paleoenvironment of the Iriomote Formation from mollusks. They suggested that it ranged from fresh and brackish water to shallow marine (<50 m depth). An unanswered question is the provenance of the Iriomote formation. At present, Iriomote Island is isolated from a siliciclastic source terrane because the Okinawa trough separates the Ryukyu Arc from the Eurasian Continent. Consequently, identification of the origin of the clastic detritus in the Iriomote Formation will permit better understanding the origin of the Ryukyu Islands. This study addresses the major mineral composition of sandstones of the Iriomote Formation to better understand the provenance of the Iriomote Formation.

Section snippets

Geological setting

Iriomote Island lies at latitude 24°15–25′ N, longitude 123°40–55′ E (Fig. 1). The strata on the island are divided into 6 formations in ascending order: the Tomuru Formation of the Ishigaki Group, the Miyaragawa Formation and the Nosoko Formation of the Miyara Group, the Iriomote Formation and the Sonai Conglomerate of the Yaeyama Group, and the Sumiyoshi Formation of the Ryukyu Group. They crop out from the northeast to the west, respectively, and are separated by unconformities (Nakagawa et

Sandstone composition

The major mineral composition of sandstone is largely controlled by the tectonic setting (Crook, 1974, Schwab, 1975, Dickinson and Suczek, 1979, Dickinson et al., 1983). Many workers have analyzed both modern sands of known tectonic setting and ancient sandstones whose tectonic setting has been inferred in order to clarify the relationship between sandstone composition and tectonics that characterize the provenances and basins.

Paleocurrents

Paleocurrents are inferred from the dip directions of cross-stratification and lamination. The strike of the ancient slope is inferred from the orientation of the soft-sediment slump fold axes. We measured the dips and strikes of lamina planes and orientation of the slump fold axes. All measurements are corrected for structural tilts.

Trough cross-stratification indicates paleocurrent flow from the north or northeast to the south or southwest (Fig. 2A). The sense of wave oscillation inferred

Discussion

Most of the Iriomote Formation sandstones plot in the transitional continental field of Dickinson et al. (1983) (Fig. 6A). The compositional data of the Iriomote Formation, combined with their shallow marine depositional setting, all suggest that these sandstones were deposited along the margin of a continent that was the source area. In Fig. 6B, we compared the sandstone compositions of the Iriomote Formation with those of the Japan Islands of similar age. The ages of sandstones we compared

Summary

Most of the sandstones of the Iriomote Formation, Ryukyu Islands, Japan, are classified as feldspathic arenites indicative of a ‘continental’ provinance. The well-rounded and well-sorted grains in the sandstones suggest long distances of transport. The sediment supply for the Iriomote Formation was from the north-northwest. These results indicate that the provenance of the Iriomote Formation was the Eurasian Continent. Our study emphasizes the importance of recognizing continental-derived

Acknowledgements

Thanks to Iriomote Station, Tropical Biosphere Research Center, University of the Ryukyus, for providing a comfortable research environment. We also would like to thank Dr Toru Tamura, Kyoto University, for constructive and helpful comments on our study, Drs Kenneth Ridgway, Purdue University, Lee Suttner, Indiana University-Bloomington, for reviewing the manuscript, and Dr Kevin Burke, University of Houston for patient editing.

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