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Home , Marine regression, Ryukyu Islands

58 [Vol. 11,

18. Geological History of the Ryukyu -`

By Shoshiro HANZAWA. (Comm. by H. YABE, M.LA., Feb. 12. 1935.) The geological history of the Ryukyu Islands begins with the Permian Period. The marine Permian formation (slate, , limestone with Neoschwagerina) was intruded by igneous rocks during late Permian or early Mesozoic time, thus forming the nucleous of the future Ryukyu Cordillera. After a long period of subaerial denudation, the older rocks suffered from crustal movements during a certain part of the Mesozoic Era ; that is, they were in parts contorted, faulted, and regionally metamorphosed into crystalline schists and phyllites. The great Ryukyu Cordillera thus arose very nearly from the present site of the Ryukyu Islands. Owing to the absence in these Islands of any formation younger than the Permian and older than the Upper Eocene, the exact age of the intense crustal deformation is uncertain. The Ryukyu Cordillera, after subjection to prolonged subaerial denuda tion, was divided into a number of mountain masses. Long after the subaerial denudation, the erosional surface of the Permian formation was transgressed by the sea in the Akitu Period (the Japanese Palaeogene), during which the Miyara beds, comprising the Pellatispira limestones of Isigaki and Kobama, was deposited. Stampian and Aquitanian rocks are missing both in the Ryukyu. Islands and in . During these periods orogenesis, dynamo , and marine regression occurred in Taiwan. In the Ryukyu Islands during the same periods, similar land conditions prevailed, while the Miyara beds were tilted and denuded, but not metamorphosed. These stages correspond to the Takatiho Period of Emergence. During the Neogene (=the Miduho Period of Submergence),

1) This is a very condensed summary of the results obtained by my geological studies of the Ryukyu Islands prosecuted during the years 1929 to 1933, with the financial support of the Foundation for the Promotion of Scientific and Industrial Research of and the Sait6 Gratitude Foundation (Sendai), to which councils, I should like to express my gratitudes in this place. My sincere thanks are also due to Prof. H. Yabe for his kind advise and criticisms on the whole subject of the studies. Full accounts will be given in my paper entitled "Topography and of the Ryukyu Islands " shortly to be published in the Science Reports of the Tohoku Imperial University, Second Series (Geology), Vol. XVII, with 13 plates and 5 maps. No. 2.] Geological History of the Ryukyu Islands. 59 excluding the Aquitanian stage, the Ryukyu Cordillera was widely inundated by the transgressing Neogene sea, in which the of the Yaeyama coal-bearing beds, the Simaziri beds, and the Sonai conglomerate were laid down in upward succession. The Yaeyama coal-bearing beds comprises volcanic material in its lower part and the Simaziri beds the same in its upper part, so that during the Neogene, volcanic activities broke out twice in the Ryukyu Islands. In Taiwan, the Kaizan beds, which are of the same age as that of the Yaeyama coal-bearing beds, are accompanied by basalt, andesite, and their tuffs in the lower part. According to Mr. K. Tan's recent geological studies of the Daiton volcanoes in the itself, its volcanic activities began in the later part of the Byoritu stage, during which the Byoritu beds, correlative with the Simaziri beds were deposited. In the post-Simaziri stage, the whole series of the Neogene deposits were dislocated, although with less intensity than that during the post-Syokkozan stage (the land stage immediately following that, during which the Syokkozan conglomerate was deposited) in the island of Taiwan. In the same stage, the Neogene sediments were extensively exposed to subaerial denudation. If we are willing to admit that the submarine valleys adjacent to Taiwan, and often traceable to as great a depth as 700 m or more, were formed subaerially in this period, then we must also admit that Ryukyu Islands were once elevated more than 700 m above its present position. Prof. H. Yabel' has already pointed out that, in the last continental stage of the Japanese Islands, the entire region of Taiwan, the Ryukyu Group, Kyushu, Sikoku, Honsyfl, Hokkaido, the Tisima (Kurile) Group, all lying within the 720 m submarine contour line, were dry land connected with the Asiatic continent, while the easternmost part of the Tung-hai, the greater part of the Japan Sea, and the southernmost part of the Ochotsk Sea were almost, if not entirely, land-locked sea basins. The geological history of the Ryukyu Islands, after the post-Simaziri stage, is largely bound up with the shifting of strand lines. The erosional land surface of the deformed Neogene rocks was invaded by the advancing sea, in the shallower part of which the Ryukyu limestone, about 100 m thick, began as coral reefs, when the upper courses of the deep drowned valleys were either buried in some places or remained as coral -passages in others, just as we frequently find in the case

1) H. Yabe : The Latest Land Connection of the Japanese Islands to the Asiatic Continent. Proc. 5 (1929), 168. 60 S. HANZAWA. [Vol. 11, of the recent coral reefs of the South Seas. In this period, only the tops of Yaku, Takara, Kotakara, Amami, Toku, Okierabu, Okinawa, Kume, Iheya, Tonaki, Tokasiki, Mae, Yukan, Kuba, Isigaki, and Iriomote showed themselves as small islands above the water ; all the other islands being submerged below sea level. Following this was the land emergence, when the Ryukyu limestone showed above sea level, re juvenating the topography. This was the post-Ryukyu stage of emergence, during which Takara and Kotakara were united into a single island, and similarly all the islands of the Osima-Okinawa group as well as those of the Sakisima group, into two other islands respectively. The erosional surface of the three large newly formed islands, was again transgressed by a non-coral sea that cut the successive coastal terraces in its periodical retreats leaving behind the Kunigami gravel on the terraces as and fluviatile deposits. At maximum inundation, the tops of only Yaku, Takara, Kotakara, Amami, Toku, Okinawa, Iheya, Kume, Tonaki, Tokasiki, Isigaki, and Iriomote remained as islands, all the other islands being entirely drowed in the sea. A crustal disturbance that set in later, tilted the Kunigami gravel along with the Ryukyu limestone. In this, the post Kunigami stage of emergence, Isigaki was connected with Taketomi and Kuro ; Oki nawa with its dependent (excluding Tuken and Kudaka) ; and Kume with its dependent islets ; Iheya with Noho and Gusitya ; and Tokasiki with Kuro, Zihusi, Hukase, and . The other islands were separated from one another by the sea. The seas lying between Okinawa and Miyako, between and Taiwan, between Yoron and Okierabu, and between Amami and Yaku and to the north of Kume, were in all probability deepened by the latest tectonic movement. As to the Tokara between Amami and Yaku, however, with depth as much as 2,000 m in its greater part, it is supposed that it was in existence at the time of Prof. H. Yabe's latest continental stage of the Japanese Islands as an opening, permitting communication between the land-locked sea inside the Ryukyu Islands and the Pacific Ocean. This land connection in the post-Kunigami stage just mentioned is an inference based on the present submarine configuration of the Ryukyu Islands. The subsequent history of the Ryukyu Islands resembles those of the island of Taiwan and the main islands of Japan, namely, that positive shift of sea level, 20m. or less, occurred, resulting in land- No. 2.] Geological History of the Ryukyu Islands. 61 submergence topography, with fringing reefs and equivalent beach deposits formed around the newly submerged islands. Later, the sea level shifted in a negative sense, the fringing reefs and beach deposits were elevated to heights of from 2 to 20 m. Recent coral reefs are today growing luxuriantly along the margins of all the islands south of the Tokara Strait.