Geographical Review of Vol. 58 (Ser. B), No. 2,185-194,1985

Late Holocene Coastal Morphology and Sea-level Records

on Three Small Islands, the South Ryukyus, Japan*

Yoko OTA, 1) Paolo A. PIRAZZOLI, 2) Toshio KAWANA3) and Hiroshi MORIWAKI 4)

The Holocene coastal morphology of three small islands, Kohama, Kuro and Hateruma, of the southern part of the , Southwest Japan, is discussed in relation to paleo sea-level

indicators. Three islands are located between about 24•‹0•L N and 24•‹20•L N, and are largely under lain by Pleistocene coral limestones. They are fringed by emergent coastal features such as marine

terraces and marine notches. Emergent marine deposits and fossils are also preserved. Present elevations of these landforms and fossils generally range from 1.0 to 1.5m above mean sea level

(AMSL) and C-14 dates for five samples are younger than ca. 4,200yBP. Sea level was slightly higher than at present, about 1m AMSL from ca. 3,300 to 800yBP in Kohama and Hateruma and about 1.5m AMSL at about 4,200yBP in Kuro Island. No definite evidence is found to con

firm the culmination of postglacial sea-level rise which is usually observed on the Japanese coast at about 6,000yBP. Age of placement and orientation of large boulders scattered on the terrace up to 22m high on Hateruma Island probably suggest their tsunami origin, associated with the

1771 earthquake.

Channel with different height, fringed by the I. Introduction Holocene coral reefs on some islands (e.g. OTA and HORS,1980). Recent coral reefs in The Ryukyu Islands are situated southwest the Ryukyus show a fringing reef type except of mainland Japan, and extend about 1,200km in the areas off the (the between Taiwan and Kyushu. The zonal South Ryukyus) and off Kume Island (the structure and the main geomorphological units Central Ryukyus), where small-scale barrier in the adjacent area of the Ryukyu Islands are reefs 10 to 20m deep are developed. The as follows (from west to east): Tunghai Shelf Holocene sea-level stands in the entire Ryukyus on the East China Sea, Okinawa Trough (max have been discussed by PIRAZZOLI(1978), OTA imum depth: about 2,200m), Quaternary vol and HORS(1980) and KOBAet al. (1982), men canic front, Ryukyu Geoanticline (correspond tioning that, in general, the Holocene sea-level ing to the main parts of the Ryukyu Islands), stands in the South Ryukyus are lower than and Ryukyu Trench (maximum depth : 7,881m) those in the North and the Central Ryukyus. on the Philippine Sea. The Ryukyu Islands This paper deals with the Holocene sea-level are divided into three groups (the North, the records in Kuro, Kohama and Hateruma Central and the South Ryukyus) by two major Islands, the South Ryukyus, based on the left-lateral strike-slip faults: the Tokara Chan survey of marine notches, beachrock, coral nel and the Kerama Gap (or Miyako Depres fossils and intertidal barnacles. In addition, sion) (KONISHI,1965). large boulders on Hateruma Island are dis Pleistocene Ryukyu Limestone occurs cussed in relation to tsunami. Elevations, throughout the Ryukyus southwest of Tokara measured by hand-levels in relation to the pre

1) Department of Geography, Yokohama National University, Yokohama 240, Japan. 2) C. N. R. S.-INTERGEO, 191 rue St-Jacques, 75005 Paris, France. 3) Department of Geography, University of the Ryukyus, Okinawa 903-01 Japan. 4) Department of Geography, Tokyo Metropolitan University, Tokyo 158, Japan. * A contribution to the IGCP Project No. 200: "Sea-level correlations and applications" 186 Y . OTA et al.

Table 1. Tidal range in studied area based on the local datum at Ishigaki Port, .

(1): Mean values compiled from the Okinawa Branch of the Japanese Meteorological Association (1981) . The MHH(LL)WSL corresponds to the average highest (lowest) level reached at each spring tide . (2): The average annual and spring values are calculated from the tide table of the year 1981 (see the above reference) .

Figure 1. Index map of South Ryukyu Islands . Bathymetries are shown in meters.

sent sea-level, were corrected to the elevations omote Islands. The Sekisei Lagoon , less than above mean sea level (AMSL), with reference 20m deep, is surrounded by barrier reefs. to tide records at Ishigaki Island (Table 1). Several islands within the lagoon are composed Samples collected for radiocarbon dating, were of Paleozoic rocks, truncated and capped by analysed by the Japan Radioisotope Associa Pleistocene coral limestones of various ages, tion and the GIF Laboratory (France). Field and surrounded by fringing reefs (Figure 1). work was carried out in March, 1981. Hateruma Island is located about 25km The area studied is shown in Figure 1. south of the lagoon, close to the Ryukyu Kohama and Kuro are small islands in the Trench. This island is entirely underlain by Sekisei Lagoon, which was named by EGUCHI Pleistocene coral limestone unconf ormably (1974), between tile large Ishigaki and Iri overlying Pliocene sedimentary rocks, and Coastal Morphology in the South Ryukyus 187 probably belongs to the "trench margin" zone minor emergence has occurred in the late (Zone I of OTAand HORI, 1980). In contrast, Quaternary. Kohama and Kuro Islands are a part of the Sea-levelindicators and their interpretation "outer rise" zone (Zone II) . The localities of the detailed observation are shown in Figure 2. II. Kohama Island Loc. 1: Beach deposits up to 2.7m about mean sea level (AMSL) including fragments of General coral and shells were observed at the foot of a Kohama Island (approximately 24•‹20•LN, sea cliff, cut in Paleozoic rocks. The upper 124•‹E, 10.3km2 in area) is located at the limit of the beach deposit may correspond to northern part of the Sekisei Lagoon. Undulat storm deposits. No radiometric age is avail ing surfaces of Pleistocene coral limestone able from this locality. surround a small and steep hill(99.4m in height) Loc. 2: A well-defined gently seaward slop which is composed of Paleozoic rocks (Figure ing surface, the inner margin of which is 1.9m 2). The absolute ages of the Pleistocene AMSL, is developed along an embayment of surfaces are unknown. At the northern the western coast. At the outer margin of coast, steep sea cliffs predominate, with the surface a concrete breakwater separates emerged marine notches near their base, but the surface from a mangrove swamp. Loc. 2 low flat surfaces less than 5m in elevation is east of the breakwater and about 1m AMSL occur along the other coasts (Figure 2). The at the top of the surface. At this locality, existence of these surfaces suggests that shells of Batillaria zonalis, typical of the inter tidal zone, were collected and dated at 2,580 •} 60yBP (N-4255). A small and higher rem nant of flat surface at 2.9m AMSL was also

Figure 2. Kohama Island showing topography, localities discussed in the text and elevation data of coastal features re

lated to relative sea-level movement. Figure 3. Kohama Island, Loc. 5, Arrow shows Contour interval is 10m, drawn from topographic map at a small Cyphastrea microatoll dated 1:50,000 scale. 1. Sea cliff 2. Holocene terrace 3. Sand dune 4. Locality at 3,300•}80yBP and remnants of a number 5. Elevation of marine terrace (in meters AMSL) former reef about 0.2m AMSL. 6. Elevation of retreat point of notch (in meters AMSL) The water level is 0.05m AMSL (photo P. P.). 188 Y. OTA et al .

observed, although no outcrop of terrace de Port (Table 1). This estimation seems to be posits was found. consistent with SCOFFINand STODDART's(1978) Loc. 3 and Loc. 4: A well-defined terrace is assertion: the tidal level of microatolls are also observed along the eastern embayment , situated just below MLWSL. with sand dune at its outer edge. The inner Northern and southern coasts: Along the margin of the terrace is 1.1m AMSL at Loc. 3 northern coast and in the central part of thee and 1.8m AMSL at Loc. 4. Scattered shells on southern coast, emerged notches cut in Paleo the surface near Loc. 3 were dominated by zoic rocks or Pleistocene coral limestone were Melanoides tuberculatus chinensis (fresh water observed at the foot of the sea cliff. Elevations species) with other species characteristic of of retreat points in notches were measured mangrove swamps (e.g. Vittaina plumbea). Be at ten localities. They range from 0.3 to cause the inner margin of the surface is clearly 1.1m AMSL (Figure 2), with a predominant parallel to the coast, a marine origin of the elevation of 0.7 to 1.1m AMSL, suggesting surface is probable. probable correlation with the terrace at Loc. 2 Loc. 5: Remnants of a former reef slightly and 4. Therefore, sea-level about 2,600 to emergent at mean tide occur near the shore. 3,300yBP was about 1 m higher than at present Here, a small fossil coral about 25cm in in Kohama Island and a relative sea-level diameter and 15cm high, still in growth posi fall took place after that time. A higher tion, reaches 0.2m AMSL, and displays a sea-level older than about 3,000yBP may be typical microatoll shape (Figure 3). A sample inferred from the presence of higher beach or from the microatoll, collected at 0.2m AMSL bench at Loc. 1 (2.7m) and Loc. 2 (2.9m). has been dated at 3,300•}80yBP (N-4258) . Definite evidences of marine origin or ages The former MSL can be estimated about 0 .8 are not yet available. to 0.9m above the present, in terms of the fact that the flat surfaces of microatolls off

Yonehara, Ishigaki Island, are situated about III. Kuro Island 35 to 45cm above the local datum at Ishigaki

Figure 4. Kuro Island showing topography, and sample locality , contour interval is 5m Map drawn from a topographic map at 1:50,000 scale. Coastal Morphology in the South Ryukyus 189

General Islands (KAWANA, 1982). The floor of the notch Kuro Island is situated in the southeastern es is often pitted, indicating that subaerial part of the Sekisei Lagoon. Fore-reef slopes, lowering processes are now active. These notch except the northern ones, continue to deep es are, therefore, considered indicative of a fore-arc slopes (Figure 1). Kuro is a low island sea-level stand slightly higher than the present. about 4km in diameter, reaching 13.7m AMSL Emerged beachrock occurs on the north in the west but generally less than 10m AMSL western coast of the island. Beachrock eleva and without surface drainage. A fringing tions range from 0.5 to 2.2m AMSL. Some reef, up to 1km wide with a crest-moat system lower parts of the beachrock have been broken occurs except to the north where the moat into slabs by erosion, but the upper parts are gently lowers to the bottom of the Lagoon clearly in situ and emergent. Figure 5 indi (Figure 4). FURUKAWA(1979) has revealed that cates the relation between the beachrock and the island is composed of Pleistocene Ryukyu the notch at Loc. 1 shown in Figure 4. Be Limestone with coralline, sandy foraminiferal, cause beachrock fills the notch, the formation algal ball, and muddy facies, exceeding 64m of the notch must predate at least a part of in thickness, and that Holocene sand dunes the beachrock. A shell specimen of Hippopus fringe the island especially in the northern and hippo pus (Linne) collected from the beachrock northeastern coast. Marine notches occur at 1.8m AMSL at Loc. 1 was dated at 4,220•} along the coast, and beachrock is found at the 80yBP (N-3965). northwestern coast. In several places along the shore of Kuro Sea-levelindicators and their interpretation Island, emerged fossil acorn barnacles have Emerged notches: Elevations of the retreat been observed in growth position on sheltered points of notches vary between 0.5 and 1.5m faces of sea caves or rock crevices. Fossil AMSL, i.e. up to 0.7m above the present mean Octomeris sulcata (Nilsson Cantell) was found highest high water spring level MHHWSL at approx. 1.4 to 2.45m AMSL. O. Sulcata (Table 1). The upper parts of the floor of the is a typical intertidal varnacle. In Kuro notches are colonized by Bostrychia tenella Island living O. sulcata occurs up to about (Vahl) J. Agardh, which usually reaches an 0.85m AMSL, although this barnacle usually upper limit between mean sea level (MSL) and forms colonies in the middle part of the tidal MHHWSL in the central and southern Ryukyu range (PIRAZZOLIet al., 1985). Fossil bar

Figure 5. Beachrock and a notch at Loc. 1 in Figure 4.

The notch is filled by beachrock. A shell from beachrock 1.8m AMSL (S. L.•}1.8ml was dated at 4,220•}80yBP. The highest elevation of the beachroclc is 2.2m AMSL, and the

elevation of the retreat point(R. P.) of the notch is 1.0m AMSL (photo T. K.). 190 Y. OTA et at.

nacles occur at the same level as notches and normally occurs between MHWSL and are indicators of former sea-levels. However, MLWSL, the upper limit (2.2m AMSL) all barnacle samples were too small to allow of the beachrock investigated in Loc. 1 radiocarbon dating. corresponds to a former MSL at approx. Interpretation: The late Holocene sea-level 1.5m AMSL (Table 1). This level has history in Kuro Island can be summarized as been dated at ca. 4,220yBP. follows 4. The final slight sea-level fall to its present 1. Based on the elevation of the notches and level was probably a gradual phenomenon the age of the beachrock at Loc. 1, because the floor of the notches grades to mean sea level prior to 4,220•}80yBP the present reef flat. was about 1.0m higher than the present. 2. The depths of the notches vary around the IV. Hateruma Island island from 1 to more than 3m (Oeogr. Club, Univ. Ryukyus, 1077), and deeper General undercuts may have been destroyed by Hateruma Island, 14km2 in area, is situated the collapse of the roof. A rate of notch at about 24•‹N and 124•‹E, and is the southern development is estimated to be 1mm per most emerged coral island in the Ryukyus. year in tropical limestones (KAYE,1959; The island is composed entirely of Pleistocene HODGKIN,1064; PIRAZZOLI,in press). coral terraces. Many studies of the coral Therefore, the sea level 0.5-1.5m higher terrace chronology and tectonic movement

than the present seems to have lasted have occurred at this important locality (e .g., several thousand years in Kuro Island. KAWANA and OSHIRO, 1918a, b; OTA et al., 1978a, 3. Assuming that cementation of beachrock b; KONISHI, 1980; OTA and HORI, 1980). The

Figure 6. Holocene coastal features in Hateruma Island. Terrace classification is after OTA et at. (1978b). Coastal Morphology in the South Ryukyus 191 terraces are divided into seven levels, with island (Loc. 1 and Loc. 2 in Figure 6). At Terrace III, (20 to 40m high), composed of Loc. 1, the highest elevation of beachrock crest thick coral reef limestone, correlated to the is 1.35m AMSL. The shell fragment of major stage of the Last Interglacial (OTA and Hippopus hippopus (Linne) collected at 1.0m HORI, 1980; Figure 6). The correlation has AMSL is dated at 1,430•}70yBP (N-4256). subsequently been confirmed by Uranium At Loc. 2 beachrock slabs are developed be series dating of corals of Terrace III (OTA tween -0.3 and +3.5m AMSL (Figure 7). et al., 1982; OMURA,1983). Holocene coastal A Hippopus Hppopus (Linne) collected slight features are shown in Figure 6. A fringing ly below the surface of the beachrock, at about reef with a width of a few hundred meters 0.6m AMSL, has been dated at 2,200•}75yBP surrounds the island, except along the south (N-4257). It is difficult to ascertain whether eastern coast. Sand dune and sand beach a higher slab of beachrock extending landward occur locally. up to approx. 3.5m AMSL is in situ, or if it Sea-level indicators and their interpretation is a broken part of the lower beachrock pro Emerged marine notches are frequently de veloped around the island. The elevation of their retreat points varies between 0.5 and 1.25m AMSL, but most commonly between 0.8 and 0.9m AMSL, indicating that relative sea level at the time of the formation of notches was higher than at present. Fossil barnacles Octomeris sulcata, collected from growth posi tion at 1.5•}0.15m AMSL in a rock crevice of Loc. 3, have been dated at 780•}60yBP

(GIF-5635). At that time sea-level was slight ly higher than present and approximately Figure 7. Hateruma Island, Loc. 2. Beachrock developed between -0.3 and •{1.8m AMRL. The coincident with the level of the emerged water level is at about -0.5m AMSL. A is an isolated slab notches. Emerged beachrock occurs locally of beachrock extending (out of the photo) up to 3.5m AMSL on the southern and northern parts of the (photo P.P.).

Figure 8. Aerial view of "Tsunami boulders" scattered on the flat surface near Takanazaki in the southeastern coast of Hateruma Island (photo H. M.). 192 Y. OTA et al .

Table 2. Radiocarbon dates of samples collected in Kohama , Kuro and Hateruma Islands.

jected upwards by a tsunami wave or a ty 9m on the downtlirown side (OTA and HORI , phoon. The latter possibility would be in 1980). There are no cliffs landward that may accord with the lack of evidence for a Holocene have supplied these boulders by erosion sea-level higher than the notch levels. , "TsunamiFeatures" and the boulders cannot be reached by present normal wave action. Measurement Dune like ridges and large boulders of coral of the direction of the long axis of fifteen limestone have been recognized along the boulders ranged from N20•‹E to 75•‹E and southeast coast (OTAand HORI,1980; Figures 6 averaged N50•‹E, consistent with deposition and 8). Characteristic features are: by a violent wave action coming from the 1. The ridges, up to forty meters wide and southeast. several meters high, are distributed parallel to 3. The distribution of the ridges and large the shoreline in dune-like forms, covering the boulders is limited to the area near Takanazaki , step separating the two lowest marine terraces. on the southeastern coast of the island . A These ridges are composed of sands and tsunami is the only event that is likely to gravels with large boulders, up to two meters transport and distribute the boulders in the across. C-14 dates of coral fragments col fashion described. lected from the ridge are less than 200yBP C-14 dates of coral fragments in the ridge

(OTAet al., 1980: Table 2). suggest the tsunami occurred about 200yBP . 2. Large boulders of coral limestone sev A large tsunami, accompanying a great eral meters across, are scattered on the flat earthquake is recorded in the South Ryukyus surface, which extends seaward of the ridges, in 1771. Boulders of coral limestone, similar as far as the present sea cliff (Figure 8). The to those in Hateruma Island, are described inner margin of this surface reaches 22m in from the southeastern part of Ishigaki Island elevation on the upthrown side of a fault and and their origin has been ascribed to the 1771 Coastal Morphology in the South Ryukyus 193

Tsunami (MAKING,1968; KATO and KIMURA, Japan, to Yoko OTA, and by The Centre National de 1983). The ridges and boulders in the south la Recherche Scientifique (France) and The Japan eastern part of Hateruma Island almost Society for the Promotion of Science to Paolo A. PIRAZZOLI. The authors would like to express their certainly have the same origin. thanks to Dr. Toshiyuki YAMAGUCHI(Chiba Uni versity, Japan) for identification of barnacles and V. Summary Dr. Yoshiaki MATSUSHIMA(Kanagawa Prefectural Museum, Japan) for identification of shells. Thanks 1. Marine terraces, marine notches, beach are also due to Mr. K. BERRYMAN(New Zealand rock, in situ coral reef and emerged shellbeds Geological Survey) for his critical reading of the are used as indicators of Holocene sea-levels in manuscript and refining English. Lastly, we are Kohama, Kuro and Hateruma Islands, the indebted to Mrs. Y. SAKAMURA,for typewriting and South Ryukyus. A sea-level slightly higher drawing. than the present one, about 1 m AMSL is References found from ca. 3,300 to 800yBP in Kohama and Hateruma Islands and about 1.5m AMSL DELIBRIAS,G. and PIRAZZOLI, P. A. (1983) : Late at about 4,200yBP in Kuro Island (Table 2). Holocene sea-level changes in Yoron Island, the It is, however, difficult to distinguish local Ryukyus, Japan. Marine Geology, 53, M7-M16. differences in these three islands, owing to the EGUCHI, M. (1974) : Scleractinian corals in the sea limited number of observations and some un of the proposed site for the sea park around Kerama certainty in the value of some coastal features and Yaeyama islands. In: Report of the basic as indicators of former sea-level. Late Holo investigation and the planning of the sea park around Kerama and Yaeyama Islands, Ohinawa Prefec cene emergence is very small. ture, 37-62 (in Japanese). 2. Shorelines which records the culmina FURUKAWA,H. (1979): Quaternary geologic history tion of Postglacial (Jomon) transgression at of the Ryukyu Islands. Bulletin of the Science and about 6,000yBP have been found along Engineering Division, University of the Ryukyus many parts of the Japanese coastline. Such (Mathematics and Natural Sciences), 27, 99-161. evidences of shorelines, however, have not been Geographical club of University of the Ryukyus observed in these three islands. The buried (1977): Notches on Kuro Island. Report of the notch in Kuro Island, covered by beachrock Geography of University of the Ryukyus, 12, 27-38 at ca. 4,200yBP, may correspond to this early (in Japanese). transgression. HODGKIN,E. P. (1964): Rate of erosion of intertidal 3. The relative sea-level drop causing the limestone. Zeitschrift fur Geomorphologie, N. F., 8, emergence of marine features in several Ryukyu 385-392. KATO, Y. and KIMURA,M. (1983): Age and origin of islands is a recent phenomenon. Slight emer so-called "Tsunami-ishi", Ishigaki Island, Okinawa gence dated at ca. 800 to 600yBP occurs not Prefecture. Journal of the Geological Society o only on Hateruma Island, but also on Yoron Japan, 89, 471-474 (in Japanese). Island (DELIBRIASand PIRAzz0LI, 1983), on KAWANA,T. (1982) : Vertical distribution of several central Okinawa Island (KAWANAand PIRAZZOLI, kinds of organisms attaching to notches along the 1983) and on Tarama Island (PIRAZZOLIet al., coasts of Yoron, Okinawa, Miyako and Ishigaki 1984). islands, the Ryukyus. Geological Studies of Ryukyu 4. Deposits found on the southeast coast Islands, 6, 107-113. of Hateruma Island, can be interpreted as the KAWANA,T, and OSHIRO, I. (1978a) : Preliminary result of a tsunami, which accompanied the report of the geomorphology and geology of Hate ruma Island, the South Ryukyus. Bulletin of great earthquake of 1771. Okinawa Prefectural Museum, 4,1-16 (in Japanese). (Received March 9, 1985) KAWANA,T. and OSHIRO, I. (1978b): Topography and (Accepted August 7, 1985) geology of Hateruma Island, -with special reference to the late coral reef de

Acknowledgments posits-. Geological Studies of Ryukyu Islands, 3, 139-146 (in Japanese with English abstract). Research fund was partly supported by Grand-in KAWANA,T. and PIRAZZOLI,P.A. (1983): Late Holocene Aid for Cooperative Research A (No. 56380027), crustal movements in Okinawa Island, the Ryukyus, 194 Y. OTA et al.

Japan. Abstracts of International Symposium on morphology and deformation of the raised coral Coastal Evolution in the Holocene, Tokyo, 53-56 . reefs in Hateruma Island, Ryukyu Islands, Japan. KAYE, C.A. (1959): Shoreline features and Quater Abstracts of the Annual Meeting of the Association nary shoreline changes Puerto Rico. U. S. Geo of Japanese Geographers, 15, 120-121 (in Japanese) . logical Survey Professional Paper, 317B, 49-140 . OTA, Y, and Horn, N. (1980): Late Quaternary KOBA,M., NAKATA,T, and TAKAHASHI,T. (1982): Late tectonic movements of the Ryukyu Islands, Japan. Holocene eustatic sea-level changes deduced from Daiyonki Kenkyu (The Quaternary Research), 18, geomorphological features and their 14C dates in 221-240 (in Japanese with English abstract). the Ryukyu Islands, Japan. Palaeogeography OTA, Y., HORI, N. and OMURA,A. (1982): Age and de Palaeoclimatology Palaeoecology, 39, 231-260. formation of marine terraces of Hateruma Island, KONISHI, K. (1965): Geotectonic framework of the Southwest Japan. XI INQUA Congress, Moscow, Ryukyu Islands (Nansei-shoto). Journal of the Abstracts II, 232. Geological Society of Japan, 71, 437-457 (in Japanese PIRAZZOLIP. A. (1978): High stands of Holocene with English abstract). sea levels in the northwest Pacific. Quaternary KONISHI, K. (1980): Diverse plate convergence as Research, 10, 1-29. deduced from raised coral reefs since the last PIRAZZOLI,P. A. (in press): Marine notches. In O . van interglacial. Daiyonki Kenkyu (The Quaternary de PLASSCHE(ed.): Manual for sample collection Research), 18, 241-257 (in Japanese with English and evaluation of sea-level data. Geo Abstracts (Geo abstract). Books), Norwich. MAKINO,K. (19681: The great tsunami of 1771 in the PIRAZZOLI,P. A., KAWANA,T. and MONTAGGIONI,L. F. Ryukyu Islands. 447p. (in Japanese). (1984): Late Holocene sea-level changes in Tarama OMURA,A. (1983): New information on radiometric Island, the Ryukyus, Japan. Chikyu-Kagaku ages of fossil coral from the Hateruma Island, (Earth Science), 38, 113-118. Ryukyu Islands. Daiyonki Kenkyu (The Quaternary PIRAZZOLI, P. A., DELIBRIAS, G., KAWANA, T. and Research), 22, 19-22 (in Japanese). YAMAGUCHI,T. (1985): The use of barnacles to OTA, Y., MACHIDA,H., HORI, N., KONISHI, K. and measure and date relative sea-level changes in the OMURA,A. (1978a): Late Quaternary tectonic move Ryukyu Islands, Japan. Palaeogeography Palaeo ment deduced from the raised coral reefs in Ryukyu climatology Palaeoecology, 49, 161-174. Islands, Japan. 66p. Report to the Ministry of SCOFFIN,T. P, and STODDART,D. R. (1978): The nature Education, Science and Culture, Japan (in Japanese). and significance of microatolls. Philosophical OTA, Y., HoRI, N, and TAMURA,A. (1978b): Geo Transactions Royal Society of London, B 284, 99-122.

琉球列島南部における完新世の海岸地形および海水準

太 田陽子1)・P・ ピラ ツオ リ2)・河名俊 男3)・森脇 広4)

琉 球 列 島南 部 の 小 浜 島 ・黒島 ・波照 間 島 の3島(第1 ビー チ ロ ッ クや フ ジ ツボ が や は り海抜 約1mの 高 さ に見 図)に つ い て,海 成 段 丘,離 水 ノ ッチ,ビ ー チ ロ ッ ク, 出 され る.以 上 の よ うに,こ れ らの3島 に お い て は 約 貝 ・フジ ツボ ・サ ン ゴ化石 な ど,完 新 世 に形 成 され た と 4,000年 前 以 降 に 今 よ り約1m高 い位 置 に海 面 が あ り, 思 わ れ る海 岸 地 形 ・堆積 物 ・化石 を調 査 した.調 査 した そ れ 以 降わ ず か な が ら離水 した こ とが 認 め られ る。 しか 島の 地 形 分 類 図,試 料 採 取 地 点 な どは第2, 4, 5図)に, し,日本 の各 地 にみ られ る縄 文 海 進 最 盛 期(約6,000年 前) 試 料 のC-14年 代 は第2表 に示 され る.こ れ らの3島 で を示 す 資 料 は これ ら3島 か ら現在 の とこ ろ見 出す こ と は は 約4,000yBPか ら1,000yBPの 間 の海 水 準 は 今 よ りわ で き な か った.な お 波照 間 島 の南 東 岸,高 那 崎 付 近 の海 ず か に 高 い 位 置 に あ った と思 わ れ る.す なわ ち,小 浜 島 抜 約20mに 達 す る平 坦 面 上 に サ ン ゴ石灰 岩 の 巨礫 が 多 で は 海 成 段 丘 堆 積 物 中 の 貝 や原 地 性 の サ ン ゴのC-14年 数 み られ る.こ れ はそ の 配 列 の方 向やC-14年 代か ら, 代 はそ れ ぞれ 約2,600yBP, 3,300yBPで,旧 海 面 は 海 1771年 の 明和 地 震 に よる津 波 の堆 積 物 で あ る こ とが 明 ら 抜 約1mの 位 置 に あ る.黒 島 で は ビー チ ロ ッ ク中 の 貝化 か に な った. 石 のC-14年 代 は 約4,200yBPで,旧 海 面 の 年 代 を 示 す

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