喜界島東岸, 志戸桶付近の完新世サンゴ礁段丘の形成と離水過程-ボーリング資料に基づく再検討 Holocene Sea Level and Tectonic History Related to the Formation of Coral Terraces at Kikai Island, Northern Ryukyu Islands
喜界島北東部の志戸桶海岸において,完新世サンゴ礁のボーリング調査および地表調査,サンゴの<sup>230</sup>Th/<sup>234</sup>U年代測定などに基づいて,後氷期海進に伴うサンゴ礁の形成過程と,その後の離水に伴う段丘の分化を論じた.本地域でのサンゴ礁の成長は,海面上昇におくれたcatch-up typeである.そのために,最高位のI面にはサンゴ石灰岩はわずかしかみられない.それに代わって,5.3kaに離水したII面がサンゴ石灰岩の堆積面で,礁池と礁嶺を伴う典型的な隆起サンゴ礁の形態をとる.段丘III,IVは礁斜面に発達した狭い波食面で,一部に若いサンゴ石灰岩を伴う.段丘の分化は間欠的な離水,おそらく地震隆起によるが,II面を除いて離水期を確定できなかった.さらにパプアニューギニア,ヒュオン半島と完新世サンゴ礁段丘との比較により,サンゴ礁の成長の速度や段丘の形態の差を明らかにした.ヒュオン半島では,最高位の完新世段丘はkeep-up typeのサンゴ礁からなる堆積性の面であり,地震隆起による多段化が顕著で,その離水期が確定できたことが,喜界島との大きな相異である.
Kikai Island in the northern Ryukyu chains, 80km from the Ryukyu Trench, has the highest uplift rate in Japan, 1.8m/ka since isotope stage 5e. Four subdivided Holocene terraces, from the highest terrace I to the lowest IV, fringe Kikai Island and record the Holocene uplift. These terraces were previously described and considered as evidence for coseismic uplift and sea level fluctuation since the culmination of the Postglacial transgression by Nakata <i>et al.</i> (1978) and Ota <i>et al.</i> (1978), on the basis of surface observation. Here, in addition to the surface data, we use subsurface data to infer the history of formation and emergence of Holocene coral reef terraces.<br>We used five drilling cores at the Shidooke coast, northeastern Kikai Island. These drillings reached the base of the Holocene sediment, with maximum thickness of 26m. The basal topography beneath the Holocene sediment was reconstructed. Holocene sediment is classified into five lithofacies: A (coral limestone), B and C (detrital limestone), D and E (detrital limestone with terrestrial fragments). Forty <i>in situ</i> corals are dated by the α-spectrometric <sup>230</sup>Th/<sup>234</sup>U method. Isochron for Holocene sediments indicates upward reef growth from ca. 10ka to ca. 6.5ka, followed by seaward growth since that time.<br>The highest Terrace I is mostly underlain by non-coral sediment of Facies C. In contrast, Terrace II, the widest and with moat and crest system, is underlain by Facies A, typical coral limestone. Thus, Terrace II is a constructional terrace of coral limestone which emerged at 5.3ka. Narrow terraces III and IV are essentially cut surfaces into the steep fore reef slope of Holocene coral tract, characterized by Facies A but with some younger corals at their outer margin. The timing of their emergence is not determined at moment, but is estimated to be 3.1ka and younger than 2.6ka, respectively.<br>Holocene coral terraces at Kikai Island are compared with those at Huon Peninsula, Papua New Guniea, which is located in the core region of reef growth. Uplift-corrected height of the transgressive corals at Kikai is generally lower than the predicted eustatic sea level curve, implying that coral growth did not keep pace with sea level rise and probably should be interpreted as a catch-up type. In contrast, transgressive corals at Huon closely coincide with the predicted sealevel curve, which suggests that they are a keep-up type. Repeated coseismic uplifts commonly occurred at Kikai and Huon, but were more evident at Huon, where the uplift rate is higher than at Kikai. One of the significant differences is that the highest Holocene terrace at Huon is a transgressive constructional one, underlain by a transgressive reef, while that of Kikai is represented by a constructional terrace of noncoral sediments, reflecting a slower reef growth than that of Huon Peninsula.
第四紀研究 39(1), 81-95, 2000-02-01
Japan Association for Quaternary Research