Shimajiri Group Research Articles

Mechanism of denitrification in subsurface-dammed Ryukyu limestone aquifer, southern Okinawa Island, Japan

Denitrification crucially regulates the attenuation of groundwater nitrate and is unlikely to occur in a fast-flowing aquifer such as the Ryukyu limestone aquifer in southern Okinawa Island, Japan. However, evidences of denitrification have been observed in several wells within this region. This study analyzed environmental isotopes (δ15NNO3 and ẟ18ONO3) to derive the rationale for denitrification at this site. Additionally, the presence of two subsurface dams in the study area may influence the processes involved in nitrate attenuation. Herein, we analyzed 150 groundwater samples collected spatially and seasonally to characterize the variations in the groundwater chemistry and stable isotopes during denitrification. The values of δ15NNO3 and δ18ONO3 displayed a progressive trend up to +59.7 ‰ and + 21 ‰, respectively, whereas the concentrations of NO3−–N decreased to 0.1 mg L−1. In several wells, the enrichment factors of δ15NNO3 ranged from −6.6 to −2.1, indicating rapid denitrification, and the δ15NNO3 to δ18ONO3 ratios varied from 1.3:1 to 2:1, confirming the occurrence of denitrification. Denitrification intensively proceeds under conditions of depleted dissolved oxygen concentrations (<2 mg L−1), sluggish groundwater flow with longer residence times, high concentrations of dissolved organic carbon (>1.2 mg L−1), and low groundwater levels during the dry season with precipitation rates of <100 mm per month (Jun–Sep). SF6 analysis indicated the exclusive occurrence of denitrification in specific wells with groundwater residence times exceeding 30 years. These wells are located in close proximity to the major NE–SW fault system in the Komesu area, where the hydraulic gradient was below 0.005. Detailed geological and lithological investigations based on borehole data revealed that subsurface dams did not cause denitrification while the major NE–SW fault system uplifted the impermeable basement rock of the Shimajiri Group, creating a lithological gap at an equivalent depth that ultimately formed a sluggish groundwater area, promoting denitrification.

Calcareous nannofossil biostratigraphy of the upper Cenozoic in the Okinawa-Miyako Submarine Plateau in the Ryukyu Islands

The upper Miocene to Pleistocene Shimajiri Group, composed mainly of siltstone and sandstone, occurs on some of the Ryukyu Islands in southwestern Japan. This group is thought to have been deposited in a shelf-slope to fore-arc basin setting before the accumulation of Pleistocene coral reef deposits (Ryukyu Group). We investigated the calcareous nannofossil biostratigraphy of the Shimajiri Group in the Offshore Okinawa 1-x well, drilled near the northeastern margin of the Okinawa-Miyako Submarine Plateau, Ryukyu Islands. A sample from 280 m, at the base of Ryukyu Group, contains Gephyrocapsa parralella, which first occurred at 0.987 Ma. Discoaster quinqueramus and D. berggrenii, which define both the top and bottom of the calcareous nannofossil zone NN11, are found throughout the Shimajiri Group, consistent with a late Miocene age between 8.10 Ma and 5.53 Ma. A large sedimentary gap between the Shimajiri and Ryukyu groups suggests that part of the Okinawa-Miyako Submarine Plateau may have been above sea level during the early Pliocene, implying deposition of the Shimajiri Group here was completed earlier than in other regions. This study provides key constraints on the Cenozoic geological history and phylogeography of the Ryukyu Islands.

Shimajiri Group equivalent sedimentary rocks dredged from sea knolls off Kume Island, central Ryukyus: Implications for timing and mode of rifting of the middle Okinawa Trough back‐arc basin

AbstractThe late Cenozoic geohistory of the Ryukyu arc is closely related to the rifting history of the Okinawa Trough. The submarine geology and stratigraphy of areas around Kume Island, which is situated near the eastern rifted margin of the middle Okinawa Trough, provide key constraints to understand the timing and mode of Okinawa Trough rifting. Here we report the lithology of sedimentary rocks dredged along slopes of ~1000‐m‐deep sea knolls located north and northwest off Kume Island, and their depositional ages determined by calcareous nannofossils and strontium (Sr) isotope analyses. Various types of sedimentary rocks, such as siltstone, very fine‐grained sandstone, medium‐grained sandstone, fossiliferous coarse‐grained sandstone, and tuffaceous sandstone, were recovered at two dredge sites. These sedimentary rocks are lithologically similar to those in the Aka Formation and a part of the Maja Formation of the Shimajiri Group in nearby Kume Island. Calcareous nannofossils and Sr isotope analyses indicate their depositional ages from the early Pliocene to the early Pleistocene, which are generally consistent with those of the Aka Formation. The finding of the dredged rocks similar in lithology and ages to the Aka Formation indicates that marine deltaic area continued toward north and northwest from Kume Island during these periods. The presence of the Shimajiri Group equivalent sedimentary rocks at the dredge sites are likely related to the main rifting of the Okinawa Trough after ca. 2 Ma in the central Ryukyus.

1 喜界島・沖縄島・宮古島の島尻層群浮遊性有孔虫と島尻変動

1 喜界島・沖縄島・宮古島の島尻層群浮遊性有孔虫と島尻変動

Calcareous nannofossil assemblages of the upper Miocene to Pliocene Shimajiri Group on Okinawa-jima, Ryukyu Islands, southwestern Japan

We investigated the calcareous nannofossil biostratigraphy and paleoceanography of the Shimajiri Group in two exploratory wells (depths, 1243 and 1800m) drilled on southern Okinawa-jima, Ryukyu Islands, southwestern Japan. Three late Miocene to Pliocene datum planes were identified: the first occurrence of Amaurolithus spp. (7.42Ma), the last occurrence of Discoaster quinqueramus (5.59Ma), and the first occurrence of Ceratolithus rugosus (5.12Ma). These datum planes were also identified in two other deep wells on southern Okinawa-jima. Nutrient levels (reconstructed from the relative abundances of small Reticulofenestra spp., and the relative abundances of Discoaster spp. plus Sphenolithus abies) suggest that deposition of the Shimajiri Group (<8.29–3.50Ma) occurred under conditions of increasing eutrophication. The eutrophication was likely caused by shallowing of the sedimentary basin, but a stepwise shift was associated with an abrupt eutrophication event that occurred in the northwestern Pacific Ocean at 5Ma.

Subsidence of the Miyako‐Sone submarine carbonate platform, east of Miyako‐jima Island, northwestern Pacific Ocean

AbstractBathymetric mapping and observations of the seafloor using a remotely operated vehicle (ROV, Hyper‐Dolphin 3K) were carried out on the slopes of the Miyako‐Sone submarine platform, east of Miyako‐jima in the Ryukyu Islands, northwestern Pacific Ocean. The bathymetric map indicates that terraces are present at water depths of approximately 140 m, 330 m, 400 m, and 680 m on the northwestern slope of the platform. A number of NW–SE trending lineaments, probably faults, extend perpendicular to the axis of the Ryukyu Island Arc. Two ROV surveys were conducted at water depths ranging from 519 m (on the slope) to 121 m (shallowest part of the platform). The surveys revealed that well‐indurated carbonate rocks are exposed at terrace margins and on upper slopes, and that the lower slopes are covered with modern sediments consisting of unconsolidated, coarse‐sand‐sized bioclastic carbonates. Calcareous nannofossils from the well‐indurated carbonate rocks indicate a Middle–Late Pleistocene age, which suggests that the rocks correlate with the Quaternary reef and fore‐reef deposits of the Ryukyu Group (Ryukyu Limestone) on the Ryukyu Islands. No siliciclastic deposits corresponding to the upper Miocene–lower Pleistocene Shimajiri Group (as exposed on Okinawa‐jima and Miyako‐jima islands) were recovered during the surveys. Coeval well‐indurated carbonate rocks, all of which formed in a similar sedimentary environment, have been downthrown towards the west due to displacements on the western sides of normal faults. Subsidence of the Miyako‐Sone submarine platform was the result of large vertical displacements on such normal faults. The timing of initial subsidence cannot be tightly constrained, but the presence of the youngest limestone at progressively lower levels towards the west suggests the subsidence continued until after 0.265 Ma.

亜熱帯湿潤地形の発達と地すべり -沖縄島, 島尻層群地域-

亜熱帯湿潤気候下の沖縄島中南部地域において, 地形特性, 地形変遷および地すべりについて調査を行い, 本地域での地すべり抽出手法を確立することを目的として, 地すべりの発生機構について論じた。亜熱帯湿潤気候下の島尻層群分布域では, 地すべり地形と類似する非地すべり地形が混在し, 写真や地形図による判読では地すべりを誤読することがある。誤読を避けるためには, 地すべり地形および半円状谷頭地形の形成機構の理解と基盤岩についての現地調査が重要となる。段丘由来の階段状地形では, 急斜面部で崩壊性地すべり, 緩斜面部で崩積土地すべりが発生するが, これらの規模は相対的に小さい。渓流の侵食等によって, 段丘地形が失われて形成される長大斜面では, 規模の大きな初生型泥岩地すべりが発生する。中南部東域の南地区のメサ状斜面では, 新里層下部層が侵食や地すべりの抵抗体となり, 中腹付近に急崖が形成される。急崖が崩壊等で失われ, 上部斜面に新規の侵食が及んでいる箇所において, 初生型泥岩地すべり発生の危険度が高くなる。

Late Miocene and Pliocene Ostracoda from the Shimajiri Group, Kume-jima Island, Japan: Biogeographical significance of the timing of the formation of back-arc basin (Okinawa Trough)

The Shimajiri Group, sporadically distributed in Kume-jima Island, Okinawa Prefecture, Japan, is mainly formed from clastic sediments deposited during the Late Miocene (7.8–7.2 Ma, the Maja Formation) and the Middle Pliocene (about 3.2 Ma, the Aka Formation). Twenty-one samples of these sediments were taken from the Maja and the Aka Formations. Of the 102 ostracod species recorded, most of them are well known on the Japanese mainland today. Three ostracod biofacies were delineated using Q-mode cluster analysis. Modern analog technique (MAT) was used to infer the environment during the deposition of the Shimajiri Group. The Maja Formation was deposited in a littoral temperate environment near the land mass situated to the east of the continent of Asia. The Aka Formation was deposited in the temperate shallow sea in front of the delta which protrudes from the vast continental delta-faced deep sea. The depositional environment of the Aka and the Maja Formations is very closely correlated with the first stage of the formation of the Okinawa Trough. This did not increase in depth until the Middle Pliocene.

沖縄本島南部に分布する島尻層群新里層最上部および知念層の堆積残留磁化測定

沖縄本島南部に分布する島尻層群新里層最上部および知念層の堆積残留磁化測定

沖縄における知念層と島尻層群の層位学的関係

沖縄における知念層と島尻層群の層位学的関係

琉球列島久米島に分布する新第三系海成島尻層群の石灰質ナンノ化石年代に関する新事実

Calcareous nannofossil biostratigraphy was studied for the Neogene marine Shimajiri Group distributed in Kume-jima Island, Ryukyu Islands, southwestern Japan. Out of 21 samples examined in this study, 14 samples yielded calcareous nannofossils. As was reported in a previous study, Zones CN9 and CN12 of Okada and Bukry (1980) were recognized in this study. In addition, Subzone CN11b (early Pliocene) was newly identified in the lower parts of the sequence where the ages were previously assigned to the late Miocene (upper part of Zone CN9 or older). This result shows the difficulty of establishing the stratigraphy of the subjected group based mainly upon lithotratigraphy, particularly in the lower part. Additional biostratigraphic data are needed to establish the reliable stratigraphy of the marine Shimajiri Group in Kume-jima Island.

Pleistocene reef development in the southwest Ryukyu Islands, Japan

Calcareous nannofossil and sedimentological studies of the Pleistocene Ryukyu Group, based on 13 cores and surface outcrops on Irabu-jima and Shimoji-jima, the Ryukyu Islands, southwestern Japan, suggest that these sediments were deposited at 1.5–0.3 Ma and unconformably overlie the Pliocene Shimajiri Group. The Ryukyu Group consists mainly of reef complex deposits, which accumulated in a wide range of depositional environments, from shallow reef flat to deep insular shelf. Carbonate lithofacies representing 10 depositional environments were delineated by comparison with the present-day marine sediments and biota around the Ryukyus. Five coral assemblages were defined by species composition and the morphology of fossil forms within the coral limestone, each indicating a particular environment. The Ryukyu Group in the study area is composed of 13 lithologic units. A complete succession of units commences with coral limestone (LST or TST) grading upward into rhodolith and larger foraminiferal limestones (TST) overlain by coral limestone (HST). Episodic subaerial exposure and subordinate karstification commonly occurred after deposition. The study area subsided during the deposition of units 1–12 and then was uplifted during deposition of unit 13. Reef development may have been dominated by obliquity or precessional cycles (sixth- and seventh-order cycles, respectively) for the first 0.5 million years (∼1.5–1 Ma), after which it may have responded to fifth-order (eccentricity) cycles.

Seismic record of tectonic evolution and backarc rifting in the southern Ryukyu island arc system

The southern Ryukyu island arc system is located at a convergent plate margin where the Philippine Sea Plate is subducting under the Eurasia Plate. We have conducted multi-channel seismic reflection surveys to study tectonic evolution and backarc rifting of the southern Ryukyu island arc system using the R/V Tansei-maru of the Ocean Research Institute, University of Tokyo, in June 1993 and 1994. We describe systematically a complete cross-section from the East China Sea continental shelf to the Ryukyu trench from the viewpoint of seismic stratigraphy. Seven major seismic units and three stages in the tectonic evolution of the system are identified: (1) during stage 1 from Late Miocene to earliest Pleistocene, pre-rift deposits of the Shimajiri Group accumulated over a wide region from the East China Sea continental shelf to the forearc region; (2) stage 2 is defined by a series of tectonic processes involving crustal doming, erosion, subsidence, and sedimentation, in association with initial rifting of the southern Okinawa Trough during most of Early Pleistocene time; and (3) the backarc rifting is still in progress and syn-rift sedimentation has been under way since the Late Pleistocene (stage 3). A new significant observation lies in the fact that the Pliocene Shimajiri Group is not distributed in most of the present-day southern Okinawa Trough. This implies that the backarc rifting of the southern Okinawa Trough was probably initiated after the deposition of the Shimajiri Group, that is, during the Early Pleistocene. Crustal-scale simple shear allowing an asymmetrical half-graben structure, rather than pure shear, governed the initial rifting of the southern Okinawa Trough. The possibility, however, cannot be excluded that pure shear associated with symmetrical rifting may be predominant in the present-day rifting of the southern Okinawa Trough as a result of northwestward migration of the rifting axis. The average extension rate of the southern Okinawa Trough is estimated to be approximately 1–2 cm/year on the basis of fault geometry observed on the acoustic basement.

Cookeolus spinolacrymatus sp. nov., a Late Pliocene priacanthid fish from Okinawa, Japan

A new fossil priacanthid fish,Cookeolus spinolacrymatus, is described on the basis of a single specimen from the Late Pliocene Shinzato Formation, Shimajiri Group, Miyagi-shima, Okinawa, Japan. This species is distinguished from the Recent speciesC. japonicus (Cuvier, 1828) by having a strong spine on the anterior portion of the lacrymal. The specimen was identified as a small benthic adult, or large juvenile, based on comparison with pelvic fin length development in the Recent species.

Niphon macrocephalus sp. nov., a Late Pliocene serranid fish from Okinawa, Japan

A new fossil serranid fish,Niphon macrocephalus, is described on the basis of a single specimen from the Late Pliocene Shinzato Formation, Shimajiri Group, Miyagi-shima, Okinawa, Japan. This species is distinguished from the Recent speciesNiphon spinosus Cuvier, 1828, in having the following four characters: (1) large head, (2) long frontals, (3) rounded posteroventral angle of maxilla, and (4) large 3rd hypural.

Early Pleistocene birth of the Okinawa Trough and Ryukyu Island Arc at the northwestern margin of the Pacific: evidence from Late Cenozoic planktonic foraminiferal zonation

Widespread development of the upper Pleistocene Ryukyu Limestone suggests the occurrence of prototypes of the Okinawa Trough and Ryukyu Island Arc, barkarc-basin and forearc at the northwestern margin of the Pacific, before its deposition. Prior to the deposition, however, bathyal sediments of the Shimajiri Group were distributed over the arc region. On southern Okinawa Island, this group is divided into 11 planktonic foraminiferal zones, some of which may be not globally valuable but regionally useful, ranging in age from late Pliocene to earliest Pleistocene. This zonal scheme is applied to zoning 421 outcrop samples and a geologic map is succesfully obtained. The map indicates the development of many faults that cut only the Shimajiri Group but not the overlying Ryukyu Limestone and majority of these faults run in perpendicular to the arc extension. Similar faults are also recognized in the lower Pliocene to lowermost Pleistocene Shimajiri Group on Miyako Island (southern Ryukyu Island Arc), in a submarine seismic profile, which was correlated with well stratigraphy, at the east of Miyako Island, and in the upper Pliocene to lowest Pleistocene Somachi Formation on Kikai Island (northermost central arc). The structures observed in the Shimajiri Group and its equivalent may have been formed by an upward warping of the arc region associated with subsidence of the trough region. Considering the pre-Shimajiri basement geology as a whole, therefore, it is concluded that the initial appearance of this backarc basin and forearc occurred after the deposition of the Shimajiri and before that of the Ryukyu Limestone during the late Pleistocene (∼ 1.7 −0.5 Ma).

シーケンス層序学 琉球列島伊良部島の第四系石灰岩の堆積相と海水準変化

Thirteen boreholes were drilled on and around Irabu Island in the southern Ryukyus, from which limestones of the Pleistocene Ryukyu Group (50-110m) were recovered. The seven lithologies in the Group fall into the coral limestone facies and the rhodolith limestone facies. The former is composed mainly of coral floatstone/framestone and the latter is characterized by rhodolith-large foraminifera floatstone/rudstone. By comparing the bio-sedimentary facies of the core samples with those of present sediments around Irabu Is., the coral limestone facies typically represents a reefal-shelf area of 0-60m water depth, and the rhodolith limestone facies an open shelf of 60-150m deep.The Ryukyu Group is widely distributed on Irabu Is., gently dipping west. It can be divided into three Members, and each separated from its neighbours by a solution unconformity. The Lower Member comprises coral framestone/floatstone and overlies the Shimajiri Group with an angular unconformity, which gradually dips westward. The Middle Member is characterized by five cyclic sequences of alternating coral limestone facies (Unit C-1, C-2, C-3, C-4, C-5) and rhodolith limestone facies (Unit R-1, R-2, R-3, R-4). However, the coral limestone facies dominates in the eastern hilly area but abruptly decreases in thickness and is increasingly replaced by the rhodolith limestone facies towards the western lowland. The Upper Member is a large foraminifera grainstone/packstone. Boreholes can be correlated on the basis of lithostratigraphy and calcareous nannofossil biostratigraphy. Five nannofossil-datum levels show the Ryukyu Group to be 1.36-0.39Ma.The Ryukyu Group accumulated in a reefal to shelf environment where the water depth was increasing toward the west. At low sea level stand, the coral limestone facies extended for a distance westward and at high sea level stand, the rhodolith limestone facies was distributed widely in the area.The unconformity between the Lower and Middle Members falls between 1, 10Ma and 0.89Ma. A number of unconformities, regressive sequences, and cooling episodes are recorded at this time in Japan. According to nannofossil-datum, the lowest rhodolith limestone facies of the Middle Member may be correlated with the oxygen isotope stages 25-23 of Williams et al. (1988), and the coral-rhodolith sedimentary cycles in the Middle Member are considered to be formed in response to glacial eustacy (oxygen isotope stages 26-16).

Influx of the Kuroshio Current into the Okinawa Trough and Inauguration of Quaternary Coral-Reef Building in the Ryukyu Island Arc, Japan.

The oldest Quaternary coral-reef limestone of the Ryukyu Islands have been dated 700, 000 to 600, 000yrs BP by the electron spin resonance (ESR) method. Prior to this date, the last time fossil coral reef was built in the Eocene time. The Shimajiri Group, mudstone and sandstone of the Late Miocene to 1.6 million years ago, was deposited in bathyal ironment. A 1, 000km long land bridge, the ‘China-Okinawa Land Bridge’, was formed in the Early Pleistocene Time. Coral-reef building since then is discussed here. Coral reef could not have been built in the marginal area of coral-reef building like the Ryukyu Islands, before the Kuroshio or Japan Current entered into the Okinawa Trough situated behind the land bridge. This reason is explained using the three-dimensional flow characteristics of the present Kuroshio Current. The collision of the Luzon Volcanic Arc with Taiwan made the boundary area between the Ryukyu Island Arc and Taiwan deeply bend and pulled apart the Yonaguni Depression on it. The depression introduced the current to the back-arc basin as at the present day. As a result tropical waters swept the Ryukyu Islands, and Quaternary coral-reef building began 700, 000 to 600, 000yrs BP.

Relation Between Reorientation of Clay Particles and the Residual Strength of Mudstone of the Shimajiri Group

Relation Between Reorientation of Clay Particles and the Residual Strength of Mudstone of the Shimajiri Group

Pelecypod Fauna of the Shimajiri Group in Miyako-jima, Okinawa Prefecture, Japan

Pelecypod Fauna of the Shimajiri Group in Miyako-jima, Okinawa Prefecture, Japan