Oga Peninsula Research Articles

1009 Larval paleoecology of the late Pleistocene naticid gastropod Cryptonatica with two protoconch types from the Oga Peninsula, northern Japan

1009 Larval paleoecology of the late Pleistocene naticid gastropod Cryptonatica with two protoconch types from the Oga Peninsula, northern Japan

時間指標テフラ阿蘇３によって示唆される最終間氷期直後５ｄ期の海面低下

We have discovered a distal tephra with similar petrographic and chemical properties to the Aso-3 pyroclastic flow deposits of the Aso caldera origin at several localities of Honshu, Japan, and adjacent seas. It is concluded to be a fallout ash associated with the Aso-3 pyroclastic flow, The tephra, named Aso-3, is a cry tal vitric fine-grained ash, consisting of dacitic pumiceous and bubble-walled glass shards, and orthopyroxene and crynopyroxene as mafic phertocrysts. It is identified with the aid of combined petrographic and stratigraphic parameters : relatively high index of glass (n = 1.5131.519), low index of orthopyroxene (γ =1.7021.705) and dacitic glass composition (rich in alkali, K2O> Na2O, rich in TiO2, FeO* and CaO low abundance in light rare earth elements and characteristic low degree of angle in chondritenormalized pattern).This tephra was dated by several radiometric methods : 103±4.2 (FT), 110±3 (TL) and 123±6 (K-Ar) in ka. The stratigraphic position of this tephra is above the last interglacial culmination in Kyushu and Honshu, and below such widespread time-marker tephras as Toya, Ata, On-Pml, SK, etc. In addition, the tephra occurs in the marine isotope stage (MIS) 5d in the abyssal sediment in the northwest Pacific.The occurrence of this tephra in the Quaternary sequence in northern coastal areas of Miyazaki, south Kyushu, clearly indicates that the sea level was more than c. 70 m lower than the preceding high stand of sea level of the MIS 5e. At the coastal cliff of Kawaminami, the Aso-3 tephra occurs 21 m above the present sea level in the basal part of the valley-filling deposits (Tohriyamahama formation) which followed the prominent marine terrace, Sanzaibaru of the MIS 5e. Former shoreline of this stage was uplifted to the elevation of c. 90 m. This tephra was also recognized in the brackish to fresh water layer sandwiched between marine sediments, called Shibikawa formation in Oga Peninsula, northern Honshu. It clearly suggests significant lowering of sea level at the time of Aso-3 eruption as well.

秋田油田新第三系・第四系層序に関する最近の変革

Neogene stratigraphy of the Akita oil fields (s.l.) was first established by OHASHI (in TOYAMA, 1925) in the Oga Peninsula, and later by OMURA (1928, 1930) in and around Akita City, which is an important oil-producing area in the region. The oil-bearing marine strata in the Akita City area belong to the Onnagawa, Funakawa, Tentokuji and Sasaoka Formations, the type locality of the former two is in the Oga Peninsula. The Tentokuji and Sasaoka Formations were designated in the Akita City area and have been correlated with the Kitaura and Wakimoto Formations of the Oga Peninsula. The stratigraphic succession in the Akita City area has long been regarded as the standard succession not only for the Akita oil fields but also for the Neogene sequences along the Japan Sea coastal regions.Recent studies in magnetostratigraphy and in biostratigraphy based on diatoms, calcareous nannoplankton and planktonic foraminifera indicate that the correlation, so far established between the successions in the Oga Peninsula and Akita City area, is not valid. Based on these studies, the Onnagawa Formation of the Akita City area is correlated with the lower to middle part of the same formation of the Oga Peninsula, and the Funakawa Formation of the Akita City area is correlated with the upper part of the Onnagawa to the basal part of the Funakawa Formation of the Peninsula. The Tentokuji and Sasaoka Formations are correlated with most of the Funakawa Formation of the Peninsula, unlike the usual practice of correlating them with the Kitaura and Wakimoto Formations. It is after the deposition of the Sasaoka Formation that the Early Pleistocene Kitaura and Wakimoto Formations were deposited in a marine basin, which existed as a remnant of the earlier one, to the west of the present coast line of Akita-Noshiro.The Kitayuri thrust system along the present coast line seems responsible for the difference in the sedimentary environment of the two areas since the time of deposition of the upper part of the Onnagawa Formation in the Oga Peninsula.

褐藻イシモヅクの接合子発芽体の再生糸状体を種苗とする栽培

The zygotes grew into thick branched filaments under water temperature higher than 20°C. When the filaments were cut into small pieces with razors, they regenerated shoots and grew into new filaments. By means of cutting treatments, the filaments propagated in the culture vessels of Erlenmeyer flasks. Before cultivation, the filaments were cut into small pieces which were allowed to attach on nori-nets as seedlings according to the following procedures: the small pieces were scattered on the netting twines immersed in culture vessels fullfiled with culture medium, and kept aeration with air compressor for 2 to 3 months. The nori-nets were transplanted from laboratory culture to the sea for cultivation at Omura Bay, western Kyushu in 1980 and along Oga Peninsula, northern Honshu in 1985. Many Sphaerotrichia erect thalli appeared on the netting twines transplanted in December at Omura Bay and January at Oga Peninsula and grew to over 10cm length in 4 months at both localities.

Ages of Megata Maars in the Oga Peninsula, Japan

Ages of Megata Maars in the Oga Peninsula, Japan

Paleomagnetism of Tertiary rocks from the OGA Peninsula and the rotation of northeast Japan

We have made a paleomagnetic study of Tertiary rocks from the Oga Peninsula, northern Honshu Island. Volcanic and sedimentary rocks were sampled from 26 sites in the peninsula. The ages of the rocks range from 62 Ma to the present. The results indicate a counterclockwise rotation of Northeast Japan with respect to eastern Asia around a pole at 58°N, 149°E, between about 22 Ma and 15 Ma. The amount of the rotation is about 20°. Before the rotation, Northeast Japan was situated along the east coast of the Asian continent.

東北日本弧内帯の白亜紀‐第三紀火山岩のフィッション・トラック年代 奥尻島・男鹿半島・朝日山地

東北日本弧内帯の白亜紀‐第三紀火山岩のフィッション・トラック年代 奥尻島・男鹿半島・朝日山地

Volcanic Facies of the Tertiary Volcanic Sequences and Quaternary Kampu Volcano in Oga Peninsula, Northeast Inner Japan.

Volcanic Facies of the Tertiary Volcanic Sequences and Quaternary Kampu Volcano in Oga Peninsula, Northeast Inner Japan.

Genesis of calc-alkali andesite magma in a hydrous mantle-crust boundary: Petrology of lherzolite xenoliths from the Ichinomegata crater, Oga peninsula, northeast Japan, part II

The Ichinomegata volcano, northwestern Honshu, Japan, consisting of three explosion craters, is characterized by the presence of contemporaneous basalt (high-alkali tholeiite) and calc-alkali andesite and a variety of mafic and ultramafic xenoliths of deep-seated origin. The population of the rock types decreases exponentially as a function of increasing depth of their origin. Based on the Ichinomegata xenolith mineralogy, it is inferred that the lower crust and uppermost mantle beneath this area is partially hydrated, consisting dominantly of hornblende gabbro and hornblende-bearing spinel lherzolite, respectively. Chemical analysis on spinel-pyroxene symplectite (so called garnet pseudomorph) in some Ichinomegata lherzolites suggests a calcic-plagioclase primary chemistry rather than garnet. In lherzolite xenoliths which have undergone a preheating event, primary partial melting textures are observed. The composition of the glass formed along the grain boundaries of the partially melted lherzolites are similar to those produced in hydrous melting experiments on natural peridotite at about 10 kbar between 1000 and 1100°C. The high-alkali tholeiite and calc-alkali andesite of the Ichinomegata volcano are considered to have been formed by the following two-stage melting processes; (1) derivation of the basalt magma from partial melting of a peridotite diapir in the upper mantle at 40–50 km depth; (2) derivation of the calc-alkali andesite magma at 25–30 km depth by wet partial melting of the rocks at the mantle/crust boundary caused by emplacement of hot basaltic magma body. It is proposed that similar wet partial melting takes place more extensively beneath major island-arc volcanoes in the world, because the lower crust and the upper mantle beneath them may be hydrated due to continuous water supply from the subducting plate, and the amount of heat energy liberated at the mantle/crust boundary would be much larger in major stratovolcanoes than in the comparatively small Ichinomegata volcano. From consideration of energy balances, it is expected that the calc-alkali magma formed by this mechanism will be similar in volume to the hot basaltic magma solidified in the deep crust, and the base of the crust is the most likely site where substantial amounts of calc-alkali magma can be produced. It is emphasized that the model accords well with various constraints on the petrogenesis of calc-alkali rocks, among which most important are: (1) within the history of an island-arc volcano, calc-alkali rocks (andesite and dacite) occur in association with tholeiitic or alkalic rocks; (2) the ratio of calc-alkali rocks to other rocks (basalts and their derivatives) increases systematically with average crustal thickness beneath the volcanic belt; (3) calc-alkali rocks frequently show evidence of magma mixing.

Paleomagnetic Study on the Dike Swarm in the Oga Peninsula, Northeast Honshu Island

A paleomagnetic study has been made on 14 dikes emplaced at two formations in the Oga Peninsula. Stable remanences of seven dikes are normally magnetized, six are reversely magnetized, and one has an intermediate direction. No significant difference in the directions is observed between the normally and the reversely magnetized rocks and between the intruded formations. The veins of dikes trend northeast with the standard deviation of 4°. Mean inclination and declination of remanence for 13 dikes excluding an intermediate one are calculated to be 58.3° and -2.8°, respectively. The pole position corresponding to the mean field direction is close to that observed at the Middle Miocene formations in the peninsula and in the northern part of NE Honshu Island. Dike rocks indicate the bimodal volcanism characteristic in Middle Miocene time. The direction of stable remanences and the bimodal distribution suggest that the dikes intruded in Middle Miocene time. The dominant orientation of strikes in the Oga dikes represents one of the general trend of the Middle to Late Miocene stress field after the tectonic rotation of NE Honshu Island had completed.

Some Limnologial Remarks on Three Volcanic Lakes in the Oga Peninsula, Akita Prefecture

In this paper, the authors have tried to clarify some basic limnological features of three lakes called “Megata” in northwestern part of Oga Peninsula, especially aiming to compare the water quality in recent years with that about forty years ago (S. Yoshimura, 1936). This paper presents the result obtained by several surveys undertaken from 1973. Some results are summarized as follows:a) From this investigation, the bathymetric maps as shown in Figs. 2, 3 and 4 were obtained.b) In these three lakes, the water temperature gradient in thermocline in summer is higher than other lakes in Japan, and especially it is very high in Ichino-megata.c) According to the result of the analyses, the water quality in Nino-megata is slightly different from that of the other two lakes.d) A linear relationship is found to exist between the total amount of the precipitation in preceding winter and each content of Na+ and Cl- dissolved in the lake water in spring.

奥羽せきりょう最下部グリンタフ，大荒沢層の形成期

Remarkable fossil plants and marine animals were found from the Oarasawa Formation distributing along the upstream of the Isawa River, southwestern part of Iwate Prefecture, near Akita-Iwate border.The Oarasawa Formation, the lowest Neogene sequence in the Ou Backbone Mountain range area, has been correlated to the Nishioga Group or the Monzen Group of Oga Peninsula on the basis of characteristic lithofacies called “Oarasawa-Semi facies” without enough fossil evidence. The Oarasawa Formation outcropping along the Isawa River is composed mainly of subaqueous basaltic to andestic volcanic rocks and intercalates conglomerate, sandstone and siltstone layers, in which the fossil were newly found.The plant fossils include some characteristic species of the Daijima-type fossil flora such as Cyclobalanopsis nathorstii, Quercus miovariabilis, Liquiclamber miosinica and Smilax trinervis being determined by Professor emeritus K. Huzioka. The fossil marine animals are not abundant and poorly preserved, but the fossils contain a few common species of the Kadonosawa fauna that is nearly equivalent of the fauna in the Nishikurosawa stage.The fossil evidences show that the Oarasawa Formation is now placed in the Daijima-Nishikurosawa age. Therefore, by biostratigraphical and lithological facts, the Oarasawa Formation can be correlated consistently with the formations which were established or redefined recently, such as the Okachigawa Formation of Dewa mountoins, southern Akita Prefecture and the Sunakobuchi Formation distributting along the eastern margin of the Akita Oil Field.The litho- and biostratigraphical informations obtained recently from the lower Green Tuff, indicate that the Green Tuff volcanism took place in the Daijima-Nishikurosawa age in many places of the inland region of the northeast Japan.

Katanishi Formation in the Katanishi area, Oga Peninsula, Northeast Honshu, Japan

Katanishi Formation in the Katanishi area, Oga Peninsula, Northeast Honshu, Japan

Ecological Study of Grey Heron in Oga Peninsula, Akita Prefecture

Ecological Study of Grey Heron in Oga Peninsula, Akita Prefecture

石灰質微化石群集に基づく七谷層と西黒沢層の生層序学的考察

Biostratigraphic studies of both calcareous nannofossil and planktonic foraminifera were made on the Nanatani Formation exposed in the Imogawa section and the Nishikurosawa Formation in the Oga Peninsula, northeast Japan.The studies revealed the interrelation between calcareous nannofossil datum planes and planktonic foraminiferal ones shown by means of their occurences in ascending order as follows: Datum level a: Last appearance of Helicosphaera scissura (NN4/NN5 boundary), Datum level b: Abundant top of Discoaster def landrei (CN3/CN4) and first appearance of Orbulina suturalis (N8/N9), Datum level c: Abundant base of Reticulofenestra pseudoumbilica and first appearance of Globorotalia peripheroacuta and G. miozea (s. 1.) (N9/N10), Datum level d: Last appearance of Sphenolithus heteromorphus (CN4/CN5a or NN5/NN6).These fossil criteria indicate that the Nanatani Formation is CN3 to CN5a and N8 to N10. The Nishikurosawa Formation is the lower part of CN4 which corresponds to N9.Abrupt increment of Reticulofenestra pseudoumbilica in the late Nanatani stage implies the influence of cold water in early Middle Miocene to the area of Japanese islands which had been under the subtropical climate.

Stratigraphy of the upper Cenozoic marine strata in the Akita-Gojome area and its relation to those of the Oga Peninsula

Stratigraphy of the upper Cenozoic marine strata in the Akita-Gojome area and its relation to those of the Oga Peninsula

Thermal history of iherzolite xenoliths—I. Petrology of iherzolite xenoliths from the ichinomegata crater, oga peninsula, northeast Japan

The thermal history of four spinel lherzolites (Lhz-13. Lhz-28, Lhz-29 and Lhz-53) from tuff breccia of the Ichinomegata crater, northeast Japan, has been studied in detail. Lhz-13 and Lhz-53 showed nearly perfect chemical homogeneity of the constituent minerals, and increase of Ca near the rim of olivine is the only disequilibrium evidence observed. In addition to the Ca zoning in olivine, Lhz-28 and Lhz-29 revealed compositional zoning in the Mg/Mg + Fe ratio and Ca content in ortho- and clinopyroxenes. Lhz-13 and Lhz-53 equilibrated at about 800°C in the upper mantle, based on Fe/Mg partitioning between olivine/spinel and olivine/clinopyroxene, and on the mutual solubility of Ca between olivine and pyroxenes. Lhz-28 and Lhz-29 also equilibrated originally at about 800°C, but were preheated at about 1000°C prior to their entrapment by the ascending host magma. The Fe/Mg partitioning between olivine /spinel and olivine/clinopyroxene reequilibrated during the preheating event: however, the Ca solubility did not reequilibrate. Olivine alone has rehomogenized with a high-Ca content but pyroxenes were compositionally zoned with Ca. The preheating event, indicated by the high-Ca content in the core of olivine, is recognized from about a half of the Ichinomegata Iherzolites (50 xenoliths were studied). The duration of heating during the transport of the xenolith by the magma (estimated from the width of the Ca zoning in the rim of olivine) ranges between several hours to a year depending on the rock specimen. From the requirement to reset olivine core compositions, the duration of the preheating event was estimated as greater than 1000 yr.

Fission track ages of the Tertiary volcanic rocks in the Oga Peninsula, northern Japan

Fission track ages of the Tertiary volcanic rocks in the Oga Peninsula, northern Japan

Petrologic model of the crust and upper mantle of the Japanese Island arcs

Mafic and ultramafic xenoliths, in the Holocene calc-alkali andesite of Ichinomegata(1) crater in Oga peninsula and those in the Plio-Pleistocene alkali-olivine basalts of Oki-Dōgo island in the Japan Sea, have been studied in detail. Based on geothermometry and geobarometry, and relative abundance of the rock types of the xenoliths, petrologic models of the crust and upper mantle beneath these two areas were constructed. The crust and upper mantle beneath Ichinomegata crater are characterized by hydrous and relatively low temperature conditions. On the other hand, the crust and upper mantle beneath Oki-Dōgo island are characterized by nearly anhydrous and high temperature conditions, and presence of thick lavers of peridotite and pvroxenite cumulates in the uppermost mantle. The crust and upper mantle of the western part of the Northeast Honshū Arc can be considered as similar to those beneath Ichinomegata crater, because of the common occurrence of similar mafic xenoliths from many andesite volcanoes in this area. The crust and upper mantle of the northern part of the Southwest Honshū Arc, in the same way, can be regarded as similar to those beneath Oki-Dōgo island. Differences in amount of hydrous minerals of deep-seated rocks between the two areas can be interpreted as due to the presence of migrating water derived from the subducting Pacific plate in the Northeast Honshū Arc. Difference in slope of the geotherm may be due to the difference in temperature of the partial melt zones beneath these two areas. Bulk chemical compositions of the lower crustal materials of the Japanese island arcs, 85 mafic inclusions from 15 volcanoes, are listed, and it is concluded that they are cumulates or metamorphosed cumulates in the lower crust.

Neogene pollen stratigraphy of the Oga Peninsula, Northeast Honshu, Japan

Neogene pollen stratigraphy of the Oga Peninsula, Northeast Honshu, Japan