Kokumoto Formation Research Articles

Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba section. The sediment succession consists of a massive siltstone layer of the Kokumoto Formation, Kazusa Group. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the Byk-E tephra layer from the Chiba section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were collected at intervals of 1–10 cm from the siltstone. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. In addition, magnetization during PThD showed sharp drops around 175 °C, which decreased gradually between 175 °C and ~ 300 °C, and became unstable above ~ 350 °C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175 °C followed by PAFD was conducted. Combined analysis of remagnetization circles enables extraction of primary magnetization with improved reliability. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200 °C, 400 °C, 450 °C and 550 °C. Rock magnetic analyses and electron microscopy indicate that titanomagnetite/magnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified preserving secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested as secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The correlation of relative paleointensity with the Chiba section provides an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra. VGP latitudes are highly consistent with those from the Chiba section based on the age model, which assigns the main directional swing from reversed to normal polarities as 772.8 ± 0.5 ka.Graphical

Millennial-scale oscillations in the Kuroshio\u2013Oyashio boundary during MIS 19 based on the radiolarian record from the Chiba composite section, central Japan

Marine isotope stage (MIS) 19 is considered to be the best orbital analog for the present interglacial. Consequently, clarifying the climatic features of this period can provide us with insights regarding a natural baseline for assessing future climate changes. A high-resolution radiolarian record from 800 to 750 ka (MIS 20 to MIS 18) was examined from the Chiba composite section (CbCS) of the Kokumoto Formation, including the Global Boundary Stratotype Section and Point for the lower–middle Pleistocene boundary on the Boso Peninsula on the Pacific side of central Japan. Millennial-scale oscillations in the Kuroshio warm and Oyashio cold currents were revealed by the Tr index, which is estimated using a simple equation based on radiolarian assemblages. The estimated Tr values ranged between 0.1 and 0.8 for MIS 18 through MIS 19, with minimum and maximum values corresponding to values observed off present day Aomori (41°N) and the Boso Peninsula (35°N), respectively. The observed patterns tended to be synchronous with the total radiolarian abundance associated with their production. Multiple maxima in radiolarian abundance occurred during periods of the Oyashio expanded mode before 785 ka and during periods of Kuroshio extension after 785 ka in MIS 19. Such increases in radiolarian abundance with the Kuroshio extension during MIS 19 are likely related to improvements in nutrient and photic environments with the development of a two-layer structure along the Kuroshio–Oyashio boundary zone. A similar pattern of millennial-scale climatic changes was also recognized in a precipitation record from the Sulmona Basin in central Italy, suggesting a close relationship with the CbCS record as a result of a large-scale climate system similar to the Arctic Oscillation in the northern hemisphere.

Multiproxy sedimentological and geochemical analyses across the Lower\u2013Middle Pleistocene boundary: chemostratigraphy and paleoenvironment of the Chiba composite section, central Japan

The Chiba composite section (CbCS) in the Kokumoto Formation, Kazusa Group, central Japan is a thick and continuous marine succession that straddles the Lower–Middle Pleistocene boundary and the well-recognized Matuyama–Brunhes paleomagnetic polarity boundary. Although recent studies extensively investigated the CbCS, its chemostratigraphy, particularly around the Lower–Middle Pleistocene boundary, is poorly understood. Therefore, in this study, we performed multiproxy sedimentological and geochemical analyses of the CbCS, including the Chiba section, which is the Global Boundary Stratotype Section and Point for defining the base of the Middle Pleistocene Subseries. The aim of these analyses is to establish the high-resolution chemostratigraphy and to reconstruct the paleoenvironments of its sedimentary basin in detail. We used the K/Ti ratio as a broad proxy for the clastic material grain size of the sediments. Although the K/Ti ratio generally varies throughout the studied interval, the K/Ti ratio especially during Marine Isotope Stage (MIS) 19a shows a variation pattern like those of the foraminiferal oxygen isotope (δ18O) records. The records of the C/N ratio of bulk samples and carbon isotope ratio of the organic carbon (δ13Corg) suggest that the organic matter in the CbCS sediments during MIS 19c mostly originated from marine plankton, whereas the organic matter during MIS 18 and 19a was characterized by a mixture of marine plankton and terrestrial plants. These records are clearly indicative of changes in mixing ratio of marine vs. terrestrial organic matter in association with glacial–interglacial cycles from the late MIS 20 to the early MIS 18. In addition, we calculated the mass accumulation rates (MARs) of organic carbon, biogenic carbonate, and terrigenous material for quantitative interpretations on the paleoenvironmental changes. MAR calculations revealed that the contribution of marine organic carbon relative to terrestrial organic carbon increased during MIS 19c, and that the contribution of the terrigenous material relative to biogenic carbonate decreased during MIS 19c. Furthermore, we observed relatively large variations in the total organic carbon and total nitrogen contents during MIS 19a. These variations were probably caused by the relative decrease in bottom-water oxygen level, which is also supported by our trace-fossil data, although it is not certain whether the increase in organic-carbon flux at ~ 760 ka was due to the synchronous increase in biogenic productivity in surface water. Such a relative decrease in bottom-water oxygen level was partly due to the increased ocean stratification because of the northward displacement of the Kuroshio Extension Front.

Spatial and temporal variations in depositional systems in the Kazusa Group: insights into the origins of deep-water massive sandstones in a Pleistocene forearc basin on the Boso Peninsula, Japan

A detailed chronostratigraphic framework established by the mapping of tephra key beds and application of oxygen isotopic data allows assessment of the synchroneity and diachroneity of depositional systems formed in coastal and deep-water environments. This framework also allows estimation of the timing of active delivery of coarse-grained sediments beyond the shelf margin in relation to relative sea-level changes. The depositional processes of deep-water massive sandstones (DWMSs) are still enigmatic; their formation is a result of active delivery of sands in association with the supply of organic carbon into deep-water environments. DWMSs are also important as reservoirs for hydrocarbon explorations. This study investigated the origins of DWMSs in the upper Umegase, Kokumoto, and Chonan formations (in ascending order) of the Pleistocene Kazusa Group on the Boso Peninsula, central Japan. Each formation contains several packets of DWMSs that are interpreted to have formed in response to the progradation of gravelly shelf-margin deltas or fan deltas during the falling and lowstand stages of relative sea-level changes controlled primarily by glacioeustasy. The development of DWMSs and associated sandstone beds is interpreted to have been induced by hyperpycnal flows, in association with sediment gravity flows that were initiated by breaching and/or collapse of sandy substrates on the shelf-margin deltas or fan deltas. The timings of the initial and final deposition of the packets vary within and between the formations, and are considered to have been controlled by the interaction between allogenic and autogenic processes operating in the gravelly shelf-margin deltas or fan deltas. A muddy horizon that contains the Lower–Middle Pleistocene Subseries boundary (the base of the Chibanian Stage) in the Kokumoto Formation is also underlain and overlain by the packets and represents a deposit formed in a condensed section in an upper slope environment. This depositional setting may have favored the development of the Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene Subseries boundary in the formation.

Calcareous nannofossil biostratigraphy of the Lower\u2013Middle Pleistocene boundary of the GSSP, Chiba composite section in the Kokumoto Formation, Kazusa Group, central Japan, and implications for sea-surface environmental changes

The Chiba composite section (CbCS), in the middle of the Boso Peninsula in central Japan, was ratified as the Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene boundary, accompanied by the Matuyama–Brunhes (M–B) paleomagnetic polarity boundary in January 2020. This study examined the calcareous nannofossil biostratigraphy of the CbCS to describe potential nannofossil events and discuss sea-surface environments around the M–B paleomagnetic polarity boundary. There are no clear biohorizons at the M–B paleomagnetic polarity boundary, although a temporary disappearance of Gephyrocapsa specimens (≥ 5 μm in diameter), an important calcareous nannofossil genus in the Pleistocene, occurs just above the Lower–Middle Pleistocene boundary. Although this is a characteristic event around the M–B paleomagnetic polarity boundary, it is unclear whether the event is globally traceable.Changes in the environmental proxy taxa of calcareous nannofossils in the CbCS revealed that sea-surface environments were driven by glacial-interglacial and millennial-scale climate forces. The time-transgressive change of the Tn value, a calcareous nannofossil temperature index, is mostly concordant with the planktonic foraminiferal oxygen isotope fluctuation. Abundant occurrences of a warm-water species, Umbilicosphaera spp., indicate that the Kuroshio Current was strong after ~ 783 ka. Even the strong influence of the Kuroshio Current, cooling events related to southward movements of the Kuroshio Front occurred every 10,000 years based on the presence of a cold-water taxon, Coccolithus pelagicus. Additionally, the inflow of coastal waters strengthened after ~ 778 ka because of abundant occurrences of Helicosphaera spp. Millennial-scale sea-surface changes were also inferred from detected floral fluctuations of less than 10,000 years.Graphical abstract

A summary of the Chiba Section, Japan

We propose the extensively studied Chiba section of the Chiba composite section (CbCS) as the Global Boundary Stratotype Section and Point (GSSP) to define the base of the Middle Pleistocene Subseries and Chibanian Stage. The CbCS is a continuous and expanded marine sedimentary succession within the middle of the Ko-kumoto Formation, Boso Peninsula, Chiba Prefecture. It contains well-preserved pollen, marine micro-and macrofossils, a tightly-defined Matuyama-Brunhes (M-B) paleomagnetic polarity boundary, and numerous tephra beds, allowing the establishment of a robust and precise chronostratigraphic framework across the Lower-Middle Pleistocene boundary. Its deep-marine, open-ocean continental slope setting, coupled with high sedimentation rates with no evidence of abrupt deposition, has resulted in preserving both terrestrial and marine environmental changes in and around Marine Isotope Stage (MIS). We have performed high-resolution oxygen isotope analysis , as well as detailed sedimentological, geochemical, and biostrati-graphic studies of the CbCS, confirming the completeness of the stratigraphic interval spanning the Lower-Middle Pleistocene boundary. The M-B polarity boundary serves as the agreed primary guide for the Lower-Middle Pleistocene boundary, and the CbCS represents one of the most detailed marine sedimentary records yet obtained for this reversal, yielding an astronomical age of. ka with a duration of ca.. kyr. This section therefore offers an exceptional opportunity to calibrate the geological time scale as well as understand the dynamics of the geodynamo. The widespread Byk-E tephra lies close to the reversal, allowing precise regional lithostrati-graphic correlation. The CbCS is easy to access from international airports. The section will be permanently preserved and maintained by the local government as a natural monument. Based on these attributes , the base of the Byk-E tephra bed in the Chiba section is the best horizon for establishing the GSSP that will define the Lower-Middle Pleistocene boundary.

Paleomagnetic direction and paleointensity variations during the Matuyama\u2013Brunhes polarity transition from a marine succession in the Chiba composite section of the Boso Peninsula, central Japan

The youngest geomagnetic polarity reversal, the Matuyama–Brunhes (M–B) boundary, provides an important plane of data for sediments, ice cores, and lavas. The geomagnetic field intensity and directional changes that occurred during the reversal also provide important information for understanding the dynamics of the Earth’s outer core, which generates the magnetic field. However, the reversal process is relatively rapid in terms of the geological timescale; therefore, adequate temporal resolution of the geomagnetic field record is essential for addressing these topics. Here, we report a new high-resolution paleomagnetic record from a continuous marine succession in the Chiba composite section of the Kokumoto Formation of the Kazusa Group, Japan, that reveals detailed behaviors of the virtual geomagnetic poles (VGPs) and relative paleointensity changes during the M–B polarity transition. The resultant relative paleointensity and VGP records show a significant paleointensity minimum near the M–B boundary, which is accompanied by a clear “polarity switch.” A newly obtained high-resolution oxygen isotope chronology for the Chiba composite section indicates that the M–B boundary is located in the middle of marine isotope stage (MIS) 19 and yields an age of 771.7 ka for the boundary. This age is consistent with those based on the latest astronomically tuned marine and ice core records and with the recalculated age of 770.9 ± 7.3 ka deduced from the U–Pb zircon age of the Byk-E tephra. To the best of our knowledge, our new paleomagnetic data represent one of the most detailed records on this geomagnetic field reversal that has thus far been obtained from marine sediments and will therefore be key for understanding the dynamics of the geomagnetic dynamo and for calibrating the geological timescale.Graphical abstract.

Sedimentary processes and depositional environments of a continuous marine succession across the Lower–Middle Pleistocene boundary: Kokumoto Formation, Kazusa Group, central Japan

The Chiba composite section of the Kokumoto Formation in central Japan is a remarkable, thick marine succession and is a candidate for formal recognition as the Global Boundary Stratotype Section and Point (GSSP) for the base of the Middle Pleistocene stage. Although there have been numerous studies of the Kokumoto Formation, the sedimentology of the formation, particularly in the Chiba composite section, has been unclear. We use field observations and laboratory analyses to elucidate the sedimentary processes and depositional environments of the formation. The Chiba composite section includes the Matuyama–Brunhes paleomagnetic reversal, which is the primary guide for the Lower–Middle Pleistocene boundary, along with multiple tephra deposits that provide isochronous correlational tie points. One bed, Byk-E (also known as Ontake-Byakubi Tephra), immediately underlies the paleomagnetic reversal and hence provides a useful widespread marker for the boundary. The predominant silty beds of the Chiba composite section are intensely bioturbated and lack evidence of episodic deposition such as slumps or muddy turbidites. However, minor sandy beds intercalated within the silty section, particularly in its lower part, are either (1) fining upward sequences with faint planar or ripple laminations and an erosional base, or (2) sequences containing abundant mud clasts in a poorly sorted sand matrix with a sharp base and an irregular top surface. The silty beds are interpreted to be hemipelagite formed by deposition of fine-grained suspended material under stable and calm bottom-water conditions. The sandy deposits are interpreted to be sediment gravity-flow deposits (i.e., turbidites and debrites). These sedimentary processes and the trace fossil assemblage indicate that the Chiba composite section was formed in a continental slope environment. The depositional environments of the Chiba composite section, together with tephrostratigraphy, make it suitable for high-resolution studies on the paleoenvironment, paleoclimate, and paleoceanography of the northwest Pacific Ocean across the Lower–Middle Pleistocene boundary; it is therefore a suitable candidate to become the GSSP for the Middle Pleistocene.

High resolution stratigraphy across the early–middle Pleistocene boundary from a core of the Kokumoto Formation at Tabuchi, Chiba Prefecture, Japan

We present a detailed stratigraphy of the lithology, sedimentary facies, magnetic susceptibility, and chemical composition of a 54-m thick core sequence drilled adjacent to the Chiba section, a candidate for the Global Boundary Stratotype Section and Point of the early–middle Pleistocene boundary. Siltstones rich in trace fossils are dominant throughout the core, except for the lowermost sand beds. The Byakubi tephra is divided into three units, of which the uppermost (39.54–39.49 m core depth) is identified as a primary bed showing evidence of fallout. The preliminary Matuyama–Brunhes magnetic polarity boundary lies 1 m above the top of the fallout unit. Maximum and minimum susceptibility axes for anisotropy of magnetic susceptibility have shallow (<11°) and steep (>73°) inclinations, respectively, throughout the core. The results suggest that sedimentary structures formed in calm environments remain undisturbed in the sediment. Clustered maximum axis directions indicate the influence of a persistent NW–SE contour current. The homogeneous depositional environment for the siltstone interval is supported by relatively smooth variations in magnetic susceptibility and chemical composition. Stratigraphic correlation with published oxygen isotope data indicates that the depositional age of the core may extend from the end of marine oxygen isotope stage (MIS) 20 to just after mid-MIS 19, from which an average sedimentation rate of 2–2.5 m/ky is estimated. These results suggest that the Chiba section has a high potential to provide an excellent stratotype for the early–middle Pleistocene boundary.

Stratigraphy of the Kazusa Group, Boso Peninsula: An expanded and highly-resolved marine sedimentary record from the Lower and Middle Pleistocene of central Japan

Major climatic reorganizations, including changes in the nature of glacial–interglacial cycles through the Pleistocene, are a key issue for improving the understanding of Earth's climate system. Highly resolved marine sedimentary records are essential to reconstruct the details of these past climatic changes and investigate the mechanisms responsible for them. The Kazusa Group, located in the central part of the Japanese island chain, is well-exposed and contains a remarkably continuous and thick, deep- and shallow-water marine sedimentary succession. This group also contains well-preserved marine microfossils, pollen, paleomagnetic reversal events, geochemical signatures, and a large number of tephra beds. These features allow us to establish a robust chronological and stratigraphic framework for the Kazusa Group, and provide a rare opportunity to study oceanic and terrestrial climatic and environmental changes at high resolution especially through the Lower and lower Middle Pleistocene. In the Boso Peninsula, Chiba Prefecture, the Kazusa Group is deeply incised, yielding spectacular river-cut exposures. The Matuyama–Brunhes paleomagnetic reversal is observed immediately above the widespread Byakubi-E (Byk-E) tephra in the Chiba composite section within the Kokumoto Formation (Kazusa Group). Because the Matuyama–Brunhes boundary customarily serves as the primary guide for the Lower–Middle Pleistocene Subseries boundary, the Chiba composite section is considered an excellent candidate for its global boundary stratotype section and point (GSSP). This study reviews the published data for the Kazusa Group (hitherto almost exclusively in Japanese) in order to place the Chiba composite section within its broader depositional and chronostratigraphic context.

A marker tephra bed close to the Lower-Middle Pleistocene boundary: Distribution of the Ontake-Byakubi Tephra Bed in central Japan

Tephrochronology is an exceptionally important tool in the precise regional correlation of Early and Middle Pleistocene sedimentary strata in Japan. The present study reveals that the Yukawa tephra 5 (YUT5) derived from the Older Ontake volcano, the Nezumigawa (Nzg) and Mitamitajima (Mtj) tephras of the Ina Bazin, and the Byakubi-E tephra (Byk-E) of the Boso Peninsula are the same tephra on the basis of their lithofacies, bulk grain composition, mafic mineral composition, major element composition of hornblende, and stratigraphic relationships with the dated tephras. We propose to call the series of tephras correlated with Byk-E the Ontake-Byakubi Tephra Bed (On-Byk Tephra) following the naming convention in which the tephra name consists of the names of the source volcano and the type location. The Matuyama-Brunhes Chronozone boundary occurs just above Byk-E in the type section of the Kokumoto Formation in the Kazusa Group, which is a candidate Global Boundary Stratotype Section and Point (GSSP) for the lower boundary of the Middle Pleistocene Subseries. Therefore, On-Byk Tephra becomes a critically important marker tephra bed for the Early-Middle Pleistocene boundary in central Japan. The present study indicates that the major element composition of hornblende can be a useful tool for identification and correlation of strongly weathered tephra layers such as Nzg and Mtj in which all the volcanic glass shards have been altered.

Depositional Process of the thick siltstone of middle part of Kokumoto Formation in the Kazusa Group, the L-M Pleistocene boundary horizon

Depositional Process of the thick siltstone of middle part of Kokumoto Formation in the Kazusa Group, the L-M Pleistocene boundary horizon

地化学・貯留層シミュレーションによる二酸化炭素の地化学トラッピングの検討：東京湾岸モデル

Two dimensional numerical simulation was performed using reactive geochemical transport code TOUGHREACT based on the geology of the Tokyo Bay area to understand long-term geochemical evolution of aqueous and mineral phases after CO2 injection and to elucidate key issues affecting CO2 mineral trapping. The injected CO2 migrates upward in the reservoir (the Umegase Formation) towards the cap rock (the Kokumoto Formation) driven by buoyant force. In the CO2 plume, water pH drops from 7.8 (initial) to 4.9 by dissolution and dissociation of CO2. The acidified water containing CO2 and related species induces mineral dissolution and promotes complexing of dissolved ions, which increase the water density and cause its downward movement to the bottom of the reservoir. This process gives rise to convective mixing which further promotes CO2 solubility trapping. In the peripheral zone of the CO2 plume, the acidified water comes in contact with intact reservoir water, giving rise to the precipitation of calcite (and siderite, in lesser amount). The precipitation of calcite and other carbonates might form a low permeability barrier surrounding the CO2 plume, and is expected to increase the storage security with time. Among newly formed carbonates, calcite becomes the most dominant in a long period after injection cease. Dawsonite, which is occasionally considered to be important in trapping CO2, is found to have limited stability that requires high CO2 partial pressure at the conditions of saline aquifer storage. Our study also finds that the formation of dawsonite is in a close relation to the dissolution of plagioclase, and that its precipitation is strongly suppressed by the formation of kaolinite. After reviewing rates of dissolution of carbonates and feldspars, we conclude that the precipitation of dawsonite is controlled by plagioclase dissolution kinetics which determines the supply of Al as a major constituent of dawsonite. Plagioclase is a dominant clastic mineral in sandstones in geologically young formations of the Japanese Islands. Its dissolution could be a key issue in geochemical modeling of CO2 aquifer storage.

Geometry and internal organization of hyperpycnites associated with a shelf-margin delta, the Middle Pleistocene Kokumoto Formation on the Boso Peninsula of Japan

Some turbidite beds of the Middle Pleistocene Kokumoto Formation on the Boso Peninsula of Japan exhibit different types of vertical and lateral changes in grain size and sedimentary structures compared with the Bouma model, which is generally interpreted to represent turbidites deposited from surge-type, ephemeral turbidity currents. Some turbidite beds examined by this study are characterized by several repetitions of coarsening-to-fining upward patterns in association with climbing ripple lamination and parallel lamination. Furthermore, these turbidites are not necessarily laterally continuous in the two-dimensional cross section. Two thickest parts are separately observed within the same stratigraphic level. The geometry and internal organization of these turbidites are interpreted to be a product of hyperpycnal flows generated from a multi-source system associated with a shelf-margin delta in an upslope area during falling- and lowstand stages.

Diatom Biostratigraphy of the Kazusa Group, Boso Peninsula, Honshu, Japan.

Since the Kazusa Group is considered as the candidate to stratotype of the Early-Middle Pleistocene boundary in the Pacific region, it is urgent to investigate the Kokumoto Formation as a member formation of the Kazusa Group. The paleomagnetic boundary between C1r (Matuyama) and C1n (Brunhes) is recognized at the ash marker bed “Shirao” in the middle part of the Kokumoto Formation. The diatoms from the Otadai, Umegase, Kokumoto, Kakinokidai, and Chonan Formations along the Yoro River, in the middle part of the Boso Peninsula, about 80km southeast of Tokyo, have been studied. The marine sequences are dominated by shelf-water Paralia sulcata (Ehrenberg) Kützing indicating shallow water sedimentation. The seven diatom datum levels (biohorizons) allow precise correlation in the transitional region of the northwest Pacific Ocean. Among them the four biohorizons appear synchronous near the Early-Middle Pleistocene boundary in the Kokumoto Formation: LO (last occurrence) of Actinocyclus oculatus Jousé and LO of Rhizosolenia matuyamai Burckle at the lower part of the boundary, and LO of Nitzschia fossilis (Frenguelli) Kanaya and LO of Nitzschia reinholdii Kanaya at the upper part of the boundary. The regressions were recognized twice at the basal part of C1r. 1n (Jaramillo) in the Otadai Formation and across the C1r and C1n (Early-Middle Pleistocene boundary) in the Kokumoto Formation. The Kokumoto Formation is suitable as the candidate to stratotype of the Lower-Middle Pleistocene boundary of the Pacific region based on diatom biostratigraphy.

Tephrochronological study on the middle Pleistocene deposits in the Kanto and Kinki districts, Japan

The middle Pleistocene time range is defined here the period before the last interglacial in the Brunhes epoch. In Japan, the Shimosa-Kazusa Groups around the Tokyo Bay in Kanto and the upper part of the Osaka Group in Kinki both represent the standard middle Pleistocene sequences and have been studied in detail. These groups are characterized by the cyclic sedimentation caused by transgression and regression corresponding to climatic changes.The purpose of this paper is to attempt the correlation of the Kazusa Group and the Osaka Group by identification of widespread marker-tephras. Accurate determinations of the refractive indices of volcanic glass, orthopyroxene and hornblende, together with other determinations, have enabled successful characterization for correlation to be made for several tephra layers in southern Kanto and Kinki. The following marker-tephras are found over two districts, resulting in the establishment of several important datum planes in the middle Pleistocene sequences.The vitric tephra called Ks 11, which is sandwiched in the Kasamori Formation and its correlatives in southern Kanto, can be identified as a marker tephra of the Osaka Group called Sakura ash, in the vicinity of Osaka and Kyoto. The estimated age of this tephra, about 450, 000 years, is based on its stratigraphic relationship with the underlying dated tephra, Kinukawa ash (460, 000-470, 000 years). Stratigraphically, it is included in the deposits immediately below the transgressive horizon (Tama-f and Ma 7) and also occurrs in the biozone of Stegodon orientalis in both districts. The vitric tephra called Ku 1, sandwiched in the Kokumoto Formation, is identified and correlated with a tephra called Imakuma I ash. It is included in the deposits immediately above the Brunhes-Matuyama boundary. The very important dated tephra, Azuki ash (870, 000 years), in the Osaka Group, can be correlated with Ku 6C tephra in the lower part of the Kokumoto Formation by their peculiar petrographic properties. Both are sandwiched in the deposits below the Brunhes-Matuyama boundary.From the relationship between the identified tephras and marine sequences, it is concluded that during the period from 700, 000 years to 450, 000 years at least three interglacial-glacial cycles are recorded, and that after 450, 000 years the following major interglacial episodes are indicated; 450, 000YBP, 370, 000YBP, 300, 000YBP, 230, 000YBP, and 130, 000YBP.

Geomagnetic Reversal and Change in Foraminiferal Fauna and Sedimentary Environments

The purpose of the present study is to find the relationship of a geomagnetic field reversal to the changes in a foraminiferal fauna and the sedimentary environments. In the first part of this paper, quantitative estimations were made of the possible effects of direct and/or indirect radiations on the earth's surface caused by cosmic rays, solar flares and solar wind during the geomagnetic field reversal. The result of estimation suggested that the increased radiation during the geomagnetic field reversal could not be a factor significant to cause the appearance or extinction of marine species.A fossiliferous marine sedimentary section of the middle part of the Kokumoto Formation continuously exposed at Kamiyanagawa in the Boso Peninsula was chosen for the investigation of the behavior of the geomagnetic field during its reversal between the Matuyama and Brunhes Polarity Epochs which took place around 0.69 m.y. ago. The behavior of the geomagnetic field was established in detail through the measurement of the inclination, declination and intensity of the DRM and IsRM of the sedimentary rocks in the middle part of the Kokumoto Formation. Along the same section, analyses were made on the grain-size of the sediments, oxygen isotope ratio in the planktonic and benthonic foraminiferal tests, and the faunal assemblages of the planktonic and benthonic foraminifers.The intensity of the magnetic field fluctuated with a period of approximately 7000 years and did not disappear during the reversal. The migration of the virtual magnetic north pole occurred at the magnetic field reversal between the Matuyama and Brunhes Polarity Epochs roughly along the meridian of 120°E Long. in 4700 years. In the same duration, the depth of the water, under which the sediments were accummulated, fluctuated within the range of 200-300m with a period of approximately 7000 years. No particular relation between the magnetic field reversal and fluctuation of the water depth was recognized. The result of paleotemperature and faunal analyses of the foraminifers showed that the magnetic field reversal at the Matuyama-Brunhes Polarity Epoch boundary may have been related to the changes in the circulation pattern of the upper water-masses which caused more influx of cold water into this area in the northwestern Pacific as compared with the preceding period.

DummyGeometry and internal organization of hyperpycnites associated with a shelf-margin delta, the Middle Pleistocene Kokumoto Formation on the Boso Peninsula of Japan

Some turbidite beds of the Middle Pleistocene Kokumoto Formation on the Boso Peninsula of Japan exhibit different types of vertical and lateral changes in grain size and sedimentary structures compared with the Bouma model, which is generally interpreted to represent turbidites deposited from surge-type, ephemeral turbidity currents. Some turbidite beds examined by this study are characterized by several repetitions of coarsening-to-fining upward patterns in association with climbing ripple lamination and parallel lamination. Furthermore, these turbidites are not necessarily laterally continuous in the two-dimensional cross section. Two thickest parts are separately observed within the same stratigraphic level. The geometry and internal organization of these turbidites are interpreted to be a product of hyperpycnal flows generated from a multi-source system associated with a shelf-margin delta in an upslope area during falling- and lowstand stages.