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Temporal and spatial variations in accommodation (i.e., paleo-water depth) and sediment accumulation (amount of deposition) in the intra-arc Osaka Basin, Japan, were reconstructed from the post-glacial transgression through the sea-level highstand, a total of 9000 years. At the beginning of the marine transgressive stage (about 11,000 cal y BP), paleo-water depths were shallow and the sediment accumulation was large. The area occupied by Osaka Bay gradually extended and sediment deposition decreased from 11,000 to 6000 cal y BP. During the period of maximum transgression (6000 - 5000 cal y BP), an inner bay, Kawachi Bay with a water depth of 5 - 10 m, was expanded in the inland eastern Osaka area, and paleo-water depths reached a maximum and depositional rates reached a minimum. During the subsequent highstand and small regression (about 5000 cal y BP to the present), however, deposition increased rapidly as a result of river delta and shoreline progradations. Regional differences were observed in accommodation and accumulation between the outer bay area and the inner bay area. During both the transgressive and regressive stages, deposition decreased in the inner bay area. In contrast, in the outer bay area and in the basin overall, deposition was high during the first part of the transgressive stage but it decreased during the maximum transgression, before reaching a maximum during the subsequent highstand and regression. During the regressive stage, fluvial delta progradation led to the formation of a thick sequence of delta body sediments. Sediment accumulation was 30% - 40% higher during the regressive stage than that during the transgressive stage.

大阪平野と大阪湾域の完新統の海成粘土層の堆積環境を堆積曲線解析から求めた.その結果,海成粘土層は従来から知られていた湾底泥堆積物だけではなく,プロデルタや潮汐低地の堆積物が含まれることがわかった.特に,ここで新しく認識された潮汐低地堆積物は,淀川流域や河内平野の海成粘土層に広くみられる.この発見はまた,そうした潮汐低地の粘土層が支持地盤として非常に弱いことから,応用地質学的にも重要であるといえる., Depositional environments of the Holocene marine clay bed in the Osaka plain and Osaka bay areas were clarified based on the analysis of depositional curves. The result shows the marine beds previously reported as a bay-floor mud deposit include not only the bay-floor deposit but also prodelta and tidal-flat deposits. This tidal-flat deposit, especially mud-flat one, newly defined here is discovered widely from the so-called marine clay beds in the Yodo river and Kawachi plain. The finding is also important for the geotechnical engineering, because of the mud-flat deposit shows very poor foundation.

We acquired three geo-slicer cores along a longitudinal profile on the subaqueous portion of the Hii River delta built since the 1630s in Lake Shinji, western Japan. Grading patterns of 26 individual sand beds in the cores vary spatially. We interpret these sand beds as subaqueous flood deposits from their sedimentary structures, the geomorphology of the delta lobe, and the absence of hydrodynamic influences other than river floods entering the lake. Most sand beds at shallow depths (< 4 m below the water surface) display triple stacks of inverse-to-normal grading. Single inverse-to-normal grading appears between 4 and 5 m depth, and normal grading dominates in deeper beds. Inverse and normal grading could reflect the waxing and waning of the parent flow, respectively. We explain the occurrence of triple stacks of inverse-to-normal grading in shallower horizons and single inverse-to-normal grading in deeper horizons as consequences of lateral movement of the plunge point of flood plumes during the course of flood events. Spatially decelerating sediment-laden river plumes steeply increase their velocity after they plunge beneath the water surface. In depth-limited proximal areas of a subaqueous delta, back-and-forth translation of the plunge point over a fixed point due to the waxing and waning of river discharge leads to three cycles of waxing and waning of flow velocity, and the resulting deposits at the fixed point would have triple stacks of inverse-to-normal grading. In the distal parts of the delta, velocity of fully plunged hyperpycnal flow increases and then decreases, reflecting directly the waxing and waning of river discharge, and the resulting deposits would be composed of single inversely-to-normally graded beds. In the most distal parts, the slower initial part of the hyperpycnal flow may be overtaken by the succeeding faster part to yield a monotonically waning flow at a fixed point. This process may have resulted in sand beds with uniformly normal grading in lower (distal) horizons. Assuming that flood hydrographs for the Hii River have not changed since the 17th century, these findings mean that the momentum of river flow during floods propagates to the outflow differently at different distances from the river mouth. Our results show that in parts of the delta-front slope, the outflow of river floods accounts for the inverse-to-normal grading considered to be an important basis for identifying hyperpycnal flow turbidites. However, in other parts of this environment, grading patterns of flood deposits do not necessary reflect the flood hydrograph. Overdependence on inverse-to-normal grading in identifying hyperpycnal flow deposits may lead to inaccurate estimations of their frequency in the sedimentary record. On the other hand, special care is needed for the reconstruction of paleo-flood hydrographs using sedimentary records.

Subsurface geological cross sections of 0 - 200 m depth were constructed using a dense drilling database of the Osaka Plain in the intra-arc Osaka Basin of the Japan island arc, an active plate margin. The cross sections revealed the subsurface geological structures and the geometry of folding and faulting in the basin. The comparison between the constructed subsurface cross sections and the seismic sections of the basement and basin fills at a depth of 1500 - 2000 m showed that the basement and shallow subsurface structures are similar; however, the shallow cross sections were of higher resolution than the deep seismic profiles.

A paleo-lacustrine delta in Kyoto, Japan was reconstructed on the basis of subsurface geological and geomorphological analysis, and paleo-lake level changes were estimated from the structure of the delta. These analyses of the study region, i.e., the Oguraike reclaimed land area provided evidence that Lake Ogura existed until about 60 years ago in southern Kyoto, Japan. The Uji river delta was provided influents to this lake until ca. 400 years ago, as is indicated by an upward-coarsening delta succession of about 2 - 4 m thickness. The lake level could also have changed in the past as a result of a change in altitude of the delta-front (foreset) and delta-plain boundary, which probably reflects the lake surface elevation. About 400 years ago, the Paleo-Uji River was separated from Ogura Lake because a levee was constructed along the river for building a castle and for constructing a waterway for transportation. As a result of this construction, the lake level that was more than 13.0 m in elevation was reduced by 1.5 m. In a more ancient times, the lake level experienced two stages—one in which the elevation was more than 13.5 m, and one in which the elevation was reduced to less than 10 m. These changes in the lake level are represented by a flat surface with four steps and small cliff of height ca. 0.5 - 2 m (relative elevation) separating them, recognized at the southern lakeshore. The observation of strata along with the archaeological survey in the north of Ogura Lake reveals that the lake level was decreased ca. 800 - 680 years ago. The lake level was at its highest during two periods, the first from before the 8th century to the end of the 8th century and the second from the 14th century to 400 years ago.

Flume experiments aimed to produce flaser bedding were conducted using fine sand and clay in a circular flume. The formation process of mud drapes during the slack-water stage was revealed in detail. When the tidal current declines, a uniform mobile mud layer initially settles from suspension and drapes the entire rippled sand bed (type A mud). When the remaining flow velocity is very low, a more fluid mud begins to settle out (type B mud) that preferentially fills the ripple troughs, the ripples and mud together forming a flat surface. At slack tide, the two-phase mud drape is temporarily stationary. After the onset of the reversed flow phase, most of the type B mud is resuspended, while the type A mud is eroded from the crests, leaving behind a remnant mud drape (flaser) in the troughs that is subsequently buried by migrating ripples. Type B mud generally contains variable amounts of sand derived from eroded ripple crests, but does not show any visible internal sedimentary structures. Type A mud represents the ‘mud drapes’ commonly described in the literature, the temporary existence of type B mud having gone unnoticed because of its low preservation potential. When present, it can be recognized by its sand content and the occurrence of flame structures in ripple troughs. Tidal deposits reflecting the existence and depositional characteristics of both type A and type B mud are found in, for example, the macrotidal Oligocene Ashiya Group, Japan.

The depositional history of a delta was investigated by using radiocarbon-dated borehole cores from the Yahagi delta (Nishimikawa plain), Central Japan. In particular, historical changes in the gradient, thickness, and grain size distribution of the delta front and sediment discharge of the Yahagi River are discussed. The latest Pleistocene to Holocene sequence in the Yahagi delta is interpreted as an incised valley fill system formed after the Last Glacial and subdivided into five sedimentary facies: facies A (fluvial), facies B (estuary), facies C (prodelta), facies D (delta front), and facies E (gravelly tidal flat). In the main axis of the incised valley, facies A, facies B, facies C, facies D, and facies A are deposited, in ascending order. In the western area of the valley, facies E adjoins facies D. The delta front was formed by river floods without reworking by wave or tide. The succession was interpreted as a transgressive systems tract and highstand systems tract formed under sea-level rise until ca. 7 cal kyr BP and the subsequent highstand. After the formation of the delta in ca. 7 cal kyr BP, characteristic features of the delta front changed twice as the delta prograded. First, during 4–5 cal kyr BP, the delta front became thicker and steeper. This change was induced by differences in water depth between the middle incised valley and the outer incised valley, which resulted from a buried clinoform. Second, in ca. 3 cal kyr BP, the grain size distribution of the delta front became coarser and the sorting became poorer. The sediment discharge of the Yahagi River also increased abruptly. This change was induced by the increase of erosional capacity in the hinterland. Increasing human activities such as deforestation and poor soil conservation might have induced this change.

The strontium isotopic composition of the oceans changed markedly at the Cambrian-Ordovician transition, some 500 million years ago. This isotopic shift was greatly affected by the first bio-invasion of the land and the ensuing terrestrial-surface environments in a chain reaction that included: 1) an attenuation of weathering on land, 2) changes in the outflow patterns of fluvial floods and the circulation patterns of groundwater, 3) marked regional differentiation of coastal environments, 4) the formation of soil layers, and 5) the eutrophication of estuaries by nutrient salts of terrestrial-biosphere origin. A series of these environmental changes culminated in the marine Ordovician biodiversification, an explosive flourishing of the Paleozoic evolutionary fauna that is characterized by a variety of filter and suspension feeders. The bio-invasion onto land was one of the greatest geobiological events in the Earth's history.

Paleoclimate of interglacial Marine Isotope Stage 11 (MIS 11), about 400 ky ago was estimated using data from shallow-marine to terrestrial strata of the Japanese Islands. The reason of the estimation comes from that the paleoclimate gives analogs for the future climate, because the conditions of Milankovitch forcing of MIS 11 are similar to those of modern Holocene. The results show the MIS 11 of the Japanese Islands was warmer, with a longer interval of highstand, and higher sea levels than the other interglacials. Further investigation for the strata of MIS 11 of the Japanese Island is needed and will give us important information about our future climate.

Setting the experimental condition under partially standing wave, which little attention has been given to in spite of being found in nature, a series of wave-flume experiments of development of ripples were performed, using four kinds of sand with different diameters (0.15 mm≤D≤1.23 mm). Ripples developed under partially standing waves showed the cross-shore variations in ripple spacing, asymmetry and crest-line pattern, depending on the positions to ‘nodes’ and ‘anti-nodes’, and on grain size. Ripple spacing was the largest under nodes and the smallest under anti-nodes and was associated with the variation in the orbital diameter. Ripples with rounded crests and bifurcated crest-lines tended to form under anti-nodes.

The formation time of cross-beds was estimated by comparing fluctuations of foreset angle of cross-stratification of paleo-Kuroshio current deposits within the Pleistocene Ichijiku Formation Japan with variations in flow velocity in the present-day Kuroshio current. Foreset lamina angles were measured from a continuous 14 m long bed, and angles of cross-stratification were determined using Fourier analysis. The periodicities of Kuroshio current velocity fluctuations were also subjected to the same Fourier analysis. Comparison between the periodicities of foreset lamina angle and Kuroshio velocity revealed that the two are related with interval ratios of the peaks in periodicity of 1:1.8:3.2:4.2 and 1:1.9:3.2:4.4, respectively. These two ratios are almost identical. It is calculated that 4.0, 7.3, 12.8 and 17.0 foreset lamina were formed in 12.4, 24.0, 39.4, 55.9 h.

This study reports a newly discovered gravelly tsunami deposit from the Lower Cretaceous Miyako Group, Japan. The deposit was formed in an open shallow marine setting. The event deposit erosionally overlies shoreface deposits and shows marked lateral facies change. At the basin margin, the deposit is composed mainly of amalgamated HCS sandstones with liquefaction structures, overlain by finer sediments that contain many plant fragments or micas. Conglomerates accompanying the HCS sandstones contain molluscan fossils and many coral clasts. In the basin center, the event deposit is made up mainly of conglomerates and lenticular sandstone beds, and passes upwards into alternating sandstones and siltstones. A condensed organic debris layer is intercalated within the alternating section. Conglomerates contain abundant beach gravel, and also contain beachrock, coral blocks, and boulders. Bivalve fossils are well preserved despite their occurrence in grain-supported conglomerates. The event deposit is divided into sub-layers bounded by internal scours that are wavy and intersect. Each sub-layer consists of a conglomerate grading into a sandstone layer. Imbrications just above the scours in sub-layers show seawards paleocurrents; however, imbrications just beneath the sandstone horizons in the same sub-layers feature landward paleocurrents. Respective sub-layers in the tsunami deposit were formed by substrate erosion due to backwash flow, gravel deposition, reworking by flood flow, and sand deposition during the stagnant water period. The overall upward-fining trend reflects decline of the tsunami event. Development of the gravelly deposit in the central part of the basin and lateral facies change may be attributed to hydrodynamic response of the tsunami pulse to local bathymetry and geography.

A method for mapping of grain fabric is proposed for analysis of the cryptic internal structure of massive sedimentary units. The method is applied to the analysis of an experimental debris-flow deposit, revealing a number of characteristic features of this type of mass sedimentation. The debris flow was simulated in the laboratory using a channel inclined 30° opening onto a 10° slope, and transverse thin sections were prepared from four longitudinal points in the depositional lobe. Back-scattered electron images of the sections obtained by scanning electron microscopy were processed and analyzed by mapping of grain fabric using an automated image-analysis procedure. Although the samples appear structureless by macroscopic observation, the grain-fabric map reveals a range of sedimentary features, including distinctive lineations from lower-upcurrent to upper-downcurrent in the most proximal section representing synsedimentary thrusts, a steepening-upward trend of grain imbrication angle in intermediate samples with very low-angle imbrication in the basal horizon, indicative of high-shear-rate flow, and complex imbrication features in the most distal samples. This analysis reveals that massive debris-flow deposits actually contain a range of distinctive structures which are characteristic of the mode of deposition and which are not identifiable by visible inspection or analysis of grain size or color. The proposed method is therefore of great utility for the investigation and characterization of massive deposits.

Holocene inner-shelf storm deposits preserved beneath the Sendai coastal plain facing the Pacific coast of north-eastern Japan were formed during a transgressive–regressive cycle. The evolution of the Holocene wave-dominated depositional system along the Sendai coast is reconstructed using 76 AMS (accelerator mass spectrometers) 14C ages and the origin of bed thickness variations in the inner-shelf storm deposits is explored. The Holocene succession is

This article presents graphical methods for illustrating the dependence of fabrics on aspect ratio of grains. The method is especially useful for analyzing the massive amounts of fabric data generated by computerized fitting of ellipses to images of sedimentary grains. The methods were applied to grains in a thin section of sand taken from an experimental microdelta. The section was made vertical and parallel to the water current. The results show strongly developed fabrics that are influenced heavily by grain shape and size. The graphical representations help us grasp the influence of aspect ratio in its entirety. These results are notably not obtainable through standard analysis using rose diagrams unless a series of the diagrams are made for the intervals of aspect ratios.

Hyperpycnal flow is one type of turbidity current, which is generated at a river mouth when the suspended sediment concentration of river discharge is so high that the density of the effluent exceeds the water of the receiving basin. Generations of this type of flow have become a common phenomenon recently. A lot of hyperpycnal flow deposits are expected to be preserved naturally in stratigraphic records. Hyperpycnal flow deposits (hyperpycnites) might become an innovative factor for turbidite paradigm considering the particularity of the flow, and bring facies analysis towards the next step. In this review paper, the criteria for identifying hyperpycnal flow deposits from strata are proposed by summarizing studies on hyperpycnal flow and its deposits. Typical characteristics of hyperpycnal flow include : (1) vertical succession composed of 2 parts, inversely-graded lower part and normally-graded upper part, (2) internal scour surface, (3) repetitive alternation of fine-grained and coarse-grained layers, or laminated and massive layers, (4) abrupt pinch-out of beds, and (5) inclusion of terrestrial materials such as leaves. Hyperpycnal flow gradually waxes and then wanes to terminate in response to flood conditions, resulting in (1). The internal scour surface (2) developed in accordance with the degree of waxing. Fluctuations of flow velocity and sediment concentration due to changes of river discharge and/or internal waves can occur during the flow event, and lead to repetitive alternation of (3). Because of the fresh interstitial water, the marine hyperpycnal flow might start to lift off after losing suspended sediments to the degree that the density of the flow is exceeded by ambient saline water. This results in (4) at the lifting point. Terrestrial materials referred to in (5) are the result of the fact that the hyperpycnal flow originated from terrestrial floods.A much wider variety of deposits, which form in one history of a turbidity current, than that expected from the Bouma sequence model, is shown. Exploration of the variety has just started.

We evaluated the major mineral composition of 100 sandstone samples from the Iriomote Formation of the Yaeyama Group located at the southwestern end of the Ryukyu Islands, eastern Asia. These sandstones are feldspathic arenites. Paleocurrents inferred from these sandstones are generally from north to south. Compared with other Tertiary sandstones of the Japanese Islands arc, the Iriomote sandstones are enriched in quartz and contain less lithic fragments. The roundness and sorting of the sand grains are high, suggesting long transport distances. We interpret these compositional and textural data as indicating that the provenance of the Iriomote Formation was the Eurasian Continent.

The radiocarbon dating of sediments using marine shells involves uncertainties due to the mixed ages of the shells mainly attributed to depositional processes also known as “sedimentological time-averaging”. This stratigraphic disorder can be removed by selecting the well-preserved indigenous shells based on ecological and taphonomic criteria. These criteria on sample selection are recommended for accurate estimation of the depositional age of geologic strata from 14 C dating of marine shells.

Temporal and spatial growth patterns of the Holocene wave-dominated inner shelf to shoreface sequence in the Sendai coastal plain, Pacific coast of northeastern Japan are detailed in relation to high resolution AMS 14C data. The sequence, deposited during about 10,000 years of rising to stable relative sea level, is a 22–26 m thick, tripartite inner-shelf to shoreface unit that overlies a transgressive ravinement surface. It is composed successively of laminated fine sand (Facies B), alternating mud and laminated very fine sand (Facies C), and burrowed/laminated fine to coarse sand (Facies D). Facies B consists of transgressive ravinement deposits formed during transgression in a water depth of 12–18 m at the drill sites. Facies C is inner-shelf storm deposits deposited during continued sea-level rise to highstand. Facies D is regressive highstand shoreface deposits. Isochrons show highstand deposits were formed by seaward progradation of the inner shelf to shoreface system. The stacking of deposits during rising sea level was mainly aggradational though there was probably accompanying progradation in the innermost shelf. The significant shelfal aggradation was probably because of an abundant mud supply from coastal rivers during gradual rise of relative sea level.

In this paper, we summarize our recent studies using the high-resolution 14C dating analysis on Holocene wave-dominated coastal sequences in the Kujukuri and Sendai coastal plains, eastern Japan. Results of high-resolution 14C dating, depositional facies, grain-size and depositional-depth analyses reconstruct temporal and spatial developments of the sequences in relation to the Holocene relative sea-level change and regional sediment supplies in detail. From the comparison between the sequences, we discuss (1) how the nature and developments of the sequences were affected by the differences in style and amount of sediment supply, (2) relationship between relative sea-level change and grain-size characteristics, and (3) controls on preservation potential of wave-dominated sequence.

Horizons at 500-yr intervals for the past 6000 years can be inferred in two cores drilled along a high-resolution seismic line in the prodelta of the Kiso River, Ise Bay, central Japan. These horizon ages clearly indicate that a given seismic reflection line in the profile is isochronous. Synchronism between ages and reflectors in the succession is especially distinct for the interval from 5000 to 2500 yr BP, despite the reflectors having inclined planes and having the same ages at different depths. These data indicate that the Kiso River delta prograded to form a downlap surface before the maximum flooding stage of the post-glacial relative sea-level rise at about 6000 yr BP. Delta progradation beginning late in the transgressive stage was caused by the flow of large amounts of sediment into this steep river that is part of the active Japanese volcanic arc.

Prograding beach–foreshore deposits with a basal ravinement bed are recognized from detailed AMS 14C dating and grain size analyses of Holocene deposits of the Kujukuri coastal plain facing the Pacific coast of Japan. The Holocene deposits are about 20 m thick and consist of an upward-shoaling beach–foreshore succession that has a basal unconformity overlain by a thin layer of relatively coarse sand. The unconformity is a downlap surface, which originated from a ravinement surface. The overall succession was formed by a prograding beach–foreshore system during the highstand in sea level of the last 6000 years. Poorly preserved fossil shells, enclosed in the thin layer of coarse sand at the base of the succession, date from the transgressive stage in sea level before 6000 cal yr BP. Grain size distribution patterns are polymodal in the basal sand layer and unimodal in the remainder of the succession. Modal classes of the basal part are fine (3.3–2.6φ), medium (2.3–1.8φ), and coarse (1.2–1.0φ). The fine mode and the unimodal grain sizes define an overall upward-coarsening trend, which correlates with the upward-shoaling succession. In contrast, the medium and coarse modes are restricted to the basal sand, and are interpreted as related to the ravinement process. The thin basal sand layer, an admixture of these modal classes, is interpreted as a type of ravinement deposit preserved during sea-level highstand. No transgressive deposit was preserved on the ravinement surface because sediment supply was so small that the sediment on this surface experienced thorough reworking by storm waves.

The Plio-Pleistocene Zaimokuzawa Formation comprises marine to fluvial deposits in one of the Neogene back-arc basin around the Japanese islands. Five sedimentary facies (facies A–E) and two facies associations (FA 1 and FA 2) were distinguished in the lower Zaimokuzawa Formation. Facies A is cross-stratified sandstone induced by tidal currents. Facies B is a couplet of wave-rippled fine sandstone and siltstone. Facies C is pebbly sandstone interpreted as the deposit from a concentrated density flow and a turbidity flow. Facies D is pebbly mudstone deposited from a mud flow or debris flow. Facies E is a slumped alternation of sandstone and mudstone. FA 1 and FA 2 are typical of the lower and upper parts of the lower Zaimokuzawa Formation, respectively. FA 1 is composed of alternations of facies A and C, including facies B, D and E. Facies C and D have lobe-like geometries in FA 1. FA 2 is composed of facies B and C, and lacks facies A. Facies B and C fill concave-upward slump scars in FA 2. The constituent facies, shapes of the deposits and paleocurrent directions suggest that the lower Zaimokuzawa Formation was formed by progradation of a shallow-marine fan delta slope influenced by tidal currents, and that its lower (FA 1) and upper part (FA 2) corresponds to middle, lower and upper slope deposits, respectively. Wave and tidal currents reworked and redeposited the sediments supplied by gravity-driven processes on the delta slope to form the multiple facies associations. Grain sizes of facies A and B, which are mainly composed of sand-sized particles in spite of the presence of intervening pebbly deposits of sediment density flows, imply competence of the reworking processes. The distribution of cross-stratified sandstone is restricted to the western area, indicating a local inequality in the influence of tidal currents.

Grain fabric of deposits accumulated from a high-density surge-type gravity (turbidity) current in an experimental flume was measured. Vertical sequential change (0.2-mm interval) in imbrication shows that a bed can be divided into lower, middle, upper and uppermost parts. The lower part is characterized by both up-current and down-current imbrication with a wide range of angles. Dominant up-current imbrication and rare down-current imbrication characterize the middle part. The imbrication angle of this part tends to be smaller than in the lower part. The upper part is represented by intervals with up-current imbrication. Nearly flat imbrication is dominant in the uppermost intervals. Statistically significant preferred orientation was observed from the lower, middle and upper parts, and it deviates up to 13° in both clockwise and anticlockwise directions from the current direction. The lower and middle parts of the bed may correspond to the Bouma A-division judging from the wider range of imbrication angles and the presence of down-current imbrication which have been reported from natural turbidite beds. The upper part, which is characterized by up-current imbrication, is interpreted to be the Bouma B-division. The uppermost interval may coincide with the D-division. The episodically appearing down-current imbrication in the lower and middle parts can probably be attributed to oscillation of an interface between a denser basal layer and superjacent low-density layer of the turbidity current. Measurement of grain orientations in this and previous studies implies that at least 30° of deviation from the flow axis should be considered for paleoflow analyses based on grain fabrics.

Relative sea-level changes and variations in shoreline progradation rate over the past 6000 years are elucidated from the elevations of the beach deposits, their 14C ages, and the geographic position of each locality on the Kujukuri strand plain, Pacific coast of the Boso Peninsula, central Japan. These past sea-levels were +4 to +6 m above the modern sea-level (high-stand stage) at 6000 to 5300 calendar years B.P., +3 to + 4.5 m (stable stage) at 5000 to 3500 years B.P., declined from +1.2-+ 3.4 m to + 0.5-+ 2.1 m (falling stage) at 3300 to 2250 years B.P., and were-1.5--1 m to + 1 m (stable stage) from 1650 to 250 years B.P. ; sea level is 0 m at present. The detailed changes revealed by this new method strongly imply the existence of several rapid uplifts (0.4 to 1.2 m per event) at 5100 to 5500 years B.P., 3400 years B.P., 2400 years B.P., 1650 years B.P., and 0-250 years B.P. The uplifts were co-seismic, because the speed of occurrence seems to have been high, and the events are generally associated with so-called “tsunami deposits.” Co-seismic uplift in this region has not been reported previously from historical records or geological evidence.

浜堤平野を構成する地層の発達過程を捉える目的で,千葉県九十九里浜平野において,海岸線と直交する方向での地質断面図が描けるように,深さ数m～24mのボーリングを19本行った.得られたコア試料に対する堆積相解析の結果,この地域の表層地下に分布する地層は,鮮新-更新統を基盤とし,谷埋めエスチャリー泥層,下部外浜基底の貝殻密集砂層,下部外浜細粒砂層,上部外浜細粒～中粒砂層,海浜砂層,潟・氾濫原泥層からなることがわかった.コアから得られた73試料の14C年代値は,これらの地層が完新統からなることを示している.また,高密度の年代値から作成した“堆積曲線”によって堆積年代が連続的に求まり,地質断面図上に500年ごとの等時間線を描くことができた.その結果,約5,700年前の縄文海進最高海面期以後に,海浜-外浜システムが基盤にダウンラップしながら年1.4～1.6mの速さで前進してこの完新統が形成されたこと,海岸線の前進速度が大きいと外浜勾配が急になること,基底のダウンラップ型貝殻密集砂層には海進期のラビーンメント堆積物が混入していることなどがわかった.

Fossil ostracodes were studied in two bore hole cores (OB-1 and OB-2) from Osaka Bay off Kobe in western Japan to examine vertical changes in ostracode faunas. Fifty-two species were identified in 56 sediment samples. Based on numerical analyses of these ostracodes, the samples examined are grouped into five biofacies: BM (brackish mud), SM (shallow mud), SS (shallow sandy mud), DS (deep sandy mud) and DM (deep mud). These biofacies are distributed in upward sequence in both cores. From ca. 11, 000-10, 000cal yrs BP, the predominant species living were those dominant in enclosed inner brackish bays at water depths of 2-7m. Inner to middle, enclosed shallow muddy bay species increased at the study sites after the opening of the Akashi Strait (ca. 9, 700cal yrs BP). Ostracode assemblages changed ca. 8, 000cal yrs BP, when Osaka and Harima-nada Bays were connected with Mizushima-nada Bay. Tidal currents from the Akashi Strait have since been strong. As a result, specimens of intertidal to shoreface species were transported with sandy sediments into deeper Osaka Bay off Kobe. Thus, their individual numbers increased rapidly. A peak in the number of intertidal to shoreface species and deep mud-dwellers occurred around 5, 500cal yrs BP, suggesting that tidal currents from the Akashi Strait and paleo-water depth reached their maxima at this time (ca. 33m). These species then decreased with time, with the start of a regression. After ca. 2, 000-1, 600cal yrs BP, mud-dwellers inhabiting enclosed, organic matter-rich bays were abundant. Their numbers increased further as coarse materials from the Akashi Strait decreased and the Yodo River began to bring flood clays into the study sites. Both “form A” and “form M” of Bicornucythere bisanensis are present in both core samples. “Form M” first invaded the study sites through the Kitan Strait to the south. “Form A” came to Osaka Bay off Kobe, ca. 8, 350-8, 000cal yrs BP, and has since become the dominant of the two forms.

Tsunami have probably deposited sand sheets that are intercalated in Holocene bay-floor mud exposed on the Boso and Miura Peninsulas, facing the convergent boundary of the Pacific, Philippine Sea, and Eurasian Plates. We have identified seven sand sheets at four drowned valleys, and correlated these by 137 radiocarbon dates of shells and wood. The sheets consist of poorly sorted muddy sand and well-sorted sand layers in fining upward sequences, containing abundant transported shells, rip-up clasts and wood fragments. The sheets erosionally overlie bioturbated bay-floor mud that contains molluscan shells in life position. Most of the sheets are less than 20 cm thick and rarely more than 50 cm thick. Some molluscan shells are older in these layers than in underlying mud. Both landward and seaward paleocurrents are shown in a few cases by imbrication of shells and by low-angle wedge shaped lamination. At least five of the sand sheets contain molluscan fossils derived from rocky coasts or shore platforms, although they intercalated in mud deposited within bays, at depths of 10–15 m. Two other sand sheets are dominated by open seashore ostracode assemblages, although they were deposited in the brackish inner bay and muddy central bay. Five of the layers may correlate with emergences recorded by nearby Holocene marine terraces. These correlations suggest that great earthquakes triggered the inferred tsunami. The tsunami occurred at intervals of 300–2000 years beginning about 10,000 years ago.

Two ‘anomalous’ deposits with well-preserved sedimentary structures were discovered in a drowned valley and in relict barrier sand body deposits in the Furuya Mud and Kyomatsubara Sand of the Upper Pleistocene incised valley fills. The sedimentary features of both deposits are interpreted as ‘earthquake-induced tsunami deposits’, which were deposited in incised valley fills. The tsunami deposit in the drowned valley (Furuya Mud) is 150 cm thick and shows a contorted structured bed in the lower part and cross-laminated sand in the upper part. The tsunami deposit in barrier sand body (Kyomatsubara Sand) is 270 cm thick and shows structureless sand with mud clasts in the lower part, cross-laminated sand with mud-drapes in the middle part and mud with small wave-ripples in the upper part. Taken together, these sedimentary features suggest a model for the composite succession, which is comprised of three units: a lower earthquake-shock-influenced (anomalous) convoluted part, a middle part dominated by tsunami-generated tractional currents, and an upper suspended-sediment part. The lower part is interpreted as earthquake event deposits (seismites) and the middle and upper parts as tsunami current deposits (tsunamiites).

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well-sorted, pebble- to cobble-size gravel beds with minor sand beds. The gravel includes large-scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert-type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low-energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large-scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea-level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross-sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large-scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf-margin systems tracts.

大阪湾とその周辺には,海成や陸成堆積物からなる完新統が発達する.この地域の完新統の特徴は,海成粘土層に粗粒な部分(粗粒シルト～中粒砂)が間に挾まっていることである.それは後氷期の海進で,海面が高くなった時期に潮流や沿岸流による運搬作用が強くなって形成されたものである.

神戸沖の大阪湾底で掘られたコアの完新統の部分について, 堆積相解析を行い, 高密度の14C年代値と相対的海水準変動から, 次のような完新世の大阪湾の海況変動を復元した.11000年前に海面高度は約-50 mにあった.海進に伴い後退する三角州はエスチュアリーとなった.海面高度が約-30 mに上昇した9700年前に, 明石海峡での潮流が発生した.海面高度が約-12 mになると, 淀川河口が河内湾に退き, 備讃瀬戸が繋がり, 現在のような瀬戸内海が成立した.この時から約3000年前まで, 大阪湾全体が強い潮流の影響下にあった.5300年前～5000年前が大阪湾では最も海面が高い時期であった.1700年前頃に, 河内湾が埋め立てられ, 淀川三角州が大阪湾にまで前進し, 1000年前には神戸沖にまでその影響がおよぶようになった.

The Upper Pleistocene (the last interglacial period) of the Makinohara Upland, Shizuoka, Japan, is divided into four formations, and constructed from a drowned valley, estuary, beach-shoreface and an alluvial system. These comprise one depositional sequence. Grain-size characteristics of each facies are clarified in this study. The assemblages of each facies can be classified into two types (closely and scattered distributed groups) based on 3-D diagrams (arithmetic mean versus standard deviation versus mud content). Fluvial deposits of LST have poorly sorted fine sand, whereas fluvial deposits of HST have well sorted coarse sand. In the beach-shoreface system, grain-size gradually fines and mud content increases, from the beach to the lower shoreface. In the drowned valley- estuary system, estuary mouth bars have higher arithmetic mean values than the bay-head delta. Both environments have moderate standard deviation and mud content values. Central bay floor (central basin) and drowned valley have high arithmetic mean and mud content values, and low standard deviation values. Seaward barrier have intermediate arithmetic mean values, low standard deviation and mud content values. These facts indicate that 1) terrigenous coarse sediments were trapped in the bay-head delta and were not transported to the central basin, 2) seaward coarse sediments were not transported into the central basin (tidal inlet is an exception). The horizon of the MFS have high arithmetic mean and mud content values compared with other similar facies. Grain-size characteristics differ between the tsunami deposits in the relict barrier and drowned valley. The tsunami deposits in drowned valley include numerous suspended (from landward and/or seaward) load by oscillationary currents along the paleo-valleys, whereas the tsunami deposits in relict barriers consist of offshore sands and erosion of the barrier itself.

Studies of sequence stratigraphy in Japan began in the early 1990s with the Quaternary successions, especially the Plio-Pleistocene of the southern Kanto area. Comprehensive geologic researches in this area provided a framework for the application of the sequence-stratigraphic principles to the onshore outcrops. The outcome of such studies represents one type of variation in the standard sequence-stratigraphic model. In particular, such studies have clarified some kinds of variations in the sequence-stratigraphic tenet. That is (1) diachroneity of systems tracts, (2) spatial and temporal relationship between high-frequency depositional sequences and parasequences, (3) relative timing of turbidite deposition in a submarine fan environment, and (4) contemporaneity of lowstand depositional systems. Furthermore, sequence-stratigraphic classification of the Quaternary successions, together with tentative correlation of sequence boundaries with the oxygen isotopic sea-level index, permits detailed chronostratigraphic correlation of such successions developed in the Osaka and Boso areas. Recently, detailed measurements of depositional rates in a Holocene sequence has been available. Such measurements identified several different patterns of depositional rates that characterize each of a wide range of shallow marine and coastal depositional systems.

The Plio–Pleistocene Kakegawa Group, central Japan, consists of a third-order depositional sequence (2.6–1.0 Ma). The northwestern part of the Kakegawa sequence consists of up to 500 m of alluvial, shoreface, shelf, slope and submarine-channel facies. It contains at least sixteen upward-shallowing cycles (parasequences), the deposition of which was affected by high-frequency eustatic sea-level cycles. The lower part of the sequence is characterized by a retrogradational parasequence set, which formed a transgressive systems tract (2.2–1.75 Ma) followed by a progradational parasequence set comprising a highstand systems tract (1.75–1.4 Ma). Subsidence analysis and evaluation of changes in the shelf sedimentation rate estimated from cross-sections, suggest that formation of the third-order sequence was controlled by tectonic subsidence and variation in the sedimentation rate. Rapid subsidence and a high rate of sedimentation during 2.2–2.0 Ma resulted in deposition of the lower part of the transgressive systems tract, characterized by thick backstepping successions. The rate of subsidence decreased in the period 2.0–1.75 Ma. The sedimentation rate also decreased due to a high rate of sediment bypassing. However, subsidence was still the dominant factor, leading to the formation of thin backstepping successions. The 1.75–1.4 Ma progradational succession resulted from a combination of a low rate of subsidence and moderate sedimentation. The progradational units become thicker basinward owing to faster subsidence in the basin center. The maximum flooding surface was formed around 1.75 Ma even though subsidence was slow at this time.

The Plio-Pleistocene Kakegawa Group crops out in western Shizuoka, Japan. It consists of submarine fan and slope to alluvial deposits and forms a third-order sequence. The slope deposits of the upper Kakegawa Group (upper Horinouchi and Hijikata formations) are characterized mainly by alternations of sand and silt (turbidites). Individual sand layers commonly show either small lobe or channel features. In the upper Horinouchi (TST) and middle Hijikata formations (HST), sand turbidite packets (2–15 m) consist of small channel and lobe complexes. The slope sand turbidite packets are different between TST and HST. The differences result from the dissimilar net amplitude of sea-level excursions when fourth- or fifth-order sea-level changes superimposed on a third-order sea-level change with different trend. Other important variables are rate of sediment supply and width of shelf.

相模湾周辺では,完新世を通じて巨大地震が繰り返し発生したことが,完新世の海成段丘や歴史地震の研究から知られている.しかし,完新世の地震を示す津波堆積物などの地質学的な証拠は,ほとんど知られていない.完新世の3回の地震隆起に対応する津波堆積物を,房総半島南部の館山市周辺に分布する内湾堆積物から初めて見いだした.これらの津波堆積物は,館山市周辺で広域に追跡できる.津波堆積物は,基底が侵食面を示し上方へ細粒化する砂層や砂礫層からなり,貝化石片や木片を多量に含む.貝化石は,内湾泥底と沿岸の砂底や礫底に住む種が混合しており,海底の侵食と再堆積が生じたことを示す.また,陸側と海側への両方の古流向が堆積構造から推定される.3枚の津波堆積物は,それぞれ約6,300～6,000yrs BP,4,800～4,700yrs BP,4,500～4,400yrs BPに堆積したことが貝化石の14C年代値から明らかになった.最下位の津波堆積物は,沼I段丘,野比I段丘の離水と年代が一致する.また,中位と上位の津波堆積物は,それぞれ野比II段丘,沼II段丘の離水と年代が一致する.調査地域では,上述の津波堆積物と類似した堆積相を示す砂層や砂礫層が,約7,400～3,600yrs BPの間に100～200年に1枚の割で堆積している.これらの砂層や砂礫層の一部は,沼段丘上に分布する離水波食棚群(茅根・吉川,1986)に対応する津波堆積物の可能性がある.