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Sea-level rise and sediment supply have influenced coastal morphology and sedimentation on Bangladesh’s southwestern Ganges‐Brahmaputra‐Meghna (GBM) delta coast. Satellite images and geological core from the Haringhata coastal region were analyzed to explain the morphological changes and to understand the influences on deposits. The results derived from satellite images indicate that the southern coastline experienced a retreat that ranges between 2.3 and 2.9 km. In contrast, the eastern and western coastline advanced. The erosion and accretion ratio was 0.29 from 1977 to 1989, while the ratio was higher 2.90–4.77 from 1989 to 2020. Two sedimentary facies were identified using 130 cm thick successions. A parallel to wavy laminated bluish gray mud facies of deeper part was deposited in a marine-influenced environment. A planar to hummocky cross stratified, gray to grayish white silty sand facies of storm overwash deposits overlies the mud facies with sharp contacts. Unimodal to bimodal grain distributions of sandy sediments suggest two sources: sand derived from the beach and mud carried by adjacent tidal rivers and resuspended offshore sediment. Coastline dynamics and sedimentation of the area were influenced by inequality of accommodation and sediment supply ratio in the river mouth. This occurs due to sea-level rise and deficit in upstream water and sediment discharge. Morphological change along the southwestern GBM delta coast was not only caused by wave energy, but also by rising sea levels which shifted sediment accommodation space landward.

This study illustrates the influences of sea-level on the depositional process during the last 1000 years of the southwestern delta, Bangladesh. Sediments of eight litho-sections from landward in upper delta plain to seaward in lower delta plain along the Rupsa-Pasur river were studied. Sedimentary facies, total organic carbon, total nitrogen, δ13C value, diatom assemblages, and radiocarbon dating of deposits were carried out to determine the paleoenvironments that were influenced by the relative sea-level (RSL) change over time. During the 850–1300 AD, RSL was reached up to +80 cm higher than the present level where tidal-influenced bioturbated light yellow to gray mud deposited in the upper delta plain area. RSL was dropped up to −110 cm during 1300–1850 AD, organic-rich bluish-gray mud, mangrove peat, and terrestrial influenced yellowish-gray mud were deposited successively in the lower delta plain area, and the terrace was formed at landward due to the lowering of the base level. RSL started to rise after the period 1850 AD where tidal-influenced sediments gradually increased and deposited in the upper part at seaward and terrestrial flood sediment deposited over the erosional surface at the landward part. The estimated average sedimentation rate (1.96–2.89 mm/year) is not enough to offset the effect of subsidence and present sea-level rise over the study area. The rising trend of the sea creates inundation in the lower delta plain area, also hinders upstream water flow. For that, terrestrial flood sediments settle over the erosional surface in landward, and tidal-influenced sediment gradually onlap upon it from seaward.

This study examines the sedimentary characteristics, provenance, and depositional processes of the storm deposits from 2007 Cyclone Sidr that have been identified on the southern coast of Bangladesh. Three sedimentary units up to 70 cm thick have been identified in fifteen geological core collected at a distance of 135–277 m from the coast. Laboratory analyses of grain size, TOC, TN, δ13C, and diatom assemblages were carried out to characterize the deposits. Massive to parallel laminated bluish gray mud underlies the storm overwash deposits. White to light gray, massive to parallel laminated, normal graded 1–17 cm thick sand overlies the mud unit with sharp to erosional contact. The mean grain size and thickness of this sand unit decrease landward with increasing sorting value, where grain size distribution of sand is comparable with modern beach sand. The unimodal sand that dominated the base of the storm deposits grades into bimodal olive-gray sandy silt in the upper part of the deposits. The grain size implies that the sand carried from the beaches and mud likely sourced from the suspended, nearshore sediment of the bay and adjacent rivers. TOC/TN, δ13C values and the presence of brackish mudflat origin diatoms with silty sand laminae in bluish gray mud indicate a tide-influenced environment. Post-storm sand and sandy silt sediments unit contain freshwater and marine-brackish diatoms that have likely been deposited from flooding caused by storm surge water from the bay and overbank flooded river water due to subsequent heavy rainfall. These sediments were sorted according to storm waves and settled over the study area. This study indicates that the low-lying delta coast gets the influences of high water levels both from the bay and the river during a storm. The modern storm deposit will help to characterize the paleo/prehistoric tropical cyclone in geological time for future studies in this area.

This paper discusses relationship among depositional environments, origin of the sedimentary organic matter and diatom fossil assemblage, combined with sequence stratigraphy in an incised-valley system. Interpretation of the environments and their changes was based on sedimentary facies, total sulfur (TS) content, and diatom assemblages. The quantity and composition of organic matter were estimated for the environmental interpretation on the basis of the total organic carbon (TOC) content, reflected-light fluorescent microscopy, and stable carbon isotope ratios of the organic matter (δ13Corg). The Miocene to Pliocene Ishibane, Tatsunokuchi and Motohata formations in northeast Japan consist of estuarine and fluvial deposits that were formed during a rise and fall of sea level. The proportions of vitrinite and cutinite that are coarse-grained and terrestrial in origin are relatively high in fluvial deposits of the lowstand systems tract. The proportion of marine alginite, TOC content, and δ13Corg values increase upward in estuary deposits of the transgressive systems tract. This implies the increase in the influence of the sea and the estuarine bottom conditions becoming anoxic. The proportions of vitrinite and cutinite increase upward in the highstand systems tract, whereas the δ13Corg values and TOC content decrease upward. These reflect the strong influence of river discharge. Variations of diatom fossil assemblage of fresh water, brackish water, intertidal, coastal marine and open marine species indicate the same repetitions of transgression and regression in the Tatsunokuchi Formation. Therefore, the transgressive systems tract in the Tatsunokuchi Formation includes four cycles of transgression and regression, while highstand systems tract shows one cycle of transgression and regression. / 岩手県南部の北上市に分布する中新統～鮮新統の石羽根，竜の口，本畑層は，エスチュアリーと河川環境での堆積を示している．また，1回の海水準上昇と引き続く低下で堆積した地層で，低海水準期堆積体，海進期堆積体，高海水準期堆積体に区分できる．その周期は約100万年と推定できる．河川成堆積物からなる低海水準期堆積体では陸源有機物片を多く含み，有機物は低い安定炭素同位体比（δ13Corg）を示す．引き続くエスチュアリー環境を示す海進期堆積体では，上方に向かって，海棲有機物が増え，δ13Corgの値が大きくなり，さらに全有機炭素量も増加する傾向にある．海進初期ではこれらは，小さな増減を4回繰り返しているが，やがて最大海進面に向かって安定して増加するようになる．最大海進面より上位の高海水準期堆積体では陸源有機物量が増大し，δ13Corgと全有機炭素量は減少し，陸域からの物質供給が増加したことが示される．珪藻化石の淡水種，汽水種，潮間帯種，沿岸性種，外洋種の比率変化も同一の海進海退の繰り返しを竜の口層中に記録している．したがって，竜の口層の海進期堆積体は短周期の4回の海進海退と引き続く海進から構成され，一方，高海水準期堆積体は1回の海退を示す．これらは数10万年周期の海水準変動に対応するかもしれない．, Article, Journal of the Sedimentological Society of Japan. 70(2):63-69 (2011)

River floods influence sedimentary environments and ecosystems from the terrestrial to the deep-marine. This study documents the occurrence conditions of hyperpycnal flows generated by river floods and related organic-matter sedimentation for Holocene sediments of the Niigata Plain, Central Japan, based on detailed sedimentary facies, total sulfur and total organic carbon content, diatom assemblages and organic-matter composition. Holocene sediments of the Niigata Plain consist of sand, mud and gravel that were deposited in estuarine and fluvial systems during a sea-level rise (15 000–6800 years BP) and stillstand (after 6800 years BP) following the Last Glacial Maximum. Hyperpycnites are present in the upper part of the estuarine lagoon sediments. The depositional age is considered to be about 5000 years BP. The hyperpycnites comprise two successions of a top fining-up unit and a basal coarsening-up unit, and include abundant terrigenous organic matter and freshwater diatoms. A large volume of freshwater is inferred to have flowed into the lagoon during deposition of the upper part of the lagoon sediments. In consequence, hyperpycnal flows may have readily formed in the lagoon, because the halocline was weak. The hyperpycnal flows also produced a layer of concentrated terrigenous organic matter in the uppermost part of the hyperpycnites. The abundant organic matter on the estuarine floor is inferred to have produced anoxic bottom conditions owing to oxidative decomposition by benthic bacteria.

The Dainenji Formation is deposited on shelf environment and consists mainly of sandy siltstone and sandstone. It includes a number of volcanic ash layers, which serve as good key beds useful for precise correlation. Mappable units of submarine canyon-fill sediment are recognized in this area. A valley has a maximum depth of 100 m and is considered to be formed by submarine landslide, which took place after 2.7 Ma. Submarine canyon-fill sediments are composed of siltstone interbedded with thin sandstone layers, thick sandstone beds and slump deposits. Formation and filling of the submarine canyon might be controlled by climate cooling of latest Pliocene.

産炭地であった夕張は様々な地層と岩石が分布し地質学的魅力にあふれ，さらに日本における地質学の発展に大きな貢献をしてきた地域である．なかでも20世紀前半の夕張炭田地質構造の解明とクリッペ・ナッペの発見は，20世紀後半に日高造山論として結実し発展した．さらに，現在では地殻深部の探索，活構造や地形形成との関連，白亜紀以降の地球環境変遷と地層・資源形成の関連などが追及されている．この見学コースでは，白亜系の最上部から夾炭層である古第三系をへて新第三系までの堆積岩を堆積学とシーケンス層序学の視点から観察する．

Pliocene Dainenji Formation is distributed in the Pacific Ocean side of the Fukushima Prefecture. The depositional age is from 4.18 to 2.6 Ma. We carried out facies analysis and geochemical analysis, such as total organic carbon (TOC), total sulfur (TS), and stable carbon isotope ratio of organic matter (δ13Corg). Our results are as follows : (1) Abundant land-derived organic matter was transported to the shelf and was preserved in the sediment in warm period (3.75-3.5 Ma), (2) Productivity of marine organism increased in the early cooling period (3.5-3.35 Ma), (3) The considerable variations in TS, TOC and δ13Corg are seen in cooling stage after 3 Ma, which succeeded to the Quaternary ice age. Theses considerable variation are shown in δ18O curve of the same age.

Sequence stratigraphic and organic matter analyses were carried out on the Miocene to Pliocene Ishibane, Tatsunokuchi and Motohata formations that occur in Kitakami City, Iwate, Northeast Japan. These formations consist of nine facies and a depositional sequence that formed during a rise and fall of sea level. The proportions of vitrinite and cutinite are high in fluvial deposits of the lowstand systems tract. The total organic carbon content and the proportion of NFA (non-fluorescent amorphous kerogen) increase with transgression, because the sedimentary environment changed into an estuary with bottom anoxia. The proportions of vitrinite and cutinite increase in the highstand systems tract. These phenomena indicate the strong influence of river discharge.

Sedimentary organic matter is a potential indicator of paleoenvironments. In this study we examine the relationship among composition of kerogen (insoluble organic matter), sedimentary environments, and sea-level changes in Miocene to Pleistocene sediments of the Niigata and Akita backarc basins, central Japan. Our primary analytical tool is a ternary diagram with apexes consisting of woody-coaly organic matter, herbaceous with pollen and spores, and amorphous organic matter (AOM) with alginite (organic-walled marine microfossils). The composition of kerogen plots into distinct regions on the ternary diagram: fluvial to estuarine, prodeltaic, shelf, slope and basin-floor submarine fans, and distal basin-floor sediments. The fluvial and estuarine sediments have high proportions of woody-coaly and herbaceous organic matter with pollen and spores, and a lesser proportion of AOM + alginite, because pollen and spores were mainly deposited in estuaries. Because abundant coarse-grained, terrigenous organic matter was supplied by delta distributaries, the prodeltaic sediments have high proportions of woody-coaly organic matter. The composition of kerogen in the shelf sediments is similar to the kerogen in slope and basin-floor submarine-fan sediments, as plotted on the ternary diagram. Both kinds of kerogen accumulations have high proportions of woody-coaly organic matter and AOM with alginite and lesser proportions of herbaceous organic matter with pollen and spores. This relationship suggests that turbidity currents supplied the terrigenous sediments. The sediments on the distal basin-floor contain high proportions of AOM. Each pattern in the ternary diagram reflects a difference in hydrodynamic behavior, distance from land, and the supply of terrigenous organic matter. The sub-ternary diagram, which has apexes of WFA (weakly fluorescent amorphous organic matter), NFA (nonfluorescent amorphous organic matter) + FA (fluorescent amorphous organic matter), and alginite, further suggests the origin of AOM. The NFA in shelf sediments and WFA in distal basin-floor sediments are inferred to consist of terrestrial higher plant and marine organic matter, respectively. A 13C value of kerogen rich in NFA (-24.6 and -27.3‰) suggests land plants, whereas kerogen rich in WFA (-20.0 to -23.6‰) suggests marine plankton. These inferences agree with those derived from the sub-ternary diagram. Furthermore, compositional changes of the kerogen in turbidites reflect relative sea-level changes, as seen by shifts in compositions on the ternary diagram. The use of ternary diagrams like those used here is recommended for future studies of kerogen, depositional environment, and sea-level relationship.

A revision of the Neogene lithostratigraphy of the Akan area of eastern Hokkaido is based on a reexamination of lithofacies and diatom biostratigraphy. The Neogene strata are divided into the Chichappugawa, Tonokita, Chichappu, and Kotan formations, in ascending order. The lower Miocene Chichappugawa Formation is newly defined and named in this study. The age of the Tonokita Formation is early middle Miocene, the Chichappu Formation is middle to late Miocene and the Kotan Formation is latest Miocene to Pliocene. The relationship between the Chichappugawa and Tonokita formations is inferred to be an unconformity, which suggests tectonic movement between the early and middle Miocene. The discovery of early Miocene strata in the Akan area indicates that the early Miocene sedimentary basin in eastern Hokkaido extended toward the northeast and east, and that it was connected to the Sea of Okhotsk.

Twelve deep-sea turbidites were identified in the Holocene sequence of a core collected from the western foot of the Kaiyo Seamount, located near the fault area of the 1940 Shakotan-oki earthquake, northern Okushiri Ridge, eastern margin of the northern Japan Sea. On the basis of the geological, tectonic, and topographical settings of the site, the turbidites are interpreted as of earthquake origin (seismoturbidite). Age determinations on radiolarian remains in the sediments suggested that the uppermost sediments, younger than 3 ky BP, were lost during the coring. Four clear faunal boundaries, at 4, 6, 8.5, and 10.5 ka, were recognized. The turbidite recurrence interval in the deep-sea basin indicated an average recurrence interval for large earthquakes of 500–1,000 years around the Kaiyo Seamount during the early- to middle Holocene.

We present a model for the evolution of the shelf surface in response to marine sedimentary processes. Following morphodynamical theory, the model presupposes a characteristic configuration of the shelf surface, that in profile is a concave-up exponential curve, whose steeper inner limb is the shoreface. The profile is seen as an equilibrium response to the variables of sedimentation. The profile translates landward or seaward as sea level rises or falls, but will do so in an state of dynamic equilibrium with the shape of the profile varying according to changes in: (1) the rate of sea level change, (2) the time-averaged wave and bottom current conditions, (3) the average allochthonous sediment supply rate, and (4) the sediment grain size distribution. The governing equations for the model include an equation for time-averaged cross-shore (diabathic) sediment flux and the sediment continuity equation. p ]Simulations of continental margin profiles show that profile adjustments affect mainly the coefficient of curvature of the profile. An increase in the rate of sea level rise straightens the profile; it decreases the slope of the shoreface, but increases the shelf slope. An increase in sediment input increases profile curvature; the shoreface steepens while the shelf floor flattens. An intensification of hydraulic climate straightens the profile in a manner similar to an increase in the sea level rise rate, while an increase in grain size increases profile curvature as does an increase in sediment input.

Several third-order slope-shelf-nearshore depositional sequences are found in the Middle to Late Miocene Aoki and Ogawa Formations in the southern part of the Northern Fossa Magna region, central Japan. Each depositional sequence consists of a transgressive systems tract, varying in thickness from several tens of centimetres to about 10 m, a falling stage systems tract several tens of metres in thickness and a highstand systems tract, 20–80 m thick. The highstand systems tract and falling stage systems tract are bounded by the regressive surface of marine erosion. Each depositional sequence mainly consists of coarse-grained sediments. The materials of the transgressive conglomerate beds were derived from the gravelly foreshore and upper shoreface by transgressive shoreface erosion. The thin and conglomeratic transgressive systems tract indicates that these slope-shelf-nearshore sequences were deposited on the steep slope area under a rapid transgression. Therefore, these sequences were formed in response to excess sediment supply and a rapid rise in relative sea-level at an active margin of the backarc side of the arc-trench system.

Sequence stratigraphy has been rapidly spreading in the world and causes various sort of debates because sequence stratigraphic interpretations related to mechanisms of deposition make possible three-dimensional estimation of facies distribution and worldwide correlation of strata. The “Exxon Model”, as a basin fill model, has been paid much attention because it displays the significance in the basic concept of sequence stratigraphy and possibilities of applications. The “Exxon Model” which has been constructed as a depositional model for passive margin basins represented by gentle subsidence and relatively stable deposition, however, might not always be applicable to some sedimentary basins without careful considerations for their tectonic and depositional settings.In the Niigata sedimentary basin, as a representative back-arc basin located on central Japan, hydrocarbon exploration for several decades has resulted in a large accumulation of subsurface geologic information. Three-dimensional studies for sequence stratigraphic framework within a back-arc tectonic setting, therefore, are available in this basin.As a result of this study, 12 third and fourth order depositional sequences, which are interpreted to have been formed by relative change of sea level, are now recognized. These depositional sequences are superimposed on a part of a second order depositional sequence considered as a tectonically controlled sequence. The features of fifth or sixth order depositional sequences are superimposed locally in the deposits of the Niigata sedimentary basin. The depositional history of the Niigata sedimentary basin can be divided into following 5 stages, based on the disclosed sequence stratigraphic framework of this basin.i) stage of sedimentary basin formationii) stage of turbidite deposition at basin flooriii) stage of slope progradationiv) stage of shelf extensionv) stage of delta progradation

[Extract - Foreword] The Integrated Ocean Drilling Program (IODP) is now in the latter half of its decadal program (2003–2013). As envisioned in the Initial Science Plan (ISP), IODP expeditions take advantage of three scientific ocean drilling platforms that enable us to cover unprecedented areas of wide oceans, from ice-covered shallow water to full ocean depths. Drilling miles of depth below seafloor, now part of IODP capabilities, is the major advance from the program predecessors, the Deep Sea Drilling Project and the Ocean Drilling Program. The living Earth is a dynamic system that is continuously evolving. IODP seeks to understand this complex and unique system through scientific ocean drilling, sampling, and experimenting in deep holes, along with advancement of related scientific disciplines. IODP is an international collaboration among scientists and nations with keen aspirations to attain the scientific goals of the ISP. IODP currently includes participating members from 24 nations. The Proceedings present the scientific and engineering results of IODP drilling projects, each designed to better understand the past, present, and future of the Earth system. IODP expeditions begin with scientists who submit research drilling proposals to test new and innovative ideas, then the proposals progress to international scientific advisors (Science Advisory Structure) who nurture, evaluate, rank, and prioritize proposals. Scientists also schedule the science operations, select science party members from scores of international scientists qualified to participate, plan platform operations, ready the drillship, and choose borehole locations. The science party, collectively and individually, conducts science on board and on shore. The co-chief scientists on each expedition are responsible for synthesizing the scientific results as hallmark of expedition. Ocean-drilling achievements help us to understand and interpret phenomena in various parts of the Earth system. Achievements in the two legacy drilling programs have validated the scientific concepts behind plate tectonics, contributed to the understanding of ocean circulation changes, and extended our knowledge of long- and short-term climate change. IODP is truly an expansion and extension of the scientific research conducted by the legacy programs, engaging in cutting-edge research concerning topics of global importance. IODP drilling platform operations are conducted by three Implementing Organizations (IOs). Riserless platform operations are conducted by the U.S. Implementing Organization (USIO), comprising the Consortium for Ocean Leadership, Inc., Texas A&M University through the Texas A&M Research Foundation, and Lamont-Doherty Earth Observatory of Columbia University. Riser platform operations are conducted by the Japan Agency for Marine-Earth Science and Technology through Japan's Center for Deep Earth Exploration in cooperation with the Center for Advanced Marine Core Research at Kochi University. Mission-specific platform operations are conducted by the European Consortium for Ocean Research Drilling (ECORD) Science Operator (ESO), comprising the British Geological Survey, Bremen University, and the European Petrophysics Consortium. The European IO currently represents the ocean-drilling efforts of 16 nations in Europe, plus Canada. The discoveries presented in this volume build upon layers of knowledge and science developed over roughly the last fifty years. Expedition Proceedings are published by IODP Management International for IODP under the sponsorship of the U.S. National Science Foundation (NSF), Japan's Ministry of Education, Culture, Sports, Science and Technology, and other IODP members. The material is based upon research supported under Contract OCE-0432224 from NSF.

Integrated Ocean Drilling Program (IODP) Expedition 317 was devoted to understanding the relative importance of global sea level (eustasy) versus local tectonic and sedimentary processes in controlling continental margin sedimentary cycles. The expedition recovered sediments from the Eocene to recent period, with a particular focus on the sequence stratigraphy of the late Miocene to recent, when global sea level change was dominated by glacioeustasy. Drilling in the Canterbury Basin, on the eastern margin of the South Island of New Zealand, takes advantage of high rates of Neogene sediment supply, which preserves a high-frequency (0.1–0.5 m.y.) record of depositional cyclicity. The Canterbury Basin provides an opportunity to study the complex interactions between processes responsible for the preserved stratigraphic record of sequences because of the proximity of an uplifting mountain chain, the Southern Alps, and strong ocean currents. Currents have locally built large, elongate sediment drifts within the prograding Neogene section. Expedition 317 did not drill into one of these elongate drifts, but currents are inferred to have strongly influenced deposition across the basin, including in locations lacking prominent mounded drifts. Upper Miocene to recent sedimentary sequences were cored in a transect of three sites on the continental shelf (landward to basinward, Sites U1353, U1354, and U1351) and one on the continental slope (Site U1352). The transect provides a stratigraphic record of depositional cycles across the shallow-water environment most directly affected by relative sea level change. Lithologic boundaries, provisionally correlative with seismic sequence boundaries, have been identified in cores from each site and provide insights into the origins of seismically resolvable sequences. This record will be used to estimate the timing and amplitude of global sea level change and to document the sedimentary processes that operate during sequence formation. Sites U1353 and U1354 provide significant, double-cored, high-recovery sections through the Holocene and late Quaternary for high-resolution study of recent glacial cycles in a continental shelf setting. Continental slope Site U1352 represents a complete section from modern slope terrigenous sediment to hard Eocene limestone, with all the associated lithologic, biostratigraphic, physical, geochemical, and microbiological transitions. The site also provides a record of ocean circulation and fronts during the last ~35 m.y. The early Oligocene (~30 Ma) Marshall Paraconformity was the deepest drilling target of Expedition 317 and is hypothesized to represent intensified current erosion or nondeposition associated with the initiation of thermohaline circulation following the separation of Australian and Antarctica. Expedition 317 set a number of scientific ocean drilling records: (1) deepest hole drilled in a single expedition and second deepest hole in the history of scientific ocean drilling (Hole U1352C, 1927 m); (2) deepest hole and second deepest hole drilled by the R/V JOIDES Resolution on a continental shelf (Hole U1351B, 1030 m; Hole U1353B, 614 m); (3) shallowest water depth for a site drilled by the JOIDES Resolution for scientific purposes (Site U1353, 84.7 m water depth); and (4) deepest sample taken by scientific ocean drilling for microbiological studies (1925 m, Site U1352). Expedition 317 supplements previous drilling of sedimentary sequences for sequence stratigraphic and sea level objectives, particularly drilling on the New Jersey margin (Ocean Drilling Program [ODP] Legs 150, 150X, 174A, and 174AX and IODP Expedition 313) and in the Bahamas (ODP Leg 166), but includes an expanded Pliocene section. Completion of at least one transect across a geographically and tectonically distinct siliciclastic margin was the necessary next step in deciphering continental margin stratigraphy. Expedition 317 also complements ODP Leg 181, which focused on drift development in more distal parts of the Eastern New Zealand Oceanic Sedimentary System (ENZOSS).

The development of an estuary and the accumulation of organic matter in it are strongly influenced by variations in sea level and river discharge related to climate changes. We examined the relationship between development of a Holocene estuary and the sedimentation of organic matter. The study estuary, on the Niigata Plain, central Honshu, Japan, is wave-dominated and microtidal, with a barrier island and bay-head delta. We took two 50-m-long cores, one from an area adjacent to major river discharge and another from the basin margin. Estuarine environmental and sea-level changes were reconstructed using facies analysis. Bottom conditions, which affect the preservation of organic matter, were also examined by means of facies and geochemical analyses, including the TS (total sulfur) content and C/S (carbon to sulfur) ratio. Organic constituents of the estuarine sediments were examined with reflected-light fluorescent microscopy and geochemical analyses, including TOC (total organic carbon), C/N (carbon to nitrogen) and stable carbon isotope ratio (δ 13 C) of organic matter. Late transgressive and highstand estuaries are especially suitable for the preservation of organic matter, because they are enclosed by a barrier island that promotes anoxic bottom conditions. Nearly all of the organic matter preserved in this estuary is terrestrial origin, and a part of it is preserved as NFA (nonfluorescent amorphous organic matter), which forms under anoxic conditions. The NFA formed during the highstand stage in the basin-margin area, whereas an even greater abundance of coarse- grained organic matter accumulated during the late transgressive stage in the area with major river input. During the late highstand stage, coarse-grained terrigenous organic matter, including vitrinite and cutinite, was deposited in both the marginal area and the river- mouth area.

The Niigata back-arc basin, compartmentarized and characterized by steep slope, large sediment influx related to active island-arc and extremely large depositional rate higher than 200 cm/1,000 year, had been formed at a stroke with close relationship to Japan Sea opening in the Middle Miocene age. A large part of this basin has been filled with a thick 2nd-order tectono- sequence by present. Deposition of 3rd-order depositional sequences comprising 2nd-order tectono-sequence is interpreted to correspond with filling-up of subbasins. A lower 3rd-order depositional sequence distributes rather proximal part of the Niigata basin, and an upper one occupies distal part. Stacking pattern of the 3rd-order depositional sequences, therefore, demonstrates a large scale progradational configuration. Their component 4th-order depositional sequences, ranging several tens through hundreds meter in thickness, are suitably recognized on a set of seismic profiles. Those 4th-order depositional sequences consist of depositional systems like fan-delta, fluvial, delta, shelf, slope and basin floor on the basis of assemblages of the observed sedimentary facies and facies associations in many exploratory wells. Sedimentary facies distributions, as a conclusion, are interpreted to reflect faithfully 4th-order relative sea-level cycles in sedimentary basins in the active margin area like the Niigata back-arc sedimentary basin. 3rd-order sequences, composedmore » of those 4th-order sequences, fill subbasins progressively from proximal one to distal. In addition, process of basin evolution would correspond to 2nd-order tectonic event in the active margin basins.« less

阿寒地域新第三系の珪藻生層序学的研究から,殿来層上部はDenticulopsis lauta帯(15.9-14.9Ma)に,知茶布層下部はThalassiosira praefraga帯(24.0-20.3Ma)とT.fraga帯(20.3-18.4Ma),知茶布層中部はD. lauta帯,D.hyalina帯(14.9-13.1Ma),Crucidenticula nicobarica帯(13.1-12.9Ma),D. praedimorpha帯(12.9-11.5Ma),Thalassiosira yabei帯(11.5-10.1Ma),知茶布層上部はRouxia californica帯(7.6-6.4Ma)とNeodenticula kamtschatica帯(6.4-3.9/3.5Ma),古潭層はR.californica帯とN.kamtschatica帯にそれぞれ相当することが示された.すなわち,知茶布層は堆積時代の異なる3つの層準に分割され,阿寒地域の新第三系は下位から下部中新統の知茶布層下部,中部中新統の殿来層および知茶布層中部,上部中新統最上部から鮮新統の知茶布層上部および古潭層の順に累重することになる.以上の結果は従来の岩相層序区分とは大きく矛盾し,岩相層序の再検討が必要であることを示す.