A unified theory of sedimentary basin evolution is gradually emerging, based on our knowledge of plate tectonics and its driving forces.
The constantly changing kinematic pattern of the global plate system is driven by mantle convection. Plate convergence, transform motion and divergence lead to the development of a range of types of sedimentary basin, most of which can be classified into two broad classes-extensional basins caused by crustal stretching, and the foreland type, caused by compression and supratrustal loading.
The creation and breakup of supercontinents lead to regional and global stratigraphic cyclicity over a wide range of time scales, reflecting the effects of changing patterns of sea floor spreading on ocean basin volumes and continental margin configuration. An additional, short-term pattern of cyclicity is generated by astronomical forcing, the so-called Milankovitch effects. These are variations in the amount and distribution of solar radiation caused by tilts in the earth's orbit.

View full abstract

Three processes in petroleum geology have been tried to be modeled in these decades as a part of computer applications for actual exploration problems. They are generation, migration and accumulation processes. Simulation models have been developed indivisually in each part of these processes. Among them generation model was achieved as a most successful tool for exploration. Geochemical information can be used to predict the timing and amount of hydrocarbon generated in the basin. This model, coupled with fluid flow model in two-dimensional scheme, makes an integrated simulation tool for searching areas of high potential of petroleum accumulations.
However, when the model was applied to the real geologic situation, it was turned out that the results were so dependent upon given lithological distributions which were controled directly by input parametes. As an extention of simulation modeling, the sedimentological approach was then applied to estimate facies distributions within a basin.
The concept of Quantitative Dynamic Stratigraphy (QDS) in recently employed for this type of modeling where three modes exist: geodynamic modeling, purely-sedimentary process modeling and stratigraphical modeling. This paper demonstrates how one of the stratigraphical models works through its application to “Vail's Sequence.” This type of forward modeling functions to test independently a hypothesis proposed by the seismic stratigraphy.
The QDS models should conceptually be applicable to ancient geological deposits, however, we know so little about input parameters in the geologic past. We may be necessary to include some stochastic processes for simulating real sedimentary features as an integration of the present deterministic approaches.

View full abstract

The Lake Biwa is a deep (up to 100m) tectonic basin with rapid accumulation of muddy sediments, and show an asymmetrical profile with stacked submerged terraces on the east and steep slope on the west.
The results of acoustic survey off the Echi River and the facies analyses of sediment cores show that these terraces are composed of sand-rich deposits formed under delta, barrier, and alluvial plain environments, which are similar to those in the present time and characterized by wave-dominated edimentary structures. They are successively built up 40-m-thick coastal sedimentary prisms, where at least nine sedimentary sequences are identified. Each sequence has dominant regressive facies represented by prograding delta-front and shoreface deposits followed by lagoon or alluvial-plain deposits behind them, but fewer transgressive facies.
In addition to the stacked terraces, barrier-growth, lagoon-filling, overwash, and coastal onlap structures around the sequence boundaries can prove the water-level rise during the last 30, 000yr. Thin sand sheets or reworked sediments on the ravinement surfaces, washover deposits, and well-preserved surface topographic features are suggestive of rapid shoreface retreat or in situ drowning. The average amplitude and period of lake-level rise are estimated as 3.5m and 3, 500yr, respectively.
According to the previous works (TAKAYA and NISHIDA, 1964; YOSHIOKA, 1987; KAWABE, 1989), it is expected that the tectonic block including the drainage channel of the lake water (present Seta River) is uplifted, and that the lake basin is tilting westward to the active faults along the western basin boundary. Therefore, such a periodic rise can be sttributed to tectonically-induced relative subsidence compared to the level of the drainage channel, rather than a climatic control.

View full abstract

Active sediment accretion in the landward slope of the Nankai Trough off SW Japan has been documented by multichannel seismic reflection data which show a development of accretionary prism associated with landward dipping thrusts. It has been difficult to know the lateral extend and configuration of these thrusts which have relatively small displacement even by means of the Sea Beam survey. An oceanfloor imaging sonar system, IZANAGI, however revealed detail structures of the prism and the trough floor. Mud volcano-like morphologies were detected at the trough floor, one of which reaches 2km diameter. Deformation at the toe of the prism is characterized by ramp anticlinal ridges due to thrusting. The trend of thrust faults is affected by the orientation of oceanic basement as well as convergence direction and slope orientation. Toward the interior region of the prism, slope sediments composed of fine grained materials modify the relief of anticlinal ridges.

View full abstract

The detail observation using soft-x radiography is carried out for a cross-stratification formed in the Upper Pleistocene tidal inlet exposured at Uchijuku, Ibaraki, Japan. The foreset laminae of the cross-stratification are characterized by thin, sharp and fine-grained heavy mineral layers. The dip of laminae is constant thoughout the outcrop. These layers are straight and do not disappear even near the base of foreset. The thickness of each lamina is almost the same thoughout one set. These foreset structures indicate that each foreset lamina is formed by a grain flow.

View full abstract

The ultrasonic sieving method is a new and useful tool for grainsize analysis of fine-grained muddy sediments. The new method was compared with the traditional hydrometer method. The ultrasonic sieving method shows a little wide variation. Ultrasonic vibration is often used to disperse the sample before the hydrometer analysis. It decreasea the grainsize of muddy sediments, but the sediments attains a “terminal distribution” in grain size after about 30 minutes of vibration. The weigted mean of the analysis by ultrasonic sieving method is smaller than the analysis of unvibrated sample by the hydrometer method, and is concordant with the analysis of sample in terminal distribution by the hydrometer method.

View full abstract

This paper outlines the methodology and philosophy for modern integrated multi-disciplinary studies of hydrodynamic and sedimentary processes in coastal harbours and bays. As Japan begins to become more concerned about and aware of environmental matters, emphasis on such studies can be expected for the future. In the past most matters of environmental concern in Japan have been dealt with by the civil engineers. It is argued that for the future the best solutions arise from a multi-disciplinary approach to coastal and harbour problems, involving input from a wide range of geoscientists.
The study proposal includes review of previous work and collation of existing data; investigation of bathymetry, morphologic evolution and changes; determination of bottom sediment facies, sediment transport pathways and budgets; investigation of the water column (temperature and salinity), and surficial sediment geochemistry and ecotoxicity; inlet hydraulics, tidal, current, and wave climate measurements; numerical modelling of current hydrodynamics; numerical modelling of water quality and pollution dispersion; numerical modelling of hazard events such as storm surge and tsunami, and the effect of such events on the hydrodynamics, pollution and sediments; and review of the efficacy of the numerical models in relation to the field and empirical data, i. e. how well do the models fit perceived “reality”?

View full abstract