The stratigraphic distribution and abundance of planktonic foraminifera within sixth-order (41-k. y.) depositional sequences of the Omma Formation provide the high resolution sequence biostratigraphic information for analyzing the Sea of Japan Quaternary strata. Warm-and shallow-water dweller Globigerinoides ruber intruded into the Sea of Japan associated with the influx of the warm Tsushima Current during interglacial periods since oxygen isotope stage 57, and were regionally (Sea of Japan) extirpated at the following glacial period. The ecostratigraphic data is therefore helpful for interpreting the sequence stratigrephic framework. Planktonic foraminiferal abundance peaks associated with Gds. ruber are interpreted to correlate with the maximum flooding horizons, whereas abundance minima are associated with sequence boundaries within sixth-order depositional sequences. On the basis of the stratigraphic distribution of Gds. ruber, migration of planktonic foraminifer Globolotalia inflata (its stratigraphic distribution is used as datums in the Sea of Japan Quaternary strata) into the Sea of Japan was controlled by both flow of the Tsushima Current and barrier to migration in the from of shallowing around Tsushima Strait.
This paper describes the sequence stratigraphy of six sedimentary basins in northwest Kyushu based on offshore seismic, well, and surface geological data. And then regional stratigraphic correlation and sedimentary process are discussed. The northeastern end of the East China Sea shelf basin extends to the Fukue basin, north of Goto Islands, which mark the northeastern end of the Goto belt. To the south of Goto Islands, there also developed small Paleogene sedimentary basins called the Goto-nada, Nishisonogi, and Karatsu basins. These basins have NW-SE trending rhomboidal frames. On the contrary, NE-SW trending Paleogene sedimentary basins called the Amakusa-nada and Amakusa basins are present to the south of the Goto-nada basin. A single sequence stratigraphic framework is applicable for the strata in these basins, and the strata are subdivided into five Paleogene depositional sequences; DS-P1 to -P5 in ascending order, and three Neogene to Quaternary depositional sequences; DS-N 1 to -N3 in ascending order. The DS-P1 and -P2 (Middle Eocene) were only deposited in the NE-SW trending sedimentary basins, while the DS-P3 (upper Middle to Upper Eocene) directly overlies the pre-Tertiary basement in the NW-SE trending basins. The first marine incursion to the NW-SE trending basins from the southeast occurred probably during the deposition of DS-P3 of a late Middle to Late Eocene age. There have been some problems in the previous lithostratigraphic division in onshore northwest Kyushu such as inconsistent definition of a certain group over several basins, as well as mistaking ravinement surfaces for unconformity in some cases. These problems may be resolved with the stratigraphic scheme established here, which is applicable regionally to the Paleogene basins in northwest Kyushu. It will provide a useful tool for identifying regional tectonic events in this region.
Carbon isotopic ratios, δ13C values, of CH4 gases from their production wells in Japan were measured for discussing the generation process of natural gases in formations, based on the positive correlation between their N2/Ar and He/Ar ratios reported by Kita et al. (1999). Their δ13C values are distributed in a range of about-70‰ to-30‰, covering those of CH4 gases generated by bacterial and thermal decompositions of organic matter in formations. We found a clear relation between δ13C values of CH4 and N2/Ar ratios as well as between those of CH4 and He/Ar ratios, supporting that the correlation between N2/Ar and He/Ar ratios of natural gases reflects the decomposition process of organic matter in formation. Furthermore, the N2/Ar and He/Ar ratios increase gradually in the range of δ13C values from-70‰ to-40‰ and start to increase drastically at about-40‰. This seems to give a first criterion for defining thermogenic CH4 gases as those higher than-40‰. These relations between δ13C value of CH4, N2/Ar ratio, and He/Ar ratio of natural gases can play an important role in clarifying the chemical and geological environments of their generations and accumulations in formations.
Safety use limit of API-13Cr steel tubular in terms of Sulfide Stress Cracking (SSC) property is studied. Since SSC is a phenomenon of hydrogen embrittlement, the hydrogen content in steel is a key factor to determine SSC susceptibility. The hydrogen content entering into steel and the critical hydrogen content of steel below which no SSC occurs were experimentally determined as a function of environmental factors and yield strength. Comparison of both values of hydrogen contents enables to estimate critical environmental conditions and yield strength. Estimation results by this method coincides well with data in several literatures.
This paper describes pollen assemblage zones of the Paleogene to Quaternary in northwest Kyushu based on the result of pollen fossil analysis of samples from offshore wells and surface outcrops. And then regional stratigraphic correlation between the East China Sea and northwest Kyushu is discussed. Eight pollen assemblage zones (Pinus, Alnus-Ulmus-Zelkova, Pinus-Taxodiaceae, Taxodiaceae-Alnus, Pinus-Gramineae, Quercus-Fagus, Pinus-Picea, and Pinus-Alnus Zones in ascending order) were defined in the late Middle Eocene to Quaternary sequences in northwest Kyushu. Based on the co-occurrences of marine microfossils and other published dating data, these zones are dated as latest Middle to Late Eocene, latest Eocene to earliest Oligocene, Early Oligocene, Late Oligocene to earliest Miocene, Early Miocene, Pliocene, Quaternary, and Quaternary, respectively. The pollen assemblage zones in eastern East China Sea described by Hu and Sarjeant (1992) are correlative well with the zones in northwest Kyushu. This study reveals that palynology is a very useful tool for regional correlation of Cenozoic strata in the area between East China Sea and northwest Kyushu where non-marine deposits are dominant.
Not a few papers concerning terrestrial sequence stratigraphy have been published during the last decade, as applications using sequence stratigraphic concept have increased. This paper reviews current developments in terrestrial sequence stratigraphy in terms of sedimentation control factors and recognition methods of depositional sequences in terrestrial strata. Terrestrial sedimentary environments are divided into three territories; coastal plains, transfer zones and inland basins. Relative sea-level changes are the most important control factors in coastal plains, whereas climate and tectonic movement are critical in inland basins. In coastal plains, a depositional sequence can be recognized by such strata indicative of marine incursion as tidal, bay, estuarine or shallow marine sediments, which constitute a maximum flooding surface. Frequency and size of fluvial channels are important signals for recognition of a depositional sequence in fluvial strata, because they may have been formed in response to changes of accommodation space corresponding to relative sea-level or base level changes. In inland basins, it is necessary to recognize cycles in relation with base level, climate and sediment supply changes. In any case, we need to improve terrestrial sequence models by carrying out further case studies on a large variety of geological settings.