Fujio Masuda

Temporal development of prograding beach–shoreface deposits: the Holocene of Kujukuri coastal plain, eastern Japan

Abstract 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.

High-accuracy synchronism for seismic reflectors and 14C ages: Holocene prodelta succession of the Kiso River, central Japan

Abstract 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.

Grain fabric of experimental gravity flow deposits

Abstract 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.

A Great Revolution of the Earth-Surface Environment: Linking the Bio-Invasion Onto the Land and the Ordovician Radiation of Marine Organisms

Abstract. 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 Earths history.

Shallow-marine fan delta slope deposits with large-scale cross-stratification: the Plio-Pleistocene Zaimokuzawa formation in the Ishikari Hills, northern Japan

Abstract 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.

Paleoclimate of Interglacial Marine Isotope Stage 11 (MIS 11) from Strata in the Japanese Isalands

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.

The relationship between glacio-eustatic parasequences and a third-order sequence in the Kakegawa Group, central Japan

Abstract 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.