There seem to be some misconceptions about the definition and the criteria of antidunes and their deposits. Antidune is one of the upper-flow-regime bedforms, whose definition was limited originally to upstream-migrating bedwaves but recently includes any bedwaves in-phase with surface gravity waves in the overlying flow. The term “antidune” represents a bedform, not a sedimentary structure or a deposit.
Thin lenticular laminaset and HCS-like structure (“HCS mimics”) may be formed as antidunes as well as upstream-dipping, relatively low-angle stratification (backsets). HCS-like structure may be formed as three-dimensional antidunes in upper-flow-regime conditions, particularly in abundant suspension fallout. However, the paleocurrent direction or regional facies assemblage, for example, will be needed in practice for the recognition of antidune deposits, besides descriptions of their internal sedimentary structures and grain fabric.
Most studies were based on limited examples of diverse antidunes or only small portions of antidune deposits. The classification of antidune geometries is so insufficient that the processes, hydraulic conditions and mechanisms of the formation of sedimentary structures have been poorly understood. Detailed descriptions and analysis of the bedforms, resultant deposits and grain fabrics should be conducted based on a more sophisticated classification, from experimental or theoretical approaches.

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For the purpose of demonstrating the origin of gravels, rock composition of gravels was analyzed for gravelly strata of the Middle Pleistocene Nagahama Sand and Gravel Member distributed in the western area of the central Boso Peninsula, and Late Pleistocene Hasirimizu Gravel Member distributed in the eastern part of the central Miura Peninsula. Gravels of the Nagahama Member are mainly composed of Mesozoic and Paleozoic sedimentary rocks (62.3%), Miocene green tuffs and igneous rocks (34.8%), and Tertiary sedimentary rocks (2.8%). Gravels of the Hasirimizu Member consist of Mesozoic and Paleozoic sedimentary rocks (86.3%), Miocene green tuffs and igneous rocks (13.6%).
Judging from the geological distribution and paleogeography during the Middle to Late Pleistocene in the Kanto region, the gravels of the Mesozoic and Paleozoic rocks were derived mainly from the Kanto Mountainland, the Miocene green tuffs and igneous rocks were from the Tanzawa Mountainland, and the Tertiary sedimentary rocks might be from the Mineoka Mountainland. These results mean that the Kanto- and Tanzawa Mountainlands uplifted also in the Middle to Late Pleistocene. Relatively abundant occurrence of the gravels of the Tertiary sedimentary rocks may suggest uplift of the Mineoka Mountainland at the time of deposition of middle part of the Nagahama Member.

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Late Pleistocene Yasuda Formation distributed around the Kashiwazaki Plain in Niigata Prefecture is mainly composed of alternated beds of silt, sand, and sandy gravel, with intercalated beds of peat and volcanic ash. The Yasuda Formation is 12 meters in total thickness. On the basis of correlation of the volocanic ash layers with volcanic tephras distributed regionally, the Yasuda Formation is considered to have been deposited at the isotope substage 5c, and correlated with the Obaradai Formation distributed around Kanto Plain. Molluscan fossils and diatom ecological group show that the upper part of the Lower Yasuda Formation was deposited in a brackish environment. Spatial distribution of the altitude of this horizon implies fault movements after deposition of the Yasuda Formation.

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Sediment gravity flow deposits associating with reworked accretionary lapilli are described in the Miocene Tokibariyama Formation, in Oki Island, SW Japan. Focus beds comprise three units, and each unit is lithologically subdivided into a lower normal grading division, and an upper mud division. Reverse grading is occasionally developed in the lowermost part, and a parallel laminated division is rarely intercalated in the two main divisions. Palaeoslope direction estimated by slumping structures is concordant with the flow direction of the deposits. Lithology and palaeoflow direction indicate that the deposits must be formed by turbidity currents. Mean flow velocities estimated by grain diameter vary from 57 to 210cm/s within one flow unit. Some accretionary lapilli are brittled, and some are plastically deformed. These two types of accretionary lapilli are maldistributed depending on the subunit divisions. Namely, the brittled ones and non-deformed ones are concentrated in the lower grading division, while the ductiled ones are dominant in the upper mud division. Accretionary lapilli must be hydraulically sorted, which means that they may be used as an indicator of palaeohydrology. However, the relation between the deformation origin on the accretionary lapilli and hydrology needs further consideration.

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Taphonomic process of diatom valves was investigated with samples from a surface sediment core and a sediment trap above it in Omura Bay, western Japan. Extremely weak diatom valves of a coastal marine diatom, Chaetoceros affinis Lauder was considerably observed in the sediment core, whereas the other Chaetoceros species with weak valves like C. affinis rarely occurred even though they were found in trap samples. Biweekly sediment trap experiments revealed the season when each Chaetoceros species bloomed and deposited on seafloor. The observation suggests that the latest bloomed Chaetoceros valves (younger than 7-8 months) were preserved in the sediments. Vertical mixing of loose sediments by seawater movement and burrowing invertebrates seems to work within the depth of 5cm of surface deposits according to observation of sedimentary structures and stratigraphical distribution of C. affinis in the core. The diatom valves are well mixed in this layer within 4 weeks.

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