相対的な海面変動が支配する地層の累重と地形の形成 : わが国の沖積層の解析から

概要

論文の詳細を見る
Holocene relative sea-level changes have been reconstructed by using the relationship between carbon isotope (^<14>C) ages of sample and/or depths of occurrence. The curves for Japan show a maximum highstand at 6,000 to 5,000 yrs BP and 2 to 5 m higher than it at the present. The marine transgressive stage began at about 10,000 yrs BP at several tens of meters in depth and the following regressive stage or highstand stage was during the past 6,000 to 5,000 yrs BP. In fact, the Japanese Holocene successions are interpreted to represent a depositional sequence. Temporal changes of depositional (i.e. accumulation) rate also can be obtained for the Holocene by the numerous and highly dense ^<14>C ages and their elevations (or depths) of occurrence for a given core. A smoothed curve on the diagram of age versus elevation is called "depositional curve". The depositional curve represents a space and time trajectory of "paleo-sea floor" during the period of deposition. The paleo-depth also can be deciphered using the reconstructed depositional curve and contemporaneous relative sea-level curve of the region. The depostional curves characterize temporal patterns of depositional rates in a wide range of depositional systems, such as, delta, beach-shoreface, bay, barrier, fluvial and lake systems. The delta system indicates a pattern characterized by a high rate of accumulation during the transgressive stage, slow deposition at the highest stand in relative sea-level, and rapid accumulation during the regressive stage. There is a thick transgressive fluvial, estuary and bay deposits exist in the narrow drowned valley under the delta and an overlying highstand coarsening-upward succession of a prograding delta deposits. The depositional rate of drowned-valley fill is controlled by the paleo-depth, not by the speed of sea-level change. Thick and clean clay at the base of delta front and/or prodelta shows faster deposition. The thick and clean clay is not corresponding to MFS. During the MFS formation coarser sandy mud was deposited slowly. The timing of the formation of downlap-surface (MFS) is controlled by the rising-speed of sea-level and flux from the rives. When the sea-level rise slowed, sediment supply from the big rivers showed a relative increase, and delta's prograding began regressively, because Japan has a steep mountainous source regions. The bay system is characterized by slow transgressive and highstand deposition and followed by gradually increasing regressive deposition. The system consists of the thin transgressive and thicker regressive deposits. These coarsening and/or fining trends in the succession are not directly controlled by either rates of deposition or sea-level changes. The beach-shoreface system which typifies a wave-dominated strand plain shows a pattern with a fast rate of transgressive deposition, a slow rate of highest-stand deposition, and a fast rate of highsand to regressive deposition. Horizon of the maximum depth, boundary from upward-deeping to upward-shallowing is not equal to MFS. The system shows apparent facies-boundaries of wave-ravinement surface, surf diastem and storm surfaces. The surf diastem of facies gap between upper- and lower-shorefaces is formed by erosion in the surf zone, The movement of outer longshore-bars creates the diastem at a big storm time. Average accumulation rates decreased at the transgression and increased at the regression, but the sand/mud ratio varies little. Bed thickness of inner-shelf storm sands also shows no relationship to sea-level change. The average thickness of storm beds is the highest in the interval deposited between the time of maximum flooding and the time of early regression. This is probably because of the low preservation potential of thin beds associated with frequent, low-magnitude storms during this period of low accumulation rates and extensive reworking. The barrier system composed of thicker (several tens meters) and coarse deposits is characterized by very rapid deposition only during a transgressive stage. The curves of fluvial system show the eventual rapid deposition of channel-fill or bar deposits and the slow deposition of flood-plain mud. These two depositional rates are 100 to 1,000 times different. The depositional curves for different type of depositional systems appear to be important new data for the sequence-stratigraphic analysis of older deposits. These curves show quantitative variations in depositional rates during a transgressive-regressive cycle in different depositional systems.
日本地形学連合の論文
2007-10-25

相対的な海面変動が支配する地層の累重と地形の形成 : わが国の沖積層の解析から

スポンサーリンク

概要

著者

関連論文

スポンサーリンク