Temporal changes of a delta: Example from the Holocene Yahagi delta, central Japan

Abstract

The depositional history of a delta was investigated by using radiocarbon-dated borehole cores from the Yahagi delta (Nishimikawa plain), Central Japan. In particular, historical changes in the gradient, thickness, and grain size distribution of the delta front and sediment discharge of the Yahagi River are discussed. The latest Pleistocene to Holocene sequence in the Yahagi delta is interpreted as an incised valley fill system formed after the Last Glacial and subdivided into five sedimentary facies: facies A (fluvial), facies B (estuary), facies C (prodelta), facies D (delta front), and facies E (gravelly tidal flat). In the main axis of the incised valley, facies A, facies B, facies C, facies D, and facies A are deposited, in ascending order. In the western area of the valley, facies E adjoins facies D. The delta front was formed by river floods without reworking by wave or tide. The succession was interpreted as a transgressive systems tract and highstand systems tract formed under sea-level rise until ca. 7 cal kyr BP and the subsequent highstand. After the formation of the delta in ca. 7 cal kyr BP, characteristic features of the delta front changed twice as the delta prograded. First, during 4–5 cal kyr BP, the delta front became thicker and steeper. This change was induced by differences in water depth between the middle incised valley and the outer incised valley, which resulted from a buried clinoform. Second, in ca. 3 cal kyr BP, the grain size distribution of the delta front became coarser and the sorting became poorer. The sediment discharge of the Yahagi River also increased abruptly. This change was induced by the increase of erosional capacity in the hinterland. Increasing human activities such as deforestation and poor soil conservation might have induced this change.