The geomorphic development of coastal plains is considered to be influenced by sea-level changes associated with glacial-interglacial cycles. The author has selected the middle and lower reaches of the Toyo River as the study area to discuss the relationships between the geomorphic development of the coastal plains and sea-level changes since the Late Pleistocene, and identified the stratigraphy of the river terraces and the subsurface deposits by analyzing boring data. The main results are summarized in the following. The fluvial surfaces formed by the Toyo River are classified into five levels: highest terraces; higher terraces; middle terraces (Toyohashi terrace and Kozakai terrace); lower terraces; and alluvial surfaces, in descending order. Older fans and younger fans were formed by tributaries of the Toyo River. The highest terraces and higher terraces were formed before the oxygen isotope stage (St. 6). The middle terraces were thought to be formed after St. 5 and before St. 2. The lower terraces and alluvial surfaces were formed during the period of St. 2 and St. 1, respectively. Two sedimentary cycles associated with the glacio-eustatic sea-level changes are recognized under the alluvial and Kozakai surfaces in the lower reaches of the river, respectively. The deposits beneath the alluvial surface are composed of an alluvial basal gravel bed (ABG), lower sand bed (LS), middle mud bed (MM) and upper sand bed (US). The ABG was deposited during the lower sea level at St. 2. Radiocarbon ages on molluscs of 6640, 6620 and 6490 yr BP have been obtained from the MM. The MM is the deposit resulting in the postglacial transgression. The deposits beneath the Kozakai surfaces are divided into four members: Kozakai basal gravel bed (KBG); Kozakai sand bed (KS); Kozakai mud bed (KM); and Kozakai gravel bed (KG), in ascending order. The KBG was formed in the lower sea level at St. 6. The KM is thought to be marine sediments resulting from the transgression during St. 6 to St. 5e. The KG was deposited as a fluvial deposit during the regression after the last interglacial transgression. Geomorphic development during the period of St. 6-5e was mainly influenced by sea-level changes, in particular the rapid transgression at St. 5e. A similar process is also observed during the period St. 2-1. A rapid sea-level rise dominated the geomorphic development during the period from the glacial to the interglacial stage.
ASJC Scopus subject areas
- Geography, Planning and Development
- Earth-Surface Processes