Bay-head delia progradation and delta plain development in the ishikari lowland

Abstract

This study investigated bay head delta progradation and delta-plain development in the Ishikari lowland, northern Japan. Nine auger cores and geological samples were obtained from one site, and measurement of loss on ignition of sediments and radiocarbon dating of plant and wood fragments were conducted. Five cross sections of the lowland were also constructed using existing lithologs. The results showed that the lowland was transgressed to form a lagoon system during the early Holocene sea-level rise. As the sea level rise decelerated ca. 8.000cal BP. bay-head delta deposits began to fill the lagoon and drove an expansion of the subaerial delta plain. Peat deposits formed shortly after the emergence of the delta, indicating wetland development on the newly emergent land surface. Later, crcvassing and avulsion of the Ishikari River channel occurred in the upstream part of the delta plain before ca. 5.600-3.600calBP. as reflected by fluvial deposits overlying the peat that caps the bay-head deltaic deposits. Fluvial deposition nearly ccased after 5.600-3.600cal BP. largely due to dccrcascd precipitation from a weakened East Asian summer monsoon. During this time, the wetlands redeveloped across the now less fluvially active delta plain. Consequently, the accumulation of peat was dominant in the downstream part of the delta plain until the end of the 19th century, when intense human activities began in Hokkaido. Comparison with previous studies suggests that the low fluvial activity of the Ishikari delta plain after 5.600-3.600cal BP may also be common to other riverine delta plains in Japan. However, such low fluvial activity did not generally continue in those other systems, as fluvial activity increased again after ca. 2.000cal BP. The factors that drive such distinctions between the Ishikari lowland and other delta plains have yet to be fully understood.