VARIATIONS IN DEPOSITIONAL ARCHITECTURE OF HOLOCENE TO MODERN PROGRADING SHOREFACES ALONG THE PACIFIC COAST OF EASTERN JAPAN

Examination of the detailed facies architecture of Holocene depositional systems contributes to the understanding of the rock record when coupled with high-resolution radiocarbon dating. Case studies of the depositional succession of Holocene to modern prograding shorefaces in the Kujukuri and Sendai areas along the Pacific coast of eastern Japan are summarized and compared to determine the relationships between the depositional architecture and wave conditions, and the shoreline trajectory. These areas differ from each other in history of relative sea level and characteristics of sediment source, but their wave climates are similar. The 20-m-thick Holocene depositional succession of the Kujukuri coast overlies the Plio-Pleistocene basement upon a ravinement surface. The succession consists of basal, lower-shoreface and upper-shoreface deposits, and foreshore and backshore deposits, in ascending order. The shoreface receives sediment from the coastal cliffs, and it has prograded seaward since 6 ka, when relative sea level stopped rising and then started to fall. The shelf and shoreface were sediment-starved during transgression, when sediment was deposited near the cliffs and were not transported to the central coast. Along the Sendai coast, the Holocene marine deposits lie upon a ravinement surface cutting the latest Pleistocene to Holocene nonmarine deposits. The succession is less than 30 in thick and is composed of transgressive ravinement deposits, sand-mud alternations of stacked inner-shelf storm deposits, lower-shoreface and Upper-shoreface deposits, and foreshore and backshore deposits, in ascending order. The onset of shoreface progradation was at 7-8 ka, during a period of relative-sea-level rise, reflecting a high and continuous sediment supply from adjacent rivers. The Kujukuri shelf is sandy with a small amount of mud, but in the Sendai succession the base of the sandy shoreface is marked by the deposition of inner-shelf mud, and its water depth is 17 in, much shallower than half the length of an average deepwater wave. Thus, the depth of the base of the sandy shoreface is determined not solely by wave conditions but also by other factors, such as supply of mud and formation of sediment plumes. On the Sendai shelf, flood discharge of the relatively large rivers probably led to a well-developed sediment plume, resulting in shallow deposition of mud. The erosional boundary between upper-shoreface and lower-shoreface facies, defined as the surf diastem, shows abrupt increases in grain size and the angle of cross lamination, and there is a gap in the depositional age record in some places. These features indicate that formation of longshore bars dominated over the formation of storm-induced low-angle bedforms at this boundary. The water depth of the surf diastem is typically 5-7 m, which is approximately equivalent to the depth at which storm waves break, and to the depth limit of longshore bar migration observed along the modern coast. The spatial distribution of the surf diastem reflects the shoreline trajectory, although it is influenced partly by changes in the wave climate during progradation.