古東京湾のウェーブリップル形成の波浪条件

抄録

Paleo-wave conditions during the Shimosueyoshi Transgression (130, 000-100, 000 years B.P.) are estimated from oscillatory ripples preserved in prodelta, shoreface and tidal flat deposits of the Paleo-Tokyo Bay. Possible combinations of wave conditions and water depths that could have generated the observed ripples are determined by ripple spacing and grain size of ripple forming sediments using the method of Komar (1974) and others, which are based mainly on Airy wave theory. In addition, paleo-depths are calculated independently by the following two methods using stratigraphic thickness from the ripples to the above foreshore deposits and the height of longshore bars containing some of the ripples in shoreface deposits. Therefore, combinations of wave height and wave period under the water depth, which was estimated by the above method, can be determined.
Waves with a height of lower than 2.3 m and a period of 2-8 s are obtained from the ripples in prodelta deposits. Such waves represen “storm waves” of the present Tokyo Bay. This wave condition may have been formed by relatively small waves of post-storm stage, because the ripples occur in the upper part of storm-generated sheet sand and are covered by a clay layer deposited from suspended matter in flood fluvial water into the Paleo-Tokyo Bay. Waves with lower than 2.5 m in height and 1.5 to more than 10 s in period are reconstructed from the ripples in shoreface deposits. These waves can generally represent “storm waves” of the Tokyo Bay and “fairweather waves” of the Kashima beach facing the Pacific Ocean. Waves of smaller 1 m high and less than 5.5 seconds are reconstructed from the ripples in tidal flat deposits. These small waves are approximately equal to “fairweather waves” of the Tokyo Bay. No ripples representing more big waves, such as winter waves and typhoon-generated storm waves on the present Pacific coast, are preserved in the Paleo-Tokyo Bay sediments. This may be caused by the shallow seawater-depth, less than 10 m, of the Paleo-Tokyo Bay where big wave motion during storm event may have changed most of the bottom sediments to flat bed rather than ripples.