(5) Holocene Beach landform and sedimentary structure affected by glacial-isostatic rebound in East Antarctic

Authors

Hideki Miura (National Institute of Polar Research, E-mail: miura@nipr.ac.jp)

Jun’ichi Okuno (National Institute of Polar Research / Japan Agency for Marine-Earth Science and Technology)

Hideaki Maemoku (Hosei University)

Abstract

A well-marked stepped topography is developed along the shoreline of an inner cove of the Soya Coast, East Antarctica, the largest of which named Kizahashi Hama, meaning “stairs beach”. On Kizahashi Hama more than 10 steps can be recognized below 18m a.s.l., and each step is small and low, ranging 20-100 cm in relative height, and extends along the present shoreline parallel to the modern beach. The sediments are composed well-sorted fine-medium sand and displaying cross-lamination including Holocene in situ bivalve fossil shells (Laternula elliptica ) and with an overlying veneer of gravels. In Kizahasi Hama, a clear cliff at the high-tide level line is maintained, and the backshore is not clearly developed. This is because the beach faces a bay that is entirely covered by sea ice almost year-round; large storm surges do not produce a backshore and do not destroy the coastline even during winter strong storms. The ability of a wave to act in the foreshore of this special shore environment is limited to a tidal change and a calm wave during times of mild weather. In addition, in such a foreshore, sediment is planarized by the action of the sea ice loading. Melting snow water, including fine materials derived from the weathered basement, glacial sediments and aeolian dust, flows into the neighborhood of the coast. Afterward, these fine materials accumulate on the foreshore and upper shoreface by the actions of calm waves. Suspended materials slowly accumulate in the flat slope of the lower foreshore. By these progradation processes, the upper shoreface slope gradually migrates in the offing direction. As a result, when the almost same sea-level persists for a long time, the foreshore and upper shoreface slope gradually migrate in the offing direction and cover the lower shoreface and/or continental shelf. Therefore, these three strata, the foreshore beds and the upper and lower shoreface beds, have a relationship with hererotopic facies, and the height of foreshore bed following the present and past upper and lower shoreface beds that do progradation always converge to the almost same height as the high-tide level during the relative stable or slowly falling sea-level period. When the sea-level relatively rapid falls caused by glacial-isostatic rebound in an Antarctic coastal environment such as Kizahasi Hama, the flat side of the new foreshore makes a clear cliff close against the former upper and/or lower shoreface. The basal part of the cliff shows the height of the new high-tide level line.