Sedimentology of rocky shorelines: 2 Shoreline megaclasts on the north shore of Oahu, Hawaii - origins and history

Limestone megaclasts, up to 96 t in weight, rest on a coastal rock platform near Sunset Beach on the north shore of Oahu, Hawaii. They occur in seaward-dipping, imbricate clusters and as solitary clasts. The rock platform is subject to large swell waves and very large waves associated with winter storms and tsunami. A series of photographs, covering the period from ca. 1905 to 1996, show episodic rearrangement of megaclasts on the platform and the appearance of new megaclasts near the platform edge. The largest solitary megaclast (ca. 96 t) was emplaced on the platform between 1940 and 1950, possibly by the 1946 Aleutian tsunami and has moved twice since then in ca. 30-m jumps along two different trajectories. Larger clasts have been broken from the platform edge and emplaced on the platform only by very large wave events, whereas formation and movement of smaller tabular, clasts (ca. 30-40 t) appear to be a continuously ongoing process on the surface of the rock platform. Partial detachment of clasts by marine erosion processes along joints and cracks in the limestone bedrock facilitates their complete detachment from both the platform edge and platform surface, The larger numbers of megaclasts emplaced on the north and northwest sectors of the platform relative to other sectors are attributed to exertion of maximum wave power on these sectors due to a combination of steep slope and ramp-like submarine topography. (C) 2002 Elsevier Science B.V. All rights reserved.