Sedimentology of rocky shorelines: 3.Hydrodynamics of megaclast emplacement and transport on a shore platform, Oahu, Hawaii

Abstract

Megaclasts of well cemented coralgal limestone have been emplaced onto a shore platform by large waves on the North Shore of Oahu, HI. Emplacement and movements of the largest, ca. 96 ton megaclast have been dated using aerial photographs. Hydrodynamic forces at the low submerged shoreline cliff are computed using design wave characteristics based on linear wave theory and experimental results, considering the local wave climate and near-shore bottom topography. Force exerted by a tsunami has been estimated based on the 1946 Aleutian Tsunami. The computed forces are evaluated in terms of the initial fracturing along a given failure plane that is required in order for the detachment to occur. The analysis shows that tsunami, as well as large swell waves, are capable of quarrying the megaclast, provided that sufficient initial fracturing is present. Dislodgement and emplacement most likely occurred in a sequence during impact of a single wave on the shoreline cliff. Swell waves, however, are seldom capable of emplacing large blocks onto the platform due to their rapid disintegration after breaking, so that most blocks quarried from the cliff edge fall back onto the submarine terrace. Emplacement of large boulders seems to require extreme sea waves with periods longer than storms or North Pacific swells usually produce. Transport mechanisms on the shore platform vary, depending on megaclast shape. Sliding is a common mechanism of transport for larger and irregular megaclasts, whereas somewhat smaller and platy megaclasts are occasionally found in overturned positions.