Coastal Deposition Reefedge Megablocks

The erosional damage by powerful cyclone-generated waves and swells on the seaward margins of coral reefs produces huge quantities of sedimentary debris. The dimensions of some of this debris can be truly spectacular, in the form of massive blocks of limestone ripped from the reef front and then hurled onto the perimeter of the reef platform. On Nukutipipi atoll (20.7°S, 143.2°W) in the Tuamotu archipelago, several giant boulders were lodged onto the reef flat by one of the four tropical cyclones that affected French Polynesia in the 1982-1983 cyclone season (probably TC Orama or TC Veena in early 1983). The chunks of limestone were so big, up to 10 m long, 4 m

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Fig. 7.12. Satellite image of the Niue coastline at the capital Alofi on the west coast. immense waves generated by Tropical Cyclone Heta on 6 January 2004 overtopped the coastal cliffs, and scoured a zone to a distance of up to 200 m inland on the 23 m elevation marine terrace, flattening trees and buildings, including the hospital complex and the Niue Hotel. Base image courtesy of NOAA.

high and 30 m3 in size, that Salvat and Salvat (1992) called them megablocks. Of course not all boulders thrown up by cyclonic wave action are necessarily so large, but the occurrence of upstanding blocks perched along reef crests is nonetheless quite common. Through time the blocks get cemented onto the reef surface. In many places the blocks are isolated (Fig. 7.13), but where there are many of them they exhibit sorting, with the largest ones resting close to the reef crest, progressively decreasing in size and numbers farther back towards the lagoon.

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