THE GEOLOGICAL EFFECTS OF HURRICANES ON CORAL-REEFS AND THE INTERPRETATION OF STORM DEPOSITS

Hurricanes occur in belts 7-degrees to 25-degrees north and south of t he equator. Reefs growing in these belts suffer periodic damage from h urricane-generated waves and storm surge. Corals down to 20 m depth ma y be broken and removed, branching colonies being much more susceptibl e to breakage than upright massive forms. Sand cays may be washed away and former storm ridges may migrate to leeward across reef flats to l ink with islands. Reef crest and reef front coral debris accumulate as talus at the foot of the fore-reef slope, on submarine terraces and g rooves, on the intertidal reef flat as storm ridges of shingle or boul ders and isolated blocks of reef framework, as accreting beach ridges of leeward migrating shingle, as lobes and wedges of debris in back-re ef lagoons, as drapes of carbonate sand and mud in deep off-reef locat ions in the fore-reef and lagoonal areas. In addition to the coarse de bris deposited, other features may aid the recognition of former hurri cane events, including the assemblage of reef biota, its species compo sition and the structure of the skeletons; graded internal sediments i n framework cavities; characteristic sequences of encrusting organisms ; characteristic shapes of reef flat microatoll corals; and submarine cement crusts over truncated reef surfaces. The abundance of reef flat storm deposits whose ages cluster around 3000-4000 y BP in certain pa rts of the world most likely relate to a slight fall in relative sea l evel rather than an increase in storminess during that period. A highe r frequency of storms need not result in more reef flat storm deposits . The violence of the storm relative to normal fair-weather conditions influences the extent of damage; the length of time since the previou s major storm influences the amount of coral debris created; the lengt h of time after the hurricane, and before a subsequent storm influencc s the degree of stabilization of reef-top storm deposits and hence the ir chances of preservation.