46 



wetting and drying of the platform surface. This water-layer weathering 

 includes physical processes such as expansion, swelling, and salt 

 crystallization, and chemical processes such as hydration and oxidation. Wave 

 erosion may proceed more rapidly in weathered material, with subaerial pro- 

 cesses at the water table level possibly coming into play. In tropical calcare- 

 ous areas, solution of rocks and bioerosion appear to be important processes 

 in shore platform development. 



While the complexities of platform morphology are not well understood, 

 the shore platform functions as a dissipator of wave energy and as a pathway 

 for sediment transport at the cliff edge. The platform slope may thus influ- 

 ence the cliff, in that steep slopes would create increased longshore sediment 

 transport and promote cliff recession, whereas gentle slopes would not allow 

 sufficient longshore transport and beach progradation would result (Bradley 

 and Griggs 1976). The critical platform slope would be dependent upon the 

 shear stress at the bed by wave forces and the strength of materials making up 

 the shore platform. 



Organic Reefs 



Reefs, which are formed mainly from biogenically produced carbonates, 

 are important structures in tropical waters. Like lithified coasts, they form 

 hard bottoms, but several unique forms develop in such environments. Sever- 

 al types of marine organisms are capable of precipitating calcium and other 

 carbonates in skeletal and non-skeletal forms, but reefs of coral skeleton are 

 the most common. Corals grow most successfully in shallow, warm, mud- 

 free waters of moderate salinity, characteristically between latitudes of 30° N 

 and 30° S where such conditions occur. 



Four major forms of large-scale reefs have been identified: (a) fringing 

 reefs, (b) barrier reefs, (c) atolls, and (d) table reefs (Stoddart 1969). 

 Fringing reefs are connected with the land, whereas barrier reefs are separated 

 from land areas by a lagoon which may be several kilometers wide. Minor 

 reef shapes include ring-shaped forms on banks and shallows, and reef knolls 

 or patch reefs growing in lagoons. Like barrier reefs, atolls rise from deep 

 water, enclosing a lagoon, but unlike barrier reefs, they do not enclose land 

 within them. Table reefs rise from the sea floor as a shallow bank, being 

 capped with reef growths that appear different from the other forms. 



The reef surface has a high roughness, and will change local 

 morphodynamics by affecting the energy of incident waves and by increasing 

 turbulence. Reef form and structure are considered to be linked largely to 

 wave climate, and may show process gradients. Studies on the Grand 

 Cayman reefs show that the outer reef is dominated by wind -driven currents, 

 the inner by high-frequency waves, and the lagoon by deepwater waves and 

 tides (Roberts, Murray, and Suhayda 1975). 



Chapter 3 Variable Coastal Features 



