32 



also have varied composition, the most common being kaolinite, smectite, 

 illite, and chlorite. 



Shell fragments, coral debris, algal material, and oolites are common on 

 tropical coasts and islands, in temperate regions where shelly organisms are 

 abundant offshore, or along coasts with calcareous cliffs. The mineralogy of 

 the fragments is primarily calcareous, although dolomitic, phosphatic, and 

 siliceous materials may be present. Most calcium carbonate grains are 

 biogenic since they are made up largely of shells, skeletons, and invertebrates. 

 In some tropical areas where the water is super-saturated with calcium 

 carbonate, non-biogenic precipitates of calcium carbonate called oolites may 

 form. 



Along volcanic coasts, such as Hawaii and the north shore of Martinique, 

 black sand beaches of basalt and pumice may be dominant. Driftwood and 

 timber are common, particularly in the vicinity of lumbering areas, although 

 they can be found on virtually any coast. Along cliff coasts, logs may 

 facilitate erosion by abrasion. Other organic materials, such as peats, are 

 common coastal materials. They are formed in low-energy areas such as 

 vegetated marshes and swamps. 



Characteristics of the sediments can provide clues to their origin and 

 depositional environments. Size analysis can help distinguish beach, dune, 

 and eolian flat environments (Mason and Folk 1958), and the shape, inclu- 

 sions, and optical characteristics of a sediment may also assist in determining 

 its geologic origin. The mineralogy of the materials and heavy minerals can 

 be used as a tracer to indicate the source or provenance of sediments. Heavy 

 minerals typically form a minor constituent of the original rock and have a 

 specific gravity greater than quartz or feldspar, with a density of 

 2.8 gm/cu cm being a generally accepted lower limit (Brenninkmeyer 1978). 

 Because of their greater density, heavy minerals respond differently to the 

 sorting and concentration that occur in marine processes. The utility of 

 various heavy minerals in determining provenance varies according to their 

 occurrence (common to very rare) and their stability. Statistical techniques, 

 including factor analysis (Clemens and Komar 1988; Komar et al. 1989), may 

 be useful in determining dispersal patterns and sorting of heavy minerals. 



Just as rocky coasts may produce significant zones of unconsolidated 

 sediment because of erosion, coasts of unconsolidated sediments can develop 

 lithified zones because of diagenetic changes. One of the most common sec- 

 ondary diagenetic changes is the formation of beach rock, involving the con- 

 solidation of beach sand with an interstitial cement composed chiefly of 

 calcium carbonate (Higgins 1968). Beach rock develops best on tropical and 

 subtropical coasts, although it has also been reported on temperate coasts. 

 Most researchers think that the formation of beach rock takes place under- 

 ground, near the top of the water-saturated zone of the beach. Thus, beach 

 rock is generally not visible unless shoreline recession has occurred and the 

 overlying sediment has been washed away. 



Chapter 2 Relevant Processes and Factors 



