SECT. 3] SHALLOW- WATER CARBONATE SEDIMENTS 561 



subsequent history. To illustrate the application of this grouping to sediment 

 particles, four sediment samples will be analysed. 



The sediment shown in Fig. 4B consists largely of the segments of Halimeda 

 opuntia. From the coarse grain size alone, one might conclude that it represents 

 an accumulation in relatively agitated waters. However, because these large 

 segments are produced merely by the death and decay of the plant, agitation is 

 not required. ^— 



The sediment of Fig. 4C consists of rounded fragments of mollusk shells. 

 Because many of the mollusk skeletons (chambers) are quite strong and resis- 

 tant to breakdown and erosion, considerable energy is required to produce the 

 size and rounding observed here. It is, therefore, not surprising that this parti- 

 cular sample is a beach sand. Notice that the grain size and shape of its particles 

 are almost indistinguishable from the specimen shown in Fig. 4B. 



The sediment shown in Fig. 4D consists of fragments of the branching coral 

 Pontes divaricata and the branching red alga Goniolithon strictum, and 

 fragments of Halimeda. Here again, the coarse grain size might suggest high 

 agitation, but in fact it is the size and resistance to breakdown of these branched 

 forms which determine the size of particles produced. 



The sediment of Fig. 4E consists almost exclusively of whole and fragmented 

 thin-shelled mollusks and Foraminifera. The size and specific gravity of these 

 chamber forms control their deposition. The sample is from a beach on an 

 island in the calm waters of Florida Bay. The particles are those that can be 

 carried ashore in susjiension and stranded as the water recedes. 



The preceding examples are merely outlines of the way in which the variations 

 in resistance to breakdown of the different types of skeletons can help to explain 

 the origin and development of skeletal sediments. A more detailed and searching 

 analysis should consider the strength of fragments of skeletons and the nature 

 and intensity of the processes of breakdown. 



4. Distribution of Skeletal Particles 



The distribution of sediment-producing organisms on the sea floor is not 

 haphazard but varies systematically with changes in physical, chemical and 

 biological conditions. The specific factors that control the distributions are not 

 well understood, but it is often possible to relate the variety and abundance 

 of sessile organisms to the circulation of water. Circulation of water, or hydro- 

 graphy, as used here means the interaction of topograx^hy with water movements 

 — tides, waves, wind-drift, ocean currents, and run-off from the land — that 

 produces variations in temperature, salinity, nutrients, turbidity, and plankton. 



Thorson (1957, p. 461) has emphasized the distinction between the epifauna, 

 those animals that sit or crawl on rocks, stones, shells or vegetation, and the 

 infauna, those animals that live within the bottom sediments. The relationships 

 between the distribution of the infauna and water circulation may be estab- 

 lished in the larval stage. The larvae settle on particular types of sediment 

 because ' 'the types of bottom in the open sea will serve as rough indicators for 



