SECT. 3] SHALLOW- WATER CARBONATE SEDIMENTS 573 



The amounts of different skeletal particles are also a record of relative rates of 

 production, and they can provide a means of evaluating conclusions based on 

 short-term observations of the life cycle and density of organisms on the sedi- 

 ment surface. 



The sedimentary record of water circulation and organic production is not 

 without bias. As we have seen, varying resistance of different skeletons to 

 breakdown can distort the record toward the more resistant skeletons. More- 

 over, the sediment-churning of burrowing organisms can mix the records left 

 during widely different conditions of deposition. Despite these qualifications, 

 it is clear that the skeletal sedimentary record is a valuable source of informa- 

 tion and has the potential to provide additional data that will help us to luider- 

 stand organic populations and sedimentation. 



6. Non-skeletal Particles 



Many of the particles and features of shallow-water carbonate sediments are 

 not skeletal in the sense of the preceding section. Some, such as ooliths, are 

 generally believed to be physicochemical precipitates; others, such as the hard- 

 ened fecal pellets described by Ilhng (1954, pp. 24-25) and by Daetwyler and 

 Kidwell (1959, p. 11) from the Gulf of Batabano, owe their shape and perhaps 

 their hardness to organisms. However, because the criteria for the recognition 

 of biochemical and physicochemical precipitation are not well established, the 

 present discussion will treat them together as non-skeletal elements, following 

 Illing {op. cit.). The non-skeletal category includes all those particles and fea- 

 tures that are not skeletons or parts of skeletons. i The term is, unfortunately, 

 ambiguous, since many non-skeletal particles contain large amounts of com- 

 minuted skeletal debris; but because these aggregates are hardened by bio- 

 chemical and/or chemical precipitation, the particle as a whole is non-skeletal. 



The tw^o materials that are believed to be the products of non-skeletal 

 precipitation are ooliths and the cement between and within pellets and other 

 aggregates. A third material, the needle-shaped crystals of aragonite in the lime 

 muds of w^estern Great Bahama Bank, has long been considered a non-skeletal 

 precipitate. Lowenstam (1955, p. 270) suggested that these crystals are pro- 

 duced by the decay of lightly calcified green and red algae (sheaths and seg- 

 ments). The following abbreviated discussion will treat the possible modes of 

 origin for all three materials, their occurrence and abundance in the sediments, 

 and their relationships to the circulation of water. More detailed descriptions 

 and discussions of these non-skeletal materials will be found in the papers cited. 



A. Ooliths 



Ooliths are sand-sized sediment particles that consist of a core or nucleus 

 sm-rounded by one or more shells of aragonite (Fig. 5A). The nucleus may be a 



1 Some workers would prefer to regard the extra-cellular carbonate of calcareous algae 

 as a sort of non-skeletal precipitation produced as a result of algal photosynthesis. However, 

 because this carbonate has a distinct structure that is recognizable and varies in different 

 species, it is here considered as skeletal. 



