574 GINSBUKG, LLOYD, STOCKMAN AND MCCALLUM [CHAP. 22 



skeletal fragment, a fecal pellet, or a non-carbonate mineral grain. The shells 

 of aragonite have only concentric structure. i From their optical behavior, they 

 can be shown to consist of minute crystals of aragonite whose long axes are 

 tangential to the shells (Sorby, 1879, p. 74; IlHng, op. cit., pp. 36-38). 



Ooliths are generally believed to be the products of physicochemical 

 precipitation. The main evidence for physicochemical precipitation is indirect 

 but nonetheless rather strong: 



(1) Marine ooliths resemble those produced by artificial precipitation of 

 calcium carbonate in boilers (Cayeux, 1935) and water-purification plants. 



(2) Areas of oolith formation are characterized by shifting bottom sediments 

 and the lack of intense biological activity (Illing, op. cit., p. 43; Newell, Purdy 

 and Imbrie, 1960). 



(3) The oxygen and carbon isotope ratio of some Bahamian ooliths is different 

 from that of plants and animals living in the areas of oolith formation (Lowen- 

 stam and Epstein, 1957, pp. 371-372). 



The organic origin for ooliths has received little support from most recent 

 authors. NesteroflF (1956, p. 1049) has revived this interpretation on the basis 

 of the rings of organic matter that occur in contemporary ooliths. Newell, Purdy 

 and Imbrie (1960) have shown that some of this organic material is the remains 

 of boring blue-green algae that penetrate almost all calcium carbonate in 

 shallow-water sediments. However, these authors (ibid.) also call attention to 

 algal growths on the surface of Bahamian ooliths and suggest that these 

 growths may be responsible for some of the areas of unoriented crypto crystal- 

 line aragonite that occur within the oolitic shells. 



a. Occurrence and distribution of recent ooliths 



Regardless of whether the precipitation is chemical, biochemical, or a com- 

 bination of the two, the distribution of ooliths in contemporary marine sedi- 

 ments provides some idea of the water conditions necessary for their formation. 

 Oolitic particles or those that have an external coating of concentric aragonite, 

 regardless of thickness, occur in sediments from the Great Bahama Bank, from 

 Florida, from the Gulf of Batabano in Cuba, and from the eastern Mediter- 

 ranean off Eg3rpt and Tunisia. The Bahamian occurrence appears to be by far 

 the largest and most thoroughly studied. Ilhng {op. cit., pp. 41-42) has de- 

 scribed the somewhat confusing interpretations of early students of Bahamian 

 ooliths. More recently, Black (1933, p. 462), Illing {op.cit.,-pY>. 35-43) and Newell, 

 Purdy and Imbrie (1960) have all agreed that the oolitic sands of the surface 

 are contemporary and that they are produced by physicochemical precipitation. 

 The contemporary formation of ooliths has been verified by a radio-carbon 

 date of less than 225 years for the exterior 10% of ooliths from the Cat Cay area 

 on the northwestern margin of Great Bahama Bank (Olson and Broecker, 

 1959). 



1 The ooliths found in contemporary marine sediments do not have the radial structure 

 that is characteristic of fossil ooliths. 



