22. SHALLOW-WATER CARBONATE SEDIMENTS 



R. N. GiNSBURG, R. Michael Lloyd, K. W. Stockman 



and J. S. McCallum 



1. Introduction 



The purpose of this chapter is not to review the ])ubhshe(l literature on 

 shallow-water carbonate sediments nor to consider many of the specific 

 chemical and physical aspects of these sediments. Instead, this review will 

 present the argument that carbonate sediments contain a faithful record of the 

 hydrographic milieu and the activities of organisms at the time of sediment 

 deposition. This record is frequently more complete and more subtle than that 

 ])resented by sediments composed of terrigenous elastics, for in the latter only 

 the depositional features of the sediments — their grain size, packing and sedi- 

 mentary structures — reflect the environment of deposition. In the carbonate 

 sediments, the particles themselves are produced in the environment of de- 

 position. As a result, the variety and abundance of the constituent particles 

 can be related to hydrographic conditions. 



The material to be presented will be of most interest to geologists. Eco- 

 logists may also find useful ideas and data on the nature of the sea-floor 

 sediments as a substratum and as a long-term record of the distribution and 

 ])roductivity of shelled marine organisms. For the oceanographer this chapter 

 will suggest that variation in shallow-water sediments may reflect A^ariation 

 in gross water movements, a record that is difficult to develop from limited 

 instantaneous observations. 



The presentation is divided into two parts : the first treats the formation and 

 distribution of particles derived directly from the skeletons of marine organ- 

 isms (skeletal), and the second describes the nature and distribution of particles 

 whose size, shape and consistency are a result of biochemical and/or physico- 

 chemical processes (non-skeletal). 



2. Types of Skeletal Particles 



In many places on the shallow-sea floor, carbonate sediments are composed 

 largely or even entirely of the whole and fragmented skeletons of marine plants 

 and animals. Mollusk shells and fragments are the most obvious and familiar 

 skeletal remains in sediments, but a wide variety of marine organisms also 

 contribute — algae, corals, Foraminifera, echinoderms, worms, sponges, alcyo- 

 narians and many others. Skeletons of these organisms vary considerably. 

 They differ in size, shape, specific gravity, microstructure, mineralogy and 

 chemical composition. Many of the variations are sufficient to produce differ- 

 ences between the sediments derived from one or another type of skeleton; 

 others can be magnified by selective organic and mechanical breakdown to 

 yield difi'erent sediments. These variations in the nature of the skeletons 

 determine to a large degree the grain size, mineralogy and chemical composition 

 of skeletal sediments. 



[MS received December, 1960] 554 



