33. THE PRESERVED RECORD: PALEONTOLOGY OF 

 PELAGIC SEDIMENTS 



W. R. RiEDEL 



1. Introduction 



The i^aleontology of the deep and relatively permanent ocean basins represents 

 the last frontier of a discipline which, in the course of its development over the 

 past hundred years, has been largely concerned with fossil remains deposited in 

 epicontinental seas, on continental shelves, and in terrestrial environments. 

 Study of fossil assemblages deposited in shallow marine waters has yielded a 

 great deal of information about the history of the oceans, but this field is ex- 

 cluded from consideration here because its most significant results have recently 

 been summarized in the Treatise of Marine Ecology and Paleoecology (edited by 

 Hedgpeth and Ladd, 1957). Differences which exist between deep-sea and 

 shallow- water paleontology depend principally on the different types of organ- 

 isms constituting the bulk of the paleontologic records of the two kinds of 

 environment, and the relative stability and uniformity of the deep-sea environ- 

 ment. More superficial differences in methods of study result from the relative 

 inaccessibility of ancient deep-sea sediments. 



As the terms "pelagic" and "planktonic" tend to be confused by some geologists and 

 paleontologists, it may be well to explain their usage in this chapter. "Pelagic " is here used 

 to mean "of the open sea", as in the classical usage of the word. Thus, sediments deposited 

 near shores or in narrow bodies of water cannot be "pelagic". "Planktonic" is applied to 

 organisms to denote incapability of strong directed movement — their distribution is to 

 a great degree dependent upon water currents. Under certain circumstances, .sediments 

 containing a high proportion of planktonic microfossils can accumulate in a non-pelagic 

 environment. 



2. Fossils of Deep-Ocean Basins versus Those of Shallow Waters 



Sediments deposited in shallow marine waters present a wide range of bio- 

 genous comjjonents, from coquinas and reef deposits formed almost exclusively 

 of the remains of benthic macro-organisms to fine-grained calcareous and sili- 

 ceous sediments consisting largely of skeletons of planktonic micro-organisms. 

 Sequences of such sediments at any one locality are not generally uniform 

 through periods of many millions of years, and at any one time the areal extent 

 of a uniform sedimentary unit seldom exceeds a few hundred square miles. 

 Changes in the physical and chemical characteristics of shallow and nearshore 

 waters occur relatively frequently, affect relatively small areas, and are reflected 

 by sharp and frequent changes in the specific and general composition of the 

 associated fossil assemblages. Abundance, rarity or absence of any fossil group 

 in a shallow-water sedimentary rock is a function mainly of the contemporary 

 presence or absence of that group of organisms at the locality, dilution of the 

 fossils by inorganic sedimentary comjjonents, and processes during or after 

 lithification which may have affected the skeletal material physically or 



[MS received August, I960] 866 



