5. THE UNCONSOLIDATED SEDIMENTS 



J. I. EwiNG and J. E, Nafe 



1. Introduction 



In this discussion the unconsohdated sediments are taken to be those in 

 which the speed of compressional waves is less than about 4 km/sec. The upper 

 hmit is entirely arbitrary and has been chosen to include all the material with 

 velocities lower than that of "Layer 2", which is discussed by Raitt in the 

 next chapter. Layers in which the velocity is between 2 and 4 km/sec have been 

 described by various investigators as semi-consolidated sediments. Other 

 authors have designated layers as unconsolidated, semi-consolidated and con- 

 solidated sediments and rocks. In almost every case the designation has 

 been made solely on the basis of seismic wave velocity, since identification 

 according to sediment type, age or stratigraphic position has not been possible 

 except in certain shallow-water areas where wells have been drilled nearby. 



Most of the marine seismic measurements reported to date have been made 

 in the Atlantic and Pacific Oceans and in the shallower inter-continental seas. 

 In all areas investigated, the thickness of the unconsolidated sediments varies 

 widely from zero on some of the topographic highs to several kilometers in 

 some of the deeps and in some continental rise areas. The relationship of 

 sediment thickness and topographic setting has become increasingly more 

 understandable, and even predictable, in recent years since Kuenen introduced 

 the concept of transport of sediments by density or turbidity currents. This 

 concept has been thoroughly verified by sediment coring and seismic measure- 

 ments. It is now well known that even the coarser sediments derived from 

 continental erosion can be carried great distances from land, flowing out into 

 the ocean basins and being empounded in the deepest accessible areas (see 

 Chapter 14 of this volume). It is also known from underwater photography, 

 coring, dredging, and seismic measurements that many topographic highs are 

 devoid of even pelagic sediments, presumably being kept clean by deep currents 

 or by slumping. 



In addition to the variation of sediment thickness with topographic setting, 

 there is a marked difference in average thickness of sediments in the Atlantic 

 Ocean and in the Pacific. The Atlantic has an average of more than 1 km 

 (Ewing and Ewing, 1959), compared with less than | km in the Pacific (Raitt, 

 1956). This difference is generally attributed to the facts that the Atlantic is 

 much smaller and that more large, sediment-bearing rivers flow into it. 



The thickness of sediments cannot always be measured exactly by seismic 

 methods, owing to the difficulty of measuring the velocity, particularly in the 

 uppermost part of the section in the deep ocean. Refracted waves give accurate 

 measurements of the higher velocity layers ; but because of various factors 

 such as poor velocity or acoustic impedance contrast between the water and 

 upper sediments, or between two sediment layers, clear reflected or refracted 

 waves from those layers are sometimes not observed. Fig. 1 shows the time- 

 distance graph and structure section for a profile recorded in tlie South Atlantic 



[MS received July, 1960] 73 



