68 HISTORV OF THE OCEANS 



layers in the sediment column and further exploration may well 

 show shallower reflecting horizons. The presence, in all the ocean 

 basins, of a topmost layer on the order of 300 m in thickness is 

 important evidence on the possible depths to lithification previ- 

 ously discussed and is probably the expression of a universally 

 valid phenomenon. 



Compressional Wave Velocities in Deep-Sea Sediment and Rocks 



Unlithified Sediments 



Until about 1954, very little was known about velocities in the 

 surficial, unlithified sediments of the sea floor. Until that time, 

 readable returns were not usually obtained from seismic studies, 

 and, as a rule, velocities were assumed for these sediments. Within 

 the past few years, however, studies by Laughton (1954, 1957), 

 Nafe and Drake (1957), Sutton et al. (1957), Shumway (1956, 

 1958a, b), and Hamilton et al. (1956), plus the work at sea by the 

 Lamont Geological Observatory, Cambridge University, Woods 

 Hole Oceanographic Institution, and Scripps Institution, have indi- 

 cated that these velocities vary with \'arious physical properties 

 between 1.43 and 2.12 km /sec. The \-elocities above about 2.0 

 km /sec indicate some degree of lithification. 



Lithified Sediments 



The general subject of seismic velocities in lithified sediments 

 (sedimentary rocks) is of great importance in seismic exploratory 

 work in the oil industry, and lately, due to seismic work at sea, 

 the subject has assumed critical importance in our understanding 

 of the structure and history of the ocean basins. As a result of this 

 double interest there has been considerable laboratory and field 

 work to determine the velocity of sound in sedimentary rocks. 



The velocity of sound in rocks has been shown to increase with 

 decreasing porosity, increasing rigidity, age, mineral constituents, 

 and other factors. In shale there is a regular increase of velocity 

 with depth (as porosity decreases) and with age (see, for example, 

 Faust, 1951). For limestone, however, porosity variations con- 

 siderably outweigh the influences of difi^ering materials, pressures, 



