72 HISTORY OF THE OCEANS 



lithified section of terrigenous mud, as shown by Laughton's (1957) 

 data, which have been converted from pressure to depth (Fig. 8). 

 Ewing and Ewing (1959) give numerous examples of refraction 

 and reflections at depths below the water-sediment interface. 

 These horizons are supposed to be due to semiconsolidated sedi- 

 ment, and the foregoing analysis certainly bears them out. 



Most of the previous discussion has been concerned with con- 

 solidation and lithification in a clay-shale section. Hypotheses, and 

 the field data of oceanic seismic work, such as that previously 

 noted by Crary and Goldstein (1957) in the Arctic and by Ewing 

 and Ewing (-1959) in the Atlantic, coincide very well in indicating 

 the probabilities of depth to lithification. 



If one grants that the highest velocity expectable in a shale 

 section at relatively shallow sediment depths is less than 4 km/ 

 sec, then one has to explain the high velocities frequently found 

 under some areas of present red clay deposition, as in the north- 

 east Pacific. These velocities range from 4.5 to 6 km/sec (Raitt, 

 1956) (Fig. 9). In discussing the high- velocity second layers in 

 the Pacific the evidence is much more speculative. If the first 

 layer, alone, represents the entire sediment column, then there is, 

 indeed, an anonomalously thin layer of sediment in the Pacific. 

 The principal reasons advanced to explain this thin veneer of sedi- 

 ment have been: (1) the lower part of the sediment column has 

 disappeared owing to some catastrophic cause; (2) the lower part 

 of the column is undetectable by seismic means; (3) vast floods of 

 lava or other volcanics were spread over the ocean floors; (4) 

 the rate of deposition has been extremely low. 



As previously noted, the more important factors governing 

 depth to lithification in a calcareous section would normally be 

 the same as those involved in lithification of clay into shale, 

 namely pressure, chemical factors, and age. In areas of known 

 calcareous deposition, for example, the east Pacific calcareous 

 area, the first layer averages 260 m (variation from 200 to 340 

 m) ; in the equatorial area of maximum accumulation, three sta- 

 tions indicate a top layer of average thickness of 490 m [variation 

 380 to 570 m (Raitt, 1956)]. All these thicknesses are within the 

 previously discussed general range of thicknesses to lithification; 



