STRATIGRAPHY OF THE DEEP-SEA FLOOR 53 



solidated," "semiconsolidated," and "consolidated" sediment, with 

 the term "consolidated" indicating lithification. 



The consolidation and lithification of deep-sea sediments, the 

 velocities of compressional elastic waves to be expected as a conse- 

 quence of such changes, and the probability that the second layers, 

 in many areas, are lithified deep-sea sediments will be discussed 

 here. 



Consolidation of Sediments 



When a sand or coarse silt is deposited, it rolls into a position 

 supported by other grains; intermolecular forces are too small to 

 be effective. When a fine silt or clay is deposited, it is likely to 

 adhere to the grain with which it first comes in contact, owing to 

 intermolecular attractive forces. For a long time the structure of 

 a clay deposit was thought to be that in Fig. le. Recent work by 

 Rosenqvist (1959) and others using the electronic microscope has 

 show^n that the structure of a marine clay is more like the "house 

 of cards" structure in Fig. 1/. Each mineral grain of the clay-size 

 particles has its own adsorbed water layers which merge gradually 

 into the free water within the pore spaces. We know from analysis 

 of deep-sea clay, for example, that this pore space, filled w^ith 

 water, may comprise more than 80% of the sediment. 



When a natural or man-made load is applied to this last struc- 

 ture, the volume decreases because of the escape of water from 

 the pores. The rate of volume reduction or consolidation is pri- 

 marily dependent on the amount and rate of load application and 

 the permeability of the sediment. In a sediment with an impervious 

 base, the escape of the water is through the w^ater-sediment inter- 

 face. When a load is suddenly applied to a sediment structure as, 

 for example, during the construction of a building, the load is at 

 first borne by the porewater as excess hydrostatic pressure, but 

 it is gradually transferred to the sediment structure as the sedi- 

 ment slowly consolidates under the new load. Primary consolida- 

 tion is completed when the excess porewater pressure is zero. A 

 secondary consolidation may continue for some time; it is thought 

 to be due to the plastic defomiation of grains and of the adsorbed 



