76 HISTORY OF THE OCEANS 



clay alone, which allows time, since the Middle Cretaceous, for 

 pure deposits of calcareous ooze to have been formed. 



The view has been expressed that the second layer of high- 

 velocity material cannot be limestone because the geochemical 

 balance cannot tolerate that thickness and volume of calcareous 

 material. As is well known by geophysicists, these high velocities 

 are from the top of such a layer and have no bearing on the compo- 

 sition of the layer at depth. With this fact in mind it is no longer 

 tenable to suppose that the second layer velocity cannot indicate 

 limestone because of the geochemical balance. The total thick- 

 ness of sediments and sedimentary rocks in an ancient ocean basin 

 is likely, in many places, to be the total thickness of the upper 

 unlithified layer and the lower, higher-velocity layers. In the north- 

 east Pacific one result of this would be that the total thickness of 

 the first and second layers should be thinner in the north and 

 should progressively thicken toward the south to reflect the south- 

 ward movement of warmer isotherms during the Tertiary, and 

 higher rates of deposition in areas of calcareous deposition. The 

 calcareous surface should continue to dip to the north from its 

 present outcrop area near the equator, which has actually been 

 observed by Arrhenius (1952). 



Volume of Sediments and Ages of Ocean Basins 



Some of the other interesting results of applying soil mechanics 

 concepts to deep-sea sedimentation and stratigraphy are in stud- 

 ies of the total volume of original sediments deposited in specific 

 localities, the volume of solids present, and in studies of the ages 

 of ocean basins. Standard soil mechanics computations allow the 

 prediction of settlement in a sediment column provided the void 

 ratios before and after consolidation are known (or estimated). 

 Conversely, it is also possible to determine approximately how 

 much sediment was required to consolidate to present thicknesses 

 (Hamilton, 1959). The amounts of clay necessary to consolidate 

 to present thicknesses are surprisingly large: 1000 m of original 

 deposition will consolidate to about 500 m; it took more than 

 5000 m of clay to form a present consolidated section 2000 m thick. 



