64 HISTORY OF THE OCEANS 



An interesting and significant question in the foregoing sequence 

 of consolidation is: at what point does the clay cease to be an "un- 

 consolidated" sediment, become lithified and called "shale"? 

 This transition, according to the foregoing hypothesis, apparently 

 comes at around 35% porosity, when, due to chemical changes, 

 cementation between the grains imparts a rigidity to the structure 

 which previously was not present (Fig. 4). Skeels (1950) noted 

 shales with porosities up to 43%; Birch et al. (1942) recorded a 

 highest shale porosity of 45%. 



Obviously the change from a clay to a shale will occur at dif- 

 ferent depths and porosities dependent on pressure and chemical 

 effects, but a reduction to porosities between 35% and 45% is in- 

 dicated by hypothesis and field data to be the usual range when 

 clay turns to shale. The appearance of the porosity-depth curve 

 pre\'iously shown indicates that these porosities should be reached 

 at depths between 150 and 700 m depending on chemical factors 

 and grain densities, and with an average depth, around the area 

 of maximum curvature, of about 200 to 400 m. 



Calcareous Ooze 



The depth in a sediment column at which lithification of cal- 

 careous material begins is not as well indicated as for a clay shale. 

 Calcium carbonate is easily cemented and replaced by other 

 minerals and without great pressure; this is evidenced by the 

 formation of beach limestone and by the occurrence of high- 

 porosity foraminiferal limestones on the tops of seamounts (to be 

 mentioned later). The factors invoh'ed in the lithification of cal- 

 careous ooze should normally be the same as those for clay shale : 

 pressure, age, and chemical factors being the most important. 

 If calcium carbonate is relatively easy to convert into rock, then 

 one would expect that the depth to lithification in a calcareous 

 section would certainly be no greater (and might be less) than that 

 in a clay-shale section. This would place the expected limits on 

 the thickness of any topmost unlithified section at about the same 

 depths as in a clay-shale section, that is, 150 to 700 m with average 

 depths nearer 300 to 400 m. Areas of faster accumulation might 

 have greater thicknesses of unlithified sediment, as it is known 



