STRATIGRAPHY OF THE DEEP-SEA FLOOR 59 



computing pressures within the sediment. Several lines of evi- 

 dence indicate the probability that hydrostatic uplift is near 100% 

 in the upper sediment-rock column to the depts considered in this 

 study. In 1936, Terzaghi found hydrostatic uplift was effective up 

 to 99% in a stiff, plastic Tertiary clay of low permeability, and, 

 in concrete, with porosities as low as 13%, it was 100% effective. 

 In the field of soil mechanics, 100% hydrostatic uplift is allowed 

 regardless of the thickness of clay strata. In oil field reservoirs, 

 pressures are usually hydrostatic and not the combined pressure 

 induced by the weight of the rock and the water. In some sealed 

 reservoirs, however, the pressure is that of the rock and the water 

 (Levorsen, 1954). 



The porosity-depth curve (Fig. 4) for Tertiary sediments and 

 shale of the Gulf Coast (Dickinson, 1953) is close to the computed 

 curves for clay with 100% hydrostatic uplift. These Gulf Coast 

 sediments and rocks were deposited in continuous sequence with- 

 out major unconformities and usually have the normal or hy- 

 drostatic pressure* at depth. The position of the curve, and the 

 reservoir pressures, argue for full hydrostatic uplift of the mineral 

 grains of the rock. It should be expected that curves for shale will 

 lie on the low porosity side of those for clay: cementation should 

 effect a reduction of porosity not present in clays. 



In generalizing the consolidation characteristics of clay I used 

 one of Skempton's (1953) median curves for clay and the porosity- 

 pressure relations of thick shale sections as derived by Skeels 

 (1950) for a large number of shale sections. For calcarous ooze 

 Laughton (1957) data were used. After the overburden pressures 

 are computed,, the curves can be entered and the void ratio ex- 

 pectable under that pressure can be determined. Knowing the 

 properties, or assuming reasonable properties, for the sediment 

 at the water-sediment interface, it is then possible to construct a 

 table showing the probable variations of various properties at 

 depth in the sediment using standard soil mechanics computa- 

 tions (Tables I and II). From the tables, curves can be drawn 



* For the purposes of this paper all pressures are computed for full hydrostatic 

 uplift. 



