260 



Sediments 



measurements of specific gravity, 2.65, are 

 presented by the curves of Figure 210. 



Even though a core sample may contain 

 more than 100 per cent water by dry weight 

 (or a porosity greater than 72 per cent), a 

 section of silt-clay core 10 cm in diameter 

 and 30 cm long may be stiff enough to be 

 held at one end in a horizontal position for 

 several minutes without collapsing. Al- 

 though water may easily be extracted from 

 the top of a core by suction through a 

 microfilter or by twisting it in a cloth, water 

 is much more difficult to extract from the 

 stiffer sections of a core at depth. 



The resistance to bulk deformation is a 

 function of water content and can be ex- 

 pressed in at least two ways, plasticity and 

 shear strength, both of which are influenced 

 by thixotropy. The plasticity of sediment is 

 measured in terms of Atterberg limits (Ter- 

 zaghi and Peck, 1948, pp. 32-33). The Hquid 

 Umit is the water content (percentage dry 

 weight) at which two sections of a standard 

 pat of sediment flow to barely touch each 

 other when subjected to sharp blows under 

 standard conditions. The plastic limit is the 

 lowest water content at which the sediment 

 can be rolled into 3-mm diameter threads 



without crumbling. A sediment is consid- 

 ered to be liquid, plastic, or semisolid ac- 

 cording to whether its water content is 

 greater than the Hquid limit, between the 

 liquid and plastic limits, or less than the 

 plastic limit, respectively. Measurement of 

 Atterberg limits on several cores (Emery and 

 Rittenberg, 1952; Emery and Terry, 1956) 

 showed that the limits decrease with depth 

 in cores, but the decrease for the plastic 

 limit is commonly less than that for the 

 liquid limit and the decrease for the liquid 

 hmit in turn is less than that for the actual 

 water content (Fig. 211). Thus the basin 

 sediment usually must be classed as liquid 

 at the top, plastic at depth, and semisolid at 

 greater depths than is reached by ordinary 

 cores. Certainly the sediments are semisolid 

 when compacted to shale. Atterberg limits 

 for the basin cores are higher than those ob- 

 tained by White (1949) for pure illite and 

 kaolinite but lower than his values for mont- 

 morillonite, conforming with the known 

 mixed clay mineralogy of the basin sediments. 

 Even though the tops of cores are liquid, 

 as defined by Atterberg limits, in nature they 

 may not be mobile until after they become 

 disturbed. Lack of mobility is exhibited by 



Q3 



SANTA BARBARA SANTA CRUZ 

 4701 4700 



SANTA CATALINA WEST CORTES 

 1633 4674 



50 100 150 50 100 150 50 100 150 50 100 150 200 



SEDIMENT WATER CONTENT— % DRY WEIGHT 



Figure 211. Atterberg limits 

 measured on cores from sev- 

 eral basins as compared with 

 actual water contents of the 

 same cores. Left side of cross- 

 hatched area is plastic limit; 

 right side is liquid limit: sepa- 

 rate line is actual water content. 



