than 30%; i.e., if it is not significantly diluted by terrigenous or 

 siliceous materials. Generally, dilution by other materials is significant 

 only near shore; on abyssal plains; and in the high productivity, 

 siliceous ooze areas. The CCD has been mapped on a worldwide basis 

 and is shown in Figure 2-2; one can determine whether calcareous ooze 

 may be found by comparing the actual water depth with the CCD. 

 Calcareous ooze typically becomes more coarse as water depth decreases 

 (Figure 2-3). The equatorial Pacific typically has alternating bands of 

 siliceous and calcareous ooze. Where biogenic (calcareous and siliceous) 

 oozes are not found, one finds pelagic clay, an extremely slowly sedi- 

 mented material composed primarily of wind-blown dust. 



2.3.3 Terrigenous Sediments 



Terrigenous sediments are the most complex and varied of the 

 sediment types. The typical terrigenous material is probably a slightly 

 plastic clayey silt; however, vast sand beds and some plastic clay 

 deposits also exist. Layered deposits of sand, silt, and clay are 

 common. Sedimentation rules are difficult to define. In a stable envi- 

 ronment, grain size would decrease with distance from shore; however, 

 since dynamic processes are always active, this often does not occur. 

 If the sea level is rising (e.g., off the east coast of the United 

 States), one can almost assume that grain size will become finer near 

 shore. For any particular location, an expert is probably available to 

 estimate the types of sediments that can be expected. The charts of 

 the National Ocean Survey* also provide estimates of sediment type, 

 although some of the classifications do not relate very well to engineer- 

 ing application (e.g., "brown mud"). However, the split between sand 

 and clay or silt ("mud") appears reliable. 



Sediment classification on the basis of grain size or origin may not 

 be adequate for engineering application. The design engineer should 

 also know something about the state of the sediment. For cohesive 

 sediments, three terms are important: (1) overconsolidated, (2) nor- 

 mally consolidated, and (3) underconsolidated. 



*Formerly U.S. Coast and Geodetic Survey, U.S. Department of 

 Commerce . 



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