22 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



Geosynclines 



Geosynclines have many characteristics of basins (Jones, 1938), 

 though they are not necessarily enclosed in character. In fact they 

 are simply low areas in which thick deposits of sediments accumulate. 

 Because of the presence of cross-bedding, ripple marks, and alterna- 

 tions of sand, silt, and clay, many people regard geosynclines as essen- 

 tially shallow, but they may be deep in part. Recent work in the 

 Gulf of Mexico (Sverdrup Anniversary Volume, 1948, p. 683) has 

 shown that cross-bedded sand and silt are found in deep water in 

 the middle of the Gulf of Mexico. More likely, the depth at which the 

 deposits form in a geosycline represents a balance between rate of 

 sinking of the basin and rate of influx of debris. 



When studying the origin of sediments in ancient geosynclines, ge- 

 ologists should consider the fundamental factors of quantity and qual- 

 ity of debris supplied the basin, particularly the size of the particles, 

 and the probable currents that influenced the deposition of the sedi- 

 ments, especially the possibility of longshore currents which could 

 transport debris far from the mouths of rivers that supplied the detri- 

 tus. An understanding of the shape of the geosynclinal basin should 

 also help in interpreting the mode of origin of the sediments. 



Continental Slope 



Deposits on the continental slope, that is, on the slope from the flat 

 continental shelf down to the abyssal deeps, are conditioned by the 

 distance from land, the existence of currents or turbulence along this 

 slope, the angle of slope, and the type of debris supplied. The deposits 

 in general are fine-grained, but they may consist of sand. The sedi- 

 ments also may have unusual skewed size distributions in places where 

 material swept from the adjoining flat continental shelf is added to the 

 normal supply of debris that is deposited. The angle of slope ranges 

 from less than 1° to more than 10°. If the deposits are relatively 

 fine-grained, as they are likely to be, conditions of instability of slope 

 may prevail, with the result that considerable slumping may take 

 place (Fairbridge, 1946). Deposits in which the layers of sediments 

 seemingly have been deformed contemporaneously with sedimentation, 

 therefore, may be continental slope deposits. Some of the Franciscan 

 sediments, which are so badly distorted, may have such an origin. 



Continental Shelf 



A continental shelf extends outward from shore, 10 to 500 miles in 

 many parts of the world (Shepard, 1932). This shelf slopes only a 



