EPICONTINENTAL SEDIMENTATION 485 



shallow water, is normally characterized by finer sediment and 

 sw'ifter accumulation than its surroundings. 



The degree and depth of communication with the open sea 

 determines salinity and temperature, and as exchange deteriorates, 

 the influence of a dry or a humid climate increases. Likewise the 

 barring of ocean swell and storm waves, not to speak of tsunamis, 

 causes fundamental changes in the environment. Tidal range may 

 be increased or suppressed by various degrees of isolation. The 

 most obvious example is the sharp contrast in sedimentary condi- 

 tions between inland seas like the Baltic or Hudson Bay, on the 

 one hand, and the open shelves, on the other. The former collect 

 all sediment carried in, whereas the shelves loose the bulk of the 

 clay-sized sediment to the oceans. Partly enclosed seas like the 

 North Sea and the Java Sea are intermediate because they are 

 largely nondepositional. 



The degree of ventilation of the bottom waters is a matter of 

 great consequence to sediment formation. Solution of lime and de- 

 composition of organic matter depend on ventilation. Its absence 

 will result, in extreme cases, in suppression of benthonic life by 

 anaerobic conditions, and iron sulfides and calcareous concretions 

 may form in the sediment. 



A final aspect of topographic influence on sedimentation is the 

 amount of freshwater supply, a matter largely dependent on 

 topography. Freshwater usually means that colloidal sediment and 

 nutrients are furnished. Sand and even gravel may also be carried 

 out to sea. In estuaries and fjords brackish conditions with a spe- 

 cific fauna can be induced. Anaerobic conditions are more likely 

 to develop when surface waters are of low salinity. 



A remarkable result of the meeting between freshwater and 

 salt water is the establishment of various kinds of estuarine circu- 

 lation with outgoing brackish and inflowing sea water. In some 

 cases, such as the northern tributary of the Orinoco, this leads to 

 the accumulation of clay within the estuary during the rainy 

 season. The conditions happen to be such that removal in the dry 

 season takes place because of the more powerful wave action at 

 that time. In the Congo removal of sand appears to occur during 

 river flooding. There are also other situations in which the clay 



