EPICONTINENTAL SEDIMENTATION 483 



receive few coral larvae. Daly's mechanical abrasion after killing 

 of the corals would have left many stunted atolls and others with 

 broad platforms 100 m deep outside the rim. 



Differentiation of Epicontinental Environments 



The type of sediment accumulating in a given locality depends 

 on the two factors reviewed above, source and transportation. 

 Without sources of sedimentary matter there can be no accumula- 

 tion, and apart from sources in situ, there can be no supply with- 

 out movement of the source materials toward their ultimate desti- 

 nation. However, the immediate surroundings also play a part in 

 shaping the final product. Postdepositional solution of lime, decay 

 of organic matter, reworking by waves, currents, or organisms, 

 and desiccation may each have been active, but the environment 

 and the two other factors are not entirely independent. The area 

 in which a uniform type of sediment accumulates has a certain 

 extent, often a large one, and therefore transportation must con- 

 tinue within the boundaries of the environment. In other words, 

 there is transportation within the area itself. This may happen 

 along the bottom, or the final lap of the transportation may be 

 limited to vertical settling, e.g., for clay, windborne and volcanic 

 dust, planktonic shells, and materials for Faulschlamm. In such 

 cases, the absence of horizontal transportation along the bottom 

 is a major condition producing that special type of environment. 

 Some environments are closely linked with their source, for in- 

 stance, deltas, tidal flats, beaches, reef lagoons, reef aprons, and 

 glacial marine areas. Even more intimate is the link between source 

 and sediment when the former is in situ, which is the case with 

 organic reefs and shell beds, oolitic sands, and brackish swamps. 



Given the fact that the two major factors in supplying the sedi- 

 ment are usually closely tied up with the nature of the "environ- 

 ment," the latter concept, taken in its broader sense, should con- 

 stitute the most important element in determining what sediment 

 is finally deposited. Looked at from this angle it may next be 

 questioned what are the chief factors bringing about the differ- 

 ences between various environments. The answer to this must de- 



