404 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



it can be formed on a large scale except by the method of migrating 

 dunes. It is true that a series of migrating sand-bars might produce the 

 same, or very similar cross-bedding, except for the fact that the older 

 sand-bars are apt to be entirely removed during the progressive encroach- 

 ment of the sea, and that the areal extent of a formation built as a sub- 

 marine sand-bar can be only of very limited extent, at least in so far as 

 one of its horizontal dimensions is concerned. A structure of this kind 

 may also be formed in subaerial delta fans wherever the material of the 

 fan suffers rearrangement by wind. 



STRUCTURE OF THE MARGINAL DELTA 



A delta built into the sea or into a body of standing water is generally 

 much more complex than one of the subaerial type. In addition to the 

 subaerial portion, there is the subaqueous part, which consists mainly of 



Figure 2. — Diagram of complex Bedding 

 Characteristic of eolian deposits 



the foresets and bottomsets. The foresets will normally become tangent 

 to the bottomset beds, and as the angle of the foresets decreases toward 

 the periphery the distinction between foreset and bottomset may fade 

 away. At the point of merging of the foreset into the bottomset beds 

 there is often an excessive thickening over other parts of the same layer. 

 In some deltas foreset beds may become inseparable from top- or bottom- 

 set beds. 



Deltas built in regions of subsidence are apt to show portions of ter- 

 restrial origin, alternating with layers of sediments carrying a marine 

 fauna, thus indicating that a portion of even the topset beds may be 

 formed as submarine, shallow-water deposits. Thus occasional interpo- 

 lation of marine layers is to be expected in delta deposits on the seacoast, 

 and there should be expected more or less of a mergence of the landward 

 portion of terrestrial origin and the seaward portion of marine origin. 



