732 



PRINCIPLES OF STRATIGRAPHY 



ceeded upward by other beds of fine-grained terrigenous material 

 or by limestones derived from organic sources. Such a black 

 basal shale may also result from the washing of the residual black 

 soil from the surface of the plain into the shallow encroaching 

 sea. In any case, the basal black shale will rise diagonally across 

 the planes of synchroneity, and, although it will constitute a lithic 

 unit, it is not a stratigraphic unit, but made up of the shale ends 

 of a successive series of deeper water formations. This relation- 

 ship is shown in the following diagram. (Fig. 147.) An example 

 of this type of basal bed is found in the Eureka (Xoel) black shale 

 of the ]\Iississippi ^"alley. This formation rests generally upon 

 eroded Ordovicic strata, the contact being a disconformable one. 

 Upward it passes into limestones, which in southwestern Missouri 



KH 



Fig. 147. Diagram of overlap of marine strata on basal black shale in south- 

 western Missouri and northern Arkansas. 



are of Chouteau age, the shale itself carrying Kinderhook fossils, 

 while in northern Arkansas it is succeeded by limestone carrying 

 Burlington fossils. The same relationship exists in the southern 

 Appalachian region, where the black shale has risen into the base 

 of the Keokuk, if not higher. (Grabau-3.) 



2. Regressive Movements. 



c. Rate of Depression Is Exceeded by Rate of Supply. In this 

 case accumulation will go on so fast as to fill in the shallow shore 

 zone, when the coarse material begins to extend seaward, progres- 

 sively overlapping the off-shore deposits in a seaward direction. 

 We will thus have a gradual change in the character of the sedi- 

 ment from the lutaceous material at the bottom to arenaceous and 

 sometimes coarser terrigenous material. This material will all be 

 land-derived, and, as it is deposited rapidly, not a very thorough 

 sorting can generally be expected. As the shore migrates seaward, 

 subaerial deposits may accumulate above it. (Wilson-9: up.) 



