Stratigraphic, Structural, and Correlation Considerations 



39 



deposited sediments will be removed, and the erosion surface will be- 

 come an unconformity should deposition follow this erosion. 



Recognition of Unconformities 



There are a number of signposts that are useful in detecting the 

 presence of unconformities. Some of these, such as a basal conglom- 

 erate, have become traditions, although experience shows that they may 

 be the exception rather than the rule. There are probably more uncon- 

 formities without a basal conglomerate than with one. 



Unconformities are best recognized from direct observation in a 

 single outcrop. Such an outcrop might be observed in a road cut, a 



2800 feet more of Paleozoic strata 



Bright Angel Shale 



I 

 ^ I 



X /Archean. Granite/ - 



»:; \'\ i; _ ^ N V / \ x^^ 

 ^ -^ i ^ , ' \ ^ / , \ , I ' 



Figure 16. Diagrammatic cross section of inner part of Grand Canyon, showing Ep- 

 Algonkian and Ep-Archean unconformities. 



quarry, a surface exposure on canyon walls, a ravine, or a cliff. For ex- 

 ample, probably the most striking exposure of an unconformity can be 

 seen in the Grand Canyon of the Colorado River, in the walls of which 

 there are two unconformities exposed. Sharp ^"^ has described them 

 thoroughly; the diagrammatic section shown in figure 16 is from his 

 article. 



In the subsurface the recognition of unconformities depends on the 

 ability of the geologist to recognize any of the many criteria for uncon- 

 formity from the sparse evidence afforded by well cuttings, cores, and 



^' Sharp, R. p., Ep-Archean and Ep-Algonkian Eroiion Surfaces, Grand Canyon, Arizona: Geol. Soc. 

 Americ?i §un,, vol, 51, pp. 1235-127Q, 194Q, 



