Stratigraphic, Structural, and Correlation Considerations 



41 



modifications as shown in table 2. A common or an exclusive association 

 is indicated by xx, an occasional or rare association by x, and an associa- 

 tion that probably does not occur by a blank. A number of these criteria 

 are included in the subsequent discussion. 



Structural Features. Discordance in Dip : Lack of parallelism in beds 

 on both sides of an unconformable contact may be readily observed in 

 vertical sections such as the face of a cliff. Discordance alone should not 

 be accepted as positive proof without further observation, especially if 

 the exposure is a small one. Faulting, contemporaneous erosion, or large- 

 scale cross bedding may produce discordance. 



The presence of dip discordance in a sporadically cored subsurface 

 section may suggest, but does not prove, angular unconformity. A number 



FACIES CHANGE 



CROSSING AXIAL SURFACE 



FLOWAGE DEFORMATION 



UNDETECTED FAULT 



HOLE DEVIATION 



FALSE- BEDDING 



Figure 17. Diagrammatic sketches showing conditions involving spot cores the dip 

 variances of which may suggest unconformity. 



