Stratigraphic, Structural, and Correlation Considerations 



35 



The term "nonconformity" has been used extensively in the litera- 

 ture to include unconformable contacts of sedimentary rocks with meta- 

 morphic or plutonic rocks that have been subjected to erosion before 

 deposition of sediments. Billings ^^ says that the term is utilized most 

 satisfactorily for unconformities where the older rock is plutonic in or- 

 igin, and uses "angular unconformity" to designate the unconformable 

 contacts of discordant strata. Willis,^" however, suggests that the term 

 be used in a broader sense to include any unconformity that is not a dis- 



FiGURE 14. Core section showing the effect of sliding movement in sediments during 

 the hydroplastic stage. Structural interpretations cannot be depicted from data 

 of this type. 



conformity. This is probably the most satisfactory application of the 

 term. 



A nonconformity or angular unconformity involves a surface sep- 

 arating two or more stratal units at an angle of discordance, which may 

 vary from 90 degrees to less than 1 degree. If the latter variation exists, 

 it is sometimes difficult to determine whether the discordance angle is 



' Billings, M. P., Structural Geology, p. 243, New York, Prentice-Hall, 1942. 



' Willis, Bailey and Robin, Geologic Structures, 3d ed., rev., New York, McGraw-Hill Book Co., 1934. 



