Stratigraphic, Structural, and Correlation Considerations 



19 



expressing structural conditions, and deciphering the geologic history of 

 a region. Rock units may be objectively shown on a map and in strati- 

 graphic sections. Their boundaries do not necessarily coincide with time 

 boundaries. Several rock units may be incorporated with a time-rock unit. 

 It has been indicated ^ by the American Commission on Stratigraphic 

 Nomenclature that in treating a sedimentary formation (rock unit), the 

 following points should be considered: (1) It must contain no apparent 

 evidence of an appreciable break in deposition. (2) In its simplest form 

 it consists dominantly of one general type of rock. (3) Recognition of 

 the same formation in different areas is only justified when its essential 

 lithologic definition is applicable. (4) The upper or lower contacts of a 

 sedimentary formation may laterally transgress horizons of a neighboring 

 formation. (5) The top and bottom of a sedimentary formation are de- 

 fined either by a change in lithology or by evidence of an appreciable 



Figure 7. Time-space relationships of sedimentary deposits. Tr and Ti. represent time 

 surfaces. Changes in sedimentary rock types must always be considered in three 

 dimensions. 



interval of nondeposition. (6) A formation may hold one or more faunas 

 or floras. (7) A sedimentary formation may include minor developments 

 of volcanic rocks. (8) Pyroclastic materials, whether deposited in water 

 or on land, are to be regarded as volcanic sediments and, hence, as con- 

 stituents of sedimentary formations. 



In naming surface rock units the following points should be con- 



^ Rules of Geologic Nomenclature of the Geological Survey of Canada: Am. Assoc. Petroleum Geol- 

 ogists Bull., vol. 32, no. 3, pp. 366-381, Mar. 1948. Prepared by R. C. Moore. 



