must be more or less laterally continuous as well as restricted vertically. Un- 

 conformities of local and regional extent have often served as correlation datums. 

 Every attempt should be made to select marker beds common to more than 

 one facies, as for example, the bentonite and limestone beds shown in Figure 

 23-6. Facies fauna A and B in this figure can be used only locally. Some of the 

 more common paleontologic markers include foraminifera, ostracodes, fish teeth 

 and scales, pollen, conodonts, and macrofossils. 



MAGNITUDE OF Correlations may be of local, regional, inter- 



CORRELATIONS regional, or of intercontinental magnitude. 



Local correlations, for example, within an oil 

 field or in a small depositional basin, range from simple to complex. If stratal 

 and paleontological units are extremely discontinuous, proving contemporaneity 

 or noncontemporaneity of dissimilar lithologies and faunas is difficult. 



Regional correlations between basins of large provinces, such as the Rocky 

 Mountain Province, may be difficult, particularly if each basin has had a diff- 

 erent sedimentational and structural history. Interregional correlations, as be- 

 tween sections of the Gulf, Pacific, and Atlantic coasts, can be determined only 

 approximately — mainly on the basis of paleontology. Intercontinental cor- 

 relations involve those between continents, and under no circumstance can the 

 lithologies of the sections be considered in this type of correlation problem. 



Figure 23-7. Definition of major stratigraphic units. In all correlation work it is important 

 first to define the major lithologic or paleontologic units of the section as, for instance, 

 Units 1, 2, and 3. Following this subdivision, detailed correlations may be established 

 within the units. 



445 



