62 Subsurface Geologic Methods 



structural, and correlation values. These evaluations have been greatly 

 improved through the introduction, application, and coordination of many 

 new and revised techniques and through a more orderly and systematic ap- 

 proach to subsurface investigations. 



Correlation, as commonly implied, consists in matching up similar 

 lithologies, faunas, and floras. In certain instances this is correct; other 

 correlations, however, require the establishment of proof that deposits of 

 given characteristics are the time equivalent of contiguous deposits exhib- 

 iting entirely diff"erent lithologic and paleontologic aspects. Strata involv- 

 ing similar or even identical features do not necessarily indicate age con- 

 temporaneity. For example, an environment in a stratigraphic sequence 

 may have given rise to a particular lithology and microfauna; under sim- 

 ilar conditions these characteristics may occur stratigraphically higher or 

 lower in the section. These deposits with their lithologic and paleontologic 

 similarities may be correlated only on the basis of environment and not 

 on the basis of equivalent time. Such correlations have been made in the 

 past and are now being made, and they have led to unnecessary or even 

 disastrous development and exploration recommendations. 



To exemplify the preceding comments, one has only to review condi- 

 tions prevailing along modern coast lines, where many unlike though con- 

 temporaneous sedimentary and biologic realms are evident. Twenhofel ^^ 

 emphasizes this point by commenting: 



. . . When two deposits of the geologic column have been found to hold 

 pretty much the same organisms, it has been assumed that the two deposits 

 have synchronous relations. It is equally if not more valid to assume that the 

 two deposits were laid down under similar environments and may actually be 

 somewhat different in age. 



Such anomalies have prevailed throughout geologic time. 



As previously mentioned (p. 15), three primary types of correlations 

 must be considered in stratigraphic geology: time, time-rock, and rock. 

 Time correlations involve time span only and are based on time-rock di- 

 visions. Time-rock or time-stratigraphic correlations involve correlation 

 of variable rock types that accumulated during intervals of time. Rock 

 or lithogenetic correlations are based essentially on lithologic constitu- 

 tion. Boundaries of the last two units may transect or coincide (fig. 6) . 

 Neglect of defining these basic stratigraphic units only introduces chaos 

 to the science of stratigraphy. 



Some correlations, particularly those involving the time-rock type, 

 generally, require voluminous data before being satisfactorily and ade- 

 quately established. This problem is most critical in areas in which the 

 strata exhibit extreme and rapid vertical and lateral facies changes. In 

 areas where stratal sequences are reasonably uniform and constant, as in 

 certain parts of the Paleozoic section of the Midcontinent region, correla- 



°* Twenhofel, W. H., Report of Committee on Paleoecology, Nat. Research Council, Oct. 1935. 



