68 Subsurface Geologic Methods 



In certain instances, depending on the nature of the stratigraphic 

 section and the magnitude of the problem, a single method may produce 

 the desired correlation results. In others, a combination of one or more 

 techniques might be required to obtain the correct solution of the prob- 

 lem. It is the responsibility of the stratigrapher to know what method to 

 use and when to use it and to recognize the limitations of each. Failure 

 to recognize these limitations has often contributed to excessive explora- 

 tion and development expenditure. 



Magnitude of Correlations 



Correlations may be of local, regional, interregional, or interconti- 

 nental magnitude. 



Local correlations involve those within a restricted area, as for ex- 

 ample, an oil field or a minor basin of deposition. When suflBcient data 

 are available, correlations of this category are generally not excessively 

 difficult, although there are exceptions. When extremely detailed correla- 

 tions are required, difficulties are more frequently encountered. 



Regional correlations involve those between separated depositional 

 basins within a major province; for example, between the Los Angeles, 

 Ventura, Humboldt, and San Joaquin Valley basins of the Pacific Coast 

 province or between the various structural basins of the northern Rocky 

 Mountain area. Correlations among such basins are of variable complex- 

 ity. Certain parts of the section may offer little difficulty in correlation, 

 whereas in other parts of the section it may prove impossible to establish 

 accurate correlations. 



Interregional correlations, as between the Gulf, Pacific, and Atlantic 

 Coast areas, may be moderately dependable; others may be extremely in- 

 adequate and dubious. Owing to "far removal" and to facies variations, 

 correlation problems of this category may be extremely involved and must 

 be paleontologically controlled. 



Intercontinental correlations involve those between continents; for 

 example, between the Pacific Coast and the Paris Basin, or between the 

 Netherlands East Indies and the British West Indies. Lithologic correla- 

 tions cannot be considered. Correlations must be based entirely on pale- 

 ontologic data, which, although not very satisfactory, are generally con- 

 sidered the most dependable. Certain geologic periods have been fairly 

 well established the world over; however, smaller time intervals such as 

 the Pliocene, Miocene, Oligocene, and Eocene are open to question. The 

 more extended the correlation and the less the time interval involved, the 

 more inexact is the chronologic value of the correlation. 



Correlation Indicators 



The selection of correlation indicators depends on the magnitude and 

 character of the lithologic and paleontologic aspects of rock and time- 



