units that are separated widely, geographically and stratigraphically. Figure 

 23-5 illustrates a major down-dip facies change — limestone to shale — with the 

 latter present only in the subsurface. Data from wells 1, 2, 3, and 4 would 

 suggest a correlation as shown by the columnar sections; however, this correla- 

 tion fails to represent the true lateral equivalency relationships of the limestone 

 and shale facies. Additional wells between 1 and 2 and between 2 and 3 would 

 improve the overall correlation. Such a facies change could have resulted from 

 tectonic adjustments in the source and depositional areas, from geomorphic 

 changes and climatic fluctuations, and from changes in temperatures, pH values, 

 currents, and depths of the water under which the deposits accumulated. To 

 accurately evaluate such changes, one should prepare various types of strati- 

 graphic maps (see p. 449). Once the facies relationships have been determined, 

 it then becomes less difficult to unravel the tectonic and sedimentational history 

 of a region, the normal stratigraphic sequence, and the trend of potentially 

 petroliferous areas. The results obtained from facies studies are no more ac- 

 curate than the correlations of the stratigraphic units on which they are based. 



CORRELATION MARKERS The recognition and the definition of litho- 



logic, paleontologic, and seismic markers in 

 controlled stratigraphic sequences are of utmost importance in all correlation 

 work. These markers, including limestone, bentonite, coal or lignite, anhydrite, 

 concretions, chert, glauconite, or any other lithic or paleontologic types, 



iiiiiuinii/iiiiiihUiinrnnitiinuiiniiiUNnniiiiiuuiiijMiiuui/iiinuinii/iiiiimuiiiiniiu 



Bentonite bed 



Figure 23-6. Marker beds. The bentonite and limestone marker beds, common to the two 

 lithic facies, aid materially in establishing equivalency of the facies. Fauna A and B 

 are facies assemblages that contribute little to establishing the lateral equivalency of 

 the dissimilar deposits. They are, however, extremely important in paleontologically sub- 

 dividing their respective facies. 



444 



