942 



Subsurface Geologic Methods 



Isofacies Maps 



A single lithologic facies may include several rock types. For ex- 

 ample, one facies of a formation may consist of alternating shales and 

 sandstones, whereas another facies of the same formation may be made up 

 of shales, limestones, and evaporites. Since the ratio of one type of sedi- 

 ment to another varies within a given facies, some difficulty may develop 

 in attempting to map such a facies by the ratio or percentage method. 

 The complex facies may comprise only a small portion of the formation, 

 in which case the remaining beds would tend to mask the effects on the 

 percentage maps. A type of map called an "isofacies map" is designed 

 to show the change in lithologic characters within a facies. The method 



Figure 504. A — Isopach map of a sedimentation basin. B — Cross section 

 showing complex relationships of formations. 



