isopach map is needed. Obviously, the sum of all the thicknesses shown on a 

 complete set of isolith maps of a given stratigraphic interval equals the isopach 

 map of the same interval. 



Isofacies 



A single lithologic facies may include several rock types. For example, 

 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, lime- 

 stones, and evaporites. Since the ratio of one type of sediment 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 instance the remaining beds would 



Figure 24-29. v4 — Isopach map of a sedimentation basin. 5 — Cross section 

 showing complex relationships of formations. 



487 



