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 is simple and is essentially a special adaptation of those described above. 

 It consists in first delimiting the area and stratigraphic interval of the facies. 

 Now, this portion of the formation can be treated as if it alone were a formation, 

 and the required isopach or isolith maps are constructed accordingly. 



A form of generalized isofacies map is shown in Figure 24-28. The contours 

 show the total thickness of the formation, and the areas shown by different 

 patterns within the heavy dashed lines are the various facies of the formation. 

 These facies are as follows: 



Area 1 — Limestones and dolomites. 



Area 2 — Limestones, siltstones, and sandstones, interbedded. 



Area 3 — Limestones, dolomites, sandstones, and siltstones. 



Area 4 — Gypsum, limestones, and sandstones. 



Area 5 — Gypsum, arkosic sandstones, and siltstones. 



Area 6 — Arkoses, sandstones, and siltstones. 



Figures 24-29 to 24-33, inclusive, are a series of isolith maps of a portion 

 of a sedimentary basin. Figure 24-29 is an isopach map of the total thickness 

 of the formation, and the cross section shows the stratigraphic relationships of 

 conglomerates, sandstones, shales, limestones, and evaporites. This type of 



Figure 24-30. Sandstone isolith map of basin shown in Figure 24-29. 



488 



