Subsurface Maps and Illustrations 



943 



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 constructed 

 accordingly. 



A form of generalized isofacies map is shown in figure 503. 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: 



Figure 505. Sandstone isolith map of basin shown in figure 504. 



Area 1 — Limestones and dolomites 



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



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



Area 4 — Gypsum, limestones, and sandstones. 



Area 5 — -Gypsum, arkosic sandstones, and siltstones. 



Area 6 — Arkoses, sandstones, and siltstones. 



Figures 504 to 508, inclusive, are a series of isolith maps of a portion 

 of a sedimentary basin. Figure 504 is an isopach map of the total thick- 

 ness of the formation, and the cross section shows the svratigraphic rela- 

 tionships of conglomerates, sandstones, shales, limestones, and evaporites. 

 This type of section is somewhat too complex to analyze lithologically on 

 one lithofacies map by percentages or ratios. For this reason an isolith 



