section is somewhat too complex to analyze lithologically on one lithofacies 

 map by percentages or ratios. For this reason an isolith map is made for each of 

 the principal lithologic classes: coarse elastics, fine elastics, precipitates, and 

 evaporites. The control points used in contouring are wells whose logs have 

 been broken down according to these lithologic classes and tabluated on the 

 form shown in Figure 24-27. 



Figure 24-30 is the isolith map of coarse clastic rocks: i.e., sandstones 

 arkoses, and conglomerates. The thicknesses shown represent the aggregate 

 thickness of all rocks falling in this classification, regardless of the thickness of 

 the individual beds in which they occur. Thus, of two control points, each indi- 

 cating a thickness of 100 feet, one might be made up of two 50-foot members, 

 the other of five 20-foot beds. They have the same values on the isolith map. 



The sandstone isolith map shows not only the aggregate thicknesses of the 

 coarse elastics, but also the areas where certain kinds of sandstones predominate. 

 The shale isolith shows the aggregate thicknesses of the shales and a further 

 differentiation on the basis of color. Likewise, the limestone and evaporite maps 

 indicate areas of different types. There are many other ways of differentiating 

 within each of the lithologic groups ; and, depending on local conditions, it may 

 be advantageous to set up other main classes. These examples are given to 

 indicate how lithologic facies may be drawn on maps in both a qualitative and 

 quantitative manner. 



Figure 24-31. Shale isolith map. 



489 



