maximum development farther toward the center. Therefore, coincidence of the 

 thin and thick areas on the two maps should not be expected. This is one of the 

 reasons that the isolithic maps are of such importance in the study of stratig- 

 raphy. From the foregoing it is clear that a set of isolithic maps with their 

 opposing thin and thick sections will tend to cancel one another out in an over- 

 all isopach map of the same stratigraphic interval. In other words, the isopach 

 map may be generally featureless, whereas the series of isolithic maps reveals 

 much of the stratigraphic information sought. 



W. C. Krumbein, of Northwestern University, describes the ratio method of 

 lithofacies mapping in the American Association of Petroleum Geologists Bulletin 

 No. 10, Vol. 32, 1948. As mentioned earlier, the ratio method of mapping 

 lithofacies is essentially the same as the percentage method. However, the relative 

 spacing of contours may vary greatly. 



Figure 24-36 is an isopach map of a group of rocks consisting of sand- 

 stones, shales, limestones, and anhydrites. This map is used as a guide for 

 drawing the lithofacies map of Figure 24-37. Table 24-1 is a summary of the con- 

 trol points, showing the total thicknesses, thicknesses of clastic and non-clastic 

 rocks, the clastic percentages, and clastic ratios. The values in the last two 

 columns are obtained from the following formulas: 



Clastic ratio 



sandstones + shales 



Clastic percentage 



limestones -(- anhydrites 

 sandstones 4- shales 



sandstones -f- shales -f- limestones -\- anhydrites 



Figure 24-35. Lithofacies map. 



FACl E S 



Sandstone 

 (Brown) 



Limestone 

 (Blue) 



493 



