It should be emphasized that the alternation rate map, though very useful, 

 can be quite misleading with respect to actual stratigraphic conditions. In a 

 succession of sandstones and shales where the average thickness of the sand- 

 stones is 10 feet and the shales, 20 feet, the average of both lithologies is 15 feet. 

 However, if the alternation rate map is used in combination with an isopach 

 map and a sandstone percentage map, a fair representation of actual relation- 

 ships is obtained. 



In Figure 24-38, A, B, and C, is a series of maps consisting of an isopach of 

 the total stratigraphic interval, composed of sandstones and shales, a sandstone 

 percentage map, and an alternation map based on a 100-foot unit interval. 



Figure 24-39 shows the three maps referred to registered one over the other 

 The alternation rates are shown by dashed contours, and shading is used to 

 emphasize the region of the highest rate of alternation. At location a the total 

 thickness (from the isopach) is 500 feet; sandstones comprise 40 percent of 

 the section (from the percentage map) ; and the rate of alternation is 10 per 

 100 feet of interval. From these data it is a simple matter to determine the aggre- 

 gate thickness of sandstones (or shales) , which in this instance is 200 feet, and the 

 average thickness of the sandstone beds, which is 10 feet. At location b the thick- 

 ness of the stratigraphic interval is 700 feet, 20 percent of which is sandstone. The 

 alternation rate is about 5 per 100 feet; therefore, there are 35 beds of sandstone 

 and shale together. The average thickness of the sandstones is about 8 feet, and 

 the average thickness of the shale units is about 33 feet. 



Uses of Lithofacies Maps 



Much of the preceding discussion is devoted to lithofacies maps, how 

 they may be constructed, and what they represent. In most instances the 

 lithofacies map, regardless of the type, is only one phase of regional geologic 

 investigation and analysis. These maps do not depict a complete geologic story, 

 but, rather, present complex stratigraphic data in a simplified form so that 

 broad concepts can be developed. All the various lithofacies maps discussed 

 reveal only the averages of geologic phenomena. None is precisely specific where 



497 



