938 Subsurface Geologic Methods 



the commonest being panel maps, isometric projections, certain isopach 

 maps, and cross sections; but these methods fail in one way or another 

 to give a complete and continuous picture where facies change rapidly 

 and the section as a whole is highly diversified. Other methods invented 

 for special conditions or for a specified problem lack general applicability. 

 Most attempts to show facies changes on maps have been concerned 

 with qualitative data, rather than quantitative. However, comparatively 

 recently, greater effort has been directed toward lithologic analysis and 

 lithologic mapping on a quantitative basis, and the results of this work 

 have been gratifying. Since the scope of this book is somewhat restricted 

 to the more practical aspects of subsurface mapping, it is necessary to 

 omit certain methods which occasionally or locally do have practical ap- 

 plications but which are not generally useful in lithologic mapping. The 

 methods described below illustrate an approach to the problems which 

 may, in turn, stimulate further endeavor along similar lines. 



LiTHOFACiES Maps 



"Lithofacies map" is a comparatively new term in the geologist's 

 vocabulary. It denotes a map that shows by one means or another the 

 changes in lithologic facies of a formation, group, or system of rocks 

 within a sedimentation basin. A lithofacies map may show the different 

 facies in either a qualitative or quantitative way. Each is important in 

 its own way. The definition of the term is broad enough to include a 

 rather wide variety of maps dealing with facies changes. 



It is difficult, or impossible, to show by conventional maps what and 

 where facies changes take place in a highly diversified section, such as 

 rapidly alternating beds of shale, sandstone, limestone, and anhydrite in 

 lenses or discontinuous beds. The relationships of each of these litho- 

 logic groups to the others in the section can be shown clearly on litho- 

 facies maps. 



Figure 500 shows a well log consisting of alternating limestones, 

 shales, sandstones, and evaporites. The total thickness of this succession 

 is 478 feet. To the right of this log are four columns, each representing 

 a lithologic class of rocks. The first column contains only the sandstones 

 transferred from the well log; the second, only the limestones; the third, 

 shales; and the fourth, evaporites — all plotted in their correct thicknesses. 

 The total thicknesses are 163, 160, 110, and 45 feet, respectively; of course, 

 their sum is the thickness in the original log. Now, in the column on the 

 extreme right, these lithologic units are recombined in the simplest possible 

 manner: i.e., all of the units of one class, regardless of the thickness of the 

 individual members, are plotted as if they occurred as one thick bed. 

 Thus, the thirteen members of the original log are reduced to four in the 

 analytic log. 



In figure 501, A is a stratigraphic cross section showing normal facies 



