Figure 24-33. Evaporite isolith map. 



Figure 24-35 is a lithofacies map drawn in black and white, with certain 

 patterns representing colors, which, in turn, represent the different rock types. 

 For very simple drawings like these, the patterns are satisfactory; but, where 

 many lithologies are involved, the inevitable similarities of patterns in black and 

 white make the map difficult to read. 



Lithofacies maps of various kinds are almost indispensable in stratigraphic 

 investigations in regions where the lithologic character of formations change 

 radically from one locality to the other. Isolithic contour maps, used in con- 

 junction with maps showing mineralogic compositions of the sediments, provide 

 one of the best means of accurately locating with only a few points of control 

 the source areas of clastic sediments, the areal extent of marine, continental, or 

 fluviatile environments, and many other features vital to a thorough understand- 

 ing of sedimentation processes. 



In regions where contemporaneous strata are predominantly carbonates at 

 one locality, shales and siltstone at another, and sandstones and conglomerates 

 at a third, an ordinary isopach map can provide only a small part of the in- 

 formation needed by the stratigrapher. On the other hand, isolithic maps permit 

 each of these rock groups to be appraised separately and without the distracting 

 effect of having to deal simultaneously with the remaining rock groups of the 

 complete section. Obviously, the shorter the time range and the thinner the 



491 



