Subsurface Maps and Illustrations 



947 



An isolithic map, as explained earlier, shows the aggregate thicknesses 

 of beds in a specific lithologic class, but it does not necessarily differ- 

 entiate the rocks within this class on a mineralogic, chemical, or physical 

 basis. There are a number of ways in which this differentiation may be 

 shown. One simple method is to show such a breakdown by percentage 

 contours superimposed on the contoured isolithic map. Thus, on a sand- 

 stone isolith map the percentage of arkosic, silty, or carbonaceous sands 

 or maximum grain sizes may be shown by color bands or a second set 

 of contours. The thicknesses of rocks, alone, sometimes fail to reveal 



CLASTICS 



PRECIPITATES 



EVAPORITES 



COARSE 



FINE 



CALCITIC 



DOLOMITIC 



SULPHATES CHLORIDES 



Browns 

 Yel lo\A/s 



Grays 

 Greens 



Blues 



Purples 



Black line patterns 

 Ruling Figures 



SroWr 



Dark.' 



Mecb. 



L-Lghjb 



StsrWUTy 



YeLLow 



Cariary 



drcLx^ 



BUucJi- 



X>ar?Cy 



Med. 



Ltght, 



Green. 



Lights 



Darla 



1>Icirvrbe 



Bar'k. 



Med.. 



Light 



Sky 



PPLLSsiua 



Lvghi. 



Darh 



Dark, 



Med.. 



Ltghb 



Magenta 



Violet 



X X y X XX 

 X X X s X 

 a y X X X 



~V V V V 



V V V 



V V V V 



-1 _l _1 J J 

 -1 -J -I J -I 



+ + + + 



Figure 509. Guide for coloring lithofacies maps. 



the sources of the sediments or the conditions under which they were 

 deposited; but this information, together with that suggested above, often 

 clarifies an otherwise cloudy picture. 



A word of caution should be given in the interpretation of isolithic 

 maps. Apparent discrepancies in the locations of apparent highs and 

 lows within large basins are bound to occur if the predominance of one 

 lithologic class in one locality is replaced by that of another class at other 

 localities. For example, on a coarse-elastics isolithic map, the thickest de- 

 posits are likely to lie close to the source materials near one edge of the 

 basin and will thin out toward the central part of the basin. Conversely, 

 the limestones and evaporites are likely to reach their maximum develop- 

 ment 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 



