946 



Subsurface Geologic Methods 



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 strati- 

 graphic investigations in regions where the lithologic character of forma- 

 tions change radically from one locality to the other. Isolithic contour maps, 

 used in conjunction 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 understanding of sedimentation processes. 



Figure 508. Evaporite isolith map. 



In regions where contemporaneous strata are predominately carbon- 

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

 conglomerates at a third, an ordinary isopachous map can provide only a 

 small part of the information 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 simulta- 

 neously with the remaining rock groups of the complete section. Ob- 

 viously, the shorter the time range and the thinner the stratigraphic in- 

 terval studied, the more precise the results will be. But the different kinds 

 of facies maps are also an excellent method for studying thick series of 

 strata which, because of monotonous repetition of similar units, cannot 

 be subdivided into thinner mappable units. 



