of one map area. It naturally follows that structural as well as isopach maps 

 should be consulted in locating the geologic boundaries. For accurate results, 

 it is necessary to contour the plane of the unconformity in order to determine the 

 conditions shown in block D mentioned above. 



The uses of paleogeologic maps extend into a number of fields of geologic 

 investigation. The oil geologist applies this kind of mapping in the search for 

 oil accumulations below unconformities. Stratigraphic traps of various types, 

 buried zones of weathering, and outcrop trends of productive formations can 

 be accurately mapped in areas where a considerable number of wells have 

 been drilled. In historical geology, the paleogeologic map is not only a working 

 tool but also an indispensable illustration. The sedimentologist utilizes the 

 paleogeologic maps as an aid to working out knotty problems in structural 

 histories and source areas of sediments and finally as illustrations showing by 

 stages the progress and interruptions of sedimentation in the map area. The 

 geologist working on regional structure can use paleogeologic maps to advantage 

 in determining periods of folding and faulting and the chronologic development 

 of structure. Teachers of various branches of geologic science would find their 

 tasks diffcult were it not for maps of this general type. 



Facies Maps 



In stratigraphy it is axiomatic that the lithologies of any formation change 

 in some manner from one part of a basin to another. The degree of variation 

 and rate of change may be small or large, depending upon the physical conditions 

 of the basin and adjacent terrane, the chemistry of the waters, the climate, and 

 many other factors that determine the type of sediment laid down. Therefore, a 

 single formation may be a coarse conglomerate at one locality, a sandstone or 

 shale at another, a limestone at a third, and all three lithologies at some places. 

 Facies changes, although perhaps not so drastic as the example given, are the 

 usual and normal condition; and it is often of the utmost importance to the 

 stratigrapher to determine the characters of the variations and where they 

 occur and then to have some means of showing the change on maps. 



A large number of methods have been devised by geologists to illustrate 

 facies changes on maps — 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. Some methods invented for special conditions 

 or for a special 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 



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