926 Subsurface Geologic Methods 



In the simplest case, where the formations are parallel, where the dips 

 are constant, and where the inclination of the eroded surface is constant, 

 the widths of the bands representing the "exposed" edges of the formations 

 will be exactly proportional to the thicknesses of the formations, as shown 

 in block A of figure 493. In block B, two 100-foot members of constant 

 thickness are separated by two others which converge markedly toward the 

 "outcrop." Because of the convergence, the lowermost member dips more 

 strongly than the upper, and, therefore, its outcrop width is less. Block C 

 shows parallel beds in a truncated monocline. Because of the difference 

 in rate of dip, the uppermost 100-foot member is six times as wide on 

 the "outcrop" as the lowermost member of the same thickness and some- 

 what broader than a 200-foot bed immediately underlying it. Block D 

 shows parallel beds with constant dips, but the surface of truncation is 

 variable. The back edge of the block is an element of an inclined plane, 

 and along this line the widths of "outcrops" are proportional to the thick- 

 nesses of the beds. Toward the front of the block the surface becomes 

 terracelike, resulting in the "outcrop" pattern shown. 



These blocks illustrate four basic conditions affecting the construc- 

 tion of a paleogeologic map. Any one, or all four may be operative in 

 different portions 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 con- 

 siderable 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 work- 

 ing on regional structure can use paleogeologic maps to advantage in de- 

 termining periods of folding and faulting and the chronologic develop- 

 ment of structure. Teachers of various branches of geologic science would 

 find their tasks difficult were it not for maps of this general type. 



Cross Sections and Projections 



While subsurface maps of all kinds show geologic conditions in 

 essentially horizontal planes, sections show the details of stratigraphy or 

 structure in vertical planes. Neither the map nor the section, alone, tells 

 the whole geologic story; and, for this reason, any exhaustive subsurface 



