Paleogeologic Maps 



A geologic or areal geologic map is one that shows the present distribution 

 of consolidated rocks at the surface and immediately below the soil or uncon- 

 solidated mantle. A paleogeologic map shows the distribution of formations at 

 a surface which existed at some specific time in the geologic past. Such a surface 

 is shown in the lower block of Figure 24-6. Figure 24-23^4 is the areal geologic 

 map of the upper block, and B is the paleogeologic map of the pre-Jurassic 

 surface in the lower block. 



As might be presumed, paleogeologic maps are constructed from informa- 

 tion supplied by wells. The four wells shown in the block diagram mentioned 

 penetrate pre-Cambrian, Devonian, and Pennsylvanian beds beneath the pre- 

 Jurassic unconformity. The remaining 20 wells in Figure 24-235 encounter 

 rocks of different ages beneath the unconformity, and it is upon this type of 

 information that the map is constructed. 



Several factors control the relative breadth of the bands or areas that appear 

 on the paleogeologic map. Among these are the relative thicknesses of the 

 formations, the rates of thinning, the relative rates of dip in the different forma- 

 tions and the actual degree of dip, the character of the eroded surface, and the 

 amount and character of folding subsequent to truncation. It is well to keep 

 these conditions in mind when one is drawing a paleogeologic map, because it 

 may be necessary to interpolate several geologic boundaries between two con- 

 trol points, and any one of the conditions listed above might have a pronounced 

 effect on the map position of the boundary lines. 



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 

 24-24. In block B, two 100-foot members of constant thickness are separated by 

 two others which converge markedly toward the outcrop. Because of the con- 

 vergence, 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 it is somewhat 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 construction of 

 a paleogeologic map. Any one, or all four may be operative in different portions 



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