898 



Subsurface Geologic Methods 



typically eroded in the surface formations. In the same figure the block 

 is separated, B, to show a buried major nonconformity, the presence of 

 which is suggested nowhere in the surface geology. 



The structural contour map in figure 473 is a surface map, as the 

 datum bed crops out over a wide area, and all structural data needed for 

 the map are readily available on the surface. The structural map in figure 

 474 is constructed solely on data supplied by the 24 wells that pene- 

 trated the Paleozoic formations below the Jurassic unconformity. The 

 block diagram shows four of these wells, three of which are in the vertical 

 planes of the block. 



In the central part of the Cambrian structure map the actual datum, 

 which is the top of the Cambrian, has been destroyed by pre-Jurassic 



Figure 471. A — Actual convergence of section. B — Apparent 

 convergence of section as a result of crooked hole. 



erosion. This is shown by the fact that wells 8, 10, 16, 18 and 19 entered 

 the Cambrian directly beneath the Jurassic without first penetrating the 

 systems that overlie the Cambrian toward the edges of the map. Although 

 the contours within this area of truncation are drawn to the elevations 

 at which the Cambrian is encountered below the unconformity, they do 

 not represent the true structural configuration of the strata. In order to 

 do so, it would be necessary to reconstruct the original thickness of Cam- 

 brian throughout the area of truncation and raise the elevations accord- 

 ingly. Even this process cannot be employed in the center of the area 

 where Cambrian beds are absent, for there is no method by which the 

 depth of erosion of the pre-Cambrian rocks can be determined. 



Disconformities are frequently not recognizable in well samples or 

 on resistivity and radioactivity logs, and, because of this, so-called re- 



