Stratigraphic, Structural, and Correlation Considerations 



53 



Many subsurface faults are not reflected at the surface owing to 

 "dying out" or to truncation by buried erosional surfaces (fig. 23). Con- 

 versely, faults intersecting the surface may be absorbed by incompetent 

 beds in the subsurface and thus may have restricted vertical downward 

 extension. 



It should be recognized that faults involve surfaces and not planes. 

 Many irregularities and curvatures which these surfaces may assume make 

 their position and trend in the subsurface difficult to define. These ir- 



Figure 23. Various types of faults. A — fault surfaces may be extremely irregular; 

 the proper interpretation of details of fault zones in the subsurface requires con- 

 siderable data. B — Many faults disappear with depth owing to displacement 

 absorption by incompetent strata. C — Surface folds may be replaced by vertical 

 displacement with depth. Z) — Fault systems may be truncated by erosional sur- 

 faces. E — Favorable petroliferous structure may underlie overthrust sheets. F — 

 Major thrust faulting may create accompanying tensional fault patterns. 



