34 



Subsurface Geologic Methods 



been deposited with their stratification parallel to that of the older beds. 

 This type of unconformity has been called a parallel unconformity by 

 Lahee,^^ and they have been aptly named stratigraphic unconformities 

 by Grabau.^" 



The term "diastem" may be used to designate a disconformity whose 

 time value is less than that of a formation. 



Disconformities may be difficult to detect, especially if there is little 

 relief on the surface. Evidences of erosion, if present, may simplify the 

 task. If no erosion is detected, then fossils are the best means of deter- 

 mining disconformity. 



Nonconformity. In a nonconformity, the strata below the uncon- 

 formable contact are at an angle with those above. This type may be de- 



FiGURE 13. Cores taken two feet apart from a well section in central Sumatra. 

 Observe difference in dip; this difference may be attributed to variance in 

 competency of lithologic types, to irregularity in deposition, and perhaps to 

 preconsolidation slumping movement. If these two sections were tak n as spot 

 cores several hundred feet apart stratigraphically a nonconformity could possibly 

 be inferred. 



scriptively called an "angular unconformity." According to Grabau,^^ 

 this is the true unconformity where there is a discordance of strata, and 

 he has called them "structural unconformities" to contrast with his strat- 

 igraphic unconformities. 



^^Lahee, F. H., Field Geology, 4th ed.. New York, McGraw-Hill Book Co., 1941. 



'^ Grabau, A. W., Guide to the Geology oj the Schoharie Valley in Eastern New York: New York 

 State Mus. Bull. 92, 1906. 

 »' Idem. 



