Stratigraphic, Structural, and Correlation Considerations 33 



is given to the criteria for unconformity recognition. Considerable space 

 has been devoted to the use of unconformities in interpreting the geologic 

 record. A number of examples of unconformity-type oil fields are de- 

 scribed and illustrated. A section on the influence of unconformities on 

 ore deposits is also included. 



An unconformity has been defined by Twenhofel ^^ as a surface of 

 erosion or nondeposition separating two groups of strata. Unconform- 

 ities that are surfaces of erosion are more common than surfaces of non- 

 deposition, possibly because they are more easily recognized. Surfaces 

 of nondeposition indicate a long time break in sedimentation and may 

 leave little evidence of their occurrence in the geologic column. Such a 

 break would involve the establishment of a profile of depositional equi- 

 librium for a long period of time. 



The time value of an unconformity is the interval of geologic time 

 represented by the break. This ranges from the time required to deposit 

 a single formation to the enormous time interval represented by the un- 

 conformity between the pre-Cambrian and Pleistocene glacial deposits. 

 Twenhofel ^^ aptly likens all unconformities to the branches of a tree, 

 the trunk of which is the present land surface and whose branches spread 

 out to intercalate in all directions through the strata. Thus every un- 

 conformity directly or through another unconformity intersects the pres- 

 ent land surface. 



The relief on surfaces of unconformity may vary from peneplanation 

 to several thousand feet. The relief on the Ep-Algonkian surface of the 

 Grand Canyon is at least 800 feet, while Carboniferous sedimentary rocks 

 around Boston, Massachusetts, rests unconformably on a surface of at 

 least 2,100 feet. The amount of relief on an unconformity is no indication 

 of the time value of the unconformity. A surface of great relief might 

 represent only a small fraction of the time involved in the formation of a 

 peneplained surface of slight relief. Conversely, a surface of small relief 

 may not have passed through the initial stages of subaerial or submarine 

 erosion, and may involve only a short period of geologic time. The phys- 

 ical appearance of an unconformity should not be used to gage its time 

 value. Fossil evidence, if present on both sides of an unconformity, is the 

 most reliable criterion of time value. 



Nomenclature 



The various kinds of unconformities depend for their classification 

 upon the attitude of the strata on both sides of the unconformity, the 

 genesis of the rocks involved, and their areal extent. 



Disconformity. In a disconformity, the strata on both sides of the 

 unconformity are parallel. The older strata have lain undisturbed in 

 position during the break in deposition. The younger beds have then 



^' Twenhofel, W. H., Treatise on Sedimentation, Baltimore, Md., Williams and Wilkins Co. 1926. 

 ^'^Ibid., p. 447. 



