822 



PRINCIPLES OF STRATIGRAPHY 



Disconfonnity {Parunconfonnity, Paenaccordanz). When strata 

 are elevated without folding or other disturbances, and subjected 

 to a prolonged period of erosion, after which their truncated edges 

 are covered by other strata, either marine or nonmarine, a stratic 

 unconformity or disconformity (parunconformity, paenaccordanz) 

 is produced. Here a hiatus, measured by the length of time during 

 which the lower strata were exposed to erosion, plus the amount 

 worn away during this exposure, separates the two series. While 

 this hiatus measures the unrepresented strata it must be borne in 

 mind, however, that it does not represent the length of time dur- 

 ing which deposition was interrupted in the region in question. 

 The amount of nondeposition can be determined only when the 



Fig. 200. Diagrams illustrating the development of disconformities. 



amount of erosion during the elevation of the region is known. 

 We may assume two locations A-B, where elevation may occur or 

 some other changes by which deposition continued uninterruptedly 

 at B, while erosion replaced it at A. (Fig. 200, I-IV.) Under as- 

 sumed conditions a formation c, equal in thickness to formation a, 

 may be deposited at B, while b is eroded at A. If later deposition 

 becomes uniform again in both localities, d will rest conformably on 

 c at locality B, but disconformably on a at A. The hiatus at locality 

 A comprises formations h and c, but the actual time interval is meas- 

 ured only by the deposition of formation c at locality B. In this 

 case the erosion at A was assumed to equal the amount of deposi- 

 tion at B, and hence the hiatus at A represents twice the amount of 

 the time interval involved, as developed at B. 



If erosion at A exceeds deposition at B, then the hiatus at A 

 representing a definite time interval will be greater than twice the 

 depositional equivalent of that time interval at B, by an amount pro- 

 portional to the excess of erosion over deposition. If erosion at A 



