36 Subsurface Geologic Methods 



attributable to deformation or to variance in initial depositional dip 

 without extensive and accurate field mapping. It is even more difficult to 

 make this determination if interpretations must be made on limited sub- 

 surface evidence. (See figs. 13, 14.) 



Local Unconformity. A local unconformity is similar to a discon- 

 formity, but differs in that it is of small areal extent and represents a 

 comparatively short time interval. It is commonly the result of stream 

 action in continental deposits. During times of flood, streams may scour 

 out channels in their flood plains, which may be many feet wide and deep 

 and thousands of feet to several miles in length. As the flood subsides, the 

 channel may fill up, or it may fill up days or even years later. The strati- 

 fication above such an unconformity is parallel to that below, making it 

 a disconformity. The term "local unconformity" is used to indicate its 

 small areal and short time value. 



An example of a local unconformity in a marine section would be 

 the wave-truncated surfaces of some buried bioherms. These surfaces 

 would develop on the seaward side of the reef, as the bioherms built up- 

 ward and shoreward during a marine transgressive overlap. The bench 

 cut by wave action would represent an erosion surface of limited ex- 

 tent. If this surface was then buried by further advances of the sea, a 

 local unconformity would result. 



The erosion involved in local unconformities is accomplished by a 

 cessation of the upbuilding of the deposits and is called "contemporane- 

 ous erosion." Local unconformities may be produced by contemporane- 

 ous erosion in marine deposits by changes in strength or direction of 

 marine currents; in lake deposits by similar changes in currents; and in 

 eolian deposits by variations in wind. 



Blended Unconformity. A surface of erosion may be covered with a 

 thick mantle of residual sands and gravels that grade downward into 

 the rock from which they were derived. If they are later overlain by 

 sediments, they may become the basal member of a new formation. In 

 such a case, no distinct surface of separation can be seen, and the feature 

 is a blended unconformity. 



Genesis of Unconformities 



Continental Unconformities. Continental unconformities are those 

 whose surface is cut by a continental agent, succeeding deposits being of 

 continental origin. The agent of deposition may be fluvial, eolian, or 

 glacial. 



Aqueous: Fluvial unconformities are found in valley-flat, or in 

 alluvial-fan or deltaic deposits. 



Unconformities in valley flats may arise from scour-and-fill and 

 from the meandering of aggrading streams. In scour-and-fill, uncon- 

 formities may be abundant and the relief is apt to be locally great, and 

 correlation difficult or impossible. The relations are those of a discon- 



