Stratigraphic, Structural, and Correlation Considerations 45 



therefore, not conclusive proof of breaks in sedimentation. If tRey are 

 found in association with other chemical sediments such as manganese 

 or phosphate nodules or glauconite, an unconformity is strongly sug- 

 gested. The presence of pyrite at contacts of contrasting lithologies is 

 good evidence of unconformity. 



Fossil Evidence: Probably the most reliable means of detecting un- 

 conformities in the geologic column are the index fossils found in forma- 

 tions on both sides of the unconformity. Many extensive time breaks in 

 sedimentation, especially disconformities, if unaccompanied by erosion 

 or deformation, may leave no other proof of their presence in a series of 

 stratified rocks. There are probably many unrecognized unconformities 

 because of a lack of fossils for comparison. 



Importance of Unconformities 



Unconformities are of paramount value to the stratigrapher in sep- 

 arating units of rocks and of geologic time. They are of great importance 

 to the oil geologist because of the vast amounts of petroleum that may 

 be trapped at or near their surfaces. 



Stratigraphic. Geologists have long used diastrophism as the ulti- 

 mate basis for subdividing geologic time and the geologic column. The 

 geologic record shows evidence of rhythmic changes in sea level. These 

 changes are controlled by recurrent or cyclic periods of disturbance. A 

 geologic cycle usually starts with an unconformity and ends with an un- 

 conformity. The diastrophism that closes one cycle is usually accom- 

 panied by mountainbuilding and deepening of the ocean basins. Not 

 only are there wide spread breaks in sedimentation, but vast areas are 

 exposed to erosion. As land areas are worn down and sediments are 

 carried into the ocean basins, the seas will slowly rise and cover the 

 lower parts of the continents with shallow seas. Sediments will be de- 

 posited unconformably on the submerged erosion surface. An examina- 

 tion of the geologic record shows a great succession of marine inunda- 

 tions of the continents separated by unconformities marking the retreat 

 of marine waters. Since the oceans are freely connected, a change in sea 

 level will affect all, and major breaks will be worldwide. 



The cycles of earth movements do not follow a smooth pattern. Be- 

 tween major periods of diastrophism will be irregular periods of dis- 

 turbance involving local emergence and submergence of land areas. In 

 regions affected by these minor breaks there will be a marked hiatus 

 and unconformity. 



Breaks in the Fossil Record: The fossil record affords us the best 

 direct means of evaluating the time breaks indicated by unconformities. 

 The evolution of plant and animal forms is always greatly accelerated 

 during periods of crustal disturbance. Some forms adapt themselves to 

 the changes in environment and survive with variations in structure, 

 while others fail to adjust themselves and perish. The next invasion of 



