Laws of stratigraphy

About History of Geology - Laws of Stratigraphy

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3. Stratigraphy is the branch of Geology which deals with the
history of the rocks of the Earth’s crust with special
emphasis on their approximate time of formation and the
changes they have undergone since their formation.
Stratigraphy
Photo by W. W. Little

4. Photo by W. W. Little
Stratigraphy Records History
The stratigraphic record preserves details pertaining to major
geologic events, such as:
•Mountain-building (orogenies)
•Land-Sea Relation
•Climatic fluctuation
•Glaciation
•Epeirogeny
•Age of rocks
•Evolution and migration of plants

5. Principles of Stratigraphy
•Superposition
•Original Horizontality
•Lateral Continuity
•Crosscutting Relationships
•Faunal (biological) Succession

6. Sedimentarylayersbecomeprogressivelyyoungerupward
throughastratigraphicsequence.
Principle of Superposition

7. Atthetimeofdeposition,sedimentarylayersarehorizontal.Ifthey
arenolongerhorizontal,theyhaveundergonesometypeof
deformation.Thetwomostcommontypesofdeformationarefolding
andfaulting.
Principle of Original Horizontality

8. Tilting of Beds by Folding

9. Tilting of Beds by Faulting

10. At the time of deposition, sedimentary layers are continuous for long
distances. If they terminate abruptly, they have either undergone
deformation or they have been eroded. The two most common causes
of abrupt bed termination are faulting and unconformities.
Sedimentary layers can taper laterally into other sedimentary units.
Principle of Lateral Continuity

11. Bed Termination by Faulting

12. Bed Termination by Unconformity
Anunconformityisasurfacethatrepresentsanepisodeofmissing
geologictime.Duringtheformationofanunconformity,sedimentis
eithernotdepositedoritisdepositedandsubsequentlyremovedby
erosion.

13. Therearethreetypesofunconformitiesbasedonthetypesofrocks
involvedandtheirattitudewithrespecttoeachother.
Types of Unconformities
Disconformity (paraconformity) Nonconformity
Angular
Unconformity

14. Principle of Cross-cutting
Relationships
Arockbodythatiscut(crossed)byanotherrockbody,structure,or
unconformityisolderthanwhatiscuttingit.

18. Fossilsarefoundinaconsistentandpredictablesuccession
everywhereonearth.
Principle of Fossils or Faunal
(biological) Succession

19. Fossils may be defined as remains or impression of
ancient animals and plants which have been preserved
in sedimentary rocks by natural processes.
It is like the calendar of an year. As an year is divided
into different months, weeks, days , geological time
scale is also subdivided into smaller and smaller unit to
suit stratigraphicalstudy and correlation. The
unconformities subdivide geological time scale into
smaller units.

20. Geologic Time Scale

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