Determining geological ages

2. Determining geological ages • Relative ages – placing rocks and geologic events in their proper sequence, oldest to youngest. • Absolute dates – define the actual numerical age of a particular geologic event. For example, large dinosaurs died out 65 mya. The Lavas along Rt 22 and Rt 78 were deposited about 205 mya.

3. Relative Age Dating assigns a non-specific age to a rock, rock layer or fossil based on its position in the Strata relative to other rocks, rock layers or fossils.

4. Relative Age Dating is based on a list of principles or rules.

5. First principle of relative dating • Law of superposition • Developed by Nicolaus Steno in 1669 • In an undeformed sequence of sedimentary or volcanic rocks the oldest rocks are at the base; the youngest are at the top

6. -Superposition

7. Principle of Superposition

8. Superposition illustrated by strata in the Grand Canyon

9. 2nd principle of relative dating • Principle of original horizontality • Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)

11. 3rd principle of relative dating • Principle of cross-cutting relationships

12. 3rd principle of relative dating • Principle of cross-cutting relationships (example 2)

13. Cross-Cutting

14. An Igneous rock is always younger than the rock layer that it has intruded or cut across.

15. Principle of Cross-Cutting Relationships The dike is youngest because it cuts across layers 1-4 Layer 1 is the oldest rock layer

16. Key to Rocks Used in Diagrams

17. Limestone

20. Igneous

21. Metamorphic

22. Cross-cutting Relationship with multiple overlapping intrusions

23. Erosional Features and Faults that cut across rock layers are always younger.

24. Example of Law of Cross-Cutting Relationships Which came first, the rock layers or the faults?

25. Cross-cutting Normal Fault

28. The Law of Embedded Fragments, or Law of Inclusion, states that rocks that are embedded in another rock must be older than the rock in which it is found.

29. Inclusion

30. Examples of Law of Inclusions

31. Inclusion- Conglomerate fragments in overlying Shale

32. Inclusion-Granite fragments included in overlying Shale

33. Inclusion- Shale fragments imbedded in Granite intrusion

35. Another method of examining the Geologic Record involved examining instances where rock layers are missing (Unconformities).

36. The processes that would bring about the removal of these missing layers require large amounts of time.

37. Unconformities (loss of rock record) • An unconformity is a break in the rock record produced by erosion and/or nondeposition • Types of unconformities • Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lost • Angular unconformity – tilted rocks overlain by flat-lying rocks • Disconformity – strata on either side of the unconformity are parallel (but time is lost)

38. Layered sedimentary rocks (a) 8_9 Nonconformity Igneous intrusive rock Metamorphic rock (b) Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks (c) Disconformity Trilobite (490 million years old) Brachiopod (290 million years old)

39. Formation of an angular unconformity

40. Angular Unconformity Angular Unconformity Erosional Surface

41. Horizontal younger sediments over tilted older sediments Cambrian Tapeats sandstone over Precambrian Unkar Group What type of unconformity is this? Grand Canyon in Arizona

42. Angular Unconformity

43. Angular Unconformity, Siccar Point, Scotland

46. Disconformity

47. Development of a Nonconformity An intrusion occurs The overburden is eroded away Pennsylvanian sandstone over Precambrian granite is a nonconformity Sea level rises, new sediment is deposited

48. Nonconformity- Sedimentary Rock layers over older Igneous or Metamorphic

49. Nonconformity in the Grand Canyon - Sediments deposited over Schist

50. Cross Cutting Relationships in strata Zoroaster Granite across Vishnu Schist