The document discusses the principles of relative geologic time and methods to order geological events, including Steno's laws such as original horizontality, lateral continuity, and superposition. It highlights the concept of unconformities and their types, as well as the importance of fossils in correlating rock layers across different locations. Fossils, particularly index fossils, are crucial in defining and segmenting geologic time based on the temporal succession of species.

1. Ordering geological events in time
Relative Geologic
Time

3. What the heck happened here?!?
Can we make a relative
chronology of events?

4. Steno’s Laws and Corollaries
Principles of Relative
Time in Geology

5. • Original Horizontality
• Lateral Continuity
• Superposition
• Cross-cutting Relations
• Included Fragments
Steno’s Laws and
Corollaries
Nicolas Steno
1638-1686

6. Original Horizontality
Because of gravity, sediments are deposited
as (mostly) horizontally oriented bodies. All
tilted strata have therefore been deformed
(and usually after lithification).

7. Tilted and deformed strata, near Nizwa, Oman

8. Original horizontality (and angular unconformity),
Grand Canyon

9. Lateral Continuity
Sediments are deposited generally as tabular
sheets that extend in all directions until they run into
a barrier or decrease laterally because of a lack of
sediment supply. Breaks in the lateral continuity
happen after deposition and lithification.

11. River erosion disrupts lateral continuity

12. River erosion disrupts lateral continuity

13. Once continuous sandstones, Al Ula

14. Superposition
In a succession of sedimentary rocks that has not
been deformed, each rock unit is younger than the
unit below it, and older than the unit above it.

15. Oldest beds on the bottom, youngest at the top

16. Gorges du Todre, Jurassic of Morocco

17. Cross-Cutting Relations
Fractures, faults, and intrusions that cut
across rock are necessarily younger than the
rock through which they cut

18. The fault is younger than the rocks it cuts

19. Small-displacement faults cutting fluvial strata

20. Basalt dikes cutting through granite are younger than the granite

21. Included Fragments
Fragments of rock that are included/incorporated
within a rock are older than the rock within which
they are included (or the rock containing the
inclusions is younger than the inclusions).

25. Unconformities - “Missing Time”
• Layers of rock that were deposited without
interruption are called conformable layers
• However, periods of erosion or non-deposition can
produce substantial gaps in the stratigraphic
record
• Unconformities, or “missing time”, are surfaces
with no stratigraphic record of that duration of
geologic time at that location

26. Types of Unconformities
• Nonconformity - sedimentary strata directly overlie
basement igneous and/or metamorphic rock
• Angular Unconformity - angular relationship
between overlying and underlying strata. Implies
tilting, erosion, deposition
• Disconformity - sedimentary strata are parallel with
one another, but there is obvious evidence for
erosion at the boundary

27. Disconformity
Nonconformity
Angular Unconformity

28. Unconformities
develop over
long periods
of time

29. Angular Unconformity
Mosaic Canyon, Death Valley

30. Angular unconformity, Siccar Point, Scotland
(the most famous of angular unconformities)

33. Angular Unconformity, Grand Canyon

35. Nonconformity -
The “Great Unconformity”,
Grand Canyon
Cambrian Tapeats
sandstone overlies
Precambrian Vishnu Schist

36. The Great Unconformity (nonconformity), Grand Canyon

37. Disconformity - Here, a channel cuts down through older
strata, showing obvious evidence for erosion

39. Your job: work out the relative timing of events
Such exercises can all be done with assigning any
“absolute” numbers or ages

40. Fossils and Relative
Time
Critical for dividing geologic time into small increments
and for correlating rock strata over large distances

41. What Are Fossils?
Fossils are the remains or
traces of past living organisms
(animals, plants, microbes)

42. Correlation of Rock Layers
• Matching of rocks of similar ages in different
locations is known as correlation
• Correlation can use rock type, but commonly relies
on the presence of fossils
• William Smith (late 1700s) noted that sedimentary
strata in widely separated locations could be
identified and correlated by their distinctive fossil
content

43. Fossil Succession
• Fossil species succeed one
another in a definite and
determinable order
• Therefore, any time period
represented by rocks can be
recognized and defined by its
fossil content

44. Usefulness of Fossils Succession
in Geologic Time
• All fossil taxa (e.g., family, genus, species, etc.)
separate geologic time into three parts
• The time before that taxon evolved
• The time during which that fossil taxon was extant
• The time after which that taxon went extinct
• Thus, each fossil taxon gives us a discrete duration
of geologic time - this is used for correlation

45. • Some fossils are extremely useful in correlation.
These are referred to as “index fossils”.
• A reliable index fossil species should have the
following characteristics:
• Wide geographic range
• Abundant
• Short geological duration (rapid evolutionary change)
• Morphologically distinct - Identifiable
• Independent of rock type
Index Fossils

46. • So the best index fossils are planktonic
(floating, drifting) or nektonic
(swimming) marine organisms that
evolve rapidly

47. Some good index fossils

48. Some bad index fossils

49. Using overlapping ranges of numerous fossils are
more useful than using just a single fossil

50. Multiples fossils used for time correlation