1. Geologic Time

2. the key to the past relative time absolute time
relative time vs. absolute time
relative time
order of events or objects from first (oldest) to last (youngest)
she is older than he is; she was born first and he was born last
absolute time
age of events or objects expressed numerically
she is twenty-one and he is nineteen
study of timing of geologic events and processes is geochronology

3. relative time and relative order
apply simple concepts to determine…
• original horizontality
• superposition
• lateral continuity
• cross-cutting relationships
• inclusions
• unconformities

4. relative age dating concepts
original horizontality
all beds originally deposited in water formed in horizontal layers
sediments will settle
to bottom
and blanket
the sea floor

5. relative age dating concepts
superposition
youngest
within a sequence of undisturbed
sedimentary or volcanic rocks,
oldest rocks are at the bottom
and youngest at the top
….young upward…
lateral continuity
oldest
original sedimentary layers extend
laterally until they thin at edges
continue
continue

6. relative age dating concepts
cross-cutting relationships
a disrupted pattern is older than
the cause of the disruption
e.g. an intrusion is younger
than the rocks it intrudes

7. relative age dating concepts
inclusions
fragments of other rocks contained in a body of rock
must be older than the
host rock
e.g.
xenoliths in granite are older
than granite and
2) pieces of rock in
conglomerate are older
than conglomerate

8. relative age dating concepts
unconformities
a contact between sedimentary formations that represents a gap
in the geologic record -- “gap” represented is variable (i.e. amount of time or the amount of missing section)
different types of unconformities
conformity
• relatively continuous deposition
• deposition of a sequence of parallel layers
• contacts between formations do not represent significant amounts of time

9. conformity
from:

10. relative age dating concepts
different types of unconformities
angular unconformity
• contact separates overlying younger layers from tilted older layers
• sequence of layers
is not parallel
• contacts between formations
may represent significant
amounts of time
angular unconformity

11. angular unconformity
from:

12. angular unconformity

14. relative age dating concepts
different types of unconformities
disconformity
• contact separates beds (formations) that are parallel but not continuous in age
• sequence of layers
is parallel
• contacts between
formations
may represent significant
amounts of time
• missing time is difficult to recognize (may need other
information—paleosol
(remains of ancient soils)?)

15. relative age dating concepts
different types of unconformities
nonconformity
• strata deposited on older crystalline (metamorphic/igneous) rock
• erosion surface on igneous/metamorphic rock covered by
sedimentary rocks
• large gap in
geologic record
Magma intruding and
destroying/altering part
of the rock record
nonconformity

16. what events occur?
angular unconformity

17. what events occur?
nonconformity

18. now that we know all this…what happened?

20. deposition

21. intrusion

22. tilting and
erosion

23. subsidence
and
renewed
deposition

24. missing formation (time)?

25. dike intrusion

26. erosion and exposure

27. subsidence and deposition

28. uplift/sea level fall and river deposition

29. relative ages of the formations

30. relative age: correlation
how is this done?
faunal succession (correlation by fossils)
fossil species succeed one another through the layers
in a predictable order
index fossil
short-lived organism;
points to narrow range
of geologic time
fossil assemblage
group of fossils
associated
together