Rocks, fossils, and time

1
Rocks, fossils, and time

• Chapter 5
• continued

2
Last class

• Stratigraphy
• Unconformities
• Where sed rocks form
• Transgression/regression

3
Which rock is oldest. How do you know?
4
Correlation

• Demonstrating equivalency between rock units

5
How to correlate rocks

• Rock type
• Lithostratigraphic units

6
How to correlate rocks

• Age
• Time units
• Eon, era, period

7
Rock units dont pay attention to boundaries of
time

• Devonian and Mississippian Chattanooga Shale
• Not all same rock unit deposited at the same time

8
How to correlate rocks

• Fossils
• Biostratigraphic (fossil) correlation

9
Body fossils
10
Body fossils
11
Trace fossils
12
Trace fossils
13
Trace fossils Coprolites
14
Trace fossils Coprolites
15
Fossil preservation

• Easiest for critter with hard parts (shells,
  bones, teeth)
• Occasional soft bodied animal preserved
  (jellyfish, plant, worm)
• Fossil record biased

16
Fossils and relative dating

• Use principle of superposition with fossils
• Fossil succession
• Fossils lowest in rocks are oldest
• Sequence of fossils is consistent and predictable
• Relative ages of rocks can be correlated using
  fossils
• Lots of sandstones
• Not all with T-Rex fossils

17
Fossil correlation

• Relative ages of rocks can be correlated using
  fossils

18
Fossil succession

• Fossil assemblages succeed one another through
  time in a regular and determinable order
• Fossil succession determined geologic column

19
Type localities defining the Geologic column

• Fossil assemblages succeed one another through
  time in a regular and determinable order
• Fossil succession determined geologic column

20
How to categorize rocks

• Formation is basic unit
• Lump formations to get group
• Lump groups to get supergroups

21
Three formations in the Grand Canyon, Arizona

• Formation is basic unit
• Lump formations for a group
• Lump groups for a supergroup

22
Formations subdivided into members
23
Formations combined for Groups
24
Correlation

• Age
• Lithostratigraphic
• easiest
• Biostratigraphic
• More global

25
Easy lithostratigraphic correlation in SW U.S.
26
Lithostratigraphic correlation
27
Using a marker bed
28
Use of igneous rocks for correlation
29
Ashfall from volcanic eruptions
30
Ash layer in shale
31
Fossil correlation

• Biostratigraphic units

32
Fossil correlation

• Biostratigraphic units

33
Fossil existence in rock record
34
Correlation

• Each method has time and place
• May want to see rocks of Cambrian age
• Where is sealevel, what depth?
• May want to follow one sandstone unit
• Track sealevel changes through time
• May want to see rocks with certain fossil

35

• What can you say about sedimentation rates in the
  two columns during the time between the
  correlation lines?

36
Absolute and relative time scales
37
Absolute and relative time scales
38
Absolute and relative time scales
39
Establishing ages by bracketed intrusions

• Seds deposited
• Intrusion by igneous rock (datable)
• Erosion
• Deposition of sediment, lava (datable),
  sediment