Lecture 16:
The magnetic time scale and
magnetostratigraphy
Review of geological time scale (GTS)
Development of the geomagnetic polarity
time scale (GPTS)
Applications in geology (dates and rates)
1
Geological time scale
list of ordered events placed in a temporal
context
Eons: Phanerozoic, Proterozoic,
Eras: Mesozoic, Cenozoic, etc.
Periods: Cretaceous, Paleogene, Neogene, etc.
Series: Oligocene, etc.
Stages: Maastrichtian, Messinian, etc.
2
Stage is the fundamental unit
Defined by its base at a particular place
(global standard section and point - GSSP)
Attach a chronology using a lot of different
dating techniques (radioisotopic decay,
climatic variations with known age
dependence, magnetic stratigraphy,
progression of fossil sequences)
Most often dates are estimated by
correlation, interpolation and/or
extrapolation.
Constant revision (the official website is
stratigraphy.org)
3
The GPTS and the GTS
Identification of a particular polarity reversal
allows direct correlation and/or dating of
events globally
Increasingly, stages are defined on the basis
of magnetic reversals.
Development of GPTS and GTS have gone
hand in hand
4
Reversely magnetized rocks found in early
1900s (see Chapter 14)
But only systematic study of K-Ar dates
combined with polarities on global collection
of lava flows demonstrated reversal of
geomagnetic field
5
Mason & Raff, 1961
6
Cox et al. (1963)
7
GEOMAGNETIC DECLINATION
OBSERVATIONS
Cox et al. (1964)
Brunhes
normal epoch
Matuyama
normal epoch
Olduvai
event
N
Gauss
normal epoch
reverse epoch?
Mammoth
event
90
R
0
1.0
2.0
3.0
TIME IN MILLIONS OF YEARS
4.0
NORTH AMERICA
AFRICA
CLOSED SYMBOLS: NORMAL POLARITY
EUROPE
HAWAII
OPEN SYMBOLS: REVERSE POLARITY
note archaic use of terms “Epoch” and “event”
8
A note on terminology
epoch/event changed to chron/sub-chron in
1979
superchrons and cryptochrons
aligning anomaly terminology with
stratigraphic. Anomalies won. So now the
‘Olduvai’ is Chron C2n
9
Basic insight that proved plate tectonics
0
10 my
γ
GPTS
1000
model
Gi Ga
M B
10 my
M Ga Gi
0
γ
1000
profile backwards
0
profile
bathymetry
10
Pitman, 1966
V16-134 (62oS,91oW)
b)
Epoch
Depth in Core (cm)
Matuyama
Brunhes
a)
Pol
Event
c)
zones
Ω
0.7 Ma
Jaramillo
Ψ
χ
Olduvai
Gilbert
Gauss
2.4
Inclination
11
Mammoth
3.35
φ
13
60
25
60
13
25 13
M0
13
34
34
M25 M0
30
13
30
M0
25
5
13
13
34
0
34
25
13
13
34
0
13
-30
M0
13
34
M0
13
13
25 13
13 25
25
34
240
-30
13
25
-60
25
34 M0
270
300
34
13 25
34
-60
M0
330
0
12
Addition of biostratigraphy
Age
(Ma)
Epoch
Stage
0
Holocene
Lt. Pleist.
M. Pleist.
Pleistocene
Polarity Chron
Pt1
C2A
N20/N21
PL6
PL5
PL4
PL3
PL2
C3
N18/N19
PL1
C2
Gelasian
Pliocene
N22
C1
E. Pleist.
Planktonic
Foraminifers
Piacenzian
Zanclean
5
Messinian
C3A
C3B
N17b
N17
C4
10
C4A
Tortonian
C5
13
M14
M13
Nannofossils
NN21
NN20
NN19
NN18
NN17
NN16
NN15
NN14
NN13
NN12
alt
NN11
a
NN11
NN10
N16
N15
N14
N13
NN9
M12
M11
M10
NN8
NN7
Calibration
The polarity sequence is based on marine
magnetic anomalies for the last ~200 Myr
Prior to that, have to use terrestrial
sequences, piecemeal
Dates attached by direct or indirect radioisotopic dating “astrochronology”
14
sapropels
polarity
2.5
Age (Ma)
Gauss
Chron
precession eccentricity
3.0
15
How to do magstrat
Need to establish that magnetic record is of
polarity history and not some overprint
Need to sample sufficiently long and densely
to recover a unique period of geomagnetic
polarity history (avoiding gaps and changes in
sed rate)
Need some idea of age
Need to correlate to time scale
16
Magnetostratigraphy example in Miocene east
african rift deposits Tauxe et al. (1985)
17
Demonstrate stability
18
Demonstrate two polarities
19
Show stratigraphy
20
Tracing isochrons, Siwaliks of Punjab
Pakistan (Behrensmeyer and Tauxe, 1982)
21
22
23
24
25
26
27
similarity to Indus/Soan River system today
28
And the Indus is still doing it…
August, 2009
August, 2010
29