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Review of geological time scale (GTS)
Development of the geomagnetic polarity time scale (GPTS)
Applications in geology (dates and rates)
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Lecture 16: The magnetic time scale and
magnetostratigraphy
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.
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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)
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The GPTS and the GTSIdentification 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
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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
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Mason & Raff, 1961
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Cox et al. (1963)
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NORTH AMERICA AFRICA CLOSED SYMBOLS: NORMAL POLARITY
EUROPE HAWAII OPEN SYMBOLS: REVERSE POLARITY
Brunhes Matuyama Gaussnormal epoch normal epoch normal epoch reverse epoch?
OBS
ERVA
TIO
NS
0 1.0 2.0 3.0 4.0TIME IN MILLIONS OF YEARS
GEO
MA
GNET
IC D
ECLI
NA
TIO
N
N
90
R
Olduvai event
Mammoth event
Cox et al. (1964)
note archaic use of terms “Epoch” and “event”
A note on terminologyepoch/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
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10
bathymetry
profile
profile backwards
modelGPTS
10 my10 my
BM MGa GaGi Gi
0
0
1000
1000
γ
0
γ
Basic insight that proved plate tectonics
Pitman, 1966
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V16-134 (62oS,91oW)
Inclination
Dept
h in
Cor
e (c
m)
Brun
hes
Mat
uyam
a Ga
uss
Gilb
ert
Jaramillo
Olduvai
Mammoth
Pol Event Epoch
0.7 Ma
2.4
3.35
Ψ
χ
Ω
φ
b) c) a) zones
12240 270 300 330 0
-60
-30
0
30
60
13
13
13
13
25
25
M25
13
13
13
25
1325
34
25
13
13
34
13
M0
M0
5
13
1325
34
34
34M0
M0
M0
13
1325
34
34 M0
13 2525 34
3434
M0
13 25
-60
-30
0
30
60
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Addition of biostratigraphy
Epoch Stage Polarity ChronPlanktonic
Foraminifers
Pliocene
Pleistocene
Holocene
Tortonian
Messinian
Zanclean
Piacenzian
Gelasian
E. Pleist.M. Pleist.Lt. Pleist.
C5
C4A
C4
C3B
C3A
C3
C2A
C2
C1
N13N14N15
N16
N17
N17b
N18/N19
N20/N21
N22
M10M11M12
M13
M14
PL1
PL2PL3PL4PL5PL6
Pt1
NN7
NN8
NN9
NN10
NN11
alt NN11
a
NN12NN13NN14NN15NN16NN17NN18NN19NN20NN21
Nanno-fossils
Age(Ma)
0
5
15
20
10
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”
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precession eccentricity
3.0
2.5 A
ge (Ma)
sapr
opel
s
pola
rity
Ga
uss
Chro
n
How to do magstratNeed 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
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Magnetostratigraphy example in Miocene east african rift deposits Tauxe et al. (1985)
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Demonstrate stability
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Demonstrate two polarities
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Show stratigraphy
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Tracing isochrons, Siwaliks of Punjab Pakistan (Behrensmeyer and Tauxe, 1982)
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similarity to Indus/Soan River system today
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August, 2009
August, 2010
And the Indus is still doing it…
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