Review of geological time scale (GTS)

Development of the geomagnetic polarity time scale (GPTS)

Applications in geology (dates and rates)

1

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.

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 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

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

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6

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

11

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

13

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”

14

15

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

16

17

Magnetostratigraphy example in Miocene east african rift deposits Tauxe et al. (1985)

18

Demonstrate stability

19

Demonstrate two polarities

20

Show stratigraphy

21

Tracing isochrons, Siwaliks of Punjab Pakistan (Behrensmeyer and Tauxe, 1982)

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24

25

26

27

28

similarity to Indus/Soan River system today

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August, 2009

August, 2010

And the Indus is still doing it…