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Paleomagnetism and Magnetostratigraphy

M. Porreca - Univ. of Perugia

Paleomagnetism

Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Rocks (e.g. volcanic, sedimentary rocks) can record direction and intensity of the field as it has changed over geologic time. 

Paleomagnetism

Magnetization of a rock

Vector of magnetization recorded into a rock sample

Rock magnetization

The magnetization of a rock is described by a vector (F) and defined by:

Declination (D): azimuth

Inclination (I): dip

Intensity of magnetization (H)

Down (positive) or Up (negative)

Magnetostratigraphy : magnetism into the stratigraphic sequence

Magnetostratigraphy

GPTS: geomagnetic polarity time scale

Magnetostratigraphy

Magnetostratigraphy uses records of changes in polarity of the geomagnetic field preserved in sedimentary sequences to correlate between wells/outcrops and to date the sediment.

Individual normal (positive) and reverse (negative) polarity intervals (called "Chrons") typically range from ~10 thousand to 10 million years in duration.

Definition

Magnetostratigraphy is part of Integrated Stratigraphy

Integrated stratigraphy

Earth's magnetic field: main features

Earth's magnetic field

Magnetic field

The magnetic field can be measured in each part of the world: it changes in space and in time

Magnetic field measurement

The Earth magnetic field is a vector quantity

(magnetic field vector, F)

Declination (D): azimuth

Inclination (I): dip

Intensity of magnetization (H)

Up or Down

Models reproducing the Earth's magnetic field

Simple model

Complex model

Models

Magnetic dipole field (dipolar)

Dinamo triggered by external fluid core motion

Reversal of magnetic field on Earth

Reversals

Brunhes and Matuyama were the first researchers to find reverse magnetic polarity (these reversals are recorded into the rocks)

Magnetic field reversals and magnetic polarity scale

Magnetic polarity scale

Normal polarity

Reverse polarity

‘bar code’ of alternating normal (north directed) and reverse (south directed) polarity chrons with characteristic durations

Geomagnetic polarity time scale (GPTS)

GPTS 2000

Normal polarity

Reverse polarity

Geomagnetic time scale

Construction of the GPTS and APTS

Development of the geomagnetic polarity time scale

(GPTS) over the last half century of study.

The initial assumption of periodic behaviour (Cox, 1963) was soon abandoned as new data became available.

The first modern GPTS based on marine magnetic anomaly patterns was established by Heirtzler et al. (1968).

Subsequent revisions by other authors show improved age control and increased resolution.

A major breakthrough came with the astronomical polarity time scale (APTS) in which every individual reversal is accurately dated (e. g. Hilgen et al., 1997).

A long history.....

Astrochronostratigraphic polarity time scale (APTS)

Langereis et al. 2010

Geomagnetic behaviour: different time scales

Geomagnetic behaviour

100 yr – 10 kyr

secular variation

excursions

inversions

10 kyr – 1 Myr

(sub)chron

10-100 Myr

superchron

Secular variation of the Earth's magnetic field

Short-term variation

Secular variation

Secular variation diagram for United Kingdom for years 1600 AD to the present (Butler, 1992) - Greenwich (UK)

Paleomagnetism as dating tool

  • Present-day and historical field: excellent
  • Archeomagnetic field (0-7 kyr): very good
  • Last 5 Myr: good
  • Back in time: not good
  • Archean : mystery

Age determination

Ocean

Marine magnetic anomalies

Langereis et al. 2010

Formation of marine magnetic anomalies during seafloor spreading. The oceanic crust is formed at the ridge crest, and while spreading away from the ridge it is covered by an increasing thickness of oceanic sediments.

The black (white) blocks of oceanic crust represent the original normal (reversed) polarity of the magnetisation acquired

upon cooling at the ridge.

The black and white blocks in the drill holes represent normal and reversed polarity of the sediments.

Types of magnetization

Type of

magnetization and laboratory

The magnetization of a rock can be acquired through different processes, synchronous to or later than the formation of the rock, which produce a respectively primary or secondary magnetization.

Three types:

  • Thermal remanent magnetization (TRM)
  • Chimical remanent magnetizazion (CRM)
  • Detritical remanent magnetization (DRM)

Rocks containing magnetic minerals are able to be magnetized

Thermal remanent magnetization (TRM)

Thermal

Magnetization that is acquired by igneous rocks while they cool

TRM of the lava

Earth's magnetic field

Detritical remanent magnetization (DRM)

Sedimentary rocks, DRM reflects depositional history

Detritical

Sampling and paleomagnetic lab

How can we measure the fossil magnetism into the rocks?

1. Field survey to identify the suitable rocks for paleomagnetic analyses (e.g. lavas, clay, silt, etc.)

2. Sampling of oriented cores

3. Paleomagnetic analyses in laboratory

Laboratory techniques

Field sampling (small cores)

Sampling

Long cores and workflow

Large cores

Managing cores

Drilling

Extraction

Re-sampling

Lab measurements

Stratigraphic and sedimentological obs

L'Aquila core (LAqui-core)

Example of integrated stratigraphy

Example...

LAqui-core (Porreca et al., 2016)

core portion

stratigraphic and sedimentological chart

Paleomagnetic analysis

Paleomagnetic laboratory

Paleomag analyses

2G cryogenic magnetometer

Spinner magnetometer

Paleomagnetic procedure

Samples and paleomag data projections

Oven

Measures

Data projection

Spinner magnetometer

L. Sagnotti

Component analysis

Identification of primary magnetization:

Declination (D)

Inclination (I)

Intensity (H)

Component analysis

Case studies

Applications:

case studies

  • Neogene part of the GPTS (Monte dei Corvi section, Italy)

  • The Cretaceous-Paleocene (K-T boundary) succession (Central Apennines at Gubbio, Italy)

  • Integrated stratigraphy with orbitally controlled resolution (Triassic Newark basin, USA)

Global Boundary Stratotype Section and Point (GSSP)

https://timescalefoundation.org/gssp/

Gubbio section (central Italy)

Cretaceous-Tertiary boundary (Umbria)

a key role in three major geological controversies:

- continental drift/paleomagnetism

- the Cretaceous-Tertiary (K-T) boundary layer, related to the meteorite impact and extinction of the dinosaurs

- more recently, paleoclimate studies of the Paleocene and Eocene.

Gubbio

G. Pialli

Data

Comparison between the Upper Cretaceous polarity record from 150 m of pelagic limestone at Gubbio, and the polarity record from several hundred Kms of sea floor in three different oceans.

Magnetostratigraphy

(Lowrie & Alvarez, 1977; Alvarez, 2009)

M. Corvi (Italy)

Neogene part of the GPTS

(Monte dei Corvi section, Italy)

Normal polarity

Reverse polarity

M. Corvi

Magnetostratigraphy for the composite Monte dei Corvi beach section and calibration to the Neogene astronomical time scale.

The Tortonian GSSP is indicated by the red dashed line. The Monte dei Corvi section is suggested as the Tortonian reference section

Langereis et al. 2010

Global boundary Stratotype Section and Point (GSSP)

Newark-Hartford continental basins (USA)

Newark USA

Paleomagnetic and cycle stratigraphic analyses of nearly 7000 m of section from continuous cores in the Newark

basin and an overlapping 2500 meter-thick composite outcrop provide

an astrochronostratigraphic polarity time-scale (APTS) for practically the entire Late Triassic (Carnian, Norian and Rhaetian) and the Hettangian (233 to 199 Ma)

Kent al. 2017

Stratigraphic correlation and climate implications

Newark-Hartford APTS

Kent al. 2017

Applications

Stratigraphic

correlation

Sedimentary record of climate cycles at Pizzo Mondello (outcrop section shown in photograph at top) correlated to portion of Newark-Hartford APTS.

Correlation

Pizzo Mondello (Italy)

Marine sequence

Kent et al. 2017

Global climate change

pCO2 estimates from paleosols and the isotope 13C record from marine carbonates

Climate

Kent et al. 2017

Estimates of atmospheric pCO2 from paleosol carbonates in the Newark and Hartford basins

Late Triassic

paleogeography

Paleogeographic reconstruction

Reconstruction of Pangea is based on continental rotation parameters of Lottes and Rowley (1990) and mean global pole from Kent and Irving (2010).

Some key continental localities are indicated by filled circles (for positions at 220 Ma) connected to their relative positions at 200 Ma by open circles.

Relative positions of localities at 230 Ma and 210 Ma indicated by crosses.

Kent et al. 2017

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