Outcrop du jour:

Magnetostratigraphy:

Magnetostratigraphy

The Earth generates a magnetic field that encompasses the entire planet. To first order approximation, resembles bar magnet, slightly misaligned with Earth's rotational axis. It exists because currents in the electrically-conducting fluid outer-core produce dynamo-effect.

At a given spot, the orientation of the magnetic field is described by:


Secular changes:

The orientation of the field changes over time, including:

In the last fifty years, a new dating method has emerged that exploits global changes in Earth's magnetic field as marker beds. As we will see, changes in Earth's magnetic field create unique contemporaneous patterns that are:

Remnant magnetism:

A rock's intrinsic magnetic field that forms in alignment with Earth's magnetic field when the rock forms. This can happen in three ways:

Recall Geomagnetic reversals. The primary remanent magnetic polarity of rocks can be either:

The record of these reversals is preserved in ocean floor basalt, which both:


Magnetic Polarity Time Scale to Middle Cretaceous from
Geomagnetism and Magnetostratigraphy by Alessandro Grippo.
Because these reversals occur at irregular intervals, periods of time in which they occur have unique, bar-code-like fingerprints. In the Magnetic Polarity Time Scale an imaginary composite stratigraphic column in which deposition is constant (right), unique patterns of reversals are clear.

In principle, any stratigraphic section that preserved remanent magnetism containing a series of reversals ought to be matchable to this scale. This is the heart of magnetostratigraphy - the correlation of stratigraphic units by the matching of their unique remanent magnetic patterns. The methods strengths include:

But reality intrudes to impose limitations on their use:


And yet, decades of effort have enabled the development of an ideal Magnetic Polarity Time Scale that extends to the Carboniferous, serving as the basis for correlation of real-world sections.


Global magnetostratigraphy of the Triassic - Jurassic boundary from Donohoo-Hurley, et al.

Magnetostratigraphic nomenclature

Naming conventions for magnetostratigraphic intervals are standardized as part of the North American Stratigraphic Code, but they weren't always.


Nuts and Bolts of Sampling:

In the field:

In the lab