Geology Reference
In-Depth Information
3
Magnetostratigraphy
Abstract:
This chapter provides the fundamentals of conducting and inter-
preting a magnetostratigraphy study of sedimentary rocks. Magnetostratig-
raphy can provide an average sediment accumulation rate for the sedimentary
sequence targeted for a rock magnetic cyclostratigraphy study that will help
the investigator identify astronomically forced cycles in the cyclostratigraphy.
This chapter covers magnetostratigraphic sampling techniques and strategies,
paleomagnetic measurements and demagnetization, plotting of magneto-
stratigraphic data, and the determination of a reversal stratigraphy from
the paleomagnetic data. The chapter ends with a brief discussion of the
geomagnetic polarity time scale (GPTS) and techniques for tying the reversal
stratigraphy to the GPTS to provide absolute time for the sedimentary
sequence being studied.
3.1
Introduction
One of the most important considerations for accurately identifying
astronomically forced cycles in rock magnetic cyclostratigraphy is an
independent assignment of time to a sedimentary sequence so that the
approximate length of any cycles observed can be estimated. For example,
Olsen and Kent (1996) used the ages of geologic time scale boundaries
(Triassic-Jurassic, Carnian-Norian, Norian-Rhaetian) in the Newark
Basin lake sediments to put an approximate age scale on the sequence of
lithologically recorded, lake depth van Houten cycles. They found that
the McLaughlin cycle, which is observed to bundle the shorter van
Houten cycles in the sequence, had a duration of anywhere from 308 to
442 kyr, with a mean duration of 397.7 kyr. They identified this cycle as
the 405 kyr long eccentricity cycle. (Actually Olsen and Kent (1996) used
412.885 kyr for long eccentricity from Berger et al. (1992).) Once long
