Table

Hierarchy in Magnetostratigraphic Units and Polarity Chron (Time) Units as Recommended by the

IUGS Sub-Commission on the Magnetic Polarity Time Scale"

Magnetostratigraphic

Geochronologic

Chronostratigraphic

Approximate duration

polarity units

(time) equivalent

equivalent

(years)

Polarity megazone

Megachron

Megachronozone

10M09

Polarity superzone

Superchron

Superchronozone

10M08

Polarity zone

Chron

Chronozone

lO'-lO7

Polarity subzone

Subchron

Subchronozone

105-106

Polarity microzone

Microchron

Microchronozone

<105

Polarity cryptozone

Cryptochron

Cryptochronozone

Existence uncertain

"After Opdyke and Channell (1996).

"After Opdyke and Channell (1996).

were never established for the Brunhes, Matuyama, Gauss, and Gilbert chrons during the original development of the GPTS through polarity determinations on dated lava flows (§4.2.1). However, type localities are known for many subchrons, such as the Jaramillo Creek in New Mexico for the Jaramillo subchron. Because of the historical development of the subject, the four named chrons are used for the Pliocene and Pleistocene, with the remainder of the time scale, as shown in Fig. 4.8, being subdivided into polarity chrons designated by numbers correlated to marine magnetic anomalies (see §5.3.2 for an explanation of the nomenclature). The geomagnetic polarity history as preserved in the sea floor has become the template (a sort of type section) for the GPTS for the past 160 Myr. Thus the designation of type sections, in the classical stratigraphic sense, for the geomagnetic polarity pattern since 160 Ma is unnecessary (Opdyke and Channell, 1996). On the other hand, type sections for magnetobiochronology (the correlation of the biological record with the magnetic polarity sequence) might be desirable. For example, Opdyke and Channell suggest that a potential type section for the Paleogene might be the Contessa section at Gubbio in Italy studied by Lowrie et al. (1982).

4.3.2 Methods in Magnetostratigraphy

For times older than the present sea-floor record, the use of type sections for geomagnetic polarity history is the only way in which the record of polarity changes can be established. The establishment of polarity sequences must then be carried out using classical stratigraphic principles involving type sections. Opdyke and Channell (1996) point out that a good example of a viable magnetostratigraphic type section is that recording the Kiaman Superchron (see §4.3.6) in Australia, where the superchron is thought to be tied directly to the rock record (Irving and Parry, 1963; Opdyke et al., 1999).

Polarity subchrons as short as 20 kyr in duration are present in the polarity record, so any sampling scheme should ideally attempt to sample intervals of this duration. It is common practice when sampling marine sedimentary cores to remove samples at 10 cm spacings. Such sampling density in pelagic sediments with sedimentation rates of 10 mm kyr"1 (10 m Myr"1) would resolve polarity chrons with duration greater than -10 kyr. However, terrestrial sediments rarely have the same homogeneity as deep-sea sediments with respect to lithology and sedimentation rates. Terrestrial sections are thus usually selected for sampling for some particular reason, such as the presence of important vertebrate faunas, radiometrically dated horizons, or climatically indicative levels in loess, tills, or pollen-rich deposits.

The basic problem in magnetostratigraphic studies is that the observed series of normal and reverse polarity zones usually has a pattern that could correlate with one or more segments of the GPTS. At first a constant sedimentation rate, or rate of extrusion of lavas, is assumed and trial correlations with the GPTS are made. Such correlations are aided by additional information such as radiometric ages or biostratigraphic events in the section that have been correlated with the GPTS elsewhere. An excellent example of this is illustrated in Fig. 4.9 for the Haritalyangar section in India (Johnson et al., 1983). Here the correlation of polarity zones with the GPTS is aided by magnetostratigraphic data from Pakistan where some of the same Miocene vertebrate fossils are correlated with radiometric ages on volcanic ashes. On this basis the reversal sequence can be correlated with the GPTS quite unequivocally on the basis of pattern fit. The resulting regression is significant at the 99% level.

Using the available magnetostratigraphic data, Gradstein et al. (1994) proposed an integrated geomagnetic polarity and stratigraphic time scale for the whole Mesozoic, the framework of which involves ties between radiometric dates, biozones, and stage boundaries and between biozones and magnetic

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