Magnetic Stratigraphy

Meil D. Opdyke, James E.T. Channell

• Publisher: Academic Press
• Year: 1996
• ISBN:
  9780080535722
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Magnetic Stratigraphy is the most comprehensive book written in the English language on the subject of magnetic polarity stratigraphy and time scales. This volume presents the entirety of the known geomagneticrecord, which now extends back about 300 million years. The book includes the results of current research on sea floor spreading, magnetic stratigraphy of the Pliocene and Pleistocene, and postulations on the Paleozoic. Also included are both historicalbackground and applications of magnetostratigraphy. Individual chapters on correlation are presented, using changes in magnetic properties and secular variation.Key Features* Discusses pioneering work in the use of marine sediments to investigate the Earths magnetic field* Serves as a guide for students wishing to begin studies in magnetostratigraphy* Provides a comprehensive guide to magnetic polarity stratigraphy including up-to-date geomagnetic polarity time scales* Correlates magnetic stratigraphics from marine and non-marine Cenozoic sequences* Details reversal history of the magnetic field for the last 350 million years* Discusses correlation using magnetic dipole intensity changes* Up-to-date correlation of biostratigraphy with magnetic stratigraphy through the late Jurassic

1

Introduction and History

Neil D. Opdyke; James E.T. Channell Department of Geology, University of Florida, Gainesville, Florida

1.1 Introduction

Paleomagnetism has had profound effects on the development of Earth sciences in the last 25 years. In the early days, paleomagnetic studies of the different continental blocks contributed to the rejuvenation of the continental drift hypothesis and to the formation of the theory of plate tectonics.

Paleomagnetism has led to a new type of stratigraphy based on the aperiodic reversal of polarity of the geomagnetic field, which is now known as magnetic polarity stratigraphy. Magnetic polarity stratigraphy is the ordering of sedimentary or igneous rock strata into intervals characterized by the direction of magnetization of the rocks, being either in the direction of the present Earth’s field (normal polarity) or 180° from the present field (reverse polarity). This new stratigraphy greatly influenced the subject of plate tectonics by providing the chronology for interpretation of oceanic magnetic anomalies (Vine and Matthews, 1963). In the last 25 years, the geomagnetic polarity time scale (GPTS) has become central to the calibration of geologic time. The bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished, particularly for Cenozoic and Late Mesozoic time, through the GPTS. In most Late Jurassic–Quaternary time scales, magnetic anomaly profiles from ocean basins with more or less constant spreading rates provide the template for the GPTS. Magnetic polarity stratigraphy on land or in deep sea cores provides the link between the GPTS and biozonations/bioevents and hence geologic stage boundaries. Radiometric absolute ages are correlated either directly to the GPTS in magnetostratigraphic section or indirectly through biozonations, and absolute ages are then interpolated using the GPTS (oceanic magnetic anomaly) template. In view of the paramount importance of the calibration of geologic time to understanding the rates of geologic processes, the contribution of magnetic polarity stratigraphy to the Earth sciences becomes self-evident.

The dipole nature of the main geomagnetic field means that polarity reversals are globally synchronous, with the process of reversal taking 10³–10⁴ yr. Magnetic polarity stratigraphy can therefore provide global stratigraphic time lines with this level of time resolution. Three other techniques in magnetic stratigraphy, not involving the record of geomagnetic polarity reversals, have become increasingly important. Rock magnetic stratigraphy refers to the use of nondirectional magnetic properties (such as magnetic susceptibility and laboratory-induced remanence intensities) as a means of stratigraphic correlation. Paleointensity magnetic stratigraphy, the use of the record of geomagnetic paleointensity, and secular variation magnetic stratigraphy, the use of secular directional changes of the geomagnetic field, have been used as a means of stratigraphic correlation in Quaternary sediments.

Individual texts exist for paleomagnetism in general, such as those of Irving (1964), McElhinny (1973), Tarling (1983), Butler (1992), and Van der Voo (1993); however, none is available for the rapidly growing discipline of magnetic stratigraphy.

1.2 Early Developments

Directions of natural remanent magnetization (NRM) reverse with respect to the present ambient magnetic field of the Earth were known to early pioneers in paleomagnetic research. Brunhes (1906) reported directions of magnetization in Pliocene lavas from France that yielded north-seeking magnetization directions directed to the south and up, rather than to the north and down. He attributed this behavior to a local anomaly of the geomagnetic field. Brunhes demonstrated that the baked contacts of igneous rocks are magnetized with the same polarity as the igneous rock. This was the first use of what is now referred to as the baked contact test, which can be used to determine the relative age of magnetizations in the vicinity of an igneous contact. When lava flows are extruded, or dikes are intruded into a host rock, the temperature of the rock or sediment into which the magma is intruded is raised above the blocking temperature of the magnetic minerals. The magnetization is, therefore, reset in the direction of the field prevailing at the time of baking. Brunhes carried out this test for both normally and reversely magnetized igneous contacts.

This work was followed by that of Matuyama (1929) on volcanic rocks from Japan and north China. This study included many examples of lavas that yielded directions that were reverse with respect to the present geomagnetic field (Fig. 1.1). Matuyama correctly attributed the reverse directions of magnetization to a reversal of the geomagnetic field. Matuyama separated his samples into two groups: Group I, which were all Pleistocene in age, gave NRM directions which grouped around the present direction of the geomagnetic field. The group II directions were antipodal to those of group I and to the present directions of the geomagnetic field, and most of these lavas were older than Pleistocene in age. This was the first hint that the polarity of the geomagnetic field might be age dependent. These early studies were followed by those of Chevallier (1925), Mercanton (1926), and Koenigsberger (1938).

Figure 1.1 Directions of natural remanent magnetization in basalts from China and Japan (after Matuyama, 1929).

The modern era of studies of reversals of the geomagnetic field began with those of Hospers (1951, 1953–1954) in Iceland and Roche (1950, 1951, 1956) in the Massif Central of France, which elaborated on the early work of Brunhes. In both of these studies, the relative age and position of the rocks were fixed stratigraphically. As in Matuyama’s studies, the results indicated that rocks and sediments designated as Upper Pleistocene and Quaternary in age possessed normal directions of magnetization, whereas reverse directions of magnetization appeared in rocks of early Pleistocene or Pliocene age. Einarsson and Sigurgeirsson (1955) utilized a magnetic compass to detect reversals in Icelandic lavas and began to map the distribution of magnetic polarity zones in the rock sequences. They found about equal thicknesses of normal and reverse polarity strata, which implied that the magnetic field had little polarity bias in the Late Cenozoic. A study by Opdyke and Runcorn (1956) on lava flows from the San Francisco Peaks of northern Arizona showed that all young lavas studied were normally magnetized, whereas the stratigraphically older series of flows contained reversely magnetized lavas. Based on these studies, it was assumed that the last reversal of the field took place close to the Plio–Pleistocene boundary. Rutten and Wensink (1960) and Wensink (1964) built on Hosper’s studies in Iceland and demonstrated that the youngest lavas were normally magnetized and that older underlying lavas were reverse, and these in turn were underlain by a normal sequence. They subdivided the Plio–Pleistocene lavas into three magnetozones N₁-R₁-N₂. They correlated N₂ to the Astien and R₁ to the Villafranchian and deduced that the earliest glaciations in Iceland were of Pliocene age.

Magnetometers capable of measuring the magnetization of igneous rocks had been available since before World War II. Johnson et al. (1948) developed a rock generator magnetometer which could measure natural remanence in sedimentary rocks. Blackett (1956) developed astatic magnetometers which were a further improvement in sensitivity. Paleomagnetic study of sedimentary rocks began with the classic study of Creer et al. (1954) which documented 16 zones of alternating polarity in a 3000-m section of the Torridonian sandstone (Scotland). These rocks passed the fold test, indicating a prefolding magnetization, implying that reversals were a long-term feature of the geomagnetic field. These authors also reported reversals from rocks of Devonian and Triassic age.

Simultaneous with the developments outlined above, which seemed to favor reversal of the dipole geomagnetic field as an explanation for the observations, a discovery was made in Japan which cast doubt on the field reversal hypothesis. Nagata (1952), Nagata et al. (1957), and Uyeda (1958) demonstrated that samples from the Haruna dacite, a hyperthermic dacite pumice from the flanks of an extinct volcano in the Kwa district of Japan, was self-reversing. The striking magnetic property of this rock is that, when heated to a temperature above 210 °C and cooled in a weak magnetic field, the acquired thermal remanent magnetization (TRM) is in opposition to the applied field. Since self-reversal in...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Dedication
• Preface
• Table Key
• 1: Introduction and History
• 2: The Earth’s Magnetic Field
• 3: Magnetization Processes and Magnetic Properties of Sediments
• 4: Laboratory Techniques
• 5: Fundamentals of Magnetic Stratigraphy
• 6: The Pliocene–Pleistocene Polarity Record
• 7: Late Cretaceous–Cenozoic GPTS
• 8: Paleogene and Miocene Marine Magnetic Stratigraphy
• 9: Cenozoic Terrestrial Magnetic Stratigraphy
• 10: Jurassic–Early Cretaceous GPTS
• 11: Jurassic and Cretaceous Magnetic Stratigraphy
• 12: Triassic and Paleozoic Magnetic Stratigraphy
• 13: Secular Variation and Brunhes Chron Excursions
• 14: Rock Magnetic Stratigraphy and Paleointensities
• Bibliography
• Index
• International Geophysics Series