DOCSLIB.ORG

33. Magnetic Logging and In-Situ Magnetostratigraphy: a Field Test1

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
33. Magnetic Logging and In-Situ Magnetostratigraphy: a Field Test1 Greene, H.G., Collot, J.-Y., Stokking, L.B., et al., 1994 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 134 33. MAGNETIC LOGGING AND IN-SITU MAGNETOSTRATIGRAPHY: A FIELD TEST1 P. Roperch,2 V. Barthès,3 J. Pocachard,3 J.-Y. Collot,2 and T. Chabernaud4 ABSTRACT During Ocean Drilling Program Leg 134 (Vanuatu), geological high sensitivity magnetic tools (GHMT) developed by CEA-LETI and TOTAL were used at two drill sites. GHMT combine two sensors, a proton magnetometer for total magnetic field measurements with an operational accuracy of 0.1 nanoteslas (nT), and a highly sensitive induction tool to measure the magnetic susceptibility with an operational accuracy of a few I0"6 S1 units. Hole 829 A was drilled through an accretionary prism and the downhole measurements of susceptibility correlate well with other well-log physical properties. Sharp susceptibility contrasts between chalk and volcanic silt sediment provide complementary data that help define the lithostratigraphic units. At Hole 83 IB magnetic susceptibility and total field measurements were performed through a 700-m reef carbonate sequence of a guyot deposited on top of an andesitic volcano. The downhole magnetic susceptibility is very low and the amplitude of peak-to-peak anomalies is less than a few 10~5 S1 units. Based on the repeatability of the measurements, the accuracy of the magnetic logging measurements was demonstrated to be excellent. Total magnetic field data at Hole 83IB reveal low magnetic anomalies of 0.5 to 5 nT and the measurement of a complete repeat section indicates an accuracy of 0.1 to 0.2 nT. Due to the inclination of the earth's magnetic field in this area (~-40°) and the very low magnetic susceptibility of the carbonate, the contribution of the induced magnetization to the total field measured in the hole is negligible. Unfortunately, because the core recovery was extremely poor (<5%) no detailed comparison between the core measurements and the downhole magnetic data could be made. Most samples have a diamagnetic susceptibility and very low intensity of remanent magnetization (< I0"4 A/m), but a few samples have a stable remanent magnetization up to 0.005 A/m. These variations of the intensity of the remanent magnetization suggest a very heterogeneous distribution of the magnetization in the carbonate sequence that could explain the magnetic field anomalies measured in these weakly magnetized rocks. INTRODUCTION 118 (Pariso et al., 1991) with a University of Washington and U.S. Geological Survey three-component magnetometer and susceptibility Magnetostratigraphic investigations of sedimentary and volcanic meter. These magnetic tools were developed to study volcanic base- sequences and studies of seafloor marine magnetic anomalies have ment rocks and are not suitable for use in weakly magnetized sedi- enabled the construction of a geomagnetic polarity reversal time scale ment. Advances in technology and the interest of the oil industry in that covers the last 165 m.y. (Cox, 1983; Berggren et al., 1985). logging that could constrain age and sedimentation rate have stimu- Magnetic measurements of core sediment recovered by the Deep Sea lated the development of borehole magnetic sensors that could deter- Drilling Project (DSDP) and the Ocean Drilling Program (ODP) have mine an in-situ magnetostratigraphy in sediment. This paper focuses permitted magnetostratigraphy to become a powerful tool for be- on magnetic logging tools recently developed by CEA-LETI and tween-core correlations and absolute dating. Comparison of the ob- TOTAL (Pocachard et al., 1991) that were used during Leg 134. served pattern of magnetic reversals recorded by a sedimentary To determine the magnetostratigraphy the primary information section with the reference geomagnetic time scale provides strong that needs to be measured is the remanent magnetization of the rocks. constraints of biostratigraphic markers. However, the determination In a laboratory experiment it is easy to screen the earuYs present of magnetostratigraphy from core samples is a time-consuming labo- magnetic field and to measure the remanent magnetic field directly ratory procedure and depends upon core recovery. using a magnetometer. In contrast, magnetic field measurements in a Measurements of numerous physical and chemical properties of borehole include contributions from the earth's present magnetic rocks in the boreholes have played a major role in the mining and oil field, as well as the remanent magnetization field, and the induced industry. In contrast, the scientific community involved in the DSDP magnetization field. The main magnetic field that originates in the and ODP scientific research programs has until recently emphasized earth's core varies slowly (secular variation), and it can be considered the study of cores. Before Leg 134 magnetic logging experiments were a constant for the duration of the logging experiment (a few hours). conducted during DSDP and ODP legs only for volcanic units charac- In contrast, the field of external origin has diurnal variations with terized by strong magnetic anomalies. The first experiments were periods and amplitude that cannot be neglected during logging. conducted on DSDP Legs 68 and 69 by Ponomarev and Nechoroshkov Variations up to several tens of nanoteslas (nT) can be observed dur- (1983) and later on DSDP Leg 78 (Ponomarev and Nechoroshkov, ing stormy magnetic periods. The magnetic anomalies expected in a 1984). A gyroscope-oriented three-axis borehole magnetometer and a borehole depend on the variations of the natural remanent magneti- susceptibility tool were used in mid-Cretaceous basalts at Hole 418 A zation vector of the rocks as well as the susceptibility contrasts (ODP Leg 102; Bosum and Scott, 1988). More recently, the in-situ between sediment layers. Magnetic anomalies of a few hundred of magnetic properties of a gabbro were investigated during ODP Leg nanoteslas are expected in volcanic rocks with magnetization on the order of 1 A/m, whereas sediments with magnetization of about 10~3 A/m would produce magnetic anomalies from a few I0"1 nT to 1 nT. Deep-sea sediments have remanent magnetizations of I0"3 to I0"2 Greene, H.G., Collot, J.-Y., Stokking, L.B., et al., 1994. Proc. ODP, Sci. Results, 134: College Station, TX (Ocean Drilling Program). A/m whereas limestones carry lower magnetizations. Thus, magne- 1 2 ORSTOM, BP 48, 06230 Villefranche-sur-Mer, France. tometers with a sensitivity of I0" nT are required for conducting 3 CEA-LETI, Centre cTEtudesNucléaires de Grenoble, 85X - 38041 Grenoble, France. downhole magnetic logging in sediment. 4 Borehole Research Group, Lamont-Doherty Earth Observatory, Columbia Univer- sity, Palisades, NY 10964, U.S.A. An accurate retrieval of the direction of the remanent magnetiza- tion vector from borehole measurements requires the record of the 577 P. ROPERCH ET AL. three components of the field (Parker and Daniell, 1979; Gallet and neers (Pocachard et al., 1991). The generic name of the total field Courtillot, 1988). Pozzi et al. (1988) have shown numerical models magnetometer is nuclear resonance magnetic tool (NRMT); the sus- of the expected magnetic field components in a hole drilled through ceptibility magnetic tool is SUMT. These instruments are packaged horizontal layers, and Gallet and Courtillot (1988) extended the according to Schlumberger standards, and data are recorded every calculations to homogeneous dipping layers. The numerical model of 6 in. in accordance with Schlumberger procedures. a magnetic anomaly across a polarity transition indicates that a The SUMT is a low-frequency induction tool. The volume meas- polarity chron thicker than 3 times the borehole diameter will be fully ured is a cylinder approximately 0.5 m in radius and 1.5 m in length. resolved at the 95% confidence level. Amultisensor probe that would The susceptibility tool provides information about the electrical con- provide the three components of the magnetic field as well as the ductivity. We used this information to match the depth from the other horizontal gradients across the borehole would be a very powerful Schlumberger resistivity tools. Before the data processing, the output tool. However, directional fluxgate sensors have an accuracy no better of the SUMT has an offset of about -2500 ppm. The data processing than 1 nT and are sensitive to temperature changes. Another problem consists mostly in removing a drift linked to temperature changes. is the accuracy in the orientation of the magnetometers. For example, The zero value that corresponds to the transition from paramagnetism in a total field of 50,000 nT an accuracy of less than 1 nT on each to diamagnetism is estimated at a given temperature. The transfer rate component would require an orientation with an accuracy of 0.001°. from ppm to S1 units is about 1 ppm to I0"6 S1. Augustin (1990) These limits in directional magnetometers and in orientation of the compared core measurements with SUMT results from drill holes in tools have led the CEA-LETI and TOTAL groups to develop a total the Bassin de Paris and showed that SUMT variations are linearly magnetic field magnetometer with high resolution. The high-resolu- related to susceptibility contrasts. The NRMT consists of a Over- tion records provide valuable information from which qualitative hauser proton magnetometer with a peak-to-peak measurement noise interpretations can be attempted, assuming that the magnetization better than 0.1 nT. vector is homogeneous within a single layer of infinite extent (Tab- bagh et al., 1990; Pozzi et al., unpubl. data). MAGNETIC SUSCEPTIBILITY LOGGING (SITE 829) Assuming a simple model of infinite homogeneous layers, a simple relationship exists between the direction of the magnetization The major lithostratigraphic units recovered during drilling at Site (Mx, My, Mz) and the resulting magnetic induction (Bx, By, Bz) at the 829 correspond to a Pleistocene clayey volcanic silt sequence from 0 center of a borehole, thus to 60.5 meters below seafloor (mbsf) above a foraminiferal chalk of late Oligocene age from 60.5 to 99.4 mbsf.
Load more

Recommended publications
  • Equivalence of Current–Carrying Coils and Magnets; Magnetic Dipoles; - Law of Attraction and Repulsion, Definition of the Ampere
    GEOPHYSICS (08/430/0012) THE EARTH'S MAGNETIC FIELD OUTLINE Magnetism Magnetic forces: - equivalence of current–carrying coils and magnets; magnetic dipoles; - law of attraction and repulsion, definition of the ampere. Magnetic fields: - magnetic fields from electrical currents and magnets; magnetic induction B and lines of magnetic induction. The geomagnetic field The magnetic elements: (N, E, V) vector components; declination (azimuth) and inclination (dip). The external field: diurnal variations, ionospheric currents, magnetic storms, sunspot activity. The internal field: the dipole and non–dipole fields, secular variations, the geocentric axial dipole hypothesis, geomagnetic reversals, seabed magnetic anomalies, The dynamo model Reasons against an origin in the crust or mantle and reasons suggesting an origin in the fluid outer core. Magnetohydrodynamic dynamo models: motion and eddy currents in the fluid core, mechanical analogues. Background reading: Fowler §3.1 & 7.9.2, Lowrie §5.2 & 5.4 GEOPHYSICS (08/430/0012) MAGNETIC FORCES Magnetic forces are forces associated with the motion of electric charges, either as electric currents in conductors or, in the case of magnetic materials, as the orbital and spin motions of electrons in atoms. Although the concept of a magnetic pole is sometimes useful, it is diácult to relate precisely to observation; for example, all attempts to find a magnetic monopole have failed, and the model of permanent magnets as magnetic dipoles with north and south poles is not particularly accurate. Consequently moving charges are normally regarded as fundamental in magnetism. Basic observations 1. Permanent magnets A magnet attracts iron and steel, the attraction being most marked close to its ends.
    [Show full text]
  • Geochronological Applications
    Paleomagnetism: Chapter 9 159 GEOCHRONOLOGICAL APPLICATIONS As discussed in Chapter 1, geomagnetic secular variation exhibits periodicities between 1 yr and 105 yr. We learn in this chapter that geomagnetic polarity intervals have a range of durations from 104 to 108 yr. In the next chapter, we shall see that apparent polar wander paths represent motions of lithospheric plates over time scales extending to >109 yr. As viewed from a particular location, the time intervals of magnetic field changes thus range from decades to billions of years. Accordingly, the time scales of potential geochrono- logic applications of paleomagnetism range from detailed dating within the Quaternary to rough estimations of magnetization ages of Precambrian rocks. Geomagnetic field directional changes due to secular variation have been successfully used to date Quaternary deposits and archeological artifacts. Because the patterns of secular variation are specific to subcontinental regions, these Quaternary geochronologic applications require the initial determination of the secular variation pattern in the region of interest (e.g., Figure 1.8). Once this regional pattern of swings in declination and inclination has been established and calibrated in absolute age, patterns from other Quaternary deposits can be matched to the calibrated pattern to date those deposits. This method has been developed and applied in western Europe, North America, and Australia. The books by Thompson and Oldfield (1986) and Creer et al. (1983) present detailed developments. Accordingly, this topic will not be developed here. This chapter will concentrate on the most broadly applied of geochronologic applications of paleomag- netism: magnetic polarity stratigraphy. This technique has been applied to stratigraphic correlation and geochronologic calibration of rock sequences ranging in age from Pleistocene to Precambrian.
    [Show full text]
  • Magnetostratigraphy and Tectonosedimentology Qilian Shan
    Downloaded from http://sp.lyellcollection.org/ by guest on November 19, 2013 Geological Society, London, Special Publications Oligocene slow and Miocene−Quaternary rapid deformation and uplift of the Yumu Shan and North Qilian Shan: evidence from high-resolution magnetostratigraphy and tectonosedimentology Xiaomin Fang, Dongliang Liu, Chunhui Song, Shuang Dai and Qingquan Meng Geological Society, London, Special Publications 2013, v.373; p149-171. doi: 10.1144/SP373.5 Email alerting click here to receive free e-mail alerts when service new articles cite this article Permission click here to seek permission to re-use all or request part of this article Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes © The Geological Society of London 2013 Downloaded from http://sp.lyellcollection.org/ by guest on November 19, 2013 Oligocene slow and Miocene–Quaternary rapid deformation and uplift of the Yumu Shan and North Qilian Shan: evidence from high-resolution magnetostratigraphy and tectonosedimentology XIAOMIN FANG1,2*, DONGLIANG LIU1,3, CHUNHUI SONG2, SHUANG DAI2 & QINGQUAN MENG2 1Key Laboratory of Continental Collision and Plateau Uplift & Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Shuangqing Road 18, Beijing 100085, China 2Key Laboratory of Western China’s Environmental Systems (Ministry of Education of China) and College of Resources and Environment, Lanzhou University, Gansu 730000, China 3Key Laboratory of Continental Dynamics of the Ministry of Land and Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China *Corresponding author (e-mail: [email protected]) Abstract: Most existing tectonic models suggest Pliocene–Quaternary deformation and uplift of the NE Tibetan Plateau in response to the collision of India with Asia.
    [Show full text]
  • Magnetostratigraphy and Rock Magnetism of the Boom Clay (Rupelian Stratotype) in Belgium
    Netherlands Journal of Geosciences / Geologie en Mijnbouw 83 (3): 209-225 (2004) Magnetostratigraphy and rock magnetism of the Boom Clay (Rupelian stratotype) in Belgium D. Lagrou1*, N. Vandenberghe1, S. Van Simaeys1 & J. Hus2 1 Historische Geologie, KU Leuven, Redingenstraat 16, 3000 Leuven, Belgium 2 Centre de Physique du Globe de 1'IRM, 5670 Dourbes, Belgium * Corresponding author: D. Lagrou, presently at the Flemish Institute for Technological Research (Vito), Boeretang 200, B-2400 Mol, Belgium. E-mail: [email protected] Manuscript received: February 2004; accepted: August 2004 N G Abstract This paper presents the results of a detailed rock magnetic and magnetostratigraphic study of the Lower Oligocene Rupelian unit-stratotype. Notwithstanding the relatively low intensity of the natural remanent magnetisation and the diverse and often unstable behaviour during demagnetisation, close-spaced sampling and accurate polarity determinations allowed us to determine the magnetic polarity zonation.The recognition of the characteristic magnetic polarity and the correlation with the standard magnetobiochronologic time scale yields an accurate chronostratigraphic dating of the Boom Clay Formation. The boundary between the geomagnetic chrons C12n and C12r nearly coincides with the hthostratigraphic boundary between theTerhagen and Putte Members. Rock magnetic techniques point to magnetite and probably also iron sulphides as the main magnetic remanence carriers. These magnetic minerals could, however, not be identified with classical mineralogical
    [Show full text]
  • IRM Gets New Shielded Room and U-Channel Magnetometer
    Inside... The IRM Visiting Fellows’ Reports 2 Current Articles 5 QuarterlyFall 2008, Vol. 18 No. 3 IRM gets new shielded room and u-channel magnetometer Julie Bowles IRM The IRM is pleased to announce the recent instal- lation of a new 2G Enterprises pass-through u-channel magnetometer system (Fig. 1). The system enables the measurement of intact u-channel cores for the fi rst time at Figure 1. The new magnetometer. Visible from right to left are a the IRM, and we are excited about the ability of serving portion of the sample handling track, the degausser/ARM unit, and a new group of users. the measurement region with pulse-tube cryo-cooler. The magnetometer was installed in our new shielded room, completed in March by Gary Scott and his team with a track system for automated sample handling and from Lodestar Magnetics. The shielded room consists continuous measurement of cores or multiple discrete of two layers of transformer steel and attenuates the samples. The dynamic range of the instrument is con- background magnetic fi eld by ~250 times to a few hun- siderably greater than that of our old magnetometer with dred nano-Tesla. The room now houses both the old RF SQUIDS, which remains in heavy use. This will and new 2G magnetometers, our DTech alternating fi eld allow for the measurement of weakly magnetic samples (AF) demagnetizer, ASC paleointensity furnace, and (as and for the more rapid processing of strongly magnetic needed) our ancient but still reliable Schonstedt spinner samples, which previously had to be measured on the magnetometer.
    [Show full text]
  • North American Stratigraphic Code1
    NORTH AMERICAN STRATIGRAPHIC CODE1 North American Commission on Stratigraphic Nomenclature FOREWORD TO THE REVISED EDITION FOREWORD TO THE 1983 CODE By design, the North American Stratigraphic Code is The 1983 Code of recommended procedures for clas- meant to be an evolving document, one that requires change sifying and naming stratigraphic and related units was pre- as the field of earth science evolves. The revisions to the pared during a four-year period, by and for North American Code that are included in this 2005 edition encompass a earth scientists, under the auspices of the North American broad spectrum of changes, ranging from a complete revision Commission on Stratigraphic Nomenclature. It represents of the section on Biostratigraphic Units (Articles 48 to 54), the thought and work of scores of persons, and thousands of several wording changes to Article 58 and its remarks con- hours of writing and editing. Opportunities to participate in cerning Allostratigraphic Units, updating of Article 4 to in- and review the work have been provided throughout its corporate changes in publishing methods over the last two development, as cited in the Preamble, to a degree unprece- decades, and a variety of minor wording changes to improve dented during preparation of earlier codes. clarity and self-consistency between different sections of the Publication of the International Stratigraphic Guide in Code. In addition, Figures 1, 4, 5, and 6, as well as Tables 1 1976 made evident some insufficiencies of the American and Tables 2 have been modified. Most of the changes Stratigraphic Codes of 1961 and 1970. The Commission adopted in this revision arose from Notes 60, 63, and 64 of considered whether to discard our codes, patch them over, the Commission, all of which were published in the AAPG or rewrite them fully, and chose the last.
    [Show full text]
  • Magnetostratigraphy
    MAGNETOSTRATIGRAPHY • Correlation of strata on the basis of the magnetic properties • Critical for deriving absolute ages of strata (>100 k.y., < 150 m.y.) • Marine and Terrestrial Sediments 1 Earth's Magnetic Field: • Earth has a strong dipole magnetic field – Outer core (Fe, Ni) convection – Electric currents • 11.5° Angle between Magnetic and Geographic North poles • field reversals – irregular intervals – Not instantaneous Paleomagnetism: Normal polarity: • Inclination - angle from vertical – downward N. hemisphere – upward S. hemisphere – 90 to -90 (~latitude) • Declination - angle in the horizontal from geographic North – Normal – close to 0° – Reverse – close to 180 2 Magnetostratigraphy: Calibrating magnetic field reversals to absolute time • Additional Requirements/Assumptions – Radiometric dating – Linear spreading rates Geomagnetic Polarity Time Scale (GPTS) Cande & Kent (1995) Radiometric Calibration points 3 Remnant Magnetism - 3 types • thermal - igneous or metamorphic rocks • chemical- precipitation of hematite (Fe2O3), goethite (FeOOH) • detrital- sedimentation of iron bearing minerals (~ 1-5 µm) In deep sea sediments - – iron oxides (magnetite, Fe3O4) – iron-titanium oxides (FeTiO) • Post depositional alignment and realignment 1. bioturbation - DRM locks in below 30 cm after migration out of the bioturbation zone and through dewatering 2. compaction - shallows inclination 3. metamorphism • Polarity Records GPTS (0-5 Ma) 4 Nomenclature: Polarity Chrons, Intervals • Chron (Normal and reversed) intervals (105-106y) • Chronozone-each consists of normal chron and reversed chron • sub-chrons - short reversals (104-105y) within larger chrons • excursions - short-lived transitions (failed reversals) - difficult to identify and use for correlative purpose • transitions zones - on the order of several thousand years • polarity interval - time interval that elapses between two successive reversals (0.01 to 10's My) Numbering Scheme • 1 to 32 for the most prominent chrons (C24N, C24R) • sub-chrons C6AN, C6A.1N 5 Magnetostratigraphy: Procedures • Sampling 1.
    [Show full text]
  • Magnetostratigraphy of Permian/Triassic Boundary Sequences in the Cis-Urals, Russia: No Evidence for a Major Temporal Hiatus
    Earth and Planetary Science Letters 281 (2009) 36–47 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Magnetostratigraphy of Permian/Triassic boundary sequences in the Cis-Urals, Russia: No evidence for a major temporal hiatus Graeme K. Taylor a,⁎, Christopher Tucker a, Richard J. Twitchett a, Timothy Kearsey a, Michael J. Benton b, Andrew J. Newell c, Mikhail V. Surkov d, Valentin P. Tverdokhlebov d a School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK b Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK c British Geological Survey, MacLean Building, Wallingford OX10 8BB, UK d Geology Institute of Saratov State University, Astrakhanskaya 83, 410075 Saratov, Russia article info abstract Article history: During the last five years there has been considerable doubt over the age of the continental uppermost Received 6 June 2008 Permian Russian stages, the Kazanian and Tatarian. Traditionally they have been regarded as Late Permian Received in revised form 3 February 2009 but were re-dated as Middle Permian in the 2004 international time scale, despite fossil evidence that the Accepted 6 February 2009 Tatarian, at least, is Late Permian. These debated ages are tested by magnetostratigraphic study of five Available online 5 March 2009 sections spanning the Permian Triassic Boundary (PTB) of the SE Urals in the Orenburg region of Russia. The Editor: R.D. van der Hilst Upper Permian and Lower Triassic of this region have a well documented vertebrate fauna whose evolution has a significant bearing on our understanding of the PTB mass extinction event.
    [Show full text]
  • Magnetostratigraphy, Nannofossil Stratigraphy and Apparent Polar Wander for Adria-Africa in the Jurassic–Cretaceous Boundary Interval
    Palaeogeography, Palaeoclimatology, Palaeoecology 293 (2010) 51–75 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Magnetostratigraphy, nannofossil stratigraphy and apparent polar wander for Adria-Africa in the Jurassic–Cretaceous boundary interval J.E.T. Channell a,⁎, C.E. Casellato b, G. Muttoni b, E. Erba b a Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA b Dipartimento di Scienze della Terra “A.Desio”, Università degli Studi di Milano, via Mangiagalli 34, 20133 Milano, Italy article info abstract Article history: Existing correlations of Early Cretaceous nannofossil events to polarity zones in Italian pelagic limestones are Received 27 August 2009 the basis for correlation of polarity chrons to geologic stages, and hence calibration of the Early Cretaceous Received in revised form 12 April 2010 geomagnetic polarity timescale. Here, we present correlations of nannofossil events to polarity chrons for the Accepted 26 April 2010 Late Jurassic and Jurassic/Cretaceous boundary interval from the Southern Alps, making the case for revisions Available online 6 May 2010 of current geologic timescales in this interval. The Jurassic/Cretaceous and Kimmeridgian/Tithonian boundaries can be usefully defined at the onsets of polarity chrons CM18R and CM22R, respectively. The Keywords: Oxfordian/Kimmeridgian boundary can be assigned to CM25. An apparent polar wander path (APWP) for the Late Jurassic Jurassic/Cretaceous
    [Show full text]
  • 3. Magnetostratigraphy
    3.1 Introduction 3. Magnetostratigraphy Origin of the Earth’s geomagnetic field The geomagnetic field is generated in some poorly understood way by the motion of highly conducting nickel-iron (Ni-Fe) fluids in the outer part of Earth’ core; this motion is assumed to be controlled by thermal convection and by the Coriolis force generated by Earth’s rotation, constituting what is called a self-exciting dynamo. The Earth’s magnetic field When Earth’s magnetic field has the present orientation, it is said to have normal polarity. When this orientation changes 180°, it has reversed polarity. Studies of the remanent magnetism in igneous and sedimentary rocks show that the dipole (main) component of Earth’s magnetic field has reversed its polarity at irregular intervals from Precambrian time onward, apparently owing to instabilities in outer-core convection. Reversals of Earth’s magnetic field are recorded in sediments and igneous rocks by patterns of normal and reversed remanent magnetism. These geomagnetic reversals are contemporaneous worldwide phenomena. Thus, they provide unique stratigraphic markers in igneous and sedimentary rocks. The reversal process is thought to take place1 Lamb & Sington (1998) over a period of 1,000 – 10,000 years. Declination: The angle between geographic north and magnetic north. Inclination: The angle between the horizontal and a magnetic vector. Conventionally, a vector with a magnetic north pole dipping below the horizontal is considered positive, and an upward vector is negative. Positive inclination in the Northern Hemisphere indicates normal polarity; negative inclination means reversed polarity. Boggs (2001) 2 Present-day declination Present-day inclination Butler (1992) 3 3.2 Remnant magnetism in rocks Remnant magnetism (殘磁性) The magnetization remaining after the removal of an externally applied field, and exhibited by ferromagnetic materials.
    [Show full text]
  • Pdf/104/4/409/3381416/I0016-7606-104-4-409.Pdf by Guest on 27 September 2021 410 RIECK and OTHERS
    Magnetostratigraphy and tephrochronology of an upper Pliocene to Holocene record in lake sediments at Tulelake, northern California HUGH J. RIECK* U.S. Geological Survey, Flagstaff, Arizona 86001 ANDREI M. SARNA-WOJCICKI \ CHARLES E. MEYER 1 U.S. Geological Survey, Menlo Park, California 94025 DAVID P. ADAM j ABSTRACT correlation. Most of the tephra at Tulelake records eruptions from the nearby southern and central Cascade Range of Oregon and northern Combined paleomagnetic and tephra chronologies of one of the California, and the Medicine Lake Highland of northern California. most complete middle Pliocene through Holocene stratigraphic rec- Deposition took place during most of the past 3 m.y. within the Tule- ords yet recovered in western North America provide a reference lake basin; notable periods of slow or sporadic accumulation, or ero- section for much of northwestern North America and adjacent Pacific sion, occurred between about 620,000 and 200,000 yr B.P. and Ocean. Five long drill cores of lacustrine sediments at Tulelake, north- between about 2.5 and 2.1 Ma. Rapid deposition occurred during ern California, recovered a nearly continuous 331-m-thick record marine oxygen-isotope stage 6, between about 170 and 125 ka. Re- spanning the past 3 m.y. The Brunhes Normal-Polarity, Matuyama gional volcanism during the past 3 m.y. was markedly episodic, with Reversed-Polarity, and Gauss Normal-Polarity Chronozones are rec- notable volcanic activity from about 2.1 to 1.9 Ma and from 0.4 Ma to ognized; within these, the Jaramillo, Olduvai, Reunion(?), and the present. Kaena(?) Subchronozones are present.
    [Show full text]
  • Supplement Information
    Supplement information A shipboard composite depth scale was generated for Site U1417 to take into account the recovery of sediments from multiple holes. A continuous “splice” spanning the upper and middle Pleistocene sediments was generated from 0-224.2 m core composite depth (“CCSF-A”; Jaeger et al., 2014). Below this depth, and especially where coring switched from the APC 5 (advanced piston corer) to the XCB (extended core barrel) system, core recovery was reduced from ~97 % (APC) to ~37 % (XCB). The recovered sediments tended to be in “biscuits” of coherent sediment; without knowledge of where these biscuits had originally been distributed within the 9.5 m core, they were grouped into the uppermost portions of each core (e.g. if 3 m of sediments were recovered, they were placed into the 0-3 m depth interval for each core). This leads to a clustering of recovered sediment and proxy data (Fig. S1 and Fig. S2). There is a possibility that recovery problems such as movement of 10 sediment through the liner during sediment recovery and logging resulted in difficulties for sediment depth attribution during IODP Expedition 341. Jaeger et al., (2014) acknowledge that “In areas of lower core recovery, the exact lithostratigraphic unit boundaries may have an error range of a few meters due to recovered intervals being assumed to reside at the top of each cored interval […]” [Jaeger et al., (2014); Core Log Seismic integration section]. This problem was also been observed, and accounted for, during ODP Leg 181 at Site 1123 (Shipboard Scientific Party, 2000). 15 In this instance, correlations were made between magnetic susceptibility measured in the recovered XCB cores and measured by the logging of the drilled hole.
    [Show full text]