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Magnetostratigraphy

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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. Outcrop • hand samples • plugs taken from outcrops Oriented! dip & azimuth Magnetostratigraphy: Procedures • Sampling - drill cores 1. Discrete-cubes 2. Whole core 6 Magnetostratigraphy: Procedures Cryogenic Magnetometer: – measures weak magnetic fields • liquid He (4°K) - superconducting region around the sensors • sample magnetism - current in the superconducting coil NRM - Natural Remanent Magnet. 1. intensity 2. direction of magnetic vector DRM - Detrital Remanent Magnet. 1. AF demag 2. thermal demag Magnetostratigraphy: Procedures • Data Processing – inclination and magnetic intensity against depth 7 Magnetostratigraphy: P-E boundary • A new sub-chron?, or excursion 8 ODP Site 1208 • Inclination after AF demagnetization at peak fields of 20 mT 9 10 Willwood Formation, Bighorn Basin L. Tauxe et al. / Earth and Planetary Science Letters 125 (1994) 159-172 11 Magnetostratigrahy, Willwood Formation Magnetostratigraphic results of southern Bighorn Basin composite section plotted as VGP latitudes versus height Magnetostratigraphy Correlation to the GPTS Summary diagram showing the geomagnetic reversal time scale (GRTS) of Cande and Kent (CK92) at the bottom. Above CK92 the magnetic anomaly profile from GAL03 is plotted. Above the DSDP 550 data are VGP latitudes from the Clark's Fork Basin 12 Oxygen isotope record of benthic foraminifera from deep sea sites showing: (1) the coincidence of the Clarkforkian/Wasatchian NALMA boundary to a short-term negative excursion representing an abrupt global warming event (IETM); and (2) the coincidence of the Wasatchian/Bridgerian NALMA boundary to the beginning of the long-term Cenozoic warming peak known as the Cenozoic Global Climate Optimum (CGCO). Isotope data are from Zachos et al. (1994). Calibration of the Geomagnetic Polarity Time Scale shown at left is from Cande and Kent (1995). 13 Representative vector endpoint diagrams of paleomagnetic samples analyzed from the Wasatch, Bridger, and Green River Formations. Open (closed) symbols show vector endpoints in the vertical (horizontal) plane. (A) and (B) Wasatch Formation paleosol samples showing reverse and normal characteristic magnetizations. (C) Wasatch Formation sample that exhibits an overprint component of magnetization as well as a reversed characteristic component of magnetization. (D) Siltstone sample from the Bridger Formation showing unblocking by 5908C. (E) Green River Formation sample that exhibits relatively unstable demagnetization behavior. (G) and (F) Equal area projections where open (closed) symbols lie on the upper (lower) hemisphere of the projection. (G) Wasatch formation sample that exhibits clustering of magnetic endpoints where a Fisher mean was used to calculate a ChRM direction. (F) Wasatch Formation sample representing an example of a great circle trajectory used to infer a reversed ChRM Fig. 3. Equal area projections of NRM directions, characteristic directions after demagnetization, and mean directions for alpha sites. Open (closed) symbols lie on the upper (lower) hemisphere of the projection. Samples and sites pass the reversal test at aà 0:05 (McFadden and Lowes, 1981). The mean direction for all alpha sites when reversed sites are inverted is 348/61 Ö a95 à 4:6Ü; remarkably close to the expected early eocene direction of 349/61 (Diehl et al., 1983). 14 Fig. 4. VGP latitude and magnetic susceptibility plotted against lithostratigraphic and biostratigraphic units from the study section at South Pass. Solid squares represent alpha sites, open squares represent individual samples that exhibited stable demagnetization, and open circles represent samples that followed a great circle path toward a reverse direction during demagnetization. Correlation of the South Pass section studied here and the Bridger Basin section from Clyde et al. (1997) to the GPTS (Cande and Kent, 1995). Notice the short reversed zone in both sections that correlates precisely with Chron C23n.1r. A radiometric age of 47.96 ^ 0.13 Ma that lies about 150 m above the top of the South Pass section also supports this correlation (Murphey et al., 1999). 15 Fig. 6. Geochronology of the Wasatchian/Bridgerian framework based on the correlation shown in Fig. 5. The Wasatchian/Bridgerian boundary lies in Chron C23r in the middle of the early Eocene rather than near the early/middle Eocene boundary as previously assumed. 16.
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