GEOL 340 Sedimentology and Stratigraphy
Lecture 32
1. Magnetostratigraphy
Magnetostratigraphy
Magnetostratigraphy
use of an established reversal pattern of remnant magnetism for
dating of sediments
determined by correlation with microfossils and chemostratigraphy
Mechanism of Acquiring Magnetism
deposition of ultrafine-grained magnetite crystals (<0.5 mm)
magnetite (Fe2O3) is a strong natural magnet
Earth's magnetic field
eminates at the south pole
forces are perpendicular earth surface at the poles
forces are parallel earth surface at the equator
generated by flow of electrically conductive fluids in the outer
core
Examples
Magnetostratigraphy (Boggs p. 564-586)
a. magnetic Fe-rich minerals can preserve the orientation and field direction of the earth's magnetic field at the time of formation (deposition or precipitation)
b. paleomagnetism - reversal pattern of remnant magnetism used
for dating of sediments
correlated with microfossils and chemostratigraphy
c. reversals are a synchronous global phenomenon
Magnetism
a. Magnetic Field from the motion of negatively charged particles
(electrons, cations)
random spinning electrons cancel charges = not magnetic
alligned spinning electrons = magnetic
b. Magnetic minerals have two poles magnetite (i.e. Fe2O4)
Currie point - magnetite is crystallized and alligned (500 -
600oC)
north-seeking magnetization
hematite (Fe2O3) looses magnetism
diagenesis can demagnitize (dolomitization of the Seroe Domi
Formation)
strong external magnetic fields further allign electrons and
enhance field
c. Earth's magnetic field (Fig. 16.1, p. 566)
eminates at the South pole
for a "magnet" lines of force eminate at the North
Pole
forces are perpendicular earth surface at the poles
forces are parallel earth surface at the equator
d. Generation of earth's magnetic Field
flow of electrically conductive fluids in outer core (self-exciting
dynamo)
minerals are demagnetized below 30 km by earth's heat
so no large magnetite core
e. Magnetic characteristics
inclination - with respect to horizontal (function of latitude)
declination - with respect to vertical
poles migrate and intensity shifts
current field will disappear in 2000 years
will develop again with reversed polarity
reversal perioid = 1000 to 10,000 years
caused by variations in the outer core convection
heat-induced turbulence changes drive the reversals
normal magnetism = same as modern Earth magnetism
reversed magnetism = reversed from modern Earth magnetism
Development of Technique
a. first used to date terrestrial volcanic rocks
b. linear anomoly patterns in volcanic rocks of the sea floor (Fig. 16.7 and 16.5, p. 573)
c. land and marine sections of sedimentary rocks
Mechanism of Acquiring Magnetism in Sediments
a. deposition of ultrafine-grained magnetite crystals (<0.5 mm)
b. crystals precipitated by microbes (bacteria)
biologically-induced mineralization - not controlled by organism
biologically-controlled mineralization - intracellular
c. lock-in depth = depth below zone of intensive bioturbation
d. indentify patterns of reversals, and determine their absolute age (Fig. 16.8, p. 575)
Examples (overheads)