Geology Reference
In-Depth Information
chemical changes occur. We will see examples of sedi-
ments in which diagenesis creates new magnetic min-
erals within 1000 years of deposition and other
examples in which the diagenetic changes and the sub-
sequent CRM occur millions and tens of millions of
years after the rock is formed. The other factor to con-
sider is the duration of time during which the diage-
netic changes occur. In some cases the growth of the
secondary magnetic minerals occurs over a geologi-
cally short period of time (hundreds-thousands of
years) and the CRM provides a snapshot of the geo-
magnetic fi eld at some time after deposition. In other
cases, the growth of secondary magnetic minerals
occurs over an extended time period and the CRM inte-
grates geomagnetic fi eld behavior over that period. The
worst-case scenario is when the diagenetic changes
cause a remagnetization of the sedimentary rock at
some time long (tens to hundreds of millions of years)
after its formation and over an extended period, so that
very little useful paleomagnetic information can be
extracted from the rock. Paleomagnetists are con-
stantly checking the age of the magnetization of a sedi-
mentary rock by fi eld and laboratory tests as a way of
gauging the age, and therefore reliability, of its paleo-
magnetic direction.
Some of these fi eld and laboratory tests will be dis-
cussed in Chapter 9. More detailed and comprehensive
treatment can be found in the topics of Tauxe (2010)
and Butler (1992), which describe techniques such as
the fold test, reversal test, conglomerate test, contact
test, alternating fi eld demagnetization, thermal demag-
netization and principal component analysis. For this
topic's discussion of CRM and diagenesis, one impor-
tant fact should be remembered: the magnetic mineral-
ogy can be an important guide as to whether a
magnetization is primary (formed at a rock's deposi-
tion) or secondary. Magnetite (Fe 3 O 4 ) is almost always
a primary depositional magnetic mineral. Specular
hematite (Fe 2 O 3 ) in red terrestrial clastic rocks can also
be a depositional magnetic mineral. Iron sulfi des such
as greigite (Fe 3 S 4 ) and usually pyrrhotite (Fe 1 - x S) are
secondary magnetic minerals formed by reduction dia-
genesis in marine and lake sediments. In red beds,
submicron-sized pigmentary hematite (which gives the
rocks their red color) is secondary. It is typically euhe-
dral in grain morphology. Magnetite, as well as specu-
lar hematite, can also be a secondary magnetic mineral
that typically grows in rocks long after deposition.
We will cover this when we examine the remagnetiza-
tion of North American cratonic rocks in the Late
Paleozoic by orogenic fl uids and discuss clay diagenesis
remagnetization.
The likelihood of an early reduction diagenetic
event depends largely on the total organic carbon
content of the sediment that, in turn, depends on the
environment of the sediment's deposition. Paleomag-
netists typically collect samples from fi ne - grained
sediments because this ensures a quiet depositional
environment and good alignment of the primary depo-
sitional magnetic minerals for a strong DRM or pDRM.
The sediments and sedimentary rocks targeted by pale-
omagnetists include those deposited in deep and near-
shore marine, lake and river settings. The marine and
lake sediments usually have magnetite as the primary
magnetic mineral. The terrestrial river sediments can
have magnetite as the primary magnetic mineral, but
many ancient fl uvial sediments have become red beds
with both primary and secondary hematite as the
dominant magnetic minerals.
IRON-SULFATE REDUCTION
DIAGENESIS
In marine and lake sediments with organic material,
iron-sulfate reduction diagenesis or pyritization can
occur relatively soon after deposition. This process can
then create an early CRM through the growth of sec-
ondary magnetic minerals that formed soon after dep-
osition, but not always. In marine sediments the total
organic carbon content associated with reduction dia-
genesis lies within the range 1-2% (Japan Sea, north-
ern California coastal margin, Oman margin). Lake
sediments typically have higher total organic contents
of 1-10%; in some cases organic contents can be as
large as 20% (Bohacs et al . 2000 ). Sulfate reduction
diagenesis is biologically mediated (Roberts & Weaver
2005 ). The Desulfovibrio microbe, a Gram negative
sulfate-reducing bacterium, can be important in this
reaction (Tarling 1999 ).
The chemical reactions that occur as a result of bac-
terial decay of organic matter in marine and lake sedi-
ments cause the dissolution of magnetite particles,
followed by the creation of ferromagnetic Fe sulfi des
and fi nally the end product: non-ferromagnetic pyrite
(FeS 2 ). The diagenetic reactions include much more
than just iron and sulfate reduction, but those are the
reactions important to the diagenesis of magnetic min-
erals. A more complete context for iron and sulfate
reduction would need to include the reactions (in
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