The Paleomagnetism of Sediments and Sedimentary Rocks: Importance and Reliability - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 3

tive paleointensity records S int 200, S int 800 and S int 2000
over the past 200 thousand, 800 thousand and 2
million years, respectively.
The second important reason for the large number
of sedimentary paleomagnetic results in the Global
Paleomagnetic Database is that paleohorizontal can be
unequivocally determined from sedimentary rocks.
Knowing the paleohorizontal may seem trivial, but it
is not always straightforward to determine for igneous
rocks and it is absolutely critical for determining the
paleolatitude of the rocks from the paleomagnetic
vector, assuming a GAD fi eld. Only the bedding of sedi-
mentary rocks unambiguously gives the ancient hori-
zontal. Intrusive igneous rocks provide no record of the
paleohorizontal; it must be detected indirectly, some-
times by the rare occurrence of layered early crystal-
lized minerals (Cawthorn 1996) or by techniques like
the aluminum - in - hornblende paleobathymetric tech-
nique (Ague & Brandon 1996). One example of this
approach comes from the paleomagnetic study of the
Cretaceous Mt Stuart batholith. The paleomagnetism
of the Mt Stuart batholith provides an important pale-
omagnetic data point in the argument for large-scale
translation of Baja British Columbia along western
North America's continental margin (Cowan et al .
1997). However, its anomalous paleomagnetic inclina-
tion could just as easily be explained by wholesale
tilting of the batholith after it was magnetized. The
small amount of tilt of the batholith, 7° according to
the aluminum - in - hornblende paleobathymeter (Ague
& Brandon 1996), argues for tectonic transport. It is
not as convincing as paleohorizontal obtained from
sedimentary bedding however, thus leaving the tec-
tonic transport interpretation ambiguous. Another
example of how the paleohorizontal of intrusive
igneous rocks can only be determined indirectly comes
from a paleomagnetic study of the Eocene Quottoon
plutonic complex in the Coast Mountains of British
Columbia. In this case, 12-40° tilting of the pluton is
inferred from a regular decrease in exhumation age
from west to east across the pluton determined by a
transect of K-Ar ages (Butler et al . 2001). In the Quot-
toon study the amount of tilting has a large effect on
the interpretation of the paleomagnetic inclination of
the rocks.
Extrusive igneous rocks of course have a much
better control on the paleohorizontal, either from
direct measurement of the layering in the lava fl ows or
from the bedding of sedimentary rocks intercalated
between the fl ows, but there can still be ambiguities.

One concern to paleomagnetists is the problem of
initial dip of extrusive igneous rocks, particularly of
highly viscous volcanic fl ows such as andesitic rocks.
Strato - volcano edifi ces can have initial, non-tectonic
dips up to 35 - 42 ° . Even low - viscosity basaltic fl ows
can have dips as large as 12° (MacDonald 1972;
Francis 1993 ; Gudmundsson 2009 ). These unrecog-
nized dips contribute error to the measurement of the
paleohorizontal for extrusive igneous rocks and hence
in the paleolatitude determined from these rocks. One
good example of the possibility of unrecognized tilt in
igneous rocks comes from the work of Kent & Smeth-
urst (1998) in which the frequency distribution of
inclinations from the Global Paleomagnetic Database
are binned into different time periods (Cenozoic, Meso-
zoic, Paleozoic and Precambrian) to see if they are con-
sistent with the GAD hypothesis throughout Earth's
history. The Cenozoic and Mesozoic data have inclina-
tion frequency distributions consistent with the GAD,
but the Paleozoic and Precambrian bins have more low
inclinations than predicted by random sampling of the
GAD. Even sedimentary inclination shallowing is ruled
out as the cause because exclusively igneous results
show the effect. While Kent and Smethurst speculate
that octupolar geomagnetic fi elds contributed to the
dipole in these distant times, Tauxe & Kent (2004)
point out that uncorrected and unaccounted-for dips
of igneous rocks, both extrusive and intrusive, could
also explain the effect.
Sediments and sedimentary rocks therefore have an
important role to play in paleomagnetic studies because
of the continuous record they provide and because
their bedding planes give an unequivocal record of the
ancient horizontal.

ENVIRONMENTAL MAGNETIC
RECORD FROM SEDIMENTS AND
SEDIMENTARY ROCKS

The continuity or near continuity of the sedimentary
record, particularly when compared to igneous rocks,
also makes sedimentary rocks an important target for
paleoenvironmental studies. In environmental mag-
netic studies, the magnetic minerals of sedimentary
rocks can record paleoenvironmental conditions.
While sedimentary paleomagnetism uses directional
and intensity records of the geomagnetic fi eld for cor-
relation, dating, and paleolatitude information, envi-
ronmental magnetism uses parameters that measure