The Paleomagnetism of Sediments and Sedimentary Rocks: Importance and Reliability - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 9

explain differences in paleointensity for these geo-
graphically proximal records, it would argue that a
standard deviation of 0.38 is the best that can be
expected for the reproducibility of relative paleointen-
sity variations of the geomagnetic fi eld; this is mainly
due to recording errors.
Based on this admittedly limited, but representative,
survey of the repeatability of DRM records of paleose-
cular variation in recent wet and dry lake sediments
and wet marine sediments, DRM inclination and decli-
nation appear to be repeatable within 5-10° for either
inclination or declination; the very best records show
reproducibility from the same lake in the range 2-3°.
Clearly this shows that recent lake and marine sedi-
ments have the capability of providing highly repro-
ducible records of the geomagnetic fi eld. In all these
records, magnetite is the primary depositional mag-
netic mineral. Given the observed directional variabil-
ity of the geomagnetic fi eld over the past 5 Ma with
angular standard deviations of 10 - 17 ° , the DRMs of
recent sediments clearly have the resolution to discern
the secular variation of the fi eld.

Some interesting observations about red beds add to
the controversy. Most red beds sampled by paleomag-
netists are Paleozoic and Mesozoic in age. Most of them
are interpreted to have been deposited in fl uvial envi-
ronments (the molasse red beds of the Appalachians:
Andreas Formation, Bloomsburg Formation, and
Mauch Chunk Formation; the Colorado Plateau Paleo-
zoic red beds: Chinle Formation, Moenave Formation),
coastal marine environments (deltaic rocks: Catskill
Formation), or large lake environments (Passaic and
Lockatong Formations of the Newark basin), yet rocks
currently forming in these depositional environments
are not typically red in color. In the Global Paleomag-
netic Database the youngest sediments reported to be
red in color are typically of age at least 2 Ma. Therefore,
the red pigmentary hematite in classic red continental
sediments is most likely secondary and carries a post-
depositional chemical remanence. But what about
specular hematite grains that are interpreted to be
detrital? As shown in the previous section, the most
recent lake and marine sediments generally have mag-
netite as their primary magnetic mineral. There are no
(or at least not many) good modern examples of detri-
tal hematite grains being deposited in red sediments,
particularly in the depositional environments usually
associated with Mesozoic or Paleozoic red beds.
However, there is some very good evidence from the
geologic record that continental sedimentary rocks
that have micron-sized hematite grains carrying their
paleomagnetic signal have been magnetized by a DRM.
The presence of relatively large-grained, specular
hematite is often used as evidence for a DRM (Passaic
Formation; McIntosh et al . 1985). The best way to
determine if specular hematite is present in a rock is by
petrographic examination of a magnetic extract, but
indirect methods such as a square-shaped thermal
demagnetization intensity plot can also be used. Classic
red bed units such as the Carboniferous Mauch Chunk
Formation often show a very steep decrease in intensity
at the highest demagnetization unblocking tempera-
tures (Fig 1.2; Chapter 6). This behavior can be inter-
preted to mean that there is a very narrow grain size
distribution of the hematite carrying the highest
unblocking temperatures. Since hematite has a low
spontaneous magnetization, its grains do not become
multi-domain until relatively large grain sizes are
reached (15 microns; Dunlop & Ozdemir 1997 ) and so
all the hematite observed during thermal demagnetiza-
tion of fi ne-grained continental sediments is typically
single

DRMS IN RED, CONTINENTAL
SEDIMENTARY ROCKS

The source of the paleomagnetic signal in red, conti-
nental sedimentary rocks, i.e. red beds, is quite contro-
versial. While most geologists would agree that the
magnetite carrying the paleomagnetism of recent
marine and lake sediments is a primary depositional
mineral, the hematite in red beds could be either depo-
sitional or a secondary magnetic mineral that grows
chemically after deposition. This controversy con-
sumed paleomagnetists in the 1970s and early 1980s
and was denoted the 'red bed controversy'. Butler
(1992) gives an excellent summary of the arguments
for and against a DRM in red beds, as outlined during
the height of the red bed controversy. In fact, red beds
do carry a complicated paleomagnetic signal. Although
it has been quite clearly demonstrated that the hema-
tite in the Siwalik continental sediments in Pakistan
carries a depositional remanence (Tauxe & Kent 1984),
petrographic examination of some red sedimentary
rocks (e.g. the Moenave Formation, Molina-Garza et al .
2003 or the Moenkopi Formation, Larson et al . 1982 )
of the Colorado Plateau would argue that all the hema-
tite grains are chemically produced rather than depo-
sitional grains. domain. The highest - unblocking - temperature