Post - Depositional Diagenesis and Chemical Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 76

red beds and showed, petrographically, that the hema-
tite carrying the secondary magnetization is authi-
genic and derived from alteration of ferromagnesian
silicates.
Cox et al . (2005) conducted a detailed study of the
remagnetized Devonian red beds of the Hampshire For-
mation in West Virginia. They studied this unit where
it outcropped in the more interior Alleghanian Plateau
and in the folded Valley and Ridge province. Petro-
graphic examination showed authigenic specular hem-
atite cement and submicron hematite pigment. Fluid
inclusion studies did not show warm orogenic fl uids, as
had been shown for other units in the remagnetized
Appalachians, but instead suggested that the second-
ary hematite was formed by a mixture of methane-
saturated formational fl uids and meteoric fl uids. Cox et
al . see a two-step process for the remagnetization of the
Hampshire. First, methane-fl uids, perhaps driven
through the rocks by orogenesis, caused dissolution of
depositional Fe-oxide magnetic minerals putting Fe
into solution. Meteoric fl uids that arrived during sub-
sequent post-orogenic uplift of the rocks then caused
oxidation of the Fe in solution to precipitate authigenic
hematite. Although the authors state that the standard
warm orogenic fl uid model cannot be invoked to explain
the secondary magnetization of these red beds, clearly
late - stage fl uid fl ow (in this case involving hydrocar-
bons) could have caused the remagnetization.
Stamatakos et al . (1996) give important evidence for
the timing of the Kiaman remagnetization and its rapid
occurrence. They examined the age of remagnetiza-
tion from fold tests applied to remagnetized rocks along
a transect that crossed the folded rocks of Pennsylva-
nia. They found that rocks in the hinterland (eastern
PA) had post-folding magnetizations while rocks closer
to the foreland (central and western PA) had syn- and
even pre-folding Kiaman remagnetizations (Fig. 6.6).
This observation suggested that orogenic fl uids moved
quickly through the rocks over 20 million years during
the formation of the Appalachian fold and thrust belt,
catching a 'snapshot' of the folding at different stages
in development. This observation was indirect evi-
dence of a fl uid cause of the remagnetization, even in
siliciclastic rocks since Stamatakos et al . used both car-
bonates (Greenbriar, Helderberg, Clinton, Trenton, and
Allentown/Leithsville) and siliciclastic red beds (Mauch
Chunk, Catskill, Andreas, Bloomsburg, Rose Hill and
Juniata) in their study.
Additional evidence for a basinal fl uid cause of the
remagnetization comes from McCabe et al . ' s (1989)

0.010 spu

KNOX DOL
baq 1.1c

Jrs/Js = 0.536
Hcr/Hc = 2.30

3000
Sause

Fig. 6.5 Hysteresis loop of remagnetized Knox Dolomite
from Jackson (1990) showing wasp-waisted behavior of a
remagnetized carbonate rock. M Jackson, Diagenetic sources
of stable remanence in remagnetized Paleozoic cratonic
carbonates: A rock magnetic study, Journal of Geophysical
Research , 95, B3, 2753-2761, 1990. Copyright 1990
American Geophysical Union. Reproduced by permission
from American Geophysical Union.

remagnetized North American carbonates. They found
that strongly remagnetized carbonates had nearly
horizontal minimum principal axes, indicating a sec-
ondary (perhaps tectonic) fabric and weakly and non-
remagnetized carbonates that typically had primary
fabrics with minimum axes perpendicular to the
bedding plane. The explanation for the initially observed
primary fabrics in the Onondaga now becomes clearer;
AMS preferentially measures the fabric of the coarser
magnetic particles, perhaps the depositional magnetite
in the rocks. AAR shows that these magnetic particles
appear to have a primary fabric. The secondary fi ne-
grained single-domain magnetite particles have a sec-
ondary magnetic fabric that the AMS measurements
may have missed. Remanence anisotropy is therefore
superior to AMS for detecting the fabrics of subpopula-
tions of the magnetic grains in a rock. The remanence
anisotropy of the high - unblocking - temperature hema-
tite in red beds can be a good indicator of their primary
origin.
One of the problems that McCabe and Elmore
pointed to was the lack of any petrographic evidence
about the partial remagnetizations observed in the
siliclastic rocks suffering from Late Paleozoic remag-
netization. Lu et al . (1994) studied the hematite
remagnetization in the Silurian Rose Hill Formation