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purpose of this study wes to determine whether
reliable paleomagnetic data can be acquired from the
Gravina-Nutzotin belt in southeastern Alaska that wiU
at unblocking temperatures of 450O to 5 0 0 (table ~ ~
20). 7he two samples collected from one dike did not
yield 8 reliable magnetic direction. Two of the three
I
i
ald in the resolution of the much4ebated time of
accretion of southern Alaska terranes. We chase
Marsh Island for three reasons; (1) The age of the
rocks is precisely known (Albian); (2) ancient
horizontal and stretigraphic younging directions can be
determined from bedding; and (91 the rocks are known
to be the least recrystallized part of the Gravina-
Nutzotin b%lt in southeastern Altlska.
A total of 33 samples were collected from four
b& at two locabties (fig. 78). Bed 1 is a 50- to 60cm-thick
medium*ained lithic sandstone, interbedded
with graded turbidites and shale. Beds 2 through 4 are
poorly sorted coarse-grained tuffaceous pebbly
sandstone containing angular chts of green volcanic
rock, interbedded with graded turbidites, shale, and
massive volcanic mudflow deposits containing volcanic
blocks as much as 5 m wide. Bed 1 was sampled at
regular intervals (17 samples) tor a distance of about
10 metfrs between two andesite dikes, which were also
sampled, to determine whether the unit had been
thermally remagnetized by the dikes. Beds 2 through 4
(Loc. 2, fig. 781 were sampled at a locality 0.5 km away
Prom bed 1. The 2.54-crn-diameter cores were cut into
2-cm lengths. AU samples were subjected to stepwise
thermal demagnetization. Replicates of these samples
were subjected to stepwisk alternating field (A.F.)
cleaning procedures; all magnetization measurements
were made on a cryogenicmagnetometer. 1ngenerd,
the cleaning procedure ended when the magnetisation
appeared to reach a stable end paint md was below 10
percent of the natural remartent magnetization (NRM)
or the initial magnetic-intensity value, or when
unstable magnetic behavior &came apparent,
bed represents an imtant in geologic timer therefore,
the sample directions were everaged by bed to provide
a spot reading of the geomagnetic field.
Samples fmm bed 1 responded well to thermal
demegnetization. Stable magnetic directions appeared
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132'40'
56°K
samples from the second dike, however, yielded stable
directions that are similar to the magnetic direction of
the nearest sandstone sample in bed 1, 2 cm away. We
conclude that the second dike thermally remagnetized
bed 1 near the contact but that the thermal effec 1 of
the dike dissipated withln 2 m. Bed 1 thus passes the
baked-oontact test (McBlhhy, 1973). Most of the
samples from bed 1 and the two dlkes underwent
rotational rernanent magnetization (RRM) at
coercivities of 20 to 80 mT during A.P. treatment or
did not reach a stable end point, and so these A.P. data
were not used in calculation of the ancient-field
direction. One sample, however, did reach a stable
end paint and yielded a magnetic direction similar to
that of its thermally demagnetized counterpart
Cores from beds 2 through 4 responded well to
the A.P. cleaning but behaved erratically during
thermal treatment. A few of the A.F.4emagnetized
samples were suspected of acqubing RRM at
C~rcivities of 30 to 50 mT. Most of the samples
displayed a stable magnetic dkection before suspected
RRM obscured the NRM; these directions were
comparable to RRM-free samples In the same bed.
One thermally treated sample cleaned to a stable end
point that compared well with its A.P.4ernagnetized
repLicate.
To estimate the average Albim (Early
cretaceous) geomagnetic field, the mean of the four
bed means was calculated (table 21)- mean
mapetic direction for the Marsh bland site was found
not to cofncide with the pwsent field. -,the 5-
value (pisher's precision parameter; see McEUlimy,
1973) for the mean of the bed means, corrected for
tilting, was greater than the individual bed kvalue.
BQ~W the WdN attitudes vary by so, a
reabtic fold test is not possible.
Having passed the baked+ontoct test, and in
view of the absence of present-field signature, we
consider the magnetic directions from Marsh Island to I
represent a measure of the mcient (Albian)
geomagnetic field. However, because only four beds
or time horizons are reprmented, It may be th0t the
secular varlation of the geomagnetia field was not
averaged out. Therefore, we regard these results as
preUmlnary and likely to be modified somewhat by
more complete sampling of stratigraphic section.
Within the limitations of thls study, we offer two
possible Interpretations of the mean magnetic
direatLon. The flrst interpretatJon assumes that the
magnetic vector obtained from the Marsh Island
sampla represents original dctritd rernanent I
megnetization (DRM1. If so, then the equivelent
paleolatitude of the vector Inclination is lo0.
MoreOver,becausethesero~k~are~ian,theywere
56'06
? .5 1 KJLDMEJERS
almost certainly deposited withln the Cretaceous longnormal-polarity
interval (Irvhg and Wah, 1976).
As such, the vector polarity Is unambiguous and
represents 10" north of the paleoequator. Even if *is
paleolatitude estimate Ls slightly In error, owing to a
poor time averaging of the geomagnetic field, the
.
?&-Part of the Petembhg A-3 quadrangle,
showing loeations of palesmagnetic-aample sites on
Marsh Island.
estimate stu suggests that the Qravlna-Nutzotin belt
and, thus, WrangeUia and the Alexander terrane were
far south of their present Latitude during Early
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