Paleomagnetic analysis of Chickaloon Formation volcanic ashes: Implications for dating the Paleocene

1
Paleomagnetic analysis of Chickaloon Formation
volcanic ashesImplications for dating the
Paleocene-Eocene boundary
April 2007
Alison Alden Spruce Department of Earth
Sciences Advisor Professor Will Clyde
Results Volcanic Ash 1 Volcanic Ash 1 samples
were taken from the upper Premier coal group
corresponding with the lower Eocene epoch.
Samples from this ash exhibit some of the
strongest initial intensities, with values
ranging from 0.525 mA/m to 0.727 mA/m. Data for
the three samples are remarkably similar, with
each demonstrating values comparable to the
present day field. Due to a lack of secondary
magnetic component in these samples, and the
obvious relation with the present field, these
samples are interpreted to represent solely the
present-day overprint. A vector endpoint diagram
of sample 1-2 is shown in geographic coordinates
as representative of the group (Figure 4). All
three samples are plotted on an equal-area
projection, with the 95 confidence interval of
the mean shown the present-day field has been
superimposed for comparison (Figure 5). Volcanic
Ash 4 Volcanic Ash 4 samples were taken from
the lower Premier coal group corresponding with
the upper Paleocene boundary. Samples from this
ash exhibit some of the weakest initial
intensities, with values ranging from 0.071 mA/m
to 0.122 mA/m. Of the three samples suitable for
measuring, two exhibited similar vector endpoint
plots, and differences in the third may be
attributed to alteration from weathering. The
two similar plots each exhibited two distinct
components, with one corresponding to an
overprint direction and the other being
suggestive of a ChRM. Vector endpoint diagrams
of sample 4-2 are shown as representative of the
group, with geographic coordinates shown to
demonstrate the overprint component, and tectonic
coordinates shown to demonstrate the ChRM
component (Figures 6 and 8). Both samples are
plotted on equal-area projections, with the 95
confidence interval of the means shown the
present-day field and past field have been
superimposed for comparison on the overprint and
ChRM diagrams, respectively (Figures 7 and
9). Volcanic Ash 5 One sample was taken from
Volcanic Ash 5 in the Eska coal group
corresponding with the Paleocene epoch. This
sample exhibited the strongest initial intensity,
with a value of 0.887 mA/m. The shallow
inclination of the sample corresponds neither
with the present-day field or expected past field
for a location of such high latitude, and may
represent a period of anomalous field activity
often associated with periods of geomagnetic
reversals or a local lightening strike. The
vector endpoint diagram of sample 5-1 is shown in
tectonic coordinates (Figure 10), as well as
plotted on an equal-area projection (Figure
11). Volcanic Ash 6 One sample was taken from
Volcanic Ash 6 in the upper Premier group
corresponding with the lower Eocene epoch. This
sample exhibited the weakest initial intensity,
with a value of 0.037 mA/m. Due to its very weak
intensity, meaningful demagnetization data was
unattainable from this sample. The vector
endpoint diagram shown is characteristic of a
sample with no stable remanence magnetization
(Figure 12).
Abstract The Paleocene-Eocene boundary represents
a period of significant mammalian evolution,
which resulted in the explosive radiation of
several orders of placental mammals. Despite the
obvious importance of the Paleocene-Eocene
boundary in understanding mammalian origins, the
geological record available for this time has
made it difficult to determine its absolute age.
The Chickaloon Formation of south-central Alaska
represents a geological record of the
Paleocene-Eocene boundary, and may possess the
volcanic ashes needed for radiometric dating
(Figure 1). A pilot paleomagnetic analysis of
these volcanic ash beds was undertaken to
determine if these ashes do indeed bracket the
Paleocene-Eocene boundary, and thereby establish
the viability of these samples for dating
purposes. Additionally, the results of this
study can be used to determine whether these
ashes can be used to calibrate the Geomagnetic
Polarity Timescale (GPTS).
Figure 4. Vector endpoint diagram for Sample 1-2
shown in geographic coordinates. This sample is
representative of the Ash 1 group. Similarities
with the present-day field suggest that this ash
bed is characterized by a magnetic overprint.
Figure 5. Equal-Area projection of all three
samples from Ash 1 shown in geographic
coordinates. The 95 confidence interval of the
mean is shown, with the present-day field
superimposed in red for comparison.
Figure 1. Location of the Chickaloon Formation
in south-central Alaska.
Figure 6. Vector endpoint diagram for Sample 4-2
shown in geographic coordinates. This sample is
representative of the Ash 4 group, and separate
groupings of data points suggest two magnetic
components with one corresponding to a
present-day overprint and the other suggestive of
ChRM.
Background The Chickaloon Formation is a
terrestrial sedimentary sequence in south-central
Alaska composed primarily of claystone,
siltstone, and sandstone layers and interbedded
with thick coal beds in the upper layers of the
sequence. Exposures of this formation are found
in coal mine outcrops in the Matanuska Valley, 70
km east of Anchorage (Figure 2). Samples were
taken from volcanic ashes in two coal groups,
bracketing the Paleocene-Eocene boundary. Four
samples were taken from two ashes in the upper
Premier coal group, corresponding with the lower
Eocene epoch. Four samples were taken from one
ash in the lower Premier coal group,
corresponding with the upper Paleocene epoch. A
ninth sample was taken from an ash in the Eska
coal group, from lower in the Paleocene epoch.
(Figure 3)
Figure 7. Equal-Area projection of overprint
components from both samples in Ash 4 shown in
geographic coordinates. The 95 confidence
interval of the mean is shown, with the
present-day field superimposed in red for
comparison.
Figure 8. Vector endpoint diagram for Sample 4-2
shown in tectonic coordinates. This sample is
representative of the Ash 4 group, and separate
groupings of data points suggest two magnetic
components with one corresponding to a
present-day overprint and the other suggestive of
ChRM.
Figure 9. Equal-Area projection of the ChRM
components from both samples in Ash 4 shown in
tectonic coordinates. The 95 confidence
interval of the mean is shown, with the past
field superimposed in green for comparison.
Conclusions Samples within each volcanic ash of
the Chickaloon formation of south-central Alaska
show reasonable consistency of remanence magnetic
behavior however, there are significant
differences in the behavior of different ashes.
The dominance of the overprint magnetization in
Ash 1, although not excluding the possibility of
a reverse polarity ChRM, could not confirm it
either. Similarly, despite the presence of a
reverse polarity signature in Ash 5, the lack of
correlation with either the present or past
field, made association of that polarity with the
samples ChRM unreliable. Additionally, the
weakness of Ash 6 produced an unmeaningful
magnetic signature. Therefore Ash 4 was the only
ash in the formation with a magnetic signature
suggestive of a primary reverse polarity ChRM.
In conclusion, the lack of consistently stable
remanence magnetization makes it unlikely that
these beds will provide a reliable
magnetostratigraphic record for conclusively
determining the viability of these samples as
brackets for the Paleocene-Eocene boundary.
Figure 2. Eska pit coal mine in south-central
Alaska. Volcanic ashes from the Chickaloon
formation exposed in this mine may provide
radiometric dates for the Paleocene-Eocene
boundary.
Figure 3. Stratigraphic relationship of the
volcanic ash samples within the Chickaloon
Formation with respect to the Paleocene-Eocene
boundary.
Methods When volcanic ashes are deposited in a
sedimentary sequence, the magnetic minerals
within the ashes can align themselves to the
geomagnetic field at the time, creating what is
known as a characteristic remanent magnetization
(ChRM). Occasionally more magnetically
susceptible minerals within the ashes may realign
themselves to the present day field, creating a
magnetic overprint. Through incremental
demagnetization processes, different components
of remanent magnetization can be identified,
allowing the interpretation of the ashes ChRMs.
  The boundary of the Paleocene-Eocene boundary
is defined by a long period of reversed polarity.
Therefore, if these ashes do indeed bracket the
Paleocene-Eocene boundary, they should
consistently exhibit reverse polarity ChRMs.
Due to the weakness in the initial magnetic
intensities of these ashes, alternating-field
methods were used for the demagnetization of the
samples.   By placing the samples in a tumbler in
an alternating magnetic field, minerals within
the field become randomly oriented netting their
magnetic contribution to zero. Different
minerals are susceptible to this process at
different intensities, allowing the incremental
application of this process to produce a record
of the changes in the magnetic properties of the
ashes. This information was then plotted on a
vector endpoint diagram, which was interpreted to
identify the different components of remanent
magnetization.
Figure 10. Vector endpoint diagram for Sample 5-1
shown in tectonic coordinates. The low
inclination of this sample lacks similarities
with either the present-day field, or expected
past field for a location at such high latitude.
These inconsistencies may be explained by
anomalous field activity associated with a period
of geomagnetic reversal or a local lightening
strike.
Figure 11. Equal-Area projection of Sample 5-1
shown in tectonic coordinates.
Acknowledgements I would like to express my
appreciation to Ross Secord for providing the
samples for this study, as well as my thanks to
my advisor, Professor Will Clyde, for his help
throughout the entire research process.
References Triplehorn, D.M., Turner, D.L., and
Naeser, C.W., 1984, Radiometric age of the
Chickaloon Formation of south-central Alaska
location of the Paleocene-Eocene boundary,
Geological Society of America Bulletin, v. 95, p.
740-742.   Butler, R.F., 1998, Paleomagnetism
Magnetic Domains to Geologic Terranes, University
of Arizona.
Figure 12. Vector endpoint diagram for Sample 6-1
shown in tectonic coordinates. The scattered
arrangement of data points indicates that the ash
does not record a stable remnant magnetization.