MAGNETOSTRATIGRAPHY OF THE CHINLE AND MOENKOPI STRATA CORED AT
COLORADO PLATEAU CORING PROJECT SITE 1A, NORTHERN PETRIFIED FOREST
NATIONAL PARK
by
Hesham M. A. Buhedma
APPROVED BY SUPERVISORY COMMITTEE:
___________________________________________
Dr. John W. Geissman, Chair
___________________________________________
Dr. Robert J. Stern
___________________________________________
Dr. Ignacio Pujana
Copyright 2016
Hesham M. A. Buhedma
All Rights Reserved
To William J. Fulbright
MAGNETOSTRATIGRAPHY OF THE CHINLE AND MOENKOPI STRATA CORED AT
COLORADO PLATEAU CORING PROJECT SITE 1A, NORTHERN PETRIFIED FOREST
NATIONAL PARK
by
HESHAM M. A. BUHEDMA, BS
THESIS
Presented to the Faculty of
The University of Texas at Dallas
in Partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE IN
GEOSCIENCES
THE UNIVERSITY OF TEXAS AT DALLAS
JANUARY 2016
ACKNOWLEDGMENTS
I’m immensely grateful to my friend and thesis adviser Dr. John Geissman for his continued
support and guidance. I’m also indebted to the rest of the committee, Dr. Robert Stern and Dr.
Ignacio Pujana for their valuable comments, patience and understanding. I also would like to
thank Ms. Kelly Ables and Ms. Julia McIntosh for their friendship and essential assistance in the
lab. Finally, special thanks are extended to the Fulbright program and Amideast, without whom
none of this would have been possible.
v
MAGNETOSTRATIGRAPHY OF THE CHINLE AND MOENKOPI STRATA CORED AT
COLORADO PLATEAU CORING PROJECT SITE 1A, NORTHERN PETRIFIED FOREST
NATIONAL PARK
Hesham M. A. Buhedma, BS
The University of Texas at Dallas, 2016
ABSTRACT
Supervising Professor: John W. Geissman, PhD
The Triassic Period (ca. 251.9-201.3 Ma) is bounded by two of Earth’s largest mass extinctions,
suffered several giant bolide impacts and eruption of three large igneous provinces, and
witnessed evolution of the main components of modern tetrapod communities, and yet has
sparse geochronologic calibration. To remedy this problem, the US NSF- and ICDP-funded
coring of Phase 1 of the CPCP was completed in late 2013, with the recovery of two major cores
(6.35 cm diameter: 1A, 518m length and 2B, 253m; 31km apart) from the north and south ends
of Petrified Forest National Park, spanning nearly the entire Triassic section (Chinle and
Moenkopi formations.). Typically, multiple specimens were collected from the same core
segments, to test for internal consistency of paleomagnetic and rock magnetic results, then
subjected to progressive thermal demagnetization or a combination of alternating field (AF)
followed by thermal treatment. For the mudstones, siltstones and sandstones of the Chinle
Formation, NRM intensities are highly variable and range from 128.7 to 0.07 mA/m and bulk
susceptibilities range from ~ 2 x 10-2 to ~ 5 x 10-5 SI volume units. For indurated hematitic
siltstones to medium grained sandstones of the Moenkopi Formation, NRM intensities are less
vi
variable and range from 0.05 to 53.01 mA/m and bulk susceptibilities are far less variable, with
values between ~3.0 X 10-4 and ~0.5 x 10-5 SI volume units. Progressive demagnetization of
accepted specimens typically isolates magnetizations of north declination and shallow to
moderate positive inclination (interpreted as normal polarity) and antipodes (reverse polarity).
IRM acquisition and backfield demagnetization experiments demonstrate both hematite and
magnetite as magnetic phases. An attempt was made to correlate current version of the CPCP 1A
magnetostratigraphy with the astronomically tuned Newark Basin record.
vii
LIST OF FIGURES
Figure 1.1 Location of CPCP 1A core drill site .............................................................................14
Figure 1.2 Generalized Stratigraphy of the Triassic section in the PFNP. Modified from Martz et
al. (2012) ............................................................................................................................18
Figure 2.1 An example of demagnetization data from a specimen from a core segment that is
suspected to be a result of an artificial error during the coring process. Orthogonal
demagnetization diagram (Zijderveld, 1967) showing response to progressive thermal
demagnetization. Filled (open) circles correspond to the projection of the magnetization
vector on a horizontal (east-west vertical) plane in geographic coordinates. Also shown
are equal area projections with filled (open) circles on lower (upper) hemisphere. ..........22
Figure 3.1 (a, b) Equal are projection of interpreted Triassic age magnetization directions from
two specimens measured in this study. Filled (open) circles represent projection on lower
(upper) hemisphere. c) Paleopoles of North America from the Middle Triassic to the
Lower Jurassic surrounded by 95 percent confidence limits based on average of poles
from Van der Voo (1990). Modified from Molina-Garza el al. (2003). ............................26
Figure 3.2 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing
response to progressive thermal demagnetization for specimens from the Petrified Forest
Member. Filled (open) circles correspond to the projection of the magnetization vector on
a horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates.
Also shown are equal area projections with filled (open) circles on lower (upper)
hemisphere along with normalized magnetic intensity decay plots...................................27
viii
Figure 3.3 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing
response to progressive thermal demagnetization for specimens from the Sonsela
Member. Filled (open) circles correspond to the projection of the magnetization vector on
a horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates.
Also shown are equal area projections with filled (open) circles on lower (upper)
hemisphere along with normalized magnetic intensity decay plots...................................28
Figure 3.4 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing
response to progressive thermal demagnetization for specimens from the Mesa Redondo
Member. Filled (open) circles correspond to the projection of the magnetization vector on
a horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates.
Also shown are equal area projections with filled (open) circles on lower (upper)
hemisphere along with normalized magnetic intensity decay plots...................................29
Figure 3.5 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing
response to progressive thermal demagnetization for specimens from the Holbrook
Member. Filled (open) circles correspond to the projection of the magnetization vector on
a horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates.
Also shown are equal area projections with filled (open) circles on lower (upper)
hemisphere along with normalized magnetic intensity decay plots...................................30
Figure 3.6 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing
response to progressive thermal demagnetization for specimens from the Moqui/Wuptaki
members. Filled (open) circles correspond to the projection of the magnetization vector
on a horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates.
ix
Also shown are equal area projections with filled (open) circles on lower (upper)
hemisphere along with normalized magnetic intensity decay plots...................................31
Figure 3.7 Plots showing results of IRM acquisition and backfield demagnetization experiments
for specimens from Petrified Forest (a), Sonsela (b, c, d), Mesa Redondo (e) and
Moqui/Wuptaki (f) .............................................................................................................33
Figure 3.8 Representative lower hemisphere, equal-area projections of anisotropy of magnetic
susceptibility (AMS) data that were accepted accepted for polarity information based on
demagnetization behavior specimens as grouped my member. The AMS fabric at each
locality is consistent with that of detrital sedimentary rocks. ............................................36
Figure 3.9 Anisotropy of magnetic susceptibility data for all specimens measured. Outliers have
been omitted for clarity. a) Shape Parameter vs. Degree of anisotropy (Jelinek, 1981). b)
Lineation vs. Foliation (Flinn, 1962). ................................................................................36
Figure 3.10 Bulk Susceptibility vs. Depth for all specimens collected as grouped by member. a)
including all data from specimens from Sonsela Member. b) excluding data from Sonsela
Member. .............................................................................................................................36
Figure 3.11 NRM Intensity vs. Bulk Susceptibility for each specimen measured as grouped by
member. Linear trend lines with corresponding R2 values are shown for each set of
data. ....................................................................................................................................36
Figure 3.12 Partial magnetic polarity stratigraphy of CPCP 1A core next to a generalized
stratigraphy of the northern PFNP and U-Pb zircon dates. Red dashes indicate sampling
location. ..............................................................................................................................36
x
LIST OF TABLES
Table 1 Bulk susceptibility data for each member sampled. .........................................................35
xi
CHAPTER 1
INTRODUCTION
Lasting about 50 million years, the Triassic Period was a time of great climatic and biotic
changes on the continents, all of which shaped the remainder of the Mesozoic world. This period
witnessed the evolutionary appearance of modern terrestrial biota, saw the ascent of the
dinosaurs and was punctuated by two mass extinction events (Erwin, 1994; Raup, 1979). This
study centers on the magnetostratigraphy of the succession of Triassic age strata (Chinle and
Moenkopi formations) that was cored in the Petrified Forest National Park, Arizona, USA, in
Fall, 2013, (Figure 1.1) as part of the Colorado Plateau Coring Project (CPCP). This area has
been the subject of study by many workers for over a century, but limitations regarding the
distribution of outcrop exposure itself have presented several ongoing challenges, which have
made geologic investigations incomplete and ambiguous. The best exposures in the study area
are either inaccessible in steep cliffs or severely weathered, making geologic observation at the
appropriate level of confidence and resolution unattainable. Such limitations have resulted in an
uncertain temporal context for parts of the Chinle and Moenkopi strata, produced ambiguous
global correlations, and cast doubt on paleolatitudinal position, which hinder the integration of
the voluminous amount of information gathered from the region. Key concerns will not be
resolved until a proper chronostratigraphic framework is available.
To begin to address these issues, the Colorado Plateau Coring Project (CPCP) was
initially proposed by Olsen et al. (1999), and was intended to provide a pristine, uninterrupted
section of as much of the Triassic succession as possible, in the selected areas where
12
superposition is not in question. A continuous core has the potential of providing an
unambiguous, high resolution magnetic polarity stratigraphy, which is the focus of this MS
thesis, that can be integrated with geochronologic, paleoenvironmental and paleontologic
information. Coupled with outcrop data, the information can be used to generate a reference
section with a globally exportable absolute age time scale providing the needed temporal context
for global correlation. One version of the geomagnetic polarity time scale (GPTS) for the Late
Triassic and Early Jurassic (~235 – 200 Ma) was produced from the astronomically calibrated
results of the Newark Basin Coring Project (Kent et al., 1995; Olsen and Kent, 1996; Kent and
Olsen, 1999; Olsen and Kent, 1999; Olsen and Kent, 2000) and outcrop studies in the Hartford
Basin (Kent and Olsen, 2008). The magnetic polarity stratigraphy of the cored sequence from
Chinle and Moenkopi formations from the Petrified Forest National Park area may have the
potential of serving as a test of the robustness of the Newark Basin astrochronologic record for
the Late Triassic.
The site of the first phase of the CPCP is located in the Petrified Forest National Park in the
south part of the Colorado Plateau (Figure 1.2), a high standing physiographic feature of
relatively undeformed Phanerozoic flay-lying strata with an average elevation of 1500 m.
Basement is comprised of Precambrian metamorphic and igneous rocks covered by a sequence
of Neoproterozoic to Cenozoic sedimentary rocks and a volumetrically thin section of Neogene
igneous and sedimentary rocks. Outcrops provide excellent exposure of Triassic strata in the
Petrified Forest National Park area (Martz and Parker, 2010; Martz el al., 2012), although surface
exposures are considerably weathered. The exposed strata are parts of the Triassic Chinle At the
CPCP1A coring site at Chinde Point, Chinle Formation strata are disconformably overlain by the
13
Neogene Bihadochi Formation and some thin mafic lava flows. Structurally the area is very
simple, with strata that dip very gently to the north as a part of an east–west oriented homocline.
Figure 1.1 Location of CPCP 1A core drill site
14
1.1
Geologic Background
The Moenkopi Formation is of Middle to Late Triassic age (Mckee, 1954; Stewart et al., 1972)
and crops out extensively in Northern Arizona but it is not exposed within PFNP. In northeast
Arizona, the Moenkopi strata consist of a suite of marine and continental deposits that represent
deltaic, fluvial, near shore and shoreline environments (Baldwin, 1973) and was originally
subdivided into three members by Hager (1992). The lower and middle members were later
renamed by Mckee (1954) to Wuptaki and Moqui, respectively, while retaining the original
name of the youngest member (Holbrook). A complete section of all three members was
recovered from the CPCP 1A core. The Moenkopi is bounded by unconformable contacts at the
top and bottom, with the Chinle Formation and the Coconino Formation, respectively. The
Wuptaki Member is chiefly composed of pale reddish-brown siltstone and contains a
stratigraphically significant bed of sandstone that extends even beyond where Wuptaki member
pinches out and is used for regional correlation (Stewart et al., 1972). At Chinde Point, the lower
contact of the Wuptaki is disconformable with the Coconino sandstone, while the upper contact
with the Moqui Member is conformable and gradational. The Moqui Member is composed of
reddish brown, horizontally laminated siltstones with grayish bands of gypsum. Both contacts of
the Moqui Member are conformable. McKee (1954) defined the lower contact as the first
occurrence of primary gypsum above the Wuptaki lower massive sandstone and the upper
contact as the last occurrence of primary gypsum. These boundaries are sometimes difficult to
distinguish in the field and were later defined by Purucker et al. (1980). They defined the bottom
of Moqui as the first deposits of siltstone above the lower massive sandstone and the top as the
deposits just under the upper massive sandstone of the Holbrook member. The Holbrook
15
Member consists of thinly to thickly bedded layers of pale reddish-brown siltstone interstratified
with pale brown, dark yellowish brown, and pale red sandstone. The lower contact of this
member corresponds with the lowest sandstone lens overlying the Moqui Member. The upper
contact is disconformable and is marked by prominent channels filled with sandstones and
conglomerates of the Chinle Formation (Stewart et al., 1972).
The Chinle Formation is of Late Triassic age and comprises a series of fluvial, lacustrine
and flood plain sediments (Stewart et al, 1972; Blakely, 1989; Dubiel, 1989) deposited east of a
Cordilleran volcanic arc on the western margin of Pangea (Blakey and Gubitosa, 1983). An
almost complete section of Chinle strata is exposed within the boundaries of PFNP but only the
Owl Rock Member is exposed at the drilling site CPCP 1A, at Chinde Point (Martz and Parker,
2010). The initial nomenclature was proposed by Gregory (1917) but that has experienced many
revisions since then (Cooley, 1957, 1959; Billingsley, 1985; Murry, 1990; Heckert and Lucas,
2002; Woody, 2006; Martz and Parker, 2010) and even after a century of geologic study, the
lithostratigraphic framework of the Chinle is still a matter of dispute. An excellent review of
these stratigraphic revisions was compiled by Martz and Parker (2010) and the latest revision
recognized by them is used in this thesis (Figure 2). The four Chinle members encountered in the
1A core in ascending order are: Mesa Redondo Member, Blue Mesa Member, Sonsela Member,
and Petrified Forest Member.
The Petrified Forest Member is a packege of pale-gray to red mudstones and pale-gray to paleyellowish sandstones. The basal contact with the Sonsela Member is gradational and the upper
contact with the Owl Rock Member is sharp but conformable (Heckert and Lucas, 2002). The
Sonsela Member is composed of light gray and yellowish gray, fine to medium grained , cross
16
bedded sandstones, interbedded with conglomerates and blue, gray, and purple mudstones
packaged between the thick sandstones sequenses (Heckert and Lucas, 2002). The sandstones of
the Sonsela tend to be coarser and more compositionally mature than other sandstones within the
Chinle Formation. The lower contact of the Sonsela Member with the Blue Mesa Member is
conformable with the sanstones of the former filling incised channels in the latter. The boundary
between these two members has been a matter of some debate. Lucas (1993) proposed the
existance of an unconformity at the base of the Sonsela Member that he called (Tr-4), citing the
existance of downcutting and reworked strata at the top of Blue Mesa, the major change in
lithology, and vertebrate fauna above and below the proposed unconformity as supporting
evidence (Lucas, 1993; Heckert and Lucas, 1996). This interpretation was later questioned by
several independent studies (Herrick, 1999; Woody, 2006; Martz, 2008) that demonstrated that
the existance of the Tr-4 uncoformity is unlikely. The Blue Mesa Member consists of
fossileferous, banded, gray, blue, purple and grayish-green mudstones with thin siltstones and
friable lenticular sandstones. The Mesa Redondo Member consists of reddish-brown siltstones
and mudstones as well as sandy conglomerates (Martz et al., 2012).
17
Figure 1.2 Generalized Stratigraphy of the Triassic section in the PFNP. Red dashes indicate
sampled intervals. Modified from Martz et al. (2012).
18
1.2
Previous Paleomagnetic Work
The Moenkopi Formation has been the subject of numerous paleomagnetic investigations, going
back to early as 1956 (by Keith Runcorn), and has been the subject of many studies since then
(Baag and Helsley, 1974; Helsley, C. E., 1969; Helsley and Steiner, 1974; Purucker and
Shoemaker, 1980; Steiner and Shoemaker, 1993; Molina-Garza et al., 1991, 1993; Larson et al.,
2012). Overall, Moenkopi strata have typically proven to be an excellent recorder of remanent
magnetization of Middle Triassic age, exhibiting both normal and reverse polarities of
remanence. Notably, Moenkopi Formation strata were part of the centerpiece of the longstanding debate over the actual timing of magnetization acquisition in hematitic detrital
sedimentary rocks (Elston and Purucker, 1979; Liebes and Shiva, 1982; Purucker and Elston,
1980; Walker et al., 1981).
Several paleomagnetic studies of Chinle Formation strata have shown that many of its hematitic
mudrock components are reliable and geologically stable recorders of remanent magnetization
(Graham, 1955; Reeve and Helsley, 1972; Reeve, 1975; Bazard and Butler, 1991; Steiner and
Shoemaker, 1993; Kent and Witte, 1993; Molina-Garza et al., 1991, 1993; Steiner et al., 1993;
Steiner and Lucas, 2000; Molina-Garza, 2003; Zeigler et al., 2005). Magnetic polarity
stratigraphy sequences have been generated from many localities but attempts at regional to
global correlation based on lithologic and biostratigraphic criteria have faced difficulty.
19
CHAPTER 2
METHODS
The CPCP 1A core was shipped from the Petrified Forest National Park to the University of
Texas High-Resolution X-ray Computed Tomography Facility. After scanning, the core was
shipped to the University of Minnesota, LacCore Facility in Spring, 2015 where it was halved,
logged, and scanned. Subsequently, standard 2.54 cm diameter specimen plugs were cored using
a drill press equipped with a non-magnetic diamond drill bit, from the suitable CPCP core
segments at the Rutgers University Core Repository. Typically, at least two and often several
specimens were cored from the same core segment to assess internal homogeneity of results.
Standard right cylindrical specimens were prepared after coring. Initial measurements on all of
these standard specimens include bulk magnetic susceptibility and anisotropy of magnetic
susceptibility using AGICO KLY3A or MFK-1A susceptibility units. Selected specimens from
each core segment were then subjected to progressive thermal demagnetization with a minimum
of 19 temperature steps to, when warranted, about 688o C to fully resolve the characteristic
magnetization of each specimen, which may have been acquired during or soon after deposition.
Progressive thermal demagnetization was carried out using one of three ASC TD-48 thermal
demagnetizers.
A fraction of the specimens was subjected to alternating field demagnetization (AF), then
detailed isothermal remanent magnetization (IRM) acquisition to saturation (SIRM) and
backfield demagnetization. All remanence measurements were conducted using a pulse cooled
DC-SQUID based superconducting rock magnetometer (2G Enterprises). IRM acquisition
20
measurements were carried out with an ASC multi-coil impulse magnetizer and measurements
were made on AGICO JR5A or JR6A spinner magnetometers. Orthogonal vector component
diagrams (Zijderveld, 1967) and principal component analysis (Kirschvink, 1980) were
generated from the results of the thermal demagnetization experiments using the Agico Remasoft
3.0 software (Chadima and Hrouda, 2006) and the results of the IRM experiments were plotted and
interpreted on a program written by the author of this thesis on MATLAB.
The initial (ideal) sampling scheme was to collect specimens, or a suite of specimens, at about
0.3 m intervals to achieve a nominal sampling density comparable to that of the Newark
astronomically calibrated GPTS (1 sample per ~20 Ka) which would have precluded aliasing
during correlation. In reality, the CPCP 1A core, in particular through Chinle Formation strata,
includes many intervals that are either far too friable to sample with conventional methods or
that are dominated by medium to coarse grained, non-hematitic sandstones as well as other rock
types that are not conducive to the retention (or acquisition) of a primary remanence. Material
from many of these intervals has proven to exhibit uninterpretable response in progressive
thermal demagnetization. In addition, many specimens yield well-defined magnetizations in
progressive demagnetization, yet their directions are inconsistent with an expected Triassic
magnetic field direction, so a standard yet non-stringent acceptance criteria was introduced to
objectively evaluate the data. Consequently, specimens with inclinations below 35o and
declinations within 50o of North (normal polarity) or within 50o of South (reverse polarity) were
accepted, as a first pass, to estimate magnetic polarity zonation. This results in the inclusion of
54 percent of the specimens analyzed and allowed a sufficient number data points for a magnetic
polarity stratigraphy to be approximated for at least a part of the CPCP 1A core. About 21
21
percent of the rejected specimens yielded shallow inclinations consistent with the expected
Triassic age magnetization but yielded declinations oriented close to east-west. I interpret this
population of data to reflect core orientation errors during the actual drilling process (Figure 2.1).
A list of these specimens/core segments is included in the appendix.
Figure 2.1 An example of demagnetization data from a specimen from a core segment that is
suspected to be a result of an artificial error during the coring process. Orthogonal
demagnetization diagram (Zijderveld, 1967) showing response to progressive thermal
demagnetization. Filled (open) circles correspond to the projection of the magnetization vector
on a horizontal (east-west vertical) plane in geographic coordinates. Also shown are equal area
projections with filled (open) circles on lower (upper) hemisphere.
22
CHAPTER 3
PALEOMAGNETIC AND ROCK MAGNETIC RESULTS
3.1
Thermal Demagnetization Results
During the Triassic, the American Southwest was located at very low latitudes (Figure 3.1).
Moenkopi and Chinle strata were thus deposited near the equator and are expected to yield
magnetizations, if primary, with shallow inclinations that are north or south directed. Response
to thermal demagnetization by specimens from many intervals of Chinle strata is less than
favorable. These samples commonly exhibit erratic demagnetization behavior with laboratory
unblocking temperatures showing a distributed behavior with much of the remanence unblocked
below 600 oC and a poorly defined remanence. This behavior is consistent among specimens
collected from the same core section and is likely either the result of a magnetization overprint
that is younger in age than Triassic or simply, the presence of a magnetic mineralogy that is
incapable of retaining an ancient field direction. Demagnetization results are discussed by each
member of the Triassic section.
About 72 m of sedimentary section was recovered from the Petrified Forest Member. The
mudstone segments are too friable to be cored, so sampling was limited to the more indurated
gray sandstone at a sampling rate of ~0.5 m on average. Natural remanent magnetization (NRM)
intensities range from 0.28 to 13.64 mA/m. A total of 73 specimens were demagnetized but only
30 met the acceptance criteria described above. The magnetization directions from the accepted
specimens are poorly defined (Figure 3.2) with an average MAD (maximum angular deviation)
23
of 26.4o but their polarity is unambiguous and the results show that the sampled section is
dominated by normal polarity.
A total of 51m of the Sonsela Member was recovered, with 25 out of 59 specimens yielding
acceptable results. NRM intensities are varied and range from 0.07 to 128.7 mA/m. Typical
demagnetization data from specimens that yield acceptable results (Figure 3.3) show that an
appreciable fraction of the NRM is unblocked above 600 oC. The quality of demagnetization
behavior varies, with more consistent and interpretable results from the more (visibly) hematitic
specimens. Overall, thermal demagnetization first isolates a magnetization component with
declinations directed NW to NE and of moderate to steep positive inclination, and then a ChRM
(characteristic remanent magnetization) with NNW declination with shallow to moderate
positive inclination (normal polarity) the decays to the origin and antipodes (reverse). Only six
specimens were collected from the Blue Mesa Member but only two were demagnetized and
both yielded uninterpretable results. NRM intensity ranges from 0.3 to 0.63 mA/m.
About 32 m of strata were recovered from the Mesa Redondo Member, with 39 out 73 specimens
yielding acceptable demagnetization results and directions suggesting that the NRM is
dominated by normal polarity magnetozones. Only six specimens yielded reverse polarity
magnetizations. Specimens with declinations ranging from NNW to NNE and of shallow to
moderate inclination are interpreted to be of normal polarity (Figure 3.4). Those specimens with
SSE to S declinations and shallow to moderate negative inclination are considered of reverse
polarity.
A total of 57.5 m of section was recovered from the Holbrook Member siltstones and sandstones.
A total of 60 out of 98 demagnetized specimens was accepted for polarity interpretation.
24
Specimens from this member typically yield well defined, interpretable demagnetization
behavior (Figure 3.5). Thermal demagnetization isolates a north directed magnetization overprint
with moderate positive inclination that is typically unblocked below 600 oC. Further
demagnetization to 680 oC reveals a ChRM with NNW direction and shallow positive inclination
and antipodes (Figure 3.5). The available data from the Holbrook Member section define one
reverse polarity magnetozone in the upper 33 m, followed by a lower normal polarity
magnetozone. NRM intensities range from 0.13 to 10.98 mA/m
A total of 51 m of well indurated hematitic siltstones and mudstones was recovered of the
Moqui/Wuptaki Members. A total of 79 specimens were demagnetized, but only 40 were
accepted based on demagnetization response and/or direction of magnetization isolated.
Specimens from this member yield well-resolved magnetization components with an average
MAD value of 7.5o and NRM intensity that ranges from 0.05 to 53.01 mA/m. ChRM is usually
unblocked by about 640 oC (Figure 3.6). Magnetization is typically characterized by (relatively)
shallow inclinations with north or south directed declinations. One reverse polarity magnetozone
is identified at the top of this member and a normal polarity magnetozone continues to near the
bottom of the core, where an interval of six specimens with south directed ChRM and shallow
inclination define a reverse polarity magnetozone.
25
Figure 3.1 (a, b) Equal are projection of interpreted Triassic age magnetization directions from
two specimens measured in this study. Filled (open) circles represent projection on lower
(upper) hemisphere. c) Paleopoles of North America from the Middle Triassic to the Lower
Jurassic surrounded by 95 percent confidence limits based on average of poles from Van der
Voo (1990). Modified from Molina-Garza el al. (2003). Abbreviations: Tr, Triassic; J, Jurassic;
l, lower; m, middle; u, upper.
26
Figure 3.2 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing
response to progressive thermal demagnetization for specimens from the Petrified Forest
Member. Filled (open) circles correspond to the projection of the magnetization vector on a
horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates. Also shown
are equal area projections with filled (open) circles on lower (upper) hemisphere along with
normalized magnetic intensity decay plots.
27
Figure 3.3 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing
response to progressive thermal demagnetization for specimens from the Sonsela Member.
Filled (open) circles correspond to the projection of the magnetization vector on a
horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates. Also
shown are equal area projections with filled (open) circles on lower (upper) hemisphere
along with normalized magnetic intensity decay plots.
28
Figure 3.4 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing response to progressive
thermal demagnetization for specimens from the Mesa Redondo Member. Filled (open) circles correspond to the
projection of the magnetization vector on a horizontal (east-west vertical) plane. All diagrams shown in geographic
coordinates. Also shown are equal area projections with filled (open) circles on lower (upper) hemisphere along with
normalized magnetic intensity decay plots.
29
Figure 3.5 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing response to progressive thermal
demagnetization for specimens from the Holbrook Member. Filled (open) circles correspond to the projection of the magnetization
vector on a horizontal (east-west vertical) plane. All diagrams shown in geographic coordinates. Also shown are equal area
projections with filled (open) circles on lower (upper) hemisphere along with normalized magnetic intensity decay plots.
30
Figure 3.6 Examples of orthogonal demagnetization diagrams (Zijderveld, 1967) showing response to progressive
thermal demagnetization for specimens from the Moqui/Wuptaki members. Filled (open) circles correspond to the
projection of the magnetization vector on a horizontal (east-west vertical) plane. All diagrams shown in geographic
coordinates. Also shown are equal area projections with filled (open) circles on lower (upper) hemisphere along with
normalized magnetic intensity decay plots.
31
3.2
Isothermal Remanent Magnetization (IRM) Experiments
IRM acquisition experiments to saturation or near-saturation and backfield demagnetization
reveal the presence of both a high coercivity magnetic phase (hematite) and a low coercivity
phase (magnetite) as principal magnetic minerals present in varying proportions, based on rock
type. Sandstone specimens from the Petrified Forest and the Sonsela members rapidly acquire an
IRM over low fields (Figure 3.7 a, b, c) with 60 to 90 percent of the total IRM acquired below
500 mT. This behavior suggests the dominance of magnetite in these rocks. A SIRM is
demagnetized rapidly during backfield demagnetization with coercivity of remanence (Bcr)
values consistently less than 300 mT.
Hematitic mudstone and siltstone specimens from the Sonsela, Mesa Redondo, Holbrook, and
Moqui/Wuptaki members (Figure 3.7 d, e, f) yield similar IRM experiment results. The
acquisition curves are typically concave downwards in fields less than 200 mT, but most
specimen show result in less than 25 percent of the total IRM in a field of about 300 mT. There
is a slight change in inflection over the range of fields between 600 and 1000 mT, and then a
continued increase in IRM to saturation or near-saturation by about 2600 mT. During backfield
demagnetization, these specimens lose magnetization slower than sandstones and yield values
Bcr between 420 and 800 mT. These results are consistent with hematite as an important
magnetic phase.
32
Figure 3.7 Plots showing results of IRM acquisition and backfield demagnetization experiments for specimens from Petrified Forest
(a), Sonsela (b, c, d), Mesa Redondo (e) and Moqui/Wuptaki (f)
33
3.3
Anisotropy of Magnetic Susceptibility (AMS)
AMS data were obtained for all the standard specimens obtained from the PFNP 1A core and
reveal a nearly vertical minimum susceptibility axis, and horizontal to shallowly inclined
maximum and intermediate axes for both coarse and fine-grained rocks (Figure 3.8), as well as a
general oblate to prolate magnetic susceptibility ellipsoid (Figure 3.9).
Bulk susceptibility data show that sandstones (e.g., of the Petrified Forest and Sonsela members)
have higher average values than siltstones and mudstones, which are more common to other
members (Table 1). The highest bulk susceptibility values are from sandstones in the Sonsela
Member with an average of 370E-06 SI volume units. Bulk susceptibility values from this
member are highly variable, however, with a range of three orders of magnitude and the highest
values originating from the interval between 224 m and depth 240 m (Figure 3.10). The lowest
bulk susceptibility values are from the Moqui/Wuptaki intervals of the Moenkopi Formation,
which are dominated by gypsum, with a value of -1.37E-06 SI volume units. The lowest average
bulk susceptibility of any member is that of the Mesa Redondo Member with a value of 120E-6
SI volume units. Bulk susceptibility shows a crude positive relationship with NRM as shown in
plots for each separate member (Figure 3.11). Date from the Mesa Redondo Member are the
exception, where the relationship between bulk susceptibility and NRM intensity is poorly
defined, with an R2 values of 0.0157, while data from the Petrified Forest Member show the
highest R2 value of 0.645. Specimens with the lowest NRM and bulk susceptibility typically
reveal the most incoherent demagnetization behavior, resulting in most rejected specimens. The
opposite relation is also true.
34
Member
Table 1 Bulk susceptibility data for each member sampled.
Average (x10-6 SI
Standard
Range
N (specimens)
Volume Units)
Dev.
Petrified Forest
191
56
91.6 - 313
55
Sonsela
370
2020
58.9 - 9850
178
Blue Mesa
151
49
114 - 250
6
Mesa Redondo
120
187
63.9 - 933
71
Holbrook
159
38
40.3 - 259
93
Moqui/Wuptaki
139
71
-1.37 - 318
121
35
Figure 3.8 Representative lower hemisphere, equal-area projections of anisotropy of magnetic susceptibility (AMS) data that
were accepted accepted for polarity information based on demagnetization behavior specimens as grouped my member. The
AMS fabric at each locality is consistent with that of detrital sedimentary rocks.
36
Figure 3.9 Anisotropy of magnetic susceptibility data for all specimens measured. Outliers have
been omitted for clarity. a) Shape Parameter vs. Degree of anisotropy (Jelinek, 1981). b)
Lineation vs. Foliation (Flinn, 1962).
37
Figure 3.10 Bulk Susceptibility vs. Depth for all specimens collected as grouped by member. a)
including all data from specimens from Sonsela Member. b) excluding data from Sonsela
Member.
38
Figure 3.11 NRM Intensity vs. Bulk Susceptibility for each specimen measured as grouped by
member. Linear trend lines with corresponding R2 values are shown for each set of data.
39
3.4
Discussion
Progressive thermal demagnetization of specimens from core segments obtained from Chinle
strata from CPCP 1A core yield relatively low quality results, compared to previous studies of
the Chinle strata conducted in other areas. Response to progressive thermal demagnetization is
frequently erratic with MAD values above 15o on average and in some cases, there is no evidence
of a decay of the NRM to the origin. IRM experiments also show that these rocks contain
abundant magnetite. Core segments from the Chinle strata are too friable to be sampled with
conventional methods and this results in, at present, to significant gaps in the
magnetostratigraphic record of the CPCP 1A core. Future work using alternative approaches may
resolve this matter. Results of thermal demagnetization of specimens collected from the
Moenkopi Formation show that these siltstones and mudstones are generally stable recorders of
the Triassic magnetic field. Rocks of the Holbrook Member yield the best results and define an
upper reverse polarity magnetozone and a lower normal polarity magnetozone. The
Moqui/Wuptaki members generally yield well resolved magnetization components but many
core segments yield magnetization directions that are inconsistent with the expected Triassic
magnetic field which leads to the large zones that are considered uninterpretable. Two reverse
and normal polarity magnetozones are clearly recognized near the top of the section. A magnetic
polarity record based on available data has been generated (Figure 3.12) and is placed next to
published U-Pb zircon dates acquired from tuffaceous sandstone beds at or near the PFNP from
Ramezani et al. (2011) and Atchley et al. (2013). The significant gaps in CPCP 1A core made it
difficult to correlate the entire core with previous studies of the Triassic magnetostratigraphy. A
crude attempt to correlate the current results with the astronomically tuned data from the Newark
40
Basin Coring Project is shown in Figure (3.13) and show promise for a more complete version of
the CPCP 1A magnetostratigraphy in the future. Interestingly, the Holbrook Member upper
reverse polarity magnetozone possibly correlates with the one large uninterpretable zone in the
Newark Basin astrochronology record. The Triassic section recovered from the CPCP 1A core
still has potential to add more to the accepted Triassic GPTS, if the challenge of sampling and
measuring the friable red mudstone intervals can be overcome.
41
Figure 3.12 Partial magnetic polarity stratigraphy of CPCP 1A core placed next to a generalized
stratigraphy of the northern PFNP and published U-Pb zircon dates. Red dashes indicate sampled
intervals.
42
Figure 3.13 Possible correlations of the available CPCP 1A core magnetostratigraphy with
the Newark Basin record based on pattern matching and published U-Pb ages.
43
CHAPTER 4
CONCLUSIONS
One of the objectives of the CPCP was to generate a continuous high resolution
magnetostratigraphy of the Chinle and Moenkopi strata in the PFNP and use this
magnetostratigraphy to export the U-Pb zircon ages measured in strata exposed in the area. This
was hampered by the fragile character of the most promising intervals of the Chinle Formation
(very hematitic friable mudstones) for paleomagnetic study and only a partial
magnetostratigraphy was obtained. Further complicating things, is the mineralogy of the sampled
section. Thermal demagnetization revealed overlapping unblocking spectra and a significant
percentage of the magnetization was unblocked well below the maximum laboratory unblocking
temperatures of hematite. IRM experiments reveal the presence of a magnetic mineralogy with
abundant low coercivity magnetite, lending evidence to the possibility that detrital magnetite is
the principal carrier of the remanent magnetization. Analysis of the accepted remanent
magnetization data from the Petrified Forest Member sandstones reveals one normal polarity
magnetozone. Rocks of the Sonsela Member are mostly of normal polarity with one reverse
polarity interval. The boundaries of these magnetozones are ambiguous and more sampling is
needed for an accurate positioning of magnetozone boundary. Mesa Redondo Member is
dominated by normal polarity, except for a reverse polarity interval at ~400 m depth.
The indurated siltstones of the Moenkopi lend themselves to more robust demagnetization
behavior and a ChRM was easily isolated from most accepted specimens. The sampled section of
the Holbrook Member reveals one upper reverse polarity magnetozone and a lower reverse
44
magnetozone. The sampled Moqui/Wuptaki interval reveals one reverse polarity magnetozone at
the top and normal polarity magnetozone that continues to near bottom of the core. This
magnetozone is interrupted by significant uninterpretable zones where well-resolved
magnetization directions are isolated but are inconsistent with the accepted Triassic age magnetic
field.
45
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50
BIOGRAPHICAL SKETCH
51
CURRICULUM VITAE