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Manuscript Number: EPSL-D-08-00570
Title: Magnetostratigraphy of the Early Triassic from Chaohu (China) and its implications for the Induan -
Olenekian stage boundary
Article Type: Regular Article
Keywords: Chaohu, Lower Triassic, Paleomagnetism, Olenekian, magnetostratigraphy
Corresponding Author: Mr Sun Zhiming, PhD
Corresponding Author's Institution: Institute of Geomechanics, CAGS
First Author: Sun Zhiming
Order of Authors: Sun Zhiming; Mark W. Hounslow; Junling Pei; Laishi Zhao; Jinnan Tong
Abstract: A magnetostratigraphic study was performed on the lower 44 m of the West Pingdingshan section
near Chaohu city, (Anhui province, China) in order to provide a magnetic polarity scale for the early Triassic.
Data from 295 paleomagnetic samples is integrated with a detailed biostratigraphy and lithostratigraphy. The
tilt-corrected mean direction from the West Pingdingshan section, passes the reversal and fold tests. The
overall mean direction after tilt correction is D=299.9º, I=18.3º (κ=305.2, α95=1.9, N=19). The inferred
paleolatitude of the sampling sites (31.6ºN, 117.8ºE) is about 9.4º, consistent with the stable South China
block (SCB), though the declinations indicate some 101o counter-clockwise rotations with respect to the
stable SCB since the Early Triassic. Low-field anisotropy of magnetic susceptibility indicates evidence of
weak strain. The lower part of the Yinkeng Formation is dominated by reversed polarity, with four normal
polarity magnetozones (WP2n to WP5n), with evidence of some thinner (<0.5 m thick) normal
magnetozones. The continuous magnetostratigraphy from the Yinkeng Formation, provides additional high-
resolution details of the polarity pattern through the later parts of the Induan into the lowest Olenekian. The
magnetostratigraphic and biostratigraphic data shows the conodont marker for the base of the Olenekian
(first presence of Neospathodus waageni) is shortly prior to the base of normal magnetozone WP5n. This
provides a secondary marker for mapping the base of the Olenekian into successions without conodonts.
This section provides the only well-integrated study from a Tethyan section across this boundary, but
problems remain in definitively relating this boundary into Boreal sections with magnetostratigraphy.
State Key Laboratory of Geological Processes and Mineral Resources ,
China Universit y of Geosciences, Wuhan, 430074, China
phone: +86-10-68422365, fax: +86-10-68422326
EPSL:Dear Editor:
Please find enclosed manuscript entitled as “Magnetostratigraphy of the Early Triassic from Chaohu (China) and its implications for the Induan -Olenekian stage boundary” by Z. Sun, Mark W. Hounslow, J. Pei et al., we hope you to consider for publish in EPSL.
The names and addresses of my six suggested reviewers are listed as follows:
Robert Scholger, Institute of Geophysics, University of Leoben, Peter-Tunner-Strasse 25, 8700 Leoben, [email protected] Muttoni Department of Earth Sciences, University of Milan, via Mangiagalli 34, I-20133 Milan, ItalyEmail:[email protected] GalletEquipe de Paléomagnétisme, Institut de Physique du Globe de Paris, UMR CNRS 7154, 4 Place Jussieu, 75252 Paris Cedex 05, [email protected] Dr. J. BesseLaboratoire de Paleomagnetisme et Geodynamique, Department de Geomagnetisme et Paleomagnetisme (URA CNRS 729), Institut de Physique de Glove, 4 Place Jussieu, 75252 Paris Cedex 05, Francee-mail: [email protected]. Xixi Zhao Institute of Tectonics and Department of Earth Sciences,University of California, Santa CruzCA 95064, USAe-mail: [email protected] SzurliesInstitut fur Geologische Wissenschaften, Universitat Halle, DomstraMe 5, 06108 Halle/Saale, Germanye-mail: [email protected] you very much. I look forward to your reply.
Sincerely yours,Zhiming Sun
Cover Letter
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Magnetostratigraphy of the Early Triassic from Chaohu (China) 1
and its implications for the Induan - Olenekian stage boundary2
3
Zhiming Sun1, Mark W. Hounslow2, Junling Pei3, Laishi Zhao1, Jinnan Tong1, James G. Ogg44
1. State Key Laboratory of Geological Processes and Mineral Resources , China Universit y of Geosciences, Wuhan, 5
430074 , China6
2. Centre for Environmental Magnetism and Palaeomagnetism, Geography Dept, Farrer Avenue, Lancaster University, 7
Lancaster, UK., LA1 4YQ.8
3. Key Laboratory of Crust Deformation and Processes, Institute of Geomechanics, CAGS, Beijing, China9
4. Dept. Earth & Atmospheric Science, Purdue University, West Lafayette, IN 47907 USA10
11
12
Corresponding author:13
Zhiming Sun14
State Key Laboratory of Geological Processes and Mineral Resources , 15
China Universit y of Geosciences, 16
Wuhan, 43007417
China18
phone: +86-10-68422365, fax: +86-10-6842232619
e-mail: [email protected]
* Manuscript
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Abstract21
A magnetostratigraphic study was performed on the lower 44 m of the West Pingdingshan section near Chaohu city, 22
(Anhui province, China) in order to provide a magnetic polarity scale for the early Triassic. Data from 295 paleomagnetic 23
samples is integrated with a detailed biostratigraphy and lithostratigraphy. The tilt-corrected mean direction from the West 24
Pingdingshan section, passes the reversal and fold tests. The overall mean direction after tilt correction is D=299.9º, I=18.3º 25
(κ=305.2, α95=1.9, N=19). The inferred paleolatitude of the sampling sites (31.6ºN, 117.8ºE) is about 9.4º, consistent with 26
the stable South China block (SCB), though the declinations indicate some 101o counter-clockwise rotations with respect to 27
the stable SCB since the Early Triassic. Low-field anisotropy of magnetic susceptibility indicates evidence of weak strain. 28
The lower part of the Yinkeng Formation is dominated by reversed polarity, with four normal polarity magnetozones (WP2n 29
to WP5n), with evidence of some thinner (<0.5 m thick) normal magnetozones. The continuous magnetostratigraphy from the 30
Yinkeng Formation, provides additional high-resolution details of the polarity pattern through the later parts of the Induan 31
into the lowest Olenekian. The magnetostratigraphic and biostratigraphic data shows the conodont marker for the base of the 32
Olenekian (first presence of Neospathodus waageni) is shortly prior to the base of normal magnetozone WP5n. This provides 33
a secondary marker for mapping the base of the Olenekian into successions without conodonts. This section provides the only 34
well-integrated study from a Tethyan section across this boundary, but problems remain in definitively relating this boundary 35
into Boreal sections with magnetostratigraphy.36
37
Key words: Chaohu, Lower Triassic, Paleomagnetism, Olenekian, magnetostratigraphy38
39
1. Introduction40
Magnetostratigraphy can be used as an essential tool for chronostratigraphic correlations between rocks from different 41
environments including those that are unfossiliferous or have poor fossil preservation. The Lower Triassic 42
magnetostratigraphy has been studied in many places, where progress has been made in constructing a magnetic polarity 43
scale for the Permian-Triassic boundary interval either from marine (Heller et al., 1988,1995; Haag et al., 1991; Li et al., 44
1989; Steiner et al., 1989; Chen et al., 1994; Embleton et al., 1996; Zhu et al., 1999; Scholger et al., 2000; Gallet et al., 2000; 45
Hounslow et al., 2008) or terrestrial rocks (Molostovsky, 1996; Szurlies et al., 2003; Steiner, 2006; Szurlies, 2007). In spite of 46
this, discussions continue about how to precisely correlate marine and continental facies using magnetostratigraphy near the 47
Permian-Triassic boundary (PTB) interval and the remainder of the Lower Triassic (Steiner, 2006; Szurlies, 2007; Hounslow 48
et al., 2008). This in part reflects the debate about the placement of the base of the 2nd stage of the Triassic, the Olenekian, as 49
well as differing quality, quantity and types of secondary biostratigraphic constraints in these Early Triassic successions. In 50
addition, paleomagnetic studies from some continuous marine sections have differing magnetostratigraphic records (e.g. the 51
Induan global stratotype section and point (GSSP) at Meishan; Yin et al. 2005 ) perhaps because of undetected 52
remagnetization (Li et al., 1989; Zhu et al., 1999; Steiner, 2006).53
Recent investigations were carried out on the west Pingdingshan section to provide detailed biostratigraphic and 54
lithostratigraphic information for the Induan–Olenekian boundary (Tong et al; 2003, 2005, 2007; Zhao et al., 2007, 2008). 55
Based on this robust lithostratigraphic and biostratigraphic framework, a precise positioning of paleomagnetic data was 56
achieved, resulting in a detailed composite magnetic record. Along with biostratigraphic data, the magnetic record of these 57
lowermost Triassic sediments are important in that they provide an integrated magnetostratigraphic and biostratigraphic 58
reappraisal that allows the recognition of the Induan– Olenekian boundary in a Tethyan section. This is all the more 59
important, in that the ratified GSSP for the Olenekian at Mud (Spiti, India) is unlikely to ever have a magnetostratigraphy 60
because of low grade metamorphism (Krystyn et al., 2007)61
62
2. Geological setting63
The West Pingdingshan section (location 31.6ºN, 117.8ºE), located near Chaohu city, Anhui province, China, consists 64
of interbedded calcareous mudstones and limestones. The section is one of the well-exposed Lower Triassic successions in 65
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South China (Fig. 1), which was deposited in a carbonate ramp setting on the Lower Yangtze Block, within the low-latitude 66
eastern Tethyan archipelago. The Lower Triassic sections near Chaohu city have been extensively investigated using a 67
variety of detailed lithostratigraphic, biostratigraphic and chemostratigraphic tools (Tong et al., 2003, 2005, 2007; Zhao et al., 68
2007, 2008). The Lower Triassic succession at Chaohu yields abundant fossils, which provides a comprehensive 69
biostratigraphy marked by conodonts and ammonoids (Fig. 2). 70
Information is presented on a set of palaeomagnetic samples collected from the lower 44 m of the West Pingdingshan 71
section, from the latest Permian into the Olenekian. The latest Permian ammonoids occur in the highest beds of the Dalong 72
Formation just 11.5 cm below the “boundary clay bed”. Beds containing Claraia and Ophiceras occur about 50 cm above the 73
“boundary clay bed”, which is located at 0.33 m in the section. More extensive biostratigraphic data on the overlying Induan 74
is summarized in Zhao et al. (2007). The conodont specimens from the West Pingdingshan section are not clearly darkened 75
and have a conodont alteration index of about 2.0, which is consistent with alteration index’s of 1.5 to 2.0 in the general study 76
region (Wang, 1993). The bedding in the West Pingdingshan section dips at some 78-88o towards the E, forming part of the 77
core of the Majiashan-Pingdingshan synclinorium.78
79
3. Sampling and paleomagnetic procedures 80
A total of 347 drill-plugs were collected for palaeomagnetic investigations from the Yinkeng Formation. These were 81
collected, using a gasoline-powered drill, and were oriented using a magnetic compass. Samples were collected at 10-15 cm 82
intervals. The sampling interval covers the stratigraphic interval from just below the lithologic PTB (at 0. 33m), to just above 83
the base of the Olenekian (at first occurrence (FO) of Neospathodus waageni eowaaageni, morphotype of Ns. waageni).84
Fifty-two drill-plugs did not produce suitable paleomagnetic specimens for the magnetometer. The suitable remaining 85
295 specimens underwent stepwise thermal demagnetization using 15-20 steps in an ASC TD-48 oven with an internal 86
residual field less than 10nT. Remanent magnetization was measured with a 2G cryogenic magnetometer at the 87
palaeomagnetic laboratory of the Institute of Geomechanics, CAGS in Beijing. The magnetometer is located inside a 88
Helmholtz coils that reduces the ambient geomagnetic field to around 300nT. Remanent component directions were 89
determined by principal component analysis, as implemented in the Enkin suite of software. The software of Cogné (2003) 90
and Kent et al (1983) was used in the analysis of the resulting demagnetisation data. A KLY-4 Kappabridge susceptibility 91
system was used to measure the anisotropy of magnetic susceptibility (AMS) of 137 specimens, prior to thermal 92
demagnetisation, to assess if these rocks have suffered substantive tectonic strain. The magnetic mineralogy of representative 93
lithologies was studied using stepwise thermal demagnetization (Lowrie, 1990) of isothermal remanent magnetization (IRM). 94
The IRM was imparted using an ASC scientific pulse magnetizer (Model IM-10-30). Thermal demagnetization of the 95
three-component IRM used fields of 0.12 T, 0.4 T and 1.2 T.96
97
4. Magnetic mineralogy results98
The anisotropy degree (P’) of the samples ranges from 1.001 to 1.057 (Fig. 3a). The mean anisotropy degree is 1.013. 99
The anisotropy shape factor (T) varies widely, mostly independent of P’, and is distributed mostly in the oblate field (T >0; 100
Fig. 3a). The principal anisotropy directions (K1) are dispersed but show some evidence of a preferred N-S orientation (Fig. 101
3b). The K3 directions are on average perpendicular to the bedding plane. The low degree of anisotropy and the reasonably 102
large scatter in the maximum susceptibility axis directions within the bedding plane, indicates these rocks have not 103
experienced large amounts of tectonic strain, although the orientation of the K1 axes approximately parallel to the 104
Majiashan-Pingdingshan synclinorium axis is evidence of weak strain, not out of the ordinary for such gently folded rocks.105
Thermal demagnetization of the soft (<0.12 T) and medium (0.12-0.4 T) fractions shows an unblocking temperature of 106
560-580°C, indicative of magnetite. The hard fraction (0.4-2.7 T) shows a distinct unblocking temperature between 107
630-660°C, which is probably indicative of hematite (Fig. 4).108
109
5. Palaeomagnetic Results110
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The specimens have initial natural remanent magnetisation (NRM) intensities between 10-4 and 10-3 A/m. Thermal 111
demagnetization isolated three magnetisation components: 112
(a) Firstly a low-temperature component (component A: LTC) is isolated in all samples below about 300°C. The mean 113
direction of this component before tilt correction is D=3.0°, I=41.5° (N=295 and α95=1.3°), and is similar to the present-day 114
field direction (D=355.4° and I= 47.3°; Fig. 5a, 6), which is inferred to be the origin of this component. 115
(b) A second component (component B: MTC) is determined mostly between the 300-480°C demagnetisation steps, by 116
a well-defined linear segment on the orthogonal vector diagrams in nearly all specimens. This magnetisation component 117
largely dominates the NRM. This component is NNW and down-directed in geographic coordinates and easterly and 118
down-directed in stratigraphic coordinates (Fig. 5b, 6). The Fisher precision parameter of the mean direction of the MTC 119
component changes from 42.2 before bedding correction to 40.3 after bedding correction, indicating a slightly tighter 120
directional dispersion in situ coordinates (Fig. 5b). The mean magnetization direction at 350o, +32o (k=42.2, α95= 1.4) in 121
geographic coordinates is not distinct from that expected for the Jurassic-Cretaceous of the South China block (SCB). Hence, 122
we interpret this component B as probably a remagnetization acquired during the Jurassic-Cretaceous period, after tilting of 123
the beds.124
(c) Thirdly a high-temperature component (component C: HTC). This component is mostly present between 125
480-580°C. Its direction is of dual polarity, and is interpreted as a Triassic magnetisation (Fig. 6, 8). Only some 66% of 126
samples showed evidence of this magnetisation component, the remaining specimens were dominated by component B until 127
complete demagnetisation. Component C has a strong overlap of unblocking temperature with component B in a large 128
majority of specimens at and above 480°C. Only some 11% of specimens show clear linear segments, separating component 129
C from component B on Zijderveld plots and principle component (PCA) analysis (Fig. 6, 7). 130
Categories of demagnetisation behavior were visually assigned (assisted by the PCA analysis) to several classes of 131
demagnetization data shown by the specimens. 132
Firstly, a category indicating no component C could reliably be interpreted from the specimen data (‘MTC only’ in Fig. 133
9). This type of behaviour is dominant in the lower 5 m of the section (which is more weathered than the overlying parts), and 134
above 20 m in the section (Fig. 7). Some 34% of specimens possess this type of behavior.135
Secondly, two classes (good and poor) of ChRM line-fit data (Fig. 6, 9), which are exclusively present in the levels 136
between 5 and 30 m (Fig. 9). Some 11% of samples possess this type of behavior.137
Thirdly, specimen data, which showed evidence of incomplete separation of components B and C, but which showed 138
evidence of great circle trends towards either the reverse or normal polarity directions of component C (‘GC trends’ in Fig. 9). 139
Two sub-categories of good and poor behavior were evaluated, based on the amount of approach towards the component C 140
dual polarity directions (Fig. 8, 9). Some 55% of specimens possess this type of behavior. Great circles were predominantly 141
fitted through the higher temperature demagnetisation steps as an estimate of these great circle trends.142
The reversal test (McFadden & McElhinney, 1990) has been performed on the component C mean direction. The test 143
indicates a positive reversal test (Ra), with less than 5o degrees between inverted antipodal mean directions (γObs=2.3; 144
γCritical=4.4). The tilt test on the site-mean directions of ChRM is also positive at the 95% level of confidence according to the 145
criteria of McFadden and Jones (1990) (Xi2Is=6.8, Xi2Tc=1.8, Critical 95%=5.07). These findings indicate the primary nature 146
of the ChRM from the West Pingdingshan section. The site-mean directions were determined using the combined ChRM 147
line-fit and fitted great circle data using the method of Mcfadden and McElhinny (1988), as implemented in the Cogné (2003) 148
software (Fig. 8). The averaged site-mean direction for component C is Dg=285.1°, Ig=-62.7°, κg=193.7, α95=2.4° before tilt 149
correction, and Ds=299.9°, Is=18.3°, κs=305.2, α95=1.9°, N=19 after tilt correction (Fig. 8a, Table 1). The average direction of 150
each group of sites was calculated using the great circles (re-magnetisation circles) /fixed points to evaluate one of the means 151
displayed in Fig. 8a, for example the samples in bed AC14 (Fig. 8b). The determined paleopole lies at 30.3°N, 19.9°E with 152
A95=1.4° (dp/dm=1.0/2.0). The paleolatitude of 9.4° for the section is not significantly different (at 95% confidence level) 153
with that predicted for the stable South China Block (Heller et al., 1988, 1995; Steiner et al., 1989). Hence, we infer 154
component C is a Lower Triassic magnetisation, acquired prior to folding. However, the mean directions for the ChRM are 155
substantially different in declination from the Lower Triassic means for the South China block, a result of 101.2° (±3.3°) 156
anti-clockwise vertical axis rotation with respect to the stable South China block. Similar anticlockwise rotations occur 157
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further south adjacent to the Tanlu fault (Tan et al., 2007). These are probably associated with local rotation of the Chaohu 158
area, during docking with the nearby North China block, along the Tanlu fault. 159
160
6. Magnetostratigraphy and global magnetostratigraphic correlation161
The ChRM directions were converted to VGP latitude (displayed as filled circle in Fig. 9). For those specimens 162
possessing great-circle behaviour, the point on the fitted great circle closest to the section mean direction was used to163
calculate the VGP latitude (displayed as open circle in Fig. 9). The magnetic polarity of the section is dominated by reverse 164
polarity, with three normal polarity magnetozones represented by three or more specimens (WP2n, WP3n and WP4n). 165
Normal magnetozones WP1r.1n and WP5n are defined by only two specimens. A number of tentative normal polarity 166
sub-magnetozones, represented by a single specimen are also present within WP4r, with only that in bed 19 defined by PCA 167
line-fit data (Fig. 9). 168
The magnetic polarity at the correlated base of the Olenekian (at the first occurrence of morphotype Ns. waageni 169
eowaggeni in subbed 24-16) is probably reversed, since underlying sub-bed 24-15 and overlying sub-bed 24-17 are reversed, 170
with both sampled intervening beds not containing any evidence of Triassic component C. The base of the overlying normal 171
magnetozone W5n is ~2.5 m above the FA of Ns. waageni eowaggeni, although a substantial number of specimens in the base 172
of bed 25 posses no component C. Hence, the base of magnetozone WP5n provides a good secondary marker a little above the 173
base of the Olenekian in the West Pingdingshan section.174
The dominance of reverse polarity in the lower part of the section (i.e. that below bed 15, Figs. 9 & 10) would suggest, 175
according to the magnetostratigraphy, that this interval is entirely late Griesbachian, since no substantive evidence of the 176
equivalent of normal polarity magnetozone LT1n, which characterizes the basal Induan, occurs in the base of the section (Fig. 177
10). It is perhaps possible the normal magnetozone WP1r.1n, some 2.0 m above the “PTB set” (Peng et al., 2001), may 178
represent part of LT1n and the remainder of the basal Griesbachian normal magnetozone is obscured by the scarcity of 179
polarity data in this interval (Fig. 9). The basal part of the West Pingdingshan section was only uncovered in recent years, and 180
not much biostratigraphic work has been done on it, but neighboring sections such as the north Pingdingshan and west 181
Majiashan sections have been studied in detail at the boundary, with no evident breaks in sedimentation.182
The magnetostratigraphy of the Gaundao section is ambiguous around the Induan-Olenekian boundary (IOB), but the 183
positive carbon isotopic excursion allows approximate correlation from Gaundao to the West Pingdingshan and Bulla/Suisi 184
sections (Payne et al. 2004; Horacek et al. 2007; Tong et al. 2007; Richoz et al. 2007; Fig. 10). The magnetostratigraphy of the 185
upperpart of the West Pingdingshan section, has a close correspondence in polarity style to that at Hechuan, as does the 186
reverse-polarity dominated interval at Guandao, which covers the upper range of the conodont Ns. dieneri (Fig. 10). The most 187
complete magnetostratigraphy across the IOB occurs in the Boreal realm from the Sverdrup Basin in Canada and Spitsbergen 188
in arctic Norway (Ogg & Steiner, 1991; Hounslow et al., 2008). However, correlation of the IOB onto the 189
magnetostratigraphy of the boreal sections is problematic with two possible solutions. 190
Both Ns. kummeli, Ns. dieneri and Ns. svalbardensis are known from the P. candidus Zone in Canada (Orchard & 191
Tozer, 1997), which suggests the reverse polarity interval covering beds 18-20 at West Pingdingshan (Fig. 10) represents the 192
lower part of magnetozone LT2r (GC2r at Griesbach Creek; Sc2r at Creek of Embry and Vh4r at Vikinghøgda; Fig. 10) in the 193
Boreal composite magnetic polarity timescale (MPTS). Ns. dieneri and Ns. cristagalli are present in the Canadian V. 194
sverdrupi Zone (Orchard & Tozer, 1997) and in sections on Spitsbergen V. spitzbergensis (probable synonym of V. 195
sverdrupi) occurs with Ns. pakistanensis, Ns. dieneri, and Ns. aff. svalbardensis (Nakrem et al., in-review), which suggests 196
the boundary between the Ns. cristagalli and Ns. pakinstanensis conodont zones at the GSSP in Mud (Krystyn et al. 2007), 197
occurs within the Boreal Sverdrupi Zone. Within Canadian sections Ns. waageni first occurs within the Euflemingites 198
romunderi Zone, whereas in Siberian sections Ns. waageni first occurs some 20% through the H. hedenstroemia Zone (Dagis, 199
1984), indicating that the IOB probably occurs within the lower part of the H. hedenstroemi Zone. This suggests that the 200
magnetozone WP5n at West Pingdingshan is probably the equivalent of magnetozone LT4n in the boreal MPTS (option in 201
Fig. 10), and hence the IOB is within the uppermost part of LT3r. A consequence of this biostratigraphic-driven correlation is 202
that the equivalent of LT3n in the MPTS appears to be unconvincingly detected at West Pingdingshan- possibly represented 203
by WP4n in bed 19? (Fig. 10).204
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A second, lower placement of the IOB, relative to the magnetostratigraphy is also possible (Option in Fig. 10). Since 205
the ammonoid control at Griesbach Creek is based on spot occurrences, its not clear where the base of the H. hedenstroemi206
Zone is located, its may be that it is located below the base of the Smith Creek Member in the upper part of GC2r (Fig. 10). 207
Consequently, it is possible that WP5n is the equivalent of GC3n (LT3n in MPTS). Two features support this possibility; 208
firstly, this correlation is more consistent with the magnetostratigraphy, in that no substantive normal magnetozone is 209
detected in beds 18 to 24 at West Pingdingshan. Secondly, at both the Griesbach Creek and Vikinghøgda sections close to the 210
lower boundary of GC3n and Vh5n (LT3n in MPTS) is a major transgressive surface, which may be the equivalent of that 211
seen at Chaohu and Mud close to the IOB (Guo et al., 2007; Krystyn et al 2007).212
With these uncertainties in correlation of marine sections in mind, its premature to attempt mapping of the 213
cyclo-magnetostratigraphy from the Buntsandstein (Szurlies, 2007) into the marine sections. In addition much reliance has 214
been placed on relating the Italian sections at Bulla and Siusi (Scholger et al., 2000) to age-calibrate the Buntsandstein cycles 215
and lithostratigraphy (Szurlies et al., 2003; Szurlies 2007), yet the isotopic curves from these Italian sections clearly located 216
the IOB higher in the Bulla/ Siusi sections than that used by Szurlies (2007).217
218
7. Conclusion219
A dual polarity Triassic magnetisation can be extracted from the West Pingdingshan section, in spite of minor 220
associated tectonic deformation, and partial remagnetisation. The Early Triassic magnetization is masked by a strong 221
Jurassic-Cretaceous overprint magnetization, acquired post-tilting, which variably masks the Early Triassic magnetisation. 222
Nevertheless some 66% of specimens display evidence in the demagnetisation diagrams of characteristic polarity, either 223
through conventional magnetisation component isolation, or great circle trends. The correlated conodont marker for the base 224
of the IOB in the GSSP at Mud (FA of Ns. dieneri s.l.) indicates that at West Pingdingshan the IOB is some 2.5 m below a 225
normal magnetozone, which provides a secondary marker, for mapping the base of the Olenekian into successions without 226
conodonts. Around the IOB the West Pingdingshan section can be correlated confidently to other Tethyan Lower Triassic 227
successions, but which however lack the corroborative magnetostratigraphic details shown at Chaohu. Problems remain near 228
the Permian-Triassic boundary in the West Pingdingshan section, due to inadequate recovery of Triassic magnetisations from 229
specimens. Problems also remain in relating the Induan-Olenekian boundary interval into the boreal sections in Canada and 230
Spitsbergen with magnetostratigraphy. 231
232
Acknowledgements233
This study was supported by NSFC (40102021), Leo Krystyn provided useful discussion on biostratigraphic correlations.234
. 235
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Figure captions:340
Fig. 1 (a) simplified geological map of Chaohu area and (b) Geographic location. 341
Fig. 2 Vertical range and zonation of conodonts, ammonoids and bivalves assemblage at West Pingdingshan section, Chaohu, 342
Anhui Province. Modified after Zhao et al. (2008). Bold lines highlight the occurrence of the most important 343
ammonoids and conodonts used to identify the Induan-Olenekian boundary. 344
Fig. 3(a) Anisotropy of magnetic susceptibility (AMS) plots of shape parameter T versus anisotropy degree P’, (b) 345
stereographic projection of AMS principle axes in stratigraphic coordinates for specimens from the Yinkeng 346
Fm. K1 (square): maximum axis, K3 (circle): minimum axis, K2 (triangle): intermediate axis. 347
Fig. 4 Representative plots of thermal demagnetisation of 3-axis isothermal remanent magnetization, 348
using magnetizing fields of 0.12 T (soft), 0.4 T (medium) and 2.7 T (hard). these plots show Tc at ~ 349
560-580 and 630-660.350
Fig. 5 (a) Equal-area projections of a) the low temperature (LTC or A) component and b) the mid temperature 351
(MTC or B) component. Star is the geocentric axial dipole field, and square the present-day field directions. 352
Fig. 6 Representative orthogonal demagnetization diagrams (in stratigraphic coordinates). The characteristic 353
magnetisation (component C) directions in the specimens displayed were determined by principle 354
component line-fits. a-e: good quality line-fit from 480C through the origin; f-i: poor quality line-fit from 355
480C through the origin; (d), (e), (i): Normal polarity; (a)-(c), (f)-(h): Reverse polarity. Demagnetization 356
steps in °C in all plots. 357
Fig. 7 Equal-area projections of the magnetisation directional tracks during thermal demagentization of representative 358
samples with a variable content of component C (all in stratigraphic coordinates). G1: good quality great circle data; 359
G2: poor quality great circle (with only short paths towards directions that are consistent with component C being 360
present). Plot c) is for a normal polarity sample, all others for interpreted reverse polarity specimens. In general, these 361
specimen show motion along a great-circle paths in a southeasterly direction, trending towards negative inclination or a 362
northwesterly direction with a postive inclination. The great-circle path is best defined from about 480C to 580C. 363
Fig. 8 (a) Equal-area stereographic projection of mean directions of high temperature component C for 364
stratigraphic groups of specimen data. Lower (upper) hemisphere directions are marked with closed (open) 365
symbols. (b) Equal-area stereographic projection of the site ( AC14) showing the great circles 366
(re-magnetisation circles) /fixed points used to evaluate one of the means displayed in Fig. 8a. Ellipses are 367
95% confidence cones of group means. Stars=mean directions of these dual polarity magnetisations. 368
Fig. 9 Demagnetisation behaviour (see text), virtual geomagnetic pole (VGP) latitude, and the magnetic polarity 369
interpreted in the West Pingdingshan section, along with summary biostratigraphic data (Zhao et al., 2007). 370
Magnetozones defined by no adjacent specimens of the same polarity, are indicated by half bars, with full 371
bars indicating two or more adjacent specimens of the same polarity. Full grey bars indicate adjacent 372
specimens with only component B present. The major N-R magnetozone couplets have been labeled WP 373
(for West Pingdingshan), for ease of description.374
Fig. 10 Comparison of the magnetostratigraphy at Chaohu with other studies of marine sections through the latest 375
Permian and Lower Triassic. Data for polarity columns: Guandao (Lehrmann et al., 2006), Hechuan (Steiner 376
et al., 1989), Abadeh (Gallet et al., 2000), Bulla/Siusi (Scholger et al., 2000; Perri & Farabegoli, 2003; 377
Horacek et al., 2007). Griesbach Creek, Smith Creek and Creek of Embry (Ogg & Steiner, 1991 modified by 378
Hounslow et al 2008), Vikinghøgda and composite magnetic polarity timescale (MPTS) from Hounslow et al. 379
(2008).380
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Table 1 381
Site-mean paleomagnetic results from the West Pingdingshan section at Chaohu, Anhui Province. 382
383
Bed/Site n Dg (o) Ig(
o) Ds(o) Is(
o) 95(o)
ac24 9 107.3 67 124.7 -17.1 30.9 5.4
ac23 11 106 63.9 122.5 -19.5 42.5 4.6
ac22 12 100.3 62.9 117.9 -19.8 23.5 5.6
ac21 8 107.8 67.2 121.8 -17.9 13.3 8.4
ac20 7 116.2 63.9 124.8 -19.2 24.9 7.9
ac19 10 303.5 -61.6 308.2 20 28.5 5
ac18 12 114 62.4 123.3 -19.2 22.2 5.2
ac17 9 267.9 -66 294.4 14.5 14.7 7.5
ac16a 7 278.1 -70.7 300.3 14.1 16.7 9.7
ac16b 7 292.8 -61.8 301.6 24.7 8.3 15.3
ac15 11 100 62.5 116.2 -21.4 10.5 9.8
ac14 14 94.6 62 115.9 -17.5 50.6 3.4
ac13 11 96.6 61.7 117.7 -16.8 25.3 5.6
ac12 10 91.7 58.2 113.3 -17.5 39.6 4.2
ac11 7 98.2 62.8 119 -16.4 28.5 6.4
ac10 10 107.6 54.7 116.8 -20.4 20.8 6.4
ac9 10 109.3 55.3 118 -20.3 30.8 4.8
ac8 10 103.9 60.3 117.5 -14.5 13.4 7.9
ac7 10 297.8 -59.9 304.1 16.4 12.3 7.8
mean 19 285.1 -62.7 - - 193.7 2.4
- - 299.9 18.3 305.2 1.9
n: number of specimens used to calculate mean; Dg, Ig, Ds, Is: declination and inclination in geographic and stratigraphic 384
coordinates respectively; κ: the best estimate of the Fisher precision parameter; α95: the radius of the 95% cone of 385
confidence. 386
387
2.38
1.62
1.76
2.85
2.76
2.19
3.52
2.57
2.47
2.47
3.24
3.33
2.39
1.62
2.13
2.56
1.961.87
3.06
3.81
3.95
2.130.992.311.511.97
1.49
9. 93
0.10
Fe Fe Fe
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
2120
19
18
17
16
15141312
11
1
0
2
4
6
8 m
.
Neo
spat
hodu
s ku
mm
eli
Ns.
pos
tero
long
atus
Ns.
sp.
Con
odon
t Zon
e
Ns.
dien
eri
.
Gui
chie
lla a
ngul
ata
a
6-105 0.20
2-4 0.15
Upp
erP
erm
ian
Cha
nghs
ing
Dal
ong
H.ty
pica
lisH
. sp.
Isar
cice
lla?
sp.
Neo
gond
olel
la c
arin
ata
Ng.
kry
styn
i
Ng.
sp.
Ng.
pla
nata
Ng.
orc
hard
i
Cla
raia
hun
anic
a
Cla
raia
gre
isba
chi
Cla
raia
sta
chei
Cla
raia
radi
alis
Cla
raia
gre
isba
chi
Cla
raia
aur
itaC
lara
ia c
once
ntric
aC
lara
ia h
ubei
ens
isC
lara
ia g
reis
bach
i
Eum
orph
otis
cf.
vene
tina
Eum
orph
otis
hua
ncan
geni
sE
umor
phot
is in
aequ
ieco
stat
a
Oph
icer
as s
p.Ly
toph
icer
as?
sp.
Lyto
phic
er a
s? s
p.
Kon
inck
ites
sp.
Prio
nolo
bus
sp.
Cly
pece
ras
cf. l
entic
ular
e
Neo
spat
hodu
s ch
i
Ns.
die
neri
M1
Ns.
die
neri
M1
Ns.
die
neri
M2
Ns.
die
neri
M3
Ns.
cha
ohue
nsis
Ns.
aff.
cris
taga
lli
Ns.
n. s
p. F
Ns.
aff.
cha
ohue
nsis
Ns.
con
cavu
sN
s. n
. sp.
K
Ns.
waa
geni
eow
aage
ni
Par
achi
rogn
athu
s cf
. tric
usoi
datu
sP
laty
villo
sus
cost
atus
Ns.
n. s
p. G
Ns.
n. s
p. H
Ns.
Ton
gi
Pl.
ham
adai
Ns.
aff.
dis
cret
us
Ns.
pec
ulia
risN
s. a
lber
ti
Cly
pece
r as
sp.
Euf
l em
ingi
tes
sp.
Pse
udos
agec
er a
s sp
.
Pre
fl or
iani
tes
cf. s
trong
iP
refl
oria
nite
s? s
p.P
seud
osag
ecer
as
sp.
Bak
evel
lia c
osta
ta
Arc
toce
r as
cf. l
olou
ense
Flem
ingi
tes
ellip
ticus
Flem
ingi
tes
kaoy
unlin
gens
isFl
emin
gite
s sp
.
Pse
udoc
ellit
es s
p.
Uss
uria
? sp
.O
wen
ites?
sp.
Mee
koce
r as
sp.
Euf
l em
ingi
tes
cf. t
sote
ngen
sis
Cly
pece
r as
cf. k
wan
gsie
nsis
Die
nero
cer a
s di
ener
Pse
udos
agec
er a
s ts
oten
gsen
e
Neo
spat
hodu
s w
aage
niA
nasi
birit
esO
phic
er a
s - l
ypto
phic
er a
sC
lara
ia s
tach
ei -
C. a
urita
Prio
nolo
bus
- Gyr
onite
sE
umor
phot
is in
aequ
i coa
stat
a - E
. hua
ncan
gens
isFl
emin
gite
s - E
ufl e
min
gite
sA
mm
onoi
d Zo
neB
ival
ve A
ssem
blag
e Zo
ne
Ram
iform
ele
men
ts
Ns.
nov
aeho
lland
iae
+ N
s. p
akis
tane
nsis
Ns.
spi
tiens
isN
s. a
ff. w
aage
ni w
aage
niN
s. n
. sp.
I
Adu
ncod
ina
unic
osta
Par
achi
rogn
athu
s sp
.
Ns.
waa
geni
waa
geni
Pbs
idon
ia m
inar
Cla
raia
sp.
Gui
chie
lla s
ubro
tund
aE
umor
phot
is s
p.
Pos
idon
ia s
p.
Pos
idon
ia c
ircul
aris
Arc
toce
r as
sp.
Ow
enite
s pa
kung
ensi
sK
onim
kite
s lo
low
ensi
sP
roph
ingi
toid
es s
p.W
asac
hite
s sp
.H
emip
rioni
tes
sp.
Juve
nite
s cr
ient
alis
Die
nero
cer a
s sp
.D
iene
roce
r as
cf. o
vale
Kon
inck
ites
cf. l
oloe
nsis
Ns.
waa
geni
waa
geni
Ns. waagenieowaageni
Ns. dieneri M3
Ns. dieneri M2
Ns. dieneri M1
Ns. kummeli
Ng. krystyni
H. typicalis
Indu
an
Low
er T
riass
ic
Ole
neki
an
Yink
eng
Form
atio
n
Ser
ies
Sta
ge
Form
atio
n
Bed
Thic
knes
s (m
)
Lith
olog
y
Lith
olog
y
Lim
esto
ne
Nod
ular
lim
esto
ne
Arg
illac
eous
lim
esto
ne
Mud
ston
e
Cla
ysto
ne
Sili
ceou
s m
udro
ck
FigureClick here to download Figure: Fig 2.pdf
0
90
180
270
0.90 0.95 1.00 1.05 1.10-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
P
T
(a)(b)
FigureClick here to download Figure: fig 3.pdf
0
1
2
IRM
(x 1
0 m
A/m
)
0 100 200 300 400 500 600 700AC17-10
0
1
2
3
4
5
6
IRM
(x 1
0 m
A/m
)
0 100 200 300 400 500 600 700AC24-19 C0
hardmediumsoft
hardmediumsoft
(a) (b)
Temperature ( ) C0Temperature ( )
FigureClick here to download Figure: Fig 4.pdf
0
90
180
270
0
90
180
270
LTC before bedding tilt correction LTC after bedding tilt correctionNumber of samples plotted :295
Dg=3.0Ig=41.5k=38.5a95=1.3
Ds=89.2Is=33.5k=36.7a95=1.4
0
90
180
270
0
90
180
270
MT before bedding tilt correction
MT after bedding tilt correctionNumber of samples plotted :251
Dg=349.6Ig=32.3k=42.2a95=1.4
Ds=84.6Is=47.1k=40.3a95=1.4
(a)
(b)
FigureClick here to download Figure: Fig 5.pdf
N EUp
S
WDown
AC1-85-TC Scale: 1e-3 A/m
N EUp
S
WDown
AC12-2-TC Scale: 1e-4 A/m
N
EUpS
WDown
AC16-9-TC Scale: 1e-4 A/m
N
EUp
S
WDown
AC16-18-TC Scale: 1e-4 A/m
N EUp
S
WDown
AC1-67-TC Scale: 1e-3 A/m
N EUp
S
WDown
AC11-6-TC Scale: 1e-3 A/m
N EUp
S
WDown
AC14-5-TC Scale: 1e-4 A/m
N
EUp
S
WDown
AC16-15-TC Scale: 1e-4 A/m
NRMT580
NRM T480
T590
NRM
T480
T580
T480
T580
NRM
T480
T640
NRMT480
T580
NRM
T480
T580
NRM
T480
T580
NRM
T480
T580
NRMT480
T580
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
N EUp
S
WDown
AC14-13A-TC Scale: 1e-4 A/m
NRM
FigureClick here to download Figure: Fig 6.pdf
AC11-3
0
90
180
270
AC13-10
0
90
180
270
AC16-10
0
90
180
270
AC24-24
0
90270
AC15-8
0
90
180
270
AC23-6
0
90
180
270
G1(a) (b)
(c)
(d) (e) (f)
NRM
T580
G1 G1
G1 G2 G2
NRMT580
T550
T450
NRM
NRM
T580
T550
NRM
T580
T520
NRM
T510
FigureClick here to download Figure: Fig 7.pdf
0
90
180
270
0
90
180
270
HTC before tectonic dip correctionHTC after tectonic dip correction
Dg=285.1Ig= -62.7k=193.7a95=2.4n=19
Ds=299.9Is=18.3k=305.2a95=1.9
site:Ac14
0
90
180
270
site:Ac14
0
90
180
270
HTC before tectonic dip correction
HTC after tectonic dip correction
(a)
(b)
FigureClick here to download Figure: Fig 8.pdf
H. t
ypic
alis
H. t
ypic
alis
Oph
icer
as s
p.
AmmonoidsConodonts
WP
1rW
P1r.1
nW
P3
WP
4W
P2
WP
5n
Ng.
kry
styn
i
Ng.
kry
styn
i
Cla
raia
spp
.
Ns.
kum
mel
i
Ns.
kum
mel
i
Ns.
dis
cret
us
Ns.
pos
tero
long
atus
Ns.
aff
cris
tagl
liN
s. d
iene
ri M
1 to
M3
Ns.
die
neri
Kon
icki
tes
sp.
Euf
lem
ingi
tes
Flem
ingi
tes
spp.
Prio
nolo
bus
sp.
Ole
neki
an
Mag
neto
zone
s
Mag
netic
Pol
arity
Indu
an
Goo
d
Goo
dP
oor
Poo
r
VGP latitudeLine-fits
PCA line-fit
Point on great circle
GC trendsOnly MTC
Demagnetisation behaviour
0
10
20
30
40
Hei
ght (
m)
Yink
eng
Fm
Ns.
waa
geni
s.l.
Ns.
waa
geni
111213141516
17
18
19
20
21
22
2324
25
Bed
No
Zone
s
‘Lith
olog
ic’ P
TB
-90 -60 -30 0 30 60 90
Normal polarityReverse polarity
Uncertain polarity
10 (m
)
Dalong Fm
Ng.
pla
nata
FigureClick here to download Figure: fig 9.pdf
LT1
n.1r
LT2
LT3
LT4
LT5
LT6
Griesb
ach
ian
Smith
ian
Dien
eria
n
Indu
an
Induan
Ole
nekia
n
Olenekian
Mag
neto
chro
n
Abad
eh(Ir
an)
Bulla
& S
iusi
(Italy
)
5(m)
0
Oph
icer
asPr
opty
chite
s
H. p
arvu
s &
H.
typ
ica
lisN
s. d
iene
ri
O. bore
ale
E.
rom
un
de
ri
A.b
Konic
kites
Delta
dale
n M
em
ber
Lusi
tania
dale
n M
em
ber
Vh1r
Vh2
Vh3
Vh4
Vh5
Vh6
Vh7
Vh8
Ng. m
eis
hanensis
,N
g. ta
ylo
rae
B. ro
senkra
ntz
iN
s. svalb
ard
ensis
A. tardus
C.
sta
ch
ei
C.
sta
ch
ei
(m)
10
0
20
Vikinghøgda(Svalbard)
MPTS
West Pingdingshan
c
Campil Mb
25
(m)
0
H.
pa
rvu
s H
. p
rae
pa
rvu
s H
. ty
pic
alis
I. is
arc
ica
I. s
tae
sch
ei
H. aequabili
sZ
one
H. ance
ps
Zone
P. obliq
ua
Zone
Ma
zzin
Mb
TS
eis
Mb
Andrez
c
~45
m g
ap
c
Talus breccia-no sampling
Lower Guandao
Ns.
b
rans
oni
gr.
0
30
(m)
Sh
ale
Sh
ale
Ng
. ch
an
gxin
ge
nsisH
. p
arv
us
Ns.
die
ne
ri
Ns.
crista
ga
lli;
Ns.
pa
kis
tan
en
sis
Ns.
dis
cre
tus;
Ns.
co
nse
rva
tivu
s
Ns.
wa
ag
en
i
c
c
c
South ChinaSverdrup Basin (Canada)
Creek of Embry
Griesbach Creek
Smith Creek
Mag
. Zo
ne (S
C)
Mag
. Zon
e (C
E)
A.n
E.r, M.g, A.b
(m)
Sm
ith
Cre
ek M
bC
onfe
dera
tion
Pt
0
50
3
2
1
E.r
(m)
Sm
ith
Cre
ek M
b
0
100
7
6
5
4
3
2
1r
Co
nfe
de
ratio
n P
t. M
b
Ma
g.
Zo
ne
(G
C)
O. concavum
O. commune
V. sverd
rupi
H. he
dens
troem
i
O. boreale
?
+
?
?
?
?
E. c
irrat
us
25
50
(m)
0
Sm
ith C
reek M
b
3
2
1
B.
str
iga
tus
Confe
dera
tion P
t. M
b
11
22
E.r Euflemingites romunderi=A.b = Arctoceras blomstrandi
A.n = Arctoprionites nodosus
13C Isotope positive peak 13C Isotope negative peak
(o=organic matter, c=carbonate)
Normal polarityReverse polarity
Uncertain polarity
Sampling gap
Magneto correlations
Other correlationsAmmonoid horizon/ rangeConodont horizon / range
Transgressive surface
Condensed
HiatusM. g= Meekoceras gracilitatus
Hechuan
F1
F2
Jia
lingjia
ng F
m
H.
pa
rvu
s
J1
F4
F3
100
0
(m)
Feix
ianguan F
m
H. ty
pica
lis
WP1
rW
P3W
P4
WP2
WP5
n
Ng. k
rystyn
iNs
. kum
meli
Ns. d
iener
i
Mag
neto
zone
s
Ns. w
aage
ni
11
1r.1
n
1213141516
17
18
19
20
21
22
2324
25
Bed
No
Con
odon
tZo
nes
10 (m
)
Dalong Fm
??
FigureClick here to download Figure: fig 10.pdf
Supplementary material for on-line publication onlyClick here to download Supplementary material for on-line publication only: Fig 1.pdf
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