Full text of DOI:10.1007/BF03187035

an isotopic age obtained from tephras near Jinganshan,
Smith et al. inferred it as the Aptian stage[8]. Recently
Swisher et al. (1999) obtained the 40Ar/39Ar age of
(124f0.1) Ma (mid-Early Cretaceous)[9], also Wang et at.
Magnetic polarity ages of the
fossil-bearing strata at the Si-
hetun section, West Liaoning:
A preliminary result
(2001) reported the U-Pb age of (125.2f0.9) Ma of zir-
con separated from tuff of Sihetun vertebrates horizon[10]
.
As there is still no systematic paleomagnetic study on the
Yixian Formation, it is necessary and important to provide
paleomagnetic evidence for the age determination of the
fossil-bearing interval. In this note we present new pa-
leomagnetic results for the fossil-bearing interval and ba-
salts at the Sihetun section.
PAN Yongxin1, ZHU Rixiang1, John Shaw2
& ZHOU Yaoxiu3
1. Institute of Geology and Geophysics, Chinese Academy of Sciences,
Beijing 100101, China;
2. Geomagnetism Laboratory, University of Liverpool, Liverpool L69
7ZE, UK;
1
Sampling and experimental methods
3. The Aviatic Remote Sensing Center, Ministry of Land and Resources,
Beijing 100083, China
Correspondence should be addressed to Pan Yongxin (e-mail: yxpan
@mail.c-geos.ac.cn)
The Sihetun section is located 20ü25 km south of
Beipiao City. It can be divided into the upper basalt inter-
val, fossil-bearing interval and the lower basalt interval.
The fossil-bearing interval is approximately 30 m thick,
composed of light-gray shale, siltstone, silty mudstone
and gray tuffs. Abundant fossils such as Probaicalia viti-
mensis, Cypridea liaoningensis and Manchurochelys sp
and others are well-reserved. Importantly, the top part of
this interval yields the most famous ‘feathered’ dinosaurs.
The exposed lower basalt interval is over 30 m thick. Us-
ing a portable gasoline-powered drill, 45 and 64 orientated
cores were collected from the upper and lower basalt in-
tervals, respectively. 33 orientated hand samples were
collected along fossil-excavation pit walls, at about 0.5 m
spaces in 12 m sequence in the fossil-bearing interval,
because highly laminated and easily weathered rocks pre-
vented from drilling. Meanwhile, 230 non-orientated bag
samples were correspondingly collected (at about 0.05 m
spaces) for environmental magnetic studies. Unfortunately,
the lower part of the fossil-bearing interval is not accessi-
ble for sampling. Only 8 orientated samples were obtained
from a fine-grained sandstone bed of this part.
Abstract
Rock-magnetic and palaeomagnetic studies
have been carried out on the interval of famous fos-
sil-bearing sedimentary rocks and its overlying basalts and
underlying basalts at the Sihetun section, West Liaoning
Province. Normal polarity was obtained for the sedimentary
interval and the underlying basalts, while reversed polarity
was found in the overlying basalts. Taking account of the
new 40Ar/39Ar ages (Swisher et al., 1999), we classified the
fossil-bearing sedimentary interval into the Barremian M3n
zone (Early Cretaceous age). Several abnormal horizons on
magnetic properties, probably corresponding to the tuffs,
were observed in the fossil-bearing sedimentary interval.
This implies that the massive bio-extinction may link to
dramatic environmental changes that were caused by vol-
canic eruptions.
Keywords: magnetic polarity age, Jehol fauna, Sihetun section.
With increasing impacts from the westward ancient
Pacific plate, the northeastern China region was over-
whelmed by intensive magmatic, tectonic and sedimentary
basin-forming activities during the late Mesozoic. The
corresponding strata are composed of extensive volcanic
and sedimentary rocks. A wide range of fossils, famously
known as Jehol Fauna including early birds, flowers and
All measurements of sedimentary rocks were con-
ducted at the Palaomagnetic Laboratory of the Institute of
Geology and Geophysics, the Chinese Academy of Sci-
ences (CAS). Alternating field demagnetization of the
natural remanent magnetization (NRM) and remanece
measurements was performed on a 2G cryogenic magne-
ü
6]
mammals are well preserved in sedimentary intervals[1
.
This well-preserved assemblage of fossils and their related
massive bio-production and extinction records during this
specific period provide a good target for the establishment
of biological evolutions and ancient environmental
changes.
The fossil-bearing interval at the Sihetun section of
the Lower Yixian Formation yields the most famous
‘feathered’ dinosaurs Sinosauropteryx, Protarchaeopteryx,
Caudipteryx, etc.[3,4]. Despite the importance of this fossil
assemblage, the age of this fossil-bearing interval is not
completely settled. Chen et al. assumed it to be the Juras-
sic-Cretaceous (K-J) boundary[3], while Hou et al.[1] and
Ren et al.[7] classified it as the late Jurassic time. Based on
tometer in a field-free space (˘300 nT . Frequency-de-
)
pendent magnetic susceptibility (Ffd) and temperature-
F
dependent behavior of magnetic susceptibility ( -T) were
measured on a Bartington MS2 susceptibility meter and a
KLY-3s (CS-3) kappa-bridge. Saturation isothermal re-
manent magentization (SIRM) and anhystertic remanence
(ARM) were produced in a 2G660 pulse magnetizer and a
laboratory-built instrument. Remanences were measured
on a JR-5A automatic spinner megnetometer.
Stepwise demagnetization and remanence experi-
ments of all basalt rocks were performed in the Geoma-
gentism Laboratory of the University of Liverpool, using
Chinese Science Bulletin Vol. 46 No. 17 September 2001
1473

NOTES
Fig. 1. Rock magnetic results of representative samples. (a) and (b) are temperature-dependent susceptibility curves; (c) and (d), ther-
mognetic curves of samples from the lower and upper basalt intervals, respectively; (e) and (f), hysterestic loops of basalt samples.
a tumbling a.f. demagnetizer and spinner magnetometer.
mineral with lower unblocking temperature (e.g. JFe2O3)
Hysteretic loops and thermomagnetic curves were meas-
ured on a Variable Field Translation Balance (VFTB).
Data were processed by the VFTB-Analyzer software.
co-ex- isting with titanomagnetite (fig. 1(d)). A small
number of samples were dominated by multi-domain ti-
tanomagnetite (fig. 1(e)). For the basalt samples, the satu-
ration isothermal remanent magnetization ranges from
2
Results and discussions
(3.4ü5.2)hꢀ10ꢀ2 Am2gkgꢀ1, and remanence coercivity
from 15 to 35 mT.
Stepwise alternating field demagnetization was con-
ducted on all sedimentary samples and selected basalts
F
-T curves show that titanomagnetite may be the
main remanece carrier of sedimentary rocks with rela-
tively low Curie temperature (Tc) and concentration. A
distinct susceptibility peak around 500ü530ć on heat-
ing curves may suggest a transformation from siderite to
magnetite[11] (fig. 1(a), (b)). Pseudo-single domain (PSD)
titanomagnetite in basalts were shown by hysterestic pa-
rameters and thermomagnetic behavior, with ratios of re-
manence coercivity to coercivity (Hcr/Hc), 2.1ü4.9, re-
with a maximum peak field of 150 mT at steps 5ü10 mT.
A total of 25 sedimentary samples yielded stable direc-
tions of characteristic remanence (ChRM) component (fig.
2). Others failed due to very weak natural renament mag-
netization ((˘0.4ü1.1)h10ꢀ8 Am2gkgꢀ1 . All basalt
)
samples yielded stable ChRM directions. Parallel checks
of thermal demagnetization on 8 sister samples showed
consistent results with the alternating field experiments.
Viscous remanent magnetization (VRM) was usually
cleaned in fields less than 15 mT in af demagnetization or
manence to saturation remanence (Mrs/Ms), 0.05ü0.14,
and Tc, 520ü565ć. We noted that most samples from the
upper basalt interval also show a drop around 350ć on
the thermomagnetic curves, indicating another magnetic
1474
Chinese Science Bulletin Vol. 46 No. 17 September 2001

250ć in thermal demagnetization. A ChRM component
was usually isolated between 300ü520ć; regardless of
its belonging to the upper or lower basalt intervals.
Fig. 3. Result of magnetic polarity of the Sihetun section.
young for the fossil-bearing interval. Also, the age of the
first stage in the Yixian Formation (˚129.8 Ma) proposed
by Xu et al. (1997) is also too old due to lack of accurate
dating[13]
.
Although the lower basalt interval has the same po-
larity as the sedimentary rock interval in this section, it is
a premature to assume that they belong to the same polar-
ity zone before accurate isotopic dating from these basalt
rocks. In other words, it is difficult to tell the age differ-
ence between the sediments and the underlying basalt
lavas. Regarding the upper basalt interval, we noted that
the reversed polarity is similar to that of Zhuanchengzi’s
lavas, which belong to the second stage proposed by Xu et
al. (1997). Combining with the 40Ar/39Ar age determined
by Smith et al. (1995) from Jingangshan volcanic rocks
(the third stage of Xu et al’s), we supposed that the upper
basalt interval is likely to belong to the M1r chron zone,
124ü123.6 Ma. This assumption needs to be proved by
temporal controls, considering the uncertainty of those
volcanic rocks (eruptive or shallow intrusive).
Rock-magnetic studies on sedimentary samples in-
dicate that the magnetic mineral concentration is very low.
Their average values of low-field susceptibilities, fre-
quency-dependent susceptibilities, SIRM and ARM are of
5h10ꢀ5 SI, 1.4%, 4.5h10ꢀ3A/m and 1.5h10ꢀ3A/m. In-
terestingly, eight abnormal horizons of rock-magnetic
properties are observed in the fossil-bearing interval (fig.
4), if classified by 5-fold susceptibility value (arbitrary).
Those horizons also have unusually high average values
of SIRM and ARM, up to 8- and 11-fold, respectively.
Those samples are yellow-brown color and strongly
Fig. 2. Typical demagnetization projection plots. The solid (open)
symbols refer to the projection in the horizontal (vertical) plane. “sedi”
refers to sedimentary rocks, b05-7 and b06-1 are basalts from the upper
and lower intervals, respectively.
Fig. 3 shows the magnetic polarity zones determined
by the corresponding ChRM components. Both the fos-
sil-bearing sedimentary interval and the lower basalt in-
terval have normal polarity, while the upper basalt interval
has a reversed polarity. We noted that the top three sedi-
mentary samples showed abnormal directions. Those sam-
ples were likely to be baked by the overlying basalt lava’s
heat and produced a secondary stable overprint. A
possibility of polarity transition record cannot be com-
pletely ruled out at this stage.
Recently, Swisher et al. (1999) obtained an age of
(124.6f0.1) Ma (40Ar/39Ar method) from 34 single
sanidine crystals on a tuff close to a fossil-bearing layer at
the Sihetun section and adjacent Jianshangou section.
With reference to this new age[9], we assume that this fos-
sil-bearing interval belongs to the Barremian stage, 124.7
ü124.0 Ma[12], Early Cretaceous time.
Based on this result, an estimation of the Late Juras-
sic age[1,7] is too old while an Aptian stage[8], controlled by
younger volcanic rocks at a far distance section, is too
Chinese Science Bulletin Vol. 46 No. 17 September 2001
1475

NOTES
gation and evolution of fauna living in this ancient lake.
Therefore, it is supposed that deep earth’s processes may
be linked to the massive bio-production and extinction via
volcanic eruptions during the special geological time[14]
.
Because faulting and erosion seriously broke the
fossil-bearing strata in this area into pieces, it is difficult
to illustrate fully environmental significance of the fos-
sil-bearing strata based on this section only. A synthetic
investigation combining with palaeoecology, sedimentol-
ogy, environmental magnetism and geochemistry covering
adjacent areas is required in future.
Acknowledgements We are grateful to Guo Youzhao, Zhang Xiping
and Yang Guiqing for their kind help in arduous field sampling. Dr. Liu
Jian and Dr. Sun Zhiming are thanked for their help in this study. Pan
Yongxin thanks the Royal Society Royal Fellowship of UK. This work
was supported by the National Natural Science Foundation of China
(Grant No. 49834001) and the National Key Basic Research Program
(Grant No. G1999075509) and the CAS (Grant No. KZ951-A1-401).
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(Received January 11, 2001)
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