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Journal of Earth Science, Vol. 26, No. 2, p. 236–245, April 2015
Printed in China
DOI: 10.1007/s12583-015-0535-x
ISSN 1674-487X
The Relationship between the “Green-Bean Rock”
Layers and Conodont Chiosella timorensis and Implications
on Defining the Early–Middle Triassic Boundary in the
Nanpanjiang Basin, South China
Chunbo Yan1, 2, Haishui Jiang*1, 3, Xulong Lai*1, 3, Yadong Sun1, Bo Yang1, 2, Lina Wang1, 3
1. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China,
2. Wuhan Center of Geological Survey, Wuhan, 430205, China
3. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
ABSTRACT: The Triassic “Green-bean Rock” (GBR) layers were widely recognized around the
Early–Middle Triassic boundary interval in the Nanpanjiang Basin, South China. To determine the
precise relationship between the GBR layers and the first appearance datum (FAD) of the conodont
Chiosella timorensis, four Lower–Middle Triassic sections from the Nanpanjiang Basin, including the
Gaimao, Bianyang II, Zuodeng and Wantou sections have been studied in detail. Detailed conodont
biostratigraphy convinces us that there is no exact temporal relationship between the GBR layers and
first occurrence of Ch. timorensis. Moreover, the numbers of the GBR layers are different from the
place to place within the Nanpanjiang Basin, and the time span of the GBR layers was much longer
than previously estimated. Global correlations show that the FAD of Ch. timorensis is contemporaneous
basinwide and worldwide and more suitable marker defining the Olenekian-Anisian boundary
(Early–Middle Triassic boundary) than any other proxies.
KEY WORDS: Early–Middle Triassic boundary, Green-bean Rock, Chiosella timorensis, Nanpanjiang
Basin.
0
INTRODUCTION
The complete Lower–Middle Triassic marine successions
are well exposed in South China, especially in the Nanpanjiang
area, which provide useful materials for studying the biostratigraphy and eventstratigraphy of the Lower–Middle Triassic and
biotic recovery after the end-Permian mass extinction (Chen
and Benton, 2012). High-resolution U-Pb dating on zircon from
the volcanic ash beds near the Permian-Triassic boundary has
played an important role in constraining the age of the
Permian-Triassic boundary and associated bioevents as well as
the duration of some key conodont zones (e.g., Burgess et al.,
2014; Shen et al., 2011; Bowring et al., 1998). Similarly, several volcanic ash beds, used for the U-Pb dating test on zircon,
can also be found from the Lower–Middle Triassic interval.
Previously, one or more special layers of these volcanic tuff
beds, traditionally named the “Green-bean Rock”, was referred
to as the marker of the Olenekian-Anisian boundary (OAB) in
the Yangtze Platform region and Nanpanjiang Basin (Yao et al.,
2011; Lehrmann et al., 2006, 2005; Wang et al., 2004; Yin and
*Corresponding author: [email protected]; [email protected]
© China University of Geosciences and Springer-Verlag Berlin
Heidelberg 2015
Manuscript received December 20, 2014.
Manuscript accepted February 25, 2015.
Tong, 2002; Guan et al., 1990). The so-called “Green-bean
Rock” (GBR) was named for containing the bean-like quartz
particulates (Guan et al., 1990). In fact, the GBR is the light
yellow or yellow green volcanic potassic crystal pyroclastvitroclastic tuff produced by acidic volcanic eruption, containing illite and a small amount of quartz, montmorillonte, and
also a little bit volcanic glass, muscovite and kaolinite (Wang et
al., 2004; Guan et al., 1990). However, the bean-like quartz
particulates are not usually present in the GBR in the Nanpanjiang Basin. The light yellow or yellow green volcanic ash beds
were also referred to as the GBR in the Nanpanjiang Basin
(Lehrmann et al., 2006).
Biostratigraphically, the conodont Chiosella timorensis
has long been considered as the index taxon for determining the
OAB (e.g., Grãdinaru et al., 2007, 2006; Orchard et al., 2007a,
b). Up to date, the Global Stratotype Section and Point (GSSP)
for the OAB, however, has not been determined yet, although
the conodont evolutionary lineages as the important biostratigraphic criteria defining the OAB have been well studied in the
Deşli Caira Section of Romania (Grãdinaru et al., 2007, 2006;
Orchard et al., 2007a) and Guandao Section of Luodian,
Guizhou (Orchard et al., 2007b; Wang et al., 2005).
Interestingly, abundant Chiosella timorensis and other
conodonts are commonly present in these so-called GBR layers
in the Nanpanjiang Basin. The former is accepted widely as a
lithostratigraphic marker of the base of the Anisian in the field,
while the latter is the potential biostratigraphic marker of the
Yan, C. B., Jiang, H. S., Lai, X. L., et al., 2015. The Relationship between the “Green-Bean Rock” Layers and Conodont Chiosella
timorensis and Implications on Defining the Early–Middle Triassic Boundary in the Nanpanjiang Basin, South China. Journal of
Earth Science, 26(2): 236–245. doi:10.1007/s12583-015-0535-x
The Relationship between the “Green-Bean Rock” Layers and Conodont Chiosella timorensis
OAB. Then, what is the precise relationship between the GBR
layers and the first occurrence of Ch. timorensis?
Lehrmann et al. (2005) reported for the first time the U/Pb
age from several layers of GBR near the Lower–Middle Triassic boundary interval in the Guandao Section. Of these five
GBR layers in the Lower Guandao Section, the radiometric age
of the uppermost GBR layer is 246.5±0.14 Ma. The first occurrence (FO) of Ch. timorensis in the Upper Guandao Section
is 3 m lower than the third GBR layer, which was dated as
247.8±0.074 Ma, indicating that the FO of Ch. timorensis is
earlier than 247.8±0.074 Ma. Subsequently, Lehrmann et al.
(2006) reported the U/Pb age from four GBR layers in the
Lower Guandao Section: 247.38±0.10, 247.32±0.08,
247.13±0.12, and 246.77±0.13 Ma in an ascending order. The
FO of Ch. timorensis is located between Tuff-2 and Tuff-3
(Lehrmann et al., 2006), implying an age of between
247.32±0.08 Ma and 247.13±0.12 Ma. Bucher et al. (2007) has
calibrated the U-Pb dates derived from these GBR layers to the
evolutionary sequences of Ch. timorensis, and estimated that
the FAD of Ch. timorensis in the Lower Guandao Section was
about 247.2±0.2 Ma. In contrast, Ch. timorensis occurred earlier than 247.8±0.074 Ma in the Upper Guandao Section.
Bucher et al. (2007) interpreted these conflicted occurrences of
the FAD of Ch. timorensis to come forth due to the following
three reasons. Firstly, the U/Pb age yielded from the GBR layer
is correct, while the FAD of Chiosella timorensis is diachronous; secondly, some U/Pb age values are incorrect while the
latter is isochronous; thirdly, the worst situation is that both
have problems.
Given the phenomenon that the horizons of the first appearance of Ch. timorensis does not coincided with the zircon
U-Pb dating of the GBR layers in the Nanpanjiang Basin areas,
some researchers (Ovtcharova et al., 2010; Bucher et al., 2007)
therefore questioned if the FAD of Ch. timorensis can be used
as the marker of the OAB (e.g., Goudemand et al., 2012a; Ovtcharova et al., 2010). However, detailed conodont stratigraphy
have been proved that can play very important roles in determination of the Permian-Triassic boundary (e.g., Zhang et al.,
2014; Jiang et al., 2011, 2007) or the age of microbialite deposition in the aftermath of the end-Permian mass extinction in
South China (Jiang et al., 2014). Here, we document the detailed stratigraphic distributions of Ch. timorensis and associated conodonts near the OAB from four Lower–Middle Triassic
sections in the Nanpanjiang Basin areas. The detailed conodont
biostratigraphy provides determination of the precise relationship between the GBR layers and the FAD of Ch. timorensis.
Moreover, the potential of the FAD of Ch. timorensis as the
marker defining the OAB is also assessed in a global context.
1 GEOLOGICAL AND STRATIGRAPHIC SETTINGS
OF THE STUDIED SECTIONS
These four study sections are all located in the
Nanpanjiang Basin, southern Guizhou and northwestern
Guangxi regions, South China (Fig. 1).
The Gaimao Section (26°26'48.59''N, 106°44'36.83''E) of
the Huaxi District, Guiyang City, Guizhou Province is located
at the northern edge of the Nanpanjiang Basin. The Gaimao
area was situated at the southern part of the Yangtze Platform
237
during the Early–Middle Triassic (Feng et al., 1997). Thus, the
Early–Middle Triassic successions comprise platform facies
carbonates. The OAB succession is dominated by dolomitic
limestone and dolomite. One GBR layer (5 cm thick) is present
near the Early–Middle Triassic boundary (Figs. 2a (close range),
2b (distant view)). Previously, the Early Triassic trace fossils
(Luo et al., 2007; Wang, 1987) and the earliest Triassic chert
event (Yang et al., 2012) were reported from this section.
The Bianyang Section (25.6446°N, 106.6191°E) of the
Luodian County, Guizhou Province is located at the northern
part of the Nanpanjiang Basin during the Early–Middle Triassic
(Feng et al., 1997). The Bianyang II Section and the Bianyang
Section (in term of Yan et al., 2013) are located in the same
place, but the former is usually covered by vegetation and
cropland resulted undiscovered before. The OAB succession at
the Bianyang II Section is dominated by bioclastic limestone,
siliceous nodular limestone, striated limestone, chert and calcareous mudstone (Figs. 2c, 2e). A total of five GBR layers
(Figs. 2c, 2e) and two tuffaceous sandstones are exposed near
the OAB. As in a basin facies (Lehrmann et al., 2003; Feng et
al., 1997), the Bianyang area has been relatively detailed researched, including conodont sequences from the uppermost
Permian to Middle Triassic (Yan et al., 2013; Jiang, 1980),
microfacies and biotic recovery after the end-Permian mass
extinction (e.g., Song et al., 2011), inorganic carbon isotope in
the whole Early Triassic (Sun et al., 2012; Meyer et al., 2011),
and conodont oxygen isotope data and their indication for the
sea water palaeotemperature in the Early and earlier Middle
Triassic (Sun et al., 2012). Nevertheless, no conodonts were
found around the OAB succession in this area.
The Zuodeng Section (23.4537°N, 106.9933°E) of the
Tiandong County, Guangxi Autonomous Region is located
southern part of the Nanpanjiang Basin. As in a basin facies
(Lehrmann et al., 2003; Feng et al., 1997) too, the Zuodeng
Section has been reported conodont sequence and correlated
ammonoid zones in the Early Triassic before (Yang et al., 1986).
The Zuoden II Section, equivalent to the upper part of Zuodeng
Section in Yang et al. (1986), is dominated by striated limestone and silty mudstone. The GBR layer with silty mudstone
above it lied at the upper part of this section. It is a pity that few
conodont materials were obtained from the upper part of this
section. And more, the FAD of Ch. timorensis was marked by a
question mark in Yang et al. (1986). Subsequently, carbon isotope data (Tong et al., 2007) and conodont oxygen isotope
curve in the Early Triassic (Sun et al., 2012) concerning this
section have been reported. The Zuodeng III Section, locating
about 150 m on the southwest of the Zuodeng II Section, is
dominated by striated limestone, silty mudstone and chert. One
GBR layer (2 m thick) is present near the Early–Middle Triassic boundary (Fig. 2d).
Another basin facies section, the Wantou Section
(24.5915°N, 106.8625°E) of Jinya, Fengshan County, Guangxi
Autonomous Region is located middle part of the Nanpanjiang
Basin (Lehrmann et al., 2003; Feng et al., 1997). This section is
dominated by striated limestone, chert, siliceous nodular limestone, siliceous banded limestone and so on (Fig. 2f). There are
five GBR layers appeared around the OAB successions. While
conodont sequences are absent in this section. Zhang (1990)
238
Chunbo Yan, Haishui Jiang, Xulong Lai, Yadong Sun, Bo Yang and Lina Wang
Figure 1. Paleogeography of the study region during the Early Triassic. (a) Paleogeography of the Yangtze Platform and
Nanpanjiang Basin; (b) paleogeography location of the Gaimao, Guandao, Bianyang II, Ganheqiao, Wantou and Zuodeng III
sections in the Nanpanjiang Basin during the Early Triassic. GBG. the Great bank of Guizhou, modified from Lehrmann et al.
(2003).
first reported the conodont zonations in this area. Recently, a
series of reports about the Early Triassic and Lower–Middle
Triassic boundary in this section and surrounding area have
made a lot of achievements (Goudemand et al., 2012a; Ovtcharova et al., 2010, 2005; Galfetti et al., 2008, 2007a; Brayard
et al., 2007). A series of zircon U-Pb dating from the volcanic
tuff beds during the Lower and Middle Triassic strata have been
reported and the correlated chronostratigraphic framework of
the Early Triassic was set up (Ovtcharova et al., 2010, 2005;
Brayard et al., 2007). Based on the established high-resolution
ammonoid biostratigraphy, Galfetti et al. (2008, 2007b) discussed the climatic change in the later period of Early Triassic.
Moreover, Goudemand et al. (2012b) have reported conodont
clusters from Lower Triassic strata in this area.
2
CONODONT OCCURRENCES
Total 67 limestone samples (each weighting about 5–8 kg)
were collected from four sections. All samples were broken into
small fragments. Then they were dissolved by diluted acetic
acid (~10%). Subsequently, heavy liquid separation was used to
concentrate conodonts (see Jiang et al., 2007). A total of 16 128
conodont elements were obtained, including 3 137 P1 elements,
among which, these elements can be identified belong to genera
Chiosella, Icriospathodus, Neospathodus, Novispathodus, Triassospathodus, etc. (Figs. 3 and Fig. 4).
At Gaimao, four important conodont species: Icriospathodus collinsoni, Icriospathodus? crassatus, Triassospathodus homeri and Novispathodus abruptus are identified
(Fig. 4). However, the limitation of this section is lack of Ch.
timorensis. These conodonts are typical of the latest Spathian
conodont faunas in South China (i.e., in the Chaohu Section,
Zhao et al., 2007; the Qingyan Section, Ji et al., 2011; the
Bianyang Section, Yan et al., 2013). As demonstrated in the
Gaimao Section (Fig. 4), these conodont species were derived
from the beds aroud the GBR layer. The latter therefore are
latest Spathian in age at this section.
At Bianyang II Section, the important conodont species
include Nv. abruptus, Tr. homeri, Tr. brevissimus, Tr. symmetricus, Ns. curtatus, Tr. brochus, Ic.? crassatus, Ch. gondolelloids,
and Ch. timorensis (Fig. 4). The key conodont species, such as
Tr. homeri, Tr. brochus, and Ch. timorensis constrain the GBR
layers as the latest Spathian (Olenekian) to earliest Anisian in
age.
Eight conodont species Nv. pingdingshanensis, Nv. abruptus, Tr. symmetricus, Ic. collinsoni, Ic.? crassatus, Ns. curtatus,
Tr. homeri and Tr. brochus are identified at Zuodeng II Section
(Fig. 4). Five conodont species: Nv. abruptus, Tr. symmetricus,
Tr. brochus, Ch. gondolelloids and Ch. timorensis are identified
at Zuodeng III Section (Fig. 4). Of these, Nv. pingdingshanensis, Ic. collinsoni, Tr. homeri and Tr. brochus are normally index
species defining the Spathian, while Ch. timorensis marks the
beginning of the Anisian worldwide (Grãdinaru et al., 2007,
2006; Orchard et al., 2007a, b) (Fig. 4). Accordingly, the GBR
layers range from the Spathian (Late Olenekian) to Anisian in
Zuodeng.
Previously, no Middle Triassic conodonts have been reported from the Wantou Section although ammonoid biostratigrphy has been well studied, and radiometric ages have
also been obtained from the GBR layers near the Lower–
Middle Triassic transition. Some important conodont species:
Nv. abruptus, Tr. symmetricus, Ic.? crassatus, Ns. curtatus, Tr.
homeri, Tr. brochus, Ch. gondolelloids, and Ch. timorensis are
first reported from this section in this study (Fig. 4). Of these,
Ns. curtatus, Tr. homeri, Tr. brochus, and Ic.? crassatus constrain the studied interval as the late Spathian in age, while the
presence of Ch. timorensis suggests an earliest Anisian age.
The GBR layers therefore are Late Spathian–earliest Anisian in
age in Wantou. The combination of ammonoid zones, conodont
The Relationship between the “Green-Bean Rock” Layers and Conodont Chiosella timorensis
239
Figure 2. Photographs and details of outcrops. Green pentagons and red triangle stand for the position of the GBR layers and
the FAD of Chiosella timorensis, seperately. (a)–(b) Lime dolomite and dolomitic limestone around the GBR layer, Gaimao
Section;(a) close range, (b) distant view. (c), (e) Striated limestone, chert and calcareous mudstone, Bianyan II Section. From
the (c) and (e), we can see the first to third and fifth GBR layer. (d) Striated limestone, chert and silty mudstone, Zuodeng III
Section. The GBR layer is about 2 m thickness. (f) Striated limestone, chert, siliceous nodular limestone and siliceous banded
limestone, Wantou Section. The third to fifth GBR layers are been marked in the picture.
zones and radiometric ages from the Wantou Section enables us
to define precisely the OAB.
3 DISCUSSION AND CONCLUSIONS
3.1 The Relationship between the GBR Layers and Conodont Chiosella timorensis
Here, we combined all biostratigraphic data obtained from
the four study sections and some typical sections already reported from the Nanpanjiang Basin areas (Fig. 4). At the platform facies Gaimao Section, no Ch. timorensis was found in
association with the GBR layers, instead, a lot of elements belonging to Icriospathodus (Ic.)? crassatus, Ic. collinsoni, Tri-
assospathodus (Tr.) brevissimusi and Tr. homeri have been recognized. Correlated with the Early Triassic conodont sequences
worldwide (e.g., Yan et al., 2013; Zhao et al., 2008; Krystyn,
2005; Sweet et al., 1971), these conodont species suggest that
this strata interval should correspond to the middle and late
Spathian substage of the Olenekian Stage, but not Anisian. At
the Bianyang II Section, the FAD of Ch. timorensis is 50 cm
high above the fifth GBR layer. At the Zuodeng II Section, Ch.
timorensis is not present near the GBR layer. While at the Zuodeng III Section, the FAD of Ch. timorensis is 2.5 m lower than
the bottom of the 2-m-thick GBR layer. At the Wantou Section,
five GBR layers (labelled Tuff-1, Tuff-2, Tuff-3, Tuff-4, and
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Chunbo Yan, Haishui Jiang, Xulong Lai, Yadong Sun, Bo Yang and Lina Wang
Figure 3. SEM photos of conodonts from the Early–Middle Triassic boundary interval of Gaimao, Bianyang II, Wantou and
Zuodeng sections in the Nanpanjiang Basin. All specimens are preserved in the School of Earth Sciences, China University of
Geosciences, Wuhan, Hubei Province. Scale bar=400 µm. 1–2. Novispathodus pingdingshanensis (Zhao and Orchard); 1a)
lateral view, (1b) lower view, from Luolou Formation of Zuodeng II Section, ZDY0328; 2a) lateral view, (2b) lower view, from
Luolou Formation of Zuodeng II Section, ZDY0330; 3. Novispathodus radialis (Zhao et al.), lateral view, from Luolou Formation of Zuodeng II Section, ZDY1070; 4. Triassospathodus brevissimus (Orchard), lateral view, from Luolou Formation of
Zuodeng II Section, ZDY1069: 5–6. Icriospathodus collinsoni (Solien), upper view, 5. from Luolou Formation of Zuodeng II
Section, ZDY0340; 6. from Huaxi Formation of Gaimao Section, GMY21011; 7. Icriospathodus? crassatus (Orchard), upper
The Relationship between the “Green-Bean Rock” Layers and Conodont Chiosella timorensis
241
view, from Anshun Formation of Gaimao Section, GMY2047; 8–9. Triassospathodus homeri (Bender), 8. lateral view, from
Luolou Formation of Bianyang II Section, BYY6008; (9a) lateral view, (9b) lower view, from Luolou Formation of Zuodeng II
Section, ZDY3057; 10. Neospathodus pusillus Orchard, (10a) lateral view, (10b) lower view, from Luolou Formation of Bianyang II Section, BYY5097; 11–12. Neospathodus curtatus Orchard, (11a) lateral view, (11b) lower view, from Luolou Formation of Wantou Section, WTY2212; (12a) lateral view, (12b) lower view, from Luolou Formation of Zuodeng II Section,
ZDY0370; 13. Novispathodus abruptus (Orchard), (13a) lateral view, (13b) lower view, from Luolou Formation of Zuodeng II
Section, ZDY0357. 14. Triassospathodus symmetricus (Orchard), (14a) lateral view, (14b) lower view, from Luolou Formation
of Wantou Section, WTY2216; 15–17. Triassospathodus brochus (Orchard), (15a) lateral view, (15b) lower view, from Luolou
Formation of Zuodeng III Section, ZDY2127; (16a) lateral view, (16b) lower view, from Luolou Formation of Wantou Section,
WTY2202; 17. lateral view, from Xinyuan Formation of Bianyang II Section, BYY6024; 18–19, 24. Chiosella gondolelloides
(Bender), (18a) lateral view, (18b) lower view, from Wantou Section, WTY2140; 19. lateral view, from Luolou Formation of
Wantou Section, WTY2203; 24. lateral view, from Wantou Section, WTY2138; 20–23. Chiosella timorensis (Nogami), 20.
lower view, from Luolou Formation of Zuodeng III Section, ZDY1075; (21a) lateral view, (21b) lower view, from Xinyuan
Formation of Bianyang II Section, BYY6015; (22a) lateral view, (22b) lower view, from Wantou Section, WTY2132; 23. lower
view, from Xinyuan Formation of Bianyang II Section, BYY6022; 25. Neospathodus clinatus Orchard, (25a) lateral view, (25b)
lower view, from Wantou Section, WTY2137.
Tuff-5; Lehrmann et al., 2006) and a 2.6 m thick igneous clastic
sandstone cropped out in the field. The FAD of Chiosella
timorensis is situated between Tuff-3 and Tuff-4. In addition,
the Ganheqiao Section of Wangmo, Guizhou, locating in the
Nanpanjiang Basin during the Early–Middle Triassic (Fig. 1)
also yields conodont Ch. timorensis near the GBR layers. Here,
the FAD of Ch. timorensis is 50 cm lower than the base of the
GBR-bearing interval, which is up to 6.1 m thick (Yao et al.,
2011, 2004). Although the thickest GBR layer or its equivalent
igneous clastic sandstone has been treated as the marker the
base of the Anisian Stage, their correlations with the FAD of
Ch. timorensis are still disputed. At Ganheqiao and Zuodeng III
sections, the FAD of Chiosella timorensis is beneath the thickest GBR layer or its equivalent the igneous clastic sandstone.
The opposite situation occurs in the Wantou Section (Fig. 4).
Therefore, several primary conclusions could be reached.
Firstly, numerous GBR layers were deposited during the
Early–Middle Triassic boundary interval. Secondly, there is no
compliance order between the GBR layers and the FAD of Ch.
timorensis. The FAD of Ch. timorensis could occurred before
the GBR layers (Ganheqiao and Zuodeng III Section), or after
the latter (Bianyang II Section), or within the sedimentation of
numerous GBR layers (Guandao (Orchard et al., 2007b; Wang
et al., 2005) and Wantou Section). Ovtcharova et al. (2010)
argued that the GBR layer is not suitable to be treated as a
standard defining the OAB in the Nanpanjiang Basin, because
the GBR layers possess a rather long duration (>0.4 Ma). In
fact, the duration of the GBR layers (from 248.12±0.28 to
246.77±0.13 Ma) lasted probably extended to about 1.5 Ma.
Is the FAD of Ch. timorensis synchronous in the Nanpanjiang Basin? Lehrmann et al. (2005) has pointed out that the
U/Pb age values of the GBR layers in Lower and Upper Guandao sections were just preliminary results. It required further
complementarity and improvement to confirm the stratigraphy
boundary finally. After that, Lehrmann et al. (2006) has published new U/Pb age data only in Lower Guandao Section.
Therefore, it could be deduced that the age value of
247.8±0.074 Ma from the Upper Guandao Section need to be
reconsidered. Bucher et al. (2007) has inferred that the FAD of
Ch. timorensis occurred in 247.2±0.2 Ma. This result is reinforced by the occurrence of both radiometric ages and Ch.
timorensis in the Wantou Section. If combining a series of U/Pb
age obtained from the GBR layers and other volcanogenic
rocks of the Jinya region, Fengshan County, Guangxi (Galfetti
et al., 2008) with the updated conodont biostratigraphy documented in this study, clearly, the GBR layer yielding U/Pb age
of 248.12±0.41 Ma is 1.5 m below the FAD of Ch. timorensis,
while the GBR layer 1.5 m above the FAD of the same conodont species is 246.83±0.31 Ma. Lithology of this interval remains unchanged. Thus, their sedimentation rate should be
stable. Consequently, the date of the FAD of Ch. timorensis is
estimated as 247.43±0.36 Ma. Considering the error range, this
age value could correlate well with that (247.2±0.2 Ma) from
the Lower Guandao Section. Therefore, we considered that the
FAD of Ch. timorensis should be isochronous and some radiometric ages from the GBR units require further study.
3.2
The Definition of the Olenekian-Anisian Boundary
Although conodont Chiosella timorensis has been widely
accepted as one key index taxon defining the OAB (Ji et al.,
2011; Grãdinaru et al., 2007, 2006; Orchard et al., 2007a, b),
three approches have been proposed to define this boundary.
Firstly, ammonoids zonation is an important auxiliary index
defining the OAB (Grãdinaru et al., 2007). Secondly, the systematic magnetostratigraphic study at Deşli Caira Mountain
Section of Romania, a candidate stratotype section of the OAB,
shows that this boundary is placed between two short normal
magnetic polarities (Grãdinaru et al., 2007; Orchard et al.,
2007a), while the FAD of Ch. timorensis also appears in this
part of the strata. Ammonoid Paracrochordiceras-Japonites
assemblage has been recognized from the bottom of Anisian
Stage (Grãdinaru et al., 2007). This is also true that the FAD of
Ch. timorensis appears the negative polarity strata in Guandao.
Paleomagnetism variation trend of these two sections correlate
well with that of West Tethys regions, including Bulgaria
(Muttoni et al., 2000) and Poland (Nawrocki and Szulc, 2000).
Thirdly, since a relatively detailed conodont sequences have
already been established in all kinds of lithology (especially the
limestone), conodont evolutionary lineages are very significant
for defining the OAB.
Figure 4. The relationship between the conodont sequences and the GBR layers at Gaimao, Bianyang II, Wantou and Zuodeng sections in the Nanpanjiang Basin (the age
value and ammonoid Zonations at Wantou Section from Galfetti et al. (2008, 2007b); green and black pentagon standing for “Green-bean rocks” and grey white-grey black
tuff or volcanogenic sandstone). ①. Ch. timorensis; ②. Tr. brochus; ③. Tr. homeri; ④. Ic. collinsoni; ⑤. Nv. pingdingshanensis.
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Chunbo Yan, Haishui Jiang, Xulong Lai, Yadong Sun, Bo Yang and Lina Wang
The Relationship between the “Green-Bean Rock” Layers and Conodont Chiosella timorensis
In some sections, Ch. timorensis occurs in the ammonoid
haugi Zone, diagnostic of the latest Spathian substage. For
these reasons, Goudemand et al. (2012a) considered that Chiosella timorensisis was not fit for confirming the base of the
Anisian Stage, and expected the possibility of diachronous FAD
of Chiosella timorensis worldwide. However, Sun et al. (2013)
has discussed the flaws of ammonoid zones in Early Triassic. It
is not common to use this defective zonation scheme to question a more reliable conodont zonation scheme. The FAD of
key fossil is normally used to define the boundary for the synchrony of that in geological scale. It should be noted that some
species have wide stratigraphic distributions and probably extended to a high stratigraphic level. This is probably due to the
mixture of relic species with nascent species, for example of
the occurrence of conodont Ch. timorensis in the ammonoid
haugi Zone.
To date, no ammonoid fossils were found around the OAB
in the Guandao sections, which have detailed conodont biostratigraphic control (Orchard et al., 2007b). Alternatively,
abundant conodonts were newly obtained from the Wantou
Section (Figs. 3, 4) from which abundant ammonoids have
been reported from the Lower–Middle Triassic strata (Galfetti
et al., 2007a). As a result, the occurrence of both conodonts and
ammonoids can be correlated with one another (Fig. 4). Clearly,
the FAD of conodont Chiosella timorensis is about 0.5 m
higher than the top of the hauzi Zone.
Accordingly, we suggest that the FAD of Ch. timorensis is
still an appropriate mark defining the OAB. The reasons are as
below. (1) At Deşli Caira Mountain Section of Romania
(Grãdinaru et al., 2007, 2006; Orchard et al., 2007a), Guandao
Section (Orchard et al., 2007b; Wang et al., 2005) and the four
sections studied in this paper, accompanying with the appearance of Ch. timorensis, Triasspathodus elements disappeared
gradually. This conodont faunal turnover occurred clearly
around the OAB. (2) The strata yielding elements of Ch.
timorensis are usually equivalent to the relatively positive range
of carbon isotope curves. This trend is not only reflected at
Bianyang II Section and Wantou Section (Sun et al., 2012), but
also the Gandao Section (Sun et al., 2012; Meyer et al., 2011;
Payne et al., 2004) and Desli Caira Mountain Section of Romania (Grãdinaru et al., 2007). (3) As mentioned above, the FAD
of Ch. timorensis, with the age of 247.2±0.2 Ma, is better to
define the OAB than the GBR layers in the Nanpanjiang Basin.
4 TAXONOMIC NOTES
Chiosella gondolelloides (Bender, 1967) (Fig. 3: 18–19, 24)
1967
Spathognathodus gondolelloides n. sp.-Bender, p.
529–530, Pl. 5, Figs. 17, 19, 20.
2005 Neospathodus gondolelloides (Bender)-Wang et al., p.
621, Pl. 1, Figs. 13, 15.
2007a Chiosella gondolelloides (Bender)-Orchard et al., p.
345, Fig. 5, Figs. 13–15, 23–25.
2007b Chiosella gondolelloides (Bender)-Orchard et al., p.
353, Fig. 6, Figs. 22–29.
Diagnosis This speices is characterized by a relative elongate
and asymmetric segminate P1 element. A low lateral rib developed along most of its length in later growth stages. Denticles
fused moderately, increasingly reclined posteriorly. The oval
243
basic cavity expanded beneath the posterior 1/3 to 1/2 of the
element.
Remarks Ch. gongolelloides can be distinguished from the
similar Tr. symmetricus and Tr. brochus by the former having a
lateral rib. Additionally, Tr. symmetricus is shorter and higher
than this species. Ch. gongolelloides also can be distinguished
from Ch. timorensis by the latter having a very narrow or rudimentary platform, which could extends to the posterior edge of
the element.
Chiosella timorensis (Nogami, 1968)(Fig. 3: 20–23)
1968 Gondolella timorensis n. sp.-Nogami, p. 127, Pl. 10,
Figs. 17–21.
2005 Neospathodus timorensis (Nogami)-Wang et al., p. 621,
Pl. I, Figs. 14, 17, 19.
2007a Chiosella timorensis (Nogami)-Orchard et al., p. 345,
Fig. 5, Figs. 16, 17, 31–34.
2007b Chiosella timorensis (Nogami)-Orchard et al., p. 353,
Fig. 6, Figs. 32–34, 36–38.
2011 Chiosella timorensis (Nogami)-Ji et al., p. 218, Fig. 4,
Figs. 10–11.
2012 Chiosella timorensis (Nogami)-Goudemand et al., p.
203, Fig. 2, Figs. 1–14; Fig. 3, Figs. 1–8.
Diagnosis This speices is characterized by an asymmetric segminate or segmiplanate P1 element with a very narrow or rudimentary platform, which could extends to the posterior edge of
the element. A distinct rib through its length can be seen in
lateral view. The relatively high carina bears six to seventeen
denticles.
Remarks Orchard (1995) did not consider that the Ch. gongolelloides is an early growth stage, or junior synonym of Ch.
timorensis. He pointed out that the stratigraphic position of the
holotype of Ch. gongolelloides is beneath that of the first of Ch.
timorensis.
ACKNOWLEDGMENTS
This study was supported by 973 Program (No.
2011CB808800), the Natural Science Foundation of China
(Nos. 41172024, 41272044, 41402005), and the “111” project
(No. B08030), the ‘Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) and
the State Key Laboratory of Biogeology and Environmental
Geology, China University of Geosciences (No. GBL11202).
Thanks are also due to Xiaodan Liu and Zhiguo Li for their
assistance in field work. All SEM pictures were taken at the
State Key Laboratory of Geological Processes and Mineral
Resources (China). We thank two anonymous reviewers for
their helpful comments and constructive suggestions.
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