Precambrian Research 324 (2019) 1–17
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Precambrian Research
journal homepage: www.elsevier.com/locate/precamres
Repositioning the Great Unconformity at the southeastern margin of the
North China Craton
T
⁎
Bin Wana, , Qing Tangb, Ke Panga, Xiaopeng Wanga,c, Zhian Baod, Fanwei Menga,
⁎
Chuanming Zhoue, Xunlai Yuana,c,f, Hong Huad, Shuhai Xiaob,
a
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese
Academy of Sciences, Nanjing 210008, China
b
Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA
c
University of Chinese Academy of Sciences, Beijing 100049, China
d
State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, China
e
CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and
Paleoenvironment, Chinese Academy of Sciences, Nanjing 210008, China
f
Center for Research and Education on Biological Evolution and Environment, Nanjing University, Nanjing 210023, China
A R T I C LE I N FO
A B S T R A C T
Keywords:
Neoproterozoic
Great Unconformity
North China Craton (NCC)
Gouhou Formation
Detrital zircon geochronology
The Great Unconformity between the Precambrian basement rock and Cambrian sedimentary cover has been
extensively documented in Laurentia and may provide insights into the environmental context of the Cambrian
explosion. A similar unconformity has been known in the North China Craton (NCC) and it offers an opportunity
to constrain the geographic extent and geochronological magnitude of the Great Unconformity. However, the
placement and magnitude of the Great Unconformity in the NCC, particularly at the southeastern margin of the
NCC, is uncertain. For example, in the Huaibei region of the southeastern margin of the NCC, this unconformity
has been variously placed beneath or above the Gouhou Formation, which preserves critical micro- and macrofossil assemblages with great paleobiological and biostratigraphic significance. This uncertainty significantly
hampers our ability to fully appreciate the geological history of the NCC. To resolve this issue, we carried out an
integrated biostratigraphic, sedimentary petrological and detrital zircon geochronological investigation on the
Gouhou Formation and the overlying Houjiashan Formation in the Huaibei region. Our data suggest that the
Great Unconformity lies within the Gouhou Formation and represents a ca. 200–300 Myr depositional gap: the
Lower Member of the Gouhou Formation is of Tonian age whereas the Middle-Upper members of the Gouhou
Formation are of early Cambrian. Therefore, we propose that the Gouhou Formation be redefined to contain only
the Lower Member, and the Middle-Upper members be included in the Houjiashan Formation. Our analysis also
confirms that the Great Unconformity can be widely recognized along the southeastern margin of the NCC. The
recognition of the Great Unconformity in the Huaibei region helps to clarify the tectonic history, basin development, stratigraphic correlation, and paleogeographic reconstruction of the southeastern margin of the NCC
during the Tonian–Cambrian periods.
1. Introduction
In many parts of the world and particularly in Laurentia, the
Precambrian–Cambrian boundary is represented by a major unconformity—known as the ‘Great Unconformity’—between the
Precambrian basement rock and Cambrian sedimentary cover (Brasier
and Lindsay, 2001; Yochelson, 2006; Peters and Gaines, 2012;
Karlstrom et al., 2018). It has been argued that geological processes
responsible for the Great Unconformity also set the environmental stage
⁎
for the Cambrian explosion (Peters and Gaines, 2012). However, the
global nature of the Great Unconformity has not been documented yet
and hence it is uncertain whether the insights learned from Laurentia
can be extended globally.
The North China Craton (NCC) is one of the oldest cratons in the
world (Zhao et al., 2005), preserving critical geological and paleontological records of the early Earth. It has long been known that the
Precambrian-Cambrian boundary lies in an unconformity in the NCC
(Chen et al., 1980; Guan et al., 1988; Xiao et al., 1997, 2014; He et al.,
Corresponding authors.
E-mail addresses: binwan@nigpas.ac.cn (B. Wan), xiao@vt.edu (S. Xiao).
https://doi.org/10.1016/j.precamres.2019.01.014
Received 13 October 2018; Received in revised form 15 January 2019; Accepted 24 January 2019
Available online 25 January 2019
0301-9268/ © 2019 Elsevier B.V. All rights reserved.
Precambrian Research 324 (2019) 1–17
B. Wan et al.
Fig. 1. Geological maps. (A) Tectonic map of China showing major cratons and orogens. TC – Tarim Craton, NCC – North China Carton, SCC – South China Carton.
After Zhao et al. (2005). (B) Tectonic subdivision of the North China Craton. HBPO – Hidden basement in the Paleoproterozoic orogens, HBEB – Hidden basement in
the Eastern-Western blocks, EBJB – Exposed basement in the Eastern-Western blocks, EBKB – Exposed basement in the Khondalite Belt, EBEB – Exposed basement in
the Eastern-Western blocks, EBTCO – Exposed basement in the Tran-North China Orogen. Modified after Zhao et al. (2012) and Liu et al. (2017). (C) Geological map
of Huaibei region with section localities (stars).
paleo-weathering crust between the Cambrian Fujunshan Formation
and the Neoproterozoic Jingeryu Formation of the Qingbaikou Group
(Xiang, 1981).
However, the existence, placement, and magnitude of the Great
Unconformity at the southeastern margin of the NCC are uncertain,
because the age of the Gouhou Formation, which directly underlies
unambiguously Cambrian strata of the Houjiashan Formation, is a
matter of debate. The depositional age of the Gouhou Formation has
been variously interpreted as Tonian (Qian et al., 2001; Li et al., 2013),
2017). In this regard, the NCC offers an opportunity to constrain the
geographic extent and geochronological magnitude of the Great Unconformity. In the NCC, Proterozoic sedimentary packages, overlain by
early Cambrian strata, were deposited along the cratonic margins, e.g.,
the Jixian area at the northern margin, western Henan Province at the
southern margin, Liaoning and Shandong provinces at the northeastern
margin, and Huainan and Huaibei regions at the southeastern margin
(Xiao et al., 2014). The Great Unconformity is much more easily recognized at the northern margin of the NCC because of the presence of a
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southeastern margin of the NCC, about 100 km west of the Tan–Lu Fault
Zone (Fig. 1B). In this region, Mesoproterozoic(?)–Neoproterozoic and
lower Paleozoic strata expose widely forming the Xuzhou–Suzhou
Nappe (Wang et al., 1998), where early Neoproterozoic diabase dikes
(Wang et al., 2012) and Mesozoic granite complexes (Yang et al., 2008)
intrude the Neoproterozoic and Paleozoic strata, respectively (Fig. 1C).
Cryogenian–Ediacaran (Wang et al., 1984; Cao, 2000), Ediacaran–early
Cambrian (Xu, 1997), or early Cambrian (Xing, 1989; Zang and Walter,
1992). Recently, Xiao et al. (2014) and Tang et al. (2015) argued that
the Gouhou Formation is Tonian in age on the basis of biostratigraphic
and chemostratigraphic data, and they placed the Great Unconformity
between the Gouhou and Houjiashan formations. Significantly, the
Gouhou Formation contains organic-walled microfossils characteristic
of the Tonian Period (Yin, 1985; Qian et al., 2002b; Tang et al., 2015),
including the acanthomorphic acritarch Trachyhystrichosphaera that is
common in the Proterozoic but has never been known from the Cambrian (Tang et al., 2013, 2015; Baludikay et al., 2016). On the other
hand, He et al. (2017) argued that the Gouhou Formation was Cambrian in age because of the presence of Cambrian detrital zircons, and
they placed the Great Unconformity between the Gouhou Formation
and the underlying Jinshanzhai Formation. He et al. (2017) argued that
Trachyhystrichosphaera could have been reworked from older sediments. However, this is unlikely because reworked microfossils such as
organic-walled acritarchs are typically included in clasts, whereas
Trachyhystrichosphaera fossils in the Gouhou Formation reach over
100 μm in size and they are preserved in mudstone and fine-grained
siltstone with grain sizes much < 100 μm. Thus, the discrepancy in the
age of the Gouhou Formation and the placement of the Great Unconformity in the Huaibei region is a prominent problem. It hampers
our ability to evaluate the sedimentary history and paleogeographic
configuration of the NCC during the Precambrian–Cambrian transition,
and it impedes a full understanding of the paleobiological and biostratigraphic significance of the fossil assemblage preserved in the
Gouhou Formation.
To address this problem, we carried out an integrated biostratigraphic, sedimentological, and detrital zircon geochronological investigation on the Gouhou and Houjiashan formations in the Huaibei
region at the southeastern margin of the NCC. The Gouhou and
Jinshanzhai sections, which are the type sections of the Gouhou and
Jinshanzhai formations, respectively, were systematically measured
and sampled. Combined with published data, our study provides unambiguous evidence to constrain the placement and to scale the magnitude of the Great Unconformity in the Huaibei region. Our study show
that the Lower Member of the Gouhou Formation contains preCryogenian detrital zircons and Proterozoic microfossils such as
Trachyhystrichosphaera, the Middle Member includes Cambrian detrital
zircons, and the Upper Member contains bioturbated carbonates and
possible trace fossils. Thus, the Great Unconformity is placed within the
Gouhou Formation and represented by a conglomerate bed at the base
of the Middle Member. Guided by the new data, we propose that the
Gouhou Formation be redefined to include only the Lower Member and
that the Middle-Upper members be included in the Houjiashan
Formation. Our study presents another example of interdisciplinary
investigation to resolve the depositional age of Precambrian strata that
are largely poorly constrained or contradictorily interpreted (e.g., Xiao
et al., 2016).
2.2. Stratigraphic sequences
Meso-Neoproterozoic–lower Paleozoic successions are well exposed
in the Huaibei region, mainly including the Huaibei Group and overlying Cambrian strata (Regional Geological Survey Team of Jiangsu
Bureau of Geology, 1978; Wang et al., 1984). The Huaibei Group
consists of 13 lithostratigraphic formations, and is characterized by
siliciclastic rocks in the lower part (Lanling, Xinxing and Jushan formations), thick carbonates in the middle part (Jiayuan, Zhaowei,
Niyuan, Jiudingshan, Zhangqu and Weiji formations), and carbonates
intercalated with siliciclastics in the upper part (Shijia, Wangshan,
Jinshanzhai and Gouhou formations) (Xiao et al., 2014). The Jinshanzhai and Gouhou formations of the Huaibei Group, as well as the
lower Cambrian Houjiashan Formation, are relevant to the focus of this
study. These stratigraphic units are best exposed and accessible near the
Chulan to Langan towns in Suzhou, northern Anhui Province (Fig. 1C).
The Jinshanzhai Formation at its type section (i.e., the Jinshanzhai
section in Langan town of Suzhou county; 33°54.901′N, 117°17.005′E)
is about 23 m thick and consists of 0.7-meter-thick thin bed cherty
conglomerate at the base, 3-meter-thick greenish shale interbedded
with glauconitic quartz sandstone in the lower part, and 19-meter-thick
purple stromatolitic dolostone in the upper part (Fig. 2). The carbonaceous compression fossils Chuaria, Tawuia, and Sinosabellidites have
been reported from the lower shale beds (Wang et al., 1984; Qian et al.,
2009), and stromatolite forms such as Boxonia, Linella, and Xiejiella
from the upper dolostone (Qian et al., 2002a). The youngest detrital
zircons from quartz sandstone in the lower part are ∼820 Ma (Yang
et al., 2012; He et al., 2017). These paleontological and detrital zircon
geochronological data suggest an early Neoproterozoic age for the
Jinshanzhai Formation.
The Gouhou Formation at its type section (i.e., the Gouhou section
in Chulan town of Suzhou county; 33°59.563′N, 117°18.007′E) is about
120 m thick and consists of three lithological members (Figs. 2 and 3A)
(Heituwo-Lushan section in Regional Geological Survey Team of
Jiangsu Bureau of Geology, 1978; Langan section in Xiao et al., 2014).
The Lower Member is 44 m thick and consists of yellowish-greenish
shale/mudstone interbedded with thinly bedded quartz sandstone and
limonite lenses or concretions (Fig. 3B–C). Mudcracks and ripples are
common in sandstone interbeds. This member was described as Bed 15
in Regional Geological Survey Team of Jiangsu Bureau of Geology
(1978). The Middle Member is composed of 33-m-thick, thinly-bedded
reddish mudstone/siltstone interbedded with grey argillaceous limestone (Fig. 3D–E). Halite pseudomorphs are common in both siltstone
and limestone beds. This member was described as Bed 16 in Regional
Geological Survey Team of Jiangsu Bureau of Geology (1978). The
Upper Member consists of 38-m-thick, greyish thickly-bedded argillaceous limestone in the lower part (Fig. 3F) and mottled dolostone in
the upper part (Fig. 3G). This member was described as Beds 17–18 in
Regional Geological Survey Team of Jiangsu Bureau of Geology (1978).
In the Chulan area, the Jinshanzhai and Gouhou formations are in
conformable contact, with interbedded carbonates and siltstones at the
transition between these two formations. We note that the Gouhou
Formation at the Jinshanzhai section in the Jinshanzhai area is merely
∼50 m thick and only consists of the reddish Middle Member and
greyish Upper Member (Regional Geological Survey Team of Anhui
Bureau of Geology, 1977).
The lower Houjiashan Formation (Fig. 3H) was not measured in its
entirety. It begins with 5-m-thick, yellowish dolomitic siltstone and
silty dolostone at the base (Fig. 3I–J), overlain by 0.7-m-thick reddish
2. Geological setting
2.1. Tectonic background
The North China Craton (NCC) is a major tectonic unit in China
(Fig. 1A). The NCC can be divided into two Archean-Paleoproterozoic
blocks, the Eastern and Western blocks, which are separated by the
Paleoproterozoic Trans–North China Orogen (Fig. 1B). Further, the
Eastern Block contains two sub-blocks, the Longgang Block in the
northwest and the Nangrim Block in the southeast, which are separated
by the Paleoproterozoic orogenic Jiao–Liao–Ji Belt (Zhao et al., 2005,
2012; Liu et al., 2017). The boundary of the southeastern margin of the
Eastern Block is marked by the Tan (Tancheng)–Lu (Lujiang) Fault
Zone. The study area (Xuzhou area in northern Jiangsu Province and
Huaibei area in northeastern Anhui Province, China) is located at the
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Fig. 2. Stratigraphic columns of the Gouhou and Jinshanzhai sections in the Huaibei region, showing sample horizons (arrows) and lithostratigraphic sequence of the
Jinshanzhai, Gouhou, and lower Houjiashan formations.
carbonaceous compression fossils were collected from the Lower
Member greenish shale of the Gouhou Formation. Four slabs containing
trace fossils were collected from the Upper Member argillaceous limestone of the Gouhou Formation and the lower limestone of the
Houjiashan Formation. Numerous conical or tubular shelly fossils were
also collected from upper limestone of the Houjiashan Formation. Two
siltstone samples (15-LGGH-01 and 15-LGGH-02) were collected from
the basal yellowish dolomitic siltstone of the Houjiashan Formation for
organic-walled microfossil analysis using a low manipulation maceration technique (Butterfield et al., 1994; Tang et al., 2013). All illustrated specimens are reposited in Nanjing Institute of Geology and
Palaeontology (NIGPAS).
Petrographical samples were collected from the Gouhou and
Jinshanzhai sections. These include samples of the conglomerate bed at
the base of the Middle Member of the Gouhou Formation, halite
pseudomorphs from the Middle and Upper members of the Gouhou
Formation and the lower limestone of the Houjiashan Formation, and
limestone (Fig. 3K), and succeeded by 19-m-thick, thickly-bedded
mottled intrasparitic and intrasparruditic limestone with extensive
bioturbation (Fig. 3L). Chancellorid-like small shelly fossils (Xiao et al.,
2014), trilobites (Zhang and Zhu, 1979), and trace fossils (Miao, 2014)
have been reported from limestone unit of the Houjiashan Formation,
suggesting an early Cambrian age (Series 2, Stage 4, ca. 514–509 Ma).
Regional mapping shows that the Houjiashan Formation overlaps on
different formations of the Huaibei Group (from the Gouhou Formation
to the Jiayuan Formation) at different localities in the Huaibei regions,
suggesting that the basal contact of the Houjiashan Formation is an
unconformity, and this is supported by the presence of a basal conglomerate bed at some sections (Li et al., 2013).
3. Material and methods
All paleontological samples were collected from the Gouhou and
Houjiashan formations at the Gouhou section. About 100 macroscopic
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Fig. 3. Outcrop photos (all except C, E, and J) and thin section photomicrographs (C, E, and J) of the Gouhou Formation (A–G) and Houjiashan Formation (H–L) at
the Gouhou section in Chulan Town, Huaibei region. (A) Field view of the Gouhou Formation, showing the three lithological members. (B–C) Detrital zircon sample
16-GH-03 from the Lower Member, showing yellowish-green shale/mudstone interbedded with thinly-bedded sandstone (B; sample bag in upper right 22 cm wide)
and cross-polarized photomicrograph of sampled quartz sandstone (C). (D–E) Detrital zircon sample 16-GH-02 from the Middle Member, showing reddish mudstone
and siltstone interbedded with grey argillaceous limestone (D; field notebook 12.5 cm wide) and plane-polarized photomicrograph of sampled siltstone (E). (F–G)
Upper Member, showing grey thick-bedded argillaceous limestone in the lower part (F; scale bar in centimeters) and mottled dolostone in upper part (G; rock
hammer 27 cm long). (H) Field view of the Houjiashan Formation, with yellow lines bracketing a distinctive pink limestone. Rock hammer 27 cm long. (I–J) Samples
16-HJS-01 and 16-HJS-02 from the basal silty unit, showing yellowish dolomitic siltstone and silty dolostone at the base (I; field notebook 12.5 cm wide) and planepolarized photomicrograph of sampled dolomitic siltstone (J). (K–L) Pink limestone (K; lens cover 6.1 cm in diameter) overlain by grey mottled intrasparitic and
intrasparruditic limestone with extensive bioturbation (L; scale bar in centimeters). (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
detrital zircon geochronological analysis. Samples 16-HJS-01 and 16HJS-02 were collected from dolomitic siltstone in the basal Houjiashan
Formation, and they were combined into a single sample 16-HJS-01&02
for detrital zircon analysis because they are stratigraphically close
(Figs. 2 and 3I–J). Sample 16-GH-02 was collected from siltstone in the
mottled dolostone of the Upper Member of the Gouhou Formation, as
well as mottled limestone of the Houjiashan Formation. Petrographic
thin sections and polished slabs were prepared and investigated under
transmitted and reflected light microscopes.
Five siliciclastic samples were collected from the Gouhou section for
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abundant Cambrian fossils. Trace fossils (Fig. 4G) (Miao, 2014),
Chancellorid-like small shelly fossils (Xiao et al., 2014), conical shelly
fossils (Fig. 4H), and trilobites (Zhang and Zhu, 1979) have been reported from the limestone unit of the Houjiashan Formation. These
fossils collectively suggest an early Cambrian age (Cambrian Stage 4,
514–509 Ma).
lower part of the Middle Member of the Gouhou Formation (Figs. 2 and
3D–E). Samples 16-GH-03 and 15-LGGH-03 were collected from two
quartz sandstone beds in the middle and top of the Lower Member of
the Gouhou Formation (Figs. 2 and 3B–C). The samples were crushed
and processed using heavy liquid and magnetic methods to separate
zircons. Zircon grains were then randomly handpicked under a binocular microscopy, embedded in epoxy resin, and polished, and examined under reflected and transmitted light microscopes. Cathodoluminescence (CL) images of the zircons were acquired on a Quanta
400 FEG electron microscope equipped with Mono CL3+ (manufactured by Gatan Inc., USA), and were used to identify appropriate
sites for in-situ laser ablation analysis. U–Pb isotopic compositions of
zircons were analyzed on Agilent HP 7500 ICP-MS coupled to a 193 nm
ArF-excimer laser ablation system in the State Key Laboratory of Continental Dynamics and Department of Geology at Northwest University,
Xi’an. All analyses were carried out with a spot size of 30 μm, a laser
frequency at 6 Hz, and a total of 40 s ablation time. Zircons 91,500 and
GJ-1 (Yuan et al., 2004, 2008) and silicate glass NIST SRM 610 (Pearce
et al., 1997) were used as external standards, and 29Si as an internal
standard. Calculation of zircon isotope ratios and zircon trace elements
was performed using GLITTER 4.4 (Macquarie University), and zircon
ages were calculated using Isoplot 3.70 (Ludwig, 2008). The maximum
deposition ages were constrained by the youngest graphical age peak
controlled by more than one grain age (Dickinson and Gehrels, 2009) as
implemented in Isoplot.
4.2. Sedimentary petrology
4.2.1. Conglomerate bed
Our field investigation confirms the occurrence of a conglomerate
bed in the basal Middle Member of the Gouhou Formation at the
Gouhou section, which is correlated with the conglomerate bed at the
base of the Gouhou Formation at the Jinshanzhai section reported by
He et al. (2017).
At the Gouhou section, the conglomerate bed occurs in the basal
Middle Member of the Gouhou Formation, marking the boundary between the Lower and Middle members of the Gouhou Formation. This
boundary is a sharp contact between yellowish-greenish shale/mudstone of the Lower Member and reddish mudstone/siltstone of the
Middle Member (Fig. 5A). The conglomerate bed is 0.7 m thick, reddish
in color, and contains angular to sub-rounded granules and pebbles.
Within the conglomerate bed, the maximum clast size decreases upsection from pebbles (30–50 mm in diameter, Fig. 5B) to granules
(1–5 mm in diameter, Fig. 5C). Most clasts are fragments of quartz
sandstone (Fig. 5D), which are lithologically identical to and were
probably derived from quartz sandstone beds of the Lower Member
(Fig. 3C).
At the Jinshanzhai section, the conglomerate bed occurs at the base
of the Gouhou Formation, just above the Gouhou-Jingshanzhai
boundary (Fig. 5E). Here, the lower Gouhou Formation consists of
reddish mudstone interbedded with grey argillaceous limestone, which
is lithologically similar and probably equivalent to the Middle Member
at the Gouhou section. The conglomerate bed at Jinshanzhai is 0.3 m
thick, consisting of reddish matrix with small angular gravels
(Fig. 5F–G). Clasts in the conglomerate bed include limestone and
stromatolite fragments (Fig. 5H), which were likely derived from the
underlying Jinshanzhai Formation. Although the clasts in the conglomerate beds at the Gouhou and Jinshanzhai sections are mostly locally derived, they are likely correlated considering that the overlying
stratigraphy is similar at both sections, suggesting that the Lower
Member is missing at the Jinshanzhai section.
4. Results
4.1. Biostratigraphy
4.1.1. Gouhou Formation
Abundant macroscopic carbonaceous compression fossils such as
Chuaria and Tawuia are preserved in shales of the Lower Member
(Fig. 4A). In addition, 20 taxa of organic-walled microfossils have also
been reported from this member, including the acanthomorphic acritarch Trachyhystrichosphaera aimika with sparse processes (Fig. 4B), the
sphaeromorphic acritarch Valeria lophostriata with a concentrically
striated vesicle (Fig. 4C), and the herkomorphic acritarch Dictyosphaera
tacita with a reticulate vesicle (Fig. 4D) (Tang et al., 2015).
Organic-walled microfossils have also been reported from the
Middle Member, but they are dominated by the smooth-walled
Leiosphaeridia and no specimens of Trachyhystrichosphaera aimika,
Valeria lophostriata, or Dictyosphaera tacita have been found (Tang et al.,
2015).
Possible trace fossils were found for the first time from thin-bedded
argillaceous limestone in the lower part of the Upper Member. They are
horizontal tunnels preserved on the top bedding surface, ca. 2 mm wide,
and irregularly curved, with possible crossing-cutting relationship
(Fig. 4E). Most tunnels are preserved as positive epireliefs, whereas few
of them shown as negative furrows, suggesting that they are animal
burrows. In addition, the Upper Member contains beds of mottled dolostone, which is interpreted as bioturbated dolostone. The mottled
dolostone will be described in greater detail below under the section of
“Sedimentary petrology”.
4.2.2. Halite pseudomorphs
Halite pseudomorphs are present in both clastic and carbonate intervals of the Gouhou and Houjiashan formations at both the Gouhou
and Jinshanzhzai sections. They occur in reddish siltstone (Fig. 6A) and
interbedded grey argillaceous limestone (Fig. 6B) of the Middle
Member of the Gouhou Formation, greyish thin-bedded argillaceous
limestone and dolostone of the Upper Member of the Gouhou Formation (Fig. 6C), and greyish limestone of the lower Houjiashan Formation
(Fig. 6D). The halite pseudomorphs are cuboidal or hopper in shape
(Fig. 6C) and millimeters in size. They are mostly solitary, but occasionally form clusters (Fig. 6B).
4.2.3. Mottled fabrics
Mottled fabrics occur in dolostone of the uppermost Gouhou
Formation and limestone of the lower Houjiashan Formation. Similar
fabrics are common in lower Cambrian carbonates (e.g., the Fujunshan
and Zhushadong formations) in the NCC and are often interpreted as
bioturbation structures (Qi et al., 2014). This interpretation is followed
here.
Mottled limestone in the Houjiashan Formation shows an irregularly
disturbed appearance on weathered outcrops (Fig. 7A). In polished
slabs, the mottled limestone consists of irregular patches of variegated
structures (Fig. 7B–C). In thin sections, the mottled limestone consists
4.1.2. Houjiashan Formation
The yellowish dolomitic siltstone in the basal Houjiashan Formation
has long been overlooked by previous researchers or was placed in the
uppermost Gouhou Formation (Xiao et al., 2014). Two samples (15-LGGH-01 and 15-LG-GH-02) from this unit yielded organic-walled microfossils, including the sphaeromorph acritarch Leiosphaeridia tenuissima (Fig. 4F), which is characterized by thin, smooth-walled vesicles with a diameter of 75–141 μm. These organic-walled microfossils,
however, are not biostratigraphically diagnostic.
The overlying intrasparitic and intrasparruditic limestone of the
lower Houjiashan Formation is extensively bioturbated and bears
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Fig. 4. Fossils from the Gouhou and Houjiashan
formations at the Gouhou section in the Huaibei region. (A) Chuaria and Tawuia from the Lower
Member of the Gouhou Formation. (B–D)
Trachyhystrichosphaera aimika, Valeria lophostriata,
Dictyosphaera tacita, respectively, from the Lower
Member of the Gouhou Formation. After Tang et al.
(2015). (E) Possible trace fossils preserved on the top
bedding surface in the lower part of the Upper
Member of the Gouhou Formation, showing the irregularly curved burrows with possible crossingcutting relationship (arrow). (F) Leiosphaeridia tenuissima from the basal dolomitic siltstone of the
Houjiashan Formation. (G) Horizontal trace fossils
from the lower Houjiashan Formation. (H) Conical
shelly fossils from the lower Houjiashan Formation.
4.3. Detrital zircon U–Pb geochronology
of patches of opaque (organic?) material in a micritic matrix (Fig. 7D).
Mottled dolostone in the uppermost Gouhou Formation has fabrics
similar to those of mottled limestone of the Houjiashan Formation.
These mottled fabrics are mutually exclusive with the halite pseudomorphs. On outcrops, the sedimentary beds are disturbed and sometimes amalgamated (Fig. 7E). In polished slabs, the mottled dolostone
consists of variegated patches of dark-colored material (Fig. 7F–G). In
thin sections, the mottled dolostone is characterized by irregularly
shaped and dark-colored dolomicritic or muddy patches (Fig. 7H).
Stylolites are commonly present and often cross-cut the mottled fabrics
(Fig. 7F and H).
The similarity between the mottled fabrics in the Gouhou and
Houjiashan formations indicates that they have a similar origin, both
being bioturbation structures (He et al., 2017). The mutually exclusive
occurrence of mottled fabrics and halite pseudomorphs in the carbonate-rich beds of the upper Gouhou Formation is consistent with a
bioturbation origin, as bioturbators were probably excluded from hypersaline environments. Thus, the uppermost Gouhou Formation may
be of Cambrian age.
U–Th–Pb data of all analyzed detrital zircon grains are provided in
Supplementary Table 1. Two types of zircons with different age ranges
can be recognized in all samples. Most Neoproterozoic-early Cambrian
zircons are less rounded and have well-defined oscillatory zoning and
bright luminescence, whereas older zircons are more rounded and have
poorly-defined oscillatory zoning and darker luminescence (Fig. 8). Th/
U ratios are > 0.4 for most zircon grains of both age groups (Supplementary Table 1), indicating a magmatic origin. U–Pb ages of all
samples are plotted as concordia diagrams (Fig. 9) as well as histograms
with relative probability curves (Fig. 10). Samples from the Houjiashan
Formation (16-HJS-01&02) and the Middle Member of the Gouhou
Formation (16-GH-02) have similar age distribution patterns and the
youngest zircons are of early Cambrian in age. In contrast, age spectra
of samples from the Lower Member of the Gouhou Formation (15LGGH-03 and 16-GH-03) are dominated by a prominent peak at
920–930 Ma and lack any Cambrian grains.
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Fig. 5. The conglomerate bed in the Middle Member
of the Gouhou Formation at the Gouhou section
(A–D) and the Jinshanzhai section (E–H). (A)
Outcrop photo of the conglomerate bed at the base of
the Middle Member. Dotted yellow line marks the
boundary between the Lower and Middle members.
(B–C) Magnifications of labeled boxes in (A), showing
pebbles (white arrows) and the upsection decrease in
pebble size. (D) Plane-polarized photomicrograph of
the pebble marked by the left arrow in (B), showing
that it was probably derived from quartz sandstone in
the underlying Lower Member. Opaque minerals are
hematite cement. (E) Outcrop photo of the conglomerate bed at the base of the Middle Member of
the Gouhou Formation. The Lower Member is missing
here and the conglomerate bed directly rests on the
Jinshanzhai Formation, with the boundary marked
by dotted yellow line. (F–G) Magnifications of labeled boxes in (E), showing pebbles (white arrows)
and upsection decrease in pebble size. (H) Cross-polarized photomicrograph of the conglomerate bed
showing stromatolite fragments (black arrows) and
limestone clasts (white arrows) possibly sourced from
the underlying Jinshanzhai Formation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
article.)
4.3.2. Middle Member of the Gouhou Formation (16-GH-02)
Zircons from sample 16-GH-02 are very similar to 16-HJS-01&02 in
mineralogical characteristics. A total of 120 zircon grains were analyzed, and 107 analyses yield ages with a concordance of 90–110%. The
valid ages range from ca. 515 Ma to ca. 3400 Ma: two of them (1.9%)
are early Cambrian, 11 (10.2%) are Neoproterozoic, and 94 (87.9%) are
pre-Neoproterozoic. Th/U ratios range from 0.19 to 2.86, and most
zircon grains (89; 83.2%) have high Th/U ratios (> 0.4), typical of
igneous zircons. Similar to sample 16-HJS-01&02, the concordant zircons show five major age populations peaking at 524 Ma, 918 Ma,
1458 Ma, 1881 Ma, and 2502 Ma. The youngest single grain zircon age
is 515.4 ± 9.6 Ma and the youngest graphical age peak is 524 Ma,
suggesting a maximum depositional age of early Cambrian for the
Middle Member.
4.3.1. Houjiashan Formation (16-HJS-01&02)
Zircons from samples 16-HJS-01&02 are mainly subhedral to
rounded and have dark luminescence, although some are euhedral to
subhedral with brighter luminescence and oscillatory zoning. They are
typically < 50 μm in size, although a few are > 100 μm in size.
A total of 220 zircon grains were analyzed, but only 54 analyses
(24.5%) have a concordance of 90–110%, most likely because many
zircons are not large enough for LA-ICP-MS dating (Schaltegger et al.,
2015). The valid concordant ages range from ca. 510 Ma to ca.
2900 Ma: two of them (3.7%) are early Cambrian, 12 (22.2%) are
Neoproterozoic, and 40 (74.1%) are pre-Neoproterozoic. Th/U ratios
range from 0.18 to 1.83, and most zircon grains (10; 81.5%) have high
Th/U ratios (> 0.4), suggesting an igneous origin. The concordant
zircons show five major age populations peaking at 510 Ma, 945 Ma,
1641 Ma, 1886 Ma, and 2481 Ma. The youngest single grain zircon age
is 510.2 ± 3.31 Ma and the youngest graphical age peak is 510 Ma,
which constrains the maximum deposition age of the Houjiashan Formation.
4.3.3. Lower Member of the Gouhou Formation (15-LGGH-03 and 16-GH03)
Zircons from sample 15-LGGH-03 and 16-GH-03 are mostly euhedral to subhedral with bright luminescence and oscillatory zoning.
The Neoproterozoic zircons are 80–200 μm in size, whereas older
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Fig. 6. Halite pseudomorphs from the Gouhou and
Houjiashan formations at the Gouhou section in the
Huaibei region. (A–B) Halite pseudomorphs from
reddish siltstone and interbedded grey argillaceous
limestone of the Middle Member of the Gouhou
Formation. (C) Halite pseudomorphs from greyish
thin-bedded argillaceous limestone and dolostone of
the Upper Member of the Gouhou Formation. (D)
Halite pseudomorphs from greyish limestone of the
lower Houjiashan Formation.
zircons are smaller and 50–100 μm in size.
A total of 70 and 120 zircon grains were analyzed for 15-LGGH-03
and 16-GH-03, respectively, and 60 (85.7%) and 116 (96.7%) analyses
have a concordance of 90–110%. The two samples have nearly identical
age distributions, with most concordant ages in the Neoproterozoic
(151 of 176 analyses, 85.8%), although the concordant ages range from
ca. 830 Ma to ca. 2600 Ma. Th/U ratios range from 0.19 to 2.86, and
most zircon grains have high Th/U ratios (> 0.4) typical of igneous
zircons. The age spectra of concordant zircons are dominated by a
single population peaking at 928 Ma and 921 Ma for 15-LGGH-03 and
16-GH-03, respectively, with the youngest single grain zircon ages at
855.2 ± 9.27 Ma and 830.3 ± 10.06 Ma, constraining the maximum
depositional age of the Lower Member.
conglomerates can be correlated between the two sections and they
represent a basal conglomerate bed following a regional depositional
gap.
A regional depositional gap in the Gouhou Formation is consistent
with the presence of halite pseudomorphs in the Middle-Upper members of the Gouhou Formation and the lower Houjiashan Formation.
Halite pseudomorphs are indicative of evaporitic sedimentary environments (Schreiber and El Tabakh, 2000). Thus, the presence of
halite pseudomorphs in the Middle-Upper members of the Gouhou
Formation and the lower Houjiashan Formation indicates an evaporitic
and possibly restricted marginal marine depositional environment,
sharply different from the depositional environment represented by
greenish grey shales in the Lower Member of the Gouhou Formation.
The restricted evaporitic environment may also account for the general
lack of Cambrian marine shelly fossils in the Middle-Upper members of
the Gouhou Formation despite their Cambrian age.
Finally, regional correlation of halite pseudomorphs and mottled
fabrics indicates an early Cambrian age for the Middle-Upper members
of the Gouhou Formation. Halites and associate pseudomorphs are very
common sedimentary structures in the lower Cambrian successions in
the NCC. For example, halite pseudomorphs have been reported from
the lower Cambrian Zhushadong Formation in western Henan Province
and the Houjiashan Formation in the Huainan area of northern Anhui
Province (Zhu and Ma, 2008; Wang et al., 2013). Additionally, mottled
fabrics in the uppermost Gouhou Formation are indicative of moderate
bioturbation, and such fabrics are common in lower Cambrian carbonates in the NCC, for example, the Fujunshan (or Changping) Formation
at the northern margin of Yanshan area (Zhang et al., 1981) and the
Zhushadong Formation in western Henan Province (Qi et al., 2014).
Thus, regional stratigraphic correlation is consistent with an early
Cambrian age for the Middle-Upper members of the Gouhou Formation.
5. Discussion
5.1. Age of the Gouhou Formation
When considered together, our data indicate that there is a depositional gap between the Lower and Middle members of the Gouhou
Formation, and that the Lower Member is late Tonian in age and the
Middle-Upper members are early Cambrian in age. This interpretation
is supported by the following sedimentological, biostratigraphic, and
detrital zircon geochronological evidence.
5.1.1. Sedimentary petrological evidence
Sedimentary petrological investigation of the Gouhou and
Jinshanzhai sections reveals a depositional gap and sharp environmental change between the Lower Member and Middle-Upper members
of the Gouhou Formation, and suggests the Middle-Upper members of
the Gouhou Formation may be early Cambrian in age.
Of critical importance is the conglomerate bed in the Gouhou
Formation at both the Gouhou and Jinshanzhai sections. The conglomerate bed at the Jinshanzhai section has been previously reported
and was regarded as the base of the Gouhou Formation (He et al.,
2017). The conglomerate bed at the Gouhou section was briefly mentioned in published literature (Xu, 1997), but it has not been described
in detail and its significance has not been discussed. Our field investigation confirms the presence of the conglomerate bed at the type
section of the Gouhou Formation. Given that the conglomerates at the
Gouhou and Jinshanzhai sections are both immediately overlain by
reddish siltstone of the Middle Member, we consider that the
5.1.2. Biostratigraphic evidence
Current and previously published biostratigraphic data confirm that
the Lower Member and Middle-Upper members of the Gouhou
Formation are, respectively, Tonian and early Cambrian in age, with a
depositional gap in between.
The Lower Member of the Gouhou Formation contains the ChuariaTawuia macrofossil assemblage and the Trachyhystrichosphaera-ValeriaDictyosphaera microfossil assemblage. The Chuaria-Tawuia assemblage
is characteristic of the Proterozoic, and particularly Tonian, successions
(Hofmann and Rainbird, 1994; Steiner, 1996). Similarly,
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Fig. 7. Mottled limestone and dolostone from the
lower Houjiashan Formation (A–D) and the Upper
Member of the Gouhou Formation (E–H) at the
Gouhou section in the Huaibei region. (A) Outcrop
photo of mottled limestone. (B–C) Slabs cut perpendicular and parallel to bedding plane, respectively,
showing variegated pattern of mottled limestone. (D)
Cross-polarized photomicrograph of a vertical thin
section of mottled limestone. (E) Outcrop photo of
mottled dolostone. (F–G) Slabs cut perpendicular and
parallel to bedding plane, respectively, showing
variegated pattern and stylolite (arrow). (H) Crosspolarized photomicrograph of a vertical thin section,
showing dolomitic micrite and stylolite (arrow).
of the Gouhou Formation is Tonian in age and the limestone of the
Houjiashan Formation is Cambrian in age. Detrital zircon ages (see
below) also suggest that the Middle-Upper members of the Gouhou
Formation and the basal siltstone of the Houjiashan Formation are
Cambrian in age. The lack of characteristic Cambrian fossils in these
units is probably related to the evaporitic and restricted marginal
marine depositional environments as evidenced by the abundance of
halite pseudomorphs.
Trachyhystrichosphaera, Valeria, and Dictyosphaera are only found in the
Proterozoic. Importantly, T. aimika is restricted to the latest Mesoproterozoic and Tonian (Tang et al., 2013; Tang et al., 2015; Baludikay
et al., 2016). These fossil assemblages indicate that the Lower Member
of the Gouhou Formation is Proterozoic and probably Tonian in age.
The Middle and Upper members of the Gouhou Formation contain,
respectively, the sphaeromorphic acritarch Leiosphaeridia and possible
trace fossils. Leiosphaeridia has a long stratigraphic range from
Proterozoic to early Paleozoic (Jankauskas et al., 1989). Thus, it does
not provide any useful biostratigraphic constraint on the age of the
Middle Member. The possible trace fossils, together with the bioturbated mottled dolostone, in the Upper Member indicate a depositional
age no older than the terminal Ediacaran because relatively complex
animal burrows first appear in the terminal Ediacaran Period (Jensen
et al., 2006; Chen et al., 2013; Chen et al., 2018).
In the basal Houjiashan Formation, only Leiosphaeridia was found,
which has no biostratigraphic significance. Limestone of the lower
Houjiashan Formation contains trilobites, shelly fossils, and trace fossils, suggesting an early Cambrian age (Series 2, Stage 4).
In summary, biostratigraphic data indicate that the Lower Member
5.1.3. Detrital zircon geochronological evidence
U–Pb geochronology of detrital zircons has become a valuable approach to constrain maximum depositional ages, to characterize sediment provenances, and to identify stratigraphic gaps, particularly for
Precambrian strata where biostratigraphic constraints are limited (Fedo
et al., 2003; Jones et al., 2009; Yang et al., 2012; Xiao et al., 2016).
There are several strategies to use detrital zircon U–Pb ages to constrain
the maximum depositional ages, such as (a) the youngest single grain
age, (b) the youngest graphical age peak controlled by more than one
grain age, (c) the mean age of the youngest two or more grains that
overlap in age at 1σ, (d) the mean age of the youngest three or more
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Fig. 8. Representative CL images of detrital zircons from the Gouhou and Houjiashan formations at the Gouhou section. Early Cambrian and Neoproterozoic zircons
are more euhedral, show brighter luminescence, and display better preserved oscillatory zoning than older grains. Red circles mark the location and size of laser
ablation pits with analysis numbers. Detrital zircon U–Pb ages are noted below each grain (206Pb/238U ages for zircons < 1200 Ma and 207Pb/206Pb ages for older
grains). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The youngest graphical age peaks of detrital zircon samples from
the Middle Member of the Gouhou Formation at the Gouhou section are
524 Ma and 513 Ma (Fig. 11C–D). These are consistent with detrital
zircon samples (GH02/2014, JSZG2 and JSZG3) from the Gouhou
Formation at the Jinshanzhai section published in He et al. (2017)
(Fig. 11B). A youngest graphical age peak of 510 Ma is also found in a
detrital zircon sample from the basal Houjiashan Formation (Fig. 11A).
Together with biostratigraphic data from the limestone of the Houjiashan Formation, these detrital zircon data suggest that the MiddleUpper members of the Gouhou Formation and the basal Houjiashan
Formation are lower Cambrian deposits (Series 2, Stage 4, ca.
514–509 Ma), indicating that any depositional gap between the Gouhou
and Houjiashan formations (e.g., Li et al., 2013) would be minimal (< 5
Myr in duration).
Age distributional patterns of detrital zircon samples can also be
grains that overlap in age at 2σ (Dickinson and Gehrels, 2009). Here we
choose the youngest graphical age peak to constrain the maximum
deposition age. New and previously published detrital zircon geochronological data confirm and refine the age interpretations based on
biostratigraphic data as described above.
The youngest graphical age peaks of the two samples from the
Lower Member of the Gouhou Formation are 928 Ma and 921 Ma
(Fig. 11E–F), with the youngest single grain zircon ages of
855.2 ± 9.27 and 830.3 ± 10.0 Ma, respectively. Quartz sandstone
samples from the lower Jinshanzhai Formation give a youngest graphical age peak of 823 Ma and a youngest single grain zircon age of
800.5 ± 9.5 (Yang et al., 2012; He et al., 2017) (Fig. 11G). Together,
these data provide a maximum depositional age of 823 Ma for the
Lower Member of the Gouhou Formation, consistent with a Tonian age
based on biostratigraphic data.
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Fig. 9. U–Pb concordia plots for detrital zircon samples from the Gouhou and Houjiashan formations at the Gouhou section. Insets show Neoproterozoic-Cambrian
grains. N indicates the number of analyses with 90–110% concordance. Data ellipses represent one sigma errors.
Based on the sedimentological, biostratigraphic, and detrital zircon
geochronological data, the Middle-Upper members of the Gouhou
Formation are early Cambrian deposits (probably Series 2, Stage 4;
514–509 Ma). If so, then the negative δ13Ccarb excursion in the MiddleUpper members of the Gouhou Formation must be correlated with one
of the early Cambrian excursions. In this regard, it is intriguing that
negative δ13Ccarb values have also been reported from other early
Cambrian carbonates in the NCC, such as the lower Fujunshan or
Changping Formation (Xiao et al., 2017). We also note that the “AECE”
(Archaeocyathid Extinction Carbon isotope Excursion) or “ROECE”
(Redlichiid-Olenellid Extinction Carbon isotope Excursion) events may
be global-scale negative δ13Ccarb excursions in Cambrian Stage 4 strata
(Zhu et al., 2006; Fan et al., 2011), and can be potential correlatives of
the negative excursion in the Middle-Upper members. Alternatively, the
negative δ13Ccarb excursion in the Middle-Upper members of the
Gouhou Formation may represent a local event, given that these units
may have been deposited in restricted evaporitic environments. These
possibilities need to be tested in the future with tighter chronostratigraphic constraints.
used to determine sediment provenances (Fedo et al., 2003; Thomas,
2011). The age distributional patterns of detrital zircon samples from
the Middle Member of the Gouhou Formation are almost identical to
that of the Houjiashan Formation (Fig. 11A–D), but drastically different
from those of the Lower Member of the Gouhou Formation (Fig. 11E–F).
This difference indicates a fundamental change in sediment provenance
and is consistent with a significant depositional gap between the Lower
and Middle members. Also, the similarity in detrital zircon age distributions of samples from the Middle-Upper members of the Gouhou
Formation and the Houjiashan Formation indicates that they share a
similar sediment provenance, consistent with the inference that there is
no significant depositional gap between these units.
In summary, detrital zircon geochronological and paleontological
data suggest that the Lower Member of the Gouhou Formation is late
Tonian in age (< 823 Ma and > ca. 720 Ma), and the Middle-Upper
members are early Cambrian in age (< 512 Ma), with a depositional
gap of ca. 200–300 Myr in between.
5.1.4. Chemostratigraphic correlation
A negative δ13Ccarb excursion (down to –4‰) has been reported
from carbonate of the Middle-Upper members of the Gouhou Formation
(Xiao et al., 2014). This negative excursion was correlated with the
Bitter Spring anomaly in South Australia, which has been constrained
between 811.5 ± 0.3 Ma and 717.4 ± 0.1 Ma (Macdonald et al.,
2010). In light of the new detrital zircon ages and biostratigraphic data
presented in this paper, this correlation needs to be reconsidered.
5.2. Repositioning the Great Unconformity
5.2.1. The Great Unconformity at the southeastern margin of North China
Craton
Our data require us to reposition the Great Unconformity at the
Gouhou section and at the southeastern margin of the NCC. Previous
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Fig. 10. Relative probability plots and age histograms of detrital zircon samples from the Gouhou and Houjiashan formations at the Gouhou section. Graphical age
peaks are marked on the relative probability curves. Number of analyses with 90–110% concordance (n) is marked for each sample. 206Pb/238U ages are reported for
zircons < 1200 Ma and 207Pb/206Pb ages for older grains.
there is a significant unconformity between the Gouhou and Houjiashan
formations. The lower Cambrian Houjiashan Formation at many localities in the Huaibei and Huainan regions consists of the upper carbonate unit and the lower siliciclastic unit with a basal conglomerate
bed (Xu, 1958; Zhang et al., 1979; Qian et al., 2001; Li et al., 2013).
However, at the Gouhou section in Chulan area, no conglomerate occurs at the base of the Houjiashan Formation. Instead, the Middle
Member of Gouhou Formation is composed of siliciclastic rocks and has
a basal conglomerate bed, which may be correlated with the lower siliciclastic unit of the Cambrian Houjiashan Formation (Xu, 1997). In
studies placed the Great Unconformity in Huaibei region between the
Gouhou and Houjiashan formations (Xiao et al., 2014; Tang et al.,
2015) or between the Jinshanzhai and Gouhou formations (He et al.,
2017). In light of new biostratigraphic and detrital zircon geochronological data presented in this paper, the Great Unconformity in the
Huaibei region is repositioned within the Gouhou Formation, between
the Lower and Middle-Upper members. It is estimated that this unconformity represents ∼ 200–300 Myr of missing sedimentary record.
Li et al. (2013) demonstrated that the Houjiashan Formation regionally onlaps on different units of the Huaibei Group and argued that
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Fig. 11. Integrated litho-, bio-, and chronostratigraphy of the Neoproterozoic–early Cambrian succession in the Huaibei region. Stratigraphic column is the same as
that of the Gouhou section in Fig. 2, but the Gouhou Formation is redefined and the Houjiashan Formation is revised here so that the Great Unconformity sits at the
boundary between these two units. Biostratigraphic, sedimentary petrological, and detrital zircon U–Pb geochronocal data are marked against the stratigraphic
column. Detrital zircon panels B, C, and G are calculated from analyses with 90–110% concordance published in He et al. (2017) and Yang et al. (2012). 206Pb/238U
ages are reported for zircons < 1200 Ma and 207Pb/206Pb ages for older grains. Letters A–G to the right of stratocolumn mark the stratigraphic horizons of detrital
zircon samples and correspond to age spectra in panels A–G. Dashed line labeled B denotes the stratigraphic range of three samples from the Jinshanzhai section
published in He et al. (2017).
Getun formations (Yang, 1988; Hong et al., 1991; Zhang et al., 2006),
and between the ?Cambrian Getun and Tonian Xingmingcun formations
(Duan and An, 1994; Zhu and Ma, 2008; He et al., 2017). We tentatively
place the Great Unconformity between the Dalinzi and Getun formations. This placement is supported by the following evidence. First,
there is physical evidence for an unconformity [e.g., a conglomerate
bed (Chang, 1980)] between the Dalinzi and Getun formations. Second,
the Dalinzi Formation consists of reddish calcareous siltstone and argillaceous dolomite with abundant halite pseudomorphs (Zhu and Ma,
2008), similar to the Middle Member of the Gouhou Formation and the
Zhuashadong Formation, which are considered to be lower Cambrian
strata. In contrast, the Getun Formation yields the Chuaria–Tawuia assemblage (Hong and Liu, 1987; Hong et al., 1991), similar to the Tonian
Lower Member of the Gouhou Formation and Jinshanzhai Formation.
This placement needs to be tested, confirmed, or refuted with additional paleontological and geochronological data in the future.
At the northern margin of the NCC, the Great Unconformity is found
this scenario, Li et al.’s (2013) Gouhou-Houjiashan unconformity may
actually be the same as the Great Unconformity identified here within
the Gouhou Formation at the Gouhou section.
We note that the Great Unconformity can be widely recognized in
the Huaibei and Huainan regions (Li et al., 2013), and indeed along the
southeastern margin of the NCC. For example, this unconformity exists
between the early Cambrian Liguan Formation and the Tonian Shiwangzhuang Formation in Shandong Province (Yin, 1991; Chough
et al., 2010), and between the early Cambrian Shuidong Formation and
the Tonian Qinggouzi Formation in southern Jilin Province (Yue et al.,
1990). In eastern Liaoning Province, this unconformity exists somewhere between the unambiguously lower Cambrian Jianchang Formation and the unambiguously Tonian Xingmincun Formation, with the
uncertainty resulting from the unknown age of the intervening Getun
and Dalinzi formations. It has been proposed that a major unconformity
exists between the Cambrian Jianchang and ?Tonian Dalinzi formations
(Yao, 1979; Chang, 1980), between the ?Cambrian Dalinzi and ?Tonian
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et al., 2012; He et al., 2017; this paper), but also record a rifting event
that was related to the break-up of Rodinia and created a rift basin in
which Tonian sediments were deposited.
This rift basin extended along the southeastern margin of the NCC.
Tonian strata in the Huaibei and Huainan regions in Anhui Province, in
eastern Shandong Province, in the southern Liaoning Province, and in
the Pyongnam depression in North Korea may have all deposited in this
rift basin but had been subsequently offset by the Tanlu Fault. This
megabasin is sometimes known as the Jiaoliao-Xuhuai paleo-basin (Xu,
1997; Peng et al., 2011b). Tonian stratigraphic successions in the
Jiaoliao-Xuhuai paleo-basin, including the presence of the Great Unconformity, are generally similar (Xu, 1997; Cao, 2000; Xiao et al.,
2014).
In the Huaibei region, Tonian deposition continued until at least
823 Ma, the maximum age constraint of the redefined Gouhou
Formation. The redefined Gouhou Formation is thus far the youngest
known Tonian strata in the Huaibei region and it occurs only in the
Chulan area, where the thickest Tonian strata is recorded in the Huaibei
region. Thus, the Chulan area likely represents the depocenter of the rift
basin in the Huaibei region. After the deposition of the redefined
Gouhou Formation, a major regional uplift occurred, creating the Great
Unconformity in the Jiaoliao-Xuhuai paleo-basin. Cryogenian and
Ediacaran rocks are missing in most areas of the Jiaoliao-Xuhuai paleobasin, with the possible exception of Ediacaran glacial deposits (e.g.,
the Fengtai Formation) in the Huainan area (Xiao et al., 2014; He et al.,
2017). The tectonic underpinning of the Great unconformity in the
Jiaoliao-Xuhuai paleo-basin is unknown. Lu et al. (2008) proposed that
this unconformity was driven by eustatic sea-level, but it cannot be
ruled out that it is a far-field expression of an orogenic event or mantle
plume activities.
Renewed deposition was resumed in the early Cambrian Period,
when extensive transgression occurred, starting the deposition of the
Houjiashan Formation in the Huaibei region. Compared with the
Jinshanzhai and redefined Gouhou formations below the Great
Unconformity, the Houjiashan (Fig. 11A–D), as well as the Dalinzi
Formation in eastern Liaoning region (He et al., 2017), is not only
characterized by the early Cambrian aged (524–510 Ma) detrital zircons, but also contains ∼ 750–600 Ma detrital zircons suggesting a
Cryogenian–Ediacaran provenance. The Cryogenian–Ediacaran provenance may have derived from Gondwana, as the northern margin of
East Gondwana (e.g., northern India) was apparently affected by
Cryogenian magmatism (Van Lente et al., 2009; Rao et al., 2012; Meert
et al., 2013; de Wall et al., 2018). Additionally, samples from the
Houjiashan (Fig. 11A–D) and Dalinzi formations (He et al., 2017) share
strong similarities in detrital zircons age distribution with lower Cambrian strata of the Nagaur Sandstone of the Nagaur Group in northern
India (McKenzie et al., 2011b). These similarities confirm the close
affinity between the southeastern NCC and East Gondwana in the early
Cambrian Period when Gondwana was finally amalgamated (Li et al.,
2008; McKenzie et al., 2011a; Han et al., 2016).
One notable feature of the redefined Gouhou Formation is that the
U-Pb age distribution of its detrital zircons shows a single peak at
∼920 Ma, suggesting a single dominant provenance (Fig. 11E and F).
This is in sharp contrast to the multiple peaks in detrital zircon age
distributions of the underlying Jinshanzhai Formation and the overlying Houjiashan Formation. The difference between the redefined
Gouhou Formation and the Houjiashan Formation can be understood as
a reflection of sediment provenance change related to the Great Unconformity and associated tectonic activities. It is possible that another
unconformity may exist between the Jinshanzhai Formation and the
redefined Gouhou Formation. However, until convincing field observations are documented to support this unconformity, we here entertain the alternative possibility that the switch from multiple detrital
zircon age peaks in the Jinshanzhai Formation to a single dominant
peak in the redefined Gouhou Formation reflects dynamic sedimentary
processes in the Huaibei region during the Tonian Period. For example,
between the early Cambrian Changping (or Fujunshan) Formation and
the Tonian Jingeryu Formation in the Jixian area (Chen et al., 1980). At
the western and southwestern margins of the NCC, sporadic Ediacaran
deposits are found in some places (Shen et al., 2007; Dong et al., 2017),
but several unconformities are inferred to exist between Tonian and
Cambrian strata (Zhang et al., 1979; Zhu and Ma, 2008). Thus, the
Great Unconformity is widely recognizable in the NCC and in the
Huaibei region, it is constrained to be ∼ 200–300 Myr in duration.
5.2.2. Redefinition of the Gouhou Formation
Since the sedimentological and biostratigraphic features, as well as
the detrital zircon age spectra, are distinct between the Lower Member
and Middle-Upper members of the Gouhou Formation, it is no longer
appropriate to include all three members in the same lithostratigraphic
unit. We thus propose that the Gouhou Formation should include only
the Lower Member, and the Middle-Upper members at the Gouhou
section should be incorporated into the Houjiashan Formation (Fig. 11).
We choose to redefine the Gouhou Formation rather than moving
the Lower Member to the Jinshanzhai Formation as proposed by Xu
(1997), because the Lower Member and the Jinshanzhai Formation
seem to have distinct detrital zircon age distributions (Fig. 11E–G) and
the possibility of a depositional gap between them. In addition, although the Chuaria–Tawuia assemblage has been reported from both
the Lower Member and the Jinshanzhai Formation (Qian et al., 2009;
Tang et al., 2015), the Lower Member is characterized by the acritarch
Trachyhystrichosphaera-Valeria-Dictyosphaera assemblage (Xiao et al.,
2014; Tang et al., 2015) whereas the Jinshanzhai Formation is best
known for its beautifully preserved stromatolites (Qian et al., 2002a).
In contrast, the decision to incorporate the Middle-Upper members
of the Gouhou Formation in the Houjiashan Formation is based on their
overall similarities. Both units are lower Cambrian deposits (Series 2,
Stage 4), are characterized by similar detrital zircon age distributions,
and contain halite psuedomorphs. Importantly, the Houjiashan
Formation at its type section (i.e., Houjiashan section in the Huainan
region) is composed of a lower shale unit of the Yutaishan Member and
an upper carbonate unit of the Baiheshan Member (Xu, 1958; Zhang
et al., 1979). Thus, the Middle Member of the Gouhou Formation at the
Gouhou section can be regarded as equivalent to the Yutaishan Member
of the Houjiashan Formation at its type section. Indeed, Xu (1997)
made a similar proposal on the basis of lithostratigraphy and depositional environments to merge the Middle-Upper members of the
Gouhou Formation into the Houjiashan Formation. Similar to its type
section, the revised Houjiashan Formation at the Gouhou section consists of a lower unit of reddish mudstone (the Yutaishan Member) and
an upper unit of grey carbonate rock (the Baiheshan Member).
5.3. Sedimentary and tectonic implications
The new biostratigraphic and detrital zircon geochronological data
presented in this paper allow us to piece together the sedimentary and
tectonic history of the southeastern margin of the NCC.
It has been recently recognized that the NCC participated in the
assembly and break-up of the Rodinia supercontinent (Peng et al.,
2002). Most of the detrital zircons from the redefined Gouhou Formation are around 920 Ma and have a magmatic origin, suggesting a
dominant provenance of early Neoproterozoic igneous rocks in the
Huaibei region. Widespread mafic sills and dikes around 890–920 Ma
have been reported in the southeastern NCC (Liu et al., 2006; Peng
et al., 2011a; Wang et al., 2012; Zhai et al., 2015; Zhang et al., 2016);
Zhu et al. (2019) recently reported 913 +/– 10 Ma sills that purportedly intrude the Gouhou Formation, but their geological map shows
that these sills only intrude in the Shijia and Wangshan formations that
underlie the Jinshanzhai Formation. These sills, dikes, and related
magmatic rocks not only provide a source of Tonian zircons that are
common in Tonian siliciclastic rocks in the Huaibei Group of the
Huaibei region and Jinxian Group of the eastern Liaoning area (Yang
15
Precambrian Research 324 (2019) 1–17
B. Wan et al.
it is possible that hydrographic changes may have occurred between
these two formations, resulting in a switch in sediment source. In this
regard, it is interesting to note that major changes in sediment provenance has also been reported from the early Cambrian Sixtymile Formation in the Grand Canyon (Karlstrom et al., 2018). As shown in
Karlstrom et al. (2018), during the Cambrian Sauk transgression, their
detrital zircon samples A–B from the basal Sixtymile Formation are
characterized by a prominent peak at ∼1080 Ma whereas samples C–E
from the same formation has multiple peaks, even though no major
unconformity is apparent in this formation. This highlights the point
that differences in detrital zircon age distribution alone are not sufficient evidence for the existence of a depositional gap.
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6. Conclusions
This study provides a refined chronostratigraphic framework for the
upper Huaibei Group at the southeastern margin of the North China
Craton. Available biostratigraphic, sedimentological, and detrital zircon
geochronological data suggest a ca. 200–300 Myr depositional gap (i.e.,
the Great Unconformity) within the Gouhou Formation: the Lower
Member is of Tonian age and the Middle-Upper members are of early
Cambrian. We propose that the Gouhou Formation be redefined to
contain only the Lower Member, and the Middle-Upper members be
included in the Houjiashan Formation. Thus, the Great Unconformity is
placed between the redefined Gouhou and Houjiashan formations in the
Huaibei region, and it can be widely recognized along the southeastern
margin of the NCC. Age distributions of detrital zircons provide critical
insights into changes in sediment provenance related to the break-up of
Rodinia and the final assembly of Gondwana. The recognition of the
Great Unconformity in the Huaibei region helps to clarify the tectonic
history, basin development, stratigraphic correlation, and paleogeographic reconstruction of the southeastern margin of the NCC during
the Tonian–Cambrian periods.
Acknowledgements
This work was supported by grants from Chinese Academy of
Sciences (XDB26000000, QYZDJ–SSW–DQC009), National Natural
Science Foundation of China (41502010, 41602007, 4151101015, and
41672025), Jiangsu Provincial Department of Science and Technology
(BK20161615 and BK20161090), NASA Exobiology and Evolutionary
Biology Program and National Science Foundation (EAR-1528553). We
are grateful to Jianhua Li of the Geological Survey of Jiangsu Province
for field assistance, to Hujun Gong and Xinyi Ren of Northwest
University for technical support, and to Dr. Santosh K. Pandey, Dr.
Tianchen He, and an anonymous reviewer for useful comments.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.precamres.2019.01.014.
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