Journal of Earth Science, Vol. 25, No. 2, p. 241–251, April 2014
Printed in China
DOI: 10.1007/s12583-014-0418-6
ISSN 1674-487X
Cretaceous Deepwater Lacustrine Dedimentary
Sequences from the Northernmost South China Block,
Qingdao, China
Tuoyu Wu1, Yongtao Fu*2
1. Department of Earth and Environmental Sciences, University of Winsor N9B 3P4, Canada
2. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences,
Qingdao 266071, China
ABSTRACT: A sequence of terrigenous siliciclastic rocks crop out at Baxiandun, Qingdao, near the
Mesozoic collisional boundary between North China block (NCB) and South China block (SCB).
These low-grade metamorphosed siliciclastic rocks are dominated by greywacke with shale, manganiferous fine-grained sandstone, arkose and conglomerate layers. There are two basic interpretations about the formation of these rocks. One considered that this sequence was formed within NCB,
and is part of the Cretaceous Laiyang Group and Qingshan Group fluvial facies and volcanic debris
facies, as shown on the Shandong Regional Geological Map. Another opinion suggested that these
rocks represent turbidity depositional systems in the slope and the basin facies was mainly deposited
in Ordovician. Based on field observation, petrological analysis, and most importantly, geochemical
results in this study, the sedimentary strata at Baxiandun Section mainly consist of siltstone, sandstone and mudstone lithologies. They are dominated by deepwater debris and turbidity deposits in
the slope and base of a lake. The U-Pb detrital zircon dating of the rocks at the Baxiandun Section
indicates that the source rocks are very complex and their ages are varied from Archean to Early
Cretaceous. The youngest age of the terrigenous detritus could represent the age of the sedimentary
strata. Therefore, we infer that the sedimentary rocks belong to Early Cretaceous deepwater lacustrine sedimentary sequences and have multiple sources origined from the erosion of the Sulu UHP
orogen and South China block margin.
KEY WORDS: Cretaceous, turbidite, deepwater lacustrine, detrital zircon, South China block.
1
INTRODUCTION
Deepwater lacustrine deposits have been studied in the
Cretaceous Songliao Basin and Eogene Bohaiwan Basin because of their importance as a potential oil reservoir (Peng,
2011; Deng, 2009; Wang et al., 2009). However, few deepwater lacustrine sequences have exposed to topographic surface in these two sedimentary basins. Comparatively, a deepwater sedimentary sequence of terrigenous siliciclastic rocks
within the Sulu UHP terrane (Fig. 1a) provided good example
to study deepwater lake sedimentation and record important
information on regional tectonic evolution (Fig. 1b).
The outcrops of the sedimentary sequence cover an area
of 4 km2 at Baxiandun, about 20 km east to downtown
Qingdao (Fig. 1c). For the convenience of clarifying the geological properties of the sedimentary strata distributed in
Baxiandun, it is temporally called Baxiandun strata in this
*Corresponding author: [email protected]
© China University of Geosciences and Springer-Verlag
Berlin Heidelberg 2014
Manuscript received September 1, 2013.
Manuscript accepted January 3, 2014.
paper. The Geological Bureau of Shandong Province (1991)
considered the strata are fluvial facies and volcanic debris
facies which belong to Cretaceous Laiyang Group or Qingshan Group; whereas Fu and Yu (2010) argued that these
rocks represent Ordovician turbidity deposits. In their opinions,
these sedimentary sequences can be compared to the Ordovician Yuqian and Changwu groups exposed in the northwestern
part of Zhejiang Province (Zhang et al., 1982). The author
primarily agreed with Fu and Yu’s (2010) argument, and further noted that it should correspond to the underlain Ordovician sequence in South Yellow Sea Basin (Wu et al., 2010).
However, after revisiting the field area and pertinent geochronological data coming out, the author found that the
sedimentary environment and formation age of the Baxiandun
strata need to be redefined.
The purpose of this paper is to clarify sedimentary facies
and geochronology of the strata based on our field sedimentary observation and the detrital zircon testing result of the
sedimentary rocks at the Baxiandun section. This work will be
helpful to understand the deepwater sedimentation in the lake
and thus provide constraints on the tectonic evolution of the
boundary between the North China Block (NCB) and South
China Block (SCB).
Wu, T. Y., Fu, Y. T., 2014. Cretaceous Deepwater Lacustrine Sedimentary Sequences from the Northernmost South China Block,
Qingdao, China. Journal of Earth Science, 25(2): 241–251, doi:10.1007/s12583-014-0418-6 Tuoyu Wu and Yongtao Fu
(UHP) metamorphism occurring at ca. 240–225 Ma (Zheng,
2008). Because of the collision, the Jiaonan Group is strongly
metamorphosed, forming coesite-bearing eclogites preserved
at Yangkou (Wang et al., 2010; Liou and Zhang, 1996; Ye et
al., 1996), which is about 10 km north to the Baxiandun Section (Fig. 1c). Afterwards, the retrograde metamorphism occurred in this Group, and the current metamorphic degree
observed within the rocks of Jiaonan Group is lower amphibolite facies. At about 190 Ma, exhumation of the upper crust
brought the metamorphic rocks to the surface (Geological
2
REGIONAL GEOLOGICAL BACKGROUNDS
The Baxiandun Section is located in the southeastern
Sulu orogen which is the collision boundary between SCB and
NCB (Wang et al., 2010; Fig. 1a). The basement of the section
is mainly composed of the Archean to Early Proterozoic
Jiaonan metamorphic rocks (Fig. 2), which mainly deposited
at about 1.8 billion years ago (Geological Bureau of Shandong
Province, 1991). Owing to the collision taking place in the
Late Triassic, the Jiaonan Group was involved in the subduction from SCB to NCB, which resulted in ultrahigh-pressure
125°E
120°
115ºE
(a)
(b)
360 km
Bohai
Bay
Nort
Yantai
low
h Ye l
40ºN
0
asi
Sea B
North China block
Fig. 1b
Dabie UHP terrane
m
36°
an
e
Qi
o ro g e n ic
JiashanXiangshui
t
b e l fault
lt 3
be
Qingdao
lt
be
t
c
i
lif
ph
up
South Yellow Sea
or
m
Northern Basin
ta
-
ina
Ba
lu
Su
Cretaceous pluton
Jurassic pluton
South China block
Early Paleozoic UHP belt
overprinted by Mesozoic UHP belt
Pre-Mesozoic craton
Triassic pluton
UHP metamorphic
rock
0
120°30 '
+
120°45 '
+
+
+
+
+
+ +
+
Baxiandun
Jiaozhou
Bay
+
+
Yellow Sea
+
0
120°15 '
+
120°30 '
+
Monzonitic granite
Alkaline granite
porphyry
Baxiandun strata
Sinian gneissic aegirine bearing alkaline granite
Neoproterozoic gneissic
two - mica monzonitic
granite
Mesoproterozoic
serpentinized peridotite
Geological symbols
+
Qingdao
1
Quaternary cover
Cretaceous Qingshan
and Wangshi groups
Syenogranite
Alkaline granite
Wanggezhuang
Yangkou
100 km
Fault
Tan-Lu fault
2 Wulian-QingdaoWeihai fault
3 Haizhou-Siyang fault
4 Jiashan-Xiangshui
fault
Geological units
Cenozoic cover
Cretaceous
sedimentary rock
Proterozoic basement
Archean basement
(c)
120°15 ' E
36°20 ' N
Yellow
Sea
4
liy
Qinling UHP terrane
Ch
ai
Laiyang
sin
an
Fig. 1c
tra
al
l
ao
ul
ntr
Ji
Qingdao
30ºN
Ce
Korea
2
1
Sulu UHP terrane
Wulian-QingdaoWeihai fault
Tanlu fault
n
36°N
242
10 km
+ + +
+ +
Fine grained texture
Medium grained texture
Mafic dyke
Eclogite
Phenocryst
bearing texture
Fault
120°45 ' E
36°N
+
Figure 1. (a) Tectonic sketch of East China (after Xu et al., 2009); (b) simplified geological map of eastern Shandong Province (After Xie et al., 2012; Fu and Yu, 2010; Ma, 2002); (c) regional geological map of Qingdao and its periphery (after
Wang et al., 2010; Geological Bureau of Shandong province, 1991).
Cretaceous Deepwater Lacustrine Sedimentary Sequences from the Northernmost South China Block
Bureau of Shandong Province, 1991).
Qingdao and its vicinity have been mapped as the east
margin of Mesozoic Jiaolai Basin (Fig. 1b) (Zhang et al.,
2008). The evolution of the basin can be divided into three
stages: (1) Early Cretaceous (120–135 Ma) Laiyang stage
extensional faulted basin, where river and lacustrine facies
deposited; (2) Early Cretaceous Qingshan stage (106–120 Ma)
continental rift basin, where four eruptive cycles of volcanic
Chrono-stratigraphic unit
Era
Period
Cenozoic
Neogene
Mesozoic
Proterozoic
Rock unit
(thickness)
Cretaceous
Stratigraphic
column
Quaternary
cover
(8 – 15 m)
Gray greenish gray clay bearing sandy
gravel, silt, sand sediments
Wangshi
Group
(1 000 –
3 000 m)
Mainly red conglomerate, glutinite,
sandstone, siltstone and mudstone,
interbedded by graynish green
siltstone and mudstone
Qingshan
Group
(1 000 –
3 000 m)
A formation of volcanic eruption
volcanic tuffaceous conglomerate,
siltstone, arkose, lithic sandstone,
agglomerate breccias, rhyolitic
tuff, etc.
N
N
N
Jiaonan
Group
(>5 527 m)
K
N
N
N
N
K
N
N
N
K
N
N
K
N
N
N
N
N
Neoarchean
N
K
K
N
K
N
Clay
Gravel
Sand
Arkose
Conglomerate
Breccia
Glutinite
Lithicsandstone
Sandstone
Siltstone
Shale
N
Biotiteplagioleptynite
Quartzite
Marble
Micaleptynite
N
Biotiteplagiogneiss
Leucolepynite
Biotiteleptynite
Mica
monzonitic
gneiss
Unconformity
Silt
Mudstone
N
N
N
N
L4: mica schist, mica leptynite,
leucoleptynite, interbedding
marble and quartzite;
L3: biotite leptynite, leucoleptynite,
interbedding biotite schist, marble,
muscovite leptynite;
L2: mica monozonitic gneiss, biotiteplagiogneiss, leucoleptynite, biotiteschist;
L1: biotite-plagiogneiss, biotite
plagioleptynite gneiss, biotite
monozonitic gneiss, interbedding
amphibolite, leucoleptynite and
marble
N
N
N
Lacustrine deposits of gray and
yellownish green sandstone, shale,
conglomerate, dark siltstone;
fluvial deposits of brown, yellow,
green, grayish yellow sandstone,
pebbly sandstone, siltstone and shale
N
N
N
N
Laiyang
Group
(1 600 –
2 300 m)
N
N
Stratigraphic description
Paleoproterozoic
Tuffaceousconglomerate
Tuff
Biotiteschist
K
N
K
N
Biotite
monzonitic
gneiss
N
N
N
N
K
N
N
N
K
N
243
deposits developed; (3) Late Cretaceous (65–88 Ma) Wangshi
stage dextral transtensional pull-apart basin, where a river face
red clastic rocks featuring arid climate formed (Zhang et al.,
2003; Lu and Dai, 1994; Geological Bureau of Shandong
Province, 1991). Accordingly, Laiyang Group, Qingshan
Group and Wangshi Group have been built up to characterize
the difference in sedimentation of Jiaolai Basin (Fig. 2). It was
observed that each group is separated by unconformity, and
N
Archean
Muscovite
monzonitic
gneiss
Amphibolite
K
N
K
N
Muscovite
leptynite
Figure 2. Lithostratigraphy of Qingdao and its periphery (after Li et al., 2008; Geological Bureau of Shandong Province,
1991; Zhang and Liu, 1991).
Tuoyu Wu and Yongtao Fu
244
sedimentary layers dip very gently to the southeast (Fig. 3). Fu
and Yu (2010) distinguished the lower and middle sequences as
coarse to medium grained clastic rocks bearing turbidities. Here
we focus on description of the upper sequence in the Baxiandun
Section (Figs. 4, 5, 6). The geomorphology of the exposed
strata consists of fault scarp, bench platform and sea cliff (Figs.
4, 5, 6), plus isolated outcrops on surrounding hills. In the upper sequence it can be divided into nine layers based on their
colors, sedimentary structures and lithological characters. The
characteristics of each layer are described as follow.
Layer 1 occurs at a cliff 4 to 6 m above sea level (Fig. 4a).
The layer is dominated by interbedded thick quartzose arkose
and nodule or band shaped chert bearing purple mudstone.
Weak metamorphism and silification occur in this layer (Fig.
4b).
Layer 2 has a thickness of 3.3 m, and is dominated by interbedded thick quartzose greywacke and nodule or band
shaped chert bearing purple mudstone (Fig. 4c). The base of
this layer shows well-developed ripple cross bedding (Fig. 4d)
and typical Bouma sequence (Ta–Td). Ta is characterized by
massive or normally size-graded, sandy Bouma Ta division; Tb
is a representative of parallel laminated, sandy Bouma Tb division; Tc reveals ripple/climbing-ripple laminated/convoluted,
sandy Bouma Tc division; Td is mainly constituted of parallel
laminated to massive siltstone, which can be compared to
Bouma Td division (Fig. 4d).
Layer 3 consists of interbedded gray thick quartz sandstone and black shale in the lower part of the section. The layer
has well developed ripple cross bedding (Fig. 5a). Layer 4 consists of interbedded gray middle to thin sandstone beds and
quartzite. The layer shows well developed hummocky cross
bedding in the gray quartzite (Fig. 5b). A conjugate joint set
with the representative attitudes of 89°∠85°, and 280°∠68°
are commonly present in the whole layer, reflecting domination
of E-W compressional stress field.
the whole basin is bounded by strike-slip faults (e.g.,
Wulian-Weihai-Qingdao fault on the southeast, Tanlu fault on
the west) (Fig. 1b). The basement of JiaolaiBasin includes both
Sulu UHP terrane and NCB, and its main source was suspected
to be Sulu orogen (Gu et al., 1996). Ever since Tertiary, the
Jiaolai Basin has been progressively uplifted and shrunk, so
only local sedimentary deposition recorded within this area.
Magmatic activity occurred frequently from Archean to
Cenozoic in the Shandong Peninsula. The most widespread
intrusions are separately Neoproterozoic granitic gneiss
(700–800 Ma) (Xue et al., 2006) and Mesozoic granites. The
Mesozoic granites mainly includes Late Triassic M-type granite
(225–205 Ma), Late Jurassic S-type granite (160–150 Ma), and
Early Cretaceous I-A type granite (130–105 Ma) (compiled by
Zhang and Zhang, 2007). In Qingdao and its proximal area, the
intrusive rocks are dominated by Laoshan granite, a set of
quartz monozite, biotite monozitic granite, syenogranite
(calc-alkaline) and alkali granite (alkaline) rock suites with
U-Pb zircon of Early Cretaceous (146–110 Ma) (Zhao et al.,
1997).
The strata preserved in Qingdao and adjacent areas are illustrated in Fig. 2. The basement of the strata is formed by the
Jiaonan Group metamorphic rocks. Subsequently, Cretaceous
Laiyang Group, Qingshan Group, and Wangshi Group deposited in succession. Due to the intrusion of Laoshan granite and
subsequent uplift of Jiaolai Basin, Tertiary deposits are lacking
in these areas and Quaternary sediments are locally preserved
(Geological Bureau of Shandong Province, 1991; Zhang and
Liu, 1991).
3 DESCRIPTIONS OF THE SEDIMENTARY ROCKS
3.1 Description of the Baxiandun Section
The strata in the Baxiandun Section preserve a finingupward sequence (Fu and Yu, 2010). The sedimentary rocks
vary from pebble and grit stone at lower sequence to massive
sandstone, siltstone and mudstone at top of the section. The
120°
1 000 m
0
A
Yakou
B
Abandoned
barrack
Baxiandun
C
C
N
C
N
C
N
N
N
N
N
N
Mn
N
N
Mn
N
160° 30°
Qingshan Bay
Breakwater
A
Yakou
Baxiandun
0
B
1 km
Mn
N
Si
Mn
N
110° 12°
N
Siltstone
Si
Si
Si
Si
Si
Si
Siliceous
shale
Mudstone
Greywacke
Sandstone
Lithicsandstone
Conglomerate
Mn
Mn
Mn
N
N
Mn
N
4°
Mn enriched
greywacke
Granite
Arkose
Thrust
N
N
N
Si
Mn
110°
Quartz
sandstone
Si
Si
N
Mn
N
Touchstone
Bay
Abandoned
barrack
N
N
N
Figure 3. Geological profile from Yakou to Baxiandun. Inset map indicates the positions of profile and the Baxiandun Section.
Cretaceous Deepwater Lacustrine Sedimentary Sequences from the Northernmost South China Block
(a)
245
(b)
(c)
(d)
Figure 4. Field photos of siliciclastic rocks at the Baxiandun Section. (a) The distant view of the Layer 1, 111°∠12°; (b) pebbly quartzite with fine-grain graded bed upward at upper sequence of the Layer 1; (c) the medium range view of the Layer 2,
111°∠12°; (d) well developed Bouma sequence (Ta–Td) in the Layer 2.
(a)
40 m
0
Layer 9
Layer 8
Layer 7
Layer 6
60 cm
Layer 5
(b)
Figure 5. (a) Medium range view of the Layer 4; (b) hummocky cross bedding layer in the gray quartzite. Two
groups of conjugate joints are well developed in the Layer 4.
Selective measured attitude of the joints: 80°∠85°;
280°∠75°; 281°∠65°; 285°∠61°.
Figure 6. Sedimentary sequences in the Baxiandun cliff.
246
Tuoyu Wu and Yongtao Fu
Layers 5, 6, 7, 8 and 9 crop out clearly on the Baxiandun
cliff, which is about 160 m in height (Fig. 6). The sequence is
generally consisted of interbedded gray middle to thin quartz
sandstone and thin black or tawny siliceous mudstone or shale
smaller upwards (Fig. 6). Among all these layers, Layer 5
shows well-developed ripple cross bedding and Bouma sequences at base. The well developed Bouma sequences include
bands. The grain size of the strata becomes progressively Ta to
Te, especially Tc. Tc has ripple/climbing-ripple laminated/convoluted, sandy Bouma Tc divisions. Conjugate joints
with gentle mode are pervasive in the whole sequence of strata.
4 INTERPRETATION OF SEDIMENTARY FACIES
AND ENVIRONMENT
4.1 Field Interpretation
Deepwater lacustrine systems are characterized by turbidity
and debris flow deposits (Weimer and Link, 1991). The sedimentation in most of them tends to be fine-grained sequences.
The deepwater sedimentary facies in some Tertiary lacustrine
basins are gravel-rich, turbidity deposits (Sun et al., 2007; Yan et
al., 2005; Zhao et al, 2005). The thickness of sequences in balanced-fill lakes developed when the rate of sediment and water
supply are equal to potential accommodation throughout time.
Water inflow is periodic, and can match outflow through time,
though there is considerable fluctuation. According to the observation in the Baxiandun Section, all evidences imply that the
sedimentary sequence was formed by deepwater turbidity sedimentation. The sedimentary rocks are dominated by the dense
interbedded quartzose sandstone and chert enriched mudstone or
shale. There are few fossils in the sedimentary layers. Sedimentary structures are widely distributed and dominated by Bouma
sequences and ripple cross bedding, and graded bedding in Layer
1 to Layer 5. According to the paleogeography in East China, we
infer that the sequences could be deepwater lake deposits.
4.2
Petrogeochemical Features of the Rock at Baxiandun
The major and minor elements of the rocks at Baxiandun
have been analyzed in Fu and Yu (2010) and Wu et al.’s (2010)
papers. According to the analyses, the collected samples are
mainly shale, greywacke, and arkose. High ∑REE content
(146.75–245.78 ppm), high La content (30.35–61.39 ppm), and
high value of (La/Yb)N(Cl) (8.68–21.89 ppm) indicates that the
characteristics of passive continental margin (Wu et al., 2010).
Relative higher value of (La/Yb)N(PAAS) (0.95–2.03) implies
considerable terrigenous source contribution (Wu et al., 2010).
However, plot on (Fe2O3+MgO) vs. TiO2 coordinate and
La-Th-Sc triangular diagram show a continental arc environment and an active continental margin (Fu and Yu, 2010).
In this paper, the SiO2/Al2O3 vs. K2O/Na2O and the
(Fe2O3+MgO)/(SiO2+K2O+Na2O) plot are utilized to specify
the depositional tectonic setting of Baxiandun clastic rocks. The
SiO2/Al2O3 vs. K2O/Na2O relationship suggests that the source
of the Baxiandun rocks were most likely derived from an
evolved arc setting with a supply of felsic-plutonic detritus
situated along an active continental margin (Fig. 7a). This suggestion is also supported by the petrological analyses of the
Baxiandun samples. In the (Fe2O3+MgO)/(SiO2+K2O+Na2O)
diagram, the proportion of quartz relative to feldspar, and the
relative
petrologic
evolution
of
contributing
arcs
(mafic or felsic) are indicated by Al2O3/SiO2 and
(Fe2O3+MgO)/(SiO2+K2O+Na2O), respectively (Fig. 7b). It is
seen that most of the samples at Baxiandun mainly plot in the
evolved island arc (EIA) field, but a few samples in the immature island arc (IIA) field and in the mature magmatic arc
(MMA) field, which further suggests that the Baxiandun sedimentary rocks were derived from complex source regions.
In Fu and Yu’s (2010) paper, negative anomaly of Ce has
been used as the main evidence for the conclusion of marine
facies for the Baxiandun strata. However, sedimentary rocks in
both seawater and fresh water have very low concentration of
Ce due to their short lifetime in these kinds of environments, in
which Ce3+ can be quickly oxidized to Ce4+, and then adsorbed
by Fe-Mn oxide. Hence, Ce can be eliminated from water in a
short time, which results in deficiency of Ce in both two kinds
of environments (Murray et al., 1991). Murray’s research also
shows that lacustrine environments have a relative larger
Ce/Ce* value (average is 1.03). Thereby, the larger the Ce/Ce*
value, the more impact of terrigenous materials have on the
0.4
12 (a)
(b)
IIA
8
EIA
PM
Al 2O 3/SiO 2
SiO 2/Al 2O 3
0.3
0.2
MMA
4
0.1
A2
A1
ACM
0
0.01
0.1
1
K 2O/Na 2O
10
100
0
0.00
0.05
0.10
0.15
(Fe 2O 3+MgO)/(SiO 2+K 2O+Na 2O)
0.20
Figure 7. (a) Discrimination diagram to indicate the tectonic setting with SiO2/Al2O3 vs. K2O/Na2O diagram. A1. Evolved arc
setting, with supply of felsic-plutonic detritus; A2. arc setting, with supply of basaltic and andesitic detritus; ACM. active
continental margin; PM. passive continental (Roser and koersch, 1986). (b) Discrimination diagrams to indicate the tectonic
setting with Al2O3/SiO2 vs. (Fe2O3+MgO)/(SiO2+K2O+Na2O). IIA. Immature island arc; EIA. evolved island arc; MMA. mature magmatic arc (Kumon et al., 1992).
Cretaceous Deepwater Lacustrine Sedimentary Sequences from the Northernmost South China Block
formation of the rocks. Because the ranges of PASS standardization and CI chondrites standardization for siliciclastic rocks
at Baxiandun are 0.94–0.98 and 0.89–1.02, respectively (Wu et
al., 2010), the Ce/Ce* anomaly of the rocks should belong to
weak negative anomaly, which indicates that the debris of the
rock origin was deposited in a deepwater lacustrine sedimentary
facies.
5
GEOCHRONOLOGY OF THE SEQUENCE AT
BAXIANDUN SECTION
5.1 Methodology and Data
For the sake of clarifying the formation of the sedimentary
sequence at the Baxiandun Section, a representative arkosic
sandstone sample, which was collected at the bottom of the
section, was used for detrital zircon dating.
Zircon grains were separated at the laboratory of the Institute of Oceanology, Chinese Academy of Science. Firstly, the
sample was ground to 0.1–0.2 mm grains. Then the grains were
separated by magnetic, electromagnetic, dielectric and heavy
liquid processes, and hand-picked at random under a binocular
microscope.
U-Pb dating and trace element analyses of zircon were
conducted synchronously by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) at the State Key
Laboratory of Geological Processes and Mineral Resources,
China University of Geosciences, Wuhan. Measurement of the
single grain zircon U-Pb ages was determined by the isotopic
dissolution method using the procedure of Krough (1973). The
techniques of zircon solution and U and Pb extraction were
improved upon. The 208Pb-235U mixing spike was taken as the
dissolution dose (Li et al., 1995). After the solution was evaporated, U and Pb were mixed with silica gel-phosphoric acid
solution and placed on a single rhenium band. U-Pb isotopic
ratios were measured by a VG-354 thermionic mass spectrometer with a high precision Daly detector. Mass discrimination and system errors of all U-Pb data were corrected and total
Pb blanks over the period of the analysis ranged from 0.002 to
0.004 ng. The isotopic composition of radiogenic Pb is determined by subtracting first the blank Pb and then the remainder,
assuming a common Pb composition at the time of initial crystallization, determined from the global single stage model.
Calculations were performed using computer software program
PBDAT (Ludwig, 1993).
All 119 single grains were dated from the sample. Most
of these grains have larger Th/U ratio (≥0.4) and characteristic
oscillatory zonal structure in CL images, which indicates their
igneous origin. The variation of grain’s data age from 2 569 Ma
to 125 Ma indicates that the source of sedimentary rocks is very
complex. Zircon is a heavy mineral resistant to chemical
weathering, so it is a common component in siliciclastic sedimentary systems. Zircon in a sedimentary system can be derived from very old sources, stored in sedimentary strata, and
recycled back into the system one or more times. Second-cycle
zircons may be widely distributed and give information about
the ultimate source, but not the proximate one. Conversely,
first-cycle zircons are derived from weathering of Proterozoic
magmatic or metamorphic rocks (Link et al., 2005; Morton and
Hallsworth, 1999).
247
All of these processes may affect the accuracy and validity
of zircon ages to some extent. In order to reduce the effect of
Pb loss, the 207Pb/206Pb age was used to represent the ages of
old zircons with a higher Pb content (JAN01F83, 2 569 Ma).
Because the 206Pb/238U age of younger zircons has a relatively
higher precision in the isotope dilution method using Pb spike
H208 and U spike H235 than using the 207Pb/235U and
207
Pb/206Pb age, the 206Pb/238U age was taken as the age of
younger zircons with a relatively lower radioactive Pb content
(125 Ma). In this situation, 206Pb/238U age with a higher precision is more reliable (Cawood and Nemchin, 2000). The low
level discordance of younger zircons shown in the Concordia
plot (Fig. 8a) also supports that the 206Pb/238U age is more significant than the 207Pb/235U or the 207Pb/206Pb age. According to
their U-Pb ages, 108 of the 119 zircon grains separated from the
Baxiandun Formation fall into three groups: 125–259 Ma
(Jurassic–Early Cretaceous) (Fig. 8d); 674–793 Ma (Neoproterozoic) (Fig. 8c); 1 827–2 157 Ma (Paleoproterozoic) (Fig.
8b). In addition, a few other zircon grains fall in the age ranges
of 2 200–2 600 Ma (Neoarchean to Early Paleoproterozoic) and
430–502 Ma (Late Ordovician). Reflecting from the U-Pb age
distribution diagram (Fig. 9), almost half of the grains have
207
Pb/206Pb ages ranging from 125 to 259 Ma and one-third
have 207Pb/206Pb ages ranging from 1 827 to 2 699 Ma. The
relatively high degree of discordance exhibited by some grains
with low U contents indicates that these grains have suffered Pb
loss during a metamorphic/tectonic event. Since detrital zircons
constitute a mixture of grains of different ages, the time of Pb
loss is difficult to determine. Besides, more than 10 zircon
grains have 206Pb/238U ages ranging from 674–793 Ma and
there are four zircons with ages ranging 430–502 Ma.
5.3 Discussion of the Geochronology of Sedimentary Sequences
Most of zircons data indicate the Mesozoic source rocks.
The topographic relief was greatest during the Middle to Late
Jurassic and paleocurrents were from east to west (Dong et al.,
2008). The mass occurrence of Jurassic zircons and the lack of
Late Paleozoic detrital grains (400–250 Ma) suggest that the
detritus are from the adjacent Sulu orogenic belt, and the
Jiaodong micro-block, which means that the NCB became a
major source of sediment for the basin. Late Triassic and Jurassic plutonic rocks were eroded and the erosion products deposited in the Lower Cretaceous sediments indicating that intense
uplift occurred and the Meso-Cenozoic tectonic reactivated the
adjacent Jiaodong orogenic belt.
The second important age distribution (1 827–2 157 Ma)
concentrated in Paleoproterozoic metamorphic rocks in the
adjacent orogenic belts, especially Paleoproterozoic Jiaonan
Group and Wulian Group. More than 10 zircon grains with
674–793 Ma may represent the Sinian gneissic aegirine bearing
alkaline granite sources from the proximal orogen due to the
Jinning movement (Mish, 1942) at the southern margin of the
North China Craton (700–950 Ma) (Hu et al., 1996).
The most interesting observation is that there are four zircons grains with ages ranging 430–502 Ma. It is inferred that
the Late Ordovician source rocks could be sedimentary rocks
deposited along the South China block margin. But the biggest
248
Tuoyu Wu and Yongtao Fu
(a)
(b)
Data point error are 68.3% conf.
0.40
0.6
2 600
0.5
Mean=1 926 ± 53 Ma
MSWD=1.7; n =12
2 100
0.38
2 200
Pb/ 238U
0.4
1 800
0.3
206
1 400
206
Pb/ 238U
Data point error are 68.3% conf.
2 000
0.36
Data-point error symbols are 1σ
1 900
0.34
2 100
2 060
0.2
2 020
1 000
1 980
0.32
0.1
1 800
1 940
1 900
600
1 860
1 820
0.0
2
0
8
6
4
207
10
12
0.30
4.4
14
Pb/ 235U
1 780
4.8
(d)
(c)
Data point error are 68.3% conf.
0.15
Mean=763 ±19 Ma
MSWD=1.3; n =22
5.2
5.6
6.0 6.4
207
Pb/ 235U
7.2
Data point error are 68.3% conf.
0.025
860
Mean=130.5 ±1.6 Ma
MSWD=1.3; n =42
150
0.14
6.8
0.023
820
740
0.12
880
Data-point error symbols are 1σ
Pb/ 238U
780
0.021
130
206
206
Pb/ 238U
140
0.13
Data-point error
symbols are 1σ
148
144
140
840
700
0.019
800
136
132
128
720
680
0.10
0.85
120
750
0.11 660
640
0.95
1.05
1.15
1.25
207
Pb/ 235U
1.35
1.45
110
0.017
0.07 0.09
124
120
116
0.11
0.13
207
0.15 0.17
Pb/ 235U
0.19
0.21
Figure 8. (a) Concordia plot for detrital zircons from the Baxiandun Formation sample. The frame shows different age
groups; (b) the concordia plot for the age group of 1 827–2 157 Ma (Paleoproterozoic); (c) the concordia plot for the age
group of 674–793 (Neoproterozoic); (d) the concordia plot for the age group of 125–259 Ma (Jurassic–Early Cretaceous).
70
60
Relative probability
Number
50
40
30
20
10
0
0
400
800
1 200
1 600 2 000
Age (Ma)
2 400
2 800
3 200
Figure 9. Relative probability plots of U-Pb ages for concordant detrital zircons in the sample.
issue is that no Paleozoic rocks are found in the vicinity of the
study area. Some scientists (Fu and Yu, 2010) argued that it can
be compared to Yuqian Group and Changwu Group, both of
which also show well-preserved turbidites in South China. The
problem is that those two groups are both deep sea flysch facies,
but there are substantial terrigenous materials inside the rocks
Cretaceous Deepwater Lacustrine Sedimentary Sequences from the Northernmost South China Block
Early Cretaceous
South Yellow Sea Basin
Southern Sulu Wulai-QingdaoUHP terrane
Weihai fault
Early Cretaceous
Jiaolai Basin
249
Qixia magmatite 130 Ma
dome
150°
(km)
0
Baxiandun
10
20
20 km
30
Mesozoic
granite
Haizhou group
HP rock
MarbleUnknown
amphibolite
Basement
NW trending shearing relavant
with Late Triassic dome and
UHP rock obduction
Penglai Group
slate-sandstone
UHP granulite
Gravatational sliding
relavant with Late
Triassic dome
Gecun Group
clastic rock
Foliated
migmatite
Eclogite
Laiyang Group
clastic rock
Jiaonan group
UHP rock
Sinitral
fault
Lake
Figure 10. 3D model showing the tectonic and depositional environment of erosion of Sulu UHP terrane as the lacustrine deposits at Baxiandun formed (modified after Lin et al., 2003).
at Baxiandun, which indicates that the sedimentary environment at Baxiandun is not far away from a continental margin.
The age constraint is provided by the youngest detrital
zircons found in the Early Cretaceous which provides a maximum limit to the age of deposition. The youngest zircon found
in the Baxiandun Section has an age of 130 Ma, which provides
a maximum age limit of deposition. Notably, 57 U-Pb dating
data concentrated on the time range of 130–146 Ma, indicating
that substantial detritus was formed within the Early Cretaceous
period. Given that East China was uplifted and formed a landscape of high mountains and a large lake at that period, it is
most likely that the sequences at Baxiandun were a deepwater
lacustrine facies which could compare with Laiyang Group in
Jiaolai Basin.
bly related to the exhumation of late Early Cretaceous Laoshan
granite.
6
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