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1. Introduction
Microcontinents preserve records of continental rifting and breakup, seafloor spreading, and microconti-
nent accretion, and thus provide insights into the paleogeography and evolution of continental margins
(e.g., Borissova etal.,2003; Carter etal.,2014; Waldron & van Staal,2001). The South China Sea (SCS) is one
of the largest marginal seas in the western Pacific (Figure1) and is the result of continental rupture during
the Late Eocene with seafloor spreading spanning the Early Oligocene-Middle Miocene (∼33–16Ma), fol-
lowing rifting and thinning of the continental lithosphere, which dates back to the Latest Cretaceous (e.g.,
Barckhausen etal.,2014; Briais etal.,1993; Larsen etal.,2018; C. F. Li etal.,2014; Taylor & Hayes,1983).
Ocean spreading led to the formation of conjugate continental margins (Figure1). Of these, the northern
margin has seen most study (e.g., C. F. Li etal.,2014, 2015; Z. Sun et al., 2018; P. Wang,2012; P. Wang
etal.,2000), to the extent that relatively little is known about the formation and development of the south-
eastern margin represented by the Palawan microcontinental block (e.g., Yumul etal.,2003). The Palawan
microcontinental block spans the area of Palawan, Mindoro, western Panay, and the Romblon Islands, in
Abstract The Palawan microcontinental block is thought to have separated from the South China
margin due to seafloor spreading and opening of the South China Sea. However, it is uncertain when
and from which section the Palawan microcontinental block rifted from the South China margin and
little is known about sediment routing across the rifted margin before continental breakup. To address
these aspects, we studied the biostratigraphy and provenance of syn-rift sedimentary rocks collected from
the Panas-Pandian Formation in central-southern Palawan. Micropaleontological evidence indicates a
Middle Eocene-earliest Oligocene (47.7–32.9Ma) age for the Panas-Pandian Formation. Based on this
and the oldest age of the post-rift Nido Limestone (∼32Ma), the breakup unconformity on the Palawan
microcontinent block is dated around 33–32Ma. This timing of breakup unconformity is close to that
of the Pearl River Mouth Basin (∼30Ma) and IODP Site U1435 (∼34Ma), suggesting a conjugate
relationship between the Palawan microcontinental block and the Pearl River Mouth Basin. Trace
fossils and benthic foraminifera from the Panas-Pandian Formation indicate a middle bathyal to abyssal
environment on the continental slope of the South China margin. Multidisciplinary provenance analysis
reveals that the Panas-Pandian Formation was derived from both local Mesozoic basement uplifts and the
interior Cathaysia Block. It indicates that a paleo-Pearl River has been established at least since the Middle
Eocene (47.7–41.9Ma) and could deliver sediments from the interior Cathaysia Block to the continental
slope, across the wide rifted margin with a low topographic gradient.
CHEN ET AL.
© 2021. American Geophysical Union.
All Rights Reserved.
Stratigraphy and Provenance of the Paleogene Syn-Rift
Sediments in Central-Southern Palawan: Paleogeographic
Significance for the South China Margin
Wen-Huang Chen1,2,3,4 , Yi Yan1,2,4 , Andrew Carter5 , Chi-Yue Huang6 ,
Graciano P. Yumul Jr.7 , Carla B. Dimalanta8 , Jillian Aira S. Gabo-Ratio8,
Ming-Huei Wang9, Duofu Chen10 , Yehua Shan1 , Xin-Chang Zhang1 , and Weiliang Liu11
1Key Laboratory of Ocean and Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences, Guangzhou, China, 2CAS Center for Excellence in Deep Earth Science, Guangzhou, China, 3Southern Marine
Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, China, 4Innovation Academy of South
China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou, China, 5Department
of Earth and Planetary Sciences, Birkbeck, University of London, London, UK, 6School of Ocean and Earth Science,
Tongji University, Shanghai, China, 7Cordillera Exploration Company Incorporated, Taguig, Philippines, 8Rushurgent
Working Group–Tectonics and Geodynamics Academic Group, National Institute of Geological Sciences, College of
Science, University of the Philippines Diliman, Quezon City, Philippines, 9Exploration and Development Research
Institute, CPC Corporation, Taiwan, Miaoli, Taiwan, 10Shanghai Engineering Research Center of Hadal Science and
Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai, China, 11School of Marine Sciences,
Sun Yat-Sen University, Zhuhai, China
Key Points:
• Age of breakup unconformity of
Palawan microcontinent block
(33–32Ma) suggest its conjugate
relationship with Pearl River Mouth
Basin
• Both local basement uplifts and
interior Cathaysia Block supplied
syn-rift sediments to central-
southern Palawan on the continental
slope
• Sediment routing across the South
China margin implies a wide rifted
margin with a low topographic
gradient before seafloor spreading
Supporting Information:
Supporting Information may be found
in the online version of this article.
Correspondence to:
Y. Yan,
yanyi@gig.ac.cn
Citation:
Chen, W.-H., Yan, Y., Carter, A., Huang,
C.-Y., Yumul, G. P., Jr., Dimalanta,
C. B., etal. (2021). Stratigraphy and
provenance of the Paleogene syn-rift
sediments in central-southern Palawan:
Paleogeographic significance for
the South China margin. Tectonics,
40, e2021TC006753. https://doi.
org/10.1029/2021TC006753
Received 6 FEB 2021
Accepted 13 AUG 2021
Corrected 1 OCT 2021
Article was corrected on 1 OCT 2021.
See the end of the full text for details.
10.1029/2021TC006753
RESEARCH ARTICLE
1 of 27
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the southwest Philippines, and the Reed Bank (Figure1) (Hinz & Schlüter,1985; Holloway,1982; W. N. Liu
etal.,2014; Yumul etal.,2009). The Palawan microcontinental block is commonly thought to be a conti-
nental fragment that rifted and drifted away from mainland China during seafloor spreading of the SCS
helped by the southward subduction of the Proto-SCS beneath the Cagayan Ridge (Holloway,1982; Taylor
& Hayes,1983; Yumul etal., 2003,2009). The geology of the Palawan microcontinental block, especially,
Figure 1. Geological map of Southeast Asia surrounding the South China Sea adapted from Yan etal.(2018). The boundary of the Palawan microcontinental
block outlined with red dashed line was modified from W. N. Liu etal.(2014). Also shown are the ocean drilling sites and industrial boreholes referred to
in the text. Boreholes drilling the Cretaceous sediments in the northern South China Sea margin with published detrital zircon data (Shao, Cao, etal.,2017)
are marked by purple diamonds. In the Pearl River Mouth Basin and the Taixinan Basin, boreholes drilling the Eocene syn-rift sediments with published
detrital zircon U-Pb data (Shao etal.,2016,2019; Shao, Cao, etal.,2017) are marked by green and blue diamonds, respectively. For the well offshore Palawan:
B-1=Busuanga-1. JFZ=Jiangshao Fault Zone; NEPR=northeastern tributaries of the Pearl River (the Bei and Dong Rivers); HK=Hong Kong; and
RI=Romblon Islands.
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during the Eocene-Oligocene, is therefore key to understanding the early stages of rifting and the paleo-
geography of the South China margin, prior to opening of the SCS.
The Palawan microcontinental block is located east of the Dangerous Grounds and west of the Philippines
Mobile Belt (Figure1). Seismostratigraphy offshore the northwestern Palawan shelf (Franke etal., 2011)
has identified four tectonostratigraphic units: (a) Mesozoic pre-rift metasediments/sediments associated
with an active margin; (b) Latest Cretaceous-Paleogene syn-rift sediments associated with rifting; (c) Oligo-
cene-Early Miocene post-rift sediments concurrent with the drifting of the Palawan microcontinental block;
and (d) Late Miocene—Recent sediments formed during and after collision between the microcontinental
block and the Cagayan Ridge-Philippine Mobile Belt. Due to this collision, Paleogene syn-rift sediments,
known as the Panas-Pandian Formation, that were originally deposited on the southern continental mar-
gin were thrusted, uplifted, and juxtaposed with ophiolites as seen in central-southern Palawan (Aurelio
etal.,2014) (Figures2a and2b). As a sedimentary archive of the early stages of continental breakup, the
Figure 2. (a) Simplified geological map of Palawan Island adapted from Yumul etal.(2009). The poorly exposed granites of various ages are labeled by a, b, and
c. (b) Geological map of central-southern Palawan (Aurelio etal.,2014) showing the sampling sites of the Panas-Pandian Formation. The sample names of this
study, Wolfart etal.(1986) and Shao, Cao, etal.(2017) are marked in black, grey and orange colors,respectively. The sample names in italics denote sandstone
samples, while the rest represent mudstone samples. Note that four samples of Wolfart etal.(1986) were collected from small outcrops/islands (difficult to see
on the geologic map) near Quezon.
Rio Tuba
Berong
Aboabo
Aborlan
Puerto
Princesa
Babuyan
Ulugan
Bay
Beaufort Ultramafic complex
Serpentinized peridotite and dunite
Isotropic gabbro with minor layered gabbro
Espina Formation
Pillow basalt with chert and pelagic sediments
Panas Formation
sandstone interbedded with shale and mudstone
Inagauan Metamorphics
Metamorphosed Panas Formation
Pandian Formation
Arkosic sandstone with mudstone and s iltstone
Isugod Formation
Alfonso XIII Formation
Coral reef limestone grading to chalky marl
and sandstone and shale interbed
Consist of Pusok Conglomerate and Panoyan Limestone
Alluvium
Ransang Limestone
Sandy to silty limestone, equivalent to the St. Paul
Limestone in northern Palawan and Nido Limestone
offshore Palawan
Cretaceous-
Eocene
Palawan
Ophiolite
Interbedded shale and sandstone
Eocene-Early
Oligocene
Middle-Late
Miocene
Pliocene
Holocene
Early Miocene
Stavely Gabbro
^
Iwahig Formation
^
09°N 10°N
118°E
N
050100 km
Thrust Anticline Road
South China Sea
Sulu Sea
Sabang
Quezon
119°E
Locality
Sampling Sites (This Study)
^
b
Sampling Sites (Shao, Cao, et al., 2017)
Sampling Sites (Wolfart et al., 1986)
Panas-
Pandian
Formation
Rizal
RZ-01
RZ-12RZ-03a
RZ-11
RZ-10b
RZ-04b
RZ-05a
RZ-06a
RZ-07
RZ-08
RZ-13a
RZ-14b
AB-02a
AB-02b
AB-02f
RZ-13c
RZ-03b
RZ-12a
RZ-12b
RZ-12c RZ-03d
RZ-04c
RZ-05c
RZ-06c
P028
P029
P026
P027
P032
P031
PD104
PD106
St. Paul
Limestone
Ransang Limestone
Ransang Limestone
PL051 PR60
Mariquit Is.
Triple Cima Is.
PL052 PS140
PL058B
PL058D
45°
46°
80°
49°
20°
100 km
120°E
119°E118°E
117°E
12°N
11°N10°N9°N
8°N
Eocene to Holocene
sedimentary Sequences
Late Cretaceous to Eocene
Barton Group
Late Cretaceous to Eocene
Palawan Ophiolite
Late Paleozoic to Mesozoic
Malampaya Sound Group
Late Cretaceous Daroctan Granites (a)
Middle Miocene Kapoas Granites (c)
Ulugan Bay
Fault
Central-Southern Palawan
Puerto
Princessa
Northern Palawan
Figure 2b
a
08°N
Middle Eocene C. Palawan Granites (b)
a
b
c
Ganites
^
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Panas-Pandian Formation is a good place to study the paleogeography of the South China margin prior to
opening of the SCS.
While the provenance of syn-rift sediments has been used to argue for a connection between the Palawan
microcontinental block and the northern SCS margin (Concepcion etal.,2012; Shao, Cao, etal.,2017; Yan
etal., 2018), it is unknown when and where exactly the Palawan microcontinental block rifted from the
South China margin as direct stratigraphic evidence is lacking. A useful indicator would be a similar timing
of the breakup unconformity, since along the South China continental margin the breakup unconformity
shows a diachronous southwestward younging, from the Taixinan Basin to the Pearl River Mouth Basin
then to the Qiongdongnan Basin (Figure1), following the stepwise propagation of seafloor spreading from
northeast to southwest (Franke,2013; Morley,2016). Currently the breakup unconformity along the Pala-
wan microcontinental block is poorly constrained because few boreholes have been drilled into the syn-rift
sediments. As a consequence, age constraints have depended on the age of post-rift platform carbonates
(Nido Limestone) (Franke,2013). Syn-rift sediments exposed on central-southern Palawan (Panas-Pandian
Formation) are mostly unmetamorphosed and contain fine-grained sediments, and are thus suitable for
biostratigraphic study to better constrain the age of the breakup unconformity. Comparing the timing of
the breakup unconformity between the Palawan microcontinental block and different sections of the South
China continental margin would help define which section the Palawan microcontinental block came from.
Until recently, little has been known about the origin and routing of syn-rift sediments deposited across
the rifted South China margin. As the development of half-grabens and grabens are generally accompanied
by uplift and erosion of horst blocks, it is reasonable to expect the dominance of proximal sources for the
syn-rift sediments as widely demonstrated for Eocene syn-rift sediments from the Pearl River Mouth Basin
(Shao etal.,2016; W. Wang etal.,2017) and at International Ocean Discovery Program (IODP) Site U1435
(Shao, Meng, etal.,2017). Sediments sourced from the interior of continental South China with a Cathay-
sian affinity, potentially delivered by a paleo-Pearl River like the present-day northeastern tributaries (the
Bei and Dong Rivers) of the Pearl River have been detected in late Early Oligocene rocks from boreholes X28
and P33 in the Pearl River Mouth Basin (Cao etal.,2018; Shao etal.,2016; W. Wang etal.,2019) (Figure1).
Potentially, these sediments may have reached the Palawan sector of the continental margin prior to it drift-
ing away but this would depend on the geomorphology of the rifted continental margin, such as whether a
narrow, elongate deep-sea basin/gulf existed (Q. Y. Li etal.,2017; P. Wang etal.,2003). As a consequence, a
provenance study of the Paleogene Panas-Pandian Formation will help improve understanding of margin
development and sediment routing across the South China margin prior to breakup.
To address these aspects, our study examined the biostratigraphy and provenance of syn-rift sedimenta-
ry rocks collected from the Panas-Pandian Formation in central-southern Palawan. Specifically, we deter-
mined the depositional age of the Panas-Pandian Formation using new planktonic foraminiferal and cal-
careous nannofossil data together with the biostratigraphic results of Wolfart etal.(1986). This new age
helps to constrain the timing of the breakup unconformity on the Palawan microcontinental block and
therefore helps to elucidate its conjugate relationship with the northern SCS margin. Benthic foraminiferal
and sedimentological evidence are used to reconstruct the depositional environment. To constrain sediment
provenance and routing, we adopted a multidisciplinary approach using trace elements, Nd isotope, heavy
mineral assemblages, and detrital zircon U-Pb geochronology (in combination with previously published
detrital results).
2. Geologic Setting
2.1. The South China Margin
The continental margin of South China has experienced a complex tectonic evolution. It was an Ande-
an-type active margin between the Permian and mid-Cretaceous, and then evolved to a western Pacific-type
margin in the Late Cretaceous (Z. X. Li etal., 2012). The margin finally became a passive margin in the
Cenozoic owing to extension and rifting followed by opening of the SCS. The South China continent is con-
ventionally divided into the Yangtze Block in the northwest and the Cathaysia Block in the southeast, along
the Jiangshao Fault Zone (Figure1). The Cathaysia Block has a basement dominated by Paleoproterozoic to
Neoproterozoic (2,500–540Ma) units with minor Archean (3,500–2,500Ma) components, overlain by the
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Paleozoic and Mesozoic sedimentary strata (J. F. Chen & Jahn,1998; X. Xu etal.,2007; Yu etal.,2006). A
series of magmatic events, that is, Caledonian (570–400Ma), Indosinian (257–205Ma), and Yanshannian
(180–67Ma) have taken place within the Cathaysia Block since the Paleozoic (e.g., X. H. Li etal.,2007; Z. X.
Li etal.,2010). Caledonian and Indosinian granitoids are mainly distributed along the northern boundary
of the Cathaysia Block and the interior of the Yangtze Block (J. F. Chen & Jahn,1998). Mesozoic (Yansha-
nian) granitoids are extensively exposed on the Cathaysia Block as well as the northern SCS continental
margin (J. F. Chen & Jahn,1998; F. Li etal.,2018; Ye etal.,2018) and fall into two age groups: Jurassic (Early
Yanshanian) and Cretaceous (Late Yanshanian) (e.g., X. H. Li etal.,2007; Z. X. Li etal., 2010). The Taix-
inan Basin, Pearl River Mouth Basin, and Qiongdongnan Basin were regarded as the offshore extensions of
the Cathaysia Block in light of their basement (X. M. Sun etal.,2014). The Pearl River Mouth Basin has a
basement dominated by Mesozoic granitoids and sedimentary rocks, with Paleozoic metasedimentary rocks
scattered in the western part (X. M. Sun etal.,2014).
2.2. Regional Tectonics of Palawan
The northeast-southwest oriented islands of Palawan are approximately 450km long and separate the Sulu
Sea from the SCS (Figure1). In the northwest Sulu Sea basin lies the submerged Cagayan Ridge, which
is the extinct volcanic arc associated with the subduction of the Proto-SCS beneath the Northwest Sulu
Sea (Holloway,1982; Rangin & Silver,1991). Palawan is commonly divided into two discrete tectonic ter-
ranes, northern Palawan and central-southern Palawan, along the north-south trending Ulugan Bay fault
(Schlüter etal.,1996; Yumul etal.,2009) (Figure2a).
Northern Palawan is subdivided into the Malampaya Sound Group in the north and the Barton Group in the
south (Figure2a). The Malampaya Sound Group consists of Permian-Jurassic chert and Jurassic-Early Cre-
taceous terrigenous clastics, bearing Carboniferous-Jurassic limestone blocks. This group was interpreted as
the accretionary prism formed along the South China margin during the Middle Jurassic-Early Cretaceous
(Zamoras & Matsuoka,2004). However, the terrigenous clastic unit, known as the Guinlo Formation, was
recently assigned to the Late Cretaceous using the youngest detrital zircon ages (Padrones etal.,2017; Shao,
Cao, etal.,2017). The Barton Group is composed of sedimentary and low- to middle- grade metasedimen-
tary rocks and were thought to be deposited along the South China margin (e.g., Suggate etal.,2014; Walia
etal.,2012). The depositional age is poorly constrained as the Late Cretaceous based on the presence of cal-
careous nannofossil Prediscosphaera cretacea (Arkhangelsky) (Wolfart etal.,1986) and the youngest detrital
zircon ages (Walia etal.,2012). Intrusions of diverse ages, including the Late Cretaceous, Middle Eocene, and
Middle Miocene granites, sparsely crop out in northern Palawan (Padrones etal.,2017) (Figure2a). The St.
Paul Limestone north of the Ulugan Bay fault (Figure2b) represents one of the onshore correlative equiva-
lents of the Nido Limestone. The St. Paul Limestone was ambiguously assigned to the Early Miocene (Wolfart
etal.,1986) or the Late Oligocene-Early Miocene (Aurelio & Peña,2010), based on large benthic foraminifera.
To the southwest of the Ulugan Bay fault, central-southern Palawan is composed of two contrasting units,
the Cretaceous-Eocene Palawan Ophiolite and Cenozoic sedimentary sequences (Figure2a). The Palawan
Ophiolite is primarily made up of Eocene oceanic lithosphere of the Northwest Sulu Sea basin emplaced
onto the Palawan microcontinental block during Miocene arc-microcontinent collision, along with alloch-
thonous remnants of the Cretaceous Proto-SCS oceanic lithosphere (Keenan et al., 2016). The Cenozo-
ic sedimentary sequences were assumed to be an emergent imbricated thrust belt or accretionary prism
subsequent to the Miocene collision (Hinz & Schlüter,1985; Steuer etal.,2013). Some researchers (e.g.,
Lai etal.,2020) did not regard central-southern Palawan as part of the Palawan microcontinental block
and placed central-southern Palawan in the southern margin of the Proto-SCS before the elimination of
the Proto-SCS, in light of the widespread ophiolite in central-southern Palawan. However, the Palawan
Ophiolite has not only been obducted onto the Panas-Pandian Formation in central-southern Palawan,
but has also been thrust over the Barton Group and St. Paul Limestone north of the Ulugan Bay fault (e.g.,
Keenan,2016) (Figure2b). Therefore, the Palawan Ophiolite should be treated as an allochthonous block
to both northern and central-southern Palawan, and the difference between northern and central-southern
Palawan is primarily in the exhumation level of the ophiolite (Ilao etal.,2018). The exhumation level of the
ophiolite is much greater in northern Palawan than in central-southern Palawan. Moreover, the turbidites
of the Panas-Pandian Formation exposed on central-southern Palawan appear to be the onshore continua-
tion of syn-rift sediments found in half-grabens on the microcontinental block offshore Palawan (Aurelio
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etal.,2014; Ding etal.,2015; Sales etal.,1997). Therefore, we follow Hall(2002,2012) inferring that both
northern and central-south Palawan belong to the Palawan microcontinent block (Figure1).
2.3. Cenozoic Sedimentary Stratigraphy in Central-Southern Palawan
The Cenozoic sedimentary strata in central-southern Palawan include the Paleogene Panas and Pandian
Formations, the Early Miocene Ransang Limestone, and the Middle Miocene-Pleistocene collisional-re-
lated clastics and carbonates (Isugod Formation, Alfonso XIII Formation, and Iwahig Formation) (Aurelio
etal.,2014; Aurelio & Peña,2010) (Figure2b).
The Panas Formation is a turbidite sequence composed mainly of medium- to thin-bedded alterations of
sandstone, siltstone, and shale (Aurelio & Peña,2010). A wide age range spanning the Paleocene to Late
Eocene was proposed by Wolfart etal.(1986) due to the occurrence of Paleocene-Early Eocene foraminifera
and late Middle Eocene-Early Oligocene calcareous nannofossils. The Pandian Formation is dominantly
made up of massive coarse-grained sandstone with indurated mudstone and silty shale interbeds down-
section (Aurelio & Peña,2010). Considering the similarities in lithology and estimated age between the
Panas Formation and Pandian Formation, Macc and Agadier(1988) suggested that the name Pandian be
adopted for the turbidites mapped as Panas Formation, while Aurelio etal. (2014) regarded the Pandian
Formation as the strongly indurated to mildly metamorphosed facies of the Panas Formation. It is difficult
to distinguish them in the field. Therefore, for the purposes of this study, we consider them together as the
Panas-Pandian Formation. The Panas-Pandian Formation is mainly distributed around the Palawan Ophi-
olite in southern Palawan and occurs in a tectonic window near Aborlan in central Palawan (Figure2b).
The poorly exposed Ransang Limestone consists of massive to bedded limestone bearing Early Miocene
large foraminifera (Macc & Agadier,1988) and is correlative to the post-rift Nido Limestone widespread off-
shore Palawan and the St. Paul Limestone north of the Ulugan Bay fault (Aurelio & Peña,2010). A Miocene
arc-microcontinent collision led to a complex contact between the Cenozoic sedimentary sequences and
the Palawan Ophiolite. The northwestward thrusting of the ophiolite over the Panas-Pandian Formation
formed a tectonic window in central Palawan. Along the fringe of the tectonic window, the Panas-Pandian
Formation was metamorphosed (Inagauan Metamorphics, Figure2b). The thrust contact was reversely
sealed by Miocene clastics of the Isugod Formation (Aurelio etal.,2014).
3. Methods
Samples of the Panas-Pandian Formation were collected for micropaleontology, trace element, Nd isotope
and heavy mineral analyses, and detrital zircon U-Pb geochronology to reconstruct the biostratigraphic
framework, sedimentary environment, routing, and provenance of central-southern Palawan prior to the
opening of the SCS. Field investigation was mainly conducted along the coast, road cuts, and valleys to ob-
tain the freshest possible samples (Figures2b and3). Nineteen mudstone samples were collected for plank-
tonic and benthic foraminiferal and calcareous nannofossil analyses, of which 11 samples were chosen for
trace element and Nd isotopic analyses. Four sandstone samples were collected for heavy mineral analysis
and two samples were selected for detrital zircon U-Pb dating, to complement the data from sample AB-02f
reported by Yan etal.(2018).
Planktonic and benthic foraminifera were separated from 200g of mudstone using conventional meth-
ods. Foraminiferal tests from the 100-mesh screen were picked for identification and counting under a
stereomicroscope. Standard zonations of planktonic foraminifera established from low-latitude regions by
Blow(1969) and datum planes (FAD: first appearance datum; LAD: last appearance datum) as documented
by Wade etal.(2011) were followed (Figure4). Five to 10g of each sample were prepared for calcareous
nannofossil analysis employing standard smear-slide techniques. More than 100 random fields of view from
each slide were examined under a polarizing microscope with 1,600× magnification. The zonal scheme
of calcareous nannofossils proposed by Martini (1971) and datum planes compiled by Anthonissen and
Ogg(2012) were applied (Figure4).
Mudstones for trace element and Nd isotopic analyses were leached with 2N acetic acid to remove car-
bonates. Solid residues were collected after centrifuging, oven dried and ground to powder <75μm. Pow-
ders were then heated to 700°C to destroy organic material prior to element and isotopic analyses at the
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State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences (GIGCAS). Trace elements were measured on a Perkin-Elmer Sciex Elan 6000 inductively coupled
plasma mass spectrometer (ICP-MS). The analytical procedures followed the method described by Y. Liu
etal.(1996). The precision is generally better than 5%. Several USGS and Chinese rock standard references
including BHVO-2, GSR-1, and GSD-9 were repeatedly measured with the samples, yielding values gener-
ally within ±10% (RSD) of the certified values. Nd isotope measurements were performed on a MicroMass
Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS). For details of the
methods, see Wei etal.(2002). The measured 143Nd/144Nd ratios were normalized to 146Nd/144Nd=0.7219. A
standard Nd solution, Shin Etsu JNdi-1, was repeatedly measured with the samples to monitor the quality of
measurements, yielding a mean value of 0.512111±6 (2σ, N=5). The Nd isotopic composition is reported
as εNd calculated using the CHUR value (143Nd/144Nd=0.512638) given by Jacobsen and Wasserburg(1980).
Sandstones for heavy mineral analysis and detrital zircon U-Pb dating were disaggregated and then passed
through a 40-mesh sieve. Detrital heavy mineral components were preliminarily separated by centrifugal
elutriation, and further mineral separation was achieved by magnetic and electrostatic filters and heavy
liquid (bromoform, density=2.89g/cm³). At least 700 non-opaque heavy mineral grains were identified
and counted under the binocular microscope for each sample. Around 300 zircon grains were handpicked
from the heavy mineral fractions of each of the Samples RZ-01 and RZ-12b and mounted in epoxy resin
for U-Pb isotopic analysis. The location of analytical spots was established by grain cathodoluminescence
images, avoiding inherited cores where possible. Zircon U-Pb dating was performed by laser ablation-in-
ductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Key Laboratory of Mineralogy and Metal-
logeny, GIGCAS. The instrumentation is composed of an Agilent 7900a ICP-MS coupled with a Resonetic
RESOLution S155 ArF-Excimer laser source (λ=193nm). U, Th, and Pb concentrations were calibrated
using 29Si as an internal standard and the standard silicate glass NIST 610 as reference material. Zircons
91500 (Reference age 1,064±0.8Ma) (Wiedenbeck etal.,2004) and Plešovice (Reference age 337±0.4Ma)
(Sláma etal.,2008) were used as the standard. We analyzed at least 110 grains from each sample. Analyses
were conducted with a beam diameter of 30μm; each analysis includes 30s of gas blank followed by 60s
of data aquisition. Isotope ratios were calculated using ICPMSDataCal 7.7 (Y. Liu etal.,2010). The relative
age probability of detrital zircons was processed using Isoplot (Version 3.23) (Ludwig,2003). Ages with dis-
cordance >10% were excluded from the discussion. 206Pb/238U and 207Pb/206Pb ages were adopted for zircons
younger and older than 1,000Ma, respectively.
4. Results
4.1. Field Occurrence
Outcrops of the Panas-Pandian Formation are generally sparse owing to the humid tropical climate and
lush vegetation in central-southern Palawan. The observed outcrops are restricted to road cuts, coast, and
valleys and are dominated by massive sandstones and alternating interbeds of sandstone and mudstone.
The massive sandstone mainly occurs along the western coast of southern Palawan (e.g., Sites RZ-01 and
RZ-10) and is typically fresh, gray-green, and lithic-rich with occasional pebbles of limestone (Figures2b
and3a). Grading is well-developed fining upwards. Sedimentary structures of the massive sandstones are
consistent with middle fan deposits of a submarine fan. Alternations of medium- to thick-bedded sandstone
and thin-bedded mudstone are exposed along the road cuts between Quezon and Rizal, slightly east of the
coastline (Figure2b). They are yellowish-brown as a result of strong weathering (Figure3b). A rare outcrop
of relatively unweathered rock consisting of interbeds of light-gray thin-bedded sandstone and thick-bed-
ded mudstone was found at Site RZ-12 (Figure3c). Trace fossils Helminthopsis are well preserved within
sandstone soles (Figure3d). Helminthopsis is a pascichnion, a structure produced in response to feeding
during locomotory activity, of worm-like organisms (Han & Pickerill,1995). It was commonly reported from
deep-water (slope and basinal) flysch sequences, typically as an integral component of Nereites ichnofacies
defined by Seilacher(1967). The Panas-Pandian Formation inside the tectonic window in central Palawan
(e.g., Site AB-02) is very-weakly metamorphosed, composed of interbeds of highly indurated medium- to
thick-bedded sandstone with parallel bedding and thin-bedded shale (Figures3e and 3f). Based on the
above observations, these sandstone-mudstone interbeds were most likely deposited in the middle-lower
fan environment.
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The attitude of the strata between the observed outcrops is highly variable, and folds are locally developed.
In general, the strata dips 10°–70° toward northeast to southeast (Figure2b). It would be difficult to de-
termine the stratigraphic position of the samples collected from different outcrops because of the sparse
distribution of the outcrops and the variations of attitude between adjacent outcrops. Nevertheless, in light
of the general eastward dip of the strata, we infer that the massive sandstones exposed along the western
coast (e.g., Sites RZ-01 and RZ-10) and the alternating sandstone-mudstone sequences along the road cuts
between Quezon and Rizal (e.g., Sites RZ-04 and RZ-12) in southern Palawan lie in the lower part of the
Panas-Pandian Formation, while the alternating sandstone-mudstone sequences in the tectonic window in
central Palawan (Site AB-02) lie in the upper part of the Panas-Pandian Formation.
4.2. Biostratigraphic Result
Only three samples at Site RZ-12 (Figure3c) of the 19 analyzed samples yielded rare but age-diagnos-
tic planktonic foraminifera and calcareous nannofossils (Figures4, 5a and 5b). Calcareous microfossils
Figure 3. Field photographs of outcrops of the Panas-Pandian Formation. (a) Gray-greenish massive sandstone at Site
RZ-01, showing a clear graded bedding from coarse- to medium-grained sandstone upwards. The arrow (scale) has a
length of 10cm. (b) Alternations of medium- to thick-bedded sandstone and thin-bedded mudstone that are highly
weathered, at Site RZ-04. (c) Interbeds of light-gray thin-bedded sandstone and medium- to thick-bedded mudstone at
Site RZ-12. Yellow dashed line marks the bedding. (d) Trace fossil Helminthopsis well-preserved in convex hyporelief
on sandstone soles at Site RZ-12. (e) Highly indurated medium- to thick-bedded sandstone and thin-bedded shale
interbeds at Site AB-02. (f) Sandstone bed at Site AB-02 with parallel bedding. The sample names in italics denote
sandstone samples used for heavy mineral analysis and detrital zircon U-Pb dating.
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Figure 4. Datum planes of planktonic foraminifera and calcareous nannofossils (marked by short horizontal bars with numerical age), given according to
Wade etal.(2011) and Anthonissen and Ogg(2012), respectively, in the Panas-Pandian Formation. Both the age spans derived from this study at Site RZ-12 and
Wolfart etal.(1986) are shown.
P19
34.03 Ma
34.22 Ma
P17
P16
35.87 Ma
P15
P14
38.62 Ma
40.03 Ma
P13
40.49 Ma
P12
43.26 Ma
43.88 Ma
P11
P10
44.49 Ma
P9
P8
50.20 Ma
50.67 Ma
P7
52.54 Ma
54.61 Ma
P6b
P6a
NP21
34.44 Ma
NP20
NP19
~
36.97 Ma
NP18
37.32 Ma
NP17
40.40 Ma
NP16
42.87 Ma
NP15c
44.12 Ma
NP15
b
45.49 Ma
NP15
a
46.29 Ma
NP14b
47.84 Ma
NP14
a
49.11 Ma
NP13
50.50 Ma
53.70 Ma
NP12
NP11
54.17 Ma
NP10
33.89 Ma
47.84 Ma
37.75 Ma
Early EoceneMiddle Eocene Late Eocene
40.49
Acarinina bullbrooki
Acarinina cuneicamerata
50.20
Morozovelloides crassatus38.25
Globigerinatheka subconglobata
53.70
47.41
Discoaster lodoensi s
Nannoterina cristata
47.73
41.85
34.44
Discoaster barbadiensis
Discoaster kuepperi
Discoaster saipanensis
34.76
Age
Berggren
et al.
(1995) Planktonic Foraminifera Calcareous Nannofossils Martini
(1971)
Oligocene
Panas-Pandian
Formation
Ericsonia formosa
43.32 32.02
Reticulofenestra umbilica
38.25 Reticulofenestra bisecta
This
Study
Wolfart
et al.,
(1986)
NP22
NP23
32.92
32.92 Ma
32.02 Ma
32.10 Ma
P18
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in other outcrops were probably diluted by turbidite currents or lost to intense weathering. The identified
planktonic foraminifera (seven species belonging to six genera) and calcareous nannofossils (14 species
belonging to seven genera) are listed in detail in TablesS1 andS2. The planktonic foraminiferal assemblage
is dominated by those spanning the Early Eocene to the Middle Eocene, including Acarinina cuneicamer-
ata (Blow) (FAD at 50.20Ma), Acarinina bullbrooki (Bolli) (LAD at 40.49Ma), Morozovelloides crassatus
(Cushman) (LAD at 38.25Ma), and Globigerinatheka subconglobata (Shutskaya) (P9–P14; Middle Eocene)
with characteristic sub-spherical test (Figure5a). The concurrent occurrence of A. cuneicamerata (FAD at
50.20Ma) and A. bullbrooki (LAD at 40.49Ma) throughout the samples indicates a wide age range of Zones
P9–P12 (50.2–40.5Ma) (Figure4), which also partially overlaps the range of the other species. There are
calcareous nannofossil assemblage of Discoaster lodoensis Bramlette and Riedel (FAD at 53.70Ma; LAD
at 47.41Ma), Discoaster kuepperi Stradner (NP12–NP14b; Early to Middle Eocene), Discoaster gemmifer
Stradner (NP12–NP16; Early to Middle Eocene), Nannotetrina cristata (Martini) (FAD at 47.73Ma; LAD
at 41.85Ma), Sphenolithus spiniger Bukry (NP14–NP15; Middle Eocene), Discoaster barbadiensis Tan (LAD
at 34.76Ma), and Discoaster saipanensis Bramlette and Riedel (LAD at 34.44Ma; NP15–NP20; Middle to
Late Eocene) (Figure5b). Because the FAD of D. saipanensis within Zone NP15 has a large uncertainty
(Perch-Nielsen,1985), it is difficult to determine whether D. lodoensis and D. kuepperi are indigenous or
reworked. If D. lodoensis and D. kuepperi are indigenous, we could assign this assemblage to a narrow range
of Zone NP14b (47.7–47.4Ma) based on the concurrent occurrence of N. cristata (FAD at 47.73Ma) and D.
lodoensis (LAD at 47.41Ma). If D. lodoensis and D. kuepperi are reworked, we could assign this assemblage
to Zones NP14b–NP16 (47.7–41.9Ma) (Figure4) simply based on the appearance of N. cristata (FAD at
47.73Ma; LAD at 41.85Ma). Altogether, it is reasonable to assign the calcareous nannofossil assemblage to
Zones NP14b–NP16 (47.7–41.9Ma) irrespective of whether D. lodoensis and D. kuepperi are indigenous or
reworked, which is also consistent with the planktonic foraminiferal data (Zones P9–P12, 50.2–40.5Ma).
Therefore, the depositional age range of the Panas-Pandian Formation at Site RZ-12 is placed within the
Middle Eocene (47.7–41.9Ma) (Figure4) in light of the overlapping range of our results of planktonic fo-
raminifera and calcareous nannofossils.
Abundant and diverse benthic foraminifera, characterized by deep-water agglutinated taxa, were recovered
from 11 samples (Figure5c and TableS3). There are frequent occurrences of Haplophragmoides walteri
(Grzybowski), Nothia excelsa (Grzybowski), Psammosiphonella discreta (Brady), Psammosphaera irregula-
ris Grzybowski, and Trochammina globigeriniformis (Parker and Jones), with sporadic occurrences of Am-
modiscus tenuissimus Grzybowski, Haplophragmoides eggeri Cushman, Haplophragmium horridum Grzy-
bowski, Haplophragmoides spp., Pseudonodosinella elongata (Grzybowski), Reticulophragmium amplectens
(Grzybowski), Saccammina grzybowskii (Schubert), and Trochamminoides subcoronatus (Grzybowski).
These agglutinated foraminifera are common in Eocene “flysch-type” deep-water assemblages of the North
Atlantic and western Tethys (Kaminski & Gradstein,2005) and suggest an age of late Early Eocene to Late
Eocene (FigureS1). The “flysch-type” fauna indicates middle bathyal to abyssal environments with an up-
per depth limit of ∼500m (Kaminski & Gradstein,2005). There are also rare calcareous benthic foraminif-
era (Pseudonodosaria sp., Lenticulina sp., Cibicides sp., and Planulina sp.) in Site RZ-12 samples (Figure5c).
4.3. Trace Element and Nd Isotope Results
Results of trace element and Nd isotopic analyses of the silicate fraction of the Panas-Pandian Formation
mudstones are listed in TablesS4 and Table1, respectively.
Trace elements that are common in felsic rocks, Rb, Th, U, Nb, Zr, Hf, and Y in the Panas-Pandian For-
mation, except Samples RZ-03b and RZ-06c, show higher concentrations compared to the Upper Conti-
nental Crust (UCC) (Rudnick & Gao,2003) (Figure 6a). Abundances of transitional elements, V and Sc
in the Panas-Pandian Formation are lower to slightly higher than those of the UCC, while Co, Cr, and
Ni are significantly lower than those of the UCC (Figure6a). Such trace element patterns suggest felsic
source rocks. Chondrite-normalized distribution patterns of rare earth element (REE) concentration in the
Panas-Pandian Formation are similar to those of the UCC, displaying light REEs enrichment, heavy REEs
depletion, and a negative Eu anomaly (Figure6b).
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Figure 5. (a) SEM images of planktonic foraminifera recovered from the Panas-Pandian Formation are shown with scale bars of 200μm. Numbers (1–8)
recovered from Sample RZ-12c: (1–2) Acarinina bullbrooki (Bolli); (3–4) Acarinina cuneicamerata (Blow); (5–7) Morozovelloides crassatus (Cushman); (8)
Globigerinatheka subconglobata (Shutskaya). (9) Globigerinatheka subconglobata (Shutskaya), Sample RZ-12; and (10) Cataphydrax unicavus Bolli, Loeblich
and Tappan, Sample RZ-12a. (b) Polarizing photographs of calcareous nannofossil recovered from the Panas-Pandian Formation are shown with their
approximate diameter of the fossil (red bar). Numbers (1–6) recovered from Sample RZ-12: (1) Discoaster lodoensis Bramlette and Riedel; (2) Discoaster
kupperi Stradner; (3) Discoaster saipanensis Bramlette and Riedel; (4) Nannotetrina cristata (Martini); (5) Discoaster barbadiensis Tan; (6) Discoaster gemmifer
Stradner. Numbers (7–11) recovered from Sample RZ-12a: (7) Discoaster lodoensis Bramlette and Riedel; (8) Discoaster kupperi Stradner; (9) Sphenolithus
spiniger Bukry; (10) Sphenolithus moriformis (Brönnimann and Stradner); (11) Ericsonia formosa (Kamptner). Numbers (12–16) recovered from Sample
RZ-12c: (12) Discoaster lodoensis Bramlette and Riedel; (13) Discoaster kupperi Stradner; (14) Discoaster saipanensis Bramlette and Riedel; (15) Discoaster
barbadiensis Tan; and (16) Discoaster deflandrei Bramlette and Riedel. (c) Composite all-in-focus stereomicroscopic images of benthic foraminifera recovered
from the Panas-Pandian Formation are shown with scale bars of 200μm. (1) Haplophragmoides eggeri Cushman, Sample-03b. Numbers (2–3) recovered
from RZ-03d: (2) Psammosphaera irregularis Grzybowski; (3) Reticulophragmium amplectens (Grzybowski). Numbers (4–10) recovered from Sample RZ-04b:
(4) Haplophragmoides walteri (Grzybowski); (5) Psammosphaera irregularis Grzybowski; (6–7) Pseudonodosinella elongata (Grzybowski); (8) Saccammina
grzybowskii (Schubert); (9–10) Trochamminna globigeriniformis (Parker and Jones). (11) Psammosiphonella discreta (Brady), Sample RZ-04c. Numbers (12–19)
recovered from Sample RZ-05c: (12) Ammodiscus tenuissimus Grzybowski; (13) Haplophragmium horridum Grzybowski; (14–15) Haplophragmoides walteri
(Grzybowski); (16) Nothia excelsa (Grzybowski); (17) Psammosphaera irregularis Grzybowski; (18) Trochamminna globigeriniformis (Parker and Jones); (19)
Trochamminoides subcoronatus (Grzybowski). (20) Haplophragmoides walteri (Grzybowski), Sample RZ-07. Numbers (21–23) recovered from RZ-12: (21)
Cibicides sp.; (22) Lenticulina sp.; (23) Planulina sp.; and (24) Pseudonodosaria sp., Sample RZ-12c.
12
456
78910
38
45
911 12 14 15
13
1
17 19
16
20 21 22 23 24
1
11 14 15 16
b
a
c
2345678
910 12 13
3
12 67
10
18
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The 143Nd/144Nd ratios of the Panas-Pandian Formation mudstones are
concentrated in the range from 0.512162 to 0.512214, corresponding to a
εNd range from −9.3 to −8.3 (Table1).
4.4. Heavy Minerals and Detrital Zircon U-Pb Geochronology
The percentages of non-opaque heavy minerals of four sandstone sam-
ples are listed in TableS5 and shown in Figure 7. Samples RZ-10b, RZ-
12b, and AB-02f have remarkably high percentages of zircon ranging
from 73.5% to 92.8%, which is consistent with previous results (average
value=81.2%) of Shao, Cao, etal.(2017). While Shao, Cao, etal.(2017)
only find minor abundances of tourmaline (≤1.1%) and rutile (≤2.7%),
our samples RZ-10b and RZ-12b show significantly higher contents of
tourmaline (15.6%–16.6%) and rutile (4.7%–6.4%). Nevertheless, the high
ZTR index (zircon+ tourmaline +rutile) in samples RZ-10b, RZ-12b,
and AB-02f as well as samples of Shao, Cao, etal.(2017) indicate a dom-
inance of felsic provenances. Sample RZ-01 has a much lower content of
zircon (12.1%) and is dominated by unstable minerals of epidote (64%)
and garnet (9.2%), which indicate the existence of a metamorphic source.
There is also minor but noteworthy Cr-spinel (10.9%) suggesting a basic-ultrabasic/ophiolitic provenance.
The low ZTR index and high abundances of unstable minerals might reflect a short transport distance for
this sample.
Detrital zircon U-Pb age results of samples RZ-01 and RZ-12b, together with that of Sample AB-02f pub-
lished earlier by Yan etal.(2018), are detailed in TableS6. Zircon grains extracted from these three samples
are light pink, light yellow to colorless, and euhedral to subhedral. Most of the grains show oscillatory
zoning typical of magmatic origin (FigureS2). Th/U ratios of the zircons are generally >0.3 also consistent
with an igneous origin. Only three grains in RZ-01 and two grains in RZ-12b possess Th/U ratios <0.1 corre-
sponding to a metamorphic origin (Corfu etal.,2003) (FigureS3). Zircon U-Pb ages span a wide spectrum,
from 93Ma to 3,400Ma (Figures8a, 8b and8d). In Sample RZ-01, zircons younger than 200Ma make up
56% of the total grains analyzed and consist of two Mesozoic groups of 90–150Ma and 160–200Ma with
peaks at ∼110Ma and 177Ma, respectively (Figure8a). Two minor age groups are seen at 200–300Ma
and 1,700–2,100Ma. The remaining zircons show scattered age distributions from; 400 to 500Ma, 900 to
Sample no. 143Nd/144Nd SE εNd
RZ-03b 0.512177 0.000004 −9.0
RZ-04b 0.512214 0.000005 −8.3
RZ-05a 0.512192 0.000004 −8.7
RZ-06c 0.512207 0.000004 −8.4
RZ-08 0.512209 0.000005 −8.4
RZ-12 0.512180 0.000005 −8.9
RZ-12a 0.512185 0.000005 −8.8
RZ-12c 0.512181 0.000004 −8.9
RZ-13a 0.512210 0.000004 −8.3
AB-02a 0.512162 0.000005 −9.3
AB-02b 0.512169 0.000006 −9.2
Table 1
εNd Values of the Mudstone of the Panas-Pandian Formation
Figure 6. (a) Upper continental crust (UCC) (Rudnick & Gao,2003) normalized trace element diagram for the Panas-Pandian Formation mudstones,
compared with the Middle-Late Eocene sediments from IODP Site U1435 and borehole L21 (Shao, Meng, etal.,2017). (b) Chondrite-normalized REE
distribution plot for the Panas-Pandian Formation mudstones as compared with the UCC and the Middle-Late Eocene sediments from IODP Site U1435 and
borehole L21 (Shao, Meng, etal.,2017). The chondrite values are cited from S. S. Sun and McDonough(1989).
RZ-03b RZ-04b RZ-05a RZ-06c RZ-08 RZ-12 RZ-12a RZ-12c
RZ-13a AB-02a AB-02b
Rb Ba Th USc
VCr
Co Ni
Y
Zr
Nb Hf
0.1
1
Sample/UCC
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb
Lu
110 1001
000
Sample/Chondrite
UCC
b
Borehole L21IODP Site U1435
a
10
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1,100Ma, and 2,200 to 2,600Ma. In Sample RZ-12b, zircons younger than 200Ma account for 35% of the
total zircons and these comprise two groups spanning 90–140Ma and 140–200 Ma, with well-developed
peaks at ∼108Ma and ∼153Ma, respectively (Figure8b). There are also three age groups of 200–300Ma,
380–480Ma, and 500–1,200Ma, and a few ages between 1,500 to 3,400Ma. We note that the age distribution
in this sample is quite similar to that in Sample P027 of Shao, Cao, etal. (2017) (Figure 8c). In contrast,
Sample AB-02f (Yan etal.,2018) contains over 87% Mesozoic zircons clustered in 90–200Ma, with major
age peak at 109Ma and subordinate peak at 151Ma (Figure8d).
5. Discussion
5.1. Age of Breakup Unconformity of the Palawan Microcontinental Block
In classical models of continental breakup, the transition from rifting of continental crust to the onset of
seafloor spreading and drifting is often marked by a prominent breakup unconformity on the conjugate con-
tinental margins, as a result of breakup-induced isostatic uplift that leads to widespread erosion (Driscoll
etal.,1995; Franke,2013; Morley,2016). This erosional unconformity typically truncates syn-rift sediments
in half-graben basins and separates them from the drape of post-rift sediments (Franke,2013). So far, the
age of the breakup unconformity on the northern SCS margin has been extensively investigated, displaying
a diachronous southwestward younging trend consistent with seafloor spreading propagating from north-
east to southwest (e.g., Franke,2013; Morley,2016; Zhou etal.,1995) (Figure9). In the northeast, Cenozoic
strata of the Taixinan Basin exposed on the Western Foothills and the Hsüehshan Range in Taiwan show a
regional breakup unconformity between ∼39 and ∼33Ma (e.g., Huang etal.,2013,2017). To the south of
the Pearl River Mouth Basin, IODP Expedition 349 penetrated the breakup unconformity at Site U1435 and
dated it to ∼34Ma based on the marine microfossils in the post-rift sediments (Q. Y. Li etal.,2017). Within
the Pearl River Mouth Basin, the breakup unconformity was placed around the regional marker horizon T7
at ∼30Ma (e.g., C. Chen etal.,2003; Xie etal.,2014). For the Qiongdongnan Basin in the west, the breakup
unconformity was assigned to 23–22Ma (Zhou etal.,1995).
On the southern SCS margin, the breakup unconformity seen in the Reed Bank area and offshore Palawan
is directly capped by a widespread carbonate platform (Nido Limestone) (Franke,2013). Due to the scarcity
Figure 7. Heavy mineral assemblage from the Panas-Pandian Formation sandstones, as compared with the results
published by Shao, Cao, etal.(2017) (see location of their six samples in Figure2b).
RZ-01
RZ-10b
RZ-12b
AB-02f
20 40 60 80
Percentage(100%)
Sample
Zircon Tourmaline Titanite
Rutile AnataseApatite
Garnet Epidote
MonaziteAmphiboleCr-Spinel
Shao, Cao et al. (2017)
P026
P027
P028
P029
P031
P032
This study
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Figure 8.
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of boreholes penetrating syn-rift sediments, the breakup unconformity of the Palawan microcontinental
block has only been constrained by the post-rift Nido Limestone. The onset of the Nido Limestone deposi-
tion was controlled by the syn-rift deformation leading to a rugged seafloor relief and varied from the Early
Oligocene to the early Early Miocene in wells offshore Palawan (Steuer etal.,2013). Among these wells, the
base of the Nido Limestone in well Busuanga-1 offshore northwest Palawan (Figure1) has been estimated
to be as old as the Early Oligocene (∼32Ma) from the top of planktonic foraminiferal Zone N2 (∼26Ma)
located above ∼200m above the base of the limestone (Steuer etal.,2013). The limestone deposition in the
wells offshore Palawan continued until the early Middle Miocene (∼15Ma) (Steuer etal.,2013). The syn-rift
Figure 8. Comparison of U-Pb age spectra for detrital zircon from (a–e) the Panas-Pandian Formation (this study, Shao, Cao, etal.,2017; Yan etal.,2018), (f)
the modern sediments of the Red River (Clift, Carter, etal.,2006; Fyhn etal.,2019; Hoang etal.,2009; Nguyen etal.,2018; C. Wang etal.,2018), (g) the modern
drainage sediments from central Vietnam (Truong Son Belt and Kontum Massif) (Fyhn etal.,2019; Jonell etal.,2017; Nguyen etal.,2018; Usuki etal.,2013; C.
Wang etal.,2018), (h) the modern sediments of the western tributaries of the Pearl River (C. Liu etal.,2017; Zhao etal.,2015), (i) the modern sediments of the
northeastern tributaries of the Pearl River (C. Liu etal.,2017; X. Xu etal.,2007; Zhao etal.,2015), (j) the Middle-Late Eocene sediments of the boreholes in the
Pearl River Mouth Basin (Shao etal.,2016,2019; W. Wang etal.,2017), (k) the Middle-Late Eocene sediments of IODP Site U1435 (Shao, Meng, etal.,2017),
(l) the Cretaceous sediments of boreholes L35 and B23 on the northern South China Sea margin (Shao, Cao, etal.,2017), (m) the Late Cretaceous Barton
Group and Guinlo Formation of the Malampaya Sound Group in northern Palawan (Padrones etal.,2017; Shao, Cao, etal.,2017; Suggate etal.,2014; Walia
etal.,2012), and (n) the Eocene sediments of the boreholes in the Taixinan Basin (Shao, Cao, etal.,2017).
Figure 9. Simplified stratigraphic column for the Cenozoic sediments in central-southern Palawan showing the timing of the breakup unconformity at
33–32Ma, as compared with the breakup unconformity of the northern margin of the South China Sea (SCS, Chen etal.,2003; Huang etal.,2013,2017; Q. Y. Li
etal.,2017; Xie etal.,2014; Zhou etal.,1995). The Palawan microcontinental block is outlined in red dashed line, while the basins on the northern SCS margin
are outlined in white dashed lines. The pink dashed line marks the continent-ocean boundary, and the yellow lines mark the kinematic flow lines defined from
the seafloor spreading lineaments and the fracture zones within the SCS and showing conjugate segments of passive margin located north and south of the SCS
(Sibuet etal.,2016).
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Panas-Pandian Formation cropping out in central-southern Palawan might help to provide more reliable
age constraint for the breakup unconformity on the Palawan microcontinental block.
Our integrated biostratigraphic study shows that the Panas-Pandian Formation at Site RZ-12 was depos-
ited in the Middle Eocene (47.7–41.9Ma). However, Wolfart et al. (1986) reported Paleogene-Early Eo-
cene planktonic foraminifera and late Middle Eocene-Early Oligocene calcareous nannofossils from the
Panas Formation. Here, we apply the datum planes calibrated by Wade etal.(2011) and Anthonissen and
Ogg(2012) to refine the age of these microfossils. The planktonic foraminifera were only recovered from
Sample PD 104 in central Palawan (Figure2b) and comprise Subbotina velascoensis (Cushman) (LAD at
55.07Ma; P3b–P6a), Globigerina gravelli (Brönnimann) (synonym of Acarinina esnehensis [Nakkady] [P5–
P8]), Acarinina soldadoensis (Brönniman) (FAD at 57.79Ma; P4c–P9), Morozovella aequa (Cushman and
Renz) (FAD at 57.79Ma, LAD at 54.20Ma; P4c–P6b), Globanomalina chapmani (Parr) (P3–P5), Morozovella
subbotinae (Morozova) (FAD at 57.10Ma, LAD at 50.67Ma; P5–P7), and Acarinina wilcoxensis (Cushman
and Ponton) (P5–P7), which indicate a narrow age range of Zones P5–P6a (57–55Ma). However, the ap-
pearance of younger species like Subbotinae linaperta (Finlay) (LAD at 37.96Ma; P7–P15) in the same
sample is inconsistent with the narrow age range of Zones P5–P6a (57–55Ma). Moreover, a neighboring
sample (PD106) contains younger (43.3–32.9Ma) calcareous nannofossils (which will be discussed later).
Therefore, S. linaperta should be recognized as an indigenous fossil, while the Late Paleocene-Early Eocene
(57–55Ma) planktonic foraminiferal assemblage, especially S. velascoensis, M. aequa, and G. chapmani,
are of reworked origin. As a result, we do not adopt the age of Late Paleocene-Early Eocene (57–55Ma)
for the Panas-Pandian Formation. The calcareous nannofossils came from eight samples scattered across
central-southern Palawan (six from central Palawan and two from southern Palawan) (Wolfart etal.,1986)
and include age-diagnostic species like Ericsonia formosa (Kamptner) (LAD at 32.92Ma), Reticulofenestra
bisecta (Hay, Mohler, and Wade), Reticulofenestra umbilica (Levin) (FAD at 43.32Ma; LAD at 32.02Ma),
and Sphenolithus predistentus (Bramlette and Wilcoxon) (LAD at 26.93Ma). These species suggest an age of
Zones NP15c-NP21 (43.3–32.9Ma) and is younger than the microfossils (47.7–41.9Ma) recovered from Site
RZ-12 (Figure4). Therefore, these calcareous nannofossils mainly recovered from central Palawan are like-
ly to be taken from the upper part of the Panas-Pandian Formation, while Site RZ-12 at southern Palawan
might belong to the lower part of the Panas-Pandian Formation. This is consistent with the inference from
field observation that the massive sandstones and the alternating sandstone-mudstone sequences in south-
ern Palawan lie in the lower part of the Panas-Pandian Formation, while the alternating sandstone-mud-
stone sequences in central Palawan lie in the upper part of the Panas-Pandian Formation. Considering both
the present and previous results, we assign the syn-rift Panas-Pandian Formation to the Middle Eocene-ear-
liest Oligocene (47.7–32.9Ma) (Figure4). It is broadly consistent with the late Early Eocene-Late Eocene
age as indicated by the deep-water agglutinated foraminifera (FigureS1) that are widespread in our sam-
ples, although agglutinated benthic foraminifera are not as accurate and extensively used as planktonic fo-
raminifera and calcareous nannofossils for the biostratigraphy of Cenozoic marine sediments. A calcareous
nannofossil assemblage indicative of 43.3–32.9Ma was also found in the syn-rift sediments in northwestern
Mindoro (Concepcion etal.,2012). At a regional scale, this implies that the youngest syn-rift sediments
along the Palawan microcontinental block date to around 33Ma.
The onshore correlative equivalent of the Nido Limestone in central-southern Palawan, known as the
Ransang Limestone (Early Miocene), only occurs as small, patchy remnants in central-southern Palawan
(Aurelio & Peña,2010) and was not found during our fieldwork. We suggest that the Ransang Limestone
should be treated as part of the Nido Limestone spanning the Early Oligocene (∼32Ma) to the early Middle
Miocene (∼15Ma) (Steuer et al.,2013) (Figure9). Although no contact between the Panas-Pandian For-
mation and the Ransang Limestone has been observed onland central-southern Palawan, the syn-rift struc-
tures and sedimentary fills truncated by a prominent seismic reflector corresponding to the Nido Limestone
were observed in seismic profiles offshore Palawan (Aurelio etal.,2014; Steuer etal.,2013). By integrating
both the outcrop and subsurface data, the breakup unconformity of the Palawan microcontinental block
between the Panas-Pandian Formation and the Nido Limestone can be constrained to 33–32Ma (Figure9).
This age reveals that the unconformity is a short hiatus in response to the breakup of the South China mar-
gin. The sharp change in sedimentation from siliciclastics to carbonates across the unconformity reflects a
lack of terrigenous supply owing to the southward drifting of the Palawan microcontinental block following
continental breakup. For the Dangerous Ground area, the breakup unconformity is younger, with a dia-
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chronous Late Oligocene age (i.e., around 28–23Ma, Morley,2016; Steuer etal.,2014). The breakup uncon-
formity along the southern SCS margin also shows a southwestward younging trend similar to the northern
SCS margin. The age of the breakup unconformity of the Palawan microcontinental block (33–32Ma) is
different to that of the Qiongdongnan Basin (23–22Ma) (Zhou etal.,1995) but is close to that of the Pearl
River Mouth Basin (∼30Ma) (C. Chen etal.,2003; Xie etal.,2014), and IODP Site U1435 (∼34Ma) (Q. Y. Li
etal.,2017), suggesting that the Palawan microcontinental block is the counterpart of the Pearl River Mouth
Basin in the southern SCS margin. It is noteworthy that the age of the breakup unconformity of the Pala-
wan microcontinental block also overlaps with that of the Taixinan Basin (Taiwan region, 39–33Ma) (e.g.,
Huang etal.,2013,2017). It is difficult to exclude the conjugate relationship between the Palawan micro-
continental block and the Taixinan Basin only based on the age of the breakup unconformity. However, the
conjugate relationship between the Palawan microcontinental block and the Pearl River Mouth Basin is also
evidenced by the flow-line patterns defined from the seafloor spreading lineaments and the fracture zones
within the SCS (Sibuet etal.,2016) (Figure9), which also proved that the Taiwan segment, corresponding
to the Taixinan Basin in this study, has no counterpart on the southern SCS margin.
5.2. Paleoenvironment of Central-Southern Palawan in the Middle Eocene-Earliest Oligocene
The Palawan microcontinental block was part of the South China margin and attached to the Pearl River
Mouth Basin before opening of the SCS in light of the timing of the breakup unconformity and the flow-line
patterns within the SCS (Sibuet etal.,2016). The sedimentary environment of the Panas-Pandian Formation
in central-southern Palawan can therefore aid understanding of the paleogeography of the South China
margin as well as sediment routing across the continental margin. The Panas-Pandian Formation had a
middle-lower fan setting and contained trace fossil Helminthopsis typical of Nereites ichnofacies and “flysch
type” deep-water agglutinated foraminiferal assemblage. The above evidence suggests middle bathyal to
abyssal depths (>500m), such that the Middle Eocene-earliest Oligocene depositional environment was
confined to the continental slope and abyssal plain.
In contrast, the syn-rift sediments in the half grabens on the northern SCS margin are mostly deposited in
non-marine fluvial to lacustrine environments (Gong & Li,1997; Yang etal.,2012). Although the marine
influence might have expanded southwestward from the East China Sea-Taiwan region to the northern SCS
margin, it only occurred in well D21 in the Early to Middle Eocene (shelf to upper-slope environment), in
well H15 in the Middle Eocene (shelf to upper-slope environment), and in well B7 in the Late Eocene (shelf
environment) (Figure1) (Q. Y. Li etal.,2017). Microfossils from Eocene syn-rift sediments at IODP Sites
U1435, U1501, and U1504 on the distal margin near the continent-ocean boundary also denote coastal and
shelf environments (C. F. Li etal.,2015; Z. Sun etal.,2018). A deep-water environment (∼1,000m depth)
was not widespread until the early seafloor spreading stage, as revealed by the Early Oligocene post-rift
hemiplegic sediments recovered from Ocean Drilling Program (ODP) Site 1,148 and IODP Sites U1435 and
U1501 (C. F. Li etal.,2015; Z. Sun etal.,2018; P. Wang etal.,2000). IODP Expedition 368 obtained diverse
abyssal agglutinated benthic foraminifera from the Late Eocene syn-rift sediments at Site U1502, but such
deep-water conditions are linked to the final stage of continental breakup (the latest Eocene), approaching
the initiation of seafloor spreading (Jian etal.,2019; Larsen etal.,2018). On the southern SCS margin, the
Eocene-earliest Oligocene syn-rift sediments in half-grabens on the Reed Bank area and offshore Palawan
are dominated by deltaic, littoral, and shallow marine deposits, with partly lower neritic to upper bathyal
facies in the southeast (Kudrass etal.,1986; Sales etal.,1997; Yao etal.,2012). This paleoenvironment was
intermediate between that of the northern SCS margin and central-southern Palawan, showing a config-
uration with a southward deepening trend on the South China margin. The southward deepening trend
is consistent with an open ocean, named the Proto-SCS by Hinz etal.(1991) that existed on the southern
extremities of the South China continental margin at that time. Thus, central-southern Palawan was located
on the continental slope of the northern margin of the Proto-SCS (Figure12).
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5.3. Provenance of the Panas-Pandian Formation and Its Paleogeographic Implication
5.3.1. Proximal and Distal Supply From the Cathaysia Block
The trace element and REE distribution patterns of the Panas-Pandian Formation are generally similar to
those of the Middle-Late Eocene sediments from IODP Site U1435 and borehole L21 on the northern SCS
margin (Shao, Meng, etal.,2017) (Figure6) and indicate sediment supply from felsic rocks. The high ZTR
index in most of the Panas-Pandian Formation sandstones analyzed (except Sample RZ-01) (Figure7) also
denotes derivation from felsic source rocks. In the plots of Cr/V-Y/Ni and Co/Th-La/Sc, the Panas-Pandian
Formation samples fall into the field of granite or felsic magmatic rocks and is of similar range to data from
Site U1435 and borehole L21 (Figures10a and 10b). In the La-Th-Sc and Th-Sc-Zr/10 ternary diagrams,
the Panas-Pandian Formation, together with Site U1435 and borehole L21 sediments are plotted between
tectonic settings of continental island arc, active continental margin, and passive continental margin (Fig-
ures10c and10d), consistent with the complex tectonic evolution of Cathaysia Block where granitoids are
widespread. Therefore, the Panas-Pandian Formation might have a similar provenance to the contemporary
Site U1435 and borehole L21 sediments, which were mainly derived from the Cathaysia Block (Shao, Meng,
etal.,2017; see also C. Liu etal.,2017). Noteworthy is the abundance of the unstable mineral epidote in
Sample RZ-01 (Figure7) that might imply a local metamorphic source, either the pre-Cambrian metamor-
phic basement of the Cathaysia Block or the metamorphic rocks related to the Mesozoic subduction along
the South China margin, which were uplifted and exhumed by normal faults during continental rifting
(Kudrass etal.,1986). Mesozoic subduction-related ophiolites might account for common Cr-spinel (10.9%)
but since epidote and Cr-spinel are rare in all other samples (Figure7), such local metamorphic and ophi-
olitic sources would not be common to the Panas-Pandian Formation. No mudstone samples were collected
from Site RZ-01 for trace element and Nd isotope analyses because this site is dominated by coarse- to medi-
um-grained sandstone. Therefore, the ophiolitic source as reflected in the sandstone sample is not observed
in the geochemical and isotopic results.
Integrating detrital zircon U-Pb results from this study with previously published work (Shao, Cao, etal.,2017;
Yan etal.,2018), the Panas-Pandian Formation is dominated by Cretaceous (90–140Ma) and Jurassic (140–
200Ma) zircon grains, with moderate Permian to Triassic (200–300Ma), Ordovician to Silurian (350–480Ma)
and Mesoproterozoic to Cambrian (500–1,200Ma) zircon grains and minor Paleoproterozoic (1,700–2,100Ma)
and Archean to Paleoproterozoic (2,200–2,700Ma) zircon grains (Figure8e). This zircon age distribution and
the dominant Mesozoic age groups are consistent with a Cathaysian provenance. Similar age distributions are
found in modern sediments from the northeastern Pearl River tributaries draining the interior of the Cathaysia
Block (C. Liu etal.,2017; X. Xu etal.,2007; Zhao etal.,2015) (Figure8i). We can exclude the Indochina Block
and the Yangtze Block as the major source areas for the Panas-Pandian Formation by comparing probability
density plots, pie charts (Figure8), and multidimensional scaling (MDS) plot (Figure 11) of detrital zircon
U-Pb age data from the Panas-Pandian Formation with these regions. The modern sediments from the Red
River flowing between the Yangtze Block and the Indochina Block (Clift, Carter, etal.,2006; Fyhn etal.,2019;
Hoang etal.,2009; Nguyen etal.,2018; C. Wang etal.,2018), the rivers draining central Vietnam (Truong Son
Belt and Kontum Massif of the Indochina Block in Figure 1) (Fyhn etal.,2019; Jonell etal., 2017; Nguyen
etal.,2018; Usuki etal.,2013; C. Wang etal.,2018), and the western Pearl River tributaries primarily draining
the Yangtze Block (C. Liu etal.,2017; Zhao etal.,2015) only contain minor Yanshanian (80–200Ma) zircon
grains generally lower than 7% of the total zircon grains (Figures8f–8h). The abundance of the Yanshanian
zircons in the modern sediments from these rivers are significantly lower than the age groups of 200–300Ma,
300–500Ma, and 500–1,500Ma (Figures8f–8h), contrasting to the dominance of the Yanshanian zircons in
the Panas-Pandian Formation samples (Figure8e). In the MDS plot (Figure11), the modern sediments from
the Red River, the rivers in central Vietnam, and the western Pearl River tributaries are grouped together and
well separated from the Panas-Pandian Formation samples. Therefore, northern-central Vietnam (the Indo-
china Block) and the Yangtze Block could not be major sources for the Panas-Pandian Formation. Hainan Is-
land is also unlikely to be a major source of central-southern Palawan sediments because Jurassic-Cretaceous
granites in Hainan Island are less common than Triassic granites (Shao etal., 2019). By comparing detrital
zircon U-Pb age data from the Panas-Pandian Formation with modern sediments from the northeastern Pearl
River tributaries and Middle to Late Eocene sediments from the northern SCS margin (Figures8 and11), we
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Figure 10. Plots of (a) Cr/V versus Y/Ni and (b) Co/Th versus La/Sc and ternary plots of (c) La-Th-Sc and (d) Th-Sc-Zr/10 of the Panas-Pandian Formation
mudstones, as compared with the Middle-Late Eocene sediments from IODP Site U1435 and borehole L21 (Shao, Meng, etal.,2017). (e) Relative probability
plot of Nd isotope compositional ranges of the Panas-Pandian Formation mudstones. Also shown are those of the Cathaysia Block, the Yangtze Block, the
Indochina Block (Clift, Blusztajn, & Nguyen,2006 and references therein), and the Hong Kong granites (Darbyshire & Sewell,1997). The εNd values of the
sediments from the outlets of the Bei and Dong Rivers (C. Liu etal.,2017) and the average εNd value of the Hong Kong granites are marked as asterisks.
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infer that the Panas-Pandian Formation was derived from both proximal and distal source regions within the
Cathaysia Block.
It is noteworthy that Sample AB-02f in central Palawan shows a unimodal age distribution with an age peak
of ∼110Ma, which resembles zircon ages from Middle to Late Eocene sediments of IODP Site U1435 (Shao,
Meng, etal.,2017) (Figures8d and8k). The percentage of the Yanshanian zircon grains of 80–200Ma is as
high as 87% in this sample, even higher than that in Site U1435 sediments (80%). Contemporary sediments
from the Pearl River Mouth Basin (Shao etal.,2016,2019; W. Wang etal.,2017) generally show a similar
restricted age range except for a peak at 44Ma associated with syn-rift volcanism (Figure8j). Sediments
from two boreholes (ZI-2 and ZI-3) in the northern Pearl River Mouth Basin have a stronger affinity with the
northeastern Pearl River tributaries (Figure11) probably owing to their proximity to the interior Cathaysia
Block. Site U1435 and Pearl River Mouth Basin sediments were interpreted to be primarily eroded from
proximal, local basement uplifts that were mainly composed of Mesozoic granitoids (Shao etal.,2016,2019;
Shao, Meng, etal.,2017; W. Wang etal.,2017). A review of industrial wells in the Pearl River Mouth Basin
shows that 94 wells have drilled into the pre-Cenozoic basement (Ye etal.,2018) of which 85 wells inter-
sected granitoids in both structural uplifts and depressions, only two wells encountered Mesozoic sedimen-
tary rocks in structural depressions in the easternmost part of the basin. The ages of basement granitoids
from 17 wells, determined by zircon U-Pb dating ranges from 161.6 to 101.7Ma (Shi etal.,2011; C. H. Xu
etal.,2016). Within the Palawan microcontinental block, only the Daroctan Granite is of Late Cretaceous
Figure 11. Nonmetric multidimensional scaling (MDS) map (Vermeesch,2013) for detrital zircon U-Pb ages of
the Panas-Pandian Formation (this study and Yan etal.,2018, black circles; Shao, Cao, etal.,2017, orange circles),
the modern sediments from the Red River, the rivers draining central Vietnam and the western tributaries and the
northeastern tributaries of the Pearl River (WPR and NEPR), the Middle-Late Eocene sediments of the boreholes in
the Pearl River Mouth Basin (green diamonds), the Middle-Late Eocene sediments of IODP Site U1435, the Eocene
sediments of the boreholes in the Taixinan Basin (blue diamonds), the Cretaceous sediments of the boreholes on the
northern South China Sea margin (purple diamonds), and the Late Cretaceous Barton Group and Guinlo Formation in
northern Palawan. References are the same as in Figure8. The Kolmogorov-Smirnov effect size is used as the measure
of dissimilarity. The stress value is 0.066, indicating a good goodness-of-fit. Solid and dashed lines in the map indicate
the closest and second closest neighbors, respectively.
NEPR
P027
P028
RZ-01
P026
P032
P031
U1435
AB-02f
P029
0.00.2
RZ-12bZI-3
ZI-2
U-
1
ZI-4
ZI-1 L21
D2
T1
Panas-Pandian Formation
}
Pearl River Mouth Basin, Mid-LateEocene
Taixinan Basin, Eocene
WPR
С. Vietnam
Barton
Group
Guinlo
Fm.
B23
L35
0.4 0.6
-0.2-0.4-0.6
-0.6 -0.4 -0.2 0.00.2 0.4
Red River
Northern SCS margin, Cretaceous
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age (87Ma) in northern Palawan (Padrones etal.,2017), which is young-
er than the youngest detrital zircon (93.8Ma) in Sample AB-02f. There-
fore, we consider the Mesozoic granitic basement uplifts in the Pearl
River Mouth Basin as the most possible source area for Sample AB-02f.
Within and around the Pearl River Mouth Basin, detrital zircon U-Pb age
data of the Cretaceous sedimentary rock is only available from borehole
L23 in the easternmost part of the basin and borehole B23 located near
the boundary between the Pearl River Mouth Basin and the Qiongdong-
nan Basin (Shao, Cao, etal.,2017) and display a unimodal age distribu-
tion dominated by Yanshanian zircon grains (Figure8l). Such a unimod-
al age distribution is also typical of the Late Cretaceous Barton Group
and Guinlo Formation (the terrigenous clastic unit in the Malampaya
Sound Group) in northern Palawan (Padrones et al., 2017; Shao, Cao,
etal.,2017; Suggate etal.,2014; Walia etal.,2012) (Figure8m). These
Mesozoic sediments/metasediments from the northern SCS margin and
Palawan microcontinental block were interpreted to be derived from the
Mesozoic continental arc along the South China margin (e.g., Shao, Cao,
etal.,2017). They show close linkage with Sample AB-02f and Site U1435
sediments on the MDS plot (Figure11). Therefore, Sample AB-02f is also
possible to be recycled from the Mesozoic sedimentary rocks in the base-
ment uplifts in the Pearl River Mouth Basin or the Palawan microconti-
nental block, besides directly eroded from the Mesozoic granitoids.
Samples RZ-12b (Middle Eocene, 47.7–41.9 Ma) and P027 (Shao, Cao,
etal.,2017) in southern Palawan share almost the same age distribution
pattern as modern sediments from the northeastern Pearl River tributar-
ies (Figures8a, 8c, 8i and 11). They are characterized by a multimodal
age distribution, with a lower percentage of Yanshanian (80–200Ma)
ages (34%–36%), and higher percentage of grains with ages between
200–300 Ma (13%–14%), 300–500 Ma (18%–24%), and 500–1,500 Ma
(21%–27%), relative to the unimodal age distribution. This indicates that samples RZ-12b and P027 were
mainly sourced from the interior of the Cathaysia Block where the northeastern Pearl River tributaries
drain. Zircon grains of 200–300Ma and 300–500Ma might be derived from the Caledonian and Indosinian
granitoids restricted along the northern boundary of the Cathaysia Block (Figure1). Alternatively, these
zircon grains, together with the older age groups, could be also recycled from the Paleozoic and Mesozoic
sedimentary rocks, which are part of the basement of the Pearl River Mouth Basin (X. M. Sun etal.,2014)
and the Palawan microcontinental block (Franke etal.,2011; Kudrass etal.,1986). However, the Paleozoic
and Mesozoic sedimentary basement rocks seem unlikely to be the major source of Samples RZ-12b and
P027. For the Pearl River Mouth Basin, the intrabasinal uplifts are dominated by Mesozoic granitoids in
light of the drilling data (Ye etal.,2018). The Palawan microcontinent block was mostly submerged during
the Eocene and transient uplift and erosion of the Reed Bank was assumed to occur at the end of the Mid-
dle Eocene (Yao etal.,2012), which contradicts the depositional age of Sample RZ-12b within the Middle
Eocene. More importantly, the Cretaceous sediments from the Pearl River Mouth Basin and the Palawan
microcontinental block, displaying unimodal age distribution dominated by Yanshanian zircon grains (Fig-
ures8l and8m), could not supply abundant zircons older than 200Ma to samples RZ-12b and P027. The
Eocene sediments from the Taixinan Basin also exhibit a multimodal age distribution, but generally with
much less zircons younger than that of 500Ma and much more zircons older than 500Ma (Figure8n). Sed-
iments from boreholes ZI-2 and ZI-3 in the northern Pearl River Mouth Basin and from borehole D2 in the
Taixinan Basin, showing affinity with the northeastern Pearl River tributaries (Figure11), might not require
a direct sedimentary connection to central-southern Palawan. These sediments were likely to be delivered
by other small rivers draining the interior Cathaysia Block given the location of the boreholes.
Therefore, the Panas-Pandian Formation might have two major source regions, a proximal source from the
Mesozoic basement uplifts composed of granitic and possibly sedimentary rocks and a distal source from the
interior Cathaysia Block. This is also supported by the age peaks of the Mesozoic zircons. Samples RZ-12b
Figure 12. Paleogeographic reconstruction for the South China margin
during the Middle Eocene (47.7–41.9Ma) (revised from Hall,2002,2012;
Q. Y. Li etal.,2017), showing two source regions for the Panas-Pandian
Formation, a proximal source from the Mesozoic basement uplift
and a distant source from the interior Cathaysia Block. The inferred
Middle Eocene coastline is mainly based on Huang etal.(2019), Q. Y.
Li etal.(2017) and Yao etal.(2012). PRMB=Pearl River Mouth Basin;
QDNB=Qiongdongnan Basin; TXNB=Taixinan Basin; RB=Reed Bank;
and MB=Macclesfield Bank.
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and P027 with a predominantly distal source have a stronger Early Yanshanian peak at ∼150Ma, whereas
Sample AB-02f with a predominantly proximal source show a stronger Late Yanshanian peak at ∼110Ma
(Figure8), which is in agreement with the oceanward younging trend of the Yanshanian granitoids as a
result of steepening of the subduction angle of the Pacific Plate beneath Eurasia (Z. X. Li & Li,2007). The
remaining samples in central-southern Palawan might result from mixing of these two sources (Figure11).
The Nd isotope results of the Panas-Pandian Formation are less effective in discriminating the source areas,
because the εNd values of the Panas-Pandian Formation (−9.3 to −8.3) fall into all the ranges of the Cathay-
sia Block (−15.7 to 11.8), the Yangtze Block (−31.1 to 2.6), and the Indochina Block (−21.8 to 14.8) (Clift,
Blusztajn, & Nguyen,2006 and references therein). Nevertheless, they are most close to the major peak of
the Cathaysia Block (Figure10e). It is intriguing that the Panas-Pandian Formation only show small Nd
isotopic variation between the analyzed samples (Table1 and Figure10e), although two major sources are
indicated by the detrital zircon U-Pb age data. This could be explained by mixing of two sources with εNd
values similar to the Panas-Pandian Formation (from −9.3 to −8.3). The modern sediments collected from
the outlets of the Bei River and the Dong River, which might represent the average composition of the inte-
rior Cathaysia Block, show εNd values of −11.2±0.2 and −9.9±0.7, respectively (C. Liu etal.,2017) (Fig-
ure10e). These εNd values, especially that of the Dong River, are close to the εNd values of the Panas-Pandian
Formation. Although no Nd isotope data of the Mesozoic granitoids have been reported from the Pearl River
Mouth Basin and the Palawan microcontinental block yet, the Mesozoic Hong Kong granites, which crop
out in a small area along the South China coast, might be treated as an alternative to the Mesozoic grani-
toids exposed in local basement uplifts. The Hong Kong granites, predominately of Jurassic-Cretaceous age,
show a wide range of εNd values from −15.7 to −4.4 (Darbyshire & Sewell,1997) (Figure10e). The granite
samples (n=40) yield an average value of −7.9, also close to the εNd values of the Panas-Pandian Forma-
tion. However, such similarity in εNd values may have resulted from sampling bias of the granite samples.
Future Nd isotope work of the Mesozoic granitoids and the granitoid-derived sediments from the northern
SCS margin and the Palawan microcontinental block needs to be done to interpret the small Nd isotopic
variation of the Panas-Pandian Formation.
5.3.2. Paleogeographic and Tectonic Implication for the Rifted South China Margin
Although a near-modern drainage configuration of the Pearl River had not formed until the Early Miocene
(Cao etal., 2018; C. Liu et al., 2017), a paleo-Pearl River confined to the Cathaysia Block like its mod-
ern northeastern tributaries was previously proposed to be established since the late Early Oligocene (Cao
etal.,2018; Shao etal.,2016). This scenario is suggested by the large set of a southward-prograding deltaic
sequences observed in seismic profiles across the southern Pearl River Mouth Basin (Pang etal.,2009) and
by the multimodal age spectra of detrital zircons in the Lower Oligocene samples of borehole X28 and P33
in the Pearl River Mouth Basin (Shao etal.,2016; W. Wang etal.,2019). However, in light of the similarity
in detrital zircon U-Pb age spectra between Samples RZ-12b and P027 from the Panas-Pandian Formation
and modern sediments from the northeastern Pearl River tributaries, the paleo-Pearl River can be dated
back to the Middle Eocene (47.7–41.9Ma) based on the depositional age of Sample RZ-12b. It flowed across
the rifted South China margin, delivering sediments from the interior Cathaysia Block to central-southern
Palawan on the continental slope (Figure12). This scenario is consistent with a complex Eocene south-
ward-prograding deltaic system as observed in the Reed Bank area (Yao etal.,2012). It could be deduced
that although the structural uplifts on the rifted South China margin, which act as local sediment source,
might obstruct the sediments transported from the interior Cathaysia Block, the structural depressions
might serve as passages for these sediments to be delivered to central-southern Palawan during the Middle
Eocene-earliest Oligocene. The sediment routing also argues against the existence of a narrow, elongate
deep-sea basin/gulf between the northern SCS margin and the area that became the Palawan microconti-
nental block, otherwise the sediments with affinity of northeastern Pearl River tributaries would not reach
central-southern Palawan.
From the evidence presented above, we consider that the Panas-Pandian Formation was deposited prior to
SCS seafloor spreading when the South China continental margin developed into a wide rift system with a
low topographic gradient (Figure12). Such a wide rift system typically develops on hot, weak continental
crust, which is in agreement with the weak crustal rheology of the South China margin relating to its pre-
rift history where plate convergence led to crustal thickening and magmatic additions in a continental arc
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regime shortly before the onset of rifting (Brune etal.,2017; Clift etal.,2002). A deep-water basin between
the Pearl River Mouth Basin and the Palawan microcontinental block did not fully exist until the Latest Eo-
cene, as indicated by deep-water environment sediments at IODP Site U1502 (Jian etal.,2019). This change
is consistent with a phase of rapid rifting in the Late Eocene-Early Oligocene, just before final breakup and
seafloor spreading (Brune etal.,2016; Larsen etal.,2018). The rift acceleration might result from the suc-
cessive weakening of the rift center due to necking and strain softening with continued deformation (Brune
etal.,2016).
6. Conclusion
Our stratigraphic and provenance work on the syn-rift Panas-Pandian Formation from the southern margin
of the Palawan microcontinental block improves the understanding of the paleogeography of the South
China margin prior to opening of the SCS. Biostratigraphic constraints from planktonic foraminifera and
calcareous nannofossils show that the Panas-Pandian Formation was deposited in the Middle Eocene-ear-
liest Oligocene (47.7–32.9 Ma). Therefore, the breakup unconformity on the Palawan microcontinental
block, between the syn-rift Panas-Pandian Formation and the post-rift Nido Limestone (as old as ∼32Ma,
Steuer etal., 2013) is constrained to 33–32Ma. This age is close to the age of the breakup unconformity
of the Pearl River Mouth Basin (∼30Ma) and IODP Site U1435 (∼34Ma), suggesting that the Pearl River
Mouth Basin is the conjugate margin of the Palawan microcontinental block, which is also supported by
the flow-line patterns within the SCS (Sibuet etal.,2016). Trace fossil Helminthopsis and deep-water agglu-
tinated benthic foraminifera observed in the Panas-Pandian Formation indicate middle bathyal to abyssal
environment (>500m water depth), which locates central-southern Palawan on the continental slope of
the South China margin in the Middle Eocene-earliest Oligocene. All the results of trace elements, Nd
isotope, heavy mineral assemblage, and detrital zircon U-Pb geochronology indicate the Cathaysia Block as
the potential source area of the Panas-Pandian Formation. The detrital zircon U-Pb age distribution of the
Panas-Pandian Formation further suggest two major source regions, a proximal source from local Mesozoic
basement uplift (granitic and possibly sedimentary rocks) and a distant source from the interior Cathaysia
Block. The distant source indicates the establishment of a paleo-Pearl River like its modern northeastern
tributaries at least since the Middle Eocene (47.7–41.9Ma). It could deliver sediments across the South
China margin from the interior Cathaysia Block to central-southern Palawan on the continental slope. This
source-to-sink transport process implies a wide rifted margin with a low topographic gradient generated on
the hot, weak continental crust of South China before the seafloor spreading.
Data Availability Statement
All the data used in this study are provided in Supporting Information and can be also found in Mendeley
Data (http://dx.doi.org/10.17632/d6r9ggdbw4.1).
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Acknowledgments
This research was financially
supported by the National Natural
Science Foundation of China (grants
41606068, 42076053, U1701641 and
41972049), research grants from the
Key Special Project for Introduced
Talents of Southern Marine Science and
Engineering Guangdong Laboratory
(Guangzhou) (GML2019ZD0202
and GML2019ZD0205) and from the
Innovation Academy of South China
Sea Ecology and Environmental Engi-
neering, Chinese Academy of Sciences
(ISEE2020YB07), and the Research
Fund Program of Guangdong Provincial
Key Laboratory of Marine Resourc-
es and Coastal Engineering (grant
GDKLMRCE1804). This is contribution
No. IS-3064 from GIGCAS. We thank
Pacle Nichole Anthony D., Villaplaza
Barbie Ross B., and Tian Zhixian for
their assistance in the fieldwork. We
also appreciate Sun Shengling, Zhang
Le, Wu Dan, and Yao Deng for their
help on the analyses in GIGCAS labo-
ratories. We are grateful to Schildgen
Taylor and Yan Zhen for their editorial
handling and to Kirstein Linda and an
anonymous reviewer for their valuable
and constructive comments, which
have substantially improved the study.
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Erratum
In the originally published version of this article, the age of samples from Site RZ-12 was incorrectly listed
as 47.7–42.1 Ma. This article has been corrected to list the correct age, which is 47.7–41.9 Ma. This version
may be considered the authoritative version of record.
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