Deciphering the Early Cretaceous transgression in coastal southeastern China: Constraints based on petrography, paleontology and geochemistry

Palaeogeography, Palaeoclimatology, Palaeoecology 317–318 (2012) 182–195

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Deciphering the Early Cretaceous transgression in coastal southeastern China: Constraints based on petrography, paleontology and geochemistry Guang Hu a, Wenxuan Hu a, b, Jian Cao a, b,⁎, Suping Yao a, Xiaomin Xie c, Yongxiang Li a, Youxiang Liu a, Xueyin Wang a a b c

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing Jiangsu 210093, China Institute of Energy Sciences, Nanjing University, Nanjing 210093, China Wuxi Research Institute of Petroleum Geology, SINOPEC, Wuxi Jiangsu 214151, China

a r t i c l e

i n f o

Article history: Received 19 August 2011 Received in revised form 26 December 2011 Accepted 2 January 2012 Available online 8 January 2012 Keywords: Early Cretaceous transgression Biomarker Carbon isotope U–Pb zircon dating Coastal southeastern China

a b s t r a c t The occurrence of a transgression in coastal southeastern China during the Early Cretaceous has been debated. This question was addressed here through the use of petrographic, paleontological and geochemical studies of ten representative Lower Cretaceous outcrop sections in the study area. The petrographic results suggest that limestones in the Shipu section of northeastern Zhejiang Province were predominantly deposited in a tidal flat environment. The paleontological study reveals marine red and brown algae in black shales and mudstones of the Yong'an and Chong'an sections of the western Fujian Province. In the biomarker study of black shales and mudstones, the widespread detection of gammacerane, in combination with ratios of tricyclic terpane C26/C25 b 1.3, hopane C35S/C34S > 0.8 and hopane C29/C30 b 0.8, demonstrates that the study area was influenced by a transgression. A carbon isotopic study provides additional evidence of the transgression, including positive carbon isotopes of the majority of calcareous mudstone and limestone samples (approximately 2.5‰), and correlation of the values between saturated (− 30.01‰ to − 22.87‰) and aromatic (− 29.42‰ to − 21.35‰) hydrocarbons of shale and mudstone samples. Thus, transgression did take place in coastal southeastern China during the Early Cretaceous, and was broadly simultaneous (from 119 ± 3 Ma to 99 ± 3 Ma) in different depositional regions based on zircon U–Pb dating. Under this isochronous framework, a paleogeographical limit of the transgression was tentatively proposed for the first time. The northern boundary extends at least to 29° N, whereas the western boundary is limited to the southeastern side of the Wuyi Mountains. Regional tectonic subsidence together with the overall high sea level may be the main driving force for the transgression. These results have broad implications for regional studies and for Cretaceous paleogeographical studies in general. © 2012 Elsevier B.V. All rights reserved.

1. Introduction It has been widely acknowledged that the Cretaceous was a period of exceptionally high sea-level (Haq et al., 1987; Skelton et al., 2003), and thus, inundations by sea water were frequent in the main continental margins (Cooper, 1977; Hallam, 1977; Matsumoto, 1980), such as during the transgression in northeastern China (Sha et al., 2008). However, evidence of transgression is less clear in southeastern China where the Upper Mesozoic is mainly characterized by terrestrial pyroclastic rocks interbedded with silica rocks under a general tectonic setting of high uplift after the Early Mesozoic Indosinian Orogeny (Wang and Zhou, 2002) and of volcanism stimulated by the westward subduction of the Paleo-Pacific plate below the Eurasian continental margin

⁎ Corresponding author at: Department of Earth Sciences, Nanjing University, Nanjing 210093, China. Tel.: + 86 25 83686649 (Office); fax: + 86 25 83686016. E-mail address: [email protected] (J. Cao). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2012.01.008

(Gilder et al., 1996; Li, 2000; Wang and Zhou, 2002). Therefore, southeastern China during the Late Mesozoic has been suggested to have been characterized by continental sedimentation (Chen, 1979; Li, 1996; Chen, 2000), However, this perspective was questioned as some works have provided evidence of marine deposition/transgression, especially in coastal southeastern China during the Early Cretaceous. The two fossil discoveries in the eastern Zhejiang Province are the most representative of these works including marine fish fossils in the Lower Cretaceous Guantou Formation in the Xiaolin Basin (Zhang and Zhou, 1977) and marine stromatolites, coccolithes, diatom and serpulid fossils in the Shipu Group limestones of the Shipu outcrop section (Xu and Zheng, 1989)(Fig. 1). According to these reports, sea water may have at least invaded into the eastern Zhejiang Province during the Early Cretaceous. Hence, the depositional paleogeography, as well as the tectonic paleogeography, of coastal southeastern China should be reinvestigated. However, there have been few studies on the critical issue of the occurrence of an Early Cretaceous transgression.

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Fig. 1. Sketch map showing present-day Early Cretaceous remained basins (mainly after the Petroleum Geological Survey of China's National Geological Administration, 1975), ten representative outcrop sections and boundary of the transgression in the coastal southeastern China. According to rock types, sedimentary associations and the geotectonic positions, the study area can be subdivided into five depositional regions (A–E). Sections ①–③, ④–⑥, ⑦–⑨ and ⑩ are located in the regions A, B, C and D, respectively. Ten representative outcrop sections include: ① the Shipu section of the Guantou Formation in the Shipu city of the northeastern Zhejiang Province; ② the Shangzhang section of the Guantou Formation in the Xianju city of the northeastern Zhejiang Province; ③ the Changtan section of the Guantou Formation in the Taizhou city of the northeastern Zhejiang Province; ④ the Laozhu section of the Guantou Formation in the Lishui city of the northern Zhejiang Province; ⑤ the Xiahuyuan section of the Guantou Formation in the Liucheng city of the northern Zhejiang Province; ⑥ the Hushan section of the Guantou Formation in the Suichang city of the northern Zhejiang Province; ⑦ the Chong'an section of the Guantou Formation in the Wuyi Mountains of the northwestern Fujian Province; ⑧ the Julan section of the Bantou Formation in the Julan city of the western Fujian Province; ⑨ the Yong'an section of the Bantou Formation in the Yong'an city of the western Fujian Province; and ⑩ the Yiyang section of the Shixi Formation in the Yiyang city of the northeastern Jiangxi Province.

In addition to the two aforementioned areas, other areas, such as the Bantou Formation in the western Fujian Province and the Shixi Formation in the eastern Jiangxi Province, also contain several sets of black or gray-black mudstones that were deposited coevally with the Guantou Formation in Zhejiang Province (Jiangxi Geology And Mineral Resources Bureau, 1984, 2008; Fujian Geology And Mineral Resources Bureau, 1985, 1997; Zhejiang Geology And Mineral Resources Bureau, 1989, 1996). However, whether transgressions took place in these areas remains unclear. In this paper, we conducted a study to elucidate the possibility of an Early Cretaceous transgression in coastal southeastern China. Firstly, we investigated almost all the present-day basins of Early Cretaceous age in the study area. Then, based on the geological reconnaissance, ten representative well-preserved Lower Cretaceous outcrop sections were

selected for an integrated study of the petrography, paleontology and geochemistry with the objective of determining whether a transgression occurred, the timing constraints and the paleogeographical limits of the transgression. 2. Geological setting and outcrop sections The study area of coastal southeastern China commonly refers to the area defined by the Jiangshan–Shaoxin Fault and Ganjiang Fault as the northern and western boundaries, respectively. The NNEtrending Zhenghe-Dapu Fault runs through the central part of the area (Fig. 1). The area contains several tens of Early Cretaceous NE-trending rift basins, which were formed due to the lithospheric thinning and extension triggered by the westward subduction of the

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Paleo-Pacific plate (Wang and Zhou, 2002; Shu et al., 2009). These basins were substantially reshaped by later tectonic activities (Shu et al., 2009). From these basins, we selected ten representative outcrop sections with good exposure and preservation of Early Cretaceous strata (Fig. 1).

As shown in Figs. 2 and 3, the lithology of the study area can be generally divided into three types of lithofacies: (1) volcaniclastic rocks, (2) detrital rocks with little volcanic debris, and (3) carbonate rocks. The volcaniclastic rocks, including pyroclastic rocks and tuffaceous detrital rocks, occur widely across the study area, and the

Fig. 2. Photographs showing outcrop sections and the deposited rocks. (a) the Shipu section ①; (b) the Shangzhang section ②; (c) the Hushan section ⑥; (d) the Chong'an section ⑦; (e) the Yong'an section ⑨; (f) the Yiyang section ⑩. See location of the section in Fig. 1.

Fig. 3. Lithology, sedimentary cycles and stratigraphic correlation of the Late Aptian to Albian transgressions in the coastal southeastern China with eustatic sea-level and total crust production. sample locations being marked.

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proportion of volcaniclastic rocks decreases in the westward direction. The detrital rocks in the area, including gray sandstones and siltstones, dark gray mudstones and black shales, are mainly distributed to the west of the Zhenghe-Dapu Fault. The carbonate rocks, including limestones and calcareous mudstones, primarily interbedded, crop out in several sections, such as the Shipu, Xiahuyuan and Yiyang sections. In general, five fining-upward cycles can be recognized in each section except for the Yiyang section, and they are well correlated (Fig. 3). According to the lithologic characteristics, facies associations and geotectonic natures, the study area can be subdivided into five depositional regions as A–E shown in Fig. 1. Each depositional region contains 1–3 representative outcrop sections, except area E due to the wide coverage of volcanic strata. Region A is located to the east of the Zhenghe-Dapu Fault of the eastern Zhejiang Province, and includes three representative outcrop sections ①–③. The stratigraphic succession of the Lower Cretaceous Guantou Formation is mainly composed of volcaniclastic rocks intercalated with detrital sedimentary rocks, such as tuffaceous sandstones and shales. The carbonates are mainly comprised of stromatolites, marls and bioclastic limestones. One distinctive feature of the rocks in this region is that they have the lowest proportion of the black, grayish-green mudstone among the five regions. Five finingor deepening-upward cycles can be recognized in each section (Fig. 3). Region B, which also has three representative outcrop sections, including sections ④–⑥, is located to the west of the Zhenghe-Dapu Fault. The Guantou Formation in this area mainly consists of volcano-sedimentary rocks and normal sedimentary rocks. This lithological feature is similar to that of region A. However, the proportion of black and grayish-green fine-grained rocks is higher, and the limestone interbeds are absent. Occasionally, calcareous mudstone nodules or interlayers can be found in the Xiahuyuan section. The fining-upward cycles in each section are similar to those of the region A (Fig. 3). Region C, which also having three representative outcrop sections, including sections ⑦–⑨, is situated between the Zhenghe-Dapu Fault and the Wuyi Mountains of the Fujian Province. The strata here are called the Bantou Formation, which generally corresponds to the Guantou Formation in the regions A and B. The Bantou Formation is mainly composed of blackish gray mudstones, siltstones and black shales. Both the volcano-sedimentary rocks and calcareous mudstones are observed less frequently here than in regions A and B. However, the rock structure and cycles are similar to those of regions A and B (Fig. 3). Region D has only one representative outcrop, section ⑩, where the exposed Shixi Formation corresponds to the Guantou Formation. However, the lithology is distinctly different and is mainly characterized by red and brown sandstones, siltstones and pebbly sandstones. The grayish green mudstones and calcareous mudstones occasionally occur in the lower part of the section. Region E in the eastern Fujian Province is bounded by the Zhenghe-Dapu Fault to the west (Fig. 1). The strata in this region that are comparable to the Guantou and Bantou formations mainly consist of terrestrial pyroclastic deposits. Thus, the rocks may not preserve significant information on the transgression. 3. Samples and methods Integrated petrographic, paleontological and geochemical analyses were conducted. The sampling strategies vary according to different research purposes, lithologies and method requirements. The samples of limestones and calcareous mudstones were prepared for petrographic and carbon isotope geochemical studies. The samples of black shales and mudstones were collected for paleontological studies, and biomarker analyses and carbon isotope analyses of

bitumen extracts. In addition, the samples of volcaniclastic rocks were collected for zircon LA-ICPMS U–Pb dating. A total of 236 samples were collected for thin section observations to precisely characterize the petrographic features. Then, appropriate samples were selected for different analyses according to requirements of the research objectives and methods. The sampling locations are shown in Fig. 3 and mainly included 27 limestone and calcareous mudstone samples for carbon isotope analyses, 20 black mudstones and shales for biomarker and carbon isotope analyses and 6 tuff samples for zircon U–Pb dating analyses. Thin sections preparation and petrographic observations were made and identified at the State Key Laboratory for Mineral Deposits Research of Nanjing University. Acid-macerated thin sections of the black shale and mudstone samples were prepared for paleontological study at the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences. The limestone and calcareous mudstone samples that were collected for carbon isotope analysis were ground to less than 100 mesh, and were then soaked in diluted hydrogen peroxide for 72 h prior to carbon isotope measurements. After lyophilization, the powders were allowed to react with 100% H3PO4 under a vacuum for 12 h, and the produced CO2 was introduced into a MAT 252 mass spectrometer for the measurement of the 13C/ 12C ratios. The δ 13C values were reported in per mil relative to V-PDB (Vienna Peedee belemnite), and the precision of the carbon isotope measurements were better than ±0.1‰. The mass spectrometer analyses were conducted at the State Key Laboratory for Mineral Deposit Research of Nanjing University. The rock samples intended for biomarker and carbon isotope analyses were ground to 100 mesh and the powdered samples were extracted using a Soxhlet apparatus with chloroform for 72 h. The extractable organic matter was deasphalted by precipitation with n-hexane followed by filtration. The deasphalted extracts were fractionated by column chromatography using alumina over a silica gel. Saturated hydrocarbons, aromatic hydrocarbons and non-hydrocarbons were obtained by successively eluting with n-hexane, toluene and chloroform/methanol (98:2), respectively. The saturated hydrocarbons were analyzed using a HP 6890 mass spectrometer (GC–MS) equipped with a DB-5MS capillary column (30 m ×0.25 mm×0.25 μm). The gas chromatography (GC) oven temperature was programmed to increase from 80 °C to 300 °C at 3 °C/min, and then maintain at this temperature for 30 min. Helium was used as the carrier gas. The mass spectrometer was configured for electron ionization at 70 eV with an ion source temperature of 300 °C. The GC–MS system was operated in the full scan mode from m/z 20 to m/z 750. The GC–MS analyses were performed at the Wuxi Research Institute of Petroleum Geology, SINOPEC. Stable carbon isotope compositions of extracted bitumen were determined using the static combustion method described by Engeland and Maynard (1989). The saturated and aromatic hydrocarbons were combusted in quartz tubes at 800 °C for 10 min. After cooling, the produced CO2 was transferred directly to the inlet system of a Finnigan MAT 252 mass spectrometer for the determination of carbon isotope ratios. The carbon isotopic values are expressed relative to the PDB carbonate. These analyses were performed at the Wuxi Research Institute of Petroleum Geology, SINOPEC. Tuff samples for Zircon U–Pb dating were ground to approximately 60 meshes. Approximately 200 zircons were separated from each sample using standard density and magnetic separation techniques. Cathodoluminescence (CL) images were obtained to select appropriate target sites for U–Pb dating. Zircon U–Pb analyses were performed using Agilent 7500a ICP-MS. The laser ablation system delivers a beam of 213 nm UV light with a diameter of 20–30 μm and an energy of 10–20 J/cm 2. Each analysis lasted 100 s (40 s and 60 s to obtain the background and signal, respectively). The mass discrimination of the mass spectrometer and residual elemental fraction were corrected

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by calibration against a homogeneous zircon standard, GEMOC/GJ (609 Ma). Mud Tank (735 Ma) was analyzed in each run as an independent control of the reproducibility and instrument stability. Hence, every run contained approximately 10 samples, 4 standards and one independent standard analysis. The detailed analytical procedures were similar to those described by Jackson et al. (2004). The raw ICP-MS data were processed using GLITTER software. Common Pb was evaluated using the method described by Andersen (2002). The age calculation and plotting of Concordia diagrams were completed using Isoplot software (ver. 2.49) (Ludwig, 2003). The sample preparations and analyses were performed at the State Key Laboratory for Mineral Deposits Research of Nanjing University.

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detrital nuclei are crusted with sparry calcites (Fig. 4d). The diameter of the ooids ranges between 250 μm and 500 μm. These features indicate that the oolitic limestones were deposited in a warm and carbonatesaturated aqueous environment between the wave base level and the low tide mark (Tucker and Wright, 1990; Reading, 1996). 4.1.4. Bioclastic limestone The bioclastic limestones with marine bioclastic detritus, such as ostracode and brachiopod shells, are mainly gray massive biomicrites. The matrix is micrite with a relative abundance of approximately 50% (Fig. 4e). An open shallow marine depositional environment below the wave base level is indicated by the relative proportions of the bioclasts and the matrix (Tucker and Wright, 1990; Reading, 1996).

4. Evidence of transgression 4.1. Petrographic evidence of limestones and calcareous mudstones Limestone is a typical lithofacies of marine deposits. Therefore the petrographic feature of limestones and calcareous mudstones were carefully studied from outcrop investigation to thin section observation and identification. Limestones in coastal southeastern China only occur in the middle and upper parts of the Shipu section, and include stromatolites, algal-bound limestone, oolitic limestone, bioclastic limestone, micritic limestone and marl. In contrast, calcareous mudstones are locally present in the Xianju Shangzhang and Songyang Xiahuyuan sections of the Zhejiang Province and the Yiyang section of the Jiangxi Province (Fig. 3). 4.1.1. Stromatolites Stromatolites are well developed and are usually associated with conglomerates, sandstones and siltstones (Figs. 4a, b). The stromatolites are 5 cm to 0.5 m in height, and 20 cm to 2 m in diameter. The growth laminae are mm to cm thick. Columnar and domal stromatolites are the most commonly observed types of stromatolites. In addition, plate-like stromatolites are also found. In thin section, the algal layers are observed to alternate with the bound carbonate or terrigenous detritus layers, thereby reflecting microbial growth via daynight and tidal cycles (Radtke and Golubic, 2011). In the marmorization zone, most of the aragonite has been transformed to calcite. The morphology of the stromatolites varies throughout the vertical profile of the section, in response to environmental conditions. For the plate-like stromatolites, small corrugated and undulated laminas and tepee structures are indications of an upper intertidal flat environment with low-energy and intermittent exposure (Reading, 1996; Nichols, 2009). In contrast, the discrete columnar stromatolites and the sand-pebble size grains between the columns indicate that they were formed in relatively higher-energy locations, e.g., a subtidal flat (Reading, 1996; Nichols, 2009). Therefore, the stromatolites are believed to be formed in an intertidal to subtidal environment. 4.1.2. Algal boundstone The algal boundstones occur as laminar beds intercalated within the irregular horizontal mudstones and siltstones. Shrinkage cracks of surface mats due to evaporation demonstrate that the depositional environment varied from muddy intertidal to supratidal (Tucker and Wright, 1990). The black alga layers alternate with the gray boundstone layers. Volcanogenic and carbonate detritus present in the black algal layers are observed along the bedding (Fig. 4c). In addition, the sand shadow structures associated with stromatolites are observed on certain sandy bed surfaces. They are indicatives of a supratidal to shallow subtidal marine environment (Bottjer and Hagadorn, 2007). 4.1.3. Oolitic limestone The oolitic limestones are mainly oosparites. The proportion of matrix is less than 15%. Approximately 90% of the ooids with volcanogenic

4.1.5. Micritic limestone The micrite limestones are gray and either are massive or have laminar horizontal beds. The abundance of the micrite matrix is greater than 90%. This high matrix level reflects a low-energy water environment. According to the facies association of the micrite limestone, it can be inferred that the limestones were formed in the upper part of intertidal flat or below the storm wave base level (Tucker and Wright, 1990; Reading, 1996). 4.1.6. Calcareous mudstone The calcareous mudstones mainly include calcareous nodules and lenses (Figs. 4g, h). Calcareous mudstones can form in various environments, such as shallow marine, fresh water and evaporative environments. Here, the nodular, lenticular and layer structures indicate that the micrite calcites are crystallized and deposited from the carbonate saturated water (Tucker and Wright, 1990). Due to the very fine granularity, it is difficult to precisely identify the depositional environments solely based on the petrographic observations. 4.2. Carbon isotopes of limestone and calcareous mudstone According to the petrographic study of the limestones and calcareous mudstones, it is clear that the main depositional environment of the Shipu section is a tidal flat. In contrast, the environment of the calcareous mudstones in the Shangzhang, Xiahuyuan and Yiyang sections remains uncertain. Consequently, the carbon isotopic compositions of limestone and calcareous mudstones have been analyzed because the isotopic signature can provide clues to the depositional environment (Peters et al., 2005). The results of carbon isotope analyses illustrate that the δ 13C values of 15 calcareous mudstone samples and 12 limestone samples are between − 4‰ and 2.5‰ (PDB standard) (Fig. 5a). By reference to the statistics of Hudson (1977), most of the samples scatter within the ranges for marine limestone, and approximately 55.5% of the δ 13C ratios are greater than the average value of marine limestones (Fig. 5a). This result indicates that most of the limestones and calcareous mudstones are influenced by sea water, which agrees well with the results of the above petrographic study. 4.3. Fossils in black mudstone and shale Thin section observations of the acid maceration of representative mudstone and shale samples show that there are typical marine red and brown algae in the Bantou Formation shales in the Yong'an and Chong'an sections in addition to the terrestrial fossils (e.g., higher plant fragments). This observation provides further evidence of a marine influence. The red algae have a double-layer structure of external perithallus and internal hypothallus (Figs. 6a, b). The inner layer is composed of a series of light and dark bands. The cells are parallel to the long axis of the thallus and show a regular and extremely small-scale boxwork structure. The external perithallus has rounded or nodular

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Fig. 4. Photographs showing limestones and calcareous mudstones in the Shipu, Xiahuyuan and Yiyang outcrop sections. (a) plate-like stromatolite; (b) columnar stromatolite; (c) algal boundstone; (d) oolitic limestone; (e) bioclastic limestone; (f) micrite limestone; (g) calcareous mudstone interlayer; (h) calcareous mudstone lens. (a)–(f) are from the Shipu section. (g) and (h) are from the Xiahuyuan and Shangzhang sections, respectively. The photographs are all rock observation except for (d), which is observed under cross-polarized light. See Figs. 1 and 3 for location of the sections and samples, respectively.

conceptacles, as shown in the middle and lower right of Fig. 6a. The red algal fragments founded in this location may be coralline red algal grains based on the above diagnostic structures (Peter and Dana, 2003). Red algae commonly live in a marine environment that receives blue sun light, and certain species may live at depths of 125 m or more (Peter and Dana, 2003). The brown algae are sheet-like which clear boundaries and outlines, and are several μm in thickness. Therefore, it can be inferred that these fragments are not amorphous organisms. The polygonal and meshing cells are commonly less than 10 μm in diameter. The margins of the cavities (likely reproductive organs) are thicker (Figs. 6c, d). Brown

algae have adapted to a wide variety of marine ecological environments, including slope, platform and lagoon areas (Lee, 2008). Thus, transgression is further supported by the presence of brown algae. 4.4. Biomarkers of black shale and mudstone As discussed above, the petrography and carbon isotopes of the limestone, as well as the fossils in the black mudstone and shale, provide important evidence of a transgression. However, typical limestones occur only in the Shipu section, whereas fossils are not common and cannot be readily identified due to poor preservation.

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greater than 0.50 often reflects a saline lacustrine environment. Therefore the water environment is interpreted to have been brackish in regions A, B and C, but relatively saline in region D. This result further indicates that region D shows a relatively less influence due to transgression than regions A, B and C where the transgression increases the salinity of the lacustrine water. A similar phenomenon, with a transgression resulting in a relatively high concentration of gammacerane, has been widely reported in the Lower Cretaceous Qinshankou and Nengjiang formations in the Songliao Basin of northeastern China (Zhang et al., 1999; Feng et al., 2009). Previous studies have also suggested that some certain biomarker ratios can reflect the influence of a transgression, such as tricyclic terpane C26/C25, homohopane C35 S/C34 S and hopane C29/C30. According to statistical result of more than 500 crude oil samples collected worldwide, Zumberge (1987) and Peters et al. (2005) suggested that most crude oils of marine origin have a ratio of tricyclic terpane C26/C25 less than 1.3, hopane C29/C30 from 0.3 to 0.8, and homohopane C35 S/C34 S from 0.2 to 2.0. In contrast, crude oils of a lacustrine origin have a relatively high ratio of tricyclic terpane C26/C25 (generally greater than 1.3) and relatively low ratios of homohopane C35 S/C34 S from 0.2 to 0.8. When Comparing our results, all samples, except for Wy-8 and K1bn-1 from the Chong'an and Yong'an sections, respectively, in region C have the values of tricyclic terpane C26/C25 values less than 1.3 (Table 1), thereby implying the influence of a transgression. With respect to the other two ratios, most samples plotted in the marine zone or the mixed marine and lacustrine zone, whereas the two samples from the Yiyang section plotted in the carbonate zone (Fig. 8). Therefore, it can be inferred that a transgression did take place in coastal southeastern China during the Early Cretaceous, and had a greater influence on regions A, B and C than on region D. 4.5. Carbon isotope of bitumen extracted from black shale and mudstone

Fig. 5. Carbon isotopic compositions of limestones, calcareous mudstones and bitumen extracted from black shales and mudstones. (a) distribution of carbon isotope values of limestones and calcareous mudstones in the Shipu (sp), Shangzhang (sz), Xiahuyuan (xhy) and Yiyang (yy) sections. According to the statistical research of Hudson (1977), most samples are located in the range of marine limestones or the zone having marine influences. (b) cross plot of carbon isotope values of saturated and aromatic hydrocarbons extracted from 17 representative black shales and mudstones from 8 sections. The dashed line (δ13Caromatic = 1.14∗ δ13Csaturated + 5.46) is the boundary between marine and terrestrial environments (Sofer, 1984). All the values plot in the marine zone. See Figs. 1 and 3 for location of the sections and samples, respectively.

Carbon isotope values of bitumen extracted from black shale and mudstone can also provide evidence of transgression (Peters et al., 2005). The carbon isotopic compositions of saturated and aromatic hydrocarbons extracted from 17 representative mudstone and shale samples were analyzed. The value of δ13C values for saturated hydrocarbon ranges from −30.01‰ to −22.87‰ with an average of −28.08‰, whereas the value of δ13C for aromatic hydrocarbon ranges from −29.42‰ to −21.35‰ with an average of −26.30‰. Fig. 5b presents the correlation between the carbon isotopes of the saturated and aromatic hydrocarbons. The dashed line (δ13Caromatic =1.14×δ13Csaturated +5.46) is the boundary between marine and terrestrial environments (Sofer, 1984). As shown in Fig. 5b, all 17 samples plotted below the dash line in the marine zone, which is indicative of the influence of a transgression. 5. Timing of the transgression

Thus, to characterize the transgression in areas without limestones and fossils, an organic geochemical analysis of the black mudstones and shales was carried out, including the examination of biomarkers and carbon isotopes of extracted bitumen. Representative analyzed mass chromatograms and biomarker parameters are given in Fig. 7 and Table 1, respectively. As shown in Fig. 7, gammacerane was identified in all of the analyzed samples. The gammacerane index (gammacerane/C30 hopane) is a fairly good proxy for the stratified water column in both marine and nonmarine depositional environments (Sinninghe-Damsté et al., 1995). In this case, the gammacerane index ranges from 0.11 to 0.23 with an average value of 0.15 in the depositional regions A, B and C (Table 1). In contrast, the detected two values in region D are 0.44 and 0.46. According to Fu et al. (1991) and Peters et al. (2005), a moderate gammacerane index generally between 0.10 and 0.50, is indicative of a brackish water environment, whereas a value equal to or

Based on the evidence and discussion presented above, it is clear that a transgression did occur in coastal southeastern China during the Early Cretaceous, and the western-most Yiyang section experienced a relatively small influence of the transgression. However, it is unclear whether these transgressions in different depositional regions took place coevally. Therefore, knowledge of the timing of the transgression is a prerequisite for discussing the paleogeographical limits of the transgression. Furthermore, the timing is also essential for determining the origin of these transgressions. The widely developed tuff interlayers in sedimentary rocks provide suitable sampling targets for determining the transgression timing (Fig. 3). Ensuring each depositional region has one or two control sections, five typical sections were selected at random. A total of 262 zircons were selected from six samples, which were collected from the outcrops, for U–Pb dating analyses. The analyses under the background noise were discarded. The remaining data are near to the Concordia Line, thereby indicating a

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Fig. 6. Photomicrographs showing red and brown algae in the Yong'an and Chong'an sections. (a) and (b) red alga fragments in the Yong'an and Chong'an sections, respectively; (c) and (d) brown alga fragments in the Yong'an and Chong'an sections, respectively. Transmitted light. Samples are from acid maceration of the mudstones and shales. See Figs. 1 and 3 for location of the sections and samples, respectively.

good preservation of the primary structure and composition of the zircons (Fig. 9). However, the wide range of U–Pb age and the large MSWD variation (Table 2) result in the improbability of the weighted mean 206Pb/ 238U age as time constraints. This may be related to the complex origin of the zircon grains of the pyroclastic rocks (Nichols, 2009). Pyroclastic rocks can trap lithic fragments from underlying strata during volcanic eruption and deposition. The youngest 206Pb/238U age of the magmatic zircons is in generally the most valuable evaluation of the depositional age (Su et al., 2010). According to these considerations, the final results are listed in Table 2. The zircon U–Pb dating reveals that the starting ages of the five successions are roughly isochronous at 115 ± 2 Ma −119 ± 3 Ma, whereas the age of the Xiahuyuan section (section ⑤ in region B; Figs. 1 and 3) is slightly younger at 110 ± 3 Ma. This age is consistent with the geological investigation. In the Xiahuyuan section, the lower part contacts the underlying strata via a fault. Therefore, it is reasonable that the age of the base in the section is younger than the ages of the other sections. With respect to the ending age of the Guantou Formation, there is one control age (99 ± 3 Ma) in the top of the Shipu section. This age is applicable to all sections because the tops of the sections are generally isochronous based mainly on three lines of evidence. First, all of the sections developed in one tectonic-sedimentary system and are overlain by Upper Cretaceous fluvial and alluvial pebbly sandstones or contemporaneous heterotopic pyroclastic rocks according to the field survey. Second, an overall uplift occurred across the southeastern China during the Late Cretaceous, and a regional parallel unconformity between the Early and Late Cretaceous implies that the erosion of different sections should be roughly equal (Lapierre et al., 1997). Third, the stratigraphic studies indicate that all of the sections have five fining-upward cycles, and the mudstone and siltstone of the uppermost cycle are intact (Fig. 3). Hence, the tops of all outcrop sections should be isochronous.

In summary, the transgression in different depositional regions are roughly isochronous, and the accurate time constraint is from 119 ± 3 Ma to 99 ± 3 Ma, i.e., from the late Aptian stage to the Albian stage of the Early Cretaceous (Ding and Li, 1999; Luo and Yu, 2004). 6. Paleogeographical limits of transgression Zircon U–Pb dating suggested that different regions of coastal southeastern China have been influenced by transgression during the same transgressive episode from the Aptian to Albian ages. With respect to its paleogeographical limits, many researchers believed that transgression in the northern area was limited to the Taixinan Basin and the PeiKang High; this conclusion was based on the Lower Cretaceous marine strata revealed by the wells in the Pengjiayu Sag (south of the East China Sea shelf Basin) and the PeiKang High in western Taiwan (Yuan et al., 1987) (Fig. 1). Our studies, however, suggested that Early Cretaceous marine deposition expanded widely in the Zhejiang and Fujian provinces from the late Aptian to the Albian ages. In addition, the strata influenced by the transgression are isochronous to the first calcareous nannofossils zone that occupies the depth from 1463 m to 1701 m of well PK-2 in the Peikang High (Fig. 1) (Huang, 1978; Matsumoto, 1979; Zhou, 2002). Furthermore, in well WH-1 (Fig. 1), the Cretaceous rocks contain a set of oolitic limestones that are similar to those from the Shipu section. The nuclei of the ooids in both well WH-1 and the Shipu section are the volcanic lithic or crystal fragments, which appear to originate from a contemporaneous volcano eruption. Therefore, the strata recording the transgressions in the Zhejiang and Fujian provinces may be extensions of the marine deposits in the Peikang High. In summary, the Early Cretaceous transgression expanded at least to the Shipu region in the Zhejiang Province (approximately 29° N;

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191

Fig. 7. Representative m/z 191 and 217 mass chromatograms of saturated hydrocarbon extracted from black shales or dark mudstones from the sections in regions A–D. See Figs. 1 and 3 for location of the sections and samples, respectively.

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Table 1 Representative biomarker parameters of saturated hydrocarbon extracted from black shale and mudstone. See Figs. 1 and 3 for location of the sections and samples, respectively. Depositional region

A

B

C

Section

1. Shipu

2. 3. 4. 5.

Xianju Changtan Laozhu Xiahuyuan

6. Hushan 7. Chong'an

8. Julan 9. Yong'an

D

10. Yiyang

Sample

xs-1 sp-04 sp-16 sz-48 ct-9 ls-7 xhy-15 xhy-39 lsw-27 wy-8 wy-29 wy-41 wy-47 jl-7 bt-9 k1b-16 k1bn-1 k1bn-9 yy-44 yy-49

N-alkane peak

C25 C25 C25 C18 C25 C18 C18 C18 C18 C23 C18 C18 C18 C16 C24 C18 C16 C18 C18 C20

Pr/Ph

0.45 0.45 0.41 0.28 0.30 0.20 0.37 0.23 0.35 0.60 0.44 0.51 0.46 0.90 0.57 0.59 0.65 0.42 0.77 0.21

ααα 20R regular sterane (%) C27

C28

C29

0.31 0.31 0.31 0.34 0.43 0.43 0.34 0.37 0.38 0.45 0.45 0.43 0.42 0.41 0.44 0.42 0.44 0.46 0.36 0.36

0.24 0.24 0.25 0.24 0.23 0.24 0.24 0.25 0.22 0.23 0.25 0.24 0.25 0.24 0.24 0.24 0.23 0.23 0.25 0.25

0.45 0.45 0.45 0.42 0.34 0.33 0.42 0.38 0.4 0.32 0.3 0.33 0.33 0.35 0.32 0.34 0.33 0.31 0.39 0.39

Fig. 1). In fact, Chen (1989) has speculated that the Early Cretaceous transgression likely extended from western Taiwan to as far as 28° N. To date, there has been little discussion of the western boundary. Our study indicates that the transgression spread widely across coastal southeastern China, and the intensity of the transgression was reduced westward. In the eastern-most region (i.e., the Shipu section), the depositional environment is inferred to be predominantly of tidal flat origin, and typical marine limestones are widely developed. In addition, marine fish fossils have been found in the neighboring area (Zhang and Zhou, 1977). To the west, in the depositional regions B, C and D,

Sterane C30 4-methyl/C29 (S + R)

Tricyclic terpane C26/C25

Hopane C35/C34 22S

C29/C30

0.29 0.26 0.25 0.24 0.22 0.23 0.22 0.18 0.25 0.26 0.22 0.18 0.18 0.27 0.27 0.24 0.23 0.24 0.16 0.18

0.74 0.66 0.67 0.80 1.28 1.24 0.94 1.21 1.03 1.31 1.24 1.25 1.27 1.26 1.17 1.12 1.36 1.20 1.13 1.11

2.33 0.68 2.19 0.54 1.69 0.65 3.57 0.53 2.04 0.85 0.46 8.37 0.55 0.59 0.7 2.26 0.5 1.5 1.24 0.53

0.76 0.55 0.53 0.52 0.67 0.72 0.66 0.57 0.64 0.58 0.59 0.58 0.51 0.57 0.62 0.59 0.65 0.58 1.12 1.03

Gammacerane/ C30 hopane

0.20 0.14 0.12 0.12 0.11 0.11 0.11 0.21 0.12 0.16 0.13 0.15 0.17 0.12 0.12 0.15 0.17 0.14 0.46 0.44

the calcareous mudstone was deposited in a marine environment, as indicated by the occurrence of red and brown algae. However, a westward decrease of the influence of seawater is indicated by multiple lines of evidence. In the western-most depositional zone, region D, which is located west of the Wuyi Mountains, the marine calcareous mudstones interlayer is locally present in the lower part of the section. In addition, a significantly abundant level of gammacerane was identified with a ratio of gammacerane/C30 hopane greater than 0.4 (Table 1, Fig. 7). These findings suggest a depositional environment of an evaporated saline lake with only a limited marine influence. It is consistent

Fig. 8. Cross plot of hopane C29/C30 vs. homohopane C35 S/C34 S of saturated hydrocarbon extracted from black shale and mudstone. Data from Table 1. The depositional setting interpretations are mainly after Zumberge (1987) and Peters et al. (2005). See Figs. 1 and 3 for location of the sections and samples, respectively.

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193

Fig. 9. LA-ICP-MS U–Pb concordia diagram of zircons.

with the lithologic feature in the Yiyang section, where the redbrownish siltstones, mudstones and sandstones occur widely. Based on these observations, the western boundary of the transgressions may be limited to the southeastern side of the Wuyi

Mountains (Fig. 1). Sea water might have intruded into the hinterland from southeast to northwest, and the flooding may have finally ended at 29° N and at the southeastern side of the Wuyi Mountains (Fig. 1).

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Table 2 Analytical results of zircon U–Pb dating. See Figs. 1 and 3 for location of sections and samples, respectively. Section

Sample

Locality in section

Analyzing point

Abandoned analyses

Age ranges

MSWD

Weighted mean age

Youngest age

Shipu Shipu Shangzhang Xiahuyuan Chong'an Yiyang

Sp-02 Stw-02 Fs Xhy wys Yy

Top Base Base Base Base Base

32 20 70 45 35 42

0 2 0 1 1 2

99 ± 3 Ma–131 ± 6 Ma 119 ± 2 Ma–228 ± 4 Ma 115 ± 2 Ma–168 ± 13 Ma 110 ± 3 Ma–162 ± 3 Ma 117 ± 2 Ma–141 ± 5 Ma 117 ± 3 Ma–171 ± 3 Ma

3.6 55 6.6 49 6.2 23

112 ± 2 Ma 138 ± 4 Ma 117 ± 2 Ma 131 ± 3 Ma 131 ± 3 Ma 132 ± 3 Ma

99 ± 3 Ma 119 ± 2 Ma 115 ± 2 Ma 110 ± 3 Ma 117 ± 2 Ma 117 ± 3 Ma

7. Geological implications According to regional geological investigation, the strata involved in the transgression are sandwiched between Jurassic-Lower Cretaceous terrestrial pyroclastic deposits and Upper Cretaceous red alluvial–fluvial sedimentary rocks (Jiangxi Geology And Mineral Resources Bureau, 1984, 2008; Fujian Geology And Mineral Resources Bureau, 1985, 1997; Zhejiang Geology And Mineral Resources Bureau, 1989, 1996). Hence, the transgression may have geological implications regarding tectonic changes in southeastern China during the Early Cretaceous. It is generally accepted that a transgression is the result of complex geological processes, such as relatively high sea level controlled mainly by the eustatic sea level, tectonic subsidence and sediment supply. The correlation of the transgression with the eustatic sea level curve established by Haq et al. (1987) shows a eustatic sea level fall to be co-eval with the transgression (120–100 Ma), whereas the two stages of relatively higher sea level correspond to the Lower Cretaceous pyroclastic deposits and the Upper Cretaceous alluvial– fluvial deposits (Fig. 3). Therefore, the Early Cretaceous transgression in the coastal southeastern China seems not to be attributable to a change in eustatic sea level and, therefore, was a likely consequence of regional subsidence. Further evidence in support of this conclusion is the variability in the thicknesses of the different sections (Fig. 3), which implies differences in accommodation and hence the likely effects of syndepositional faulting. Nevertheless, a relatively high eustatic sea level during the Cretaceous was a necessary condition for the transgression (Haq et al., 1987). In addition, the transgression events coincided with a higher rate of global crust production (Fig. 3). Therefore, given the rapid westward subduction of the Paleo-Pacific plate at a low angle (Engebretson et al., 1985; Zhou and Li, 2000), the transgression may have been caused by intensive regional tectonic subsidence associated with the active subduction of the Paleo-Pacific plate. In turn, the sea eventually invaded the sagging areas of modern coastal southeastern China. 8. Conclusions It has been generally accepted that Early Cretaceous pyroclastic sediments in coastal southeastern China were deposited in the intermontane depressions or coastal glens; however, several studies, mainly based on fossil records, have suggested that sea water could have invaded certain limited areas. However, there have been few systematic studies on this contentious issue to date. This question was addressed for the first time in the present study based on investigations of ten carefully-selected and representative Lower Cretaceous outcrop sections, mainly involving studies of their petrography, paleontology and geochemistry. (1) Petrographic investigations demonstrate that the limestones in the representative Shipu section of the northeastern Zhejiang Province were formed mainly in a tidal flat environment. Paleontological studies of black shales and mudstones reveal marine red and brown algae in the representative Yong'an and Chong'an sections of the western Fujian Province.

Biomarkers of black shales and mudstones and carbon isotopes of saturated and aromatic hydrocarbons reflect the wide influence of the transgression over the entire coastal southeastern China. (2) A relatively accurate time constraint (from 119 ± 3 Ma to 99 ± 3 Ma) of the transgression was obtained according to U–Pb dating of 262 zircon grains. Under the isochronous framework, the paleogeographical limits of the Early Cretaceous transgression were determined combined with the regional stratigraphic correlations. The northern and western limits were identified as approximately 29° N and the southeastern side of the Wuyi Mountains, respectively. (3) The correlation between the transgression with the eustatic sea-level curve and tectonic events suggests that the transgression was caused by regional tectonic subsidence stimulated by the rapid westward subduction of the Paleo-Pacific plate accompanied by the generally high Early Cretaceous eustatic sea level. Acknowledgments We would like to thank Prof. Dave Bottjer, Pro. Peter Skelton, and an anonymous reviewer for constructive reviews. Detailed reviews of Peter Skelton especially help to improve the article. Prof. Lizheng Bian is specially thanked for guidance and assistance in the identification of marine red and brown algae. Prof. Wenlan Zhang and Dr. Zhengyu He are thanked for help in the Zircon U–Pb dating experiments. They are all from School of Earth Sciences and Engineering, Nanjing University. Dr. Yuqiao Gao is thanked for assistance during field work, who is from Research Institute of Exploration and Exploitation, East China Petroleum Bureau, SINOPEC. Thanks are also extended to Dr. Yang Pu from Nanjing University of Information Science and Technology and Xiaoqing Zhu from Nanjing University for manuscript draft. This work was jointly funded by the Major State Basic Research Development Program (973 project, Grant No. 2012CB214803) and the National Natural Science Foundation of China (Grant Nos. 41072090 and 40872086). References Andersen, T., 2002. Correction of common Pb in U–Pb analyses that do not report 204Pb. Chemical Geology 192, 59–79. Bottjer, D.J., Hagadorn, J.W., 2007. Mat growth features. In: Schieber, J., et al. (Ed.), Atlas of Microbial Mat Features Preserved within the Silicilastic Rock Record. Elsevier, Amsterdam, pp. 53–71. Chen, P.J., 1979. An outline of palaeogeography during the Jurassic and Cretaceous periods of China with a discussion on the origin of Yantze River. Acta Scientiarum Naturalium Universitatis Pekinessis 03, 90–109 (In Chinese with English abstract). Chen, Y.H., 1989. Cretaceous–Tertiary paleogeography of the east China SEA. Acta Sedimentologica Sinica 7 (4), 69–76. Chen, P.J., 2000. Paleoenvironmental changes during the Cretaceous in eastern China. In: Okada, H., Mateer, N.J. (Eds.), Cretaceous Environments of Asia. Elsevier, New York, pp. 81–90. Cooper, M.R., 1977. Eustacy during the cretaceous: its implications and importance. Palaeogeography, Palaeoclimatology, Palaeoecology 22, 1–60. Ding, B.L., Li, Y.X., 1999. New advances of Cretaceous study in Zhejiang. Volcanic Geology and Mines 20 (4), 241–286 (In Chinese with English abstract). Engebretson, D.C., Cox, A., Gordon, R.G., 1985. Relative motions between oceanic and continental plates in the Pacific Basin. Geological Society of America Special Paper 206, 1–59.

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