Early Cenozoic paleontological assemblages and provenance evolution of the Lishui Sag, East China Sea

1. Introduction

Lying along the eastern South China Continental margin, the East China Sea Shelf Basin was deposited with thick Mesozoic-Cenozoic sediments, reaching a maximum 20 000-m-of-succession. It has been widely accepted that this area is of great significance in the petroleum exploration undertakings (Shen et al., 2021; Zhang et al., 2021a, b). Due to the complexity of the plate interactions during Mesozoic-Cenozoic (Li and Li, 2007; Li et al., 2017a, 2018; Cui et al., 2021a), both East China Sea Shelf Basin and the surrounding orogenic uplifts greatly due to multiple transverse and vertical movements since their initiation. Therefore, there remaining arguments were the tectonic background, tectonic affinity and sedimentary transport-infilling processes of the East China Sea Shelf Basin.

Since the late Mesozoic, the western Pacific margin gradually switched from the Andean-type to the Western Pacific-type continental margin (Li et al., 2018; Shao et al., 2019b, 2022; Cui et al., 2021a; Zhao et al., 2021b; Tian et al., 2021). This tectonic shifting resulted in a series of rifting processes within the Eastern Asian continental margin. Due to the sudden eastward retreat of the Pacific Plate, the inversion structures of the East China Sea Shelf Basin also experienced eastward migration (Zhou et al., 2002; Zhang et al., 2016, 2011; Jiang et al., 2020). In general, the East China Sea Shelf Basin was divided into the Taibei Depression, Zhoushan Uplift and East Zhejiang Depression, and the sedimentary sequences grow thicker from the west to east (Liu et al., 2020; Jiang et al., 2020).

Sedimentary archives were considered as significant geological evidence for studying the basin evolution and the corresponding geotectonic activities (Shao et al., 2009, 2019a; Cui et al., 2018, 2019; Zhang et al., 2021a). Both sedimentary environment and provenance evolution reconstruction are important subjects of the basin analysis (Meng et al., 2021; Cui et al., 2021b; Zhang et al., 2021; Feng et al., 2021). According to previous studies, the Lishui-Jiaojiang Sag was mainly influenced by adjacent paleo-uplifts which consisted of Mesozoic magmatic bodies (Jiang et al., 2020; Tang et al., 2018). The Xihu Sag of the East China Sea Shelf Basin, on the other hand, was featured by long-distance transport with high-maturity sandstone sediments (Liu et al., 2020). However, there were still much disagreement regarding the provenance patterns and sedimentary infilling processes, such as the basinal paleo-orogenesis, areas from the North China to the Korean Peninsula, further inland materials delivered by the Changjiang River (Yangtze River), etc. (Kwon and Boggs, 2002; Qing et al., 2017; Wang et al., 2018).

By conducting micropaleontological studies and detrital zircon U-Pb age spectra on the Lishui Sag sediments, the main aims of this study were to investigate the sedimentary environment and provenance evolution of the Taibei Depression of the East China Sea Shelf Basin during Paleocene-Eocene. This study also revealed the sedimentary infilling processes and the corresponding controlling factors, which largely lays a good foundation of upcoming petroleum exploration.

2. Geological background

The Lishui Sag is located in the southwestern Taibei Depression of the East China Sea Shelf Basin (Fig. 1). Geographically, it is next to the Zhejiang-Fujian Uplift to the west, connected to the Jiaojiang Sag, Qiantang Sag and Haijao Low Uplift to the northeast, proximal to the Yushan Low Uplift, Fuzhou and Minjiang Sag to the east. The Lishui Sag initially was formed as a half graben-style rifting basin during Cenozoic underlain by the Mesozoic basement remnant. The entire basin was featured by the generalized pattern of “faulting in the east, onlapping in the west”. It had a relatively low-gradient slope on the western side. The area of Lishui Sag was 1 500 km² and was divided by two sags separated by the central buried-hill and draping tectonic units.

Figure  1.  Simplified geological map and sample locations of the basin in the East China Sea. The samples of datum sources of the Mesozoic Volcanic rock from costal Zhejiang-Fujian were as follows: A1, A2, A3 and A4 were adopted from Xing et al. (2008); B1, B2, B3 and B4 were adopted from Liu et al. (2012); C1, C2, C3, C4 and C5 were adopted from Cui et al. (2010); D1 was adopted from Wang et al. (2003); and G1 was adopted from Dong et al. (2010). The datum sources of Changjiang River, Oujiang, and Taiwan were adopted respectively from Zheng et al. (2013), Xu et al. (2007), and Hou et al (2021).

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The Lishui Sag was mainly deposited with Paleocene and Eocene strata, and displayed sedimentation hiatus of upper Eocene (Pinghu Formation) and Oligocene (Huagang Formation) intervals. The Eoceneocene sequences contrast with the overlying Miocene strata by an angular unconformity (Guo et al., 2015; Zhong et al.,2018) (Fig. 2). Scholars used integrated geological techniques such as core, logging and seismic data to analyze the lithology and sedimentary environment evolution of the Lishui Sag, and believed that the sedimentary environment was changed from continental facies in the early Paleocene (Yueguifeng Formation) to marine facies after a transgression occurring in the middle Paleocene (Lingfeng Formation) (Jiang, 2003; Tian et al., 2012; Zhong et al., 2018). Yueguifeng Formation was deposited in the lacustrine-facies environment during the early Paleocene, which lies in an angular unconformity with the Mesozoic basement. Yueguifeng Formation mainly consisted of dark brown and black brown mudstones interlayered with light grey, greyish fined or medium-grained sandstones and dark coal seams. Lingfeng Formation was deposited in the marine-facies setting during the late Paleocene. It is mainly composed of grey to dark grey mudstones, greyish siltstones and sandstones. The overlying Mingyuefeng Formation was also deposited in the late Paleocene in the regression environment. Mingyuefeng Formation is featured by two inverse grading patterns, with siltstones in the lower layer and coarse sandstones interlayered with dark coals in the upper section. Oujiang Formation was deposited in the early Eocene and is in an angular unconformity with the overlying sequences. Its lower part consists of greyish, grey fined to medium-grained sandstones interlayered with grey and brown mudstones and multiple layers of dark coal seams and thin laminae of calcareous sandstones, while the upper part forms light grey and grey mudstones interlayered with greyish sandy bioclastic limestones and abundant autogenic glauconites. The lower section of Wenzhou Formation onlaps with the Yushan Low Uplift, and was widely distributed in the Taibei Depression. It was deposited in a marine-facies setting during middle Eocene, and includes grey, light greenish-grey mudstones and sandstones with sandstone interlayers. To be noted, multiple layers of 1-2 meter-of-dark coal successions were observed within the lower Wenzhou Formation. Generally, the lower Wenzhou sequence generated an inverse sedimentary cycling from littoral-neritic to swamp facies. The upper Wenzhou Formation, on the other hand, was distributed in the scattered areas composed of light grey mudstones. The lower part formed light grey, greyish-white sandstones with conglomerates, limestones and minor glauconites.

Figure  2.  Stratigraphic framework of the Lishui Sag and sampling information.

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The Lishui Sag might was influenced by several potential provenances, including the coastal South China, the underlying basement and its surrounding paleo-uplifts, the Haijiao Low Uplift to the north, the Yushan Low Uplift to the northeast and the other intrabasinal Mesozoic magmatic bodies. The coastal South China mainly exposes Jurassic–Cretaceous volcanic and volcaniclastic rocks, in addition to scattered Precambrian metamorphic units (Ma, 2006; Yu et al., 2009). Jurassic and Cretaceous volcanic rocks from coastal Zhejiang-Fujian showed large numbers of late Yanshanian zircon (93.3−137 Ma), and a small amount of early Yanshanian zircon (149.8−178 Ma) through Ar-Ar isotopes and detrital zircon U-Pb dating (Table 1) (Wang et al., 2003; Xing et al., 2008; Dong et al., 2010; Cui et al., 2010; Liu et al., 2012).

Table  1.  Dating data table of the Mesozoic volcanic rock from coastal Zhejiang-Fujian

Sample name	Dating method	Age/Ma	Reference	Number of particles	Location
A1	Zircon U-Pb dating	130.1±4	Xing et al., 2008	15	Xianyou, Fujian Province
A2	Zircon U-Pb dating	143±7	Xing et al., 2008	6	Xianyou, Fujian Province
A3	Zircon U-Pb dating	162±4	Xing et al., 2008	13	Shekou, Fujian Province
A4	Zircon U-Pb dating	150±5	Xing et al., 2008	15	Shekou, Fujian Province
B1	Zircon U-Pb dating	177±1	Liu et al., 2012	21	Maonong, Zhejiang Province
B2	Zircon U-Pb dating	93.8±0.6	Liu et al., 2012	15	Xiaoxiong, Zhejiang Province
B3	Zircon U-Pb dating	135.3±10.9	Liu et al., 2012	20	Dashuang, Zhejiang Province
B4	Zircon U-Pb dating	138±1	Liu et al., 2012	20	Dashuang, Zhejiang Province
C1	Zircon U-Pb dating	120.0±1.4	Cui et al., 2010	15	Wenxi,Qingtian, Zhejiang Province
C2	Zircon U-Pb dating	109.4±9.3	Cui et al., 2010	2	Jingjia Moutain, Yongjia, Zhejiang Province
C3	Zircon U-Pb dating	118.1±2.3	Cui et al., 2010	12	Dadilin,Tiantai, Zhejiang Province
C4	Zircon U-Pb dating	104.1±3.5	Cui et al., 2010	3	Xuantandi, Fenghua, Zhejiang Province
C5	Zircon U-Pb dating	102.1±2.2	Cui et al., 2010	4	Jingling, Xinchang, Zhejiang Province
D1	Zircon U-Pb dating	117.7±2.7	Wang et al., 2003	8	Fuyang, Zhejiang Province
G1	Ar-Ar dating	93.4	Dong et al., 2010	19	Dongji island, Zhoushan, Zhejiang Province

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There were also minor Paleozoic gneiss and granulite rocks in this area with ages of 400 Ma and 339 Ma, separately (Ma, 2006). The modern Changjiang River sediments were characterized by multimodal zircon U-Pb age patterns, including high peaks of Indosinian (261 Ma) and Caledonian (417 Ma) periods in addition to abundant Precambrian zircons (900 Ma) (Zheng et al., 2013; Zhao et al., 2015). The Oujiang River was featured by predominant Yanshanian population centered at ca. 136 Ma and secondary Paleoproterozoic cluster at ca. 1 847 Ma, indicating a provenance of the coastal South China (Xu et al., 2007). The Lishui Sag basement consisted of Mesozoic extrusive and intrusive rocks as well as Mesoproterozoic metamorphic suites. Borehole cores obtained from drilling activities revealed that a few exploration wells such as Wells L36-2 and L1 encountered black biotite amphibole-plagioclase gneiss of a crystallization age of 1 832 Ma (Fig. 3). Well W4 and many other exploration wells were drilled into Yanshanian granites (108−112 Ma), granodiorites and intermediate extrusive rocks. Both Haijiao Low Uplift and Yushan Low Uplifts were dominated by Mesozoic U-Pb populations, with Haijiao Low Uplift showing a prominent Indosinian peak of ca. 216 Ma and Yushan showing Yanshanian cluster at ca. 120 Ma. Notably, they commonly exhibited Paleoproterozoic (1 800−1 831 Ma) and secondary Jinningian populations (732−773 Ma). However, Haijiao Low Uplift was featured by almost identical peaks of Paleoproterozoic and Indosinian zircons (Fig. 3).

Figure  3.  Zircon U-Pb spectra of the Lishui Depression basement and potential source terranes. The samples datum sources of the Mesozoic volcanic rock from coastal Zhejiang-Fujian were adopted from Xing et al. (2008), Liu et al. (2012), Cui et al. (2010), Wang et al. (2003), and Dong et al. (2010). n is the number of concordant analyses.

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3. Samples and methodology

Twelve borehole samples were sampled from both East China Sea pre-Cenozoic basement and Cenozoic sedimentary successions (Yueguifeng, Mingyuefeng, Oujiang and Wenzhou Formations) by the Shanghai Branch of China National Offshore Oil Corporation (CNOOC) Co., Ltd.. Their assigned stratigraphic ages were based on unpublished seismic and paleontological data from CNOOC. Zircon grain collection, mounting, Cathodoluminescence (CL) imaging and LA-ICP-MS isotopic dating were conducted at the State Key Laboratory of Marine Geology, Tongji University. Thermo Elemental X-Series ICP-MS coupled to a New Wave 213 nm laser ablation system was used. In the laser ablation process, helium gas was used as carrier gas, and argon gas was used as compensation gas to adjust sensitivity. The laser beam was set to a 10 Hz ablation frequency with a 30 μm spot size. Each analysis included a background signal of 30 s and a sample signal of 70 s. The international standard zircon 91500((1065.4±0.3) Ma) was used as an external standard together with zircon standard Plešovice ((337.1±0.4) Ma) to monitor the accuracy of the results of the test.

U-Pb isotopic ratio and age calculations were determined using software ICPMSDataCal, and common Pb correction was performed using method suggested by Andersen (2002) method. According to the suggestion of Cao et al. (2017), calculated ²⁰⁶Pb/²³⁸U ages were adopted for zircons younger than 1000 Ma, while ²⁰⁷Pb/²⁰⁶Pb ages were adopted for ones older than 1000 Ma (Shao et al., 2016, 2019b). The accepted ages were selected from a subset of both ≤10% discordance and ≤10% uncertainty (1 σ).

In general, foraminifera, calcareous nannofossils and dinoflagellates were typically considered as the most reliable indicators of a marine-influence environment. To be more detailed, all foraminifera and all pelagic nannofossils were good indicators of the full marine environments from continental shelf to slope. In most cases, these biotic assemblages occur with each other in most marine sections. However, a small group of foraminifera and most dinoflagellates were identified in a wider range from lacustrine to brackish marine environmental settings. This brackish living mode established them as coastal marine or distal marine-influence proxies (Wang et al., 1982; Li et al., 2017b). A total of 140 samples from Well W13 were analyzed using foraminifera, calcareous nanofossil and marine dinoflagellate assemblages to study the sedimentary environment. Notably, these analyses were carried on cutting samples, which may resulted sediment mixing and analytical bias. Therefore, these results should be treated as generalized or less reliable. To minimize the negative impact of possible sediment mixing, the last occurrence of diagnostic fossils was focused. Even in that case, the accuracy of most datum levels remains unclear due to the nature of cutting samples. Well W13 was investigated with marine microfossil records of the Paleocene–Eocene strata, and the results were analyzed together with those of Well L1 (Li et al., 2017b). We referred to the geologic timescale of Gradstein et al. (2012) for the ages of fossil datum and other events.

4. Result

4.1 Marine sedimentary records

Based on the drilling results of about 40 locations, the southern and central East China Sea were widely deposited with marine deposits containing Paleocene–Eocene foraminifera and pelagic nannofossils. Those records were mostly from the Taibei Depression in the southwestern East China Sea in addition to several drill-holes within other southern depressions. Both Wells L1 and W13 were identified with the occurrence of foraminifera Pseudohastigerina pseudomenardii, Acarinina spp., and Morozovella spp. and nannofossils Fasciculithus tympaniformis, Sphenolithus primus, and Neochiastozygus spp., commonly indicating a marine environment during the late Paleocene–Eocene (Fig. 4).

Figure  4.  Marine fossil records and sea level changes of the Lishui Sag.

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4.2 Detrital zircon U-Pb age spectra

The sedimentary cutting samples of the Lishui Sag in this study were sampled from Drill-holes L35, L36-1 and W26 (Fig. 5). L35 and L36-1 were located within the western region, while W26 lies in the east (Fig. 1).

Figure  5.  Detrital zircon U-Pb age spectra of Cretaceous to upper Eocene samples from the Lishui Sag.

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Sample L35-1 was collected from the Yueguifeng Formation at the depth of 3762.41 m. Zircon grains were mostly euhedral to subhedral in shape and commonly display clear core-rim oscillatory structures. They have relatively variable Th (17.02×10⁻⁶−4239.58×10⁻⁶) and U (419.48×10⁻⁶−1148.82×10⁻⁶) concentrations, yielding Th/U ratios mostly falling between 0.20 and 3.69. The total of 101 test results were magmatic origin. Therein, 94 test results indicated that ²⁰⁶Pb/²³⁸U ages were between 65 Ma and 279 Ma, except one age was 450 Ma. The ²⁰⁶Pb/²⁰⁷Pb ages of remaining 6 spots were between 1829 Ma and 1278 Ma. In general, Sample L35-1 was featured by the predominance of Yanshanian peak centered at ca. 109 Ma, along with scattered Paleoproterozoic (ca. 1815 Ma) zircons (Fig. 5). Samples L35-2, L35-3 and L35-4 were collected from the Lingfeng (3 344.44 m), Mingyuefeng (1 856 m) and Oujiang (1 565 m) Formations, respectively. Their zircon crystals were subhedral to subrounded in shape with modestly clear concentric zoning. Th and U concentrations were varied within 53.29×10⁻⁶–4 089.54×10⁻⁶, 110.97×10⁻⁶−1 262.39×10⁻⁶, respectively. The majority of Th/U ratios for zircons with magmatic origin were ranging between 0.11 and 3.24. The ages from 213 effective tests were ranged from 78 Ma to 167 Ma, excepted 10 spots of with ²⁰⁶Pb/²⁰⁷Pb ages of 2 555–983 Ma. These samples formed three narrow clusters centered at ca. 110 Ma, 111 Ma and 105 Ma, respectively.

Sample L36-1 was collected from the western Lishui Sag and sampled at Wenzhou Formation on the depth of 1 130 m. Zircon grains of L36-1 were mostly euhedral to subhedral in shape and commonly display clear core-rim oscillatory structures. Their relatively variable Th (80.32×10^–6–2 092.53×10^–6) and U (51.94×10^–6–1 358.19×10^–6) concentrations yield at Th/U ratios of 0.55–4.09, implying magmatic origin. A total of sixty-four (54) tests indicated that ²⁰⁶Pb/²³⁸U age were ranged between 90 Ma and 162 Ma, excepted one inherited grain with ²⁰⁶Pb/²⁰⁷Pb age of 1 884 Ma. Similarly, L36-1 resembles with L35 drilling samples by generating a unimodal group at 108 Ma (Fig. 5).

A total of four (4) samples were obtained from borehole W26. The lowermost sample W26-1 was collected from the Shimentan Formation (Cretaceous) at the depth of 4005 m. A total of 102 effective spots had relatively variable Th (17.75×10^–6–4437.80×10^–6) and U (27.97×10^–6–1494.23×10^–6) concentrations. The Th/U ratios for these samples were ranged between 0.42 and 2.97. Only one sample yielded at Th/U ratio lower than 0.1, possibly reflecting a metamorphic origin. The ²⁰⁶Pb/²³⁸U ages for ninety-seven (97) tests were from of 75–253 Ma excepted of five (5) tests with ²⁰⁶Pb/²⁰⁷Pb ages of 1887–2544 Ma. W26-1 was characterized by a bimodal pattern with 114 Ma and 177 Ma clusters. Sample W26-2 was obtained from the Yueguifeng Formation at the depth of 3645.10 m. The ²⁰⁶Pb/²³⁸U ages of ninety-seven (97) tests were 79–259 Ma, and generated a major 106 Ma population and a substitute peak of ca. 163 Ma (Fig. 5). Sample W26-3 was collected from the Mingyuefeng Formation at the depth of 2774.56 m. A total of 71 effective spots indicated magmatic origin with Th/U ratios of between 0.21 and 1.86. Likewise, Sample W26-3 showed a typical unimodal pattern with a predominance of Yanshanian peak centered at ca. 103 Ma. The uppermost sample W26-4 was collected from Wenzhou Formation at the depth of 1620 m. Ninety-seven (97) effective spots had relatively lower Th (41.78×10^–6–1629.77×10^–6) and U (50.41×10^–6–1279.59×10^–6) concentrations, and Th/U ratios mostly fall between 0.11 and 5.45, reflecting magmatic origin. To be noted, Sample W26-4 displayed a unique multimodal pattern by generating a major Yanshanian peak of ca. 118 Ma in addition to a modest group of zircons from ca. 163 Ma to 1913 Ma (Fig. 5).

5. Discussion

5.1 Potential source areas

The Lishui Sag was located along the eastern margin of the South China Continent, which was dominated by Mesozoic igneous bodies and Paleozoic sedimentary rocks interspersed with Precambrian metamorphic rocks (Yang et al., 2022). Large-scale fluvial networks flowed across the extensive South China and erode large amounts of detritus into the offshore sedimentary basins, such as the Oujiang River, the Changjiang River, etc. (Xu et al., 2007; Zheng et al., 2013). Haijiao and Yushan Low Uplifts, on the other hand, lie to the north and east, separately. A large angular unconformity was revealed between Mesozoic or even older basement and the Paleogene sedimentary cover. However, constrained by the limitation of the ECSSB basement-penetrated drillings and low precision of the dating techniques, these possible provenances have long been roughly investigated via magnetic anomaly data, indirect postulations from the continental outcrops, etc. (Chen et al., 2002). Zircon dating of basal magmatic rocks showed that the basement of Lishui Sag was mainly composed of the Yanshanian and Indosinian granite (Guo et al., 2015), and a small amount of Archean metamorphic rocks.

Basement of borehole L1 was an early collected sample with biotite plagioclase gneiss rocks, yielding a wide Paleoproterozoic Rb-Sr age span of 1970−1806 Ma. Occurrence of these late Paleoproterozoic zircons also seems to be consistent with the similar gneiss rocks from drill-hole L36-2 with 1 832 Ma (Fig. 3). Closely adjacent to the Haijiao Low Uplift margin, Sample N1 was sampled from sediments deposited in a fan delta environment directly after the initial rifting stage (Fig. 3). It was generated from a nearby source and mainly consist of conglomerates with mudstone, siltstone and sandstone matrix, as well as volcanic interlayers. Therefore, this sample was used to represent the provenance feature of the nearby Haijiao Low Uplift. It develops Indosinian (ca. 216 Ma) and Paleoproterozoic (ca. 1 831 Ma) populations of almost identical percentages, in addition to scattered groups of Jinningian and Archean-aged zircons. Another lowermost Pinghu sample B1 proximal to the Yushan Low Uplift, consisting of conglomerates bearing fan delta facies, otherwise has been confirmed greater overprinting influence by the Mesozoic arc magmatism. Its overall U-Pb combination pattern was featured by overwhelming Yanshanian zircons compared to the other tiny Indosinian, Jinningian and late Paleoproterozoic clusters. Similarly, Sample B-1 represented the provenance feature of the nearby Yushan Low Uplift (Fig. 3).

5.2 Paleo-environmental changes during Paleocene-Eocene

Figure 4 showed a series of micropaleontology indicators in order to investigate potential paleo-environmental changes. The foraminifera, nannofossil and dinoflagellate contents fluctuated with time, and possibly suggested certain land-sea interaction during the deposition of Lingfeng-Wenzhou Formations. In general, Well W13 showed that the Lishui Sag developed nannofossils of zones NP-NP15 during Paleocene-Eocene and revealed two maximum transgression intervals (Fig. 4). In addition, both foraminifera and dinoflagellate occurred simultaneously at ca. 61 Ma, reflecting the initiation of marine-facies environment. On the one hand, the Mingyuefeng and upper Oujiang Formations were characterized by high peaks of fossil contents, and suggest two maximum transgression periods. The lower Oujiang Formation, on the other hand, rarely generated any fossil records, indicating a maximum regression stage. Located in the eastern Lishui Sag, Well L1 was featured by an identical paleontological combination pattern. Therefore, the entire Lishui Sag might had been under control of a homogeneous sedimentary environment during this time (Fig. 4).

5.3 Provenance distribution patterns

The zircon U-Pb age spectra displayed both temporal and spatial variations in this study, and possibly suggested different transport pathways during Paleocene-late Eocene. There is a clear distinction of provenances between the eastern and western sediments of the Lishui Sag. To be in detail, Samples L35 and L36-1 were commonly featured by unimodal peak patterns with age populations at ca. 105−111 Ma (Fig. 5). Obviously, the western Lishui Sag was largely influenced from the nearby Yanshanian magmatic rocks to the west, excluding the possibility of inland transport, such as the Oujiang River.

Located in the east, drill-hole W26 exhibited apparent differences of the U-Pb age combination pattern with regard to their Shimentan-Mingyuefeng Formation (from late Cretaceous to late Paleocene) sediments. Two extremely high peaks of early Yanshanian (177 Ma) and late Yanshanian (114 Ma) zircons were observed in Sample W26-1. A dominant late Yanshanian (106 Ma) and a secondary early Yanshanian (177 Ma) clusters were identified in Sample W26-2. W26-3, on the other hand, generated a unimodal peak of late Yanshanian (103 Ma). These samples indicated that the sediments were derived from proximal Yanshanian magmatic rocks in the eastern areas (Fig. 5).

The Paleocene and Eocene depositions are were also marked by a distinct provenance change for the eastern Lishui Sag. The uppermost sample W26-4 exhibited a multimodal pattern featured by the prominent Yanshanian peak centered at 118 Ma, and scattered clusters of ca. 163 Ma and 1 913 Ma. It is likely that the Yushan Low Uplifts might have initially provided clastic materials to the eastern sag (Fig. 5). The provenance change also implies that there were possibly large rivers delivering sediments from the north, such as the Yushan Low Uplifts, into the southern basins.

5.4 Sedimentary environmental evolution

The Lishui Sag was initiated as a back-arc basin during the late Mesozoic (Suo et al., 2012; Ren, 2018), and mainly derived sediments from the adjacent magmatic arc since its origination. During Paleocene, the Lishui Sag was dominated by marine-facies environment (Fig. 6a). A group of deltas accumulated within the western area, while a fan delta was formed in the east suggesting a proximal source (Fig. 6a). During the early-middle Eocene, the entire sedimentary environment remained in a relatively steady state. There were modestly increasing number of deltas in the western sag, mainly sourced from the Yanshanian granitic bodies of the Zhejiang paleo-orogenesis. However, the northeastern sag was marked by extensive delta plains which largely derived from the Yushan Low Uplift (Fig. 6b). Due to the Yuquan movement in the late Eocene, the Lishui Sag was regional uplifted, and the sea water regressed eastward, the basin was exposed and eroded (Fig. 6c), and missed the Oligocene–late Eocene sedimentation, until the Miocene basin reaccepted the deposition again (Zhang et al., 2013a, b). Featured by transitional-facies depositions of Paleocene-Eocene (Zhao et al., 2021a), the Lishui Sag thus bears significant potential for source rock and oil-gas reservoir accumulation.

Figure  6.  Evolutionary stages of sedimentary environment during Paleocene to Eocene.

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6. Conclusions

A comprehensive study of micropaleontology found that a large number of foraminifera, calcareous ultra-microscopy and marine ditch dinoflagella were developed at Lishui Sag in Paleocene and the early Miocene, which indicated that the coastal marine environment was develop at the sag during this period, and experienced two obvious high sea level events.

A source-to-sink comparative analysis to restore the provenance evolution of the Lishui Sag during Paleocene–Eocene. The Paleocene sediments in the Lishui Sag were mainly from the coastal area of South China and its surrounding paleo-uplifts of Yanshanian magmatic rocks. However, its eastern area partly showed Indosinian populations compared to the dominance of Yanshanian clusters in the west. Entering the Eocene, the sediments in the western area of Lishui Sag showed a single peak of Zircon around 106−108 Ma, and still was influenced by the Yanshanian magmatic rocks along the coast of Fujian and Zhejiang. Meanwhile, the upper Eocene Wenzhou sediments in the eastern area were featured by plentiful Precambrian zircons in addition to the large Indosinian-Yanshanian peaks, indicating a possible impact from the Yushan Low Uplift to the east.

Therefore, it is likely that the eastern Lishui Sag generated large river systems as well as deltas during this time. The Lishui Sag experienced uplifting and exhumation in the late stage of the late Eocene and was not deposited with sediments until Miocene.