Cyclic Changes in Sea Level and Sequence Stratigraphy During the Latest Pliensbachian–Early Toarcian (Early Jurassic) in the Southern Qiangtang Basin (Eastern Tethys): Geochemical and Mineralogical Perspectives

Abstract

The Pliensbachian–Toarcian boundary and early Toarcian events indicate significant environmental and oceanographic instabilities attributed to the emplacement of the Karoo–Ferrar large igneous province and subsequent greenhouse gas emissions. These geologic processes influenced carbon cycle perturbations and global warming, consistent with phases of a sea level rise. This study presents a high-resolution dataset of total organic carbon (TOC) and bulk rock geochemistry and mineralogy from a complete upper Pliensbachian–Toarcian interval of the Quse Formation at the Qixiangcuo section in the Southern Qiangtang Basin. The Qixiangcuo section consists of carbonate and siliciclastic organic carbon-poor sediments deposited in a shallow-shelf setting in the eastern Tethys Ocean. Chemostratigraphic data, including Ti, Zr, U, Ca, Mn, and Sr and their ratios normalized to Al, record characteristic changes linked to sea level evolution and resulting depositional sequences. Trends in these geochemical data allow for the subdivision of the Quse Formation into nine complete third-order transgressive–regressive sequences, referred to as Pliensbachian sequences PSQ1 and PSQ2, Toarcian sequences TSQ1 to TSQ7, and one incomplete sequence. Elemental proxies indicative of terrigenous detrital input and sediment grain size along with a mineralogical composition of quartz, plagioclase, and clay minerals exhibit similar trends. Increased values of these proxies suggest a sea level fall and the deposition of regressive systems tract (RST) sediments, with peak values indicating a maximum regressive surface (MRS), and vice versa for transgressive systems tract (TST) sediments and the maximum flooding surface (MFS). On the contrary, rising trends in calcite content and carbonate-bound elements indicate phases of a relative sea level transgression, reaching maximum values at the MFS, while declining trends mark a sea level regression. The Sr/Ca ratio exhibited inverse patterns to the carbonate proxies, in part, with rising values indicating a sea level fall and vice versa.

1. Introduction

The early Jurassic (late Pliensbachian–Toarcian) was marked by environmental and paleoceanographic perturbations [1]. These changes were driven by the emplacement of large igneous provinces, such as the Karoo–Ferrar, associated with the breakup of Gondwana and simultaneous greenhouse gas emissions [1,2,3]. These geologic processes had profound impacts on both terrestrial and marine ecosystems, such as global warming and disruptions in the carbon cycle [4,5,6], accelerated hydrological cycles and intensified weathering conditions [7,8,9,10], faunal extinctions and turnovers among benthic organisms [11,12], and high-amplitude sea level cycles [9,13,14,15]. The late Pliensbachian to early Toarcian provides evidence for two events, the Pliensbachian–Toarcian boundary event and the early Toarcian Jenkyns event, both marked by global negative carbon isotope excursions recorded in marine and lacustrine sedimentary archives [4,5,6,16,17].

Sea level changes are crucial for understanding the dynamics of oceanic anoxic events (OAEs), as they influence basin configurations, sedimentary depositional patterns, and the distribution of biotic communities [18]. Cyclical patterns of transgression–regression events can be interpreted to reflect changes in major geological processes such as tectonic activity (i.e., tectono-eustasy), waxing and waning of continental ice caps (i.e., glacio-eustasy), and/or fluctuations in groundwater storage (i.e., aquifer-eustasy) [19,20,21]. These sea level variations can be reconstructed from geological archives using proxies such as sediment facies and lithostratigraphy, geochemical analyses, mineralogical composition, and paleontological data. Recent investigations in elemental geochemical proxy data and their implications for reconstructing sea level changes and sequence stratigraphy are rapidly growing [15,22,23,24,25,26,27,28,29,30,31,32,33,34]. These studies include changes in the bulk geochemical composition of elements such as Si, Al, Ti, K, Zr, Fe, U, Mn, Ca, and Sr.

This study focuses on the lower Jurassic (upper Pliensbachian–Toarcian) strata of the Quse Formation in the Qixiangcuo section, which is located in the Southern Qiangtang Basin within the eastern Tethys region (Figure 1). The study involves the analysis of total organic carbon (TOC), mineralogical composition, and inorganic bulk rock geochemistry. Previous studies in the Southern Qiangtang Basin have predominantly examined carbon cycle perturbations, paleocenographic conditions, and continental weathering dynamics [10,35]. However, there is a notable lack of investigations into relative sea level fluctuations and sequence stratigraphic interpretations from the upper Pliensbachian–Toarcian interval. To date, this has been limited to one study by Mansour et al. [15], which addressed the lower Toarcian organic carbon-rich oil shales of the Quse Formation in the Northern Qiangtang Basin (Bilong Co area) based on palynology and bulk geochemical data.

The current study forms an integral part of a focused project investigating the stratigraphic succession of the upper Pliensbachian–Toarcian Qixiangcuo section in the Southern Qiangtang Basin (Figure 1). The research involves a high-resolution sampling strategy throughout the Qixiangcuo section to elucidate the evolution of sea level cycles and establish a robust sequence stratigraphic framework. This is achieved through detailed analyses of bulk rock elemental and mineralogical data and TOC content. Additionally, the results of this study are compared with data from the Toarcian strata of the Quse Formation in the Northern Qiangtang Basin. By comparing datasets from the Southern and Northern Qiangtang terranes, this study enhances our understanding of the evolution and regional variations in sea level cycles during the late Pliensbachian to early Toarcian.

Figure 1. (A) Regional map of the Tibetan Plateau showing the location of the Qiangtang Basin and adjacent structural orogenic belts and bounding suture zones [36]. ATF: Altyn Tagh Fault; NQB: Northern Qiangtang Basin; SQB: Southern Qiangtang Basin; AKMS: Aylmaqin–Kunlun–Mutztagh suture; IYS: Indus–Yalu suture; BNS: Bangong–Nujiang suture; GLS: Ganzi–Litang suture. The orange solid circle represents the Bilong Co area. (B) Major structural features and location of the current study Qixiangcuo section as well as the Sewa and Suobucha sections in the Southern Qiantang Basin [37].

Figure 1. (A) Regional map of the Tibetan Plateau showing the location of the Qiangtang Basin and adjacent structural orogenic belts and bounding suture zones [36]. ATF: Altyn Tagh Fault; NQB: Northern Qiangtang Basin; SQB: Southern Qiangtang Basin; AKMS: Aylmaqin–Kunlun–Mutztagh suture; IYS: Indus–Yalu suture; BNS: Bangong–Nujiang suture; GLS: Ganzi–Litang suture. The orange solid circle represents the Bilong Co area. (B) Major structural features and location of the current study Qixiangcuo section as well as the Sewa and Suobucha sections in the Southern Qiantang Basin [37].

2. Regional Geologic and Stratigraphic Evolution

The Qiangtang Basin is considered a transitional tectonic domain within the Eastern Gondwana supercontinent. During the early Permian, tectonic shortening resulted in the fragmentation of the Qiangtang Basin into the Northern and Southern Qiangtang terranes via the Longmu Co–Shuanghu suture zone, which is located in the central part of the basin [38]. This suture zone formed during the convergence and collision between Gondwana and Pan-Cathaysia [38]. This was followed by the rapid separation and drifting of the Qiangtang Basin from Gondwana during the late Permian [39,40]. In the early Triassic, further northward drifting and collision occurred between the Hoh Xil–Songpan block and the Qiangtaing Basin, resulting in the development of a syn-tectonic foreland basin and a thrust-fold belt [41]. During the late Triassic, the Qiangtang Basin experienced basin inversion and subsidence due to complex intracontinental rifting and the rapid expansion of the Bangong–Nujiang Ocean [42]. This tectonic evolution continued into the early Jurassic, further spurring significant changes in the paleogeography of the basin and the formation of a hemipelagic open shelf sea in the Southern Qiangtang Basin [6]. Contemporaneously, the Northern Qiangtang Basin represented a lacustrine environment [40]. During the early Jurassic, the Southern Qiangtang Basin was part of the northeastern Tethys margin.

The studied Qixiangcuo section is located in the Southern Qiangtang Basin, which is delineated by several key geological units such as the Bangong–Nujiang suture zone and the Lhasa Blocks to the south, and the Longmu Co–Shuanghu suture zone, the Central Uplift, and the Northern Qiangtang Basin to the north (Figure 1). The northwestern margin of the Southern Qiangtang Basin is marked by the Jinsha suture zone. These sutures and geological boundaries outline the extent of the Southern Qiangtang Basin within the broader tectonic framework of the region.

In the Southern Qiangtang Basin, such as the Suobucha area, a thick succession of siliciclastic and carbonate rocks was deposited from the upper Paleozoic (Permian) to the Cenozoic (Figure 1). The studied Qixiangcuo section is located in the Suobucha area (Figure 1). The Qixiangcuo section is situated close to the Lower Jurassic Sewa [35] and Suobucha sections [10]. The predominant lithology of the Qixiangcuo section belongs to the Jurassic Quse Formation (Figure 1 and Figure 2). The Lower Jurassic strata of the Quse Formation are characterized by bioclastic and micritic limestones, organic carbon-poor mudstones, and marlstones deposited in a shallow marine environment (Figure 2). The lithologic composition of the Quse Formation in the Qixiangcuo section reflects a comparable stratigraphic composition to sediments recovered from the Sewa and Suobucha sections. However, the Toarcian strata of the Quse Formation in the Bilong Co area are dominated by thick intervals of oil shales, organic carbon-rich mudstones, and calcareous mudstones [6,7]. The stratigraphic composition of the Qixiangcuo section comprises bioclastic limestones at the interval from 0–48 m, transitioning to white, dark- to light-gray micritic limestones at the interval from 48.1–83 m (Figure 2). The upper interval is characterized by alternating layers of dark gray and yellow organic carbon-poor mudstones and marlstones (Figure 2).

Within the Suobucha area, the Quse Formation has yielded some invertebrate mollusc fossils, including bivalves and ammonites. Biostratigraphic analysis of the adjacent Suobucha section revealed an ammonite assemblage of Cleviceras, Eleganticeras, and Harpoceras groups within Lower Jurassic strata of the Quse Formation [43]. These ammonite fossil species were used to indicate an early Toarcian age for the interval of the Quse Formation in the Suobucha area, which was further supported by a record of a strong negative carbon isotope excursion (CIE) [10]. Similar records of the former ammonite groups alongside the bivalves of the Bosistra group as well as a negative CIE were also reported from the Sewa area, to the west of Suobucha (Figure 1), in the Southern Qiangtang Basin [35].

Within the Qixiangcuo section, the Bosistra and Harpoceras groups were documented in the studied interval of the Quse Formation at a height of 80–90 m, suggesting an early Toarcian age of the mudstone and marlstone strata [44]. Additionally, strata at the uppermost part of the Qixiangcuo section (at ca. 128 m) yielded the age-diagnostic ammonite species Stephanoceras sp. (Figure 2) [44], which infers a Middle Jurassic age, with its first occurrence marking the Bajocian in Western Europe [45]. Age constraints of the Quse Formation can further be elucidated from biostratigraphic data from the adjoining Sewa and Suobucha sections [35,36,43]. Zhao et al. [43] established a robust ammonoid biostratigraphic scheme for the Suobucha section. The presence of the ammonoids Eleganticeras sp., Harpoceras sp., and Cleviceras sp. in the Suobucha section aligns with their first occurrence in the H. exaratum subzone [43]. The age-diagnostic ammonite species Harpoceras falciferum is commonly used to infer an early Toarcian age in Western Europe [1,46]. The falciferum zone can be discriminated into H. exaratum and H. falciferum subzones [46]. The correlation between the recovered marker ammonoids of Harpoceras and Bosistra in the Quse Formation of the Qixiangcuo section can be correlated with contemporaneous lower Toarcian strata in the Suobucha area, thereby providing evidence for the early Toarcian age of the mudstone and marlstone interval in the studied succession.

Figure 2. Lithostratigraphic chart (A) and representative field photographs (B–G) of the Quse Formation in the Qixiangcuo section, Southern Qiangtang Basin. (A) Lithostratigraphic column of the Qixiangcuo section with sample positions in corresponding field photographs (represented by the letters D across G) and recovered ammonite fossils in the study area [44] and those recorded from coeval strata in the adjacent Sewa and Suobucha sections [35,36,43]. The black squares indicate the locations of marker ammonoid fossils found from in the Suobucha and Sewa areas. (B) Google Earth image of the Qixiangcuo section in the Southern Qiangtang Basin. (C) Panorama field photograph of the study section, illustrating some positions of fieldwork sampling exaggerated in photographs D–G and the points at which the samples were collected. (D–G) Field photographs of representative outcrop strata with their lithofacies changes and related samples. f? = fault. Aa.-Baj. = Aalenian–Bajocian.

Figure 2. Lithostratigraphic chart (A) and representative field photographs (B–G) of the Quse Formation in the Qixiangcuo section, Southern Qiangtang Basin. (A) Lithostratigraphic column of the Qixiangcuo section with sample positions in corresponding field photographs (represented by the letters D across G) and recovered ammonite fossils in the study area [44] and those recorded from coeval strata in the adjacent Sewa and Suobucha sections [35,36,43]. The black squares indicate the locations of marker ammonoid fossils found from in the Suobucha and Sewa areas. (B) Google Earth image of the Qixiangcuo section in the Southern Qiangtang Basin. (C) Panorama field photograph of the study section, illustrating some positions of fieldwork sampling exaggerated in photographs D–G and the points at which the samples were collected. (D–G) Field photographs of representative outcrop strata with their lithofacies changes and related samples. f? = fault. Aa.-Baj. = Aalenian–Bajocian.

3. Material and Methods

In this study, a high-resolution fieldwork sampling was carried out from the Qixiangcuo section within the Suobucha area at coordinates 32°31′32.2″ N and 89°56′36.5″ E. A total of 199 fresh rock samples were collected from the Quse Formation at a short stratigraphic spacing (0.2–2 m) and a total thickness of 129.5 m, spanning from the upper Pliensbachian to the Toarcian (Figure 2). Prior to analysis, weathered surfaces were removed to clear the samples and minimize sedimentary rock alterations due to weathering processes. Subsequently, all rock samples underwent crushing and powdering to ensure representative sample material for comprehensive geochemical analyses.

3.1. Whole-Rock Geochemistry

For whole-rock geochemical composition, a total of eighty-three rock samples were measured for major and trace elements using an 8900 Inductively Coupled Plasma–Mass Spectrometry device (ICP-MS Aligent, Santa Clara, CA, USA) at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Each powdered sample (40 mg) was first weighed into 15 mL PTFE bombs and digested in a mixture of 0.5 mL of 8 mol/L HNO₃ and 1 mL of 28 mol/L HF acids. The PTFE bomb of each sample was sealed, transferred to a steel jacket, and heated at 190 °C for 24 h. The bomb was then placed on a hot plate for further heating at 150 °C to eliminate the solutions and dry the powdered fractions. HNO₃ acid (2 mL of 8 mol/L) was added to the bomb and transferred to the steel jacket for heating at 150 °C for 5 h. Furthermore, the previous step was repeated by further additions of HNO₃ acid (5 mL of 3.2 mol/L). The solution of each sample was then transferred to a PET bottle and diluted in a 2% HNO₃ solution. Analytical precision and quality assurance were monitored using USGS BHVO-2 and NRCG GSR-3 standard reference materials [47]. The analytical precision of the measured values was better than 5%. The elemental geochemical data suggest distinct behaviors within each sedimentological group, providing valuable insights for tracking paleoenvironmental and sea level changes [23,24,27,28,34].

3.2. Total Organic Carbon (TOC) Analysis

Within the Qixiangcuo section, 199 rock samples underwent total organic carbon (TOC) analysis. Prior to analysis, about 100 mg of each powdered sample was digested in concentrated HCl (34%) for 10 h to eliminate the inorganic carbon present in the sediments. The mixture was subsequently rinsed with distilled water to remove any remnants of HCl. The powdered fraction was then dried at 60 °C for 4 h. The TOC content of all samples was measured using a LECO CS-200 carbon–sulfur analyzer (LECO Corporation, St. Joseph, Michigan, USA) at 1200 °C. The analytical precision was better than ±0.1 wt.%. The TOC analysis was carried out at the Exploration and Development Research Institute, PetroChina Huabei Oilfield Company.

3.3. Whole-Rock Mineralogy

Within the Qixiangcuo section, a total of 34 rock samples were analyzed for their bulk-rock mineralogical composition. These samples were chosen based on the cyclical changes observed in the lithological composition, as well as the fluctuations in major and trace element concentrations throughout the studied succession of the Quse Formation. Before analysis, all samples were crushed into small particles of 200 mesh. An aliquot of 5 g of each sample was analyzed using the back pressure method in a Rijaku SmartLab9 X-ray diffractometer. Because each type of mineral shows a specific XRD pattern, the mineral peak intensity was correlated with the bulk mineral composition, which was used to calculate the mineral content. The bulk mineralogical composition analysis was performed at the SGS–CSTC Standards Technical Services Co., Ltd., Beijing, China.

4. Results

Stratigraphic variations in the TOC content and the bulk geochemical composition of major and trace elements, including Si, Al, Ca, Ti, Zr, Mn, U, and Sr, were recorded (Figure 3, Figure 4 and Figure 5). All samples from the Qixiangcuo section exhibited an overall TOC content below 0.5 wt.% (Figure 3). The lowest TOC values (0.2–0.03 wt.%, average 0.1 wt.%) occurred within the upper Pliensbachian bioclastic limestone interval (0–36 m). A slightly higher TOC content, ranging from 0.3 to 0.1 wt.% (average 0.2 wt.%), was recorded within the lower Toarcian micritic limestone interval. The highest values of TOC (0.45–0.09 wt.%, average 0.2 wt.%) were documented within the lower Toarcian mudstone–marlstone interval (81.5–126 m) (Figure 3).

Here, the results of the elemental concentrations and their ratios used to reconstruct the third-order transgressive–regressive sequences, such as Zr, Ti, U, Mn, Ca, Sr, Si, and Al, are presented and discussed (Figure 4 and Figure 5). Within the Qixiangcuo section, elements such as Zr, Ti, and U showed typically similar patterns across the stratigraphic succession (Figure 4). The concentrations of Zr, Ti, and U were in the range of 148.6–8.9 ppm (average 95.2 ppm), 0.44–0.03 wt.% (average 0.9 wt.%), 2.6–0.4 ppm (average 1.7 ppm), respectively. These elements showed successive cyclic fluctuations, with the highest concentrations typically found within the mudstone/marlstone intervals (Figure 4). The lowest values of Zr, Ti, and U occurred within the upper Pliensbachian–lowermost Toarcian bioclastic and micritic limestone intervals (Figure 4). The Zr/Al and Ti/Al ratios also exhibited slightly similar trends throughout the Qixiangcuo section. The Zr/Al and Ti/Al ratios were in the range of 19.7–12.9 (averaging 14.5) and 0.062–0.038 (averaging 0.048), respectively, within the bioclastic limestone of the upper Pliensbachian, with a clear long-term falling trend toward the lowest values around the Pliensbachian–Toarcian boundary transition. Successive cyclic changes in the Zr/Al (15.9–13.3, averaging 14.4) and Ti/Al ratios (0.048–0.041, averaging 0.043) were evident within the lower Toarcian mudstone/marlstone interval, mirroring the trends seen in Zr, Ti, and U but with lower amplitudes (Figure 4).

The CaCO₃ (calculated from CaO) and Mn contents, along with the Mn/Al ratios, showed considerable variability throughout the Qixiangcuo section. The carbonate content showed slightly similar trends to the Mn/Al ratio and Mn content. In the lower Quse Formation, the bioclastic and micritic limestones showed a slight decreasing trend in the CaCO₃ content, in contrast to a significant decline in the Mn and Mn/Al values (Figure 5), which were in the range of 91.3–62.8 wt.% (averaging 74.7 wt.%), 0.1–0.03 wt.% (averaging 0.05 wt.%), and 0.08–0.01 (averaging 0.03), respectively. The highest carbonate content and Mn/Al ratios were observed within the bioclastic limestone interval (0–48 m) (Figure 5). The lower Toarcian mudstone/marlstone interval showed the lowest values for both the carbonate content and Mn/Al ratios, with values ranging from 73–5.3 wt.% (averaging 22.2 wt.%) and 0.05–0.01 (averaging 0.013), respectively (Figure 5). Throughout the Quse Formation, the highest Mn content was found within the mudstone/marlstone interval, with values ranging from 0.17–0.05 wt.% (averaging 0.09 wt.%). The Sr/Ca ratio generally showed opposite trends to the carbonate content across most parts of the studied succession (Figure 5). In the bioclastic/micritic limestone interval, the Sr/Ca ratios showed a gradual decrease from 65.2 to 38.2 (Figure 5). This was followed by highly variable Sr/Ca ratios ranging from 82.5–33.9 (averaging 52.9), with cyclical alternations that corresponded to the lithological changes between the mudstone and marlstone intervals. Based on the stratigraphic variations in the elemental geochemical composition, the upper Pliensbachian–Toarcian Qixiangcuo section can be subdivided into nine third-order transgressive–regressive sequences and one incomplete sequence (Figure 5). The geochemical composition of elements Si, Al, Ca, Ti, Zr, Mn, U, and Sr are shown in

Table 1. The geochemical composition of major and trace elements and ratios used to delineate the third-order transgressive–regressive sequences of the Quse Formation within the Southern Qiangtang Basin.

Samples	TOC	SiO2	Al2O3	CaCO3	Ti	Ti/Al	Zr	Zr/Al	U	Sr/Ca	Mn	Mn/Al
	wt.%	wt.%	wt.%	wt.%	wt.%		ppm		ppm		wt.%
QX199	0.16	24.08	6.14	59.13	0.16	0.048	48.00	14.77	0.99	44.21	0.115	0.035
QX197	0.19	24.07	7.39	57.83	0.17	0.044	54.55	13.95	1.10	39.38	0.084	0.021
QX194	0.22	32.86	11.55	39.35	0.25	0.041	79.98	13.09	1.49	38.90	0.050	0.008
QX191	0.27	32.59	12.40	31.88	0.29	0.044	93.19	14.20	1.81	33.90	0.048	0.007
QX188	0.14	16.25	2.87	72.96	0.07	0.045	21.42	14.08	0.57	49.00	0.071	0.047
QX185	0.18	55.46	18.90	5.33	0.43	0.043	144.73	14.47	2.44	82.51	0.081	0.008
QX183	0.25	37.13	12.13	35.19	0.27	0.042	88.13	13.72	1.64	43.84	0.151	0.024
QX181	0.24	53.78	17.47	7.46	0.44	0.048	145.54	15.74	2.47	65.63	0.098	0.011
QX179	0.29	37.66	11.44	36.85	0.27	0.044	85.13	14.07	1.68	46.50	0.165	0.027
QX177	0.19	52.95	18.01	9.92	0.42	0.044	137.57	14.43	2.29	57.30	0.073	0.008
QX175	0.38	54.37	15.92	10.44	0.40	0.048	134.25	15.94	2.30	54.63	0.096	0.011
QX173	0.19	55.05	15.38	12.34	0.36	0.044	123.18	15.13	1.98	53.95	0.094	0.012
QX171	0.28	54.05	18.27	8.67	0.43	0.045	145.73	15.07	2.39	54.26	0.099	0.010
QX169	0.17	55.20	17.64	8.22	0.43	0.046	148.56	15.92	2.36	57.87	0.073	0.008
QX167	0.23	56.18	15.48	10.59	0.34	0.042	119.46	14.58	1.97	57.23	0.087	0.011
QX166	0.22	55.24	16.54	9.39	0.39	0.045	133.34	15.24	2.20	59.16	0.077	0.009
QX164	0.19	53.43	17.95	9.27	0.41	0.044	144.25	15.18	2.45	57.41	0.069	0.007
QX162	0.22	54.45	18.12	8.53	0.42	0.044	140.57	14.66	2.33	59.67	0.068	0.007
QX160	0.13	53.42	17.73	10.10	0.40	0.043	135.24	14.41	2.31	58.72	0.069	0.007
QX158	0.13	45.36	15.78	21.46	0.37	0.044	121.36	14.53	2.08	49.79	0.091	0.011
QX156	0.12	36.41	12.10	37.99	0.28	0.044	89.25	13.93	1.57	47.44	0.110	0.017
QX154	0.17	48.90	17.48	14.56	0.39	0.042	130.50	14.11	2.29	51.12	0.067	0.007
QX152	0.18	52.71	17.94	9.97	0.39	0.042	138.09	14.54	2.29	58.14	0.057	0.006
QX149	0.20	51.24	17.59	11.81	0.39	0.042	132.81	14.27	2.21	52.92	0.068	0.007
QX147	0.24	54.14	17.60	9.18	0.41	0.044	136.89	14.70	2.22	52.65	0.060	0.006
QX145	0.27	50.04	16.37	16.06	0.38	0.044	124.57	14.38	2.00	49.66	0.100	0.012
QX143	0.10	35.69	11.75	39.31	0.26	0.043	83.06	13.36	1.44	47.43	0.143	0.023
QX141	0.11	38.58	12.89	33.82	0.30	0.044	94.12	13.79	1.63	47.54	0.131	0.019
QX139	0.09	43.60	14.14	25.92	0.33	0.044	105.05	14.04	1.79	49.66	0.111	0.015
QX137	0.14	44.65	14.94	22.74	0.35	0.044	112.62	14.25	1.88	48.45	0.115	0.015
QX135	0.16	47.74	15.83	19.55	0.37	0.044	120.10	14.34	1.94	50.82	0.090	0.011
QX133	0.27	50.67	17.58	13.01	0.41	0.044	132.93	14.29	2.14	59.56	0.098	0.010
QX131	0.33	55.97	15.25	11.69	0.35	0.043	119.36	14.79	1.94	71.61	0.080	0.010
QX129	0.29	51.97	17.90	10.33	0.39	0.042	134.22	14.17	2.28	55.18	0.064	0.007
QX127	0.25	51.61	17.74	10.93	0.40	0.043	138.20	14.72	2.38	65.98	0.065	0.007
QX125	0.28	51.63	17.14	11.96	0.38	0.042	131.80	14.53	2.26	61.90	0.062	0.007
QX123	0.23	50.61	17.37	12.52	0.40	0.043	132.95	14.47	2.30	58.47	0.062	0.007
QX121	0.29	46.41	16.87	17.54	0.38	0.042	124.74	13.97	2.18	48.28	0.074	0.008
QX119	0.22	43.89	15.67	22.62	0.34	0.041	112.59	13.58	2.02	48.42	0.088	0.011
QX117	0.31	42.68	15.21	24.80	0.34	0.042	110.81	13.77	2.13	46.00	0.093	0.012
QX115	0.16	36.77	12.78	35.79	0.29	0.043	95.65	14.14	1.77	43.76	0.113	0.017
QX113	0.10	37.76	13.19	33.56	0.31	0.044	98.79	14.16	1.71	46.21	0.114	0.016
QX111	0.14	36.47	12.75	36.31	0.29	0.043	95.32	14.12	1.76	45.55	0.115	0.017
QX109	0.10	36.93	13.10	34.54	0.30	0.044	97.65	14.09	1.77	47.15	0.111	0.016
QX107	0.21	39.37	13.60	31.60	0.31	0.042	98.63	13.70	1.74	45.42	0.112	0.015
QX105	0.23	36.79	12.65	35.79	0.30	0.044	93.40	13.95	1.70	44.35	0.118	0.018
QX103	0.24	38.90	13.45	32.46	0.30	0.042	97.34	13.68	1.67	44.42	0.115	0.016
QX101	0.24	52.66	16.78	12.27	0.38	0.043	130.99	14.75	2.26	53.13	0.077	0.009
QX099	0.26	54.02	18.36	7.98	0.43	0.044	144.29	14.85	2.51	62.26	0.065	0.007
QX097	0.36	52.71	18.09	9.45	0.42	0.044	140.57	14.68	2.39	56.90	0.070	0.007
QX095	0.45	53.84	18.16	8.26	0.43	0.044	146.36	15.22	2.47	57.63	0.076	0.008
QX093	0.41	54.37	18.73	6.88	0.44	0.045	147.79	14.91	2.55	71.61	0.069	0.007
QX089	0.33	53.56	15.29	14.02	0.36	0.044	122.56	15.14	2.11	60.88	0.109	0.013
QX085	0.35	54.80	17.98	8.03	0.40	0.042	139.20	14.63	2.38	62.89	0.061	0.006
QX083	0.24	54.99	18.74	6.48	0.42	0.042	143.07	14.42	2.49	62.29	0.054	0.005
QX080	0.22	53.21	18.41	9.48	0.42	0.043	139.42	14.31	2.46	54.75	0.061	0.006
QX078	0.37	51.80	18.08	10.63	0.42	0.043	138.87	14.51	2.45	52.17	0.066	0.007
QX076	0.21	51.50	16.99	12.12	0.42	0.046	137.98	15.35	2.40	54.09	0.081	0.009
QX074	0.28	41.62	14.87	25.91	0.33	0.041	109.94	13.97	2.01	45.16	0.105	0.013
QX072	0.21	38.04	13.33	31.04	0.29	0.042	99.93	14.17	1.84	43.75	0.110	0.016
QX070	0.16	18.47	6.38	63.85	0.14	0.041	45.13	13.36	1.03	42.13	0.076	0.022
QX069	0.20	23.60	7.82	56.25	0.17	0.041	55.16	13.33	1.15	40.29	0.072	0.017
QX066	0.18	20.56	6.15	62.30	0.13	0.041	43.71	13.43	0.99	46.73	0.127	0.039
QX063	0.29	36.72	12.28	35.28	0.27	0.041	86.47	13.30	1.54	37.65	0.079	0.012
QX060	0.13	17.32	5.11	69.03	0.12	0.044	38.68	14.30	0.81	45.33	0.112	0.041
QX057	0.20	15.36	3.42	73.87	0.08	0.043	24.04	13.28	0.46	53.08	0.095	0.053
QX054	0.19	15.51	4.72	71.37	0.12	0.046	36.53	14.62	0.70	42.70	0.037	0.015
QX051	0.25	16.50	5.32	68.42	0.13	0.045	40.31	14.32	0.79	41.30	0.036	0.013
QX048	0.25	16.55	5.37	68.72	0.12	0.043	39.87	14.03	0.76	39.86	0.036	0.013
QX045	0.21	18.18	6.16	64.51	0.15	0.045	46.74	14.34	0.88	40.09	0.034	0.010
QX042	0.26	19.47	6.49	62.77	0.15	0.045	48.12	14.01	0.91	39.20	0.035	0.010
QX039	0.22	17.15	5.53	68.79	0.14	0.046	43.10	14.73	0.82	38.15	0.034	0.012
QX036	0.20	18.46	6.37	63.38	0.16	0.047	50.22	14.90	0.95	38.74	0.037	0.011
QX033	0.21	17.27	6.11	66.09	0.15	0.045	46.98	14.53	0.86	38.30	0.039	0.012
QX030	0.19	17.03	5.42	68.81	0.13	0.045	40.87	14.25	0.80	41.38	0.052	0.018
QX026	0.16	15.67	4.69	71.74	0.09	0.038	31.88	12.85	0.59	45.47	0.099	0.040
QX023	0.17	12.44	3.19	79.00	0.07	0.040	22.14	13.12	0.52	47.31	0.081	0.048
QX020	0.18	16.63	3.07	73.89	0.08	0.052	26.05	16.03	0.53	46.76	0.061	0.038
QX017	0.25	6.33	1.76	87.83	0.05	0.051	14.08	15.12	0.36	53.26	0.037	0.040
QX012	0.04	7.00	1.67	87.14	0.05	0.062	17.43	19.72	0.86	62.28	0.070	0.080
QX009	0.04	6.69	1.62	86.61	0.05	0.059	13.89	16.20	1.01	59.74	0.065	0.076
QX005	0.05	4.93	1.24	91.27	0.04	0.058	10.69	16.28	0.66	65.17	0.048	0.073
QX001	0.05	5.98	1.08	90.42	0.03	0.057	8.90	15.57	0.66	59.84	0.031	0.054

.

The bulk-rock mineralogical composition of the Quse Formation samples provides insights into the sea level and environmental evolution that occurred during deposition. The bulk mineralogical composition of quartz, plagioclase, K-feldspar, dolomite, calcite, and clay minerals is shown in

Table 2. The bulk mineralogical composition used to reconstruct the third-order transgressive–regressive sequences of the Pliensbachian–Toarcian succession of the Quse Formation within the Southern Qiangtang Basin.

Samples	Quartz	Plagioclase	Calcite	Clay Minerlas	K-Feldspar	Dolomite
	wt.%	wt.%	wt.%	wt.%	ppm	wt.%
QX191	17.7	3.4	25.2	20.9	0.4	32.4
QX188	11.2	4.1	72.2	0.8	-	11.7
QX185	43.1	2.7	-	37.6	0.7	15.9
QX179	21.9	4.7	47.0	25.5	0.2	0.7
QX171	33.4	8.2	11.7	46.1	0.6	-
QX166	36.0	8.2	12.7	42.5	-	0.6
QX162	36.1	7.6	12.1	43.5	0.7	-
QX156	19.7	4.1	47.3	28.6	0.3	-
QX149	30.7	8.5	17.5	42.6	0.7	-
QX147	33.4	7.9	11.3	46.2	0.6	0.6
QX143	19.3	4.8	50.0	25.6	0.3	-
QX137	26.1	6.7	31.2	35.6	0.4	-
QX133	32.1	7.4	20.7	39.3	0.5	-
QX129	30.6	7.7	14.8	46.9	-	-
QX127	30.6	8.3	14.8	45.6	0.7	-
QX123	30.1	7.2	17.9	44.4	0.4	-
QX121	25.6	5.6	25.7	42.5	0.6	-
QX117	21.5	5.0	32.9	40.3	0.3	-
QX113	22.2	5.0	42.4	30.0	0.4	-
QX109	19.5	4.3	44.2	31.7	0.3	-
QX105	18.7	4.9	46.3	29.9	0.2	-
QX101	32.3	7.8	16.9	42.6	0.4	-
QX097	30.8	7.0	12.9	48.7	0.6	-
QX093	31.6	7.3	8.8	51.8	0.5	-
QX085	32.7	7.8	11.6	47.2	0.7	-
QX080	30.1	7.3	13.1	48.9	0.6	-
QX076	30.7	7.7	16.8	44.3	0.5	-
QX074	21.6	5.4	32.5	39.3	0.3	0.9
QX072	21.9	4.9	41.3	30.8	1.1	-
QX063	18.7	5.0	43.5	29.6	0.1	3.1
QX042	9.0	3.1	69.0	13.6	-	5.3
QX036	7.3	2.7	64.5	13.8	0.2	11.5
QX012	3.5	2.4	92.2	1.9	-	-
QX009	3.8	2.5	90.2	2.0	-	1.5

. At the upper Pliensbachian interval, the samples showed a relatively high content of calcite, ranging from 90.2–69 wt.% (Figure 6). This was followed by a significant decline in calcite within the Toarcian mudstone/marlstone strata, varying from 50–8.8 wt.% (averaging 25.8 wt.%), before rising again to 72.2 wt.% at the topmost layer of the succession (Figure 6, Table 2). On the contrary, the contents of quartz and clay minerals exhibited patterns that were typically opposite patterns to that of calcite (Figure 6). The quartz content increased substantially from the carbonate-dominated (9.0–3.5 wt.%, averaging 5.9 wt.%) to the mudstone-dominated facies (43.1–11.2 wt.%, averaging 27 wt.%) (Figure 6). The clay minerals content was low in the micritic and bioclastic limestones, with values ranging from 13.8–1.9 wt.% (7.8 wt.% on average), compared to moderate abundances in the mudstone/marlstone interval, where the values ranged from 51.8 to 0.8 wt.% (34.1 wt.% on average). Plagioclase had generally low values in the micritic/bioclastic limestones (3.1–2.4 wt.%, averaging 2.7 wt.%), in contrast to slightly higher values in the mudstone/marlstone interval (8.5–2.4 wt.%, averaging 5.8 wt.%) (Figure 6). Dolomite was reported from the lower and upper carbonate-dominated layers, with values in the range of 32.4–0.6 wt.% (7.7% on average). The K-feldspar content occurred in trace amounts.

5. Discussion

5.1. Sequence Stratigraphic Development and Model Used

The sequence stratigraphic architecture and cyclicity of sedimentary rock records are mainly driven by the interplay between autogenic and allogenic processes [48]. Several key factors influence sea level cyclicity, including regional-scale tectonic activities and plate movements, low- to high-amplitude eustatic sea level fluctuations, and climate evolution over short- to long-term timescales [48,49,50,51,52]. Additionally, factors such as terrigenous sediment supply, wind energy regimes, sedimentation rates, sediment compaction and loading rates, authigenic sedimentological processes, and basin subsidence/uplift significantly impact sequence stratigraphic evolution [15,53,54]. Therefore, the development of sea level cycles and sequence stratigraphy is intricately linked to these processes.

Sequence stratigraphic models include depositional, genetic, and transgressive–regressive sequence models [55,56]. This study employed the transgressive–regressive sequence stratigraphic model, which defines stratigraphic bounding surfaces and systems tracts relative to transgressive–regressive sea level cycles. The transgressive–regressive model comprises two main systems tracts: the transgressive system tract (TST), which forms under deeper accommodation space during the highest sea/lake levels and is bounded at the top by a maximum flooding surface [57,58]; and following the TST, there is a facies shift toward shallower or more terrigenous sediments, representing the regressive system tract (RST). The RST is bounded at its upper part by subaerial erosional unconformities or their correlative conformities, referred to as the maximum regressive surface [57]. The RST usually includes sediments deposited during the highstand and the early stage of the lowstand phases sea level fluctuations [56].

Subaerial unconformity surfaces can be developed in diverse environmental settings, including downstream fluvial systems [50], through fluvial incision [59], or during falling stage of sea levels that expose carbonate platforms [56]. The maximum regressive surface (MRS) marks the termination of a transgressive–regressive sequence cycle and serves as the boundary for each stratigraphic sequence. It represents the most prominent break in sea level cycles and sedimentation, indicating a shift from a sea level fall to rise [55]. The maximum flooding surface (MFS) marks the transition from a sea level transgression to regression, as well as the peak landward extent of shoreline invasion [51]. While both the MFS and MRS can be identified based on facies changes, palynological data, electrical logging data, and/or outcrop sections, detecting the MRS can be particularly challenging in deeper-water facies environments [55].

The resolution of sea level cyclicity and the reconstruction of stratigraphic sequences depend largely on the acquisition and types of data acquired. In this study, we applied a high-resolution geochemical approach to reconstruct sea level cycles and establish a sequence stratigraphic framework for the upper Pliensbachian–Toarcian shallow marine shelf deposits of the Quse Formation at the Qixiangcuo section [10,35,40,60]. For the Qixiangcuo section, we employed the third-order transgressive–regressive sequence stratigraphic model and its subdivision into the TST and RST and bounding surfaces.

5.2. Chemostratigraphic Expression of System Tracts and Bounding Surfaces

Elemental concentrations normalized to Al, in an attempt to estimate their enrichment relative to the average shale, can lead to spurious correlations between unrelated variables [61]. This issue is particularly pronounced when the coefficient of variation of Al differs significantly from that of the normalized element concentration [34]. Therefore, it is recommended to use both the relative abundances of raw element profiles and their normalized profiles to Al when comparing trends, especially in sequence stratigraphic reconstructions and bounding surfaces (Figure 4 and Figure 5). Relying solely on ratios normalized to Al may potentially obscure meaningful patterns in the dataset. Previous studies have instead employed trace element normalization to Ti as proxies for authigenic enrichments [62].

Several studies have applied the chemostratigraphic approach to reconstruct transgressive–regressive systems tracts and bounding surfaces worldwide [15,22,23,24,25,26,27,28,29,30,31,32,33,34]. In marine environments, strontium (Sr), an alkaline element, shows complex geochemical behaviors influenced by seawater chemistry and biological processes [63]. It is found in elevated concentrations in pelagic–hemipelagic carbonate sediments during phases of sea level lowstands and can serve as a proxy for reconstructing sea level changes relative to the Ca concentration [22,31]. The Sr/Ca ratio has been widely used in sea level and sequence stratigraphy studies, where high Sr/Ca ratios indicate a relative sea level fall and vice versa (Figure 5) [15,22,24,28,33]. Additionally, the highest values of Sr/Ca ratios in a stratigraphic sequence can indicate the MRS.

In shallow-shelf environments, the carbonate content can serve as a sensitive proxy for short-term sea level changes. Elevated carbonate values can be indicative of a phase of sea level highstand. In the siliciclastic–carbonate alternations found in shallow-shelf sedimentary records in northern Egypt, Mansour et al. [32] demonstrated a typical example of increasing carbonate content correlating with higher relative abundances of gonyaulacoid dinoflagellate cyst forms. This trend indicates a TST that reaches maximum values where the MFS is identified. Similarly, the carbonate-rich Sewa Formation in the Bilong Co area of the Northern Qiangtang Basin showed cyclic changes in its carbonate content, with a long-term increase in carbonate values reflecting sea level transgressions [15].

Manganese (Mn) is a highly insoluble element under well-oxygenated conditions, typically occurring in marine sediments as oxyhydroxides, and it is often found in association with carbonate and detrital fractions, as well as labile organic matter [64]. Mn solubility increases under oxygen-deprived conditions, leading to higher concentrations in anoxic bottom and pore waters [24]. The Mn content in pelagic carbonates is also influenced by the rate of terrigenous sediment influx. However, Mn levels are susceptible to diagenetic alterations, and anomalous Mn values may indicate the presence of a diagenetic Mn pump, which should be considered [24]. In sedimentary sequences, increasing trends in the Mn content and Mn/Al ratios typically characterize sediments deposited during the TST, with peaks in Mn and Mn/Al marking the MFS or sea level highstands [31]. Conversely, sediments of the RST usually show declining Mn contents and Mn/Al ratios, with minima occurring at the MRS [28,65]. However, caution should be considered when using Mn and Mn/Al ratios to reconstruct sea level variations, as the interpretation can be complicated by the diverse sources of Mn entering the basin.

Aluminum (Al) is typically associated with aluminosilicate clay mineral particles, whereas the variations in zirconium (Zr) and titanium (Ti) are the result of concentrations in heavy minerals, such as silt- and sand-sized detrital zircon particles [66], Ti-bearing heavy minerals such as ilmenite, pyroxene, titanite, augite, and rutile, as well as aluminosilicate minerals [67]. The stratigraphic variations in Zr and Ti, along with their detrital elemental ratios normalized to Al, serve as indicators of changes in grain size, current energy, relative sea level fluctuations, and variations in terrigenous detrital input from source areas [34,67,68]. In sequence stratigraphic reconstructions, increasing abundances of Zr and Ti, as well as higher ratios of Zr/Al and Ti/Al, indicate strata deposited during phases of sea level regression and/or lowstands [31]. Conversely, decreasing values characterize sediments deposited during phases of sea level rises and transgressions [25,26,29]. Uranium (U) is another important element primarily used to interpret paleoredox conditions, with stratigraphic variations in U concentrations also linked to sea level fluctuations [67]. An increase in the U concentration under well-oxygenated conditions signifies a phase of a relative sea level fall, whereas a decrease occurs during phases of a sea level rise. Recently, Mansour et al. [15] correlated U trends with Sr/Ca ratios, which typically showed opposite trends compared to Mn and carbonate contents. The lowest U values were consistent with the MFS, while the highest values were indicative of the MRS.

5.3. Sea Level Change and Sequence Stratigraphy of the Quse Formation

Within the Qixiangcuo section, stratigraphic variations in the geochemical data of terrigenous (Ti, Zr, U, and Al, Figure 4) and carbonate proxies (Ca, Sr, and Mn, Figure 5) have led to the subdivision of the upper Pliensbachian–Toarcian interval of the Quse Formation into nine complete third-order transgressive–regressive sequences and one incomplete sequence. The upper Pliensbachian interval is discriminated into two third-order sequences (PSQ1 and PSQ2), while the Toarcian interval comprises seven sequences (TSQ1 to TSQ7), and one incomplete sequence was deposited during the Bajocian (Figure 4, Figure 5 and Figure 6). In the Qixiangcuo section, the Ti, Zr, and U profiles exhibit similar patterns, indicating a coupled relationship among these parameters, which is driven by the response of terrigenous detrital supply to sea level changes (Figure 4). Additionally, the Ti/Al and Zr/Al ratios follow slightly similar trends, suggesting their association with sediment grain size and terrigenous supply [68]. This relationship is further supported by strong linear correlations observed between Ti and Zr (r = 0.998), Ti and U (r = 0.98), and Zr and U (r = 0.99) (Figure 7). Conversely, there is a moderate correlation between the U and TOC contents (r = 0.44, Figure 7) and a strong negative correlation between the U and carbonate contents (r = −0.97, Figure 7). These correlations indicate that the U content primarily reflects changes in terrigenous detrital input in response to sea level variations rather than being controlled by marine organic matter and redox conditions [67]. This suggests that U is a reliable proxy for tracing changes in sea level and sequence stratigraphic reconstructions. However, caution should be taken when using the U concentration as a proxy for tracing sea level changes and sequence stratigraphic reconstructions due to its complex geochemical behavior. Detrital minerals such as clay minerals, quartz, feldspar, and plagioclase are controlled by the weathering processes of parent rocks that erode and transport into a range of depositional environments [69]. These minerals form a major component of both marine and terrestrial sedimentary rocks. Generally, the calcite distribution in the Quse Formation is inversely correlated with the quartz, clay minerals, plagioclase, and feldspar contents (Figure 6, Table 2).

The TST sediments of the stratigraphic sequences in the Quse Formation are characterized by a long-term decline toward minimum values of the Ti, Zr, and U concentrations and/or their ratios Ti/Al and Zr/Al (Figure 4). Such decreasing trends are prominent in most stratigraphic sequences of the Toarcian interval. For example, the TST sediments of TSQ1 show gradual declines in Ti/Al (from 0.047 to 0.043), Zr/Al (from 14.9 to 13.3), Zr (from 50.2 ppm to 24.0 ppm), Ti (from 0.16 wt.% to 0.08 wt.%), and U (from 1 ppm to 0.5 ppm) (Figure 4). The lowest values in these terrigenous detrital proxies are used to identify the maximum landward invasion of the shoreline, marking the MFS. Similar patterns are observed in the TST sediments of TSQ2, TSQ4, and TSQ7 and to some extent in TSQ3, TSQ5, and TSQ6, where decreasing trends in terrigenous detrital elements are consistent [34]. However, the TST sediments of PSQ1 and PSQ2 exhibit stable and/or increasing values of Ti, Zr, and U (Figure 4). This behavior can be attributed to the predominance of carbonate deposition over terrigenous detrital input, as indicated by the pronounced carbonate values (average 74.4 wt.%).

Furthermore, the TST sediments are dominated by decreasing trends in quartz, clay minerals, and plagioclase contents, with the lowest values characterizing the MFS (Figure 6). For example, the TST sediments of TSQ3 show a decline in quartz (from 32.7 wt.% to 18.7 wt.%), clay minerals (from 51.8 wt.% to 29.9 wt.%), and plagioclase (from 7.8 wt.% to 4.9 wt.%) (Figure 6, Table 2). Similar patterns are also observed within the TST sediments of TSQ4, TSQ5, TSQ6, and TSQ7, where the lowest values mark the MFS of these sequences (Figure 6). In contrast, the calcite content shows a typically opposite pattern to phyllosilicate minerals. Within the TST sediments of TSQ3, the calcite content increases from 8.8 wt.% to 46.3 wt.%, indicating a phase of a sea level rise, with the highest value marking the MFS. Similarly, the TST sediments of the stratigraphic sequences TSQ4, TSQ5, TSQ6, and TSQ7 exhibit increasing trends in the calcite content, reaching maximum values at the MFS (Figure 6, Table 2).

In the Qixiangcuo section, the Mn and Mn/Al profiles show trends that are slightly similar to those of the carbonate content, whereas the Sr/Ca ratio exhibits opposite trends across most parts of the studied interval. This observation aligns with a strong positive correlation between Mn/Al and the carbonate content (r = 0.77, Figure 8), suggesting a coupled relationship primarily controlled by changes in terrigenous detrital supply, rates of carbonate production, and sea level fluctuations. The lack of correlations between the Mn and carbonate contents (r = −0.22) and Al (r = 0.20) indicates the complex behavior of Mn sources, which include contributions from oxyhydroxides. Hydrothermal sources, such as submarine volcanism and mid-ocean ridge hydrothermal events, also play a significant role in the Mn supply to marine environments [28,70].

The TST sediments of the Quse Formation are identified based on notable increases in the carbonate content, Mn concentrations, and Mn/Al ratios, contrasted with decreasing trends in Sr/Ca ratios. A typical example is seen in the TST deposits of TSQ3, where significant rises in the carbonate content (from 6.5 wt.% to 35.8 wt.%), Mn (from 0.05 wt.% to 0.12 wt.%), and Mn/Al ratios (from 0.005 to 0.02) are observed, alongside substantial declines in Sr/Ca ratios (from 71.6 to 44.4) (Figure 5). These highest values of Mn, Mn/Al, and carbonate content, compared to the lowest Sr/Ca ratios, indicate the farthest landward invasion of the shoreline, thus identifying the MFS of TSQ3 (Figure 5). Similar pronounced increasing trends in these marine proxies are reported within the Quse Formation, characterizing the TST deposits of PSQ1, PSQ2, TSQ1, TSQ4, TSQ5, and TSQ6 and to some extent TSQ2 and TSQ7 (Figure 5). Although the Sr/Ca ratios generally display a declining pattern across most TST deposits, in a few instances, they show an increasing trend, likely controlled by the availability and distribution of Sr in pelagic carbonates, such as in the upper Pliensbachian bioclastic limestones [22,23,28,33]. For example, the TST deposits show a gradual increasing trend toward the highest Sr/Ca ratios in TSQ1 (Figure 5).

Within the Qixiangcuo section, the RST sediments of the transgressive–regressive sequences developed in the Quse Formation display a long-term rise toward maximum values in terrigenous detrital proxies, such as the Ti, Zr, and U concentrations, as well as their Ti/Al and Zr/Al ratios (Figure 4). Pronounced increasing trends in these elements are consistently observed across several stratigraphic sequences within the studied interval. For example, the RST sediments of PSQ2 illustrate significant increases in the Ti/Al (from 0.038 to 0.047) and Zr/Al ratios (from 12.9 to 14.9), accompanied by gradual rises in Zr (from 31.9 ppm to 50.2 ppm), Ti (from 0.09 wt.% to 0.16 wt.%), and U (from 0.6 ppm to 1 ppm) (Figure 4, Table 1). Similarly, strong increasing trends in terrigenous detrital proxies are evident in the RST deposits of TSQ2, where Ti increases from 0.13 wt.% to 0.42 wt.%, Zr from 55.2 ppm to 139.4 ppm, and U from 1.2 ppm to 2.5 ppm, along with Ti/Al increasing from 0.041 to 0.046 and Zr/Al from 13.2 to 15.4 (Figure 4, Table 1). These trends suggest phases of a sea level fall [15,25,26,29,32,34]. The highest concentrations of these terrigenous detrital proxies are used to identify the maximum basinward shift in the shoreline, marking the MRS of a full sea level cycle. For example, the highest values in terrigenous detrital proxies in PSQ1, PSQ2, TSQ1, TSQ3, TSQ4, and TSQ5 are used to pinpoint the MRSs and thus the sequence boundaries. Furthermore, the RST sediments are characterized by significant increases in quartz, clay minerals, and plagioclase contents, with the highest values consistent with the MRS (Figure 6). The RST sediments of TSQ3 provide a typical example, with increasing values of quartz from 18.7 wt.% to 30.6 wt.%, clay minerals from 29.9 wt.% to 46.9 wt.%, and plagioclase from 4.9 wt.% to 8.3 wt.%. The highest values of these minerals in TSQ3 are consistent with a phase of a sea level fall toward the MRS (Figure 6). Similar patterns are observed within the RST sediments of TSQ4, TSQ5, TSQ6, and TSQ7, where the lowest values imply the MRS of these sequences (Figure 6). Conversely, the calcite content generally decreases in the RST sediments, with the lowest values characterizing the MRS. A typical example is observed within the RST sediments of TSQ4, where the calcite content declines from 50 wt.% to 11.3 wt.%. This decreasing calcite content is further recorded in the RST sediments of TSQ3, TSQ5, TSQ6, and TSQ7 (Figure 6).

The RST sediments of the Quse Formation are characterized by remarkable decreases in Mn concentrations, Mn/Al ratios, and carbonate contents, compared to notable increases in Sr/Ca ratios. A typical example is observed in the RST deposits of TSQ3, where pronounced declining trends are seen in the carbonate content (from 35.8 wt.% to 4.1 wt.%), Mn concentration (from 0.12 wt.% to 0.07 wt.%), and Mn/Al ratios (from 0.02 to 0.007), as opposed to marked increases in Sr/Ca ratios (from 44.4 to 66.0) (Figure 5). The lowest values of the carbonate content, Mn, and Mn/Al ratios, juxtaposed with the highest Sr/Ca ratios, are interpreted as indicative of a phase of a sea level fall toward the farthest basinward shift in the shoreline, thus identifying the MFS of TSQ3 (Figure 5). Further evidence of decreasing marine elemental proxies is observed during the deposition of RST sediments in PSQ1, PSQ2, TSQ1, TSQ2, TSQ4, TSQ5, TSQ6, and TSQ7 (Figure 5).

Recently, Haq [52] conducted a detailed re-evaluation of Jurassic sea level changes and associated third-order stratigraphic sequences, based on accurate chronostratigraphic data. In the Qixiangcuo section, the sequence stratigraphic framework and carbon isotope excursion events of the Quse Formation alongside the global sea level curve are shown (Figure 9). Although bulk carbonate carbon isotope (δ¹³C_carb) data are influenced by various factors, such as terrestrial input, organic matter, seawater dissolved inorganic carbon, and diagenesis, they can still provide insights into the paleo-seawater isotope signature and serve as a proxy for sea level changes [24,28,32]. Specifically, positive δ¹³C_carb trends may indicate phases of a relative sea level rise and vice versa [24,32]. Since the upper Pliensbachian–Toarcian δ¹³C_carb profile of the Quse Formation precisely correlates with coeval sedimentary successions worldwide [44], it offers a reliable chemostratigraphic tool for sea level and sequence stratigraphic correlations, particularly given the scarcity of ammonoid fossils in the Qixiangcuo section. For the Quse Formation sequences, the upper Pliensbachian sequences PSQ1 and PSQ2 may correspond with the uppermost Pliensbachian sequences JPl16 and JPl17 of Haq [52], respectively. The Toarcian sequence stratigraphy in the global sea level chart is subdivided into ten third-order sequences [52]. Among these, sequences JTo1 to JTo7 can be correlated with TSQ1 to TSQ7 of the Quse Formation in the study area. This correlation is supported by the presence of age-diagnostic ammonites, including Harpoceras sp., Eleganticeras sp., and Cleviceras sp., which are indicative of an early Toarcian age. These ammonoids are synchronous with the early Toarcian ammonite zones Harpoceras falciferum and Harpoceras bifrons of the Jurassic chronostratigraphic chart [52]. However, the long-term global sea level rise contrasts the δ¹³C_carb curve of the Quse Formation, which shows significant negative excursions during the early Toarcian (Figure 9). In the Northern Qiangtang Basin, the oil shales of the Quse Formation in the Bilong Co section, which were deposited during the T-OAE, are further subdivided into two third-order transgressive–regressive sequences (QuSQ1 and QuSQ2), bounded by a fault surface at their upper boundary [15].

Paleogeographic conditions played a prominent role in the development of the transgressive–regressive sequences of the Quse Formation in the Southern Qiangtang Basin. However, the Triassic–early Jurassic paleogeography in Tibet is poorly constrained. During the late Triassic, the Southern Qiangtang Basin underwent a tectonic inversion and subsidence, driven by complex intracontinental rifting [42]. This was followed by a great expansion of the Bangong–Nujiang Ocean (≥4000 km in width) that took place between the Southern Qiangtang Basin and the Lhasa Block [42,71]. At this time, the Qiangtang Basin and Lhasa Block were located close to 10° S, drifting rapidly toward northern latitudes. During the early Jurassic, the Southern Qiangtang Basin and the Amdo microcontinent were separated by back-arc rifting, leading to significant changes in the paleogeography of the Qiangtang Basin and the development of the Dongqiao–Amdo open-shelf seaway [6,72,73]. High-pressure metamorphic and arc-related magmatic rocks provide evidence that the lithosphere of the Bangong–Nujiang Ocean subducted northward during the early Jurassic (around 190 Ma) [72,74]. This resulted in intense structural deformation and subsidence of the Southern Qiangtang Basin, likely controlling the cyclic changes in sea level and deposition of the proposed stratigraphic sequences.

6. Conclusions

A high-resolution sampling from the upper Pliensbachian–Toarcian strata of the Quse Formation in the Qixiangcuo section of the Southern Qiangtang Basin was conducted to assess the TOC content and bulk geochemical and mineralogical composition. The stratigraphic variations in the bulk-rock elemental and mineralogical composition provided detailed insights into the chemostratigraphic expression in response to sea level fluctuations and sequence stratigraphy. Based on specific terrigenous and marine elements, such as Al, Zr, Ti, U, Ca, Sr, and Mn and their ratios, and the mineralogical composition, the studied interval of the Quse Formation is subdivided into nine complete third-order transgressive–regressive sequences and one incomplete sequence. The upper Pliensbachian interval encompasses two sequences (PSQ1 and PSQ2), while the Toarcian interval includes seven sequences (TSQ1 to TSQ7). Terrigenous detrital proxies, such as Zr, Zr/Al, Ti, Ti/Al, and U, exhibited slightly similar patterns, with relative increases together with an increase in quartz and plagioclase, indicating phases of a relative sea level fall and the deposition of RST sediments, while their highest values within a complete cycle were used to identify the MRS. This is evident in higher-resolution sequences, such as TSQ3 to TSQ7. Conversely, declining trends in these proxies marked a relative sea level rise and the deposition of TST sediments, with their lowest values indicating the MFS. Marine elemental proxies, such as the carbonate content, Mn concentration, and Mn/Al ratio, exhibited specific increasing trends, indicative of phases of a relative sea level rise and TST sediments, contrasting with Sr/Ca, which showed an inverse pattern. Decreasing values of the carbonate content, Mn concentration, and Mn/Al ratio versus increasing Sr/Ca ratios characterized phases of a relative sea level fall and RST sediments. MFSs across most stratigraphic sequences were identified based on maximal increases in the carbonate content, Mn concentration, and Mn/Al ratio juxtaposed with minimal values of terrigenous detrital proxies.