The geochemical data are often used to identify the provenance and reveal the characteristics of the source area (Roser and Korsch 1986; Cullers 1995; Armstrong-Altrin et al. 2013). As essential immobile elements, the ratios of of Al, Ti, and Zr can be used to reflect the type of the original rock (Stumm and Morgan 1981; Wesolowski 1992; Ayers and Watson 1993; Hayashi et al. 1997). In the plots of TiO2 versus Al2O3, Hayashi et al. (1997) divided the provenance into three types: mafic igneous rocks, intermediate igneous rocks, and felsic igneous rocks. And the samples of the Shashi Formation mainly belong to the source area of intermediate and felsic igneous rocks (Fig. 5A). Apart from the major elements, some trace elements (La, Sc, Cr, Hf, Th, Zr, etc.) and REEs are of great significance in interpreting the provenance and composition of the source area because of their potential nature of low mobility during the post-depositional process (Bhatia and Crook 1986; Mclennan et al. 1993). On the diagram of Th/Sc versus Zr/Sc (McLennan et al. 1993), all samples were also concentrated near the felsic source rocks, and the trend for Zr enrichment indicates that the sediment compositions of all samples may be not affected by recycling (Fig. 5B). On the diagram of La/Th versus Hf (Floyd and Leveridge 1987), all of the samples fall around the felsic arc source region (Fig. 5C). And on the Co/Th versus La/Sc diagram proposed by McLennan et al. (1993), these samples are located between andesite and felsic source rocks (Fig. 5D). In addition, the REE distribution and Eu anomaly in the sediments can also provide clues for the research of the source characteristics (Cullers 1994). The high LREE/HREE ratios and negative Eu anomalies of the Shashi Formation show the characteristics of felsic source rocks (Table 1; Fig. 4C). And the siltstone samples are mainly quartz, followed by feldspar, without basic clasts, indicating felsic source rocks. In general, these samples have the characteristics of felsic source rocks.
Previous studies have showen that the bulk-rock geochemistry of siliciclastic sediments is affected by the tectonic movement of source rocks, so it is often used to distinguish the tectonic setting of sedimentary basins (Bhatia 1983; Roser and Korsch 1986; Bhatia and Crook 1986). Some trace elements (such as La, Th, Sc, and Zr) with relatively low mobility can not only be used to trace the source rock but also be an important basis to distinguish tectonic setting. The tectonic setting of sediment development can be roughly divided into four types (Taylor and McLennan 1985; Bhatia and Crook 1986): oceanic island arc (OA), continental island arc (CA), active continental margin (ACM), and passive margin (PM). On the diagram of Th-Co-Zr/10 (Fig. 6A), most samples of the Shashi Formation fall around the ACM field; on the diagram of La-Th-Sc (Fig. 6B), most of samples are located at the CA (Bhatia and Crook 1986). And on the diagram of SiO2/Al2O3 versus K2O/Na2O (Roser and Korsch 1986) (Fig. 6C), almost all samples are located at ACM field.
Jianghan Basin is a rift basin in a composite basin-mountain system, and its sediments are provided by the erosion of elevated tectonic units that include various basement sequences. Yu et al. (2018a) identified four provenances with different orientations based on the U-Pb ages of zircons and bulk geochemical characteristics of clastic rocks of the Paleocene Shashi Formation. The oldest zircons are from Paleoproterozoic-Archean, and the geochemical data indicate that these samples are from the passive continental margin, characterized by recycled sedimentary, and are believed to have originated from the Huangling uplift in the northwest Jianghan Basin (Yu et al. 2018a). The second set of zircon ages are Neoproterozoic. During which magmatism associated with the Rodinian Supercontinent rupture was widespread in South China (Li et al. 2003a, 2003b; Zheng et al. 2004; Greentree et al. 2006), and Neoproterozoic volcanic rocks were also prevalent in the basement of the surrounding range systems of the Jianghan Basin, such as the South Qinling Belt, the Jiangnan Orogen, and Huangling uplift (Wang et al. 2007; Liu et al. 2008; Yao et al. 2013; Li and Zhao 2016). So, these samples of zircon are probably from basement of the ranges around the Jianghan Basin. The last set of zircons are from Ordovician-Triassic. The Ordovician zircons were derived from Caledonian magmatism in the Jiangnan Orogen (Shu et al. 1991; Ren 1991; Wang et al. 2011) and the North Dabashan zone of the South Qinling Belt (Chen et al. 2014; Cao et al. 2015; Wang et al. 2015), while the Triassic zircons were major derived from Indosinian magmatism in the Jiangnan Orogen (Yue et al. 1998; Wang et al. 2002, 2005). And the synthesis of geochemical data and petrographic results indicates felsic source rocks in the active continental margin and continental arc (Yu et al. 2018a). These indicate that the Jianghan Basin received detrital materials from the South Qinling Belt in the north and the Jiangnan Orogen in the south.
The siltstone samples of the Paleocene Shashi Formation in well ZK0303 also have the characteristics of felsic source rocks in the active continental margin and continental arc. According to the analysis of Yu et al. (2018), the provenance of the Paleocene in well ZK0303 should be mainly from the South Qinling Belt in the north and the Jiangnan Orogen in the south.
Palaeoweathering and palaeoclimate
In arid climate, the source rocks are mainly affected by physical weathering, in which they can only be mechanically broken down into smaller grain sizes without significant changes in mineralogical and chemical composition (Wanas and Assal 2020). And the chemical weathering plays a dominant role in humid climates and strongly controls the major and trace element composition of siliceous clastic sediments (Nesbitt and Young 1982; Harnois 1988; Middelburg et al.1988; McLennan et al. 1993; Fedo et al. 1995). It directly affects the removal of mobile elements (Na, K, Ca) and the enrichment of immobile elements (Al, Si) in the sediments (Nesbitt and Young 1982). The chemical index of alteration (CIA = Al2O3/(Al2O3 + CaO* + Na2O + K2O) × 100, Nesbitt and Young 1982; CaO*= CaO − 10/3 × P2O5, McLennan et al. 1993) and Al2O3-(CaO + Na2O)-K2O (A-CN-K) ternary diagram (Nesbitt and Young 1984) can determine the mobility of elements during chemical weathering and potassium metasomatism during diagenesis, and evaluate the weathering history and source rock composition. Besides, chemical index of weathering (CIW = Al2O3/(Al2O3 + CaO + Na2O) × 100, Harnois 1988), plagioclase index of alteration (PIA = 100× (Al2O3 - K2O)/(Al2O3 + CaO* + Na2O - K2O), Fedo et al. 1995), and index of compositional variability (ICV= (Fe2O3 + K2O + Na2O + CaO + MgO + TiO2)/Al2O3, Cullers et al. 1995) are also important bases for assessing the source area palaeoweathering.
The ratios of CIA in the all samples vary from 36.79 to 65.46 (average = 53.30) (Table 1). In general, a CIA ratio of 55 or less represents unweathered, while 100 represents fully weathered (Nesbitt and Young 1982). The lower CIA ratio indicates that chemical weathering in the source area was less during the Paleocene. In the ternary diagram of A-CN-K (Fig. 7A), All the samples are near the boundary of A-CN and are distributed along the line between clinopyroxene and PAAS, away from the A-K boundary that represents high weathering. And the ratios of CIW range from 40.23 to 78.08 (average = 61.60), the ratios of PIA range from 34.05 to 72.84 (average = 55.17), the ratios of ICV range from 1.27 to 2.39 (average = 1.66) (Table 1), They all represent a low to moderate degree of weathering in the source area (Harnois 1988; Fedo et al. 1995). In addition, the ternary diagram of Al2O3-Zr-TiO2 indicates that hydraulic sorting is not obvious (Fig. 7B, Garcia et al. 1991).
Worash (2002) believes that the distribution, composition and relative concentration of some trace elements in clastic rocks may indicate the palaeoclimate and palaeoenvironment. Zhao et al. (2007) and Cao et al. (2012) proposed using C-value as an indicator of palaeoclimate. The calculation formula of C-value is as follows: C-value = Σ(Fe + Mn + Cr + V + Ni + Co)/Σ(Ca + Mg + Sr + Ba + K + Na). This is because the Fe, Mn, Cr, V, Ni and Co elements are relatively enrich in humid conditions, while in arid conditions, evaporation precipitates saline minerals, resulting in the concentration of Ca, Mg, K, Na, Sr and Ba elements. And some current studies have shown that palaeoclimate conditions can impact the ratios of Ga/Rb, Rb/Sr and Sr/Cu in sediments (Jin and Zhang 2002; Bai et al. 2015). Gallium is related to kaolinite, suggesting strong chemical weathering associated with warm and humid climatic conditions (Roy and Roser 2013). Rubidium is enriched in illite, reflecting a dry and cold climate associated with weak chemical weathering (Beckmann et al. 2005). In the warm sedimentary environment, the ratio of Rb/Sr in sediments decreases, while the ratio of Sr/Cu increases (Lerman and Gat 1989; Cao et al. 2015). And the Sr/Cu ratio between 1.3 and 5.0 represents humid environments, while the Sr/Cu ratio is higher than 5.0 in arid climates (Lerman 1978).
On the major elements diagram (Suttner and Dutta 1986), all samples are located at arid field (Fig. 8A). And the C-value range from 0.15 to 0.48, Sr/Cu ratios range from 6.24 to 840.48, and Ga/Rb ratios range from 0.09 to 0.15 (Table 1). According to the C-value and Sr/Cu ratio (Zhao et al. 2007 and Cao et al. 2012), the climatic conditions of the Paleocene Shashi Formation were between arid and semi-arid/semi-humid (Fig. 8B). Moreover, the projection points in the Sr/Cu- Ga/Rb discriminant map indicate that the Jianghan Basin had a cold and arid climate during the Paleocene (Fig. 8C). Comprehensive geochemical indicators, climate of the Jianghan Basin was cold and arid during the Paleocene.