5.1 The significance of the development of bauxite
Based on the differences of genesis, bauxite can be divided into sedimentary bauxite, accumulation bauxite, laterite bauxite, and magmatic bauxite[33–34].
The bauxite in the study area is mainly developed at the bottom of the Wujiaping Formation, and their formation may be related to the Dongwu movement. Because of the Dongwu movement, those carbonate rocks in the Maokou Formation were exposed to the earth’s surface, suffering from weathering and denudation, and a paleo crust of weathering was formed. In the sedimentary period of the Wujiaping Formation, the bauxite was formed autochthonously or transported nearby.
Obviously, the bauxite in the WJP Formation is more likely to be sedimentary bauxite, and the distribution of the bauxite may also be the scopes of the paleo-uplift. Through the observation of thin sections, bauxite of the Wujiaping Formation is developed in the area of Shizhu-Sinan-Xuan’en, which is also the range of the paleao-uplift before the sedimentary of the WJP Formation (Fig. 3).
5.2 Paleoclimate
The values of CIA show that the paleoclimate of the bottom of the Wujiaping Formation in well XD-1 was hot and humid, which is consistent with the formation of the paleo crust of weathering. The climate of hot and humid would favor the weathering and denudation of carbonate rocks in Maokou Formation. Upward, it has gone through two circulations of warm-humid and cold-arid in the P3w1. In P3w2, the paleoclimate was cold and arid persistently. The fluctuations of CIA show that the well XD-1 has gone through the hot-warm-cold process in the Wujiaping Formation (Fig. 4).
While in the JTBP section, the values of the CIA shows that the paleoclimate of P3w1 changes from warm-humid to cold-arid. In the lower part of P3w2, the paleoclimate stayed cold and arid. While in the upper part of the P3w2, the paleoclimate converted into warm and humid. At the end of the P3w2, the climate became cold and arid again. The fluctuations of CIA show that the JTBP section has experienced the warm and humid-cold and arid-warm and humid process in the Wujiaping Formation (Fig. 5).
In general, the paleoclimate during the Wuchiapingian Stage changes from warm and humid in the P3w1 to cold and arid in the P3w2.
Table 1
Rock types of the Wujiaping Formation in the region of the eastern Yangtze Block
Rock types | Thin section photography | Size of grains | Description | Water Energy | Depositional Environment |
Bauxitite | Figure 2a-d | Silt to fined | A large number of aluminous minerals and clay can be seen, and oolitic structures are also developed | High | Shoreland |
Siliceous rock | Figure 2e | Mud (matrix) | Black-gray and lamellar structure | Low | Deep-water shelf |
Argillaceous siltstone | Figure 2f | Mud (matrix) Silt | Faint yellow, a few interlayer of gray mudstone | Low to medium | Shoreland |
Carbonaceous mudstone | Figure 2g-h | Mud (matrix) | Black, plenty of horizontal beddings | Low | Marsh of shoreland |
Siliceous mudstone | Figure 2i | Mud (matrix) Silt (siliceous radiolarian) | A few siliceous radiolarian | Low | Deep-water shelf |
Bioclastic mudstone | Figure 2j | Mud (matrix) Silt to medium (bioclasts) | Plenty of fragments of brachiopod and echinoderm (such as crinoid) | Low | Tidal flat |
Micrite | Figure 2k | Micrite (calcite) | Micrite calcite, no obvious sedimentary structures | Low | Deep-water shelf and shallow-water shelf |
Calcisphere limestone | Figure 2l | Micrite (matrix) Silt to fined (calcisphere) | Plenty of calcisphere, ellipsoidal to rounded | Medium | Shallow-water shelf |
Bioclastic limestone | Figure 2m-n | Micrite to fined (cementation) fined to medium (bioclasts) | Plenty of bioclasts | Medium to high | Shoal |
Chert nodule and clot limestone | Figure 2o | Micrite (matrix) | Chert nodules and clots, the size of them are 4 to 8cm and 5 to 16 cm individually | Low | Deep-water shelf and shallow-water shelf |
5.2 Redox conditions
In well XD-1, the values of both MoEF and UEF in the Wujiaping Formation are mostly less than 2, and the MoEF and UEF in P3w2 are relatively higher than in P3w1. It indicates that the reductibility of seawater in P3w2 was stronger than in P3w1, although the water redox conditions in both P3w1 and P3w2 were oxic. As for the ratio of MoEF/UEF, it values from 0.23 to 1.71 (avg. 0.58), which shows the same redox condition as the MoEF and UEF (Fig. 4).
In the JTBP section, the values of MoEF show the opposite way, especially in the P3w1. In P3w1, the values of MoEF shows the strong reductibility of seawater. While in P3w2, the average value of MoEF decreased, indicating the weaker reductibility than the former. The values of UEF were all less than 1, but the ratio of MoEF/UEF in P3w1 is much lager than in P3w2, which also shows the stronger reductibility of P3w1 than of P3w2. Therefore, it is obvious that the water redox conditions of the whole Wujiaping Formation in the JTBP section are anoxic, and the reductibility in P3w2 is weaker than in P3w1 (Fig. 5).
5.3 Hydrodynamic conditions
In well XD-1, the Zr/Rb ratios in P3w1 rapidly decrease in P3w2. It shows that the seawater hydrodynamic condition in P3w1 was stronger than in P3w2 (Fig. 4).
For the JTBP section, the values of Zr/Rb ratios of the whole Wujiaping Formation are very low, which indicates that the seawater hydrodynamic condition in the JTBP section was weak, according with the anoxic environment from the analysis above (Fig. 5).
5.4 Sedimentary facies
Through the observation of the thin sections and the analysis of the geochemical data, the sedimentary facies of the Wujiaping Formation in the study area are classified by shoreland, shallow-water shelf, deep-water shelf, and shoal.
5.4.1 Shoreland
Because of the Dongwu Movement, the carbonate rocks in the Maokou Formation were unevenly lifted, and suffered from the weathering and denudation, forming the paleo-weathering crust (Fig. 2a). Bauxitite are developed at the bottom of the WJP Formation in the well XD-1 and LSXP in the paleo-weathering crust. Overlying the bauxitite, argillaceous siltstones with few parallel and cross beddings are also observed (Fig. 2f), which shows the high hydrodynamic conditions in these two areas. Combined with the inheritance of the paleo-topography affected by the Dongwu Movement, we believe that shoreland are developed in the study area. Besides, based on the geochemical analysis in the well XD-1, the redox conditions are oxic, and its values of Zr/Rb shows high hydrodynamic conditions, which are also in agreement on the development of shoreland.
In the MGP, a coal seam with the thickness of forty three centimeters, interlayer of carbonaceous mudstone, is developed in the lower P3w1 (Fig. 6a-b). Further more, in the carbonaceous mudstone of SJP, carbonized plant fragments are observed in the P3w1 (Fig. 6c). Therefore, it is no doubt that the development of the marsh in shoreland.
5.4.2 Shallow-water shelf
The shallow-water shelf is the shallow water area between the storm wave base and the normal wave base, the depth of which mainly range from 10 meters to 200meters.
In the BTP, the lithology at the bottom of the WJP Formation is characterized by mustone and carbonaceous mudstone, and we prefer the shoreland of the sedimentary environment (Fig. 6d). However, adjacent to the BTP, micrite is observed in the JKP (Fig. 6e), which indicates the different sedimentary facies between them. Besides, in the same strata of the WJP Formation in SZCP and XJBP, bioclastic limestone is observed, with a few brachiopod and crinoid in it (Fig. 6f). Obviously, there must be a sedimentary facies that are mainly composed of carbonate rocks and located on the seaward side of the shoreland, that is, shallow-water shelf. Beside the carbonate rocks, clastic rocks, such as mudtone, are also observed as the interlayer between carbonates in the JKP (Fig. 6e).
5.4.3 Deep-water shelf
The deep-water shelf refers to the area below the storm wave base, where wave action is weaker compared with the shallow-water shelf.
In the JTBP, 3.6 meters thickness of siliceous rock and siliceous mudstone, interlayerd by carbonaceous mudstone, are developed at the bottom of the WJP Formation (Fig. 6g). In the siliceous mudstone, few ammonoid are recognized (Fig. 6h). Besides, the geochemical analysis shows the strong reductibility of redox conditions in the JTBP, compared with other areas. Obviously, the sedimentary facies in the JTBP is more likely to be the deep-water shelf in the P3w1.
5.4.4 Shoal
In the XYP, JKP and SZCP, heavy-layer and greyish bioclastic limestone are developed in the P3w2, and plenty of the bioclasts are observed, such as brachiopod and crinoid (Fig. 6i). Under the microscope, the bioclasts are cemented by clean calcite (Fig. 2n), which indicates the strong hydrodynamic conditions in these areas. Therefore, shoals are developed in the study area, mainly in the P3w2.
Table 2
Major elements of the Wujiaping Formation in the region of the eastern Yangtze Block
Sample | Formation | SiO2(%) | Al2O3(%) | CaO(%) | MgO(%) | K2O(%) | Na2O(%) | TiO2(%) | P2O5(%) | MnO(%) | Fe2O3(%) | CIA |
XD-1-HF1 | P3w1 | 29.22 | 18.30 | 1.32 | 2.74 | 1.80 | 0.40 | 2.30 | 0.12 | 0.44 | 28.68 | 84.84 |
XD-1-HF2 | P3w1 | 19.87 | 16.16 | 0.86 | 1.73 | 0.97 | 0.49 | 3.56 | 0.14 | 0.16 | 42.00 | 85.84 |
XD-1-HF3 | P3w1 | 50.98 | 16.18 | 3.26 | 1.27 | 1.70 | 2.37 | 2.04 | 0.32 | 0.05 | 4.84 | 62.66 |
XD-1-HF4 | P3w1 | 45.77 | 14.69 | 5.90 | 2.03 | 1.44 | 2.41 | 1.80 | 0.23 | 0.09 | 7.10 | 60.75 |
XD-1-HF5 | P3w1 | 47.18 | 18.52 | 4.79 | 1.77 | 1.98 | 2.46 | 2.22 | 0.30 | 0.07 | 5.64 | 64.39 |
XD-1-HF6 | P3w1 | 34.21 | 12.33 | 17.79 | 0.91 | 0.75 | 4.46 | 1.58 | 0.21 | 0.07 | 5.33 | 44.32 |
XD-1-HF7 | P3w1 | 49.42 | 17.34 | 2.33 | 1.47 | 2.31 | 1.45 | 1.98 | 0.22 | 0.15 | 7.93 | 70.44 |
XD-1-HF8 | P3w1 | 52.04 | 17.28 | 3.33 | 1.70 | 2.52 | 1.13 | 2.15 | 0.13 | 0.06 | 5.66 | 72.81 |
XD-1-HF9 | P3w1 | 56.92 | 15.93 | 2.67 | 1.38 | 2.31 | 1.22 | 2.01 | 0.13 | 0.05 | 4.57 | 70.96 |
XD-1-HF10 | P3w1 | 73.30 | 7.40 | 5.19 | 0.40 | 0.92 | 1.32 | 0.56 | 0.33 | 0.02 | 1.21 | 58.08 |
XD-1-HF11 | P3w2 | 54.08 | 15.86 | 5.33 | 1.52 | 1.95 | 3.11 | 1.46 | 0.18 | 0.07 | 3.74 | 56.22 |
XD-1-HF12 | P3w2 | 20.77 | 1.72 | 40.10 | 0.65 | 0.06 | 0.75 | 0.07 | 0.04 | 0.11 | 1.09 | 40.41 |
XD-1-HF13 | P3w2 | 64.30 | 2.08 | 15.80 | 0.65 | 0.28 | 0.34 | 0.07 | 0.03 | 0.02 | 0.30 | 59.39 |
XD-1-HF14 | P3w2 | 8.56 | 1.44 | 48.03 | 0.64 | 0.09 | 0.48 | 0.05 | 0.03 | 0.03 | 0.43 | 46.26 |
XD-1-HF15 | P3w2 | 21.85 | 3.39 | 36.70 | 2.00 | 0.44 | 0.62 | 0.14 | 0.03 | 0.03 | 0.82 | 57.39 |
XD-1-HF16 | P3w2 | 5.50 | 1.92 | 48.53 | 0.62 | 0.08 | 0.78 | 0.02 | 0.03 | 0.03 | 0.19 | 41.96 |
XD-1-HF17 | P3w2 | 83.52 | 0.056 | 6.22 | 0.43 | 0.03 | 0.07 | 0.01 | 0.04 | 0.01 | 0.058 | 17.74 |
JTBP-HF1 | P3w1 | 83.50 | 3.80 | 0.63 | 0.32 | 0.66 | 0.21 | 0.16 | 0.08 | < 0.01 | 1.39 | 72.98 |
JTBP-HF2 | P3w1 | 59.03 | 19.68 | 0.35 | 1.77 | 4.37 | 0.59 | 0.74 | 0.17 | 0.12 | 5.10 | 76.88 |
JTBP-HF3 | P3w1 | 68.84 | 7.68 | 1.39 | 0.52 | 1.72 | 0.60 | 0.30 | 1.33 | < 0.01 | 2.40 | 79.25 |
JTBP-HF4 | P3w1 | 75.52 | 6.29 | 0.11 | 0.37 | 1.14 | 0.58 | 0.23 | 0.05 | 0.01 | 4.25 | 73.53 |
JTBP-HF5 | P3w1 | 72.73 | 6.20 | 0.11 | 0.34 | 1.12 | 0.64 | 0.23 | 0.12 | 0.02 | 6.28 | 74.13 |
JTBP-HF6 | P3w1 | 72.03 | 8.76 | 0.88 | 0.55 | 1.95 | 0.65 | 0.32 | 0.53 | < 0.01 | 2.50 | 71.78 |
JTBP-HF7 | P3w1 | 64.32 | 9.34 | 1.16 | 0.53 | 1.76 | 0.71 | 0.36 | 0.64 | 0.01 | 5.18 | 72.36 |
JTBP-HF8 | P3w1 | 64.88 | 9.20 | 0.42 | 0.62 | 2.03 | 0.50 | 0.39 | 0.05 | < 0.01 | 1.19 | 71.52 |
JTBP-HF9 | P3w1 | 59.82 | 11.61 | 0.31 | 0.80 | 2.56 | 0.21 | 0.50 | 0.10 | < 0.01 | 2.56 | 77.17 |
JTBP-HF10 | P3w1 | 75.33 | 9.39 | 0.42 | 0.49 | 1.90 | 0.25 | 0.30 | 0.06 | < 0.01 | 2.66 | 76.50 |
JTBP-HF11 | P3w1 | 84.08 | 4.52 | 0.36 | 0.24 | 0.74 | 0.45 | 0.14 | 0.07 | 0.01 | 1.32 | 68.96 |
JTBP-HF12 | P3w1 | 68.62 | 10.12 | 1.01 | 0.58 | 2.01 | 0.46 | 0.37 | 0.07 | < 0.01 | 2.41 | 73.26 |
JTBP-HF13 | P3w1 | 89.91 | 1.85 | 0.07 | 0.11 | 0.26 | 0.42 | 0.09 | 0.02 | < 0.01 | 0.54 | 63.80 |
JTBP-HF14 | P3w1 | 77.73 | 5.77 | 0.10 | 0.36 | 1.29 | 0.54 | 0.28 | 0.02 | < 0.01 | 0.46 | 70.46 |
JTBP-HF15 | P3w1 | 76.94 | 5.42 | 0.06 | 0.28 | 1.30 | 0.50 | 0.28 | 0.02 | < 0.01 | 0.45 | 70.28 |
JTBP-HF16 | P3w1 | 31.56 | 0.98 | 22.07 | 12.15 | 0.18 | 0.14 | 0.04 | 0.04 | 0.07 | 0.41 | 59.90 |
JTBP-HF17 | P3w1 | 59.14 | 4.23 | 14.63 | 0.96 | 0.41 | 1.53 | 0.14 | 0.07 | 0.04 | 1.55 | 43.57 |
JTBP-HF18 | P3w1 | 47.66 | 4.43 | 19.83 | 0.53 | 0.50 | 1.54 | 0.14 | 0.06 | 0.02 | 1.66 | 44.13 |
JTBP-HF19 | P3w2 | 45.41 | 3.45 | 21.78 | 1.01 | 0.77 | 0.46 | 0.13 | 0.23 | 0.02 | 1.70 | 59.49 |
JTBP-HF20 | P3w2 | 13.24 | 1.41 | 40.60 | 4.50 | 0.24 | 0.25 | 0.05 | 0.04 | 0.04 | 0.54 | 56.56 |
JTBP-HF21 | P3w2 | 17.58 | 1.30 | 41.64 | 1.70 | 0.20 | 0.34 | 0.04 | 0.03 | 0.05 | 0.63 | 49.32 |
JTBP-HF22 | P3w2 | 26.46 | 2.68 | 28.68 | 7.66 | 0.60 | 0.08 | 0.09 | 0.05 | 0.07 | 1.15 | 74.56 |
JTBP-HF23 | P3w2 | 18.75 | 1.41 | 41.98 | 0.69 | 0.28 | 0.24 | 0.05 | 0.04 | 0.02 | 0.48 | 56.32 |
JTBP-HF24 | P3w2 | 18.25 | 0.86 | 42.38 | 0.74 | 0.12 | 0.23 | 0.03 | 0.09 | 0.02 | 0.51 | 49.23 |
JTBP-HF25 | P3w2 | 5.83 | 0.70 | 43.32 | 7.20 | 0.14 | 0.03 | 0.04 | 0.07 | 0.01 | 0.27 | 73.64 |
JTBP-HF26 | P3w2 | 35.53 | 0.70 | 33.83 | 0.44 | 0.16 | 0.04 | 0.02 | 0.05 | 0.01 | 0.17 | 69.64 |
JTBP-HF27 | P3w2 | 29.04 | 0.96 | 35.18 | 2.50 | 0.22 | 0.04 | 0.04 | 0.10 | 0.02 | 0.32 | 72.16 |
JTBP-HF28 | P3w2 | 33.70 | 0.85 | 33.59 | 0.27 | 0.12 | 0.05 | 0.04 | 0.10 | < 0.01 | 0.22 | 74.25 |
JTBP-HF29 | P3w2 | 88.42 | 0.64 | 2.69 | 0.20 | 0.18 | 0.06 | 0.03 | 1.09 | < 0.01 | 0.39 | 61.97 |
JTBP-HF30 | P3w2 | 13.17 | 0.18 | 44.81 | 2.47 | 0.02 | 0.02 | 0.01 | 0.02 | < 0.01 | 0.02 | 67.29 |
JTBP-HF31 | P3w2 | 19.01 | 0.19 | 43.25 | 0.88 | 0.03 | 0.03 | < 0.01 | 0.03 | < 0.01 | 0.03 | 59.14 |
Table 3
Trace elements of the Wujiaping Formation in the region of the eastern Yangtze Block
Sample | Formation | Trace elements (ppm) | Redox condition | Hydrodynamic condition |
Mo | U | Zr | Rb | MoEF | UEF | MoEF/UEF | Zr/Rb |
XD-1-HF1 | P3w1 | 0.34 | 1.64 | 489 | 44.8 | 0.12 | 0.40 | 0.30 | 10.92 |
XD-1-HF2 | P3w1 | 0.76 | 4.83 | 1180 | 19.8 | 0.31 | 1.34 | 0.23 | 59.60 |
XD-1-HF3 | P3w1 | 2.65 | 5.74 | 537 | 40.9 | 1.07 | 1.60 | 0.67 | 13.13 |
XD-1-HF4 | P3w1 | 2.06 | 4.11 | 494 | 34.7 | 0.91 | 1.26 | 0.73 | 14.24 |
XD-1-HF5 | P3w1 | 1.99 | 4.76 | 526 | 50.4 | 0.70 | 1.16 | 0.61 | 10.44 |
XD-1-HF6 | P3w1 | 2.28 | 1.93 | 298 | 18.0 | 1.20 | 0.70 | 1.71 | 16.56 |
XD-1-HF7 | P3w1 | 2.02 | 3.48 | 698 | 60.2 | 0.76 | 0.90 | 0.84 | 11.59 |
XD-1-HF8 | P3w1 | 1.15 | 3.57 | 720 | 68.2 | 0.43 | 0.93 | 0.47 | 10.56 |
XD-1-HF9 | P3w1 | 1.07 | 3.94 | 663 | 62.8 | 0.44 | 1.11 | 0.39 | 10.56 |
XD-1-HF10 | P3w1 | 0.38 | 4.28 | 186 | 24.5 | 0.33 | 2.60 | 0.13 | 7.59 |
XD-1-HF11 | P3w2 | 1.09 | 6.60 | 389 | 67.9 | 0.45 | 1.87 | 0.24 | 5.73 |
XD-1-HF12 | P3w2 | 0.49 | 1.13 | 26.1 | 2.84 | 1.86 | 2.95 | 0.63 | 9.19 |
XD-1-HF13 | P3w2 | 0.43 | 1.32 | 16.5 | 9.44 | 1.35 | 2.85 | 0.47 | 1.75 |
XD-1-HF14 | P3w2 | 2.65 | 2.64 | 28.6 | 3.68 | 11.99 | 8.25 | 1.45 | 7.77 |
XD-1-HF15 | P3w2 | 0.89 | 6.06 | 209 | 17.3 | 1.71 | 8.04 | 0.21 | 12.08 |
XD-1-HF16 | P3w2 | 0.36 | 2.04 | 41.7 | 3.12 | 1.22 | 4.78 | 0.26 | 13.37 |
XD-1-HF17 | P3w2 | 0.19 | 0.64 | 4.30 | 1.91 | 22.10 | 51.40 | 0.43 | 2.25 |
JTBP-HF1 | P3w1 | 53.4 | 0.53 | 0.46 | 23.2 | 91.53 | 0.63 | 145.92 | 0.02 |
JTBP-HF2 | P3w1 | 45.7 | 0.12 | 5.14 | 164 | 15.13 | 0.03 | 551.55 | 0.03 |
JTBP-HF3 | P3w1 | 87.4 | 0.36 | 0.82 | 58.0 | 74.12 | 0.21 | 351.61 | 0.01 |
JTBP-HF4 | P3w1 | 46.4 | 0.22 | 0.51 | 39.3 | 48.05 | 0.16 | 305.45 | 0.01 |
JTBP-HF5 | P3w1 | 148 | 0.37 | 0.48 | 38.8 | 155.48 | 0.27 | 579.31 | 0.01 |
JTBP-HF6 | P3w1 | 25.7 | 0.47 | 0.81 | 65.1 | 19.11 | 0.24 | 79.19 | 0.01 |
JTBP-HF7 | P3w1 | 53.4 | 0.55 | 0.80 | 59.2 | 37.24 | 0.26 | 140.61 | 0.01 |
JTBP-HF8 | P3w1 | 116 | 0.76 | 0.89 | 67.7 | 82.13 | 0.37 | 221.05 | 0.01 |
JTBP-HF9 | P3w1 | 43.2 | 0.61 | 1.06 | 88.5 | 24.24 | 0.24 | 102.57 | 0.01 |
JTBP-HF10 | P3w1 | 19.7 | 0.34 | 0.69 | 64.9 | 13.66 | 0.16 | 83.91 | 0.01 |
JTBP-HF11 | P3w1 | 13.6 | 0.14 | 0.34 | 25.4 | 19.60 | 0.14 | 140.69 | 0.01 |
JTBP-HF12 | P3w1 | 257 | 0.50 | 0.84 | 72.2 | 165.41 | 0.22 | 744.41 | 0.01 |
JTBP-HF13 | P3w1 | 24.7 | 0.17 | 0.15 | 9.69 | 86.96 | 0.41 | 210.43 | 0.02 |
JTBP-HF14 | P3w1 | 90.3 | 0.56 | 0.67 | 50.5 | 101.93 | 0.44 | 233.53 | 0.01 |
JTBP-HF15 | P3w1 | 334 | 0.34 | 0.54 | 46.2 | 401.38 | 0.28 | 1422.72 | 0.01 |
JTBP-HF16 | P3w1 | 72.1 | 0.05 | 0.12 | 6.00 | 479.20 | 0.22 | 2131.03 | 0.02 |
JTBP-HF17 | P3w1 | 16.6 | 0.25 | 0.37 | 18.6 | 25.56 | 0.27 | 96.17 | 0.02 |
JTBP-HF18 | P3w1 | 99.5 | 0.18 | 0.45 | 20.3 | 146.29 | 0.18 | 800.57 | 0.02 |
JTBP-HF19 | P3w2 | 112 | 0.27 | 0.38 | 30.6 | 211.45 | 0.35 | 600.77 | 0.01 |
JTBP-HF20 | P3w2 | 12.9 | 0.07 | 0.077 | 10.9 | 59.59 | 0.23 | 259.48 | 0.01 |
JTBP-HF21 | P3w2 | 2.54 | 0.09 | 0.084 | 9.78 | 12.73 | 0.31 | 40.87 | 0.01 |
JTBP-HF22 | P3w2 | 1.62 | 0.14 | 0.18 | 28.2 | 3.94 | 0.23 | 16.76 | 0.01 |
JTBP-HF23 | P3w2 | 2.57 | 0.09 | 0.088 | 14.1 | 11.87 | 0.29 | 40.90 | 0.01 |
JTBP-HF24 | P3w2 | 2.40 | 0.06 | 0.078 | 5.02 | 18.18 | 0.29 | 63.20 | 0.02 |
JTBP-HF25 | P3w2 | 2.08 | 0.03 | 0.070 | 7.15 | 19.35 | 0.20 | 97.17 | 0.01 |
JTBP-HF26 | P3w2 | 2.46 | 0.05 | 0.078 | 7.31 | 22.89 | 0.32 | 71.26 | 0.01 |
JTBP-HF27 | P3w2 | 1.69 | 0.05 | 0.068 | 10.4 | 11.47 | 0.21 | 54.39 | 0.01 |
JTBP-HF28 | P3w2 | 9.62 | 0.07 | 0.094 | 4.86 | 73.72 | 0.39 | 190.85 | 0.02 |
JTBP-HF29 | P3w2 | 6.88 | 0.12 | 0.076 | 4.07 | 70.02 | 0.84 | 83.03 | 0.02 |
JTBP-HF30 | P3w2 | 1.33 | 0.02 | 0.040 | 0.59 | 48.13 | 0.40 | 120.39 | 0.07 |
JTBP-HF31 | P3w2 | 1.47 | 0.02 | 0.046 | 0.95 | 50.39 | 0.45 | 112.05 | 0.05 |
5.5 Distribution of sedimentary facies
Based on the analysis on the sedimentary facies, during the sedimentary period of the P3w1, slow transgression occurred in the early Wuchiapinian. Most of the study area is mainly composed of clastic rock, formed in the sedimentary environment of shoreland (Fig. 7). The lithologies are mainly composed of bauxite, argillaceous siltstones, carbonaceous mustone, bioclastic mudstone and argillaceous mudstone. In the areas of Wulong, Yanhe and Nanjiang, shallow-water shelf are developed, with the lithology of micrite and bioclastic limestone. In the Wuxi and Lichuan areas, deep-water shelf were recognized. Black siliceous rocks were mainly deposited in the deep-water shelf, and complete ammonite fossils were also found. On the whole, during the sedimentary period of the P3w1 in the study area, the seawater was the deepest in the Wuxi-Lichuan area, indicating that during the Dongwu Movement, the Wuxi-Lichuan area may have formed a tectonic depression with the reductive sedimentary environment, which is conducive to the deposition of organic-rich shale.
In the P3w2, carbonates gradually replaced clastic rocks after a short period of mixed deposition of clastic rocks and carbonate rocks, and the main sedimentary facies changed to shallow-water shelf and deep-water shelf, showing a gradual deepening trend from west to east. In the shallow-water shelf, there are two patched shoals in the areas of Wulong-Yanhe and Zhenba. While in the west of the study area, shoreland are still developed, which are mainly composed of carbonaceous mudstone, and partially composed of argillaceous siltstone and a small amount of siliceous rock (Fig. 8).
5.6 Sedimentary evolution
Based on literature research and comprehensive analysis of rock types, sedimentary facies, and geochemical analysis of the Wujiaping Formation in the study area, the sedimentary evolution model of restricted platform-shelf- restricted platform was developed in the study area from the Middle Permian Maokou Formation to the Upper Permian Changxing Formation (Fig. 9).
During the period of the P2m, a restricted platform, shallow-water shelf, and deep-water shelf were developed in the study area. Some patched shoals can also be found in restricted platforms. Because of the Dongwu Movement, carbonates of the Maokou Formation were exposed to the surface and suffered from strong weathering. Therefore, bauxite was developed in some areas, and shoreland can also be found around the paleo-uplift.
In the period of P3w1, shoreland, shallow-water shelf, and deep-water shelf were developed in the study area, which was related to the transgression.
During the period of P3w2, the area of the deep-water shelf was expanded in this time. In shallow-water shelf, a few patched shoals can be found in Wulong-Yanhe and Zhenba. Shoreland-tidal flat were still developed in the Eastern Sichuan Basin.
In the period of P3c, restricted platform, bioherm, slope, and basin were developed from west to east.
To sum up, carbonate platform are mainly developed in the sedimentary period of the Maokou Formation. Because of the Dongwu Movement, most of the study area are lifted to be exposed to the surface. The carbonate platform are partially destroyed with the deposition of clastic sediments, present of mixed sedimentation.