Longbo Xu , Xingzhi Wang , Mingyou Feng , Xiaohong Liu

a School of Geoscience and Technology, Southwest Petroleum University, Chengdu, 610500, China

b Sichuan Provincial Key Laboratory of Natural Gas Geology, Southwest Petroleum University, Chengdu, 610500, China

ABSTRACT Based on stratigraphic variations of petrology, geochemistry (major, trace elements), and pyrite framboids, we study the sedimentary environment of siliceous rocks of the Wufeng Formation in Wuxi,Northeastern Chongqing, China.Coupled Al2O3/(Al2O3+Fe2O3), SiO2/Al2O3, with Al-Fe-Mn values indicate that Wufeng Formation are deposited in a continental margin and influenced by detrital input.UEF-MoEF and V/Cr-U/Th cross-plots suggest that the siliceous shale was formed in dysoxic to anoxic conditions.Redox proxies imply that the lower, the middle to upper, and the top part of the Wufeng sediments were deposited in an anoxic, dysoxic and oxic environment, respectively.Accordingly, the average size of pyrite framboids are gradually increased from bottom to top, showing that increased oxidisability.A complete vibratory third-order cycle of sea level fluctuations during the Wufeng deposition can be identified.During Wufeng to Longmaxi transition,the sedimentary environment exhibits a short-scale oscillatory pattern and was probably transformed from an outer shelf to inner shelf.Coupled multi-proxies are considered more reliable proxies for deciphering redox conditions in fine-grained sediment.

Keywords:Siliceous rocks Late ordovician Wufeng formation Multi-proxy record Pyrite framboid Sea-level fluctuations

1.Introduction

The Ordovician-Silurian (O/S) transition witnesses several geological events such as glaciation and volcanic eruptions that lead to huge changes in environment and climate,and consequent massive extinction.The Ordovician to Silurian in the Sichuan Basin are regarded as crucial strata for shale gas exploration and development in China.Lots of researches have been done on the Silurian Longmaxi Formation shale including petrology, logging, seismology,and organic geochemistry,such as lithofacies division and paleogeography [1-3], the sedimentary environment [4,5], reservoir characteristics, organic matter enrichment, source rock development,and shale gas accumulation[6-10].Previous studies suggested that the sedimentary environment of the Longmaxi deposits were influenced by sea level fluctuations, water stagnation,and tectonic activity [11-13].However, few studies have been focused on the sedimentary environment of the Wufeng Formation,and the fluctuation of sea level is vastly understudied.

The Wufeng Formation developed in Northeast Chongqing(Sichuan Basin), mainly consists of siliceous rocks/shales, they are potential for shale gas exploration and development.The geochemical characteristics of rocks often change with the palaeoenvironment and the oxidation-reduction environment during the formation of siliceous rocks [14,15].Moreover, intensive tectonism resulted in the developing of joint and fractures in the study area;the simple variation of lithology led to hardly utility traditional sedimentary facies markers, such as lithology, color and sedimentary structure,to accurately implement sedimentary environment.The main purpose of this study was to decipher the variation of the sedimentary environment of siliceous rocks of the Wufeng Formation in the Wuxi area based on multi-proxy record.To this aim,we collected 39 samples in field of Bailu (BL) and Tianba (TB) sections in the Northeast Chongqing (Southwest China) for element analyses of major and trace elements.This research may provide basis for the sedimentary facies of unconventional oil and gas exploration.

2.Geological setting

The BL and TB sections are situated in the northeast of Wuxi County,Chongqing(Southwest China).The Wuxi area is located on the southern of the Qinling Orogen and northern margin of the Upper Yangtze Plate(Fig.1a,b).The study area outcrops the Lower Palaeozoic and Permian-Triassic moderate deformation and intensity folds, including a series of reverse faults paralleling to the anticlines and synclines [16].The complex structure of the study area resulted from the superposition of early and late tectonic activity [17].The Caledonian tectonic movement during the Middle and Late Ordovician caused a transition of from craton to postuplift basin in the Middle and Upper Yangtze Block [18].The Indosinian movement in the Late Triassic directly controlled the formation and development of the basin.Compression from the Pacific Plate during the Late Yanshanian Movement results in the formation of NNE-NE-oriented structures in the study area.During the Himalayan Movement, early formed tectonic features are to a certain extent overprinted by the extrusion of the Indian Plate,forming NE-oriented low buried-hills [19].

The Wufeng Formation, including the Guanyinqiao member(K.B),is about 10-20 m thick(Fig.1c)and consists of grey and black siliceous rock (Fig.2a, Fig.2b).Argillaceous siltstone, silty mudstone, and siltstone are interbedded in the siliceous rock.Macroscopic observation shows thin dolomite belt (Fig.2c), joints(Fig.2d) and pyrite framboids (Fig.2e), and black siliceous rock is enriched in brachiopod and graptolite fossils.The Wufeng Formation conformably contacts with the underlying grey nodular limestone of Linxiang and overlying black shale of Longmaxi Formations.The major minerals of the Wufeng Formation are quartz (Qz), clay (Mnt), and feldspar (Or).Plenty of moldic pores can be observed under microscope(Fig.2f and g).Microcrystalline quartz and pyrite framboids (Py) under scanning electron microscopy also were observed (Fig.2h).

3.Methods

Total 39 samples were connected from the BL and TB sections for thin section observation, scanning electron microscope (SEM) and geochemical measurement.Thin and polished sections were prepared from 50 samples, following impregnation with blue-dye resin and staining with Alizarin Red S.Petrographic study using transmitted light and reflected light microscopy with a Olympus BX51 Microscope fitted with a high-resolution digital camera resulted in the recognition of mineral types and texture.Polished thin sections were also examined using a FEI Quanta 650 FEG SEM.Backscattered electron (BSE) images were produced to reveal micro-textures and mineral compositions.

Fig.1.Geographical location (a), regional geological structural map of Sichuan Basin (b), regional paleogeography map(c) and stratum histogram (d) of the Upper Ordovician to Lower Silurian in Northeastern Sichuan Basin, Southwestern China (modified from Qiu, 2017).KB- Guanyinqiao member.

The whole-rock X-ray diffractometer(X’Pert MPD PRO)analysis were conducted at the Southwest Oil & Gas Field Company,Exploration and Development Research Institute Analysis and Test Centre(Chengdu).The major and trace elements were analyzed at the Analytical chemistry and testing services(ALS)using a method that involves the use of Molten aluminum borate and Plasma Mass Spectrometry Analysis (Table 1).Analytical precision for trace element concentrations is better than ±5%.The REEs were normalized by Post-Archean Australian Shale (PAAS) [20] (see Table 2).

Table 1 Analytical and calculated results of major elementals and trace elements of siliceous rocks in the Wufeng Formation, BL and TB sections (Wuxi area).

Table 2 REE contents and parameters of BL and TB sections in the Wufeng Formation (Wuxi area).

The trace element concentration in sediments was composed of both detrital and authigenic components.Therefore, the detrital content(e.g.P,Ba,Cu and Ni)must be removed for effective reveal the paleoredox conditions and paleoproductivity levels.Here we use the ratio of Al and other trace elements (X/Al) to remove the detrital content (Fig.3), because it is not easily affected by weathering process and post-depositional alteration[21].The enrichment factor(XEF)is used to describe the degree of element enrichment in sediments[22].To measure the redox conditions we calculated via equation: EFX=(X/Al)sample/(X/Al)PAAS.Where X is the concentration of element X,which is normalized by the post-Archean average shale (PAAS).Values of XEF＞1.0 or＜1.0 indicate element enrichment or depletion, respectively[23-26].

Trace elements are changed during diagenetic evolution.Coupled Mn/Sr ratio with CIA values can effectively evaluate the influence of diagenesis on trace elements[27,28].The Mn/Sr ratio is ranging from 0.59 to 8.13(with average value of 2.50),and the CIA value is range from 20.72 to 76.31 (with average value of 67.44),indicate low-moderate weathering and temperate climate,and the trace element content in this paper can reflect the sedimentary redox environment.

Meanwhile, anomalies of U, Ce, and Eu are calculated by the following equations: δU = U/[1/2(U + Th/3)], δCe = Lg [3CeN/(2LaN+NdN)],and δEu=[2EuN/(SmN+GdN)][29,30](N represents the normalization value by Wright,1987; Tribovillard, 2006).And the authigenic U was calculated via Uauth= Utotal-Th/3 [31].

4.Results

X-ray diffraction analyses of whole rocks show that the mineral composition of the study sections is mainly comprised of quartz(range of 40.5 wt% to 79.6 wt% and with average of 62.5 wt%),followed by clay minerals (with average value of 22.6 wt%), and the content of anorthose is range from 5.9 wt%to 13.7 wt%(with average value of 8.7 wt%).The content of potassium feldspar is range from 0.5 wt%to 4.4 wt%with an average of 3.3 wt%.Besides,the content of pyrite is range from 1.9 wt%to 5.9 wt%,with average value of 4.2 wt%.

Major elements analysis shows that SiO2and Al2O3are thedominant constituents.The Al2O3-SiO2-CaO ternary diagram shows that SiO2ranges from 61.4 wt%to 85.3 wt%and with average of 74.98 wt%(Fig.4a).The Al2O3content served as a proxy for clay minerals,which ranges from 4.62 wt%to 16.11 wt%and with average value of 10.24%.The value of SiO2and Al2O3ratio ranges is from 4.35 to 18.21 and with average value of 9.42.Other major elements are less concentrated.CaO in the study samples is 2.08 wt%on average,ranging from 0.05 to 19.9 wt%.Fe2O3,K2O,MgO and Na2O contents have mean values of 3.41 wt%, 2.49 wt%,1.73 wt%, and 0.97 wt%,respectively.Minor elements analysis shows that the Ba content ranges from 1335 to 104μg/g and with average value of 3160 μg/g.The Mocontentrangesfrom8.18 to 82μg/g and with averagevalue of 33.80 μg/g.

The ∑REE of siliceous rocks varied greatly in the Wufeng Formation (that of BL samples is 71.50-298.08 μg/g, average value is 163.15 μg/g, and that of TB samples is 88.61-337.81 μg/g, average value is 190.47 μg/g).In the BL samples, the LREE/HREE ratio is 9.86-13.76 (mean 6.66), indicating that light REEs are relatively enriched but heavy REEs are depleted.∑REE in the lower part(BL-1 to BL-7,mean 154.28 μg/g)is lower than that in the upper(BL-8 to BL-18,mean 169.30 μg/g).Similarly,the ∑REE of the TB in the lower part(TB-1 to TB-7,mean 166.40 μg/g)is lower than that in the top(TB-8 to TB-9C, mean 226.24 μg/g).

5.Discussions

5.1.Genesis of siliceous rocks

The highest accumulation rates of Fe and Mn in the open Pacific occur along the East Pacific Rise [32].Al and Ti had the highest accumulation rates near the continents, and these elements appeared to be almost completely terrigenous. Al/(Al+Fe+Mn)＞0.5 indicates that the provenance of siliceous rock is terrigenous, however, it often represents hydrothermal processes, when the ratio less than 0.35 [33].An Al-Fe-Mn ternary diagram always is used to determine between hydrothermal and non-hydrothermal siliceous rocks[34].In addition,immobile trace elements, e.g.La, Th, Zr, Hg and Mo, are useful in discriminating tectonic settings[35].The Al/(Al+Fe+Mn)ratio ranges from 0.52 to 0.90 (average value is 0.79) in the Wuxi area, indicates the siliceous rocks were formed as non-hydrothermal or biogenic sediments(Fig.4b).Furthermore,major elements ratios,Al2O3and TiO2which is associated with terrigenous province and Fe2O3which is related with metalliferous input can distinguish continental margins from ridge and pelagic.An Al2O3/(Al2O3+Fe2O3) ratio ranging from 0.58 to 0.90, 0.4 to 0.7 or ＜0.4 indicate that siliceous rocks deposited in a continental margin, ocean basin, and mid-oceanridge environment, respectively [36].Fe2O3/TiO2and Al2O3/(Al2O3+Fe2O3) cross-plots in the BL and TB section shows the siliceous rocks are deposited in the continental margin to pelagic condition(Fig.4c).Consequently,various parameters indicated that siliceous rocks comprised sediments or biogenic silicon in a continental margin to pelagic environment.The samples were not subjected to hydrothermal processes and could be used for palaeoenvironmental analysis.

Fig.3.Cross-plots of various elements and Al in the Wufeng Formation in the BL section.

Fig.4.Siliceous rock genesis analysis charts of the Wufeng Formation in the BL and TB sections.(a)Al-Si-Ca ternary diagram and(b)Al-Fe-Mn ternary diagram(modified from Adachi,1986).(c) Determination of tectonic setting of siliceous rocks by Fe2O3/TiO2 and Al2O3/(Al2O3+Fe2O3) cross-plots (modified from Bostr¨om,1973; Murray,1994).Red circles represent BL section samples, and green triangles represent TB section samples.

5.2.Marine paleoredox conditions

The enrichment degree of trace elements in sedimentary rocks varies with redox environments.The values of XEF＞1.0 or＜1.0 indicate element enrichment or depletion, respectively [37].Furthermore,Uranium(U)and molybdenum(Mo)exhibit different geochemical behaviors during sedimentary processes.Authigenic U uptake preferentially begins at the Fe(II)-Fe(III) redox boundary under suboxic conditions, whereas authigenic Mo uptake begins later and requires the presence of H2S.Furthermore,the transfer of Mo into sediments can be accelerated by particulate shuttles,but U is unaffected by this process.Thus,MoEFand UEFare good proxies to analyse benthic redox conditions[38,39],and anoxic sediments are typically more molybdenum-enriched and uranium-enriched than suboxic sediments.From button to top of the Wufeng Formation,both Mo and U exhibit a progressive decrease in enrichment(Table 1,Fig.5).Most of the MoEF/UEFratios plot in the“unrestricted marine trend”, which is nearly 0.3 to 3 times the seawater (SW)molar ratio (～7.5-7.9).This pattern indicates that the Wufeng Formation in the Wuxi area was deposited in an open continental margin upwelling system.Specifically, the MoEFvalues is ranges from 11 to 200(with average value of 70.2),and UEFvalues is ranges from 11 to 55(with average value of 32.7)in the BL section,which indicate strongly anoxic to euxinic condition.Meanwhile,decreased MoEF(range from 8 to 216, mean 61.4) and UEFvalues(range from 8 to 125,mean 28.9)in the TB section indicate dysoxic to anoxic redox conditions and relative shallower water column than BL section during deposition.

Trace-element indices of U/Th, Ni/Co, V/Cr, and V/(V + Ni) ratio have been also used to study palaeo-redox conditions.U/Th ＞1.25,V/Cr ＞4.25,and Ni/Co ＞7 ratios indicate an anoxic condition;0.75 ＜U/Th ＜1.25,2 ＜V/Cr ＜4.25,and 5 ＜Ni/Co ＜7 infer dysoxic conditions;and U/Th ＜0.75,V/Cr ＜2,and Ni/Co ＜5 have beenwidely interpreted as indicating oxic conditions [40-42].There is a fairly good agreement between the interpretations of redox conditions using U/Th,Ni/Co, V/Cr, δCe, and δU (Table 1, Fig.6).The U/Th ratios of 1.17-2.24(mean 1.62),V/Cr ratios of 3.77-7.85(mean 6.18),and Ni/Co ratios of 17.20-151.50(mean43.83)inthelowerpartof theWufeng Formation(BL-1 to BL-7),indicating anoxic conditions.The upper-middle part of the formation(BL-8 to BL-15)had U/Th ratios of 0.75-1.67(mean 1.01) and V/Cr ratios of 2.00-4.30 (mean 3.18), indicating dysoxic conditions.While the U/Th,V/Cr ratio,and other proxies show minimum values for the Guanyinqiao member(BL-19 and TB-9),indicate that the palaeoenvironment rapidly changed from anoxic to oxic condition.To summarize,the redox data(U/Th,Mo-EF,U-EF,V/Cr,Ni/Co) indicate that sediments of the Wufeng Formation existed in overall anoxic to dysoxic conditions with some short-term redox cyclicity.Thismay have been influenced byglobal and regional events(e.g.glacial period and volcanism)and short-term sea level change.

Whereas,some redox proxies were partly inconsistent with sea level fluctuation.Notably, the inconsistency between the two curves precisely reflects the sample with high trace element or Al content, which is greatly influenced by terrigenous detritals.This further demonstrates that individual samples can be reasonably eliminated.The anomaly of high V/Cr value(samples BL-4 and BL-5), except for terrigenous detritals, may also be related to the conditions of V and Cr.The anoxic pore water of the sediments is enriched in V,but the sediments were not necessarily deposited in anoxic bottom water [43].In a reducing environment, Cr4+ion is stable and abundant in the fracture water of base rock,but less so in pore water [44].It is likely, therefore, that the high V/Cr ratio was probably due to the pore water concentration in the strata.The changes in the redox environment of the middle and upper parts of the Wufeng Formation indicated by Ni/Co values differ from the others (Fig.9), which may be related to the development of a silicate matrix and pyrite.Specifically, the contribution of the Co silicate matrix is far less than that of Ni,which can be combined with pyrite to accumulate in the deposit.These factors may increase the Ni/Co ratio[45].Generally speaking,U/Th and V/Cr ratios are highly consistent with changes in the sedimentary environment of the siliceous rocks.

Fig.5.Enrichment factor (EF) of trace elements in the Wufeng Formation in the BL section.

Fig.6.UEF versus MoEF scatter plot(log-log)for studied samples of the Wufeng formations(a)(modified from Algeo,2006)and V/Cr-U/Th cross-plots(b)in the BL and TB sections of the Wufeng Formation.Red circles indicate BL samples and green triangles the TB samples.

Pyrite framboid is another good indicator as redox parameters[46].In a euxinic setting and after achieving a critical size(6 μm±),pyrite framboids rapidly sink to the seafloor, where they cease to grow.As a result,framboids formed in such settings accumulated as small-sized (＜6 μm) populations with a narrow size distribution,characterised by a small standard deviation.In a dysoxic setting with a weakly oxygenated seafloor, pyrite framboids formed in surficial sediment attained larger sizes because of the availability of local reactants [47].We determined the particle size of 721 pyrite framboids from BL and TB sections in this area.In the BL section,the particle size of pyrite framboids is ranging from 1.59 to 22.31 μm(mainly as 3.5-6.7 μm),with an average of 5.61 μm.In contrast,the particle size of pyrite framboids in the TB section is relative larger,although the spatial trends of particle size in the two sections are generally consistent: The particle size of the pyrite framboids is ranging from 0.82 to 15.61 μm(occurring mainly as 2.99-4.49 μm),with average of 3.95 μm in the TB section.Hence,the difference of the particle size of pyrite framboids between BL and TB section implies that TB section may have been deposited in relative shallow waters condition than that in the BL section.Vertically,the particle size of pyrite framboids also exhibit a long-range oscillatory pattern in the lower part of the Wufeng Formation(BL-1 to BL-7,BL-7 to 16 and TB-1 to TB-7,TB-7 to TB-9A).In the Guanyinqiao Formation(BL-17 to BL-19, TB-9A to TB-9C), the particle size of the pyrite framboids rapidly decreased to the minimum (Fig.7).Overall, the average size of pyrite framboids in the study area is smaller in the lower part and larger in the upper part, with abrupt changes occurring locally at the O/S boundary.This change reflects the trend in paleo-water depth from deep to shallow, and the changes in pyrite size are highly consistent with other redox proxies.

5.3.Sea-level fluctuations and sedimentary model

Changes in palaeoenvironment and palaeo-water depth can also result in various in trace element contents,because the reducibility of water and enrichment of fine sediment are enhanced with increased palaeobathymetry.The finer the sediment particle size is,the stronger it adsorbs trace elements.We used geochemical characteristics and lithological associations as the main parameters to determine the sea-level fluctuations (Fig.9) and sedimentary evolution (Fig.10) of the Wufeng Formation.Combined with the analysis of global sea level fluctuations [48,49], we focused on the BL and TB sections for analysing sea-level fluctuations and divided the Wufeng Formation into a complete third-order sea level change cycles (SQ1).U/Th, V/Cr, and other redox proxies had strong responses to sea-level fluctuations.

Fig.7.Characteristics of framboidal pyrite in the Wufeng Fm.of the Wuxi area.(a) Scanning electron microscope(SEM) image of framboidal pyrites in a typical sample BL-2 from the BL section.(b)SEM image of framboidal pyrites in sample TB-7 from the TB section.(c)Pyrite-framboidal size distribution and‘box-and whisker’plots for BL section.Lithological legend and abbreviations as in Fig.1.

Fig.8.Siliceous rocks-normalized rare earth element distributions for the BL (a) and TB (b) sections of the Wufeng Formation.

Based on shallow water carbonate deposits in the Linxiang Formation, the sea surface level rose rapidly and a siliceous shelf developed.The ratio of redox proxies from the bottom to middle Wufeng Formation (BL-1 to BL-7 and TB-1 to TB-4) increased gradually(Fig.9),yield grey-black thin siliceous rocks and gradually changed upward to dark-grey thin argillaceous siltstone and silty mudstone.Redox proxies show an anoxic environment, which differed from the upper strata and had an oscillating decreasing trend.The particle size of pyrite framboid was smaller (with average size of 3.46 μm),which was generally transformed from the siliceous shelf to the silty mud shelf (Fig.10a).

In the upper Wufeng Formation (BL-7 to BL-16, TB-1 to TB-8),the relative sea level generally decreased, and the lithology mainly consisted of grey-black thin siliceous rock and silty shale with deep-grey thin argillaceous siltstone and calcareous shale.The redox proxies indicated a dysoxic environment, and the pyrite framboids size increased (average with 4.13 μm), which generally indicated the mutual environment between the siliceous and silty shale shelves(Fig.10b).The sea level fluctuations consisted of longrange oscillatory pattern of transgression and regression cycles regression.

Fig.9.Stratigraphic distribution of lithofacies and geochemical data of redox conditions of the Wufeng Formation from sections (Global sea-level variation curve and sea surface temperature refers to Kidder et al., 2016; δ13Corg data from Zhou et al., 2017).

Fig.10.Diagrammatic sketch illustrating the probable environmental evolution of the Wufeng Formation.

From the top of the Wufeng Formation to the Guanyinqiao member(BL-16 to BL-19,TB-9A to TB-9C),the relative sea level rose and then decreased rapidly.The lithology was composed of darkgrey silty mudstone and grey micrite limestone or dolomite;correspondingly, the REE patterns exhibit from a gently rightdipping V-type to flat type (Fig.8) (Guanyinqiao member), which suggests a significant change in the sedimentary environment.Redox proxies reflected the rapid change from an anoxic to a dysoxic/oxic environment.Moreover, the pyrite framboid particle size increased significantly and rapidly decreased to minimum,reflected rapid transgression - regression cycle, which was influenced by abnormal sea level changes and corresponded to the Late Ordovician bipolar cooling event (Fig.10c and d).In addition, evidence from δ13Corgsuggests that the development of euxinia is initiated at the maximum extent of the δ13Corgexcursion observed at BL and TB section[50],which inferred as a sedimentary response of sea level oscillating decreasing.This is consistent with increased availability of nutrients to drive productivity during the maximum extent of glaciation in the end of Ordovician.

In the early stage of the Longmaxi Formation,the overall redox proxies indicated dysoxic condition, and the pyrite framboid particle size was larger (with average of 4.93 μm).The sedimentary environment was transformed from deep-water shelf to argillaceous silt and mud shelves.The early transgression-regression cycle (SQ2) in this period coincided with global transgression events after the end of the Hirnantian.Overall, it occurred in the anoxic or dysoxic environment of the Wufeng Formation in Wuxi area.The oxidation of water was enhanced, and the environment was transformed into a dysoxic or oxic environment at the bottom of the Longmaxi Formation.The sedimentary environment gradually transformed from an outer shelf to an inner shelf.

This model (Fig.10) was based on a combination of several palaeoenvironmental factors, the most significant of which is relative water depth and its relationship to the vertical distribution of oxygen concentration in the water column.For the model,changes in water depth reflected relative sea level fluctuations,and the source of organic matter was assumed to be from primary productivity at the sea surface.Studies on the palaeoenvironment of the Ordovician in the Sichuan Basin and Northeast Chongqing inferred that the Wufeng Formation was mainly formed from an anoxic environment, and the sedimentary facies formed a deepwater shelf.U/Th and V/Cr values indicated that the Wufeng Formation was formed under anoxic-dysoxic condition.Oxidation of the Wufengian increased over time, and the sedimentary environment gradually changed from an outer shelf to an inner shelf.During the deposition,a global glacial period led to a drop sea level.The redox proxies consequently decreased similar to changes in value of U/Th ratio.The sea level at the top of the Wufeng Fm.decreased rapidly to lowest, and the redox proxies appeared abruptly.The sea level rose slowly during the deposition of the Longmaxi Formation.From the top of the Wufeng to the Guanyinqiao Fm., values of U/Th ratio and δU decreased rapidly.Likewise, the values of Ni/Co and V/Cr ratio also decreased, reflecting the rapid fall of sea level.Multi-proxies both indicated the rapid regression caused by the global glacial event.

6.Conclusions

(1) Siliceous rocks in the Wufeng Formation in the Wuxi area(Northeast Chongqing) were deposited in a continental margin to pelagic basin affected by detrital input.The siliceous rocks are sedimentary/biogenic in origin, and not affected by hydrothermal activity.

(2) Redox proxies and pyrite framboids suggest that the siliceous shale was mainly formed in dysoxic to anoxic conditions under a third-order cycle of sea level fluctuations during the Wufeng deposition.The sedimentary environment with increased oxidisability exhibits an oscillatory pattern and was probably transformed from an outer shelf to inner shelf.

Declaration of competing interests

The authors declare that they have no conflict of interests.

Acknowledgements

This research was funded by the National Major projects of Science and Technology(Grant No.2017ZX05001001-002)and the Project of Sichuan Key Laboratory of Natural Gas Geology of China(Grant No.2015trqdz01) and Western plan project of State Scholarship Fund.Sincere thanks go to our colleagues involved in unconventional oil and gas researching in the Sichuan Basin.We are particularly grateful to Professor Guang Hu and the anonymous reviewer for their time and valuable comments.