Stratigraphic Records of the Dynamic Uplift of the Emeishan Large Igneous Province

International Geology Review

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Stratigraphic records of the dynamic uplift of the Emeishan large igneous province

Peng Wu, Shaofeng Liu, Binghui He & Guoxing Dou

To cite this article: Peng Wu, Shaofeng Liu, Binghui He & Guoxing Dou (2016) Stratigraphic records of the dynamic uplift of the Emeishan large igneous province, International Geology Review, 58:1, 112-130, DOI: 10.1080/00206814.2015.1065515

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Download by: [Universiti Putra Malaysia] Date: 28 October 2015, At: 00:39 INTERNATIONAL GEOLOGY REVIEW, 2016 VOL. 58, NO. 1, 112–130 http://dx.doi.org/10.1080/00206814.2015.1065515

Stratigraphic records of the dynamic uplift of the Emeishan large igneous province Peng Wua,b, Shaofeng Liua,b, Binghui Hea,b and Guoxing Douc aState Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, China; bSchool of Earth Science and Resources, China University of Geosciences (Beijing), Beijing, China; cResearch Institute of Coal Geophysical Exploration of China National Administration of Coal Geology, Zhuozhou, China

ABSTRACT ARTICLE HISTORY Fluid dynamical and numerical modelling predicts a large-scale regional domal uplift prior to Received 25 April 2015 basalt eruptions in large igneous provinces, which can be readily measured when a plume head Accepted 21 June 2015 rises below a shallow marine sedimentary basin. Research on the sedimentology, biostratigraphy, KEYWORDS and isotopic chronology of the Emeishan large igneous province demonstrates that the sedimen- Emeishan large igneous tary environment in the Maokou stage is not uniform carbonate platform facies, but rather province; Maokou limestone; sedimentary facies with a north–south linear alignment and west–east different distribution syn-depositional fault; controlled by the syn-depositional normal faulting of the Changhai and Xiaojiang faults, which dynamic topography are the result of underwater dynamic uplift induced by deep mantle activity. The dynamic uplift started in the Maokou stage. Thus, thinning of the Maokou limestone was the product of the difference in the initial depositional thickness caused by the underwater uplift and post-depositional surface uplift and erosion, but post-depositional uplift was much less than kilometre scale. Sedimentary facies differentiation and tectonic–sedimentary evolution in the Maokou stage provide a constraint for the time of the initial eruption and eruption environment before and during the Emeishan basalt eruption. Small-scale magmatic activity might have already begun in the middle of the Maokou stage, whereas submarine and terrestrial sedimentary environments coexisted before and during Emeishan basalt eruption.

1. Introduction doming by an ascending mantle plume. Their evidence also included palaeo-karsts on the top of the Maokou The Emeishan large igneous province (Emeishan LIP) in Formation (He et al. 2004, 2010) and alluvial fan con- southwestern China has attracted a large number of glomerates along the eastern and northeastern sides of studies, especially over the last 20 years (Chung et al. the Emeishan LIP (He et al. 2006a, 2006b). Their model 1998; Courtillot et al. 1999;Heet al. 2003a;Xuet al. indicated the totally subaerial environment of the basalt 2004, 2014; Ali et al. 2010; Zhong et al. 2011), because eruptions. However, many scholars have raised doubts this province may be closely associated with middle regarding He et al.’s(2003a) model. The most represen- Permian mass extinction (Ali et al. 2002; Wignall et al.

Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 tative research was performed by Peate and Bryan 2009a, 2009b; Bond et al. 2010; Cao et al. 2013). (2008, 2009), who reinvestigated the Daqiao section at Nevertheless, there remains considerable controversy the edge of the strongly eroded ‘inner zone’ and recog- regarding the dynamic mechanisms that formed the nized voluminous mafic hydromagmatic deposits that Emeishan LIP and the environment before, during, and were previously interpreted as alluvial fan sediments after Emeishan volcanism (Chung and Jahn 1995; shed from a pre-volcanic domal high. Their results Thompson et al. 2001; Song et al. 2004; Zhang and clearly demonstrated that both subaerial and submarine Dong 2007; Xiao et al. 2008; Leng and Zhong 2010), environments coexisted prior to and during the initial mainly in regard to the following viewpoints. He et al. Emeishan basalt eruptions. Sun et al.(2010) also con- (2003a, 2003b, 2005, 2009) proposed a kilometre-scale firmed that many regions experienced rapid subsidence doming uplift and differential erosion model based on and were deep-water facies prior to the Emeishan basalt the biostratigraphic correlation of the Maokou eruptions, based on conodont age of the uppermost Formation immediately underneath the Emeishan beds of the Maokou Formation. Wang et al.(2014), basalts, which resulted from thermal or dynamic based on tectonic restoration and reinterpretation of

lithofacies sequences to the Binchuan section and et al. 2003a). There are many main fault belts, which Daqiao section, suggested a linear alignment of the include, from west to east, the NS-striking Jinghe– marine, submarine, and subaerial eruption environment Changhai fault (hereafter referred to as the Changhai from west to east before and during the Emeishan fault), the Mopanshan–Lvzhijiang fault (hereafter basalt eruptions. referred to as the Lvzhijiang fault), the Anninghe– Previous research focused primarily on the crustal Yimen fault (hereafter referred to as the Anninghe processes and environment after the deposition of the fault), the Ganluo–Xiaojiang fault (hereafter referred Maokou Formation and before or during the basalt erup- to as the Xiaojiang fault), and the NW-striking tions in the Emeishan LIP, but little research has been Xichang–Qiaojia fault (hereafter referred to as the conducted on the syn-depositional tectonic activity and Qiaojia fault) (Figure 1). The Chenghai, Lvzhijiang, sedimentary processes during the Maokou stage. He Anninghe, and Xiaojiang faults all have lengths of et al.(2003b, 2006a) and Li et al.(2011) briefly described several hundred kilometres, cut deep into the litho- the sedimentary environment of the Maokou Formation sphere, and control the structural evolution in the in the Emeishan LIP as a shallow marine carbonate plat- research area (Zhang et al. 1988). For convenience, form with a stable crust, uniform tectonic setting, and three sub-provinces were distinguished to describe horizontal depositional surface without intense fault the Emeishan LIP, namely the Western, Central, and activity. Peate and Bryan (2008) and Wang et al.(2014) Eastern regions based on the Chenghai and Xiaojiang referred to the possibility of differential uplift/normal faults. The Central Region overlying the ‘Panxi palaeor- faulting of the Permian Maokou limestone. Thus, were ift zone’ (Tan 1987;Heet al. 2003a) refers to the area all the research areas shallow marine carbonate platform between the Changhai and Xiaojiang faults, the facies? Were there differences in the sedimentary thick- Western Region refers to the area on the western ness, terrain, and differentiation in sedimentary facies? side of the Chenghai fault, and the Eastern Region Did pre-existing fault activity occur and control the sedi- refers to the area on the eastern side of the mentation processes during and after the deposition of Xiaojiang fault. the Maokou Formation? If so, was this fault activity The thickness and components of the Emeishan related to deep mantle activity? basalts change regularly from west to east (Xu et al. The classic mantle plume model developed by 2004). In terms of thickness, the basalt series gradually Campbell and Griffiths (1990)consideredregional becomes thinner, from over 5000 m in the west to a few domal uplift prior to basalt eruptions, which are read- hundred metres in the east. The thickest belts and ily measured, especially when a plume head rises mutation belts are consistent with the north–south below a shallow marine sedimentary basin (Griffiths Changhai and Xiaojiang faults (Zhang et al. 1988). In and Campbell 1991; Campbell 2007) with continuous regard to components, thick sequences of low-Ti volca- sediments of equal thickness. Therefore, research on nic rocks are mainly distributed in the Western Region, the tectonic activity and sedimentary environment in with some sections having subordinate high-Ti lavas at the Maokou stage can verify the potential of the the top as represented by the Binchuan and Pingchuan doming uplift model for the Emeishan LIP and the sections; picrites and low-Ti, high-Ti, and alkaline lavas accuracy of the estimated level of uplift and further coexist in the Central Region; and thin sequences of

Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 reveal the dynamic topography during and after the high-Ti volcanic rocks mainly occur in the Eastern Maokou stage (i.e. surface topography induced by Region (Figure 1). Based on the high-Ti (HT1 and HT2– deep mantle activity under the lithospheric bottom HT3) lavas that occur in the uppermost parts of the boundary (Liu and Nummedal 2004;Liu2009)), which Binchuan and Ertan sections, respectively, and a thick can help perfect the tectonic–sedimentary model of sequence of evolved alkaline lava that occurs between the Permian and fully recognize the processes and the high-Ti lavas in the Miyi section, it is possible that mechanisms that controlled the generation of the these high-Ti and alkaline lavas are relatively young Emeishan LIP. compared with the low-Ti lavas (Xu et al. 2004). Based on lithological, biogenic assembly, and contact relationships, the Permian strata underlying the 2. Geological background Emeishan basalts in the Emeishan LIP can be classified The Emeishan LIP is located in the western and south- into the Liangshan, Qixia, and Maokou formations. western parts of the Yangtze Block (Figure 1). The The Liangshan Formation is mainly composed of var- Longmenshan thrust fault and the Ailaoshan–Red ious sandstones with only local appearance of thin River slip fault are generally considered its northwes- coal sheets. Most extensive transgression event in the tern and southwestern boundaries, respectively (He late Palaeozoic occurred during the Qixia stage and 114 P. WU ET AL. Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015

Figure 1. Schematic geological map of the Emeishan large igneous province (after Zhang et al. 1988;Heet al. 2003a). Also shown is the distribution of the Emeishan basalts in southwestern China (Xu et al. 2004). The dashed lines separate the inner, intermediate, and outer zones, which are defined in terms of the erosion extent of the Maokou Formation (He et al. 2003a). The solid lines labelled with numbers indicate faults: ① Longmenshan thrust fault; ② Ailaoshan–Red River slip fault; ③ Chenghai fault; ④ Lvzhijiang fault; ⑤ Anninghe fault; ⑥ Xiaojiang fault; ⑦ Qiaojia fault. HT, high-Ti basalt; LT, low-Ti basalt; ALK, alkaline series.

reached the maximum flooding period in the early regression commenced in the middle and late Maokou stage, forming widely developed stable car- Maokou stage, and changes in the lithofacies were bonate platform facies in South China. Large-scale enhanced. In addition to the carbonate platform INTERNATIONAL GEOLOGY REVIEW 115

stage. This conclusion was based on the ‘mixed phenomenon’ of Carboniferous and Permian fossils in the conglomeratic limestone or its cements in the Shuhe Formation, such as fusulinids, including Fusulina and Triticites from the Carboniferous and Misellina and Neoschwagerina from the Permian. We observed the same phenomenon and thus accepted the stratigraphic classification of Liang et al.(1994). He et al. (2006a, 2006b) interpreted the genesis of the Pingchuan conglomerates as fillings of submarine canyons generated during domal uplift, which reflected an abrupt change in the type of sedimentation after the carbonate platform facies of the Maokou stage. Sun et al.(2010)consideredthatthe breccias were composed of varying proportions of Figure 2. Maokouan and Qixianian subseries timescale and fusulinid zones (modified from Jin et al. 1999; Sun et al. dacite, basalt, limestone, and occasionally shale clasts 2010). CS-chromostratigraphic scheme. and that the breccia beds were separated by thinner beds (0.3–12.5 m thick) of laminated, siliceous micrites.Therefore,Sunet al.(2010) interpreted the Pingchuan conglomerates as a relatively deep-water facies, platform-marginal slope sediments (calcirudite setting generated during the lifetime of the Emeishan and slide breccia) developed in Muli and elsewhere volcanism, as old as the J. xuanhanensis Zone (Chen and Chen 1987). The Maokou stage in South (approximately compared with the N. multivoluta China is generally considered as a period of great Zone). However, through an analysis of the stratigra- expansion for fusulinids, which were extremely abun- phy and sedimentology, we consider that the lithol- dant and evolved rapidly (ECS 2000;Zhuet al. 2002; ogy and origin of the Pingchuan Formation must be He et al. 2003a). Considering the overall appearance re-evaluated. and main genera of fusulinids in the Emeishan LIP Field investigation, detrital component statistics, and referring to Chinese standard fusulinid zones ziron U–Pb dating, and sedimentary facies analysis of (Jin et al. 1999) and research of adjoining areas this section show that the Shuhe Formation consists (Yang 1985;BGMRGP1987;BGMRYP1990;BGMRSPF of 41 m-thick layers of clastic rocks, which in their 1991;Yanget al. 1999;ZengandGao2005), the entirety form a normally graded bed sequence. The Maokou Formation is further divided into three bios- lower part is carbonaceous mudstone and marlstone, tratigraphic units which, from bottom to top, are the and the middle part is predominantly breccias that Neoschwagerina simplex Zone, the Afghanella schencki are occasionally separated by thinner beds of fuchsia – Zone, and the Yabiena gubleri Neomisellina multivo- shale (Figure 4a). The breccias vary in size from milli- luta Zone (Figure 2;Heet al. 2003b;Wuet al. 2014). metre scale to 5 cm, and the structure of the breccia

Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 beds includes melange accumulation, massive bedding, and no internal sorting or grading; the grain 3. Sedimentary facies and provenance analysis size of the clasts gradually decreases upwards, and of Maokou sections in the Western Region themainlithologyofthetoppartissandstoneand silty shale. The overlying Maokou Formation is domi- 3.1. Pingchuan section, Yanyuan County nated by a 173 m-thick layer of banded siliceous This section is in eastern Yanyuan County, Sichuan limestone and marlstone (Figure 4b) with thin-bedded Province, immediately west of the Chenghai fault, siliceous rock, massive siliceous nodules, and a few and sits to the north of the ‘inner zone’ of He fossils. This layer is sharply overlain by the Pingchuan et al.’s model (point A in Figure 1). The measured Formation, which is up to 224.5 m thick and contains section, which rests disconformably on Carboniferous two or three inverse graded cycles. The cycles consist limestone and is unconformably overlain by basalts, of (in ascending order) mudstone, shale, and marl- consists of the Shuhe, Maokou, and Pingchuan forma- stone, passing upward into conglomerate or con- tions (Figure 3). Liang et al.(1994) suggested that the glomeratic greywacke. The conglomerate beds are age of the Shuhe Formation should be early–middle 20–30 cm in thickness and display massive bedding, Permian, correlated with the early–middle Maokou and the contacts with upper and lower layers are flat 116 P. WU ET AL. Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015

Figure 3. The measured columnar section at Pingchuan in Yanyuan, Sichuan Province. Fm., Formation; P2β, Emeishan basalts; P2p, Pingchuan Formation; P2m, Maokou Formation; P1s, Shuhe Formation; C, Carboniferous. The black squares indicate sample level, with sample numbers shown beside the squares. INTERNATIONAL GEOLOGY REVIEW 117

Figure 4. Field photographs of the Pingchuan section: (a) sedimentary layers of conglomerate and fuchsia shale in the Shuhe Formation; (b) 173 m-thick banded siliceous limestone and marlstone in the Maokou Formation; (c) conglomerate beds in the Pingchuan section; (d) enlarged view of the red left-hand box in (c), showing conglomerate beds separated by thinner beds of fine- grained strata; (e) enlarged view of the red right-hand box in (c), showing upwardly thickening conglomerate beds; (f) brecciated and poorly sorted conglomerate clasts; (g) tuffaceous sandstone beds at the top of the Pingchuan Formation; (h) breccia beds immediately overlying the tuffaceous sandstone beds of (g); (i) enlarged view of autochthonous tuffaceous sandstone breccias derived from broken in situ tuffaceous sandstone beds; (j) enlarged view of allochthonous limestone conglomerates. Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015

(Figure 4c–4e). As the conglomerate beds thicken and altered rocks (12.7%), very little limestone (0.9%), upwards, the size of the conglomerates increases to feldspar (0.2%), and silicate (0.4%), and no volcanic 2cm,occasionallyto5cm.Theconglomerateclasts components (Figure 5a). Samples YN13062 and are brecciated and poorly sorted (Figure 4f)and YN13063, which are from thinner beds, have different mainly consist of carbonates that resemble those in compositions: YN13062 is grey–black mudstone with the Maokou and Qixia formations, as identified by the altered rocks (86.5%), quartz (6.0%), and feldspar fusulinid species. As noted by Sun et al.(2010), the (7.1%), very little silicate, and no volcanic or limestone conglomerate beds are separated by thinner beds of components (Figure 5b), whereas YN13063 is shallow fine-grained strata (Figure 4d,4e) but their lithology is greywacke with quartz (75.6%) and altered rocks neither siliceous micrites nor spiculitic cherts. (24.0%), very little feldspar and silicate, and no volca- Microscopic observation and detrital component sta- nic or limestone components (Figure 5c). The con- tistics of the conglomerate beds and thinner beds glomerate sample YN13065 at the top centre of the show that the matrix components of the conglomer- first cycle has larger-diameter components, and the ate sample YN13064 are composed of quartz (85.8%) maximum size that the conglomerates can grow is 118 P. WU ET AL.

Figure 5. Microscopic observation and detrital component statistics of conglomerate beds in the Pingchuan Formation. Q, quartz; F, feldspar; S, silicate; L, limestone; V, volcanic; AL, altered rocks.

12 cm. All the conglomerates are limestone, and the observed fusulinids were determined to be Neoschwagerina sp. with intensive abrasion marks (Figure 5d). The matrix components of sample YN13065 are basically the same as those of sample YN13064 but with more feldspar (Figure 5d). The conglomerate sample YN13067 contains more volcanic components (4.5%) but no quartz (Figure 5e). The top of the Pingchuan Formation is composed of tuffaceous sandstone beds (Figure 4g) and breccias beds (Figure 4h), which contain autochthonous tuffaceous sandstone breccias (Figure 4i) and allochthonous lime- Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 stone conglomerates (Figure 4j). Finally, the Pingchuan Figure 6. Representative cathodoluminescence images of the Formation is overlain unconformably by basalts. analysed zircon from conglomerate sample YN13064 in the One hundred and fourteen zircon grains from con- Pingchuan section. glomerate sample YN13064 were selected for LA-ICP-MS zircon U–Pb dating at the Institute of Geology and three zircon age populations (ca. 269, 667–913, and Geophysics, Chinese Academy of Sciences, Beijing. 2091 Ma (see Appendix A)). The ca. 667–913 Ma zircon Magmatic zircons are often thought to have high Th/U population that constitutes >97% of all the zircon grains ratios (>0.1) and develop internal structures with oscil- plots on the Concordia curve (n = 110), yielding a mean latory zoning (Rubatto 2002). The Th/U of almost all the 238U/206Pb age of 780.5 ± 7.5 Ma with an mean square zircon grains from this sample have Th/U ratios of >0.4 of weighted deviates value of 9.9 (Figure 7). The avail- (see Appendix A at http://dx.doi.org/10.1080/00206814. able sources for these zircons contain a continental 2015.1065515) and structures with oscillatory zoning volcanic-coarse clastic rock assemblage of the late (Figure 6); no zircon grains had Th/U ratios of <0.1, Neoproterozoic (850–700 Ma; n = 100) and a few granite which indicates an igneous origin. Among these 114 intrusive bodies of the Chengjiang period (700–650 Ma; zircon grains, only grain #113 yields a disconcordant n = 4), and a fold basement component of the age (~653 Ma). The remaining 113 zircon grains exhibit Mesoproterozoic (1700–850 Ma; n = 6). Only grain INTERNATIONAL GEOLOGY REVIEW 119

Formation are Neoschwagerina, demonstrating that the northern section of the Chenghai fault developed trench facies controlled by syn-depositional Changhai fault activity, rather than stable carbonate platform facies (He et al. 2006a), at least from the early–middle Maokou stage. The thick overlying banded siliceous limestone indicates a quiet period of fault activity under deep water. The massive bedding, disordered accumulation, and smooth bedding surface demonstrate gravity flow deposition in deep water generated by the remobilization of the Changhai fault, rather than an abrupt change in the sedimentation style. Considering the overall results of detrital component statistics and zircon U–Pb dating, we find that the quartz and altered rocks, the Figure 7. U –Pb Concordia diagram for LA-ICP-MS zircons from majority of the detrital components, may have two conglomerate sample YN13064 in the Pingchuan section. sources: one is derived from a weathered and eroded area of the early Permian Liangshan Formation, which mainly consists of quartzose sandstones, and the other is a continental volcanic-coarse clastic rock #101 yields an age of 2091 ± 8 Ma, which is significantly assemblage from the late Neoproterozoic (Zhang older than most others. Whether this age of 2091 Ma et al. 1988), which is indicated by the ca. records the older crystalline basement in the region 667–913 Ma zircon population. In addition, a few remains to be determined. Although two detrital zircons feldspar and volcanic detrital samples may be are too few to fully constrain magmatic activity of the derived from the denudation of small-scale magma- Emeishan basalts, the 267 ± 3 and 269 ± 5 Ma ages from tism in the middle of the Maokou stage (267– zircons #15 and #36, respectively, are very helpful 269 Ma). The gradually increasing upward feldspar because these ages are substantially earlier than the and volcanic detritus may demonstrate the gradual main eruption of the Emeishan LIP (259.1 ± 0.5 Ma; strengthening of volcanic activity. Zhong et al. 2014) and most likely show a small-scale magma eruption predating Emeishan igneous activity in the region. These two ages are equivalent to the 3.2. Qina section, Yongsheng County Neoschwagerina craticulifera Zone in the Maokou stage. This section lies immediately west of the Chenghai The same conglomerates are also found from north- faultandthewestofthe‘inner zone’ (point D in ern Jinhe (point B in Figure 1) to southern Shuhe (point Figure 1). The Maokou limestone in this section has a CinFigure 1) along the Chenghai fault. Further north, measured thickness of 67.8 m (excluding the bottom, similar conglomerate beds are also found to the west of Figure 8) and is conformably overlain by basalts. The

Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 Mianning but are seriously metamorphosed, preventing one to two strata in the section are composed of the identification of the stratigraphic sequence and grey–black, medium-thickness micrite limestones lithological association. Thus, the distribution range of withabundantsiliceousstripsandnodulesandno this suite of conglomerates is distributed along a narrow fossils (Figure 9a). The siliceous nodules often exhibit strip not exceeding 5 km along the west of the northern an irregular shape and parallel bedding, and indivi- section of the Chenghai fault. Detrital component sta- dual nodules vary in size from several to 30 cm and tistics of conglomerate sample YN13071 from the Jinhe are densely distributed in some places to form band section show that volcanic (66.2%), altered rocks (19%), shapes along the layer. Upwards, the strata 3–4have and feldspar (7.1%) are the main components of the micrite limestones with horizontal bedding and a matrix (Figure 5f), which suggests two possibilities: the laminated structure with multiple layers of low-grav- sampling area may be located on the top of the ity flows and turbidity currents, especially in stratum Pingchuan Formation and have formed during the vol- 4. Four periods of small-scale gravity flow developed canic eruption, or else the conglomerate is the product along stratum 4 in the thick micrite limestone with of late-stage erosion. siliceous nodules (Figure 9b). The lithology of the Research on the Pingchuan section has shown gravity flow is bioclastic limestone, mainly including that the newest fossils in the breccias of the Shuhe crinoids. The first period of gravity flow is relatively 120 P. WU ET AL.

and a smooth upper surface (Figure 9d). This layer is overlain by an erosional unconformity, draped by grey–black bioclastic limestone with abundant shallow-water fossils, including crinoids (Figure 9e). The bioclastic limestone displays normally graded bedding as identified by the grain size and enrich- ment level of crinoid fragments, namely larger grain size and higher numbers of bioclasts at the bottom, fining and attenuating upward. Strata 5–6contain grey–black siliceous limestones and silicalites with a thickness of 15 m (Figure 9f), and are intercalated by a lime conglomerate bed (1.6 m thick). All the conglomerates are grey–black micrite limestones that resemble those in the lower and upper layers, and are 10–20 cm in size and sub-rounded with abrasion borders, whereas the matrix of these rocks is composed of smaller limestone conglomerates and bioclasts, mainly crinoids (Figure 9h). The number of siliceous nodules decreases upward. Strata 7–8 change to grey–white bioclastic limestone of relatively shallow-water platform facies, with no siliceous components and a few lime conglomerates only at the top of stratum 8. The bioclasts mainly contain crinoids and fusulinids, which are well developed in some strata, and no brachiopods. Our fusulinid samples from stratum 7 and the bottom of stratum 8 yield diverse taxa, namely Neoschwagerina cf. simplex (Figure 10a), Cancelina nipponica (Figure 10b), and P. yabei (Figure 10c,10d), but not those from the top two biostratigraphic units of the Maokou Formation, which yield Afghanella, Yabeina,andNeomisellina, indicating the early–middle Maokou stage. An inter- esting finding at the top of stratum 8 is that basalt breccias, hydromagmatic deposits, and limestone conglomerates encompassed by basalts have developed in the matrix of the lime conglomerate bed (Figure 10e,10f), but all the conglomerates are

Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 bioclastic limestones belonging to the early–middle Maokou stage, as identified by fusulinids. Finally, the Maokou Formation is overlain conformably by basalts. Research on the Qina section was useful for analysing Figure 8. The measured columnar section from Qina section in the sedimentary environment and syn-depositional tec- Yongsheng, Yunnan Province. The squares indicate sample tonic activity in the Maokou stage. Confirming the sedi- level, with sample numbers shown beside the squares; the mentary environment is based primarily on the black dots indicate the locations of field photos graph. Legend is the same as in Figure 3. following two aspects: (1) the collective features of rock types and (2) the collective types of fossil (Zhu 1989). Micrite limestones and marlstones with abundant large and displays gravity flow lenses approximately siliceous nodules and no fossils in strata 1–6 indicate 3–4 cm thick and plastic crumple structures of pene- low sedimentary energy and a relatively deep-water contemporaneous deformation (Figure 9c). Three sedimentary environment rather than shallow-water periods above with individual bed thicknesses of carbonate platform facies. Small to medium-scale grav- 3–4 cm contain the lower surface of weak erosion ity flows developed in many layers of strata 3–4; these INTERNATIONAL GEOLOGY REVIEW 121 Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015

Figure 9. Field photographs of the Qina section. (a) Medium–thick micrite limestone with siliceous nodules, which are densely distributed in some layers to form band shapes along the layer; Binghui He (~1.75 m) for scale. (b) Panorama of micrite limestone layers with gravity flow and siliceous nodules; geological hammer (30 cm) for scale. (c) Enlarged view of the red lower box in (b), showing gravity flow lenses and plastic crumple structures of penecontemporaneous deformation; marker pen (14 cm) for scale. (d) Enlarged view of the red upper box in (b), showing the weak erosional lower surface and smooth upper surface; coin (2.5 cm) for scale. (e) Bioclastic limestone with normally graded bedding; marker pen (14 cm) for scale. (f) Grey–black siliceous limestones; geological hammer (30 cm) for scale. (g) Lime conglomerate bed that developed in siliceous limestones; marker pen (14 cm) for scale. (h) Enlarged view of (g), showing the matrix of lime conglomerate composed of smaller limestone conglomerates and bioclasts, mainly crinoids; coin (2.5 cm) for scale. 122 P. WU ET AL.

Figure 10. Photomicrographs of fusulinids and basalt breccias in the Qina section. (a) Neoschwagerina cf. simplex; (b) Cancelina nipponica; (c, d) Parafusulina yabei; (e) basalt breccias in the matrix of lime conglomerates; the fusulinid in the limestone conglomerate is Schwagerina sp.; (f) enlarged view of (e), showing basalt breccias and limestone conglomerates encompassed by basalt. All scales are 1 mm. Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015

rocks are overlain by bioclastic limestone with normally bioclasts entering the deep-water environment and graded bedding, which is useful for identifying turbidity eroding the micrite limestone when that was semi-soli- sediment (Shanmugam 1997). The turbidity sediment is dified during the penecontemporaneous period. Thus, thin and has a weak erosion surface. All these features these lime conglomerates represent an event deposit may be responses to frequent, low-intensity syn-deposi- dominated by Chenghai fault activity when this was tional activity of the Chenghai fault. The siliceous con- markedly increased. These rocks are overlain by shal- tent in the micrite limestone in stratum 5 above the low-water bioclastic limestone and limestone with bio- turbidity sediments increases suddenly and sharply, clasts, the fusulinid samples of which belong to the showing considerable development of siliceous strips fossil assemblage of the bottom biostratigraphic unit – and nodules and the appearance of siliceous rocks; Neoschwagerina simplex Zone; thus, the deep-water thus, stratum 5 possibly represents the maximum flood- sediments in strata 1–6 likely originated in the early ing surface of the early Maokou Formation. In addition, Maokou stage. The hydromagmatic deposits and lime- the lime conglomerates belong to intra-formational stone conglomerates, encompassed by basalts in a conglomerates that result from gravity flows of platform matrix of lime conglomerate on the top of the Qina INTERNATIONAL GEOLOGY REVIEW 123

section, indicate that small-scale magmatic activity may north of the dividing point, but there is a sharp distinc- have begun in the middle Maokou stage, and the eruption south of this point (Figure 11;Wuet al. 2014). tion environment was submarine. These obvious differences in the thickness of the It is worth noting that the age of the top of the Qina Maokou Formation on either side of the Xiaojiang fault limestone section is the early–middle Maokou stage, may be related to a single factor, such as surface ero- which resulted from a short duration of limestone sion after the deposition of the Maokou Formation (He deposition (only up to the middle Maokou stage) or et al. 2003a) or a combination of factors, such as syn- doming uplift and differential erosion after complete depositional differences in the sedimentary thickness deposition of the Maokou Formation, as proposed by and post-depositional surface erosion (Wu et al. 2014). He et al.(2006a). Therefore, several sections from both Therefore, further research was conducted to accurately the eastern and western sides of the Chenghai fault correlate the stratigraphy and palaeontology of the were investigated, and we discovered the following: Maokou limestone on either side of the Xiaojiang fault (1) on the western side of the Chenghai fault from the (Figure 11) and show that the majority of the sections in northern Youguomu section (point E in Figure 1) to the the Central Region included only the A. schencki and N. southern Reshuitang section (point F in Figure 1), espe- simplex zones, such as Laonianfang (point G in cially in the Qina section (point D in Figure 1), the top Figure 11) and Lianhe (point O in Figure 11). However, horizon of fusulinid zones reaches only the N. simplex or many sections in the Central Region, such as the A. schencki Zone and lacks the uppermost biostrati- Tangfang section in Huidong and Tongchanggou sec- graphic units of the Maokou Formation, such as tion in Huili, also have the Y. gubleri–N. multivoluta Yabeina and Neomisellina; however, on the eastern Zone, the uppermost biostratigraphic units of the side, the uppermost biostratigraphic units of the Maokou Formation. More convincingly, although the Maokou Formation developed, such as in the thickness of the Qixia and Maokou formations is 43 Pingchuanjie section (point G in Figure 1), these being and 64 m, respectively, in the Baiguo section (point H located in the inner zone and are thought to have been in Figure 11), which is located near the uplift centre of more intensively eroded than the western side; (2) the ‘inner zone’ of He et al.’s model, many fossils layers or lenses of limestone within the bottom of the belonging to the uppermost biostratigraphic units of Emeishan basalt sequence; and (3) hydromagmatic the Maokou Formation, such as Neomisellina sp., N. deposits and limestone conglomerates encompassed delicata, N. lepida, and Kahlerina sp., developed in mas- by basalts. Based on the above results, we suggest sive micrite limestone with bioclasts at the top of this that limestone deposition reached only the middle section. The complete outcrop and clear contact rela- Maokou stage; meanwhile, small-scale magmatic activ- tionship of the Baiguo section indicates that the fossils ity had begun from the northern Youguomu section to are preserved in the original horizon rather than as the southern Reshuitang section on the western side of remnants from erosion. In addition, the contact the Chenghai fault. Thus, the absence of the middle and between the basalt and underlying strata from field upper units of the Maokou Formation is related to observations is a faulted contact in the Ertan and sedimentary absence rather than to erosion after Binchuan sections located in the strong erosional zone deposition in the region. of He et al.’s model, which reflects the influence of

Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 strong late tectonic transformation and cannot be used to explain strata denudation before the basalt 4. Stratigraphic correlation and sedimentary eruptions. facies analysis of the Maokou Formation in the Meanwhile, the lithostratigraphic correlation of sec- Central and Eastern Regions tions on either side of the Xiaojiang fault shows that the Previous research found obvious differences in the thick- lithology of the Qixia Formation on either side is basi- ness of the Maokou Formation on either side of the cally the same, consisting of grey–white micrite bioclas- Xiaojiang fault (BGMRYP (Bureau of Geology and tic limestone and dolomitic limestone with a Mineral Resources of Yunnan Province) 1990;Heet al. porphyrotopic texture and a few siliceous components 2003a;Liet al. 2011): the Maokou Formation in the and siliceous nodules; thus, the Qixia Formation is a Central Region is much thinner compared with in the typical shallow-water carbonate platform facies. Eastern Region. Through detailed field investigations, However, the lithology of the Maokou Formation on we confirmed this phenomenon and suggested taking either side of the Xiaojiang fault varies greatly. In the the intersection of the Xiaojiang and Qiaojia faults as the middle–upper limestone stratigraphy of the Maokou dividing point. The thickness of the Maokou Formation Formation on the eastern side of the Xiaojiang fault, on either side of the Xiaojiang fault varies only slightly abundant siliceous strips and nodules with silicalites 124 P. WU ET AL.

Figure 11. Stratigraphic and fusulinid zone correlation of Qixia and Maokou limestone in the Emeishan LIP. P2m, Maokou Formation; P2p, Pingchuan Formation; P1q, Qixia Formation; P1l, Liangshan Formation; P1s, Shuhe Formation; 1, basalt; 2, Yabeina gubleri– Neomisellina multivoluta Zone; 3, Afghanella schencki Zone; 4, Neoshwagerina simplex Zone; 5, Qixia Formation; 6, conglomerate; A, Pingchuan in Yanyuan County; B, Xiluo in Puge County; C, Rize in Butuo County; D, Sandaoshui in Zhaotong County; E, Qina in Yongsheng County; F, Baicaoping in Yanyuan County; G, Laonianfang in Yanbian County; H, Baiguo in Huili County; I, Daqiao in Huidong County; J, Chenjiaping in Qiaojia County; K, Kuangshanchang in Huize County; L, Heiwujie in Yongsheng County; M, Pingchuanjie in Binchuan County; N, Longdonghe in Huaping County; O, Lianhe in Xundian County; P, Daibu in Huize County; Q, Dayingshang in Xuanwei County. Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015

developed locally, such as in the Kuangshanchang of thinning of original thickness in the Central Region and (point K in Figure 11) and Dayingshang (point Q in the formation of thicker deep-water facies sediments in Figure 11) sections. the Eastern Region. Surface uplift and erosion after the Based on the biostratigraphical correlation results and deposition of the Maokou Formation is not as substantial lithological variation analysis, we suggest that the Maokou as proposed by He et al.(2003a), 2005) – the scale of limestone differed greatly in its thickness and lithology on erosion is perhaps only a few tens of metres. either side of the Xiaojiang fault during deposition. The development of basinal cherty facies on platform carbonates in the Eastern Region clearly represents a consider- 5. Discussion able deepening of the water column; this facies is typically found in Permian basins in South China and has been 5.1. Regional differences in the sedimentary interpreted to have formed at water depths of at least environment in the Maokou stage 200 m (Wang and Jin 2000;Shenet al. 2007;Sunet al. He et al.(2003b, p. 195) stated that ‘the Maokou stage 2010). The variation in thickness and lithology is the result shows a sedimentary environment of shallow marine INTERNATIONAL GEOLOGY REVIEW 125

carbonate platform with stable crust, uniform tectonic But some sections, such as Tangfang, Tongchanggou, setting, horizontal depositional surface and without and Baiguo described earlier, developed very thin but intense fault activities’; therefore, the entire western complete fusulinid zones in the Maokou Formation, margin of the Yangtze plate received approximately indicating slow underwater uplift in the Maokou stage. homogeneous and isopachous Maokou limestone The syn-depositional underwater uplift zone was con- deposits (Figure 7a in He et al. 2006a). Rapid crustal trolled by normal faulting activity of the Chenghai and doming and differential erosion induced by mantle Xiaojiang faults. A complete Maokou limestone widely plume upwelling after the deposition of the Maokou developed in the Eastern Region. The middle section of Formation resulted in the regional differences in thick- the Maokou Formation, which has abundant siliceous ness seen in the Maokou limestone today (He et al. nodules and strips in many sections, reflects deepening 2003a, 2009). It can be confirmed that the stable carbo- water and the corresponding greater thicknesses of nate platform facies in the Maokou stage and homoge- limestone accumulation (Peate and Bryan 2008). Thus, neous and isopachous Maokou limestone are the critical the sedimentary facies in the Eastern Region is premise of He et al.’s model because these serve as a carbonate-platform -based with local upper slopes. simple reference point from which any subsequent The above analysis of sedimentary facies shows that change can be measured (He et al. 2009). the research area was not a uniform shallow-water plat- However, the sedimentary environment of the wes- form in the Maokou stage but was, rather, composed of tern margin of the Yangtze plate in the Maokou stage is sedimentary facies with an N–S linear alignment along different from the above case. From the large back- the Changhai and Xiaojiang faults, namely trench and ground of South China, it experienced the greatest slope facies in the Western Region, shallow-water car- transgressive period since the Palaeozoic in the Qixia bonate platform facies in the Central Region, and stage and was entirely covered by carbonates with carbonate-platform-based and local upper slopes in homogeneous lithology, lithofacies, and biota, and con- the Eastern Region; syn-depositional activity of the siderable stratum thickness. However, South China gen- Chenghai and Xiaojiang faults controlled the distribu- erally experienced regression in the Maokou stage – at tion of sedimentary facies, sediment types, and sedi- that time crustal activity increased, intensifying the dif- mentary thickness. ferences in submarine topography, lithology, lithofacies, biota, and stratum thickness (Wang et al. 1994). In our research area, the conglomerates in the Shuhe and 5.2. The time and environment of the Emeishan Pingchuan formations in the Pingchuan section are dis- basalt eruptions tributed in an approximately N–S direction along the northern section of the Chenghai fault (Figure 1), repre- Xu et al.(2001, 2003) analysed over 350 volcanic senting a suite of gravity flow deposits in trench facies samples for major and trace element composition and belonging to tectonic earthquake events (Liang and revealed spatial variations in the basalt geo- et al. 1994) controlled by syn-depositional Chenghai chemistry. They considered the domed region of fault activity in the early–middle Maokou stage. The the Emeishan LIP to comprise thick sequences fault activity was presumably the surface response to (2000–5000 m) of dominant low-Ti volcanic rocks,

Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 the initial emplacement of the deep mantle. This trench subordinate picrites, and high-Ti and alkaline lavas; facies continued towards Emeishan basalt eruptions. in contrast, thin sequences (<500 m) of high-Ti vol- The silicalites, micrite limestones with siliceous nodules, canic rocks mainly occur on the periphery of the small-scale gravity flow deposits, turbidity deposits, and domal structure (Xu et al. 2004). Wang et al.(2014) lime conglomerates in the Qina section indicate a deep- reported high-Ti basalts with many plagioclase phe- water sedimentary environment and event deposits in nocrysts in the eastern part of the Emeishan LIP, as the early Maokou stage along the middle section of the observed in the mid-upper Binchuan section, indicat- Chenghai fault. Frequent, small-scale, and lenticular ing that volcanic eruptions moved progressively event deposits indicate frequent and low-intensity syn- from west of the Panxi palaeo-uplift zone to the depositional activity of the Chenghai fault. In the east. Before or during the earlier rapid basalt erup- Central Region, the lithological features consist mainly tions of low-Ti basalts, the marine sedimentary envir- of bioclastic limestone and represent carbonate plat- onment was preserved along the western side of the form facies. The thinning or disappearance of the Panxi palaeo-uplift zone, but at the time of the later Maokou limestone in many sections of the Central high-Ti basalt eruptions, submarine and subaerial Region demonstrates that surface erosion did take environments coexisted in different parts of the LIP place, though the region of surface erosion was limited. (Wang et al. 2014). 126 P. WU ET AL.

Taking a fresh look at the spatial variations in the stage and the pillow lavas with chill rinds that formed basalt sections, combined with the distribution of the during the main basalt eruption provide key evidence of main faults in the Emeishan LIP (Figure 1), we find that subaqueous eruptions in the Western Region that pre- thick sequences of low-Ti volcanic rocks are distributed clude kilometre-scale pre-volcanic uplift over the whole mainly to the west of the Changhai fault as represented region and a fully continental eruption environment. The by the Binchuan and Pingchuan sections; picrites and control functions of the faults during deposition of the low-Ti, high-Ti, and alkaline lavas coexist in the Central Maokou Formation and eruption of the Emeishan basalts Region between the Changhai and Xiaojiang faults; and demonstrate the continuity and inheritance of the fault thin sequences of high-Ti volcanic rocks mainly occur to activity. the east of the Xiaojiang fault. In addition, the rhombic pattern and distribution in the northeast and northwest – of the Emeishan LIP indicate that the basalt eruptions 5.3. Tectonic sedimentary evolution of the were possibly controlled by a N–S extended belt and Emeishan LIP separation fracture (Zhang et al. 1988), and that the rift In the Qixia stage, a shallow carbonate platform system was connected to the rise of volcanic magma to developed across almost the whole of the western form thicker basalts on both sides of the fault. Yangtze. Because of the thicker limestone in the From our field and further microscopic observations, QixiaFormationinMiyi,Huili,Panzhihua,and we know that the basalt breccias on the top of the Qina Xichang in the middle part of the Kang–Dian palaeo- section were extremely likely to be the result of initial uplift (BGMRSP (Bureau of Geology and Mineral small-scale subaqueous magmatic activity which began Resources of Sichuan Province) 1991;ESC,2000;He in the N. simplex Zone (~268 Ma) or shortly thereafter. et al. 2003b;Liet al. 2011), it can be speculated that This magmatic activity was earlier than the main eruption this palaeo-uplift did not exist in the Qixia stage. Syn- of the Emeishan LIP (258–260 Ma, Xu et al. 2004). This depositional normal faulting movement along deep conclusion is also indicated by zircon dating data (267– regional fractures took place in the Maokou stage 269 Ma zircon population) from the Pingchuan section. along with the upwelling of hot, deep mantle mate- These age data represent not only the early stage of rial (Figure 12). These activities are intermittent fea- magmatic eruptions but also the time of Changhai activ- tures in time and non-uniform in space. The ity, which further demonstrates the relationship between outstanding characteristics are as follows: (1) the surface topography and deep mantle activity. The poten- coexistence of subaqueous turbidite fans and all tial initial small-scale underwater eruption in the Maokou types of gravity flow sediments along certain Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015

Figure 12. Schematic diagram of the tectonic–sedimentary evolution model in the Emeishan LIP. INTERNATIONAL GEOLOGY REVIEW 127

directions, especially coarse fragmental flow (Wang 6. Conclusion et al. 1994), such as in the Pingchuan section; (2) The sedimentary environment in the Maokou stage is mutation zones with different lithology, lithofacies, not uniform carbonate platform facies but, rather, and thickness, such as along the Xiaojiang fault; and sedimentary facies with an N–S linear alignment and (3) the coexistence of both high- and low-energy W–E distribution controlled by activity along the syn- sedimentary bodies, such as in the Qina section. The depositional Changhai and Xiaojiang faults. The deep- Changhai fault began moving in the early Maokou water depositions and event sediments of trench and stage to form the conglomerates in the Pingchuan slope facies developed to the west of the Changhai section and the deep-water deposition and event fault and to the east of the Xiaojiang fault, whereas deposition in the Qina section. In the Muli region underwater uplifting induced by syn-depositional nor- further west, the structural activity possibly strength- mal fault activity occurred in the Central Region. ened, which resulted in platform marginal slope and Differences in sedimentary facies and terrain resulted basin facies (Chen and Chen 1987). Activity in the in the original sedimentary thickness of the Maokou Xiaojiang fault began later than that in the limestone being thicker in the Western and Eastern Chenghai fault, which accordingly formed the rela- Regions and thinner in the Central Region. Thus, uplift tively deep-water siliceous limestone in the mid- did take place but was not a kilometre-scale domal upper section of the Maokou Formation. In the uplift after the Maokou stage. This uplift involved syn- Central Region between the Chenghai and Xiaojiang depositional underwater uplift and post-depositional faults, underwater uplifting began in the middle surface uplift. Accordingly, thinning of the Maokou Maokou stage; as a result, thinner but complete fusu- limestone was the product of the different initial linid zones in the Maokou Formation developed, such depositional thickness caused by underwater uplift as in Tangfang, Tongchanggou, and Baiguo. and post-depositional surface uplift and erosion. This Meanwhile, small-scale magmatic activity had begun uplifting process represents dynamic topography in the middle and late Maokou stage in the Western before basalt eruption, that is surface topography in Region. These surface processes must contain the response to deep mantle activity under the litho- dynamic topography in response to the deep mantle spheric bottom boundary. The time and environment activity under the lithospheric bottom boundary. of the basalt eruptions were also controlled by the Regarding how to distinguish dynamic uplift from movement of the Chenghai and Xiaojiang faults. surface processes, much work needs to be done in Small-scale magmatic activity might have begun to the future, such as palaeo-water-depth correction and the west of the Chenghai fault in the middle of the isostatic compensation. Maokou stage, whereas submarine and terrestrial sedi- Along with the intensification of the upwelling of hot mentary environments coexisted before and during mantle material after the Maokou stage, rapid uplifting the Emeishan basalt eruptions. and surface differential erosion took place in the Central and Eastern Regions to form a series of conglomerate sediments in Luji, Puge, Butuo, and Songming near the Xiaojiang fault (He et al. 2009., Wang et al. 2014., Wu et al. Acknowledgements Downloaded by [Universiti Putra Malaysia] at 00:39 28 October 2015 2014), but post-depositional uplift and erosion were We thank Professor Yu Wang for his help with the research, considerably lower than kilometre scale. Comparisons of Professor Changqun Cao, and Doctors Tao Qian and Zhou strata thickness and fusulinid zones indicate that the ero- Zhang for their helpful discussions, and Sanzhong Li and an sional magnitude may have been only dozens of metres. anonymous referee for constructive reviews. Additionally, small-scale subsidence induced by extensive mantle upwelling might have occurred along deep fractures and secondary faults (Figure 12;Zhanget al. 1988; Disclosure statement Wu et al. 2014), and the strata in the Maokou Formation are well preserved (Wang et al. 2011) in some areas of the No potential conflict of interest was reported by the authors. Central Region. Thus, the region of erosion may have been limited. Moreover, large-scale, low-Ti basalt eruptions remained in the underwater environment and formed a Funding series of pillow lavas in the Western Region. Later, erup- This research was supported by the National Basic Research tions spread progressively throughout the Emeishan LIP, Programme of China [973 Programme, number 2011CB808901] and the basalt series also gradually changed to alkaline and National Natural Science Foundation of China [numbers and high-Ti basalts. 41030318 and 91114203]. 128 P. WU ET AL.

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