Mei Mingxiang , Maurice E.Tucker

1.School of Earth Science and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, China

2.Department of Geological Sciences, University of Durham, Durham DH13LE, UK

1 Introduction*

In recent years, there has been increased interest in the well-exposed and fossiliferous Devonian strata of the Yunnan-Guizhou-Guangxi area in the southern part of Guizhou, the southeastern part of Yunnan and the western part of Guangxi (Fig.1).Earlier comprehensive sedimentologic and stratigraphic research provides a useful foundation for sequence stratigraphic studies.The most influential research includes Zhou (1992), Zhao and Ding(1996), Dong (1997)and Yin (1997)on the lithostratigraphy; Wuet al.(1987)and Zenget al.(1994)on the sedimentary facies and palaeogeography; Kuanget al.(1989),Zhonget al.(1992)and Houet al.(2000)on the biostratigraphy and chronostratigraphy; Wu (2000)on the tectonics and palaeogeography, and Chenet al.(2001a)on the palaeoclimate.Studies of the Devonian sequence stratigraphy in Yunnan-Guizhou-Guangxi area are included in the treatises published by Xuet al.(1993), Zenget al.(1994), Wuet al.(1997), Yanget al.(1999)and Meiet al.(2001).Wuet al.(1997)recognised 20 third-order sequences in the Devonian strata in South China; Meiet al.(2001)and Mei and Tucker (2007)identified 1 second-order sequence that consisted of 13 third-order sequences in the southern part of Guizhou and the northern part of Guangxi.The studies of cyclostratigraphy by Baiet al.(1995), Chenet al.(2001b, 2001c)and Gonget al.(2001)were also valuable in this research.

Fig.1 Outline map showing the sedimentary facies and palaeogeography of the Middle Devonian and locations of the principal sections logged.1-Shelf facies; 2-Mixed platform facies; 3-Open platform facies; 4-Inter-platform shelf; 5-Basin; 6-Restricted platform facies; 7-Inter-platform basin; 8-Foreshore facies; 9-Neritic shore facies; 10-Tidal flat to lagoon facies; 11-Palaeo-land; 12-The general palaeogeographical pattern of attached platform, isolated platform and basin is clearly illustrated.The black triangles refer to the locations of sections: (1)-Wudang section in Guiyang, (2)-Lüyinqiao section in Duyun, (3)-Dushan section in Guizhou, (4)-Luofu section in Nandan, (5)-De’e section in Longlin, (6)-Liujing section in Hengxian.

The Devonian in the Yunnan-Guizhou-Guangxi area is dominated by a succession of carbonates that evolved from basal transgressive clastics that gradually wedge-out towards the continental area to the north.As a result of the obvious differentiation of facies formed by the large-scale transgressive event in the Late Devonian, the third-order sequences were developed in different sedimentary environments within the large-scale palaeogeography of platforms and basins (Fig.1).Therefore, the Devonian strata in the study area provide an example for the further understanding of the depositional trend within the sequence-stratigraphic framework (Catuneanuet al., 2009).

2 Sequence stratigraphy of the attached platforms

From an examination of sections at Wudang in Guiyang,Lüyinqiao in Duyun and Dushan in Guizhou, the Devonian strata across the region show a gradual wedge-out at both the top and bottom parts of the succession from south to north across the attached platform.The lower wedge-outis transgressive and is genetically related to the Caledonian orogeny.The upper wedge-out is regressive and genetically related to the Ziyun movement.By tracing sequence boundaries and examining the facies architecture,13 third-order sequences (SQ1to SQ13)can be discerned in the Devonian strata of the Yunnan-Guizhou-Guangxi area, although it is impossible to discern all 13 sequences in every location.This illustrates the incomplete nature of the stratigraphic record as a result of local tectonics and palaeogeography (Mei, 1996; Mei and Ma, 2001).

2.1 Sequence stratigraphy of the Devonian at Wudang, Guiyang

Located near the basin margin, the Wudang section lacks some Devonian strata.These include those belonging to the Lochkovian and Pragian Stages (Lower Devonian)that constitute the third-order sequences SQ1to SQ4elsewhere, and those belonging to the upper part of the Frasnian and Famennian Stages that form the third-order sequences SQ11to SQ13elsewhere.Consequently, only 5 third-order sequences, SQ5, SQ6, SQ7, SQ9and SQ10can be discerned at the Wudang section (Fig.2).The lack of SQ8could be related to the hiatus resulting from the Haikou tectonic movement and uplift in the Late Eifelian to Early Givetian.The lack of SQ14at the base of the Carboniferous and SQ11, SQ12and SQ13in the Upper Devonian is the result of a major unconformity caused by the Ziyun movement.

The 3 third-order sequences (SQ5, SQ6and SQ7)in the lower part of the Devonian succession are mostly sandstones within the Mangshan Group.Fossil fish includeKueichowlepis sinensis,Sinopetalichthys kueiyangensis,Neoduyunaspis minuta etc.; fossil brachiopods such asOri?entospirifer wangiand plant fossils, such asZosterophyllumsp.andDrepanophycus spinaeformisalso occur.This biota indicates an Emsian to Eifelian Age.Meter-scale cycles of the clastic tidal-dynamic type (Meiet al., 2000)are well developed and constitute regular vertical stacking patterns in these third-order sequences (SQ5to SQ7).In response to the third-order sea-level rise, meter-scale cycles are marked by thickly-bedded basal subtidal sandstones; by way of contrast, thin-sandstone based cycles with thinly-bedded subtidal clastic facies developed during sea-level falls.Palaeosol beds of purple-red ferruginous sandy mudstones on the top part of SQ7demonstrate a regional unconformity which is genetically related to the Haikou tectonic movement, and which leads to the absence of SQ8.

Fig.2 Diagram showing the division of the Devonian thirdorder sequences at Wudang in Guiyang.The legend for this figure and figures 3-7 is the same as illustrated in Fig.8.SQ5 to SQ10 refer to the third-order sequences in this section.The Devonian here comprises the Mangshan Group and Gaopochang Formation (GPC).The underlying strata are the Gaozhaitian Formation (GZT)of the Middle Silurian (S2); the overlying strata are the Carboniferous (C)Xiangbai Formation (XB)marked by coal-measures.The third-order sequences show a generally upward-shoaling succession of sedimentary facies, in which meter-scale cycles show a regular vertical stacking pattern: Stsubtidal flat, It-intertidal flat, Sup-supratidal flat.In the chronostratigraphic system, Dt-the Datangian Series of the Carboniferous, Js-Jiusian Stage; Em-Emsian Stage, Eif-Eifelian Stage,Giv-Givetian Stage, Frs-Frasnian Stage.I-conformity of the Haikou movement, II-conformity of the Ziyun movement, III-curve of the third-order sea-level changes.

The upper part of the Devonian in the Wudang section consists of dolomites of the Gaopochang Formation, and two third-order sequences (SQ9and SQ10)are recognised.The lower part of each third-order sequence is mainly formed of thickly-based meter-scale cycles of the carbonate peritidal type; the upper part is marked by the development of the thinly-based variety.Hence, each third-order sequence consists of an upward-shoaling succession of facies.Fossil brachiopods such asStringocephalusin sequence SQ9indicate a Late Givetian age;Cyrtospiriferin SQ10indicates Early Frasnian.

2.2 Sequence stratigraphy of the Devonian at Lüyinqiao, Duyun

The Devonian strata at Lüyinqiao can be grouped into three units: a lower part of transgressive sandstones of the Mangshan Group; a middle part of dolomites of the Dushan Formation, and an upper part of limestones of the Wangchengpo Formation.Seven third-order sequences from SQ4to SQ10can be distinguished (Fig.3).As at Wudang, there are no strata at Lüyinqiao corresponding to SQ1to SQ3at the base of the Devonian, nor to SQ11to SQ13in the Upper Devonian.At the base of the Carboniferous,there is no equivalent of SQ14as a result of uplift and erosion related to the Ziyun tectonic movement.

Meter-scale cycles of the clastic dynamic-type are developed in the transgressive Mangshan sandstones.Their regular vertical stacking patterns constitute 3 third-order sequences from SQ4to SQ6.Fish fossils,e.g.,Kue?ichowlepis sinensis, and brachiopods such asOrientospiri?fer wangiandEuryspirifersp.indicate a Late Pragian to Early Eifelian Age.

Brachiopods such asStringocephalus obesusandSt?ringocephalus burtiniin the Dushan Formation suggest a Late Eifelian to Givetian Age.Three third-order sequences from SQ7to SQ9can be discerned in the Dushan Formation.Each of them is composed of meter-scale peritidal carbonate cycles arranged in a regular vertical stacking pattern of transgressive, thickening-upward, thick-bed based cycles and regressive, thinning-upward, thin-bed based cycles with more dolomites.In general, compared with the equivalent strata at Wudang, the intensity of dolomitisation at Lüyinqiao is weaker.

The Wangchengpo Formation of the Upper Devonian constitutes sequence SQ10.Its transgressive systems tract (TST)and early highstand systems tract (EHST)are mainly composed of limestones of open-platform facies.In its late highstand systems tract (LHST), dolomites of restricted-platform facies are developed.A regular verticalstacking pattern of meter-scale cycles is observed within SQ10; in the TST, the meter-scale cycles are thick-bedded,open-platform carbonates and thin-bedded dolomites or dolomitic limestones of restricted-platform facies, which indicates the rare “exposed beats”.The subtidal meter-scale cycles in the EHST are thick-bedded packstones and grainstones of open-platform facies with interbedded shelf marls, which shows the clear “drown beats”.Meter-scale cycles of the thin-based peritidal carbonate type with dolomites are developed in the LHST, which demonstrate the obvious “exposed beats”.

Fig.3 Diagram showing the third-order sequences at Lüyinqiao in Duyun.The strata are divided into the Mangshan Group, and Dushan and Wangchengpo Formations (WCP).The underlying strata belong to the Silurian Wengxiang Group (WX), and the overlying strata are the Carboniferous Xiangbai Formation (XB).SQ4 to SQ11 are the third-order sequences marked by various facies successions;the abbreviations for the facies are as follows: St-subtidal-flat facies, It-intertidal flat facies, Sup-supratidal-flat facies, Op-open-platform facies, Tf-tidal-flat facies, Rp-restricted platform facies.Compared with those at Wudang, one difference is that SQ4 is Pragian in age (Pra).The symbols of I to III and the abbreviations of chronostratigraphy are the same as in Fig.2.

2.3 Sequence stratigraphy of the Devonian at Dushan, Guizhou

Compared with Wudang and Lüyinqiao sections, the Devonian strata at Dushan are more completely developed.10 third-order sequences can be distinguished within the Danlin, Shujiaping and Longdongshui Formations of the Lower Devonian; the Bangzhai and Dushan Formations of the Middle Devonian and the Wangchengpo, Yaosuo,Zhewang and Gelaohe Formations of the Upper Devonian(Fig.4).

Fig.4 Diagram showing the third-order sequences at Dushan in Guizhou Province.Apart from those marked in the figure, other abbreviations for the lithostratigraphic units are as follows: WX-Wengxiang Group (Silurian), SJP-Shujiaping Formation, LDS-Longdongshui Formation, BZ-Bangzhai Formation, WCP-Wangchengpo Formation, ZW-Zhewang Formation, GLH-Gelaohe Formation,TBG-Tangbagou Formation (Carboniferous); 1 to 4 refer to four members of the Dushan Formation: 1-Jipao Member, 2-Songjiaqiao Member, 3-Jiwozhai Member, 4-Hejiazhai Member.Besides those illustrated in Fig.2, other facies codes are as follows: Sh-shelf facies, Bh-biohermal limestones, Tf-tidal-flat facies.Moreover, I-unconformity of the Haikou tectonic movement, II-unconformity of the first phase of the Ziyun movement, III-unconformity of the second phase of the Ziyun movement, IV-curve of third-order sea-level changes, Yg-the Yanguanian Series (Carboniferous), Tbg-Tangbagouan Stage (Carboniferous), Fam-Famennian Stage; other chronostratigraphic abbreviations are the same as those in Figs.2 and 3.

The Danlin Formation forms sequence SQ4and fossils present in the overlying Shujiaping Formation suggest a Pragian age.Conglomerates at the base of SQ4unconformably overlie the Silurian Wengxiang Group, as a result of uplift due to the Caledonian orogeny.Palaeosol facies composed of ferruginous sandstones and mudstones at the top of SQ4indicate a phase of exposure.The general features of the facies succession of SQ4are similar to those from SQ4to SQ6at the Lüyinqiao section described above.

At Dushan, the Shujiaping and Longdongshui Formations constitute SQ5, in which brachiopods,e.g.,Euryspiri?ferandZdimir,etc.and corals such asDushannophrentis cystotabulatacan be collected.Accordingly, the sequence belongs to the Emsian.The transgressive sandstones of the Shujiaping Formation (Fm.)make up the transgressive systems tract (TST)of SQ5, in which are developed many meter-scale cycles of the clastic tidal-dynamic type.The limestones of the Longdongshui Formation constitute the highstand systems tract (HST)of SQ5, in which thick-bedded massive biohermal limestones of open-platform facies and thin-bedded dolomitic limestones of semi-open platform facies form numerous meter-scale cycles of carbonate peritidal type.In the transitional period from TST to HST, a condensed section (CS)of SQ5is recognised consisting of marls and calcareous shales of shelf facies in the lower part of the Longdongshui Formation.Palaeosol beds composed of ferruginous sandy mudstones at the top of SQ5record subaerial exposure.

Four third-order sequences from SQ6to SQ9occur in the Bangzhai and Dushan Formations of the Middle Devonian.The Dushan Formation can be separated into Jipao Member limestones, Songjiaqiao Member sandstones, Jiwozhai Member limestones and Hejiazhai Member limestones and dolomites.The transgressive sandstones of the Bangzhai Formation constitute the TST of SQ6; the Jipao Member limestones of the Dushan Formation make up the HST of SQ6.Similar to SQ6, the TST of SQ7is composed of transgressive sandstones of the Songjiaqiao Member,and its HST consists of the limestones of the Jiwozhai Member.Therefore, the features of facies successions that make up SQ6and SQ7are generally the same as SQ5described above.At the top of SQ7, there are dolomites of restricted-platform facies (15 to 20 m thick).This shows that its top boundary is an exposure surface, similar to type I sequence boundary defined by Vailet al.(1977).This boundary also corresponds to the unconformity produced by the Haikou tectonic movement in the research area.

Two third-order sequences, SQ8and SQ9, can be discerned in the Hejiazhai Member limestones and dolomites.The general facies succession of SQ8and SQ9can be described as follows: in the deepening phase of the rising third-order sea-level TST, more thick-bedded limestones of open-platform facies are developed; in the shoaling process resulting from the fall of third-order sea-level, more dolomites of restricted-platform facies were formed, especially near the top of SQ9.

Four third-order sequences from SQ10to SQ13can be distinguished within the Upper Devonian carbonates (Fig.4).The general characters of the facies succession can be described as follows: in the TST off-platform facies biohermal limestones are developed, otherwise, more dolomites of restricted-platform facies are developed.The dolomitization is relatively more intense in the upper parts of SQ11and SQ13, where more palaeosol beds are developed.Hence,the upper boundaries of SQ11and SQ13are both long-term exposure surfaces, similar to the type I sequence boundary defined by Vailet al.(1977); they are also unconformities correlating to the first and the second episodes of the Ziyun tectonic movement.

3 Sequence stratigraphy of the isolated platforms

During the Devonian, a major sedimentary basin developed in the southern part of Guizhou, western part of Guangxi and the southeastern part of Yunnan.Its northern boundaries are two contemporaneous fault zones:the “Ziyun-Luodian-Nandan-Duan” fault zone and the“Mile-Shizong-Puan” fault zone.This sedimentary basin has been referred to as the “Dianqiangui Basin” by Zhaoet al.(1996), the “Youjiang Basin” by Zenget al.(1994),and the “Nanpanjiang Basin” by Enoset al.(1998).A variety of isolated platforms were developed within this sedimentary basin including the Jingxi Platform, Nanning Platform and Longlin Platform.Between the platforms,narrow inter-platform basins were formed.The Devonian strata are marked by carbonate facies developed in the central parts of the isolated platforms, as shown by the De’e section in Longlin, where there is an absence of Lochkovian to Pragian strata.On the margins of the isolated platforms, such as that at the Liujing section in Hengxian, the succession is more fully developed.Thus,the De’e and Liujing sections are selected to illustrate the general features of the Devonian sequence stratigraphy of the isolated platforms.

Fig.5 Diagram illustrating the third-order sequences at De’e in Longlin County, Guangxi.Apart from those lithostratigraphic units illustrated in the figure, LSG-Loushanguan Group (Cambrian), PJ-Pojiao Formation (Emsian), BZ-Baizuo Formation(Carboniferous).The brief abbreviations of sedimentary facies and the chronostratigraphic division are the same as those illustrated in Figs.2 to 4.In this section, 9 third-order sequences from SQ5 to SQ13 can be discerned.The meanings of I to IV are the same as those given in Fig.4.

3.1 Sequence stratigraphy of the Devonian at De’e, Longlin

This locality lacks Lochkovian to Pragian strata and the succession consists of the Pojiao Formation, Yintang Formation, Donggangling Formation and Rongxian Formation, with 9 third-order sequences as shown in Fig.5.The Emsian Pojiao Formation unconformably overlies the Cambrian Loushanguan Group, the long hiatus a reflection of the Caledonian orogeny.The Rongxian Formation is unconformably overlain by the Carboniferous Baizuo Formation, as a result of the Ziyun movement.

The Pojiao Formation constitutes the third-order sequence SQ5.Its TST is composed of black argillaceous lime mudstones and marls of swamp facies, passing up into open-platform carbonates, which become thickly-bedded upward.The HST is composed of numerous meter-scale peritidal carbonate cycles formed of thick-bedded open-platform facies and thin-bedded dolomitic limestones and dolomites of restricted-platform facies; the dolomite beds are more common and thicker upwards.Between the TST and HST, the black organic-rich shelf shales, 2-3 m thick, constitute the condensed section(CS)of SQ5.In SQ5, there are abundant brachiopods and bivalves including theHowellella fecunda-Reticulariopsis ertangensiassemblage, as well as theEuryspiriferassemblage withAcrospirifer subregularis, A.ordinaris, Howel?lella yukiangensis, H.papaoensis, Euspirifer wangi,andOtospirifersp.These fossils indicate a mid-Late Emsian age for SQ5at this locality.

Numerous meter-scale peritidal carbonate cycles are developed in the Yingtang Formation and Donggangling Formation.They are composed of thick-bedded massive biohermal limestones, open-platform grainstones and packstones with interbeds of thin-bedded limedolomites and dolomitic limestones of restricted-platform facies.There is a regular vertical stacking pattern of the cycles in the third-order sequences, on the basis of which 4 third-order sequences from SQ6to SQ9can be discerned in the Yingtang and Donggangling Formations.Their general characters are: the TST and the Early HST (EHST)are chiefly composed of thick-bedded cycles while the Late HST (LHST)consists of thin-bedded cycles.

The Rongxian Formation contains 4 third-order sequences from SQ10to SQ13.Their general facies characters are similar to those of SQ6to SQ9.The dissimilarities include: in the TST and EHST of SQ11and SQ12oncoid-bearing grainstones are common.At the top of SQ11and SQ13palaeosols composed of nodular ferruginous mudstones are developed, with a clear exposure surface as their upper boundary, corresponding to the unconformity of the first and the second episodes of the Ziyun movement.

3.2 Sequence stratigraphy of the Devonian at Liujing, Hengxian

The Liujing section is developed in the slope facies of the isolated platform.Fossils of both shelf and plankton biofacies are present, which makes this section a key sec-tion for stratigraphic correlation between the platform and deep-water strata (see the book on this locality by Kuanget al., 1989 for details).At Liujing, the Lower Devonian includes the Lianhuashan, Nagaoling, Yujiang and Moding Formations; the Middle Devonian contains the Najiao and Mintang Formations, and the Upper Devonian contains the Gubi and Rongxian Formations (Fig.6).Since Quaternary sediments cover most parts of the Rongxian Formation,only 11 third-order sequences from SQ1to SQ11can be recognised at Liujing.The Devonian sequence stratigraphy at Liujing is unusual: First, obvious transgressive events are indicated by the TST of SQ5and SQ8.Second,the unconformity of the Haikou movement is very clearly demonstrated at this section, as a result of the forced regression which led to deposition of dolomites, the Najiao Formation and the palaeosol beds at the top of SQ7.Third,the Devonian strata directly overlie the Cambrian Huangdongkou Formation, and the latter is marked by basinal turbidite facies.

Fig.6 Diagram showing the third-order sequences at Liujing in Heng’an County, Guangxi.Apart from those lithostratigraphic units illustrated in the figure, HDK-Huangdongkou Formation(Cambrian)and RX-Rongxian Formation (Devonian).Abbreviations for the sedimentary facies are as follows: Sup-supratidal flat facies, It-intertidal flat facies, Tf-tidal-flat facies, Per-peritidal facies, Op-open-platform facies, Srp-semi-restricted platform facies, Rp-restricted-platform facies, Dr-deep ramp facies,Mr-middle ramp facies, Sr-shallow ramp facies, Sh-shelf facies, Bs-basin facies.Loc refers to the Lochkovian Stage.Other chronostratigraphic abbreviations are the same as those illustrated in Figs.2 to 5; I-the unconformity of the Haikou movement,II-curve of the third-order sea-level changes.

The Lianhuashan Formation, generally belonging to the Lochkovian age, is marked by a set of transgressive sandstones where a few fossils such asLinguulasp.can be collected.This formation could be grouped into two third-order sequences SQ1and SQ2.The basal conglomerates at the bottom part of the Lianhuashan Formation directly lie on the Cambrian Huangdongkou Formation marked by a set of turbidites.This illustrates an unconformity due to the Caledonian orogeny characterized by long-term stratigraphic breaks and an angular unconformity.Many meter-scale cycles of clastic tidal-dynamic type are developed in SQ1and include well-developed trace fossils such asSkolithosin sandstone beds of subtidal flat facies andScoyeniain sandy mudstones of intertidal to supratidal flat facies.In the middle part of SQ2, more argillutites of shelf facies are developed and form the CS of SQ2.The lower and upper parts are mainly composed of sandstones with the development of meter-scale cycles of tidal-dynamic type.Therefore, the strata from SQ1to SQ2form a retrogradational succession.

Two third-order sequences, namely SQ3and SQ4, can be discerned in the strata of the Nagaoling Formation and the basal part of the Yujiang Formation The TST and EHST of SQ3are chiefly composed of shales of shelf facies with storm-reworked shell beds.The LHST is mainly composed of meter-scale cycles of alternating limestone-marl.The features of the TST and the EHST of SQ4are similar to those of SQ3, but the LHST is chiefly composed of thick-bedded sandstones of peritidal facies of the basal part of the Yujiang Formation.

Shelf and basinal mudrock facies in the middle and upper parts of the Yujiang Formation and siliceous dolomites of tidal-flat facies of the Moding Formation constitute SQ5.This sequence is marked by a similar symmetricalsedimentary succession as a result of long-term deepening and then shallowing due to the third-order sea-level change.The lower part of its TST is composed of sandy shales of shoreface facies, but the upper part is mainly a set of shelf mudrock facies with storm-reworked shell beds.Basinal mudrock facies occurs in the upper part of the TST of SQ5, and forms the CS.The EHST of SQ5is chiefly composed of shelf mudrock facies with thin interbeds of sandstones or limestones, and the LHST is composed of dolomites (the Moding Fm.).In the sequence SQ5, the relatively large thickness of the TST, CS and EHST that are mainly composed of shelf and basinal mudrock facies, and the relatively small thickness of the LHST composed of dolomites of tidal-flat or restricted-platform facies, implies an obvious large-scale transgressive event that leads to the clear differentiation of facies in space in the research areas.Different from the underlying strata, the LHST dolomites of SQ5with intensive dolomitization and silicification demonstrate that shallowing of the environment resulted from a third-order sea-level fall.

The Najiao Formation is composed of dolomites about 280 m in thickness, and 2 third-order sequences SQ6and SQ7can be discerned there.Brachiopods includingEo?spirifer,ZdimirandLuofugiaindicate an Eifelian age(Middle Devonian).As shown in Fig.6, the boundary between the Lower and Middle Devonian clearly lagged behind the changing depositional surface at the top of SQ5.The general characters of SQ6and SQ7can be described as follows: in the deepening period of the rise of the third-order sea-level, meter-scale peritidal carbonate cycles are composed of thick-bedded massive biohermal dolomites with interbeds of medium-to thin-bedded micritic dolomites.Hummocky and wavy stromatolites are present within the biohermal dolomites.With the shallowing of the environment due to the fall of third-order sea-level, the intensity of dolomitization increased, and fossils become rare; consequently, the meter-scale cycles are mainly thin-bedded, with clear evidence of exposure.More remarkably, palaeosol beds of 15 to 25 cm thick,marked by purple ferruginous mudstones, are developed in the top part of SQ7, and reflect the unconformity produced by the Haikou movement.

Fig.7 Diagram showing the third-order sequences at Luofu in Nandan County, Guangxi.Apart from the lithostratigraphic units named in the former figures, YL-Yilan Formation, XSDXiangshuidong Formation, LZ-Luzhai Formation (Carboniferous).Abbreviations of sedimentary facies are as follows:Bs-inter-platform basinal facies, Sh-inter-platform shelf facies, Sf-shoreface facies, Fs-foreshore facies.The chronostratigraphic abbreviations and indications from I to IV are the same as those illustrated in Figs.2 to 6.SQ1 to SQ13 refer to the 13 third-order sequences recognized in the section.

Abundant fossils are developed in the Mintang Formation including brachiopodsStringocephalusandLeiorhynchus,coralsStringophyllum,Spinophyllum,Cystiphylloides,PhillipsastraeaandTruncicarinulum, stromatoporoidsPlectostroma,Trumpetostroma,StromatoporaandActi?nostroma, all indicating a Givetian age.Two third-order sequences, SQ8and SQ9are present.The general characters of SQ8and SQ9can be described as follows: their TST are constituted by meter-scale cycles of alternating limestone-marl and shales or marls of deep-ramp or outer-shelf facies as well as nodular micritic limestones of middle-ramp facies; their HST are mainly composed of meter-scale cycles of subtidal carbonates composed of thick-bedded massive stromatoporoid biohermal limestones of shallow-ramp facies with interbeds of thin-bedded marls of deep-ramp facies with minor development of stromatoporoid reefs.Tentaculitids occur in the marls.

4 General features of sequences in inter?platform basins

Inter-platform basins are relatively deep-water regions between the isolated platforms and between them and the attached platforms.Many of these basins are similar to the starved pull-apart basin described by Chenet al.(2001c).The facies are mainly dark mudrocks that can be grouped into inter-platform shelf facies and inter-platform basin facies.The Luofu section in Nandan County is a good example to illustrate the general features.This section is located in a small basin controlled by the contemporaneous“Ziyun-Luodian-Nandan-Duan” fault zone.As shown in Fig.7, the strata at Luofu can be grouped into three parts.The lower part consists of the Lianhuanshan and Nagaoling Formations, marked by transgressive sandstones with brachiopods includingKuangsirhynchus liujingensisandOrientospirifer wangi, indicating a Lochkovian to Pragian age.The middle part is characterised by inter-platform basinal mudrocks, which can be divided into the Yilan,Tangding, Nabiao and Luofu Formations.These range from the Emsian to the Givetian.The upper part is the Xiangshuidong and Daihua Formations, grouped as the Wuzhishan Formation as a whole by some researchers.Basinal shales with siliceous interbeds and inter-platform shelf marls and limestones constitute this part.Thus, within the inter-platform basins, basinal facies are mainly developed during the Emsian to Frasnian time, corresponding to the sedimentary period with obvious differentiation of sedimentary facies in research areas.The strata at Luofu can be divided into 13 third-order sequences from SQ1to SQ13as shown in Fig.7.

There are numerous meter-scale fining-upward clastic cycles of tidal sandstones to tidal flat heterolithics in the Lianhuashan Formation, which show a regular vertical stacking pattern in each third-order sequence: in the TST and EHST, the cycles have more thick-bedded sandstones whereas in the LHST, the cycles are thinner with more heterolithics.On the basis of this stacking pattern of meter-scale cycles, 2 third-order sequences, SQ1and SQ2,can be discerned in the Lianhuashan Formation (Fig.7).

Meter-scale coarsening-upward clastic cycles with wave structures are developed in the Nagaoling Formation.These cyclic sequences are composed of silty shales of shelf facies and beds of sandstones of littoral facies.2 third-order sequences SQ3and SQ4, can be discerned in the Nagaoling Formation.They are characterized by the regular vertical stacking pattern of the meter-scale cycles: in the TST and EHST they are dominated by more thick-bedded silty shales of shelf facies; whereas the LHST consists of more thick-bedded sandstones of littoral facies.Hence,in a similar way to the facies succession of the meter-scale cycles, the successions of the third-order sequences, SQ3and SQ4, are characteristic of upward-coarsening successions, which are reversed to those of SQ1and SQ2in the Lianhuashan Formation.

The Yilan and Tangding Formations constitute sequence SQ5(Fig.7).The Yilan Formation is chiefly composed of black shales of inter-platform basinal facies, which form the TST of SQ5.In the Tangding Formation, the inter-platform basin facies consist of meter-scale cycles of alternating micritic limestones and silty marls.From the base to the top, the micritic limestone beds become thicker and lenses of storm-reworked shell beds become more common.They belong to the inter-platform shelf facies, and the HST of SQ5is mainly made up of them.Tentaculites such asNowakia subtilisandNowakia richterioccur in the TST of SQ5, and brachiopods,e.g.,Euryspirifersp.occur in the HST of SQ5, indicating an Emsian age.

Four sequences from SQ6to SQ9can be discerned in the Middle Devonian strata in the Nabiao and Luofu Formations (Fig.7).The Nabiao Formation generally constitutes SQ6.Its TST is chiefly composed of black shales and siliceous mudrocks with a few marl beds and some meter-scale limestone-marl cycles.In its HST more micritic limestone beds and marls are developed.Together with the thin-bedded shales, they constitute meter-scale limestone-marl and subtidal carbonate cycles.There are three sequences, SQ7to SQ9, within the Luofu Formation, as shown in Fig.7.The general features of the facies succession are basinal shales dominating the lower part, deposited during relative sea-level rise, and more sandy shales, muddy sandstones and siltstones of inter-platform shelf facies developed during the sea-level fall.

The Upper Devonian strata include the Daihua and Xiangshuidong Formations, in which four sequences from SQ10to SQ13can be discerned (Fig.7).Planktonic fossils are abundant and include tentaculitids such asNowakia regularisandHomoctenus acutus, as well as ammonoidsProbeloceras applanatum,Manticoceras cordatumandWocklumeria clymenia.The Xiangshuidong Formation consists of siliceous mudrocks of inter-platform basin facies and constitutes the TST of SQ10.The HST of SQ10is the basal part of the Daihua Formation, which is marked by a set of micritic limestones and marls of inter-platform shelf facies.From the middle to the upper part of the Daihua Formation, three sequences from SQ11to SQ13can be discerned.Their general characters are similar to those of SQ10: their TSTs are a succession of meter-scale cycles of limestone-marl alternations constituted by black shales and siliceous mudrocks of inter-platform basin facies with interbeds of micritic limestones of inter-platform shelf facies.Their HSTs are characterised by a succession of meter-scale cycles composed of thin-bedded marls of inter-platform basin facies and medium to thick-bedded micritic limestones of inter-platform shelf facies.

Fig.8 Diagram showing the sequence-stratigraphic framework of the Devonian in the Yunnan-Guizhou-Guangxi area.1 to 5 refer to the typical sections with detailed divisions of the third-order sequences in different sedimentary settings, and their sequence-stratigraphic features are described in the context and their locations are illustrated in Fig.1: 1-The Wudang section in Guiyang, 2-Lüyinqiao in Duyun, 3-Dushan in Guizhou, 4-Luofu in Nandan, 5-De’e in Longlin.The symbols I to IV and the chronostratigraphic divisions are the same as those in Figs.2 to 7; SQ1 to SQ13 refer to the 13 third-order sequences.

5 The Devonian sequence?stratigraphic framework

Through correlation of the sedimentary facies and sequences, a framework of Devonian sequence stratigraphy can be established, as shown in Fig.8 and Fig.9.As described in the above context, meter-scale cycles are similar to parasequences, and different types of meter-scale cycles are constituted by different successions of lithofacies (Meiet al., 2000).Systematic vertical changes in the natures of the meter-scale cycle reflect long-term onlap (transgression/relative sea-level rise)or offlap (regression/relative sea-level fall),i.e., long-term relative sea-level changes(Tucker, 2001).Therefore, the third-order sequence that is constituted by different sedimentary facies successions in different palaeogeographical settings has different vertical stacking patterns of meter-scale cycles.Two kinds of facies-changing surfaces, static and dynamic, as well as two kinds of diachronisms, diachronism of the facies-changing surfaces and diachronism of punctuated surfaces markedby sequence boundaries (Meiet al., 2001)are clearly demonstrated in Fig.9.A map of sedimentary facies and palaeogeography as shown in Fig.1, illustrates the different palaeogeographical settings of sequence-stratigraphic frameworks in the study area.

Fig.9 Diagram showing the sequence-chronostratigraphic framework of the Devonian in Yunnan-Guizhou-Guangxi area.In the figure, two kinds of facies-changing surfaces, static and dynamic, as well as two kinds of diachronisms, diachronism of the facies-changing surfaces and diachronism of punctuated surfaces marked by sequence boundaries are clearly demonstrated.1 to 5 refer to the typical sections with detailed divisions of the third-order sequences in the different sedimentary settings, and their sequence-stratigraphic features are described in the context and their locations are illustrated in Fig.1: 1-The Wudang section in Guiyang, 2-Lüyinqiao in Duyun, 3-Dushan in Guizhou, 4-Luofu in Nandan, 5-De’e in Longlin.The symbols I to IV and the chronostratigraphic divisions are the same as those in Figs.2 to 7; SQ1 to SQ13 refer to the 13 sequences.

Four third-order sequences from SQ1to SQ4composedof transgressive sandstones in the basal part of the Devonian, are distributed mainly in low-land areas (Figs.8, 9).The sandstones gradually wedge out (Figs.8, 9), as a result of the transgression after the Caledonian orogeny from the open sea towards land areas that were located to the north at the time.At Luofu, these strata occur in the Lianhuanshan and Nagaoling Formations, and four sequences from SQ1to SQ4can be discerned.They become thinner and form sequence SQ4at Dushan.However, at Lüyinqiao and Wudang near the margin of the basin, they wedge out.The central parts of most of the isolated platforms also lack these four sequences.These changing characters are clearly illustrated in Fig.9.

As shown in Figs.8 and 9, the strata of SQ5are widespread, as a result of a major transgression at the end of the Lower Devonian.For SQ5, within the inter-platform basin, shales with siliceous interbeds form the TST, and silty marls with shale interbeds of inter-platform shelf facies form the HST as described at Luofu in Nandan (Fig.7).On the attached platform, the facies of SQ5change regularly in space.Transgressive sandstones constitute the TST and open-platform limestones form the HST, as at Dushan (Fig.4).Near the basin margin, such as at Wudang and Lüyinqiao (Figs.2, 3), transgressive sandstones in the lower part of the Mangshan Formation constitute SQ5.On the isolated platforms, such as at De’e in Longlin(Fig.5), the strata of SQ5belong to the Lower Devonian.The TST contains mainly mudrocks of swamp facies,and the HST is mainly composed of open-platform limestones.Along the margin of the isolated platform, for example, at Liujing in Hengxian (Fig.6), the TST of SQ5is composed of silty shales with marly interbeds and lenses of storm-reworked shelfal shell beds.Its CS is marked by basinal shale facies, and sediments similar to the TST form the EHST.The LHST is composed of limestones overlain by siliceous dolomites called the Moding Formation.All these features of SQ5demonstrate that a large-scale transgression took place as a result of a rapid sea-level rise, and that the strata from SQ1to SQ5constitute a retrogradational succession of the third-order sequences.As a result of this transgressive event, the depositional range in the forming period of SQ5became broader than that in the forming duration from SQ1to SQ4, and the facies differentiation that is created by the attached platform-interplatform basin-isolated platform geography also became clearer (Fig.1).

As shown in Figs.8 and 9, the general features of the Middle Devonian sequences from SQ6to SQ9show a transition from clastics to carbonates.This transition is affected by uplift related to the Haikou movement that took place at the end of the Eifelian.Other features related to this tectonic event are the lack of SQ8at the Wudang section, the intensive dolomitization in SQ6and SQ7at Liujing, the presence of transgressive sandstones in the lower parts (TST)of SQ6and SQ7at Wudang and Lüyinqiao, and the limestones in the HST at Dushan.After the regressive event related to the Haikou movement, the region became a mud-free open sea, in which carbonate sediments were deposited and these constitute SQ8and SQ9.This situation continued until the Upper Devonian.Thus, the clastic strata in SQ1to SQ4form the basement to the carbonate platform, which persisted from SQ5to SQ7; the period from SQ8to SQ9marks the more mature stage of the carbonate platform.Fig.1 shows the general background of the sedimentary facies and palaeogeography, which overall is a complex of attached platforms, isolated platforms and narrow inter-platform basins, with reefs developed along some platform margins.

Sequences from SQ10to SQ13gradually wedge out from the open sea towards the basin margin as a result of uplift due to the Ziyun movement at the end of the Devonian(Figs.8, 9).Accordingly, the limestones change into dolomites.These four sequences are dominated by platform carbonate facies.The facies and palaeogeography from SQ10to SQ13are illustrated in Figs.8 and 9.Four episodes are recognized in the Ziyun movement (Meiet al., 2001)with the first and the second in the Upper Devonian represented by the upper boundaries of SQ11and SQ13as at Dushan.With the wedging out from SQ11to SQ13and SQ14(the basal sequence of the Carboniferous)from the open sea to the basin margin, the unconformities of the first, second and third Ziyun episodes are locally superimposed to form one major unconformity at the basin margin.

Sequences SQ8to SQ13constitute a broadly progradational succession.Overall, the 13 Devonian sequences in this region can be grouped into two second-order tectonic sequences, one from SQ1to SQ7and the other from SQ8to SQ13.The boundaries of these two second-order tectonic sequences are respectively as follows: the unconformity of the Caledonian orogeny, the unconformity of the Haikou movement, the unconformity of the second episode of the Ziyun movement.Therefore, this pattern as shown in Fig.10 is different from the sequence stratigraphy of Devonian presented by Ross and Ross (1988)as well as by Johnsonet al.(1985).This might suggest that the third-order relative sea level changes are controlled not only by eustasy but also by regional tectonic movements.

Fig.10 Correlation of sea-level changes between North America (the left of the figure)and the study area (the right of the figure).The curve of the sea-level changes of North America is adapted from Johnson et al.(1985).This correlation indicates that there is not only similarities but also differences.The similarities are simply shown from “[1]” to “[9]”, and the differences are exposed from “(1)”to “(7)”.I to V refer to the regional unconformities: I-the unconformity of the Caledonian orogeny, II-the unconformity of the Haikou movement, III-the unconformity of the first episode of the Ziyun movement, IV-the unconformity of the second episode of the Ziyun movement.

6 Conclusions

Thirteen third-order sequences are recognized in the Devonian strata of Yunnan-Guizhou-Guangxi area and these form two second-order sequences.This pattern is different from the sequence stratigraphy of Devonian strata presented by Ross and Ross (1988)as well as by Johnsonet al.(1985).This might suggest that the third-order relative sea-level changes are controlled not only by eustasy but also by regional tectonic movements.Definitely, further research is required, such as determining the relationship between facies changes and the response of organisms.Two kinds of facies-changing surface as well as two kinds of diachronisms in the stratigraphic records are the key to establish the sequence stratigraphic framework.Meter-scale cycles are the fundamental building block, defined by lithofacies repetitions with different facies in different palaeogeographic settings as described by Meiet al.(2000).There are regular vertical stacking patterns in both thickness and facies within the meter-scale cycles through the third-order sequences.Therefore, it can be concluded that the study of sequence stratigraphy is the study of the response of sedimentation, as well as diagenesis, to the rise and fall of relative sea level.Previously-published research confirms this,e.g., detailed discussions of the working methods and terminology for sequence stratigraphy (see Vailet al., 1977; Van Wagoneret al., 1988; Mitchum and Van Wagoner, 1991; Tucker, 2001), the relationship between sequence stratigraphy and facies models (Walker and James, 1992), and the new and evolving terminology for sequence stratigraphy (Hunt and Tucker, 1992).Therefore,the sequence-stratigraphic framework of the Devonian in the study area provides an example for the further understanding of two kinds of facies-changing surface as well as two kinds of diachronisms in the stratigraphic records.

Acknowledgements

The thesis is part of the project “Regional Geology and the Potential Analysis of Petroleum Exploration in Guizhou and Guangxi” (No.1008/2-6)financed by China Petrochemical Corporation (SINOPEC).

We thank the anonymous reviewers for their encouragement and constructive comments.We aslo thank Dr.Wang Xinwen and Dr.Wang Guibin for their help in fieldwork.The authors hereby thank Professor Wu Yi, Professor Leng Dexun, Professor Dai Shaowu, Professor Li Changquan for their generous help in original data collection.

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