Sequence-Stratigraphic Framework from the Carboniferous to the Permian Chuanshanian Epoch in Dianqiangui Basin and Its Adjacent Areas, Southwest China

INTRODUCTION

A lot of progress has been made in the study of Carboniferous sedimentology and stratigraphy in the Dianqiangui basin and its adjacent areas, which include the comprehensive summaries on chronostratig-raphy and biostratigraphy by Jin et al.(2000) and Kuang et al.(1999); the overall and systematic summaries on Carboniferous lithostratigraphy by Dong (1997), Yin(1997)and Zhao(1996); the comprehensive study on Carboniferous sedimentary facies and paleogeography by Chen et al.(1994)and Feng et al.(1998)etc.. As shown in Fig. 1, the Dianqiangui basin and its adjacent areas include the southeastern part of Yunnan, the southern part of Guizhou and the western part of Guangxi.

Figure  1.  The outline map showing the sedimentary facies and paleogeography of the Carboniferous Yanguanian epoch in the Dianqiangui basin and its adjacent areas. In the figure, the main features of the sedimentary facies and paleogeography marked by"platform-basin-hill-trough" and the general traffic location of the research areas are illustrated; the black triangles refer to the main logged sections: (1)the Wudang Section in Guiyang; (2)the Banzhuang Section in Duyun; (3)the Kaluo Section in Pingtang; (4)the Yongli Section in Nandan; (5)the Dujie Section in Long'an; (6)the Nanpo Section of Jingxi; (7)the Banwang Section in Longzhou. The chief sedimentary-facies zones are: Ca. continental area; Sa. sand beach; Tf. Clastic tidal-flat; Rp. restricted platform; Op. open platform; PS. inter-platform shelf; PM. platform margin; PB. inter-platform basin; Sh. shelf; Ba. relict deep-water basin.

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Chen et al.(1994)discerned three cycles of sealevel changes corresponding to global transgression regression. Research into Carboniferous sequence stratigraphy includes the study by Liu and Li(1994) in the southern part of Guizhou and that by Mei et al.(2001)in the southern part of Guizhou and the northern part of Guangxi. This research provides a useful foundation for the study of Carboniferous sequence stratigraphy in broader ranges. The chronostratigraphical divisions from the Carboniferous to the Chuanshanian epoch in China proposed by Jin et al.(2000)were particularly useful in this research.

The results of a more systematic study in a broader area show that the strata from the Carboniferous to the Chuanshanian epoch of the Permian constitute one second-order sequence, in which 6 third-order sequences could be discerned. The top boundary of this second-order sequence is the unconformity of the main episode of the Qiangui movement and its relative surface, and the bottom boundary is the unconformity of the main episode of the Ziyun movement and its relative surface. 25 third-order sequences from SQ₁ to SQ₂₅ could be discerned in the strata of the Late Paleozoic. Among the 25 third-order sequences, 6 of them could be discerned in the strata from the Carboniferous to the Chuanshanian epoch of the Permian, namely SQ₁₄ to SQ₁₉ (Mei et al., 2004). These six third-order sequences are similar to those of third-order T-R cycles in the Appalachian basin described by Busch and Rollins(1984).

The strata from the Carboniferous to the Chuanshanian epoch of the Permian are constituted by different stratigraphic successions in different paleogeographical backgrounds: in the attached platform they are marked by carbonates with two sets of coal measures; in the isolated platform they are characteristic of carbonates; accordingly in the setting of interplatform ditches they are characterized by dark and fine sediments that can still be grouped into two facies, the inter-platform shelf facies and the interplatform basin facies(Fig. 1). The sedimentaryfacies architectures for the third-order sequences are different in different paleogeographical backgrounds, but the processes of their depositional environmental changes due to the third-order sea level changes are simultaneous in general. Therefore, " the regularity of sedimentary-facies successions in space as well as the synchronism of environmental changes in time" (Mei and Ma, 2001; Mei, 1996)can be described as the elementary feature of the third-order sequences.

It is the key to discern two kinds of facieschanging surfaces(the static facies-changing surface and the dynamic ones)as well as two kinds of diachronisms(the diachronism of punctuated surfaces marked by sequence boundaries and that of facieschanging surfaces)in stratigraphic records(Mei and Ma, 2001; Mei, 1996). All of these features of the third-order sequences are essential factors for the establishment of the sequences framework from the Carboniferous to the Chuanshanian epoch of the Permian in Dianqiangui basin and its adjacent areas of Southwest China.

GENERAL FEATURESOF SEQUENCE STRATIGRAPHY IN THE BACKGROUND OF THE ATTACHED PLATFORM

In the background of the attached platform, two sets of coal measures are always developed in the strata from the Carboniferous to the Chuanshanian epoch of the Permian: one includes the Xiangbai Formation in Guizhou, the Wanshoushan Formation in Yunnan, and the Simen Formation in Guangxi, which developed in the lower part of the Carboniferous; the other is the Liangshan Formation that developed in the top of the Chuanshanian series of the Permian. In general, the strata become thin near the margin of the oldland; correspondingly, more strata become absent and limestones in carbonate strata are changed into dolomites. Three sections are selected to illustrate the basic features of Carboniferous sequences in the attached platform: the Wudang Section in Guiyang, the Banzhuang Section in Duyun, and the Kaluo Section in Pingtang County, Guizhou.

Divisions of Third-Order Sequences at the Wudang Section in Guiyang

At the Wudang Section, the strata from the Carboniferous to the Chuanshanian epoch of the Permian are very thin, and can be grouped into coal measures of the Xiangbai Formation, limestones with interbeds of swamp carbargillites of the Jiusi Formation and the Shangsi Formation, dolomites of the Baizuo Formation and the Huanglong Formation, limestones of the Maping Formation and coal measures of the Liangshan Formation(Fig. 2).

Figure  2.  Division of third-order sequence from the Carboniferous to the Chuanshanian epoch of the Permian at the Wudang Section in Guiyang. Similar to those illustrated in figures from Fig. 3 to Fig. 8, SQ₁₅ to SQ₁₉ refer to third-order sequences that are discerned in the strata from the Carboniferous to the Chuanshanian epoch of the Permian, and the lithological marks are the same as those illustrated in the subsequent Fig. 9. As shown in the figure, each third-order sequence is constituted by a peculiar sedimentary-facies succession and a regularly-vertical stacking pattern of meter-scale cycles. Op. open-platform facies; Sub. subtidal-flat facies; It. intertidal-flat facies; Sup. supratidal-flat facies; Tf. tidal-flat facies; Sw. swamp facies. The chronostratigraphic codes are as follows: Jis. Jiusian stage; Shs. Shangsian stage; Dew. Dewuan stage; Lus. Luosuan stage; Hus. Huashibanian stage; Dal. Dalaian stage; Xid. Xiaodushanian stage, all of the Carboniferous; Zis. Zisongian stage; Lol. Longlinian stage, both of which belong to the Chuanshanian series of the Permian. 5 third-order sequences from SQ₁₅ to SQ₁₉ are discerned and SQ₁₄ is absent. [1] to[6] are as follows: [1] the unconformity of the first episode of the Ziyun movement; [2] the unconformity of the second episode of the Ziyun movement; [3] the unconformity of the third episode of the Ziyun movement; [4] the unconformity of the fourth episode of the Ziyun movement; [5] the unconformity of the first episode of the Qiangui movement; [6] the unconformity of the second episode of the Qiangui movement; [7] the curve of the third-order sea level changes. At this section, unconformities from the first to the third episodes of the Ziyun movement([1], [2] and[3])superimpose into one unconformity.

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The Tangbagou Formation of the Tangbagouan age, Yanguanian epoch is absent in this section, as is the third-order sequence SQ₁₄. The coal measures of the Xiangbai Formation that make up SQ₁₅ directly cover on the top Devonian. Its lower to middle part is composed of muddy stones and silty shales with interbeds of coal seams and streaks; its upper part is marked by a set of carbargillites of swamp-facies with paleosol caps. Therefore, it can be concluded that the bottom boundary of SQ₁₅ is the unconformity superimposed by those unconformities from the first episode to the third episode of the Ziyun movement, in which SQ10to SQ₁₃ of the Devonian as well as SQ₁₄ of the Carboniferous are absent. The top boundary of SQ₁₅ is an unconformity of the fourth episode of the Ziyun movement.

The Jiusi Formation and the Shangsi Formation constitute SQ₁₆, in which a particular kind of meterscale cycles of the carbonate peritidal type is developed. This kind of meter-scale cycles is composed of micrites of open-platform facies, marls of restrictedplatform facies and carbargillites of swamp facies. The elementary feature of SQ₁₆ can be described as follows: in the deepening process of the sedimentary environment due to therise of the third- ordersea- level changes, thicker limestone beds were formed; in the shoaling process of the sedimentary environment due to the fall of the third-order sea-level changes, thicker carbargillite beds were developed. Thus, SQ₁₆ is characterized by a generally upward shoaling succession of sedimentary-facies.

A set of dolomites, namely, the Baizuo Formation, constitutes SQ₁₇, in which are developed a lot of meter-scale cycles of the carbonate peritidal type as described by Mei et al.(2000). They are constituted by several units of lithofacies, such as micritic dolomites of intertidal-flat facies, argillaceous dolomites of supratidal flat facies and dolomitic mudstones of lagoon facies. In the rising process of the third-order sea-level, meter-scale cycles were developed with thicker bases constituted by micritic dolomites; in the falling process the base became thinner; as a result, the generally upward shoaling facies-succession of SQ₁₇ was formed. According to the regional dolomitization in research areas represented by lots of dolomites in SQ₁₇, the corresponding strata are the Baizuo Formation in Guizhou and the Dapu Formation in Guangxi. The top boundary of SQ₁₇ could be defined as an unconformity of the first episode of the Qiangui movement by Mei et al.(2004, 2001).

The Huanglong Formation constitutes SQ₁₈, in which are developed a lot of meter-scale cycles of the carbonate peritidal type that are composed of several units of lithofacies such as bioclastic limedolomites of subtidal flat facies, micritic dolomites of intertidal flat facies, argillaceous dolomites of supratidal flat facies, and dolomitic mudstones of lagoon facies. The meter-scale cycles in the third-order sequence of SQ₁₈ are of a regularly vertical stacking pattern, which leads to a generally upward shoaling faciessuccession similar to that of SQ₁₇.

In this section, the bioclastic limestones of openplatform facies of the Maping Formation and the coal measures of the Liangshan Formation constitute SQ₁₉. The Maping Formation is 2 m to 3 m thick. Fossils, such as Pseudoschwagerina sp. can be collected. Its duration belongs to the Zisongian age of the Permian Chuanshanian epoch. This set of limestones constitutes the transgressive system tract (TST)of SQ₁₉. The Liangshan Formation has two parts: the lower part is a set of transgressive sandstones that constitute the early high-stand system tract(EHST)of SQ₁₉, and the upper part, marked by a set of coal measures with paleosol caps, constitutes the late high-stand system tract(LHST) of SQ₁₉. The lower part of the Maping Formation of the Xiaodushanian stage of the Carboniferous might be absent at the Wudang Section. This shows that the bottom boundary of SQ₁₉ is an obvious hiatus surface at the Wudang Section. In the Liangshan Formation, the succession characterized by the" transgressive sandstones-coal measures-paleosol beds" illustrates that the top boundary of SQ₁₉ is an unconformity of the second episode of the Qiangui movement that occurred in the transitional period between the Chuanshanian epoch and the Yangsingian epoch of the Permian.

Divisions of Third-Order Sequences at the Banzhuang Section in Duyun

The general stratigraphical and sedimentological characteristics at the Banzhuang Section in Duyun are similar to those at the Wudang Section in Guiyang. SQ₁₄ that is composed of the strata of the Tangbagou Formation is absent, and the Maping Formation is also thin(3 m to 5 m). Differing from those at the Wudang Section, the Jiusi Formation and the Shangsi Formation at this section are made up of a set of dolomites and the lower part of the Xiangbai Formation is composed of a set of transgressive sandstones (Fig. 3).

Figure  3.  Divisions of the third-order sequence at the Banzhuang Section in Duyun. The general features are the same as those at the Wudang Section in Guiyang illustrated in Fig. 2, except that the thickness of the Xiangbai Formation is greater and that of the Liangshan Formation is thinner. The chronostratigraphic codes and the meanings of[1] to[7] are the same as those illustrated in Fig. 2.

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The Xiangbai Formation constitutes SQ₁₅, in which several units of lithofacies can be discerned, such as sandstones of subtidal-flat facies, muddy sandstones of intertidal-flat facies, carbargillites of swamp facies, coal seams and streaks of swamp facies. These units of lithofacies constitute meter-scale cycles of clastic tidal-dynamic types. In the deepening process of the sedimentary environment responding to the rise of the third-order sea-level changes, the sandstone bed as the base of meter-scale cycles became relatively thick; in the shoaling process of the sedimentary environment responding to the fall of the third-order sea-level changes, the sandstone beds became thin, but the coal seams and streaks became thick. Therefore, the third-order sequence of SQ₁₅ is characterized by a generally upward shoaling succession. The paleosol beds developed in the top of the sequence demonstrate that the top boundary of SQ₁₅ is an exposed surface corresponding to the fourth episode of the Ziyun movement.

The Jiusi Formation and the Shangsi Formation constitute SQ₁₆, in which are developed several units of lithofacies as follows: limedolomites and dolomitic limestones of subtidal-flat facies, micritic dolomites of intertidal-flat facies, muddy dolomites of supratidal-flat facies, dolomitic mudstones of lagoon facies. These units of lithofacies make up the meterscale cycles of carbonate peritidal types. The meterscale cycles in the third-order sequence of SQ₁₆ are of a regularly vertical stacking pattern, and are marked by upward thickening dolomitic argillites beds and upward thinning limedolomites and dolomitic limestones beds.

The Baizuo Formation constitutes SQ₁₇, which is characterized by intensive dolomitization. The top boundary of SQ₁₇ is an exposure surface corresponding to the unconformity of the first episode of the Qiangui movement. SQ₁₈ is composed of the Huanglong Formation, which is also marked by a set of dolomites that is similar to that of SQ₁₇, but the intensity of dolomitization is relatively weaker than that of SQ₁₇.

Similarly to that at the Wudang Section, the limestones of the Maping Formation constitute the TST of SQ₁₉; the transgressive sandstones in the lower part of the Liangshan Formation form the EHST; the coal measures with paleosol caps in the upper part of the Liangshan Formation make up the LHST of SQ₁₉. The top boundary of SQ₁₉ is an exposure surface corresponding to the unconformity of the second episode of the Qiangui movement. We think that the boundary between the Maping Formation and the Liangshan Formation is a conversional sedimentary surface formed in the transgressive process that is genetically related to the rise of the thirdorder sea-level changes SQ₁₉, which is the result of the transformation process of the sedimentary environment from clean to muddy with the advance of the coast line from the open sea to the oldland. This opinion is different from the traditional one, which proposes that the boundary between the Maping Formation and the Liangshan Formation is an unconformity of the Qiangui movement(Dong, 1997; Yin, 1997).

Divisions of Third-Order Sequences at the Kaluo Section in Pingtang

The Carboniferous strata at the Kaluo Section are up to 1 700 m thick. Compared with those at the Wudang Section and the Banzhuang Section, they are the thickest in the setting of the attached platform. Besides, more limestones exist in the carbonate strata; the strata of the Tangbagou Formation are present(Fig. 4).

The third-order sequence of SQ₁₄ consists of the Tangbagou Formation, in which are developed a lot of meter-scale cycles of the carbonate peritidal type that are constituted by several units of lithofacies as follows: thick-bedded bioclastic limestones of openplatform facies, dolomitic limestones and limedolomites of semiopen-platform facies, micritic dolomites of restricted platform facies. There is a regularly vertical stacking pattern for meter-scale cycles in the third-order sequence of SQ₁₄ : in the deepening process as the response to the rise in the third-order sea level, the bioclastic limestone beds of openplatform facies became thick and were developed more widely; coral fossils such as Pseudourallina sp. can be collected, so the meter-scale cycles are marked by a thick base. In the shoaling process as the response to the fall of the third-order sea level, the micritic dolomite beds of restricted platform facies became thick and were developed more widely. Thus, SQ₁₄ is characteristic of a generally upward shoaling succession of sedimentary facies; its bottom boundary is an exposed punctuated surface corresponding to the second episode of the unconformity of the Ziyun movement, and the top boundary corresponds to the unconformity of the third episode of the Ziyun movement.

SQ₁₅ is constituted by the Xiangbai Formation that can be grouped into two parts: the lower part is a set of transgressive sandstones; the upper part is a set of coal measures. In the top part of SQ₁₅, a set of bauxitic-ferruginous rock-series, 2 m to 5 m thick, constitutes a typical paleosol bed. This demonstrates that this boundary is an obvious exposure surface corresponding to the unconformity of the fourth episode of the Ziyun movement.

Limestones of open-platform facies of the Jiusi Formation and the Shangsi Formation constitute the TST and EHST of SQ₁₆. The thick-bedded limestone beds of open-platform facies and the thin-bedded carbargillites of swamp facies make up a lot of meterscale cycles of carbonate peritidal types in the Jiusi Formation. In the Shangsi Formation meter-scale cycles are composed of thick-bedded limestones and thin-bedded dolomitic limestones. The strata of restricted- platform micritic dolomites belonging to the bottom part of the Baizuo Formation constitute the LHST of SQ₁₆, which demonstrates that the top boundary of SQ₁₆ is an exposure surface similar to the type Ⅱ sequence boundary defined by Vail et al.(1977). Fossils of Fusulinida such as Eostaffella hohsienica, Eostaffella mosqensis etc. can be collected in SQ₁₆, revealing that the formation duration of the sequence generally belongs to the Jiusian age and the Shangsian age of the Datangian epoch.

SQ₁₇ is constituted by the middle and upper parts of the Baizuo Formation, in which are developed meter-scale cycles of the carbonate peritidal type that consist of several units of lithofacies as follows: bioclastic limestones of open-platform facies, limedolomites of semi-restricted platform facies, and micritic dolomites of restricted platform facies. In the rising period of the third-order sea level more limestone beds were developed; otherwise, more dolomite beds were developed. Fossils of Fusulinida such as Eostaffella postmoquensis, Pseudostaffella sp. etc. can be collected in SQ₁₇, which shows that the duration is chiefly of the Luosuan age including the early Huashibanian age of the Weiningian epoch. Similar to other regions in the research area, the intensive dolomitization that occurs in the upper and top parts of SQ₁₇ indicates that the top boundary of SQ₁₇ is an obvious exposure surface corresponding to the unconformity of the first episode of the Qiangui movement.

Figure  4.  Divisions of third-order sequences from the Carboniferous to the Chuanshanian epoch of the Permian at the Kaluo Section in Pingtang. Each third-order sequence is constituted by a particular sedimentary-facies succession and a regular vertical stacking pattern of meter-scale cycles. The codes of sedimentary facies are as follows: Gb. grain beach; Bh. bioherm limestone; others are the same as those illustrated in Fig. 2. Tab represents the Tangbagouan stage, other chronostratigraphic codes are the same as in Fig. 2, SQ₁₄ to SQ₁₉ refer to 6 third-order sequences that are discerned in the strata from the Carboniferous to the Chuanshanian epoch of the Permian.[ 2] to[ 7] are the same as in Fig. 2.

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At the Kaluo Section, the Huanglong Formation constitutes SQ₁₈, which is marked by thick-bedded bioclastic limestones and packstones of open-platform facies with interbeds of thin-bedded dolomitic limestones of semi-open platform facies. These two lithofacies units form a lot of meter-scale cycles of the carbonate peritidal type. More dolomitic limestone beds are developed in the top of SQ₁₈, which indicates that the top boundary of the sequence is an exposure surface similar to that of typeⅡ defined by Vail et al.(1977). Two biozones of Fusulinida such as Fusu linella and Fusulina can be divided in SQ₁₈, which shows that the duration of SQ₁₈ belongs to the late Huashibanian age and the Dalan age of the Weiningian epoch.

The Maping Formation and the Liangshan Formation constitute SQ₁₉. The Maping Formation is an inter-system lithostratigraphic unit. Its lower part, where there are fossils of Fusulinida such as Triticites etc., belongs to the Xiaodushanian stage of the Carboniferous Mapingian series; its upper part, where there are fossils of Fusulinida such as Pseudoschwagerina, Robustoschwagerina, Sphaeroschwagerina etc., belongs to the Zisongian stage of the Permian Chuanshanian series. A lot of meterscale cycles of the carbonate peritidal type that are composed of thick-bedded and massive bioherm limestones and thin-bedded bioclastic micrites with weak dolomitization in the Maping Formation constitute the TST(the transgressive system tract) of SQ₁₉; the coal measures with paleosol caps of the Liangshan Formation form the HST(the high-stand system tract) and EHST of SQ₁₉. The paleosol beds that consist of purple-red ferruginous sandy mudstones illustrate that the top boundary of SQ₁₉ is an exposure surface corresponding to the unconformity of the second episode of the Qiangui movement. Therefore, it could be concluded that the duration of the Liangshan Formation at the Kaluo Section generally belongs to the Longlinian age of the Permian Chuanshanian epoch.

GENERAL FEATURES OF THE THIRD-ORDER SEQUENCES IN THE BACKGROUND OF AN ISOLATED PLATFORM

The region from the south-east side of the"MileShizong-Pu'an" fault zone and the south-west side of the"Ziyun-Luodian-Nandan-Du'an" fault zone to the "Qing-Fang(Qingzhou-Fangcheng)relict trough" or the" Qinzhou basin", is the Youjiang basin defined by Chen et al.(1994), the Dianqiangui basin and the Qingzhou basin by Zhao(1996)or the Nanpanjiang basin by Enos et al.(1998), in which are distributed several isolated carbonate platforms, large and small (Fig. 1). Being far away from the land-source areas, the strata from the Carboniferous to the Chuanshanian epoch of the Permian are made up of a set of carbonate rocks without the development of clastic rocks, especially coal measures. Several sections, such as the Dujie Section in Long'an, the Nanpo Section in Jingxi, and the Banwang Section in Longzhou, are selected to illustrate the general features of the third-order sequences in the strata from the Carboniferous to the Chuanshanian epoch of the Permian in the study area.

Divisions of Third-Order Sequences at the Dujie Section in Long'an

In Long'an County of Guangxi and its adjacent regions, the strata of the Carboniferous are well developed and are biostratigraphically and lithostratigraphically described in detail by Kuang et al. (1999). The strata can be grouped into several lithostratigraphic units: the Long'an Formation, the Du'an Formation, the Dapu Formation, the Huanglong Formation and the Maping Formation(Fig. 5).

Figure  5.  Divisions of third-order sequences from the Carboniferous to the Chuanshanian epoch of the Permian at the Dujie Section in Long'an. 6 third-order sequences from SQ₁₄ to SQ₁₉ are discerned at this section; they are marked by regular vertical stacking patterns of meter-scale cycles and are constituted by different sedimentary-facies successions. The stratigraphic codes and the meanings from[ 2] to [7] are the same as those illustrated in Fig. 2. Sh. shelf facies; Sl. slope facies; Bh. bioherm limestones of open-platform facies; Gb. grainstones of beach facies; Op. open-platform facies; Srp. semirestricted platform facies; Rp. restricted-platform facies.

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Two third-order sequences, SQ₁₄ and SQ₁₅, could be discerned in the lower and middle parts of the Long'an Formation. SQ₁₄ consists of lots of meter-scale cycles of the carbonate subtidal type that are constituted by micrites with interbeds of grayblack shales, in which the beds of micrites become thicker upwards. On the top are developed brecciola beds that form the cap of meter-scale cycles of the carbonate deep-water asymmetrical type. Therefore, the sedimentary succession of SQ₁₄ is marked by a generally upward shoaling from the shelf to the slope, in which coral fossils such as Pseudouralina can be collected. Thus, the duration of SQ₁₄ generally belongs to the Tangbagouan age of the Carboniferous Yanguanian epoch.

The third-order sequence of SQ₁₅ is of a regularly vertical stacking pattern of meter-scale cycles: the lower part of which is composed of a lot of meterscale cycles of the carbonate subtidal type that demonstrate the obvious" drowned beats"; in its upper part lots of meter-scale cycles of peritidal types are constituted by bioclastic limestones of open-platform facies with interbeds of limedolomites and dolomitic limestones of restricted platform facies, which demonstrates" exposed beats". Coral fossils, such as Cyathoclisia and Siphonophyllia, can be collected in SQ₁₅, which shows that the duration of the sequence is from the Jiusian age to the early Shangsian age of the Datangian epoch.

The top part of the Long'an Formation and the lower part of the Du'an Formation constitute SQ₁₆. The top part of Long'an Formation is composed of bioclastic limestones with interbeds of black marls, in which are developed meter-scale cycles of the subtidal type that illustrate the obvious " drowned beats"; this set of strata forms the TST of SQ₁₆. The lower part of the Du'an Formation is a special set of bioherm limestones with the development of a scouring surface filled by lens of oncolites and thrombolites as well as algal mats, which form the EHST of SQ₁₆. In the top of SQ₁₆ are developed limedolomites and dolomitic limestones of restricted platform facies as a result of the shoaling process formed in the forced regression, which reveals the basic features of the LHST. The obvious dolomitization demonstrates that the top boundary of SQ₁₆ is an exposure surface similar to that of typeⅡ sequence boundary defined by Vail et al.(1977). Fossils of Fusulinida, such as Eostaffella hohsinia, are developed in SQ₁₆, and coral fossils, e.g. Thysanophylloides, Yuanophyllom etc. are developed in the lower part of SQ₁₆. This shows that the duration of SQ₁₆ generally belongs to the late Shangsian age and the early Dewuan age of the Datangian epoch.

The top part of the Du'an Formation, marked by thick-bedded to massive bioherm limestones, and the Dapu Formation, marked by more dolomites, constitute SQ₁₇. The former constitutes the TST of SQ₁₇, and the latter forms the HST of SQ₁₇. In the Dapu Formation, the bioclastic limestones of open-platform facies together with dolomites of restricted platform facies constitute a lot of meter-scale cycles of carbonate peritidal types, which demonstrates that the Dapu Formation is characteristic of"exposed beats" in the HST of SQ₁₇ and that the top boundary of SQ₁₇ is an exposure surface corresponding to the unconformity of the first episode of the Qiangui movement. Fossils of Fusulinida of three biozones, such as Eostaffella mosquensis, Eostaffella postmosquensis and Pseudostaffella biozones, illustrate that the duration of SQ₁₇ generally is from the end of Dewuan age of the Datangian epoch to the Huashibanian age of the Weiningian epoch.

The Huanglong Formation forms SQ₁₈. In the TST and the EHST(the early high-stand system tract) of SQ₁₈, the thick-bedded to massive grainstones and packstones and the paleosol beds that are composed of thin-bedded purple-red ferruginous mudstones constitute a lot of meter-scale cycles of the carbonate peritidal type. The basic feature that is marked by the abrupt changing of lithofacies as well as the rapid deepening and shoaling of the sedimentary environment illustrates that these meter-scale cycles were formed in an"ice-house period" during the earth's history(Barnett et al., 2002; Mei et al., 2001; Tucker and Wright, 1990). In the LHST of SQ₁₉, the limedolomites and the dolomitic limestones constitute a lot of meter-scale cycles of the carbonate peritidal type that demonstrate obvious" exposed beats", which still indicate that the top boundary of SQ₁₈ is an exposure surface similar to that of the type Ⅱ sequence boundary defined by Vail et al.(1977). Fossils of Fusulinida, such as Fusulinella and Fusulina etc., can be collected in SQ₁₈, which demonstrates that its duration mainly belongs to the Dalan age, which includes the end of the Huashibanian age and the early Permian Xiaodushanian age.

The Maping Formation constitutes SQ₁₉, which can be grouped into three parts chronostragraphically: in the lower part, fossils of Fusulinida, such as Protricites and Triticites, are of the Xiaodushanian age of the Carboniferous Mapingian epoch; in the middle part, fossils such as Pseudoschwagerina, Robustoschwagerina and Sphaeroschwagerina are of the Zisongian age of the Permian Chuanshanian epoch; in the upper part, fossils of Fusulinida such as Parmirina and Chalaroschwagerina as well as coral fossils such as Kepingophyllum and Wentzellastraea are of the Longlinian age of the Permian Chuanshanian epoch. Therefore, the strata of the Permian Chuanshanian epoch and the late Xiaodushanian age of the Carboniferous Mapingian epoch constitute SQ₁₉. A lot of meter-scale cycles of the carbonate peritidal type form a regularly vertical stacking pattern in SQ₁₉ : the TST and EHST of SQ₁₉ are constituted by the thickbased kind of meter-scale cycles with the development of more thick-bedded to massive bioherm limestones and grain limestones of oncolite and thrombolite; its LHST is composed of thin-based meter-scale cycles with the development of more limedolomites and dolomitic limestones.

Divisions of Third-Order Sequences at the Nanpo Section in Jingxi

The Nanpo Section in Jingxi County has three characteristics distinct from those of other sections: first, a bed of volcanic rock is developed in the lower part of the Carboniferous system; second, the Maping Formation is up to 1 100 m thick, the largest in the study area; third, the top part of the Rongxian Formation belongs to the bottom of the Carboniferous system. Therefore, the strata from the Carboniferous to the Permian Chuanshanian epoch can be grouped into the Rongxian Formation, the Long'an Formation, the Du'an Formation, the Dapu Formation, the Huanglong Formation and the Maping Formation(Fig. 6).

Figure  6.  Divisions of third-order sequences from the Carboniferous to the Chuanshanian epoch of the Permian at the Nanpo Section in Jingxi County of Guangxi. Distinct from other sections, SQ₁₉ constituted by the Maping Formation is up to 1 100 m thick and can be grouped into three fourth-order subsequences. The codes of sedimentary facies and chronostratigraphy are the same as those illustrated in Fig. 5.[2] to[7] are the same as those illustrated in Fig. 2, and[8] refers to the changing curve of the fourth-order sea-level.

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A lot of meter-scale cycles of the carbonate peritidal type are developed in the strata of the top part of the Rongxian Formation, which are composed of thick-bedded to massive bioherm limestones of openplatform facies with interbeds of limedolomites and dolomitic limestones of restricted-platform facies. These meter-scale cycles are of regular vertical stacking patterns in the third-order sequence of SQ₁₄. From the bottom to the top, the limestone beds become thin; however, the beds of dolomitic limstones and limedolomites become thick. Eventually, a generally upward shoaling succession of sedimentary facies is formed. Similar to the bottom boundary corresponding to the unconformity of the second episode of the Ziyun movement, the paleosol bed constituted by purple-red ferruginous mudstones indicates that the top boundary of SQ₁₄ is an exposure surface corresponding to the unconformity of the third episode of the Ziyun movement. Coral fossils, such as Pseudouralinia, can be collected in SQ₁₄. This shows that its duration belongs to the Tangbagouan age of the Carboniferous Yanguanian epoch.

The Long'an Formation constitutes SQ₁₅, in which are developed lots of meter-scale cycles of the carbonate subtidal type that are made up of several units of lithofacies: the gray to black shales with interbeds of the tumor-shaped micrites of shelf facies, micrites and bioclastic micrites of open-platform facies. These meter-scale cycles form a generally upward shoaling succession that are marked by an upward thickening of limestones as well as upward thinning of shales. One 1.5 m to 2.5 m thick volcanic bed composed of basalt and andesitic basalt is developed in the bottom part of SQ₁₅. More limedolomites and dolomitic limestones of restricted-platform facies are developed in its top, which shows that the top boundary of SQ₁₅ is an exposure surface corresponding to the unconformity of the fourth episode of the Ziyun movement. Coral fossils, such as Cyathoclisa, Siphonophyllia, Thysanophylloides etc. can be collected in SQ₁₅, which indicates that its duration generally belongs to the Jiusian age of the Datangian epoch.

The Du'an Formation and the bottom part of the Dapu Formation constitute SQ₁₆. In the Du'an Formation, thick-bedded to massive bioherm limestones of open-platform facies with interbeds of thin-bedded limedolomites and dolomitic limestones of restrictedplatform facies make up lots of meter-scale cycles of the thick-based carbonate peritidal type, which reflects the basic characteristic of non-obvious"exposed beats" in the TST and the EHST of SQ₁₆. In the bottom part of the Dapu Formation, thin-bedded dolomitic limestones and thick-bedded limedolomites constitute the meter-scale cycles of the thin-based carbonate peritidal type, which illustrates the elementary characters of obvious" exposed beats" in the LHST of SQ₁₆ and shows that the top boundary of SQ₁₆ is an exposure surface similar to the typeⅡ sequence boundary defined by Vail et al.(1977). Fossils of coral(e.g. Yuannophyllum)as well as those of Fusulinida (e. g. Eostaffella hohsinenica, Eostaffella mosquensis etc.) can be collected in SQ₁₆, which demonstrates that its duration chiefly belongs to the Shangsian age and the Dewuan age of the Carboniferous Datangian epoch.

SQ₁₇ is composed of the middle to the top of the Dapu Formation, in which are developed lots of meter-scale cycles of the carbonate peritidal type that are constituted by several units of lithofacies: bioherm and bioclastic limestones of open-platform facies, limedolomites and dolomitic limestones of semirestricted platform facies, and micritic dolomites of restricted-platform facies. Intensive dolomitization is the basic characteristic of SQ₁₇, which illustrates that the top boundary of SQ₁₇ is an exposure surface corresponding to the unconformity of the first episode of the Qiangui movement. The main duration of SQ₁₇ belongs to the Luosuan age of the Carboniferous Weiningian epoch.

The Huanglong Formation constitutes SQ₁₈. The TST and EHST are marked by a succession of meter-scale cycles that are composed of massive bioherm limestones of open-platform facies with interbeds of thin-bedded dolomitic limestones of semi-open platform facies. The LHST of SQ₁₈ is characterized by a succession of meter-scale cycles of the thin-based kind that consist of bioclastic limestones and limedolomites. This reveals obvious"exposed beats" and indicates that the top boundary of SQ₁₈ is an exposure surface that is similar to that of the typeⅡ sequence boundary defined by Vail et al.(1977). Fossils of Fusulinida, such as Pseudostaffella, Fusulinella, Fusulina etc. can be collected in SQ₁₈. This reveals that the duration of SQ₁₈ mainly belongs to the Huashibanian age and the Dalan age of the Carboniferous Weiningian epoch, including part of the early Xiaodushanian age in the Mapingian epoch.

The Maping Formation constitutes SQ₁₉, up to 1 100 m thick at the Nanpo Section, in which are universally developed thick-bedded to massive bioherm limestones with scouring surfaces that are filled with lens of oolitic limestones or oncolite and thrombolite limestones. In the TST and the EHST of SQ₁₉, thick-bedded to massive bioherm limestones and thinbedded bioclastic limestones with weak dolomitization make up lots of the thick-based meter-scale cycles. In the LHST more limedolomite beds are developed, which reveal that the shoaling characteristics of the sedimentary environment resulted from the fall of the third-order sea level. This demonstrates that the top boundary of SQ₁₉ is an exposure surface corresponding to the unconformity of the second episode of the Qiangui movement. According to the stratigraphic texture, SQ₁₉ can be divided into three fourth-order subsequences(Fig. 6). Similar to those at the Dujie Section in Long'an, SQ₁₉ includes three parts chronostrati graphically: the lower part with the development of fossils such as Protiticites and Triticites be longs to the Xiaodushanian age of the Carboniferous Mapingian epoch; the middle part developed with fossils such as Pseudoschwagerina, Robustoschwagerina and Sphaeroschwagerina belongs to the Zisongian age of the Permian Chuanshanian epoch; the upper part with fossils such as Parmirina belongs to the Longlinian age of the Permian Chuanshanian epoch.

Divisions of Third-Order Sequences at the Banwang Section in Longzhou

The Carboniferous strata at the Banwang Section have three characteristics. First, the strata corresponding to SQ₁₄ and SQ₁₅ are absent at this section; second, the thickness of the Maping Formation is relatively thick; third, the total Carboniferous system is relatively thin(Fig. 7). Several formations are included in this section: the Du'an Formation, the Dapu Formation, the Huanglong Formation and the Maping Formation. The systematic and convenient description of sequence stratigraphy relies on the description of the Devonian Rongxian Formation. In general, the hiatus of SQ₁₄ and that of SQ₁₅ indicate a particular unconformity of the Ziyun movement.

Figure  7.  Divisions of third-order sequences from the late epoch of the Devonian to the Chuanshanian epoch of the Permian at the Banwang Section in Longzhou County of Guangxi. This section has two features: the relatively thin Carboniferous strata and the long-term hiatus between Devonian and Carboniferous. The codes of sedimentary facies are the same as those illustrated from Fig. 2 to Fig. 6. Frs. Frasnian stage of Devonian; Fam. Famennian stage of Devonian. The meanings of[ 1] to[ 7] are the same as those illustrated from Fig. 2 to Fig. 6.

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At the Banwang Section, a bed of volcanic rocks composed of basalt and basaltic andesite is developed in the carbonate strata of the Devonian Rongxian Formation, in which 4 third-order sequences could be discerned, namely SQ10to SQ₁₃ (Fig. 7). Lots of meter- scale cycles of the carbonate subtidal type, composed of thin-bedded shales of shelf facies and thick-bedded bioclastic limestones or bioherm limestones of open-platform facies, make up the TST and the EHST of SQ10, which illustrate obvious "drowned beats"; the LHST of SQ10is composed of lots of meter-scale cycles of the peritidal type, which illustrate obvious"exposed beats" that are different from the TST and EHST. The TST of SQ11is constituted by bioherm limestones of open-platform facies; its EHST and LHST consist of dolomites, forming a generally upward shoaling succession with more intensively upward dolomitization. The intensive dolomitization and the paleosol caps of SQ11indicate that its top boundary is an exposure surface that corresponds to the unconformity of the first episode of the Ziyun movement. The sedimentary-facies succession of SQ₁₂ is a generally upward shoaling succession constituted by lots of carbonate meter-scale cycles of the peritidal type. The sedimentary-facies succession of SQ₁₃ is similar to that of SQ10. More particularly, a paleosol bed constituted by bauxitic-ferruginous rock series of 1 m to 2 m in thickness is developed on the top of SQ₁₃, which illustrates a long-term hiatus between Devonian and Carboniferous at the Banwang Section. From the lack of Carboniferous strata corresponding to those of SQ₁₄ and SQ₁₅, it can be concluded that the top boundary of SQ₁₃ is a particularly composite unconformity superimposed by several unconformities from the second to the fourth episode of the Ziyun movement.

The lower and the middle parts of the Du'an Formation constitute SQ₁₆, in which coral fossils such as Yuannophyllum etc. as well as fossils of Fusulinida such as Eostaffella mosquensis etc. can be collected. So the duration of SQ₁₆ might be that from the late Shangsian age to the Dewuan age of the Carboniferous Datangian epoch. The massive bioherm limestones of open-platform facies and the thin-bedded dolomitic dolomites constitute lots of meter-scale cycles of the thick-based carbonate peritidal type, which reflects the basic characteristics of non-obvious "exposed beats" in the TST and the EHST of SQ₁₆. The thin-based meter-scale cycles make up the LHST of SQ₁₆, which shows obvious"exposed beats".

The thick-bedded bioherm limestones in the top part of the Du'an Formation constitute the TST of SQ₁₇, and the Dapu Formation mainly composed of dolomites forms the HST of SQ₁₇, in which fossils of Fusulinida such as Eostaffella postmosquensis etc. show that the duration of SQ₁₇ generally belongs to the Luosuan age of the Weiningian epoch. Intensive dolomitization formed in the upper part of SQ₁₇ indicating that its top boundary is an exposure surface that corresponds to the unconformity of the first episode of the Qiangui movement.

The Huanglong Formation constitutes SQ₁₈, in which fossils of Fusulinida such as Profusulinella, Fusulinella, Fusulina show that the duration of SQ₁₈ chiefly belongs to the early part of the Huashibanian age and the Dalan age of the Weiningian epoch. In the TST and EHST of SQ₁₈, thick-bedded to massive bioherm limestones of open-platform facies and thinbedded dolomitic limestones of semi-open platform facies constitute lots of meter-scale cycles of the thick-based carbonate peritidal type, which shows non-obvious" exposed beats". More dolomitic limestones and limedolomites are developed in the LHST of SQ₁₈, which is constituted of lots of thin-based meterscale cycles and reveals obvious"exposed beats".

The Maping Formation is about 560 m thick and constitutes SQ₁₉ at the Banwang Section in Longzhou County. There are many thick-based carbonate meter-scale cycles, constituted by thick-bedded bioherm limestones of open-platform facies with the development of scouring surfaces that are filled by lens of oncolite and thrombolite grainstones, and thinbedded limedolomites or dolomitic limestones of semi-open platform facies, which indicate the characteristics of the non-obvious" exposed beats" in the TST and the EHST of SQ₁₉. In the LHST of SQ₁₉, thin-bedded bioclastic limestones of open-platform facies and middle-bedded limedolomites and dolomites of restricted-platform facies constitute lots of thinbased carbonate meter-scale cycles, in which the very obvious"exposed beats" illustrate the characteristics of an episodic-shoaling process resulting from the fall of the third-order sea-level. They still indicate that the top boundary of SQ₁₉ belongs to an exposure surface corresponding to the unconformity of the second episode of the Qiangui movement. As shown in Fig. 7, the duration of SQ₁₉ might be from the Late Carboniferous Mapingian epoch to the Permian Chuanshanian epoch.

GENERAL FEATURES OF THIRD-ORDER SEQUENCES IN THE BACKGROUND OF AN INTERPLATFORM DITCH

An inter-platform ditch refers to areas in a deepFigure 8. Divisions of third-order sequences from the Carboniferous to the Chuanshanian epoch of the Permian at the Yongli Section in Nandan County, Guangxi. The sedimentary facies codes are as follows: Dsh. deepshelf facies; Ssh. shallow-shelf facies; Bs. basin facies; others are as illustrated from Fig. 2 to Fig. 7. Chronostratigraphic codes and meanings from[2] to[8] are as illustrated from Fig. 2 to Fig. 7. water environment among isolated carbonate platforms as well as areas between isolated platforms and attached platforms, in which dark and fine sediments mainly constitute the strata with partial development of silicalites. The Yongli Section in Nandan County, Guangxi, can be used as an example to illustrate the division of third-order sequences. At the Yongli Section, the strata from the Carboniferous to the Permian Chuanshanian epoch can be grouped into the Luzhai Formation, the Nandan Formation and the Maping Formation, in which 6 third-order sequences can be discerned(from SQ₁₄ to SQ₁₉, Fig. 8).

Figure  8.  Divisi ons of third-order sequences fro m the Carboniferous to the Chuanshani an epoch of the Permian at the Yongli Secti on in Nandan County,Guangxi. The sedi mentary facies codes are as foll ows: Dsh. deepshelf facies; Ssh. shall ow-shelf facies; Bs. basi n facies; others are as ill ustrated fro m Fig. 2 to Fig. 7.Chronostratigraphic codes and meani ngs fro m [2] to [8] are as ill ustrated fro mFig. 2 to Fig. 7.

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Two third-order sequences, SQ₁₄ and SQ₁₅, could be discerned in the lower and middle parts of the Luzhai Formation. The middle- to thick-bedded black shales of inter-platform basin facies and thinbedded micrites or marls of inter-platform shelf facies constitute lots of meter-scale cycles of the L-M type and subtidal type, which show obvious" drowned beats" in the TST of SQ₁₄. In the HST of SQ₁₄, lots of meter-scale cycles of the subtidal type are developed, that are composed of thin-bedded black shales and dark-gray middle- to thick-bedded micrites. The ratio of lithofacies units in thickness of meter-scale cycles is the reverse of the TST, which indicates the general features of the shoaling process of the sedimentary environment resulted from the fall of the third-order sea-level changes in the relatively deep paleogeographical setting.

Because of the isochrone facies-changing relationship with coal measures of the Xiangbai Formation in an attached platform, the strata of SQ₁₅ consist of more silt at the Yongli Section. From the bottom to the top, the beds of silty marls become thick, which demonstrates that SQ₁₅ is marked by the generally upward shoaling sedimentary-facies succession.

The top part of the Luzhai Formation and the bottom part of the Nandan Formation constitute SQ₁₆. Lots of meter-scale cycles of the L-M type constitute the TST of SQ₁₆, which are characterized by alternative beds of thin-bedded black silicalites of inter-platform basin facies as well as thin-bedded black siliceous marls of inter-platform deep-water shelf facies. In the HST of SQ₁₆, thin-bedded black siliceous marls of deep-water inter-platform shelf facies as well as middle- to thick-bedded dark-gray micrites of inter-platform shallow-water shelf facies constitute lots of carbonate meter-scale cycles of the subtidal type, which illustrate the upward shoaling features of a sedimentary environment.

Plankton fossils can be collected in the Luzhai Formation and the bottom part of the Nandan Formation, in which ammonite fossils can be used as the marks to confirm the times of strata that include several biozones as follows: Gattendorfia, Pericyclus, Ammonellipsites, Beyrichoceras, Goniatites, Dombarites, Eumorphoceras etc.. Therefore, it can be concluded that the duration of SQ₁₄ chiefly belongs to the Tangbagouan age of the Yanguanian epoch; that SQ₁₅ generally belongs to the Jiusian age and the early Shangsian age of the Datangian epoch; and that SQ₁₆ mainly belongs to the late Shangsian age and the Dewuan age of the Datangian epoch.

Distinct from the overlying and the underlying strata, a bed of dark limedolomites and dolomitic limestones of 10 m in thickness is developed in the middle part of the Nandan Formation, which forms particular marks to discern the top boundary of SQ₁₇ in the deep-water sedimentary background. This special set of strata constitutes the LHST of SQ₁₇, which is constituted by the imprints of the shoaling process of a sedimentary environment in the setting of an inter-platform ditch resulting from the rise in the crust that is genetically related to the first episode of the Qiangui movement. There are lots of meter-scale cycles of the carbonate L-M and subtidal type in the TST and the EHST of SQ₁₇, which are mainly constituted by the middle- to thin-bedded micrites of shallow inter-platform shelf facies with interbeds of black thin-bedded siliceous marls of deep inter-platform shelf facies. Fossils of Fusulinida such as Eostaffella hohsienica can be collected in the micrites of SQ₁₇, which demonstrates that the duration of SQ₁₇ generally belongs to the Luosuan age of Weiningian epoch that includes the late Dewuan age of the Datangian epoch and the early Huashibanian age of the Weiningian epoch.

The upper part of the Nandan Formation constitutes SQ₁₈, which can be grouped into two fourthorder subsequences. Mudstones and marls of shelf facies that are relatively thick, together with micrites of open-platform facies, constitute lots of carbonate meter-scale cycles of the subtidal type in the relative low-stand system tract(RLST) of subsequences; however, in the relative high-stand system tract (RHST) of subsequences, thick-bedded bioclastic limestones of open-platform facies and thin-bedded calcareous shales and marls of inter-platform shelf facies constitute lots of carbonate meter-scale cycles. The RHST of the second subsequence is relatively thicker. Thus, these two subsequences make up the third-order sequence of SQ₁₈. The general feature of SQ₁₈, marked by relatively more bioclastic limestones as well as relatively few shales, illustrate that the water depth of an inter-platform ditch became shallow during its formation, which also indicates that the intensity of sedimentary-facies differentiations became weak in space. Fossils of Fusulinida, such as Pseudostaffella, Profusulinella, Fusulinella, Fusulina and Protriticites can be collected in the strata, which demonstrates that the duration of SQ₁₈ generally belongs to the Dalan age of the Weiningian epoch, that can include the late Huashibanian age as well as the early Xiaodushanian age of the Mapingian series.

The Maping Formation constitutes SQ₁₉. In the TST and the EHST of SQ₁₉, the middle- to thinbedded black marls or turmor micrites of shelf facies and the middle- to thick-bedded bioclastic limestones or bioherm limestones of open-platform facies constitute lots of carbonate meter-scale cycles of the subtidal type, which show the very obvious" drowned beats". Thick-bedded to massive bioclastic limestones and thin-bedded dolomitic limestones form lots of carbonate meter-scale cycles in the LHST of SQ₁₉, which demonstrates obvious"exposed beats" and that the top boundary of SQ₁₉ is an exposed punctuated surface corresponding to the unconformity of the second episode of the Qiangui movement. Fossils of Fusulinida, such as Triticites, Pseudoschwagerina, Pamirina etc. can be collected in the strata of SQ₁₉, which illustrates that the duration of SQ₁₉ might be from the middle of the Carboniferous Mapingian epoch to the Permian Chuanshanian epoch. To sum up, even in the deep-water background of an inter-platform ditch, the clear dolomizational phenomenon on the top of SQ₁₇ and SQ₁₉ demonstrates the obvious shoaling process of the sedimentary environment that is genetically related to forced regressive events, which becomes the key to confirming the unconformities of the first and the second episodes of the Qiangui movement.

SEQUENCE-STRATIGRAPHIC FRAMEWORK FROM THE CARBONIFEROUS TO THE PERMIAN CHUANSHANIAN EPOCH

Through tracing of sequence boundaries and correlating of sedimentary-facies successions of thirdorder sequences, the sequence-stratigraphic framework from the Carboniferous to the Chuanshanian epoch of the Permian can be established in the study area(Figs. 9 and 10). This framework clearly shows two kinds of facies-changing surfaces, static vs. dynamic, and two kinds of diachronisms in stratigraphic records. The two kinds of diachronisms refer to punctuated surface diachronism, which is genetically related to the dynamic facies-changing surfaces, and the diachronism of facies-changing surface, which is genetically related to static facies-changing surfaces. As shown in Fig. 9, from the Kaluo Section in Pingtang County(far from the oldland) to the Wudang Section in Guiyang(near the oldland), the strata from the Carboniferous to the Permian Chuanshanian epoch become thin; the dolomitizational intensity becomes strong; SQ₁₄ becomes wedged out. In SQ₁₅, which is composed of coal measures of the Xiangbai Formation, and in the HST of SQ₁₉, which consists of coal measures of the Liangshan Formation, the transgressive sandstone series developed in their lower part becomes wedged out near the margin of the oldland, with decreasing thickness. From the margin of the oldland to the open sea, these two sets of coal measures are changed into carbonate strata, and the quality of coal seams or streaks becomes poor. The strata from SQ₁₄ to SQ₁₉ constitute a second-order sequence. The top boundary of the second-order sequence is the unconformity of the main episode(the second episode) of the Qiangui movement, and the bottom boundary is the unconformity of the main episode(the second episode)of the Ziyun movement.

Figure  9.  The sequence-stratigraphic framework from the Carboniferous to the Chuanshan epoch of the Permian in the Dianqiangui basin and its adjacent areas. Two basic features of third-order sequences are clearly demonstrated in the figure: the regularity of sedimentary-facies succession in space and the simultaneity of sedimentary-environment changes in time.(A)to(E)represent the main logged sections developed in different paleogeographical backgrounds with detailed descriptions of sequence stratigraphy in context: (A) Wudang Section in Guiyang; (B) Banzhuang Section in Duyun, Guizhou; (C) Kaluo Section in Pingtang County, Guizhou; (D)Yongli Section in Nandan County, Guangxi; (E)Dujie Section in Long'an County, Guangxi. Their locations are illustrated above in Fig. 1. Chronostratigraphic codes and meanings from[2] to[7] are as illustrated from Fig. 2 to Fig. 8.

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Figure  10.  Chronostratigraphic-sequence framework of the Carboniferous to the Permian Chuanshanian epoch in the study area. In the figure, (A)to(E)refer to the main logged sections as illustrated in Fig. 9, and meanings from[2] to[7] are the same as those illustrated from Fig. 2 to Fig. 9. The age values concerning the main chronostratigraphic boundaries are adapted from the" International Stratigraphic Chart"(Gradstein et al., 2004).

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In the background of isolated platforms, carbonate strata without the development of coal measures make up the third-order sequences from SQ₁₄ to SQ₁₉. SQ₁₉ (which is composed of the Maping Formation) is extensively thick, which is unique to the background of isolated platforms. It could be divided into three fourth-order subsequences as described at the Nanpo Section in Jingxi(Fig. 6)or two subsequences at the Dujie Section in Long'an(Fig. 5). Another feature of sequence stratigraphy from the Carboniferous to the Permian Chuanshanian epoch is that the bottom third-order sequences, such as SQ₁₄ and SQ₁₅, are absent in some isolated platforms. For example, at the Banwang Section in Longzhou(Fig. 7), SQ₁₄ and SQ₁₅ are absent, which reveals an obvious rise in the crust in the transitional period from the Devonian to the Carboniferous resulted from the Ziyun movement that includes four episodes.

In the background of an inter-platform ditch, dark and fine sediments make up third-order sequences from SQ₁₄ to SQ₁₉ as described at the Yongli Section in Nandan County(Fig. 8). More dark and fine sediments such as shales and siliceous mudstones of the inter-platform basin or inter-platform shelf facies are developed in SQ₁₄ to SQ₁₆; more micrites and bioclastic limestones are developed in SQ₁₇ to SQ₁₉. Both of these two groups of third-order sequences constitute a generally upward shoaling succession of sedimentary-facies that makes up a second-order sequence.

The regular vertical stacking pattern for the third-order sequences in the second-order sequence that consists of the strata from SQ₁₄ to SQ₁₉, demonstrates a regular evolutional process. The strata from SQ₁₄ to SQ₁₆ make up a retrogressive succession of third- order sequences, which is clearly indicated by the transgressively wedging-out of these three thirdorder sequences from the open sea to the margin of the oldland. But the strata from SQ₁₇ to SQ₁₉ constitute a progradational succession, which is very clearly illustrated by the progradational wedging-out of the coal measures of the Liangshan Formation in the upper part of SQ₁₉ in the setting of the attached platform. Furthermore, these changing features are reflected in the sequence-chronostratigraphic framework as shown in Fig. 10.

CONCLUSIONS

In the history of the earth, the Carboniferous is a particular time, which is marked by the universal development of glacier deposits on the southern hemisphere. Similar to those strata in other regions on the northern hemisphere, the carbonate strata of the Carboniferous have little reef and form a specially carbonate sedimentary system with high-productivity off shore(Wright, 1994), which is demonstrated by the hugely-thick carbonate strata of the Maping For mation in isolated platforms such as the Nanpo Section in Jingxi and the Dujie Section in Long'an. The special meter-scale cycles of the carbonate peritidal type that are composed of thick-bedded to massive limestones of open-platform facies, and thin-bedded paleosol beds demonstrate the nature of highfrequency sea-level changes marked by the noncontinuity of litho-facies zones and the abrupt changes of facies-changing(Mei et al., 2000; Mei, 1995; Wright, 1991). Furthermore, the large-scope falling of the third-order sea level generally corresponds to the ice ages on the southern hemisphere(Liu and Jin, 1996). As suggested by Izart et al.(2003), many third-order sequences are diachronously controlled both by global eustasy and tectonic movement, which might mean that it is very difficult to establish an isochronous-chronostratigraphic framework represented by global eustasy. Similar to the third-order sequence division proposed by Busch and Rollins (1984), our sequence-stratigraphic divisions show that the forming period of third-order sequences formed in the convergent period of Pangea is more than 10 Ma. This phenomenon could reflect one elementary feature, i. e. the period of the sea-level change cycles formed in the relatively quiet period of tectonic action is more than 10 Ma. The correlation of the third-order sea-level changes between the study area and North America proposed by Ross and Ross(1995) exposes more differences than similarities. The dominance of third-order sea-level changes (more than 10 Ma) corresponds to the non-obvious periods of sedimentary-facies differentiation in the study areas(Fig. 7), and is in great contrast with the high-frequency glaciation cycles described by many geologists such as Barnett et al.(2002), Wright and Vanstone (2001), Ross and Ross (1995), Heckel(1995), Wright(1991)and others. As suggested by Busch and Rollins(1984), this phenomenon could reflect an interesting character of third-order sea-level changes in the relatively stable period of global tectonics corresponding to the convergent period of Pangea. On the basis of the study by Chen and Zeng(1990), Izart et al.(2003)proposed that the sea-level change curves derived from South China are closer to the global eustatic fluctuations than those from other parts of the world, because South China was tectonically stable and located in the Paleotethyan Ocean at the equator during the Permo-Carboniferous. We agree with this proposition made by Izart et al.(2003). Compared with those proposed by Izart et al.(2003), there are some differences between their sequence divisions of the Permo-Carboniferous and those made by us, which might be a result of different criteria on the division of the third-order and second-order sequences. Therefore, many problems remain unsolved, such as the sedimentation response to colony extinction, the relationship between sedimentation and geological events, e.g. glacier events, etc.. Also, the regular vertical superimposition of lithofacies constitutes a variety of meter-scale cycles, and the regular vertical stacking patterns of meter-scale cycles form the thirdorder sequences with the different sedimentary-facies successions in different paleogeographical backgrounds, which are key to discerning third-order sequences.