Permian sequence stratigraphy of slope facies in southern Guizhou ...

Vol. 43

No. 1

SCIENCE IN CHINA (Series D)

February 2000

Permian sequence stratigraphy of slope facies in southern Guizhou and chronostratigraphic correlation SHI Xiaoying (史晓颖), MEI Shilong (梅仕龙) & SUN Yan (孙

岩)

China University of Geosciences, Beijing 100083, China Correspondence should be addressed to Shi Xiaoying. Received January 6, 1999

Abstract The Permian in slope facies area of southern Guizhou consists of 16 third order sequences, which can be grouped as four sequence sets and further into two mesosequences. Most of them are delineated by conformable or submarine erosion surfaces, and well controlled by conodont and fusulinid zones. Four important sea-level falls are recognized in the area, which occur respectively at the basal Sweetognathodus whitei zone, lower S.aff. hanzhongensis zone, middle Jinogondolella xuanhanensis zone and the basal Clarkina yini zone. Among the recognized transgression events, three are regarded as the most significant. They occur respectively at the basal Neogondolella exculptus zone, basal Jinogondolella nankingensis zone and the lower Clarkina postbitteri zone, and are all accompanied by faunal turnovers. The key surfaces in sequences can be used to fine-tune the boundaries of bio-chronozones, stages and series. Keywords: Permian, sequence stratigraphy, slope facies, southern Guizhou, correlation.

The marine Permian is well developed in southern Guizhou and has been studied extensively both in stratigraphy and sedimentology. In the past years, however, most of the studies concentrated on shallow-water facies, mainly from the angle of litho- and bio-stratigraphy. As far as the Permian deep-water facies in southern Guizhou is concerned, emphasis has mostly focused on the establishment of the biostratigraphic successions[1

4]

. This paper will focus on the sequence

stratigraphy of the slope facies, mainly based on the section at Nashui of Luodian County. This has been regarded as one of the most important sections for the establishment of complete marine Permian chronostratigraphic framework in South China, and has the potential to establish the correlation between shallow- and deep-water facies, as well as the chronostratigraphic correlation between the Tethys and other regions[5,6]. 1 Geologic setting Southern Guizhou is situated in southwestern part of the Yangtze Plate. During the Permian, most of the region was on the continental shelf and predominated by carbonate platform environments[5,7,8]. Studies show that there exists a reef belt in southern Guizhou[1,7,8], which goes approximately in a NE-SW direction, extending from Xinyi to Ceheng, Ziyun and Luodian, then southeastward into Guangxi Zhuang Autonomous Region (fig. 1). To the north of the belt is a vast area where the Permian is characterized by shallow water deposits, consisting typically of lightcolored carbonates rich in benthic fossils such as rugosals, brachiopods and fusulinids. Along the

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Fig. 1. A sketch map showing Permian sedimentary facies and paleogeographic outline in southern Guizhou (modified after ref. [1]).

reef belt, the Permian is characterized by sponge-algal reefs or build-ups, as well as thick-bedded grainstone, oolite and bioclastic limestones, yielding prolific benthos. In the area just south of the reef belt, is a platform foreslope belt, in which the deposits are chiefly made up of dark-colored carbonates, intercalated with shale and chert-banded limestone. Lime breccias of slump origin, carbonate turbidites and debris flow deposits are quite common, often associated with argillaceous cherts. In this belt, macrofossils are relatively rare. Fusulinids are abundant, but largely in the gravity flow deposits, while conodonts are well documented in all the rocks. Further south of the belt is the Nanpanjiang Basin, where the Permian is mainly composed of shale, siltstone and cherts. In places, slump limestone breccias and intraclastic marlite can be observed, but generally not as thick as in the foreslope belt, with gravels much smaller in diameter. From the spatial distribution of sedimentary facies it can be seen that during the Permian, southern Guizhou had formed a continental margin. From north to south, the depositional environments clearly changed from carbonate platform interior to rimmed platform margin and foreslope, then to deep-water basin. The Nashui section of Luodian is likely at a lower part of the foreslope to the basin margin, and was previously assigned to black facies area

to distinguish it from the carbonate platform

facies in the north, which is referred to as white facies area

[1,2,7]

.

As to the nature of the Nanpanjiang Basin, opinions are divergent. As no evidence of oceanic and related deposits were found in the region, it was either regarded as an intraplatform basin[1,7]

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or as an intraplate aulacogen[8,9]. In recent years, research has shed light on this topic. In northwestern Guangxi and southeastern Yunnan, large sets of radiolarian chert and pillow larva have been found in several localities about 200 km south to southwestern Guizhou[10]. Radiolarian from the cherts and a few of conodonts from the limestone interbeds have pointed to an age ranging from Carboniferous to Permian. REE and other geochemical analyses of the rocks have suggested an origin of oceanic crust to island-arc[10]. Also thick radiolarian cherts of Permian age have also been found in southwestern Guizhou as well as in the adjacent areas of northwestern Guangxi. These sediments though were deposited in the environment not as deep as those in southeastern Yunnan and southwestern Guangxi, the radiolarian fauna in them also suggest ocean background. Therefore, it is quite possible that during Permian there existed an ocean in southwest Guizhou-northwest Guangxi, which might be the eastward extension of the Paleo-tethys from the ChangningMenglian belt. The Nanpanjiang basin may have been situated at the northern part of the eastward extending seaway (fig. 2) with southern Guizhou on the north continental margin of the ocean. According to the sediments and strata of the Triassic in southwest Guizhou and northwest Guangxi, it is very likely that the eastward extending ocean was closed in Late Triassic as a result of the Indosinian Orogeny. It is generally thought that the Tethys in the Late Paleozoic was a broad shaped

ocean

[11]

V-

wider to the east and

narrowing westward (fig. 2). Recently an archiopelagic model has been proposed Fig. 2. Paleogeographic map showing major blocks of China and for the eastern Tethys[12]. In this model, their positions relative to the Pangea and some other related blocks the Yangtze, North China and the (modified after ref. [11]). CA, Cathysia; I, Indochina; L, Lhasa; NC, Cathaysia plates, as well as the adjacent North China; QI, Qiangtang; QS, Qamodo-Simao; S, Sibumasu; SC, South China; TA, Tarim; WB, Western Burma; WC, western blocks or microplates are all explained as Cimmerian continent; YZ, Yangtze. islands scattered in the Tethyan Ocean. Southwest of the Yangtze plate, it was the Qamdo-Simao-Indochina. Between them there was an ocean hundreds of km across (fig. 2). The present authors partly agree with this model, but think

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separated from the Yangtze plate in the Permian (fig. 2) and there might have only been a seaway between them without oceanic crusts. 2 Stratigraphic and sedimentary characters The Permian at Nashui of Luodian is largely made up of dark colored carbonates continued from the Carboniferous without notable unconformity. For its relative monotonous lithology, no accepted lithostratigraphic subdivisions have been proposed so far, except its lower middle part, which is named as the Nashui Formation[2]. In this paper, a new subdivision, the Naqing Formation, is suggested for the strata between the Maokou Formation (Guadalupian) and the Changxing Formation (Upper Lopingian). This formation is characterized by siliceous shale, claystone and thin bedded chert, intercalated with fine grained lithic sandstone, tuffaceous siltstone and siliceous wackstone in the upper, indicating an origin of deep water basin. Lithogically it is quite distinct from the equivalent Wujiaping Formation of platform carbonates and the Longtan Formation characterized by coal-bearing alternating facies. For the other parts of the Permian, neither the lithostratigraphic subdivisions proposed for the basin facies around Houchang-Shaiwa area nor those for the shallow platform facies in central and southern Guizhou are suitable for this area. We tentatively take the commonly used subdivision names in South China, such as the Maping, Qixia, Maokou and Changxing formations, to this area. In the Nashui section, macro-fossils are rare, but conodonts and fusulinids are prolific. According to the fossil zonation, the regional stages proposed for the Permian of South China[2,5] can be clearly recognized and well correlated with the international standards[6,13] with confidence (fig. 3). In the Nashui section, the Cisuralian (Lower Permian) is quite thick, and includes several formations (fig. 4). The Maping Formation extends from the Upper Carboniferous to Lower Permian, and is relatively uniform in lithology. Its upperpart (Zisongian Stage) is largely composed of limestones, occasionally with thin marlite and shale beds. Bioclastic wackstone and packstone of distal gravity flow origin have been recognized at several horizons. In this part, both fusulinids and conodonts are well documented, pointing to an age of Asselian to middle Artinskian (fig. 3). The Nashui Formation (upper Artinskian to lower Kungurian) has a relatively small thickness and is mainly distinguished by siliceous shale and argillaceous cherts, alternated with limestones. Its lower part, however, is predominated by thin bedded limestone with chert bands and was probably deposited under a starved and oxygen-deficiency condition caused by rapid sea-level rise. The Qixia Formation, which constitutes the major part of the Kungurian, is much thicker than the Nashui Formation and relatively monotonous, mainly consisting of limestones and rarely bearing siliciclastic and chert beds. The Guadalupian (Middle Permian) is characterized by limestone, intercalated with argillaceous cherts. Wackstone, packstone and lime-mudstone are common, bioclastic limestone and a few of grainstone beds can also be observed in places and are probably related with distal gravity flow deposits. In the Guadalupian, fusulinids and conodonts are prolific, but rarely yield

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macrofossils.

Fig. 3. Permian sequence successions in the Nashui section, Luodian, southern Guizhou and their correlation with bio- and chrono-stratigraphic subdivisions. The ages for series and stage boundaries follow those in refs. [13,14].

Lithologically the Maokou Formation (uppermost Kungurian to Capitanian) is relatively diversified, with many interbeds of shale, argillaceous chert and siltstone (fig. 5). Chert nodules and bands are quite common in the limestone. In the middle part, about 45 m massive calcirudite of debris flow origin has been identified, which is roughly equivalent to the lower Wordian and can serve as marking bed in the area. Two groups of gravels are separable in the calcirudite. One of them is 0.5 2 cm and the another 3 8 cm in diameter, both of which are poorly rounded, but moderately sorted. The lower part of the formation (uppermost Kungurian) is characterized by the alternation of packstone, lime mudstone, chert banded limestone and argillaceous chert. Distal

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Fig. 4. Cisuralian (Lower Permian) sequence successions and depositional characters at Nashui of Luodian County, southern Guizhou. Fossils mainly shown at the positions of their first appearances (FAD).

turbidites rich in bioclastics and fusulinids have been recognized in this part. The Lopingian (Upper Permian) is clearly recognized and rich in siliceous sediments. The Naqing Formation consists largely of limy-argillaceous cherts and siliceous limestone in the upper, and siliceous claystone, shale and argillaceous chert in the lower. Siltstone and lithic sandstone interbeds are also developed in its lower part. The Changxing Formation consists of chert-banded wackstone, siliceous packstone and limy cherts, with numerous shale, argillaceous chert and siltstone beds in the middle. The thick bedded limestone in the uppermost Permian is sharply overlain by the Lower Triassic ammonite-bearing shale without transition, suggesting an abrupt raise in sea-level. Only a few conodonts and fusulinids are found in the Lopingian, due to the strong silisification, however, these fossils can hardly be identified precisely. 3

Sequence succession and sea-level change cycles Based on the study of outcrop sequence stratigraphy on this section, 15 third order

depositional sequences (PSq1

PSq15) may be recognized from the Asselian to Lower

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Changhsingian (figs. 3 and 4). The upper Changhsingian has not been studied in detail because of its poor outcrop. Nevertheless, our extensive investigation on the Changhsingian in XinyuanYangchang area, Ziyun County (figs. 1 and 5), has distinguished two well defined sequences

Fig. 5. Guadalupian and Lopingian (Midde and Upper Permian) sequence successions and sedimentary characters at Nashui of Luodian County, southern Guizhou. Fossils mainly shown at the positions of their first appearences (FAD).

(PSq15 PSq16) from the platform margin reef to platform foreslope and then to the basin (fig. 6). Thus the Permian in the slope to basin margin areas of southern Guizhou can be subdivided into 16 third order sequences altogether. According to a recent study, the lower boundary of the Permian is at ca 295 Ma[14], and the top and lower boundaries of the Changhsingian are respectively at 251 and 253 Ma[15]. Therefore, most of the sequences have an average duration of ca 3 Ma for each, but the two Changhsingian sequences are much shorter, about 1 Ma each. The sixteen sequences can be further grouped into four sequence sets (Ss1 Ss4), with Ss1 being largely made up of Upper Carboniferous. Correlation of these sequences with the bio- and chrono-

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stratigraphic subdivisions are shown in fig. 3. 3.1

Sequence subdivision and boundary characters As the studied area is paleogeographically at the lower part of carbonate platform foreslope to basin margin, most of the recognized sequence boundaries either manifest as conformable surfaces or as submarine erosions. Unequivocal subaerial erosion has only been observed at the base of PSq7, of the upper part of the Qixia Formation (fig. 4). Beneath the wavy erosional surface, a caliche of 15 cm thick exists on thick bedded bioclastic calcarenite. Above the surface, a clay bed up to 30 cm thick shows up, indicating terrestrial weathering. The lower LST of PSq7 sequence is characterized by 1.5 m gravel-bearing coarse lithic sandstone to calcarenite of compound composition. The gravel and sands are mainly of quartzite and chert, generally 1

3

mm in diameter, well rounded but poorly sorted. The upper LST consists of thick bedded bioclastic packstone 10 m thick. The basal boundaries of the sequences PSq1, PSq2 and PSq3 are delineated by submarine erosion surfaces (fig. 4). Slight truncations have been recognized there, but no carbonate breccias of debris flow origin are found in their LSTs. Definite submarine erosions are identified respectively at the bases of sequences PSq8 and PSq11, where the LSTs are characterized by massive calcirudite of debris flow origin and clearly truncate the underlying bedded limestone. These aforementioned sequences should be taken as type I. The other sequence boundaries seem to be mainly conformable surfaces, though some of them have been found with distal gravity flow deposits in the LST or SMST. They are chiefly recognized by parasequence stacking patterns and interior facies architectures, as well as by abrupt change in deposition around the boundary. These may tentatively be regarded as type II sequences in this paper, as their spatial distribution has not been properly investigated yet. 3.1.1 Ss1 sequence set. It is made up of the upper part of the Upper Carboniferous and the lower Cisuralian (fig. 4). Two Permian sequences (PSq1 and PSq2) are included in this set. According to fossil zonation[1

4]

and the current Permian subdivision scheme[5,6,13], PSq1 straddles

C/P boundary (fig. 4), with its FFS quite close to the FAD of Streptognathodus nodulinearis, which is slightly above the C/P boundary[4,6,16]. Its LST is composed of 15 m packstone, in which fine intraclastic beds of gravity flow origin have been observed. The intraclast is generally 2

5

mm in diameter, with the maximum size up to 1 2 cm and volume 5% 15%. The sequence boundary of PSq1 (fig. 4) is about 8 m below the FAD of Streptognathodus wabaunensis, but 3 m higher than the FAD of Streptognathodus elongatus. The S. wabaunensis-S. nodulinearis zone was taken as the lowermost Zisongian[3,6] and was also the basal zone for the Asselian in the Urals[17]. At Aidaralash Creek of northern Kazakhstan, the GSSP for C/P boundary, the base of Permian is marked by the FAD of Streptognathodus isolatus, which is thought to be in the evolutionary morphocline of S.wabaunensis and is about 6 m above the FAD of the latter[13,18]. The basal boundary of PSq2 is just between the Streptognathodus barskovi zone and Mesogondolella

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dentiseparata zone. This position is regarded as a turning point in conodont evolution, when conodonts of Carboniferous type were completely replaced by those of the Permian type[19,20]. PSq1 and PSq2 may be taken as the highstand deposits of Ss1 sequence set and were formed during the medium-term sea-level lowering. 3.1.2

Ss2 sequence set. It comprises four third order sequences (PSq3 PSq6) and extends

from the Sweetognathodus whitei zone to the lower part of Sw. aff. hanzhongensis zone or the fusulinid Neoschwagerina simplex-Praesumatrina neoschwagerinoides zone (figs. 3 and 4), roughly covering the interval of the late Artinskian to middle Kungurian[13]. The lower boundary of PSq3 is stamped by a submarine truncation surface. Above the surface, 8 m massive bioclastic packstone occurs as LST, quite distinct from the underlying medium bedded wackstone. The FFS of PSq3 coincides with the base of the Nashui Formation. Sweetognathodus whitei has its FAD 2.2 m below the FFS. The MFS of PSq3 marks one of the most significant transgressions in the Permian and is also accompanied by a faunal turnover, starting the Kungurian. The other three sequences in this set are all well controlled by biozones (figs. 3 and 4) and can be correlated directly with those recognized at Laibin, central Guangxi[19,20]. It is worth noting that in the sequence PSq4, conodont Mesogondolella cf. gujioensis and fusulinid Brevaxina dyhrenfurthi occur just above the MFS (fig. 4), marking the beginning of the Luodianian Stage or the Chihsian Subseries[5,6]. 3.1.3

Ss3 sequence set. It comprises 5 sequences (PSq7 PSq11) and consists of the upper

Qixia Formation and Maokou Formation, roughly equivalent to upper Kungurian and Guadalupian. According to fusulinid zonation[4,5], the sequence boundary of PSq7 is 8 m above the base of Neoshwagerina simplex-Praesumatrina neoschwagerinoides zone, and about 20 m above the FAD of Sweetognathodus aff. hanzhongensis. This boundary represents one of the most significant sealevel falls in the Permian and probably had resulted in important stratal hiatus in shelf region. In the LST of PSq8, abundant fusulinids, such as Afghanella tereshkovae, Neoschwagerina cf. craticulifera and Verbeekina spp. occur, suggesting the Maokouan age. According to conodonts, however, it is far below the FAD of Jinogondonella nankingensis which has been taken as the lowermost zone of Guadalupian[6,13,19] and occurs at the MFS of PSq9 (figs. 3 and 4). The other three sequences, though yielding some conodonts, are mainly dated by fusulinids. Their correlation with biozones is illustrated in figs. 3 and 5. It is worth noting that the strata equivalent to PSq7 and probably also PSq8 seem to be absent in the Urals and in the southwest United States. In these two regions, the Mesogondonella idahoensis zone is generally taken as the upper Kungurian[3, 16], while Jinogondonella nankingensis zone as the lowermost Guadalupian. In Nashui section, however, the sequences PSq7 and PSq8 exist between these two zones (figs. 3 and 4). J. nankingensis has its FAD slightly above the MFS of PSq9 (fig. 5) and M. idahoensis has its FAD at the top of bed 56, just below the base of PSq6. In PSq7 and PSq8, three conodont zones, Sweetognathodus aff hanzhongensis, S.

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hanzhongensis-S. subsymmetricus and Mesogondonella aff. nankingensis, are clearly shown in succession (figs. 3 and 4). As the sequence boundary of PSq7 marks a significant sea-level fall world wide, we suspect that the strata equivalent to PSq7 and PSq8 might be absent in the regions for lower sea level. 3.1.4

Ss4 sequence set. It includes five third-order sequences (PSq12 PSq16). In the Nashui

section only the lower three have been studied in detail, all of them delineated by conformable surfaces at bottom. Owing to the siliciclastic and siliceous deposits, as well as deep-water environments, fossils are rare in this set. The base of PSq12 may lay in the upper Metadoliolina multivoluta zone (fig. 5). The FFS of PSq12 is in good accordance with the base of the Naqing Formation. A correlation with the sequences in Laibin, central Guangxi[19

21]

indicates that the

sequence boundary of PSq12 lays at the middle Jinogondonella xuanhanensis zone and its FFS in the lower Clarkina postbitteri zone, only 0.6 m above the FAD of this species[20,21]. The LST of PSq13 is characterized by 4

5 m fine grained quartz sandstone, quite distinct from the

argillaceous chert below and above, suggesting a sea-level fall. The LST of PSq14 consists of 9 10 m medium to thick bedded tuffaceous siltstone, sharply separated from the TST argillaceous chert. Correlation shows that the basal boundary of PSq13 is likely within the Clarkina asymmetrica zone, while that of PSq14 may sit within Clarkina transcaucasica zone. Both the LST of PSq15 and the HST of PSq14 are characterized by thick bedded limestone. The main difference is that the LST of PSq15 barely has chert bands, but bioclastics and grains, whereas the HST of PSq14 is extremely rich in chert bands and nodules. Such a difference may imply a sealevel drop and an important change in depositional environment. The base of PSq15 probably lays in the middle Clarkina inflecta zone[26,27], but its FFS is close to the base of the Changhsingian, where the fusulinid Gallowaynella sp. occurs (fig. 5). Fig. 6 shows the sequence and sedimentary characters of the Changhsingian in XinyuanYangchang area, Ziyun County, about 70 km northwest of Nashui and paleogeographically more close to carbonate platform (fig. 1). The four sections in fig. 6 constitute a transverse profile from the platform margin to basin and two sequences can be recognized. In the Xinyuan section, the Changxing Formation consists typically of massive sponge-reef limestone, suggesting a rimmed platform margin background. In this section, LST has only been observed in PSq15, where it is composed of collapsed lime breccia from the platform margin reef. Southward to Liugao and Xiayuan, slump lime breccia is very well developed, pointing to a position at carbonate platform foreslope. Gravel in the breccia is generally 2 10 cm in diameter, with the maximum up to 40 cm. In these two sections, HST is often incomplete, being truncated or partially eroded away by the overlying LST slumps. In the Shaiwa section, which is in the basin, deposits are characterized by siliceous shale, cherts, claystone and siltstone. Two major submarine erosions are observed at the lower and upper parts of the 4th Member of Shaiwa Group (fig. 6), forming the sequence boundaries of PSq15 and TSq1 respectively. About 13 m gravel-bearing siltstone to fine-gained

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Fig. 6. Changhsingian sequence successions and sedimentary characters in Xinyuan-Shaiwa area of Ziyun County, southern Guizhou.

lithic sandstone was found in the middle of the 4th Member. This gravel-bearing lithic sandstone bed is of gravity flow origin and may be taken as the LST of PSq16 without much doubt, but no eminent submarine erosion or truncation was recognized at the bottom. It is worth noting that, from the platform margin to basin, LST slumps (PSq15 to TSq1) show a clear tendency of increase in thickness, while the total thickness of each sequence continuously decreasing toward the basin. At the platform margin (e.g. Xinyuan section), LST exists only in sequence PSq15 and is characterized by collapsed lime breccia, but is completely absent in PSq16 and TSq1, both of which are marked by subaerial erosion surfaces at bottom and begin with FFS. Basinward LSTs are well developed in all the other three sequences. According to the fossils

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documented in the Shaiwa section[7], the lower boundary of PSq15 sits at the uppermost part of Wujiapingian and FFS is in accordance with the lowermost Changhsingian. Similarly, the base of TSq1 also lays in the uppermost Changhsingian and the FFS marks the beginning of the Triassic. This phenomenon seems quite common and has been noted widely at the Permian/Triassic boundary[22]. This may imply that new transgression provides the best opportunity for the dispersal of a new fauna and FFS can be used as an event marker in stratigraphic correlation and an important reference in fine-tuning bio-chronozones. 3.2

Sea-level change cycles The sea level oscillations in the Permian are quite frequent, each of the above-mentioned sequences marking a third order cycle, with a sequence set representing a super cycle in sea-level changes. Based on the present study and incorporated with the researches in other areas of South China, it seems desirable to arrange the 16 sequences into 2 mesosequences[23]. The sequence set Ss1 forms the upper part of the Late Carboniferous Mesosequence which has its lower boundary at the basal Huashibanian Stage (about equivalent to the base of Bashkirian), around 315 Ma[14]. The other three sequence sets (Ss2-Ss4) constitute the Permian Mesosequence (or the Yanghsing Sequence[19]), which was formed during a 2nd-order global sea-level cycle and roughly corresponds to the Transpecos Supercycle recognized in the North America[24]. The lower boundary of this Permian Mesosequence is correlated to the eminent discontinuity between the Maping Formation and Liangshan Formation in shelf region, and is widely recognizable in South China, North China, as well as in other regions of the world[5,13,24,25], and was previously taken as the lower boundary of Permian in China. In southern Guizhou, this boundary is clearly shown as paleokarst surfaces in carbonate platform areas (e.g. in Dushan), above which terrestrial sediments are developed with coal-seams. In the foreslope area south of Ziyun, it shows as an obvious submarine erosion surface, above which 40

120 m clastics are deposited as LST wedges. Further

southward to the Nanpanjiang Basin, it becomes a continuity surface, above which a set of thick claystone and shale are developed with abundant ammonoides. Conodont successions in the Luodian section show that the basal boundary of this mesosequence is about 6 m below the FAD of Sweetognathodus whitei, being middle Artinskian in age[13]. The upper boundary of the mesosequence may be set at very late Changhsingian, slightly below the P/T boundary. During this 2nd-order sea-level cycle, the marine transgression reached its maximum at early Guadalupian. Within this Permian Mesosequence, three major transgressions should be particularly emphasized, all of which are marked by sudden development of thin bedded chert or siliceous shale, indicating the maximum flooding deposits for the sequence sets of Ss2 to Ss4 respectively. The first significant transgression occurs at the base of bed 46 (fig. 4), slightly below the FAD of Neostreptognathodus exculptus, and accompanied with an important faunal turnover. At this horizon, schwagerinid fusulinids declined dramatically, and were replaced by Pamirina. In

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conodonts, Sweetognathodus was replaced by Neostreptognathodus, Pseudosweetognathodus and Mesogondolella. The second transgression happens at the base of bed 64 (fig. 5), about 4 m below the FAD of reliable Jinogondolella nankingensis. Here, turnover is also manifested clearly in both fusulinids and conodonts[4,19,20]. The third shows up distinctively at the bed 72-1, close to the base of Jinogondolella postbitteri zone. At this horizon, a profound change in fauna took place. More than 70% and up to 94% conodonts[19,20], and 94% ammonoids[26] in the Guadalupian became extinct or were replaced by new forms. This event has been called the “End-Guadalupian Extinction”[26,27] and is regarded as the first episode of the Late Permian mass extinction[15,27]. It is important to notice that the three transgression events are all closely related to series boundaries. The first significant transgression is in accord with the base of Kungurian and would possibly serve as a boundary to split the Cisuralian into two subseries or independent series[5,6,13]. The second one is quite close to the base of Guadalupian and the third one to the base of Lopingian. Considering the facts that the first appearance of a given fossil species may be influenced considerably by facies and depositional environments, and its first appearance in a particular area may not always represent its true FAD in evolutionary lineage, it would be more practical and preferable to define the boundary of a biochronozone at the FFS or MFS nearest to the first appearance of the fossil species. Theoretically, a major transgression would provide the best opportunity for a new fauna or species to spread and colonize. And practically it has widely been noticed that the sudden occurrence of a fresh fauna is often accompanied by a new transgression. The advantages to define the boundary of a bio-chronozone or stages at the MFS are that this position is easy to recognize and the deposition has widest distribution in space, and therefore is desirable for correlation over broad regions. Therefore, the key surfaces in sequence can be used as important references in defining bio-chronozones and stages. The aforementioned three transgression events have great potential in Permian stratigraphic correlation across continents, and can be used as markers to fine-tune the stage and series boundaries. Acknowledgements This work was supported jointly by the Ministry of Science and Technology (the SSER Project) and the Ministry of Land and Resources (Grant No. 9501105).

References 1.

Zhang, Z. H., Wang, Z. H., Li, C. Q., Permian Stratigraphy of Southern Guizhou (in Chinese), Guiyang: Guizhou Peoples’ Press, 1988, 1 113.

2.

Xiong, J. F., Zhai, Z. Q., Chen, L. Z., Boudary of transitional bed of Carboniferous-Permian in Luodian (black area), Guizhou, in Special Papers for Carboniferous Symposium of China (in Chinese), Beijing: Geological Publishing House, 1987, 234 242.

3. 4. 5. 6. 7.

Wang, Z. H., Early Permian conodonts from the Nashui Section, Luodian of Guizhou, Palaeoworld, 1994, 4: 203. Zhu, Z. L., Zhang, L. X., On the Chihsian successions in South China, Palaeoworld, 1994, 4: 114. Sheng, J. Z., Jin, Y. G., Correlation of Permian deposits in China, Palaeoworld, 1994, 4: 14. Jin, Y. G., Glenister, B. F., Kotlyar, G. V. et al., An operational scheme of Permian chronostratigraphy, Paleoworld, 1994, 4: 1. Geological Survey of Guizhou Province, The Geology of Guizhou Province (in Chinese), Beijing: Geological Publishing House, 1987, 1 698.

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Feng, Z. Z., Jin, Z. K., Yang, Y. Q. et al., Permian Lithological Paleogeography of Yunnan-Guizhou-Guangxi Region (in Chinese), Beijing: Geological Publishing House, 1994, 1 247.

9.

Sha, Q. A., Wu, W. S., Fu, J. M., A Comprehensive Study of the Permian in Guizhou and Guangxi Region

with

Reference on the Oil-Gas Potentials (in Chinese), Beijing: Science Press, 1990, 1 207. 10. 11.

Wu, H. R., Kuang, G. D., Wang, Z. C., A preliminary sutdy of Late Paleozoic tectonic-sedimentary background in Guangxi Province, Scientia Geologica Sinica (in Chinese), 1997, 32(1): 11. Metcalfe, I., Late Palaeozoic-Early Mesozoic evolution and palaeogeography of eastern Tethys, in Proceedings of the International Conference on Pangea and the Paleozoic-Mesozoic Transition (in Chinese) (eds. Yin, H. F., Tong, J. N.), Wuhan: China Univ. Geosci. Press, 1999, 131

12. 13. 14. 15. 16. 17. 18. 19.

20. 21. 22. 23. 24.

133.

Yin, H. F., Wu, S. B., Du, Y. S. et al., South China defined as part of Tethyan archipelagic ocean system, Earth ScienceJour. China Univ. Geosci. (in Chinese), 1999, 24(1): 1. Jin, Y. G., Wardlaw, B. R., Glenister, B. F. et al., Permian chronostratigraphic subdivisions, Episodes, 1997, 20(1/2): 10. Remane, J., International Stratigraphic Chart (Preliminary Edition), IUGS, 1998. Bowring, S. A., Erwin, D. H., Jin, Y. G. et al., U/Pb zircon geochronology and tempo of the End-Permian mass extinction, Science, 1998, 280: 1039. Boardman, D. B., Nestell, M. K., Wardlaw, B. R., Uppermost Carboniferous and Lowermost Permian deposition and conodont biostratigraphy of Kansas, USA, Palaeoworld, 1998, 9: 19. Snyder, W. S., Gallegos, D. M., Sequence stratigraphy along Aidaralash Creek and the Carboniferous/Permian GSSP, Permophiles, 1997, 30: 8. Chernykh, V. V., Ritter, M., Conodont biostratigraphy of Nikolsky section (southern Urals): a progress report, Permophiles, 1996, 29: 37. Mei, S. L., Shi, X. Y., Chen, X. F. et al., Permian Cisuralian and Guadalupian sequence stratigraphy in south Guizhou and central Guangxi and its relation to conodont evolution, Earth Science-Jour. China Univ. Geosci.(in Chinese), 1999, 24(1): 21. Mei, S. L., Zhu, Z. L., Shi, X. Y. et al., Sequence stratigraphy of Permian Lopingian strata in central Guangxi, Geoscience (in Chinese), 1999, 13(1): 11. Wang, C. Y., Wu, J. J., Zhu, T., Permian conodonts from the Penglaitan section, Laibin County, Guangxi and the base of the Wuchiapingian stage (Lopingian Series), Acta Micropalaeontologica Sinica (in Chinese), 1998, 15(3): 226. Tong, J. N., Yin, H. F., The marine Triassic sequence stratigraphy, Science in China, Ser. D, 1998, 41(3): 255. Wang, H. Z., Shi, X. Y., A scheme of the hierarchy for sequence stratigraphy, J. China Univ. Geosci. (in Chinese), 1996, 7(1): 1. Ross, C. A., Ross, J. R. P., Permian sequence stratigraphy, in The Permian of Northern Pangea (eds. Scholle, P. A., Peryt, T. M., Ulmer-Scholle, D.S.), Berlin: Springer-Verlag, 1995, 98 123.

25. 26. 27.

Davydov, V. I., Fusulinid biostratigraphy and the correlation of Mescovian-Guadalupian North America, Tethyan and Boreal (Russian Platform/Uralian) standards, Permophiles, 1996, 29: 47. Zhou, Z. R., Glenister, B. F., Furnish, W. M. et al., Muti-Episodal extinction and ecological differentation of Permian ammonoids, Permophiles, 1996, 29: 52. Jin, Y. G., Zhang, J., Shang, Q. H., Two phases of the end-Permian mass extinction, Can. Soc. Petrol. Geol., 1994, 17: 813.