Death in Guizhou — Late Triassic drowning of the Yangtze carbonate platform

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Sedimentary Geology 118 (1998) 55–76

Death in Guizhou — Late Triassic drowning of the Yangtze carbonate platform Paul Enos a,Ł , Wei Jiayong b , Daniel J. Lehrmann c b

a Department of Geology, University of Kansas, Lawrence, KS 66045, USA Guizhou Regional Geological Survey Team, Bagongli, Guiyang 550005, Guizhou, PR China c Department of Geology, University of Wisconsin-Oshkosh, Oshkosh, WI 54901, USA

Received 9 December 1996; accepted 22 May 1997

Abstract The Yangtze platform in south China formed a stable palaeogeographic element from the Late Proterozoic to the end of the Middle Triassic with deposition of shallow-water carbonates during much of this time. A portion of the Yangtze platform in south-central Guizhou drowned at the transition from Permian to Triassic, as the south-adjacent Nanpanjiang basin encroached about 100 km northward, but a new, stable platform margin was established that persisted through the Early and Middle Triassic. This long history as a stable carbonate platform ended at the transition from the Ladinian to the Carnian. The latest Ladinian rocks, the Yangliujing Formation, are 490 m of shoaling-upward carbonate cycles of grapestone and bioclastic grainstone, fenestral limestone, and stromatolitic dolomudstone, commonly overprinted by extensive subaerial diagenesis. The beginning of the Carnian is marked by a rapid transition to medium-dark-grey, nodular lime mudstones containing ammonoids, conodonts and thin-shelled bivalves, the Zhuganpo Formation. The upper part of this thin pelagic limestone contains many muddy intraclasts, some slightly bored and encrusted, indicating incipient cementation. The overlying Wayao Formation is a condensed black shale with thin interbeds of dark-grey, manganiferous lime mudstone near the base. Ammonoids, conodonts, thin-shelled bivalves, and articulated crinoid stems are abundant. Fine-grained greywacke with sole marks forms prominent bundles within grey, calcareous shale in the overlying Laishike Formation. Ammonoids and thin-shelled bivalves occur sporadically in this 810-m-thick unit. Calcareous shale with thicker-shelled bivalves and packages of cleaner, coarser-grained sandstone characterize the Banan Formation, 460 m thick. The sandstone units generally coarsen and thicken upward, with ripples, medium-scale trough cross-beds, and rare U-tube burrows. Quartzose, coal-bearing siliciclastics 690 m thick form the overlying Huobachong Formation. Thick-bedded, cross-stratified sandstone and conglomerates are amalgamated into thinning- and fining-upward intervals separated by blocky mudstones. This fining-upward motif continues into the overlying Erqiao Formation, but coals are lacking. At the beginning of the Late Triassic (Carnian) the previously stable Yangtze platform, on which peritidal limestones were forming, was drowned and covered by dark lime mud that was cemented into intraclasts and nodular lime mudstone. Black shale and manganiferous pelagic limestone formed a condensed interval, recording maximum submergence. Turbidite sandstone and shale of the Laishike flysch filled the accommodation space of 800 m created during drowning of the Yangtze platform, leading to deposition of shoaling-upward shelf and paralic sandstones and shales, but without significant carbonate production. The succeeding fining-upward siliciclastics are interpreted as braided-stream deposits with coals that mark minor marine incursions. The shallow-shelf and braided-stream deposits form a molasse 1500 m thick.

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0037-0738/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 0 5 - 0

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It was apparently derived from the west, in contrast to the underlying flysch where palaeocurrent directions are from the north or northeast. The entire Yangtze platform became emergent during the Late Triassic and was never submerged again. Subtle local differences in the drowning sequences indicate differential subsidence and suggest that tectonics played a role in the death of the Yangtze platform.  1998 Elsevier Science B.V. All rights reserved. Keywords: carbonate platform; China; drowning; flysch; molasse; Triassic

1. Introduction The Yangtze platform extended essentially across the entire Yangtze plate, west from the present South China Sea to the Longmen Shan, a fore-range of the Himalayas, and north from near the Vietnamese border, beyond the valley of the Yangtze River, to the Qinling Shan (Fig. 1; Wang, 1985; Yang et al., 1986; Enos, 1995). It formed a stable element in the chaotic palaeogeography of China from the Late Proterozoic (Sinian) to the end of the Middle Triassic. Shallow-water carbonates formed over this vast platform during much of this vast expanse of time, from about 850 to 230 Ma. This time interval includes a journey during the late Palaeozoic and early Mesozoic worthy of the explorer’s raft Kon-Tiki, as the Yangtze plate migrated from the northeast margin of Gondwanaland, probably adjacent to Australia, to a suture with the Sino–Korean craton and, ultimately, with Eurasia (Enos, 1995). The Yangtze platform formed a passive margin of the Yangtze plate throughout this journey; it contains the most voluminous and longest record of marine deposition in all of China (Enos, 1995). The southwestern part of the Yangtze plate was less persistently positive; basinal deposition with small, isolated platforms prevailed in the Nanpanjiang basin from the Middle Devonian through the Triassic (Fig. 1). A portion of the Yangtze platform in south-central Guizhou drowned at the transition from Permian to Triassic, as the south-adjacent Nanpanjiang basin encroached about 100 km northward (Figs. 2 and 3; Guizhou Bureau, 1987; Lehrmann, 1993; Enos, 1995). A new, stable, sigmoidal-shaped platform margin was established in south-central Guizhou and persisted through the Early and Middle Triassic (Fig. 3). In southwestern Guizhou, the Triassic platform margin essentially perpetuated the position of the Permian platform margin (Figs. 2 and 3). Deposition on the Yangtze platform continued to be dominated by shal-

low-water carbonates with intermittent terrigenous influx from the west and northwest (Wang, 1985; Wu and Yan, 1987). This long history as a stable carbonate platform ended at the transition from the Ladinian to the Carnian. This paper is an account of the waning phases of carbonate deposition in southcentral Guizhou that marked the death of this great platform and of the subsequent sedimentary history of the area during the Late Triassic when terrigenous sediments encroached onto the platform. 2. Methods The significance of the Middle and Upper Triassic sequence at Longchang, near Zhenfeng, Guizhou (LC, Fig. 3), was recognized during a reconnaissance with geologists from the Chengdu Institute of Geology and Mineral Resources and from the Guizhou Regional Geological Survey Team in September, 1988. The Longchang section has long been famous among Chinese geologists because the entire Triassic sequence is represented there, except for patchy covered intervals. Detailed work to document these initial impressions was first feasible in July, 1995. Sections of the final carbonate deposits and the overlying shale were described at Wanlan (WL), Sanhe (part of the Longchang section, LC), and Huangpingzhai (PZ, Fig. 3). The argillaceous Wayao Formation in the PZ section, now deeply weathered and oxidized, was also measured by Wei in 1991, when the exposure was fresh. Samples from these sections were studied in slab and thin section by Enos at the Institut fu¨r Pala¨ontologie, Universita¨t Erlangen-Nu¨rnberg, Germany. More extensive reconnaissance of the overlying Upper Triassic deposits at Longchang and Duanqiao (DQ) was completed in 1995. The terminal carbonate deposits were examined at Longtou (LT), near Guiyang, and briefly at Duanqiao (DQ) and Hongyan (HY), near Guanling, in 1988, 1993, and 1995 (see Fig. 3).

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Fig. 1. Tectonic elements in southern China (after Sun et al., 1989; from Lehrmann, 1993). The Yangtze block and the South China fold system (‘Huanan block’) may have been a single plate by Triassic time or they may have been sutured beginning in the Late Triassic (Hsu¨ et al., 1988). By the later suturing hypothesis the Yangtze platform would have been a separate platform from that of the South China fold system, and the northern Nanpanjiang basin in Guizhou and the southern part in Yunnan and Guangxi provinces would have been separate entities in the Middle Triassic.

3. The lithologic succession 3.1. Yangliujing Formation The upper Ladinian rocks in the interior of the Yangtze platform are the Yangliujing Formation (Table 1; Fig. 4). This carbonate unit is 490 m thick at Longchang, where it is subdivided into an upper limestone member (162 m) and a lower dolostone member (328 m), and is 318 m thick at Sanqiao, Guiyang (Guizhou Bureau, 1987) The approximately equivalent platform-margin rocks, the Longtou Formation, are 526 m thick at the type locality south

of Guiyang (LT in Fig. 3; Guizhou Bureau, 1987). The platform-interior rocks are light-grey peloidal packstone, wackestone, and minor grainstone that weather almost white. The dominant grains are peloids (fecal pellets and micritized bioclasts) and grapestone. Other intraclasts and coated grains are relatively rare. Biota is sparse; it includes gastropods, bivalve fragments, echinoderms, codiacian and dasycladacian algae, ostracodes, and foraminifers. The conodont Neogondolella foliata inclinata, a Ladinian form, was reported from near the top of the laterally equivalent Longtou Formation (Yang et al., 1995, p. 167). Bedding thickness is typically 1 to 2 m; the

58 P. Enos et al. / Sedimentary Geology 118 (1998) 55–76 Fig. 2. Late Permian facies distribution in southwestern Guizhou province. Compiled from the Geologic Map of Guizhou (1 : 500,000; Guizhou Bureau, 1987). The ‘BASIN’ corresponds to the northern end of the Nanpanjiang basin in Fig. 1. The formations, represented by patterns on the map, are essentially time-equivalent facies.

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Fig. 3. Ladinian (late Middle Triassic) facies distribution in southwestern Guizhou province. Compiled from the Geologic Map of Guizhou (1 : 500,000; Guizhou Bureau, 1987). Position of platform margin was essentially constant through Early and Middle Triassic. The ‘BASIN’ corresponds to the northern end of the Nanpanjiang basin in Fig. 1. Initials indicate locations of measured sections (LC D Longchang, PZ D Huangpingzhai, WL D Wanlan) and reconnaissance traverses (DQ D Duanqiao, HY D Hongyan, LT D Longtou). The formations, represented by patterns on the map, are essentially time-equivalent facies (Table 1): Yangliujing Formation is platform-interior limestone and dolomite; Longtou Formation is platform-margin limestone; Bianyang Formation is terrigenous basinal deposits.

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Fig. 4. Lithostratigraphy of the Upper Triassic succession at Longchang, Guizhou. The upper part of the Middle Triassic Yangliujing Formation is included. Detailed section at right is base of Wayao Formation. Formation names with approximate ages are listed in Table 1.

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Table 1 Stratigraphic nomenclature of the Triassic system, southwestern Guizhou Province a Stage

Platform interior

Platform margin

Basin

Guiyang area (platform)

Rhaetian? Norian Carnian Carnian Carnian Carnian Ladinian Anisian Late Scythian Early Scythian

Erqiao Fm. Huobachong Fm. Banan Fm. Laishike Fm. Wayao Fm. Zhuganpo Fm. Yangliujing Fm. Guanling Fm. Yongningzhen Fm. Yelang Fm.

Erqiao Fm. Huobachong Fm. Banan Fm. Laishike Fm. Wayao Fm. Zhuganpo Fm. Longtou Fm. Poduan Fm. Anshun Fm. Daye Fm.

No strata preserved No strata preserved No strata preserved Laishike Fm.

Erqiao Fm. Hiatus Hiatus Sanqiao Fm.

Bianyang Fm. Bianyang Fm. Xinyuan Fm. Ziyun Fm. Luolou Fm.

Gaicha Fm. Longtou Fm. Poduan Fm. Anshun Fm. Daye Fm.

Units considered in this paper are in italics. a Adapted from Guizhou Bureau (1987) and Yang et al., 1995.

Fig. 5. Laminated cycle cap from Yangliujing Fm. Desiccation cracks and filled fenestral pores are prominent. The spar fillings at the left (arrows) may be evaporite moulds. No other evidence for evaporites was noted in the field. Polished slab from Longchang section (LC, Fig. 3). Scale bar is 5 cm.

range observed is 40 cm to 5.5 m. Many beds or pairs of beds correspond to sedimentary cycles. The lower, thicker, portion of a typical cycle is peloidal packstone or wackestone with sparse burrows and fenestral voids that are increasingly common upward. The thin capping layer is dolomitic lime mudstone or dolostone with laminar or domed stromatolites (Fig. 5), commonly overprinted by fenestral pores, desiccation cracks, sheet cracks, buckled lay-

ers (‘tepees’), and extensive isopachous cement. The uppermost laminated cap is 25 m below the top of the Yangliujing Formation in the WL section and 4.5 m from the top of the PZ section. The metre-scale cycles of the Yangliujing Formation are interpreted as shoaling-upward, subtidal-tosupratidal platform cycles based on the shallowmarine biota in the base, upward increase in fenestral porosity, and the subaerial exposure features in

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the dolomitic cycle caps. The tepee structures overprinted on the cycle caps indicate extended subaerial exposure (cf. Assereto and Kendall, 1971; Dunham, 1972). 3.2. Zhuganpo Formation Medium-grey, nodular-bedded, muddy limestones of the Zhuganpo Formation overlie the cyclic, shallow-platform carbonates. The Zhuganpo is quite persistent in thickness and lithology. It is 55 m thick at PZ, 62 m at WL, and 73 m at LC (Guizhou Bureau, 1987, p. 292). Farther north, it is 70 m thick at Yongningzhen (YZ), 12 km west of Guanling, 120 m at Duanqiao (DQ), 10 km to the southeast, and 88 m at Hongyan (HY), about 10 km farther east (Fig. 3; Guizhou Team, 1980). The irregular, nodular beds average 25 cm in thickness. Most of the measured sections were described as molluscan wackestones or floatstones, but in thin section the thin-shelled bivalves commonly appear to be in contact to form a packstone, although the muddy matrix constitutes as much as 60 to 75% of the volume. Thinshelled bivalves are the dominant skeletal component; they include Halobia cf. subcomata, Placunopsis sp., and Leptochondria sp. (Guizhou Bureau, 1987, p. 292). Other megafossils in order of decreasing abundance are ammonoids, crinoids, gastropods, brachiopod fragments, and fish bones. Gastropods decrease in abundance upward; ammonoids and crinoids, including long, articulated stem segments, increase markedly. Microfossils include many calcispheres, few ostracodes, foraminifers, conodonts, and problematic calcareous tubes. Isolated occurrences are sponges, sponge spicules, possible stromatoporoid and bryozoan debris, and Tubiphytes fragments in the lower part of PZ section. Conodonts from the Zhuganpo Formation, including Neogondolella polygnathiformis, indicate an early Carnian age (Guizhou Team, unpubl. data, 1991; Lehrmann, 1993; Yang et al., 1995), rather than Ladinian, as concluded in earlier work (Guizhou Bureau, 1987, p. 292). Neogondolella polygnathiformis is considered to be Carnian in age by most conodont workers (Sweet et al., 1971; Ziegler, 1977; Clark et al., 1979; Wang et al., 1981), although questionable occurrences have been reported from the late Ladinian (Sweet et al., 1971).

Pellets are a significant component in the basal 10 m of the Zhuganpo Formation. ‘Intraclasts’, a few mm to cm in size, increase in abundance upward and are the dominant non-skeletal component in the upper part (Fig. 6). These are not ordinary intraclasts; they appear to have formed in situ through slight cementation, as incipient hardgrounds. They are emphasized by differential compaction of the slightly argillaceous matrix. Many such intraclasts are indistinct, typically with vague boundaries, even in thin section (Figs. 7 and 8). They are differentiated from the surrounding mud mainly by a slightly darker, denser appearance, both in outcrop and thin section (Figs. 6 and 7). Truncated grains, borings, fractures, and encrustation, features indicative of reworked lithoclasts, are relatively rare. A few intraclasts contrast with the surrounding matrix, either in the concentration of fossils or in the assemblage of fossils. This indicates either some transportation (a true clast) or nucleation of cementation by inhomogeneities, or both. A favoured site for matrix cementation was adjacent to bivalve shells (Fig. 7). A few intraclasts apparently nucleated in and around small, lined burrows (Fig. 8) or ammonoids (Fig. 9). The intraclasts are on a much smaller scale than the nodular bedding, but differential cementation may have been a factor in the bedding configuration as well, as has been suggested in other examples (Wanless, 1979; Ricken, 1986; McNeice, 1987). Pyrite occurs as disseminated microcrystals, framboidal clusters, and a few mm-scale crystals; estimated abundance is up to 2% of the volume of some samples. Chert nodules are reported from the Longchang section (Guizhou Bureau, 1987, p. 291), but observed silicification is confined to very local replacement of bivalve shells by microcrystalline quartz and single pieces of silicified wood in the PZ and WL sections. The Zhuganpo Formation is interpreted as a relatively deep-water deposit, based on the dominance of planktonic and nektonic biota. The presence of the conodont Neogondolella polygnathiformis supports a deep-water interpretation, as it is not found in shallow-marine deposits and is considered to represent a basinal biofacies (Kozur, 1976). The sparse but fairly diverse biota indicates normal bottom water at depths below the photic zone, but above the aragonite compensation depth, as the bivalves and ammonoids are reasonably well preserved. The general lack of re-

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Fig. 6. Irregular bedding surface near top of Zhuganpo Fm. with abundant intraclasts (dark) and incipient cementation of burrows. A few of the darker, apparently better cemented, clasts have sharp corners, encrustations, and edges fretted by borings. Huangpingzhai section (PZ, Fig. 3). Scale is 15 cm.

Fig. 7. Photomicrograph of intraclasts in Zhuganpo Fm. These intraclasts have probably not been transported; note the unbroken bivalve shell which encloses and probably nucleated the lower right clast. Incipient cementation (dark) is apparent adjacent to many bivalves. Arrow points out denser-textured encruster on a bivalve adjacent to cemented mud. Compaction following early cementation is shown by orientation of thin bivalve shells around the intraclasts. Scale bar is 1 mm.

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Fig. 8. Intraclast formed by incipient cementation within and around a burrow lining, possibly displaced (cf. Fig. 6). Photomicrograph; plane light. Scale bar is 1 mm.

Fig. 9. Ammonites within incipient intraclasts. Early cementation is indicated by the lack of compactional flattening of the ammonites (contrast Fig. 12 from overlying Wayao Fm.) and the slight compactional drape in the surrounding matrix. Photomicrograph; plane light. Scale bar is 1 mm.

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working suggests depths below storm wave base. The scarcity of burrows and darker shades of the Zhuganpo limestones, in contrast to the underlying shallow-water limestones, indicate dysaerobic conditions very near, if not at, the sediment surface (Byers, 1977). The intraclasts were largely cemented in situ near the sediment surface, as indicated by sparse borings and encrustations. This indicates slow sedimentation to allow for cementation of the lowpermeability muds before burial. The transition from the Yangliujing Formation to the Zhuganpo Formation is of particular interest because it reflects incipient drowning of the platform with the change from peritidal conditions to deepwater sedimentation. This transition is particularly well exposed in the WL and PZ sections, as well as at Hongyan (HY) and Duanqiao (DQ, Fig. 3), and less so in the Longchang section (Fig. 10). At PZ the contact is abrupt; thin, nodular bedding and darker shades appear at a bedding contact that also marks a sharp reduction in the number of gastropods, thick-shelled bivalves, reworked intraclasts and coated grains, and an increase in mud, incipient intraclasts, thin-shelled bivalves, and crinoids. The uppermost supratidal cap is only 4.5 m below

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this contact. In the WL section, the change from thick-bedded, very light-grey limestone to thin-bedded, medium-grey limestone is equally abrupt. In detail, however, the change to markedly muddier limestone is 1 m higher, and the onset of nodular bedding, as distinct from thin, planar and then wavy bedding, is 4 m higher. The uppermost shoalingupward cycle with a supratidal cap is 25 m below the contact, although a 40-cm interval of whitish, laminated dolomudstone with reworked intraclasts occurs 3.2 m above the contact. 3.3. Wayao Formation The dark pelagic limestones of the Zhuganpo Formation grade rapidly upward to black shale with muddy limestone interbeds of the Wayao Formation (Fig. 4). The Wayao is 26.4 m thick at PZ, 30.4 m at WL, and approximately 60 m at LC (Fig. 10; Guizhou Bureau, 1987, p. 291, reports 138 m of Wayao; but the original, unpublished measurements recognized the basal 60 m as a distinct dark-grey shale and lime-mudstone unit, which agrees with our field observations.) The shale is calcareous, dark grey to black, and distinctly fissile, except for a

Fig. 10. Upper part of Triassic section at Longchang, Guizhou. In the foreground is the upper part of the Yangliujing Fm., overlain by the Zhuganpo Fm. .Z I arrows mark basal contact), the Wayao Fm. .W /; and the Laishike Fm. .L/; with prominent ridges formed by greywacke intervals. The ridge on the distant skyline is formed by sandstone and conglomerate of the Erqiao Fm.

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few structureless intervals. It contains common to abundant thin-shelled halobiid bivalves throughout, some pectinids, and abundant ammonoids at the base, but few toward the top. The limestone distribution varies areally. Only two lenses of laminated argillaceous lime mudstone, each about 30 cm thick, are present at WL. Limestone in PZ section is confined to the lower 8.4 m, but it comprises a third of this interval in beds up to 85 cm thick. At LC muddy, laminated limestone beds up to 20 cm thick make up about a quarter of the interval at the base, but they become much thinner upward; none could be seen in the poorly exposed upper 30 m of the section. All of the limestone contains abundant halobiid bivalves, locally densely packed (Fig. 11), numerous ammonoids (Fig. 12), and patchy concentrations of articulated crinoid columnals. Microfossils include few foraminifers, calcispheres, and rare worm tubes. Fossils identified from the formation (Guizhou Bureau, 1987, p. 291; Yang et al., 1995) include: bivalves, Halobia rugosoides, H. cf. planicosta, H. ku¨i, Daonella acutifominata, Daonella bulongensis bifurcata, Rhaetinoprix avata and Posidonia sp.; ammonites, Protrachyceras costulatum, P.

douvillei, P. deprati, P. yangningensis, Joannites sp., Anolcites sp., Cyrtopleurites socius, C. bicrenatum, Pseudocarnites sp. and Trachyceras multituberculatum; conodonts; and psychotic crinoids, Traumatocrinus hsu¨i and T. cf. candex. Many of the bivalves are less than 2 mm across and have only a few growth lines, indicating juvenile forms (Fig. 12). The ammonoids are as large as 20 cm in diameter, although the modal size is about 5 cm. Black, nodular chert and microquartz replacement of bivalves are rare in the Wayao. Pyrite in the limestone forms bands and clusters of crystals that may reflect burrows. The sooty appearance of the fresh limestone and a pungent hydrocarbon aroma to the shale and limestone indicate that organic carbon content is relatively high. The weathered limestones are adorned with dendrites or stained black with pyrolusite (Fig. 11), reflecting a high content of manganese. The black shales weather reddish brown to yellow brown through oxidation of their appreciable pyrite content. The base of the Wayao Formation is marked by the abrupt appearance of shale; the underlying Zhuganpo Formation is slightly argillaceous, but lacks shale in-

Fig. 11. Densely packed, thin-shelled halobiid bivalves in thin section from the Wayao Fm. The packing reflects slow sedimentation rates of the lime-mud matrix, but the marked horizontal orientation is partly the result of compaction, evident in the draping around the burrow at the base (arrow) and fracturing of the bivalves at left centre. The dark stain of the matrix in the middle layers is from manganese oxides. Longchang section (LC, Fig. 3). Scale bar is 1 mm.

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Fig. 12. Ammonites from bedding plane of mudstone in Wayao Fm. WL section. Small bivalves (arrow) are typical of this unit. Scale bar is 15 cm.

terbeds. The transition from solid limestone to shale takes place over 8.5 m at PZ and nearly 25 m at LC, but limestone is lacking at WL, except for the two lenses mentioned above. The base of the Wayao Formation was previously identified as the base of the Carnian (Upper Triassic; Guizhou Bureau, 1987, p. 676), but the underlying Zhuganpo Formation has been placed in the Carnian as well, based on conodont biostratigraphy (Guizhou Team, unpubl. data, 1991; Lehrmann, 1993; Yang et al., 1995). The black shale interval, which marks the complete and final cessation of limestone deposition on the Yangtze platform in southwestern Guizhou, is highly condensed judging from the concentrations of planktonic and nektonic fossils, of manganese and iron, and of organic carbon in an environment that had little or no bottom life. The shale probably accumulated below the photic zone and certainly in an anoxic environment, as indicated by the concentration of organic matter and absence of bioturbation. It was, however, above the aragonite compensation depth, as the ammonoids and bivalves are exquisitely preserved and are still carbonate within centimetres of the highly weathered outcrop surface. The lack of limestone de-

position resulted from a cessation of the influx of carbonate mud exported from the moribund Yangtze platform, rather than from sea-floor dissolution. 3.4. Laishike Formation Subsequent deposits in southern Guizhou are entirely siliciclastic. Shale deposition of the upper Wayao Formation was sporadically interrupted by thin beds of siltstone and eventually terminated by thick, amalgamated beds of greywacke of fine-sand grade that characterize the Laishike Formation. The basal 45 m of this unit was measured at WL, where it is best exposed, and the remainder of the interval was examined at WL, PZ, and LC (Fig. 3). Grey, yellowish-weathering claystone and mudstone with thin interbeds of siltstone make up well over half of the unit, which is 810 m thick at Longchang. A variety of bivalves, mostly thin-shelled forms (Halobia, Posidonia, Paleoneila, Paleonucula, Chlamys?) and ammonites (Trachyceras, Cyrtopleurites, Paratibetites, Protrachyceras, Traskites, Monophyllites) are reported (Guizhou Bureau, 1987, p. 291). A distinctive feature of the unit is bundles of greywacke

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beds several tens of metres thick that form prominent ridges (Fig. 10). Four such ridges are present at Longchang; the uppermost is 135 m below the top of the formation. Many of the greywacke beds are amalgamated; maximum observed bed thickness was 3.3 m. The greywackes contain quartz of fine-sand size, small lithic fragments, white mica, and intraclasts of mudstone with soft-sediment folds. A few ammonoids, bivalves, and Zoophycos feeding traces are present. The bases of greywacke beds are commonly sharp and the tops gradational to mudstone, but any graded beds are obscured by the fine grain size. Frondescent casts (scour marks; ten Haaf, 1959, p. 30; Potter and Pettijohn, 1963, p. 126) are abundant on the bases of some beds in Longchang. Sparse sole marks, including flutes, grooves, and prod marks, were observed in all exposures. Small-scale current ripples are prominent on large exposures of bed tops above the PZ section. Larger-scale cross-beds are lacking. Palaeocurrents were generally toward the south and southwest: 248º from five flute-cast soles at Longchang, 156º from six flute casts and grooves above PZ section, 242º from seven current ripples slightly further above PZ, and a trend of 80=260º on five groove casts from WL. The mean of these and several isolated measurements .n D 26/ is 222 š 52º: The greywackes are interpreted as turbidity-current deposits, based on the diagnostic suite of sedimentary structures, the muddy texture, and the hint of graded bedding. The siltstones are probably more distal turbidity-current deposits. The claystone and mudstone represent the background hemipelagic sedimentation, as indicated by the fine grain size and the assemblage of planktonic and nektonic fossils. The Laishike Formation can be readily characterized as flysch (cf. Seilacher, 1959; Enos, 1969; Hsu¨, 1970), although it is rather thin at 810 m. The composition of the Laishike Formation at Duanqiao (DQ, Fig. 3), some 45 km to the north, is quite different. The 520-m section is now poorly exposed, but the Guizhou Team (1980) described the basal quarter as skeletal lime wackestone and mudstone, probably better referred to the Zhuganpo Formation. The overlying rocks are mostly silty calcareous shale with some silty lime mudstone. Thus, they are considerably more calcareous and lack the greywackes or any other obvious turbidites. They are interpreted as slope deposits isolated from tur-

bidity-current input. This suggests that the turbidity currents, which flowed from this general direction, either bypassed the slope and deposited on the basin floor or had an immediate source between LC and DQ, such as a submarine canyon incised into the drowned platform. 3.5. Banan Formation The Laishike flysch is overlain by mudstone, siltstone, and somewhat coarser-grained, relatively matrix-free sandstone of the Banan Formation, 465 m thick at Longchang. Sandstone beds are as thick as 1.7 m, but some intervals with rhythmic shale breaks are flaggy. Sandstone-dominated intervals 3 to 15 m thick with coarser grain size and thicker beds toward the tops (Fig. 13) form up to 14 topographic ridges within the unit (Fig. 14). Cross-bed sets about 40 cm thick with dips of 10–20º are common. Some

Fig. 13. Two coarsening- and thickening-upward cycles within the Banan Formation in a road cut at Duanqiao (DQ, Fig. 3). Lower cycle is incomplete; base is covered. From field sketch. Scale is approximate.

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Fig. 14. Ridges formed by sandstone-dominated intervals 3 to 15 m thick within the Banan Formation, Longchang section (LC, Fig. 3). Similar ridges mark the Banan outcrop belt at DQ and intervening localities.

beds have ripple-drift cross-lamination throughout. Shallow cut-and-fill structures (reactivation surfaces) and load casts are widely distributed. A combination of reactivation surfaces and gently inclined bedding with variable dips suggests hummocky cross-stratification in some thinner-bedded intervals (Fig. 13). Vertical burrows, U-shaped burrows, and bioturbation are prominent in a few beds. The sandstone is carbonate-cemented, medium- to fine-grained, grey to greenish-grey quartz arenite with locally abundant feldspar, lithic fragments, mica flakes, and dark grains (heavy minerals?). Some pebbly lenses are made up exclusively of mudstone intraclasts; others contain lithoclasts of limestone or crystalline rocks. Bivalve fragments, carbonaceous grains, and plant fragments occur sporadically. Mudstone of the Banan is generally grey, grey green, or yellow green; it is dark grey at the top of the formation (Guizhou Bureau, 1987, p. 290). It ranges in thickness from partings to decimetre beds within the sandstone to intervals up to 30 m thick with interlaminated siltstone. The mudstone is calcareous, increasingly so toward the top, and tends to weather with blocky or spheroidal surfaces,

except where bioturbation produces a rough surface texture. Siltstones are mostly ripple-laminated lenses or thin beds, exceptionally to 20 cm thick, within the mudstone. A variety of bivalves, mostly small, relatively thick-shelled forms (e.g. Myophoria, Unionites, Bakevellea, Mytilus), pectins, and a few plant remains are reported (Guizhou Bureau, 1987, pp. 290–291). Halobiid bivalves, ammonoids, and conodonts are reported only from the basal beds (Guizhou Bureau, 1987, pp. 291–292). This siliciclastic unit is interpreted as deposits of a storm- and current-swept, shallow-marine shelf. The thick-walled benthic fauna, thick, well-washed, cross-bedded sandstone, vertical burrows, and possible hummocky cross-stratification indicate relatively high energy. Ripples, particularly those in thin siltstones, could form in a variety of settings from terrestrial to abyssal, but entirely rippled cosets suggest tide- or wind-driven currents. The coarseningand thickening-upward sandstone packages suggest deltaic or coastal sequences; they appear laterally continuous in outcrop, but their geometries have not been mapped in detail. The muds presumably accumulated in deeper or more sheltered portions of

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the shelf; they were the substrate for most of the molluscan fauna. 3.6. Huobachong Formation Coarser-grained and coal-bearing siliciclastics of the Huobachong Formation covered the Banan shelf. The presence of coal seams, many of them of commercial quality in the Longtoushan area (top of LC section, Fig. 3), make a practical field distinction of the Huobachong Formation from the underlying Banan Formation as well as from the overlying Erqiao Formation. Based on this criterion the Huobachong Formation is 730 m thick in the Longchang– Longtoushan section and contains 11 coal beds up to 1 m thick (Guizhou Team, 1980, pp. 181–182; Guizhou Bureau, 1987, p. 289). Sandstone and conglomerate are the dominant lithologies in the unit; they are light-grey to white, lithic and arkosic arenites that form amalgamated beds up to 18 m thick. Composition and grain size are rather variable. Modal size ranges from fine sand to pebbles. Quartz is everywhere the principal component, but estimated content of feldspar ranges up to 25% and chert or quartzite fragments to 15%. One interval of pure quartz arenite with extensive overgrowths is so friable that outcrops are covered by white, angular quartz sand. Lenticular beds, planar beds, festooned cross-beds, and rippled surfaces are common features. Finer-grained intervals, in addition to coal, are grey-green, grey, to black, thin-bedded, calcareous claystone, silty claystone, or carbonaceous mudstone. Most of the sandstone and conglomerate is packaged in fining- and thinning-upward cycles that terminate with coal, carbonaceous mudstone, or dark-grey claystone; 25 such cycles from about 8 to 47 m thick can be recognized in the section measured by the Guizhou Team (1980, pp. 179–182). A variety of plant fossils (e.g. Cycadites, Equisetum, Neocalamites, Podozamites, Anomozamites, Pterophyllum), a number of shallow-water bivalves (e.g. Ostrea, Liostrea, Modiolus, Trigonodus, Unio, Unionites), and an inarticulate brachiopod (Lingula deitersensis) have been found within the unit (Guizhou Team, 1980, pp. 181–182; Guizhou Bureau, 1987, p. 289). Plant-root traces were observed beneath one of the few outcropping coal seams. The coarse-grained, cross-bedded, thinning- and fining-upward cycles with little fine-grained material

strongly suggest braided-stream deposits, although the thick coal seams are remarkable in such an environment. Repeated occurrences of brackish to fully marine bivalves and inarticulate brachiopods interspersed with plants and non-marine bivalves indicate a coastal setting with numerous marine incursions. Coastal swamps that were episodically overwhelmed by encroaching braided streams may have been the habitat of coal deposition, rather than the braid plain. Unfortunately, no details are available on what part of a cyclic unit contains which fossils or on the lateral continuity of the coal beds. 3.7. Erqiao Formation Deposition of coarse-grained siliciclastics continued into the overlying Erqiao Formation, but coal is confined to a basal marker bed (as used in mapping; the coal seems more appropriately considered as marking the top of the underlying Huobachong Formation). The basal unit of the Erqiao is 104 m of white, coarse-grained, lithic arenite with up to 20% quartzose rock fragments. Lenticular bedding and cross-bedding are prominent. Upward, this motif changes little, except for thin interbeds of grey siltstone and silty claystone. Two thinning- and fining-upward cycles, 75 and 45 m thick, terminate the deposition of coarse-grained siliciclastics in this incomplete section. The upper cycle ends with carbonaceous shale. The final 80 m in the exposure are grey claystone; the total exposed thickness is 340 m. All fossils reported from this exposure are of plants, except for three bivalves (Myophoriopsis, Modiolus, and Indosinion, a brackish-water form), within the lower fining-upward cycle (Guizhou Team, 1980, pp. 179–180; Guizhou Bureau, 1987, p. 289). Fossils from incomplete sections elsewhere in Guizhou are mostly plants (Calamites, Neocalamites, Equisetum, Coniopteris, Podozamites), but include a brachiopod (Howellites) and bivalves (Utschamiella and Sibireconcha, a fresh-water form). Braided-stream deposition apparently continued into the Erqiao Formation, but with a more proximal character. The brackish-water mollusks indicate that the sea had not yet retreated permanently and suggest a low-relief, perhaps coastal setting. No fossils nor diagnostic structures were observed in the poorly exposed, thick upper claystone. The Erqiao and the

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Huobachong formations constitute a typical molasse sequence, which could also include the underlying Banan Formation. 4. Discussion The lithofacies succession developed during and after the demise of the Yangtze platform is outlined in Fig. 4; the interpreted depositional environments are summarized in Fig. 15. The shallow-water and peritidal limestones of the Ladinian Yangliujing Formation are the last in a long, proud sequence of shallow-water carbonate deposition on Guizhou portion of the Yangtze platform. Limestone deposition began in the early Sinian, following continental glaciation, with a transgression that extended into the late Sinian (Late Proterozoic; Wang, 1985). Carbonate deposition was interrupted only by a few incursions of siliciclastics, notably in the Early Silurian and Early Devonian, some extrusive volcanics in the mid-Permian, and periods of exposure that included most of the Devonian and Carboniferous in the northern Yangtze platform in Sichuan (Wang, 1985; Guizhou Bureau, 1987). The southern margin of the platform sank to depths of perhaps 800 m during the Carnian. Progressive deepening in the early Carnian is indicated by the grey, nodular-bedded, pelagic limestone of the Zhuganpo Formation. The manganese-rich, pelagic limestone and black shale of the Carnian Wayao Formation are a condensed sequence that records maximum submergence. The pelagic limestone and even the condensed interval remained above the aragonite compensation depth, as documented by the common and elegant preservation of ammonoids and delicate bivalves (Figs. 9 and 12). The lithologic sequence clearly indicates deepening during deposition of the pelagic limestone, culminating in the condensed sequence; during this time rate of sediment accumulation was appreciably less than subsidence. Deposition rates exceeded subsidence during deposition of the turbidite sandstones and mudstone (Laishike Formation) as the accommodation space was nearly filled, leading to shallow-shelf deposition (Banan Formation), which only slightly exceeded subsidence rates. The order of magnitude for the water depth of the condensed sequence may be estimated from the post-compactional thickness of the

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flysch, 800 m, that filled most of the accommodation space created during the drowning of the Yangtze platform. The original thickness of this mud-rich unit would have been two or three times this figure. This estimate must be reduced by the amount of subsidence during deposition. The cause of death of the platform may have been drowning through accelerated subsidence, decrease in carbonate production, excess nutrients, or perhaps other causes, such as encroachment by siliciclastics. There is no hint of prolonged, or even intermittent, exposure in the uppermost shallow-platform limestones, such as precedes many drowning episodes (cf. Sarg, 1988; Hanford and Loucks, 1993). The supratidal cycle caps make their final appearance several metres below the first indicators of drowning, and the strong subaerial diagenetic overprint marked by tepee formation ceased many tens of metres lower in the section. This suggests incipient drowning toward the close of platform deposition. The Yangtze platform crossed the equator during a rapid northward drift from Gondwana, probably in the Permian (Enos, 1995). Abundant coal deposition in the Huobachong Formation (Norian?) suggests a continuing low-latitude position. Evaporite deposition further north in Sichuan indicates that the northern Yangtze platform was located within the arid subtropical divergence zone throughout the Early and Middle Triassic. A rapid acceleration of northward drift would apparently be required to transport the Yangtze platform of Guizhou into temperate latitudes and produce a slowdown in carbonate production, and this is contradicted by continued deposition of carbonates in Sichuan (Wu, 1989a,b). Moreover, deposition of chloralgal carbonates (Lees and Buller, 1972; Lees, 1975), marked by grapestone, dasyclad algae, and ooids, persisted until the end of platform deposition. Deposition of (periplatform?) carbonate mud in the deep-water carbonates probably reflects continuation of platform carbonate production nearby, farther into the interior of the Yangtze platform (Wu, 1989a,b). If the Yangtze platform did not starve for carbonate production, was it overwhelmed by excess nutrients? There is no discernible faunal change or increase in bioerosion toward platform termination. Phosphate concentration is lacking in the pelagic carbonates and the condensed interval.

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Fig. 15. Facies succession within the Longchang section with interpreted depositional environments. Refer to Fig. 4 for lithofacies.

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The Upper Triassic Dachstein carbonate platform in the Northern Calcareous Alps was incipiently drowned several times without unusual subsidence by encroachment of siliciclastic sediments that shut off carbonate production (Satterley, 1994). The history of the Yangtze platform was appreciably different. Pelagic carbonate mud, essentially clay-free, signals incipient drowning and the subsequent condensed interval shows that the platform was long dead before the first sign of siliciclastic influx with the Laishike flysch. Moreover, carbonate deposition continued further north in the interior of the Yangtze platform (Wu, 1989a,b) and probably contributed periplatform carbonate mud to the drowning sequence. There are subtle differences among formation contacts that mark the shift from shallow-platform deposition to pelagic deposition (Yangliujing and Zhuganpo formations, respectively), even between the closely spaced PZ and WL sections (Fig. 3). The shift appears absolutely sharp, as though it happened overnight, at PZ, whereas it is slightly attenuated at WL; the indicators of the event occur at slightly different levels as detailed above. This suggests that subsidence, which is prone to small local variations in magnitude, played an important role, perhaps the dominant role, in the drowning of the Yangtze platform. A global rise in sea level of approximately 100 m is indicated in the curves of Haq et al. (1988) extending from late Ladinian (232 m.y.) into early Carnian (229.5 m.y.), approximately coincident with the time of drowning. In contrast, Embry (1988) places a sharp eustatic fall in Arctic Canada at the Ladinian–Carnian boundary (231 m.y. on the time scale of Haq et al., 1988), followed by a major rise in the early Carnian. The rise of 100 m proposed by Haq et al. (1988) is almost exactly matched by the combined post-compaction thickness of the drowning sequence, the Zhuganpo and Wayao formations, whose facies clearly indicate deepening. The 810 m (post-compaction) of accommodation space largely filled by deposition of the Laishike flysch must have developed by subsidence during deposition of the drowning sequence, when subsidence outpaced deposition, and during flysch deposition, when the relative rates were reversed. The transition from shallow-platform to pelagic deposition at WL indicates more gradual subsidence than at PZ, where the contact is so sharp. The

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thickness of the pelagic and condensed intervals are comparable in the two sections: 62 vs. 55 m for the pelagic and 30.4 vs. 26.4 m for the condensed interval at WL and PZ, respectively. Each interval is slightly thicker at WL, where the transitional contact indicates slower subsidence. With pelagic deposition, slower subsidence should favour accumulation. At Longtou (LT, Fig. 3), south of Guiyang on the Yangtze platform margin about 150 km to the east, the apparent time-equivalent deposits to the pelagic sequence is the Gaicha Formation (Table 1), a thin (60–95 m) interval of shoaling-upward, peritidal cycles of mixed siliciclastics and carbonates. If the correlation is valid, this striking difference between the two areas would reaffirm the role of differential subsidence. However, recent results from conodont biostratigraphy indicate that the pelagic sequence, the Zhuganpo Formation, is basal Carnian, rather than Ladinian, as previously supposed (Yang et al., 1995), so the Gaicha Formation may correlate with platform carbonates of the upper Yangliujing Formation rather than the pelagic interval. Further conclusions must await resolution of the new correlation problems. Differential subsidence between the two areas is also indicated by differences in the configuration of the last of the platform-margin limestones, the Longtou Formation (Ladinian, Table 1). South of Guiyang the Longtou Formation prograded at least 5 km over the underlying basinal mudstones of Anisian age (Wei and Enos, 1991; Enos et al., 1997). In the Zhenfeng area, the Longtou shelf margin retreated; the basinal Bianyang flysch, approximate time equivalent to the Longtou Formation (Ladinian, Table 1), overstepped the underlying platform margin represented by reefs of the Poduan Formation (Anisian, Table 1). Thus the Guiyang area appears to have been more positive than the Zhenfeng area throughout the Ladinian as well as at the critical time of drowning. Shallow-platform carbonate deposition continued into the Late Triassic further onto the Yangtze platform, in the Sichuan basin (Wu, 1989a,b). The Sichuan-basin portion of the Yangtze platform thus remained in the critical depth range for carbonate production somewhat longer than the platform margin in southwestern Guizhou. Differential subsidence between the two areas is the most plausible explanation. These examples demonstrate that carbonate

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platforms can be very sensitive indicators of eustasy and subsidence because they faithfully record the points of emergence and submergence. Differential tectonic subsidence within the Yangtze platform in the Late Triassic is a reasonable expectation because of possible collisions between the Yangtze plate and the adjacent North China, Indochina, and southeast China plates (Fig. 1) as southern China was assembled and accreted to northern China during the Indosinian orogeny (Wang, 1985; Guo, 1985; Sengo¨r, 1987; Hsu¨ et al., 1988; Sun et al., 1989). Whatever the cause, the Yangtze platform was definitely dead, beyond resurrection. Although the succeeding turbidity-current deposits and mud accreted to near sea level, shallow-shelf siliciclastic sands accumulated (Banan Formation), without apparent carbonate production beyond the sparse benthic fauna, mostly bivalves. The depositional surface moved above sea level as siliciclastics of the Huobachong Formation covered the shelf. Once again, carbonate deposition was lacking except for a few marine fossils. Following these brief episodes of transgression, the southern margin of the Yangtze platform remained emergent throughout the remainder of the Mesozoic and the Cenozoic. Following the drowning of the Yangtze platform, a thin condensed interval accumulated in relatively deep water. The drowned platform was then covered by siliciclastic turbidites and hemipelagic mud, a typical flysch. The succeeding deposits were shallow-shelf sands indicating some combination of drop in relative sea level and sediment accretion. Abundant sand and gravel were delivered by braided streams and numerous, thick coal beds formed, probably in coastal swamps, during short-lived incursions of the sea. These units, the Banan, Huobachong, and Erqiao formations, constitute a molasse. This succession is a typical foreland-basin sequence (cf. Allen and Homewood, 1986; recognized as the ‘geosynclinal cycle’ by Pettijohn, 1949, p. 447). This raises the question, a foreland basin of what? At least three orogenic belts of probable Triassic age appear to be possible sources of the basin fill. The Yangtze block sutured to North China block (Fig. 1), possibly beginning in the Late Triassic (review in Enos, 1995). However, the suture zone lay well north of Guizhou, and the Sichuan portion of the Yangtze

platform, which lay between, continued to be the site of platform carbonate deposition (Wu, 1989a,b), indicating continued stability. A second possibility is a suture within the South China block, as proposed by Hsu¨ et al. (1988; see Fig. 1). Flysch sedimentation in the Nanpanjiang basin began at least as early as the Ladinian with the Bianyang Formation (Table 1). Palaeocurrent directions within the Bianyang flysch are from the east and southeast (Daniel Chaikin, unpublished data) and those in the Carnian Laishike flysch are from the northeast. These directions, if they may be projected back to a source terrain, are most consistent with South China suturing as the causal orogeny. However, the succeeding molasse appears to have been derived from the west. The molasse interval is 1500 m thick in the Longchang area where the top is eroded. Equivalent deposits in the Guiyang area are only 175 m thick in a complete section and the facies are more distal (Guizhou Bureau, 1987, p. 293). The most likely western source would be the Khamdian massif (‘oldland’), which supplied siliciclastic debris to the southwestern margin of the Yangtze platform intermittently during the Permian and Triassic (Sheng et al., 1985; Wang, 1985). The Longmen Shan, a probable Triassic suture (Sengo¨r, 1987), forms the northwestern boundary of the Yangtze plate and may have shed debris onto the western margin. It appears that the flysch was most likely derived from the east and the molasse from the west, indicating that they may be unrelated clastic wedges. Many more data are needed on the geometries, palaeocurrents, and provenance of the two units. 5. Summary and conclusions Facies succession. The prolonged passive-margin, carbonate-dominated succession at the southern margin of the Yangtze platform ended in a rapid drowning event. The overlying turbidites, shelf siliciclastics, and braided-stream deposits are characteristic flysch and molasse, a typical foreland-basin succession. The source of the clastic wedges remains conjectural. Time of death. The Yangtze platform drowned at the Ladinian–Carnian transition, based on new conodont data (Guizhou Team, unpubl. data, 1991; Lehrmann, 1993; Yang et al., 1995).

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Cause of death. Possibilities considered include drowning, starvation, nutrient excess, and tectonics. The platform drowned, but did it die first? Lack of supratidal deposits in last cycles and lack of subaerial diagenetic overprint (tepees) in upper tens of metres of platform limestone suggests incipient drowning. The Yangtze platform probably crossed the equator in the Permian and the Guizhou sector remained in the tropics through most of the Triassic, as shown by carbonate and coal deposition. Evaporite deposition throughout the Early and Middle Triassic in Sichuan indicates that the subtropical divergence zone was located to the north. Deposition of chloralgal carbonates persisted until the end of platform deposition in Guizhou. Carbonate mud in overlying pelagic carbonates indicates that the surviving platform (in Sichuan?) continued to be productive. No signal of increased nutrients was detected in this reconnaissance study. Tectonics, while not a direct cause, may have contributed to platform demise in the form of differential subsidence. Subtle differences among the three sections studied in most detail and contrasts with the succession in the Longtou area (LT) could have been caused by differential subsidence. Perhaps more significant is the major Ladinian progradation (at least 5 km) of the carbonate platform in the Guiyang syncline in contrast to the burial of the seaward margin of the Anisian platform beneath Ladinian flysch in the study area. Both lines of evidence suggest that the Guiyang area was more positive. Burial. After death, the platform was buried by 800 m of turbiditic basin fill and later by 1500 m of molasse. There was no resurrection; carbonate deposition did not resume with the return to shallow depths. Acknowledgements Reconnaissance field work on this project was supported by the National Science Foundation Committee for Scientific Communication with Peoples Republic of China, Chengdu Institute of Geology and Mineral Resources, and AMOCO Production Company. Field work in 1995 was supported by a grant from the Petroleum Research Fund of the American Chemical Society and by the Guizhou Science and Technology Association, Li Xing-min, deputy director. We thank colleagues from the

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Guizhou Regional Geological Mapping Team for discussions and support. Petrology was done at the Institut fu¨r Pala¨ontologie, Universita¨t ErlangenNu¨rnberg; thanks to Prof. Erik Flu¨gel and other colleagues. Frau Maria-Louise Neufert and Frau Christel Sporn provided some of the photographs. Critical reviews by Arthur Satterley and Brian R. Pratt improved the manuscript. References Allen, P.A., Homewood, P. (Eds.), 1986. Foreland Basins. Int. Assoc. Sedimentol. Spec. Publ. 8, 453 pp. Assereto, R.L., Kendall, C.G.St.C., 1971. Megapolygons in Ladinian limestones of Triassic of southern Alps: Evidence of deformation by penecontemporaneous desiccation and cementation. J. Sediment. Petrol. 41, 715–723. Byers, C.W., 1977. Biofacies patterns in euxinic basins: a general model. In: Cook, H.E., Enos, Paul (Eds.), Deep-Water Carbonate Environments. Soc. Econ. Paleontol. Mineral. Spec. Publ. 25, 5–17. Clark, D.L., Paull, R.K., Solien, M.A., Morgan, W.A., 1979. Triassic conodont biostratigraphy in the great basin. In: Sandberg, C.A., Clark, D.L. (Eds.), Symposium on Conodont Biostratigraphy of the Great Basin and Rocky Mountains. Brigham Young University Geological Studies, v. 26, pt. 3, pp. 179– 185. Dunham, R.J., 1972. Capitan Reef, New Mexico and Texas; facts and questions to aid interpretation and group discussion. Soc. Econ. Paleontol. Mineral., Permian Basin Sect., Midland, TX, Publ. 72-14, 235 pp. Embry, A.F., 1988. Triassic sea-level changes: Evidence from the Canadian Arctic Archipelago. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral. Spec. Publ. 42, 249– 259. Enos, Paul, 1969. Cloridorme Formation, Middle Ordovician flysch, northern Gaspe´ Peninsula, Quebec. Geol. Soc. Am. Spec. Pap. 117, 66 pp. Enos, Paul, 1995. Permian of China. In: Scholle, P.A., Peryt, T.M., Ulmer-Scholle, D.S. (Eds.), The Permian of Northern Pangea, Vol. 2. Springer, New York, pp. 225–256. Enos, Paul, Wei Jiayong, Yan Yangji, 1997. Facies distribution and retreat of Middle Triassic platform margin, Guizhou Province, south China. Sedimentology 44, 563–584. Guizhou Bureau of Geology and Mineral Resources, 1987. Regional Geology of Guizhou Province. Peoples Republic of China. Ministry of Geology and Mineral Resources, Geological Memoirs, Ser. 1 (7), 700 pp., Geologic map 1 : 500,000 (in Chinese, with English summary). Guizhou Regional Geological Survey Team, 1980. Regional Geological Surveying Report of Xingren (G-48-XXII) and Anlong (G-48-XXVIII) Quadrangles (in Chinese). Guo Han-cheng, 1985. Preliminary research on tectonic back-

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