Record of the end-Permian extinction and Triassic biotic recovery in the Chongzuo-Pingguo platform, southern Nanpanjiang basin, Guangxi, south China

Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 200 – 217 www.elsevier.com/locate/palaeo

Record of the end-Permian extinction and Triassic biotic recovery in the Chongzuo-Pingguo platform, southern Nanpanjiang basin, Guangxi, south China Daniel J. Lehrmann a,⁎, Jonathan L. Payne b , Donghong Pei c,1 , Paul Enos c , Dominic Druke a,2 , Kelley Steffen a,3 , Jinan Zhang d , Jiayong Wei e , Michael J. Orchard f , Brooks Ellwood g a Department of Geology, University of Wisconsin-Oshkosh, Oshkosh, WI 54901, United States Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, United States c Department of Geology, University of Kansas, Lawrence, KS 66045, United States d Geological Survey of Guangxi, Guilin, Guangxi, People's Republic of China e Guizhou Bureau of Geology, Guiyang, Guizhou, People's Republic of China f Geological Survey of Canada, Vancouver, B.C., Canada V6B 5J3 g Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States

b

Accepted 30 November 2006

Abstract The Chongzuo-Pingguo platform is a vast isolated platform, 180 km across, in the southern part of the Nanpanjiang basin. The end-Permian extinction is recorded in conformable sections in the northern part of the platform (Pingguo area). Upper Permian skeletal packstone contains diverse open-marine fossils including Nankinella and Sphaerulina. It is overlain by a 4.6 m thick horizon of calcimicrobial framestone constructed by globular calcified microbial framework similar to Renalcis. The PTB event horizon is interpreted to occur at the top of the packstone, coincident with the abrupt change to calcimicrobial framestone lacking Permian macrofossils. The transition from Hindeodus latidentatus to H. parvus occurs 1 m above the base of the calcimicrobial framestone, marking the conformable biostratigraphic boundary. During the Early Triassic the platform developed as a low-relief bank rimmed with oolite shoals but was bordered by a highrelief, fault-controlled escarpment along its southern margin. Platform interior facies of the Majiaoling and Beisi formations are 900 m thick and consist of thin-bedded lime mudstone, oolite, and dolostone with restricted marine biota in four shallowingupward packages that define depositional sequences. In the Chongzuo area the platform was terminated at the end of the Early Triassic by burial with up to 1600 m of felsic pyroclastic volcanics and lava flows. Transgression and shallow-marine carbonate sedimentation resumed early in the Middle Triassic followed by drowning in the Bithynian as indicated by a shift to deep-marine carbonate and clastic deposition. In the Pingguo area volcanic deposits are much thinner and only briefly interrupted carbonate accumulation. Here the platform interior drowned during a major back step early in the Middle Triassic (Anisian, Bithynian). Smaller pinnacle platforms of the ⁎ Corresponding author. E-mail address: [email protected] (D.J. Lehrmann). 1 Current address: University of Nevada, Reno, Reno, NV 89557, United States. 2 Current address: Shell Exploration and Production Company, Houston, TX 77079-1101, United States. 3 Current address: ExxonMobil Upstream Research Company, Houston, TX 77252, United States. 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.11.044

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Guohua Fm., up to 1700 m thick, continued to accumulate above the Early Triassic platform margin and were eventually drowned by the Late Pelsonian. In the Anisian, the pinnacle platforms shifted to sharply defined, reef-rimmed margins with Tubiphytes reefs. The shift coincides with accelerated metazoan biotic recovery in the Anisian. However, the reefs lack metazoan frameworks and are composed almost entirely of Tubiphytes reinforced by large volumes of marine cement. These observations indicate that controls beyond the recovery of framework-building metazoans governed the re-establishment platform-margin reefs in the Middle Triassic. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbonate platform; Reef; Permian; Triassic; Extinction; China

1. Introduction Of the five great mass extinctions of the Phanerozoic, the end-Permian extinction stands out as having greatest devastation in the marine sedimentary record, an aftermath characterized by the unusual abiotic carbonate precipitates, and an exceptionally long biotic recovery

interval (cf. Woods et al., 1999; Erwin et al., 2002; Lehrmann et al., 2003; Payne et al., 2004, 2006b; Baud et al., 2005). Although the extinction has been the subject of intensive study, the causes remain unresolved. Evidence has been published in support of a wide range of potential trigger and kill mechanisms including bolide impact, Siberian Traps eruptions, anoxia, hypercapnia

Fig. 1. Early Triassic paleogeographic map of the Nanpanjiang basin compiled from regional geologic maps (Guangxi Bureau, 1985; Guizhou Bureau, 1987). Inset is a tectonic map of south China modified from Sun et al. (1989). For the detailed distribution of Lower Triassic exposures used to constrain the distribution of platform and basin environments in this paleogeographic reconstruction see Fig. 3 in Lehrmann et al. (2005).

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(CO2 poisoning), methane release, and hydrogen sulfide poisoning linked to oceanic euxinia (e.g. Wignall and Hallam, 1992; Knoll et al., 1996; Krull and Retallack, 2000; Becker et al., 2004; Kump et al., 2005; among many others). Many studies have focused on the facies, biotic changes, geochemistry and geochronology of Permian–Triassic boundary (PTB) sections from various localities around the globe (e.g. Wignall and Hallam, 1992; Yin et al., 1996; Baud et al., 1997; Bowring et al., 1998; Jin et al., 2000; Krull and Retallack, 2000; Lehrmann et al., 2003; among many others). Recently, an increasing number of studies have been directed toward the Lower–Middle Triassic record of biotic recovery and environmental conditions in the prolonged aftermath following the extinction (e.g. Schubert and Bottjer, 1995; Twitchett, 1999; Woods et al., 1999; Lehrmann et al., 2001; Wignall and Twitchett,

2002; Fraiser and Bottjer, 2004; Payne et al., 2004, 2006b). An obstacle in understanding the end-Permian extinction has been the rarity of conformable PTB sections available for study. Even less common are the expanded, conformable and continuously exposed sections of the entire Upper Permian through Middle Triassic record needed to develop the high-resolution sedimentological, geochemical, and paleontological records required for understanding Permian–Triassic events. In previous papers we have emphasized the importance of conformable PTB sections and expanded Upper-Permian through Middle-Triassic sections of the Great Bank of Guizhou (GBG) in the northern part of the Nanpanjiang basin (Fig. 1; Lehrmann et al., 1998; Payne et al., 2004, 2006b; Lehrmann et al., 2005). The purpose of this study is to summarize the stratigraphic architecture, facies, and depositional environments from the PTB

Fig. 2. Geologic map of the Chongzuo area. Modified from Guangxi Bureau (2000, geologic map 1:500,000), using field observations made by Pei Donghong (unpublished data). Sections BM, LJ, LY, and BN are presented in Fig. 3.

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Fig. 3. Stratigraphic cross section of the Chongzuo area. Stratigraphic sections BM, LJ, and LY occur within the platform. Section BN occurs at the basin margin. Section locations are given in Fig. 2.

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through Middle Triassic of another isolated carbonate platform, the Chongzuo-Pingguo platform (CPP), in the southern part of the basin. The CPP contains continuously exposed, relatively conformable and expanded PTB through Middle Triassic sections and it is dissected by a fold that exposes a 2-D cross section revealing its architecture. Further study of this platform holds promise for understanding the environmental conditions of the extinction and biotic recovery. 2. Geological setting The Nanpanjiang basin is a deep-marine embayment in the southern margin of the Yangtze plate (Fig. 1, inset) which was located in the eastern Tethys seaway near the equator during the Permian, progressively migrated northward and eventually docked with the

north China plate during the Late Triassic (Lehrmann et al., 1998). Several isolated platforms developed within the Nanpanjiang basin during the Triassic including the Great Bank of Guizhou (GBG) in southern Guizhou Province and the Chongzuo-Pingguo platform (CPP) in southern Guangxi (Fig. 1). Each of the platforms is delineated by the regional distribution of shallow-marine carbonate platform facies and deepwater basinal facies (Lehrmann et al., 2005). The isolated platforms exhibit a pattern of greater longevity in the north, step-backed margins and pinnacle development in the south, and earlier drowning and burial by siliciclastics in the south (Lehrmann et al., 2005). Lower Triassic volcanic horizons thicken dramatically southward indicating a southerly source. These differences were interpreted to have resulted from faster subsidence rates and volcanism in the southern part of the basin

Fig. 4. Geologic map of the Pingguo area. Modified, using reconnaissance field mapping, from Guangxi Bureau (1987, 1:1000000; 2000, 1:500000) and Guangxi Bureau (unpublished geologic maps 1:200,000). Inset upper right is a detailed map of the northern pinnacle platform. Sections TP and NM are presented in Fig. 5.

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Fig. 5. Stratigraphic sections from the Pingguo area. TP section is of Permian through basal Middle Triassic strata of the platform interior. NM section is in Middle Triassic basin-margin facies south of the northern pinnacle platform. See Fig. 4 for section locations.

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caused by tectonic convergence along the southern margin of the south China plate (Lehrmann et al., 2005). 3. Stratigraphic architecture Upper Permian strata in the CPP consist of the Heshan and Dalong formations. The Heshan Fm. consists of a thin bauxitic mudstone at the base, and a thick upper part dominated by cherty limestone with lesser calcareous shale and coal seams (Guangxi Bureau, 1985). The formation ranges up to 225 m thick in the Chongzuo area and its upper part is dominated by cherty limestone with diverse shallow-marine biota of brachiopods, foraminifera, gastropods, dasycladacean algae, and corals. In the Pingguo area the top of the unit contains several felsic volcanic ash layers below a conformable PTB (Lehrmann et al., 2003). The Dalong Fm. is composed of darkcolored spiculitic mudrock, chert, and cherty argillaceous limestone with volcanic ash layers and a deeper water assemblage of ammonoids, bivalves and brachiopods (Lehrmann et al., 2005). The unit ranges up to 160 m thick (Guangxi Bureau, 1985). In the Chongzuo area regional geologic maps combine the Heshan and Dalong in one mapping unit across much of the study area (Fig. 2). However, the Pingxiang–Dongmen fault (P–D fault; Figs. 1, 2) was active during the Upper Permian affecting uplift and shallow-water deposition of the Heshan Fm. (e.g. LJ section; Figs. 2, 3) north of the fault and deeper-water deposition of the Dalong Fm. south of the fault (BM section, Lianqiao; Fig. 2). Uplift and erosion have removed Upper Permian strata north of the fault west of Chongzuo (area west of LJ section) and in the area around Longzhou where Lower Permian strata are overlain by Lower Triassic strata of the Majiaoling Fm. Regional geologic maps indicate that the Heshan Fm. has a widespread distribution in the Pingguo area that extended well beyond the area of the Lower Triassic platforms (Fig. 1; Guangxi Bureau, 2000). This change suggests that southern Guangxi may have been the site of vast shallow-marine carbonate, clastic and paralic coal environments during the Late Permian prior to the development of smaller isolated carbonate platforms in the Early Triassic. Lower Triassic formations consist of shallow-marine carbonates of the Majiaoling and Beisi formations and basinal deposits of the Luolou Fm. The Majiaoling Fm. ranges from 100 to 130 m thick in the CPP and is composed of light to medium gray, thin-bedded lime mudstone with minor oolite (Figs. 3, 5). The Beisi Fm. ranges up to 800 m thick in the CPP and is composed of lime mudstone with thick intervals of oolite, arranged

into shallowing-upward packages, and dolostone concentrated in its upper part (Figs. 3, 5). The Luolou Fm. ranges from 40 to 538 m thick and consists of black to dark-gray, laminated or horizontally burrowed lime mudstone with shale intercalations and interbeds of debris-flow breccias (Guangxi Bureau, 1985). The distribution of Lower Triassic formations from regional geologic maps and ground observations delineates the extent of the CPP (Fig. 1; Guangxi Bureau, 1985; Lehrmann et al., 2005; Guangxi Bureau, 2000). Shallowmarine carbonates of the Majiaoling and Beisi formations define the extent of the platform. In the Pingguo area the platform is bounded to the north, west, and east by basinal deposits of the Luolou Fm. (Fig. 4). The southern margin of the platform in the Chongzuo area is defined by the P–D fault which may have remained active or at least a bathymetric demarcation during the Early Triassic as evidenced by the distribution of shallow-marine Majiaoling and Beisi formations north of the fault and basinal deposits of the Luolou Fm. to the south of it (Figs. 1, 2). In the Chongzuo area the platform extends westward into Vietnam; to the east the margin is unconstrained owing to lack of outcrop (Fig. 1). A significant question is whether the Chongzuo-Pingguo area is a single isolated platform or two separate platforms. The problem arises because of the lack of preserved Triassic strata on the Nanning anticline that separates the areas (Fig. 1). Two lines of evidence suggest that it developed as a single platform: (1) reconnaissance mapping revealed shallow-marine carbonates with oolite extending to the southern extent of exposures in the Pingguo area (Fig. 1) and (2) isolated exposures 30 km west of Nanning also were interpreted to be shallow-marine carbonates of the Majiaoling Fm. (Guangxi Bureau, 2000; Pei Donghong, unpublished). The Middle Triassic (Anisian) Guohua Fm. has its type area in the Pingguo Area (Fig. 1). The Guohua Fm. consists primarily of thick-bedded massive and cyclic dolomite with restricted molluscan fauna and fenestral laminites. The formation also contains minor but significant Tubiphytes boundstone and basin-margin facies composed of pelagic limestones and interbedded debris-flow breccias. The stratigraphic architecture of the Pingguo area is revealed by a northwest trending fold that exposes a 2-D cross section (Fig. 4). The architecture is best visualized by viewing the strata down structural dip on the northeast limb of the syncline (area of TP and NM sections, Fig. 4). The Upper Permian Heshan Fm. is conformably overlain by shallow-marine carbonate strata of the Lower Triassic Majiaoling and Beisi formations. Greater abundance of oolite shoals near the northern and southern margins of

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the platform in the Majiaoling and Beisi and the lack of reefs indicate that the platform had a low-relief bank architecture during the Early Triassic. The platform continued to accumulate shallow-marine carbonate until the platform interior drowned and shifted to deepmarine siliciclastic deposition represented by the Banna Fm. in the beginning of the Middle Triassic (Anisian, Bithynian; Figs. 4, 5; top of TP section). During the interior drowning, shallow-marine carbonate deposition stepped back and continued as smaller pinnacle platforms (small isolated platforms analogous to pinnacle reefs but containing differentiated interior, reef, and slope facies) represented by the Guohua Fm. at the northern margin of the platform (NM section) and southern limit of the syncline northeast of Pingguo (Fig. 4). The pinnacle platforms of the Guohua Fm. are 1700 m thick (NM section; Fig. 5) and contain peritidal dolomite in their interiors, Tubiphytes reefs at the margins, and slope carbonates that intertongue with siliciclastic turbidites adjacent to the platforms (NM section, Figs. 4, 5). The pinnacle platforms were finally drowned and buried by siliciclastics of the Banna Fm. near the end of the Anisian, Pelsonian (Figs. 4, 5). Numerous faults dissect the Chongzuo area. Interpretation of architecture in this area was constrained by three measured stratigraphic sections in the platform interior (BM, LJ, and LY sections; Figs. 2, 3) and one section in the basin south of the P–D fault (BN section; Figs. 2, 3). Shallow-marine carbonate deposition was initiated in the Early Triassic during transgression over an unconformity above the Permian (Fig. 3). During Early Triassic deposition of the Majiaoling and Beisi formations the platform is inferred to have developed a low-relief bank type architecture bordered to the south by oolite shoals at the platform margin and perhaps an abrupt escarpment along the P–D fault (Fig. 2). The greater abundance of oolite in LY section close to the margin provides evidence for development of marginal oolite shoals (Figs. 2, 3). At the end of the Early Triassic the platform terminated by burial beneath a thick pile of felsic volcanics (Figs. 2, 3). After cessation of volcanics, shallow-marine carbonate deposition resumed briefly (Banmo member) followed by drowning and burial by deep-marine siliciclastics of the Banna Fm. in the Middle Anisian (BM section; Figs. 2, 3). 4. Lithofacies and depositional environments 4.1. Permian–Triassic boundary The Permian–Triassic boundary in the Chongzuo and Pingguo areas has been described in detail by

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Lehrmann et al. (2003). The boundary is conformable in the Pingguo area where cherty skeletal packstone of the upper Heshan Fm., with diverse open-marine fauna and volcanic ash interbeds, is overlain by calcimicrobial framestone of the basal Majiaoling Fm. The upper Heshan Fm. contains the fusulinids Nankinella and Sphaerulina indicating a Late Permian age. The endPermian extinction is interpreted to have occurred at the boundary between skeletal packstone containing diverse Permian fossils and overlying calcimicrobial framestone lacking Permian macrofossils. The biostratigraphic PT boundary is currently placed 1 m above the base of the calcimicrobial framestone on the basis of Hindeodus latidentatus within the basal meter of the calcimicrobial framestone and the first occurrence of H. parvus 1 m above the base. The skeletal packstone and the calcimicrobial framestone are interpreted to represent the continuation of shallow, open-marine environments during the end-Permian extinction; the abrupt facies change is interpreted to reflect a substantial decrease in the contribution of skeletal animals and algae to carbonate accumulation and, perhaps, a change to oceanic chemistry intolerable for much of the shallow-marine biota during the aftermath of the extinction (Lehrmann et al., 2003; Payne et al., 2006b). In TP section (Figs. 4, 5) the calcimicrobial framestone unit is 4.6 m thick and is composed primarily of chambered globular frameworks similar to Renalcis surrounding irregular cavities (Figs. 4, 5). The very base of the microbialite, however, contains digitate fabrics constructed primarily by clusters of upward radiating aragonite fans (Fig. 6A, B). The PTB is unconformable across much of the CPP in the Chongzuo area. At LY and LJ sections the Majiaoling Fm. unconformably overlies Upper Permian strata of the Heshan Fm. (Figs. 2, 3). The unconformity is overlain at LY section by a thin conglomerate containing clasts of limestone, feldspar and coal derived from erosion of underlying strata. West of LJ section and in the Longzhou area, the Majiaoling Fm. overlies Lower Permian strata. Calcimicrobial framestone was found in the base of the Majiaoling Fm., overlying the Heshan Fm. in only one unnamed locality near the southern margin of the platform (Fig. 2; UN). The beveling of the strata into the Lower Permian indicates that the unconformity resulted from tectonic uplift. The absence of the calcimicrobial facies in other sections suggests that the basal Triassic record may be missing over much of the platform. It is uncertain whether the calcimicrobial horizon is missing as the result of erosion or onlap of the Lower Triassic section along the

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Fig. 6. Lower Triassic facies. (A) Polished slab of basal Triassic, Griesbachian, calcimicrobial framestone from TP section, Pingguo area. Black areas are globular calcimicrobial clusters. Medium gray material (e.g. arrows left) is composed of aragonite fans that control overall digitate structure. Light gray is micrite internal cement. (B) Thin-section microphotograph of aragonite fans from specimen illustrated in A. (C) Thin platy-bedded lime mudstone of the Majiaoling Fm. (TP section, Pingguo area). (D) Oolite grainstone of the Beisi Fm. (northern platform margin, Pingguo area). Note giant ooids up to 1 cm across (center).

unconformity. However, the fact that the Lower Triassic is relatively similar in thickness between the LY and LJ sections (Fig. 3) indicates that there was not substantial erosional truncation of Lower Triassic strata in the region. Sections in the basin south of the P–D fault are probably conformable where the Upper Permian Dalong Fm. is overlain by the Lower Triassic Luolou Fm. (e.g. BN section; Figs. 2, 3). 4.2. Lower Triassic Majiaoling Formation The Majiaoling Fm. is composed of light to medium gray, thin-bedded, platy limestone and minor oolite. Lime mudstone beds range from 5 to 40 cm thick and are separated by argillaceous seams (Fig. 6C). The beds are typically massive, homogenous or may have wavy lamination or vague burrows. Fossils include very rare

gastropods, bivalves, and foraminifers, and one ammonoid above the calcimicrobial framestone at TP section. Argillaceous seams result in a greater magnetic susceptibility and “spikey signature” in the lime mudstone as compared with oolite intervals of the overlying Beisi Fm. (Figs. 3, 5). Oolite occurs in thin stringers commonly with scoured bases and fining upward beds. The homogenous massive beds suggest bioturbation; however, discrete burrows are rare. The low biodiversity molluscan fauna are suggestive of restricted marine circulation, but it may alternatively represent altered abundance and distribution patterns of the marine biota after the end-Permian extinction. Oolites are interpreted to have been transported into the site of deposition during storms. The predominance of fine-grained sediment and presence of fining-upward oolite beds suggests deposition in subtidal environments above

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storm wave base. The lower part of the Majiaoling Fm. in the Chongzuo area is thicker bedded lime mudstone with oolite stringers (Fig. 3; LY, LJ sections) in contrast to dominantly thin-bedded lower portion in the Pingguo area (Fig. 5; TP section). 4.3. Lower Triassic Beisi Formation The Beisi Fm. is 600–800 m thick and consists primarily of platy, thin-bedded lime mudstone and massive to cross-bedded oolite arranged into meterscale, shallowing-upward cycles and larger graining upward depositional sequences 50 to 300 m thick. The upper parts of depositional sequences commonly contain evidence of subaerial exposure such as fenestral laminates. The lower part of the formation contains shale in the Pingguo area and the upper part of the formation is dolomitized. Oolite apparently extended across the platform interior and thickens from LJ to LY section southward toward the platform margin in the Chongzuo area (Figs. 2, 3) and at the northern margin of the platform in the Pingguo area (northwest of NM section; Fig. 4) indicating the presence of shoals at the platform margin. Thin-bedded lime mudstones are similar to those described from the Majiaoling Fm. They are massive to wavy laminated and contain few burrows, thinshelled pectinoid bivalves, foraminifers, and argillaceous seams between beds. The mudstones are interpreted to represent deeper subtidal transgressive portions of depositional sequences. Thin fining upward interbeds of oolite with scoured bases are interpreted to be storm deposits. Oolites are spectacular cliff forming units and range from individual beds a few meters thick to massive amalgamated beds up to 25 m in thickness. The oolites are massive to cross-bedded grainstone and coarsen upward in some sequences (Fig. 6D). Included are “giant ooids” from 5 mm to 1 cm in diameter. Similar giant ooids have been reported from the Lower Triassic in other platforms in the Nanpanjiang basin (Payne et al., 2006a) and in Germany (Weidlich, 2005). Composite coated grains composed of grapestone-like aggregates of ooids, which are in turn enveloped in oolitic cortices, range up to 3 cm across. Additional grains include gastropods, bivalves, peloids, and intraclasts. The suite of fabrics and grains indicates high-energy subtidal to intertidal shoals. The upper parts of some oolite intervals are burrowed packstone indicating abandonment and stabilization of ooid shoals. Evidence for subaerial exposure at cycle and sequence boundaries includes ribbon rock characterized

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by flaser-laminated interlayering of lenticular and ripple cross-laminated fine peloidal grainstone with scoured bases and isopachous lime mud drapes. The ribbon rock also contains microbial laminites and dewatering structures that probably formed by desiccation. The ribbon rock is identical to Lower Triassic peritidal facies described from the Great Bank of Guizhou and is similar to that found in Lower Paleozoic tidal flat facies (Lehrmann et al., 2001). Subaerial exposure is also indicated by fenestral laminites with mud cracks capping sequences. Facies changes within depositional sequences result in a distinctive magnetic susceptibility signature (Figs. 3, 5). The lower transgressive argillaceous lime mudstone yields a spikey high magnetic susceptibility profile that changes upward into the regressive oolite with a blocky, lower magnetic susceptibility profile resulting from lower argillaceous content in the oolite. The presence of 4 or 5 depositional sequences spanning the Lower Triassic (Figs. 3, 5) indicates that they are third or fourth order (each with a duration of approximately 1 my or somewhat less). There is some ambiguity in correlation of the depositional sequences between the Chongzuo and Pingguo areas resulting from differences in facies distribution. The following describes our interpreted best correlation (sequences 1–4; Figs. 3, 5). Sequence 1 The Beisi Fm. is differentiated from the underlying Majiaoling Fm. by the greater proportion of oolite and dolomitization in its upper part. The base of the Beisi Fm. is dominated by oolite representing the regressive phase of depositional sequence 1 that began in the Majiaoling Fm. (Figs. 3, 5). Oolite beds increase in frequency and thickness and magnetic susceptibility progressively decreases upward (best developed in LY and TP sections; Figs. 3, 5). The upper oolite is cross-bedded and maximum bed thickness reaches 10 m. Giant ooids and large composite coated grains are found near the top of the sequence. Subaerial exposure is represented by ribbon rocks, microbial laminites and water escape structures capping the sequence at TP section. Sequence 2 The transgressive portion of sequence 2 begins in the Pingguo area with the deposition of an interval of marine shale 100 m thick followed by thin-bedded lime mudstone with thin oolite stringers

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(Fig. 5). The regressive portion in Pingguo consists of several shallowing upward cycles 5–15 m thick. Each cycle coarsens upward from thin-bedded lime mudstone through oolite packstone and grainstone and is capped with ribbon rock. In a few cycles the oolite coarsens upward to giant ooids and large composite coated grains in the upper part. In the Chongzuo area, sequence 2 shallows upward from a lime mudstone interval 25 m thick and coarsens upward to a package of oolite greater than 100 m thick with massive or cross-bedded oolite in individual beds ranging from 5 to 25 m thick (Fig. 3). Sequence 2 is capped by fenestral laminite at Chongzuo. The Chongzuo area apparently contains an additional sequence (2a) when compared with the Pingguo area (Figs. 3, 5). In the platform interior section at LJ, sequence 2a is characterized by a relatively thin oolite and negative magnetic susceptibility excursion within a thick interval dominated by lime mudstone (Fig. 3). At LY section, near the platform margin, sequence 2a is dominated by oolite grainstone separated from the underlying sequence by fenestral laminite. It is important to note that sequences 2 and 2A contain the greatest proportion of oolite in the Beisi Fm. which is followed upward in both the Chongzuo and Pingguo areas by greatly increased proportion of lime mudstone suggesting that this interval represents a second-order maximum regression. Sequence 3 The transgressive portion consists of more than 100 m of thin-bedded light to medium gray lime mudstone with intensive bioturbation. The unit is burrow mottled and contains abundant horizontal burrows as well as bed penetrative branched burrows with random orientations. The lime mudstones contain peloids, bivalves, gastropods, ostracodes, and foraminifers. Oolite stringers less than 0.5 m thick probably represent storm beds in deeper-water subtidal environment. The top of the sequence consists of oolite, peloidal, and skeletal packstone and grainstone 20 m and 50 m thick in the Chongzuo and Pingguo areas, respectively

(Figs. 3, 5). Sequence 3 is capped by molluscan wackestone and fenestral laminites in the Pingguo area. Sequence 4 Dolostone. The top of the Beisi Fm. consists of dolo-mudstone and packstone. The unit contains bivalves, gastropods, and the foraminifers Ammodiscus and Glomospira and the conodont Chiosella gondolelloides indicating a late Early Triassic (Olenekian) or basal Middle Triassic (Aegean) age. The fauna suggests restricted shallow-subtidal platform interior deposition. Sequence 4 is capped by microbial laminites and fenestral laminites indicating the development of tidal flats in both the Pingguo and Chongzuo areas. In both areas the dolostone is overlain by felsic volcanics (Figs. 3, 5). 4.4. Lower Triassic Luolou Formation The Luolou Fm. is widespread, spans the Lower Triassic, and is interpreted to be the basinal equivalent of the Majiaoling and Beisi formations (Guangxi Bureau, 1985). The Luolou consists primarily of laminated and horizontally burrowed, pyritic black lime mudstone with shale interbeds, shale, and debrisflow breccias. Within the limestone/shale intercalated facies the limestone to shale proportions range from 0.5 to 0.8 in sections measured south of Chongzuo. Fossils are generally extremely low in abundance, and include thin-shelled pectinoid bivalves, ostracodes, and ammonoids. Fossils become more conspicuous in the upper part at BN section (Fig. 3) and contain crinoids. The unit is interpreted to represent deep-marine, dysoxic sedimentation. At BN section along the basin-margin slope south of Chongzuo (Figs. 2, 3), the Luolou contains slump folds and several polymict debris-flow breccias. Breccias are clast supported and contain pebble to cobble sized, rounded to angular clasts composed of oolite packstone and grainstone, derived from the platform margin, and black, tabular lime mudstone, derived from the slope. Oolite clasts commonly contain leached ooids with dropped nuclei and rotated geopetals indicating lithification and vadose dissolution at the margin prior to transport. 4.5. Volcanic member of Banna Formation The volcanic member at the base of the Banna Fm. is composed of pyroclastics and lava flows of rhyolite to dacite composition (Newkirk et al., 2002; Guangxi

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Bureau, 2000). The unit is widespread in the Chongzuo area (Fig. 2) with measured thickness of 740 m at LY section (Fig. 3) and maximum thickness of 1640 m reported by the Guangxi Bureau (2000). In the Pingguo area the unit is only 6 m thick and has been mapped across the platform as a continuous horizon (Guangxi Bureau, 1985; Fig. 5). Lava flows are interpreted from the Chongzuo area based on the presence of euhedral phenocrysts, plagioclase microlites, and spherulites and a lack of glass shards or shattered phenocrysts (Newkirk et al., 2002). Pyroclastic breccias and tuff are also reported from Chongzuo (Guangxi Bureau, 2000). The thinner volcanic horizon at Pingguo is vitric tuff with abundant glass shards and shattered crystals. It is interpreted to be a water-lain ash on the basis of lamination and the fact that it is immediately underlain and overlain by marine carbonate. Several lines of evidence support the correlation of the thick volcanic succession at Chongzuo with the thinner tuff at Pingguo. (1) Biostratigraphy indicates a position near the Olenekian–Anisian (O–A) boundary in both areas. This is supported by the occurrence of Cs. gondolelloides, an index fossil that straddles the upper Olenekian to the basal Anisian (Aegean-Bithynian), immediately below the volcanics at Chongzuo and above the volcanics at Pingguo (Figs. 3, 5). Furthermore, carbonates of the Banmo member overlying the volcanics in both areas contain an assemblage of lower Middle Triassic (Bithynian) conodonts, foraminifera, and ammonoids (see section below). (2) Immobile trace elements have a similar signature in both areas indicating a similar volcanic source (Newkirk et al., 2002). (3) A preliminary zircon age for the tuff at Pingguo of ca. 247 ma corresponds to biostratigraphically constrained tuffs above the O–A boundary on the GBG (Martin et al., 2001). (4) The volcanics occur at the same lithostratigraphic position in both areas; underlain by dolostone of the Beisi Fm., and overlain by the drowning succession of the Banmo member of the Banna Fm. (Fig. 3). (5) The ash horizon at the boundary between the Beisi Fm. and Banmo member is the thickest volcanic horizon in the Pingguo platform, thus it is probable that it is equivalent to the thick pile of volcanics at Chongzuo. In Chongzuo the volcanics essentially terminated carbonate sedimentation and buried the platform. In contrast, the volcanics apparently interrupted carbonate platform growth only briefly in the Pingguo area. If our correlations are correct, the Chongzuo area is proximal to the source of volcanic ash horizons mapped across an enormous area of Guizhou, Guangxi, Yunnan, and southern Sichuan at the boundary between the Lower and Middle Triassic.

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4.6. Banmo member of Banna Formation The Banmo member overlies the volcanic member and consists of a package of shallow-marine skeletal and oncolitic limestone that changes upward into deepmarine, black, nodular-bedded lime wackestones and laminated marls marking the drowning and siliciclastic burial of a vast area of the CPP. In the Chongzuo area the lower part of the Banmo consists of rhythmic intercalation of dark gray to pinkish gray oncolitic skeletal packstone and argillaceous lime mudstone and wackestone. Lime mudstone and wackestone beds have wavy lamination and skeletal oncolitic beds are nodular-bedded with numerous argillaceous stylolite seams. The interval contains a diverse openmarine fauna of bivalves, gastropods, dasycladacean algae, crinoids, and the foraminifers Meandrospira and Pilammina (?). Oncoids have irregular cauliflower shapes, typically nucleate on bivalve or crinoid fragments, and are encrusted by cyanobacteria, foraminifera, and unidentified skeletal encrusters (Fig. 7A). This unit is interpreted to represent resurgence in carbonate sedimentation following volcanic burial (Fig. 3). A normal “healthy” carbonate platform growth was not re-established however, as evidenced by relatively thin accumulation, the dark color, argillaceous content, and lack of restricted carbonate or shoal facies. Instead the Banmo member is interpreted to represent deeper water carbonate sedimentation within the photic zone and above storm wave base. Instead of representing shallow restricted environments, the oncoids are interpreted to represent deeper-water algal nodules occasionally turned during storms. Similar algal nodules have been found in the termination sequence of other platforms in the Nanpanjiang basin (Lehrmann et al., 1998) and have been found in deep-water slope environments of modern platforms (Enos and Perkins, 1977). The oncolitic interval is overlain by black, nodular-bedded lime mudstone and wackestone. The nodular-bedded interval contains thin-shelled bivalves, and conodonts characteristic of deep-marine biofacies. This unit is interpreted to represent drowning and termination of carbonate sedimentation in the Chongzuo area and is followed by burial by siliciclastics of the Banna Fm. (Fig. 3). In the Pingguo area the Banna Fm. represents drowning of a large area of the platform as the interior was flooded and shallow-marine carbonate deposition stepped back to form smaller pinnacle platforms of the Guohua Fm. (Fig. 4). In this area the lower part of the Banmo member consists of partly dolomitized carbonate mudstones and wackestones with restricted

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Fig. 7. Middle Triassic facies. (A) Polished slab of oncolitic lime packstone from the Banmo member of the Banna Fm. Note crinoids (C) and oncoids (O). Oncoids commonly have pustular morphology (upper left). Nearly all grains have coatings of cyanobacteria, foraminifera, and unidentified skeletal encrusters. From BM section, Chongzuo area. (B) Polished slab of hardgrounds in the Banmo member of the Banna Fm. at TP section. (C) Black, nodular-bedded lime wackestone of interior drowning succession of Pingguo area. Banmo member of the Banna Fm. at TP section. (D) Laminated ammoniod-bearing marl, drowning succession of Pingguo area. Banmo member of the Banna Fm. at TP section. (E) Polished slab of Tubiphytes boundstone from massive reef facies, greater than 200 m thick along northern margin of northern Pingguo pinnacle (see Fig. 4 inset for location). Tubiphytes are small, branching, framework-building fossils (arrows). Note that the majority of this rock is composed of marine cement.

molluscan fauna that apparently represent continued platform growth following deposition of volcanic ash across the platform (Fig. 5). The presence of several hard grounds in this interval (Figs. 5, 7B) reflects pauses in carbonate-sediment accumulation. Dolomitized carbonate mudstones are followed by black, nodular lime mudstones (Fig. 7C) containing crinoids and Neogondolellid conodonts, indicating deposition in a deep-marine environment. These are finally overlain by laminated ammonoid- and radiolarianbearing marls (Fig. 7D). The succession provides relatively unambiguous evidence for termination of platform sedimentation by drowning. The Banmo member is interpreted to be Middle Triassic (Bithy-

nian) in age on the basis of the foraminifera Meandrospira and Pilammina (?), the conodonts Cs. gondolelloides and Neogondolella regalis, and the ammonoids Protrachyceras, Bulogites, and Paraceratites. The Banmo member is in turn overlain by the Banna shale member, a thick succession of shale and sandstone turbidites (Figs. 3, 5). 4.7. Middle Triassic Guohua Formation The Guohua Fm. consists of a complex of interior, reef margin, and slope facies that form at least three isolated pinnacle platforms in the Pingguo area (Fig. 4). Two pinnacles occur on the northern margin of the CPP

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in the town of Guohua and in the syncline northwest of NM section; another occurs on the southeast end of the syncline north of the town of Pingguo (Fig. 4). The pinnacle platforms range from approximately 4 to 8 km across and a thickness of 1700 m was measured at NM section. The best exposures occur in the syncline where steeply inclined strata preserve cross sections through the platform from the interior to basin margin (Fig. 4). Whereas over most of the Lower Triassic carbonate platform deposition shifted to siliciclastic basinal facies of the shale member of the Banna Fm., carbonate sedimentation continued in the areas of the pinnacles into the Middle Triassic Anisian. The isolated nature of the pinnacles is demonstrated by the pinchout of the Banna shale from TP section to the northeast and southeast against the Guohua Fm. (Fig. 4). The detailed facies distribution of the northern NM pinnacle is illustrated in the inset map of Fig. 4. The platform interior facies is composed of thick-bedded, peritidal cyclic limestone and dolomite. Cycle bases are oncolitic, intraclastic, and peloidal packstone to wackestone with few bivalves, gastropods, and dasycladacean algae. Cycle bases may contain flat-pebble conglomerates and domal stromatolites or may be extensively bioturbated. Cycle caps contain fenestral laminites. The lower part of the interior facies is limestone, the middle third is pervasively dolomitized, and the upper third consists of monotonous, massive thick-bedded peloidal and molluscan packstone without evidence of subaerial exposure. Massive Tubiphytes boundstone approximately 200 m thick occurs at the northern margin of the NM Pinnacle (Fig. 4; inset). The boundstone is composed of branching Tubiphytes frameworks reinforced by encrusting fossils such as spongiostromate algae and Bacinella as well as micritic cement (Fig. 7E). The frameworks are further reinforced by several generations of isopachous fibrous marine cement. Despite the fact that this facies apparently forms a robust platform-margin reef facing the open basin north of the CPP, the reef has extremely low biodiversity. Although in-place reefs were not found on the southeast margin of the NM platform (Fig. 4; inset), the presence of Tubiphytes boundstone clasts in slope breccias at NM section (Fig. 5) indicates that there were at least patch reefs developed along the southeast margin. In-place Tubiphytes patch reefs crop out on the northeast margin of the southerly pinnacle north of Pingguo (Fig. 4). Basin-margin facies flank the pinnacles. The best exposures, at NM section, are composed of a series of intertonguing carbonate wedges and shale tongues (Figs. 4, 5). The carbonate intervals consist of black,

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laminated lime mudstone with slump folds punctuated by polymict debris-flow breccias. The lime mudstone intervals are platy or nodular-bedded and are horizontally laminated in part. They contain crinoids, thin-shelled bivalves, radiolarians, and conodonts. Ammonoids occur in shale intervals. Debris-flow breccia beds may be as thin as a few meters, but more typically are tens of meters thick and range up to 100 m. The breccias are dominantly clast supported with pebble- to boulder-sized clasts composed of Tubiphytes boundstone, Tubiphytes fragment grainstone, oncolite-intraclastic packstone, molluscan wackestone and packstone, and black, tabular lime mudstone. The assortment of clasts indicates that erosion tapped into platform interior as well as platform margin and slope lithologies. The succession of conodonts from the NM section (Nicoraella germanica and Ni. kockeli followed by Ng. bulgarica) indicate a Bithynian to Pelsonian age (Fig. 5). The final termination of the Guohua Fm. is preserved in the southern pinnacle where the Guohua Fm. is overlain by shale in the Banna Fm. (north of Pingguo; Fig. 5). The final termination is marked by a shift to black, nodular-bedded, oncolitic wackestone followed by shale. The shift to a dark, nodular facies indicates termination by drowning. Conodonts within this interval include Ng. bulgarica indicating that the final termination occurred during the Pelsonian. Termination of the Guohua pinnacles was a significant event as it represents the end of carbonate deposition in the Nanpanjiang basin in Guangxi (Lehrmann et al., 2005). 5. Discussion: PT boundary and Lower Triassic Expanded sections spanning shallow- and deepwater environments through the Lower and Middle Triassic are essential for reaching an adequate understanding of the environmental and biological causes and consequences of the end-Permian extinction, particularly because the recovery interval was so protracted (cf. Hallam, 1991). Expanded sections provide an opportunity to distinguish facies-related artifacts from changes in the biota driven by regional or global processes. Moreover, multiple sections across an environmental gradient are ideal sources for detailed geochemical records that can be linked with the local (and global) paleontological record. Exposures of expanded sections in shallow-marine carbonate facies spanning the entire interval from the Permian through the Middle Triassic are rare, making the CPP a valuable resource for improving understanding of the endPermian extinction and its aftermath. The GBG contains perhaps the longest known Permian–Triassic record of

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shallow-marine carbonate deposition (Lehrmann et al., 1998) and is the source of a high-resolution Permian– Triassic fossil and carbon-isotope record (Payne et al., 2004, 2006b). The CPP is ideal for testing the regional generality of the paleontological and geochemical records on the GBG, as well as data from more distant localities. The biostratigraphically conformable contact between diverse, fossiliferous Upper Permian (Changhsingian) packstone and grainstone and low-diversity calcimicrobial boundstone in the Pingguo area is consistent with evidence for a geologically rapid extinction pulse (cf. Jin et al., 2000) and suggests abrupt onset of microbialite deposition after extinction. The presence of microbialites overlying the extinction horizon on all of the isolated platforms in the Nanpanjiang basin (Lehrmann et al., 2003) and the occurrence of stenohaline taxa such as echinoderms and articulate brachiopods in grainstone lenses within the microbialite indicate continued shallow, open-marine carbonate deposition across the extinction horizon rather than any significant shift in physical depositional environment (Lehrmann et al., 2003). Lower Triassic platform interior strata of the Majiaoling and Beisi formations contain a low-diversity fauna dominated by mollusks. The low overall abundance of animal and algal skeletal grains in Lower Triassic platform interior, platform margin, and basinal sediments stands in contrast to higher skeletal abundance in Upper Permian and Middle Triassic strata from the same environments. Similarly low abundance of skeletal grains was quantitatively demonstrated using point-counts of samples on the GBG (Payne et al., 2006b). Qualitative observations of Lower Triassic carbonate sections across Tethys are generally consistent with this pattern (e.g. Baud et al., 1997, 2005), although some condensed sections (e.g. Twitchett et al., 2004) contain a high concentration of fossil grains. Carbonate deposition across the CPP was dominated by lime mudstone and oolite to a much greater extent than comparable Upper Permian or Middle Triassic strata. Moreover, the presence of unusually large ooids (5–10 mm in diameter; Fig. 6D) (see Flügel, 1982, for a compilation Phanerozoic ooids by period) indicates unusually rapid growth relative to abrasion (Swett and Knoll, 1989), likely induced by high CaCO3 saturation and/or inhibition of ooid nucleation (see Sumner and Grotzinger, 1993). Qualitative field and thin-section observations indicate no significant change in the abundance of skeletal grains through the Early Triassic. The ichnofossil record, on the other hand, trends broadly from low intensity of bioturbation and exclusively

horizontal burrowing near the base of the Lower Triassic to higher intensity of bioturbation and the appearance of penetrative burrow networks higher in the Lower Triassic. Platform-margin and basinal strata (Luolou Fm.) likewise contain a low diversity biota with low skeletal abundance. Each of these observations closely parallels the record of Early Triassic biotic recovery and Lower Triassic carbonate facies previously documented on the GBG (Payne et al., 2006b) and elsewhere (e.g. Twitchett, 1999; but see Twitchett et al., 2004). The parallels include an increase in the abundance of crinoid grains high in the Lower Triassic on the basin margin in Luolou Fm. and the appearance of crinoids in the basal Anisian of the Banmo member in the platform interior. Increased abundance of crinoids in the Spathian has been observed on the GBG (Payne et al., 2006b), the western USA (Schubert and Bottjer, 1995), and western Tethys (Ramovš, 1996). The only report of high relative abundance of crinoids in the lower part of the Lower Triassic is from the Griesbachian in Oman (Twitchett et al., 2004). Two end-member explanations have been hypothesized to account for the occurrence of microbialites immediately overlying the extinction horizon (cf. Schubert and Bottjer, 1992; Baud et al., 1997; Lehrmann, 1999; Kershaw et al., 1999; Lehrmann et al., 2003; Baud et al., 2005): (1) microbialites represent a default mode of carbonate deposition in shallow-marine environments in the absence of significant skeletal production, metazoan grazing, and bioturbation, as a consequence of the extinction. (2) The microbialites reflect a change in marine chemistry governing patterns of carbonate deposition (e.g. carbonate saturation state and the concentration of inhibitors to calcium-carbonate nucleation). They do not merely occur due to the removal of skeletalcarbonate sinks. Rather, they reflect conditions generated by the same processes responsible for extinction. The occurrence of abundant aragonite fans within the base of the calcimicrobial boundstone on the CPP, many of which grow upward in discrete horizons from the substrate (Fig. 6A, B), suggests that the PTB microbialite reflects controls beyond the mere removal of skeletal sinks for calcium carbonate. Similar aragonite fans are widely distributed within the PTB microbialite unit occurring in south China, Turkey (see Kershaw et al., 1999; Baud et al., 2005), and Japan (D. Lehrmann, H. Sano, and J. Payne, unpublished observations). Similar fans are absent from younger Lower Triassic microbialites and are only known to occur locally in an outer shelf to slope setting (Woods et al., 1999). If the fans merely reflected the removal of skeletal carbonate sinks, one would expect them to continue in shallow water and in

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association with microbialites through much of the Lower Triassic given the continued low abundance of skeletal grains (Payne et al., 2006b). Furthermore, the widespread and synchronous deposition of microbialites appears to be confined to the lowermost Lower Triassic (H. parvus zone) (Lehrmann, 1999; Lehrmann et al., 2003). The fans could reflect an interval of high carbonate saturation of seawater, inhibition of other forms of carbonate deposition (e.g. lime mud), or some combination of these and other aspects of seawater chemistry. For example, the presence of metal-ion inhibitors of calcium-carbonate precipitation (e.g. Fe2+, Mg2+, Mn2+) could decrease the nucleation rate of new CaCO3 crystals and favor growth of existing crystals preferentially, yielding aragonite fans (Sumner and Grotzinger, 1993). Ferrous iron and manganese, in particular, are more soluble under low oxygen conditions, and so inhibition of CaCO3 may have been especially pronounced immediately after the extinction if low oxygen conditions were widespread (e.g. Wignall and Hallam, 1992; Wignall and Twitchett, 2002). Other possible drivers include the delivery of alkalinity from upwelling deep water (Grotzinger and Knoll, 1995; Knoll et al., 1996; Woods et al., 1999) or a weathering pulse resulting from a large input of CO2 into the atmosphere from Siberian Traps eruptions or the oxidation of large quantities of methane or organic matter (Krull and Retallack, 2000).

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implication that the return of reefs in the Middle Triassic was caused by biotic rediversification following the extinction (cf. Flügel and Stanley, 1984; Stanley, 1988). The near absence of framework-building metazoans in these reefs and dominance of Tubiphytes and penecontemporaneous cements in the reef framework indicates that the re-development of platform-margin reefs in the Middle Triassic was governed primarily by factors other than the recovery of reef building animals. The development of extensive oolite shoals containing large ooids in the Lower Triassic and reefs in the Middle Triassic composed largely of marine cement indicates that carbonate precipitation occurred readily on the margins of platforms throughout this interval and that the bulk of carbonate precipitation was non-skeletal (abiotic or biotically mediated). The shift in style of precipitation (ooids v. reef cements) as well as the concurrent shift in architecture from oolite ramps to abrupt, steep reef margins may reflect increased stabilization of platform-margin sediments by the problematic organism Tubiphytes. The relatively delicate branching frameworks of Tubiphytes were encrusted by marine cements forming relatively robust, rigid reef masses. The development of large, metazoan framework reefs dominated by scleractinian corals first began in the Late Triassic (Stanley, 1988). 7. Discussion: controls on recovery

6. Discussion: Middle Triassic Middle Triassic Tubiphytes reefs on the CPP are constrained as Anisian (Bithynian-Pelsonian) in age by conodonts from the NM section (Fig. 5). The high abundance (55–60% by volume) of penecontemporaneous and early-diagenetic cements – peloidal micritic cement, brownish fibrous calcite cement, fibrous isopachous calcite cement, and aragonite botyroids (Fig. 7E; Christensen and Lehrmann, 2004) – is consistent with observations of the Anisian reef on the GBG. In both cases, Tubiphytes is the only volumetrically significant framework element, and both reefs contain only a low diversity biota of invertebrates (ostracodes, mollusks, crinoids), dasyclad algae, and encrusting microproblematica (Bacinella, Plexoramea) (Payne et al., 2006a). The appearance of heavily cemented Anisian reef margins following Lower Triassic ramps with oolite on several platforms in the Nanpanjiang basin and worldwide (Flügel, 2002) suggests a global control on marginal reef development. The Lower Triassic has long been cited as a gap in metazoan reef development resulting from decimation of biota following the end-Permian extinction, with the

The CPP and the GBG record the end-Permian extinction and Lower–Middle Triassic recovery in unmatched detail. Strata from these platforms have provided and can provide multiple proxies for biotic recovery and environmental change through this interval, including the abundance, diversity, and distribution of skeletal grains, changing patterns of carbonate precipitation and deposition, and high-resolution geochemical records. Each of these proxy records collected to date has confirmed the geological rapidity and wide-reaching effects of the end-Permian extinction pulse. Together, they also indicate a protracted Early Triassic interval of carbon cycle instability and only limited biological recovery. Accelerated Middle Triassic recovery occurred in association with substantial change in carbonate deposition patterns and carbon cycle stabilization. Although the mechanisms linking biological recovery, carbon cycle stabilization, and the re-development of platform-margin reefs are currently poorly understood, a satisfactory model for Permian–Triassic events will only be achieved with coupled, high-resolution paleontological, sedimentary, and geochemical records similar to the ones being developed from the Chongzuo-Pingguo platform.

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8. Conclusions (1) The Chongzuo-Pingguo platform (CPP) is a vast isolated platform in the southern part of the Nanpanjiang basin of south China. The architecture and platform evolution were reconstructed from stratigraphic sections and mapping of a fold that exposes a 2-dimensional cross section through the platform. The platform evolved from a low-relief bank with oolite shoals in the Early Triassic, step back and drowning in the beginning of the Middle Triassic (Bithynian), and development of smaller pinnacle platforms with Tubiphytes reefs in the Middle Triassic (Bithynian to Pelsonian). (2) The platform contains expanded, conformable Permian–Triassic boundary (PTB) sections that closely resemble other PTB sections in the Nanpanjiang basin, around Tethys, and outside the Tethys. Diverse shallow-marine Upper Permian packstone and grainstone units are overlain by post-extinction calcimicrobial framestone with low abundance and diversity of fossil grains despite also being deposited in a shallow, open-marine setting. (3) The presence of aragonite fans exclusively in the base of the microbialite unit suggests a control of seawater chemistry on microbialite deposition beyond the mere loss of the skeletal biota. Changes in carbonate saturation of seawater or the concentration of inhibitors to CaCO3 precipitation could account for the presence of these fans. (4) The close resemblance of the Lower and Middle Triassic rock and fossil record on the CPP and the Great Bank of Guizhou (GBG), as well as more distant platforms around Tethys, suggests global controls on patterns of carbonate deposition. Lower Triassic carbonates are characterized by low skeletal content and proportional dominance by lime mud and ooids, including large ooids. (5) The re-development of platform margin reefs in the Middle Triassic was not a direct product of metazoan recovery. Instead, Middle Triassic reefs consist of heavily cemented Tubiphtyes framework. Acknowledgements This research is based upon work supported by the National Science Foundation (EAR-9804835) and by the Petroleum Research Fund of the American Chemical Society (ACS-40948-B2). This research has benefited greatly from the extensive involvement of undergraduate students as they completed independent research

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