Record of the end-Permian extinction and Triassic biotic recovery...

alaeoecology 252 (2007) 200–217www.elsevier.com/locate/palaeo

Palaeogeography, Palaeoclimatology, P

Record of the end-Permian extinction and Triassic biotic recovery inthe 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 Statesb Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, United States

c Department of Geology, University of Kansas, Lawrence, KS 66045, United Statesd Geological Survey of Guangxi, Guilin, Guangxi, People's Republic of Chinae Guizhou Bureau of Geology, Guiyang, Guizhou, People's Republic of China

f Geological Survey of Canada, Vancouver, B.C., Canada V6B 5J3g Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States

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. Theend-Permian extinction is recorded in conformable sections in the northern part of the platform (Pingguo area). Upper Permianskeletal packstone contains diverse open-marine fossils including Nankinella and Sphaerulina. It is overlain by a 4.6 m thickhorizon of calcimicrobial framestone constructed by globular calcified microbial framework similar to Renalcis. The PTB eventhorizon is interpreted to occur at the top of the packstone, coincident with the abrupt change to calcimicrobial framestone lackingPermian macrofossils. The transition from Hindeodus latidentatus to H. parvus occurs 1 m above the base of the calcimicrobialframestone, 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 high-relief, fault-controlled escarpment along its southern margin. Platform interior facies of the Majiaoling and Beisi formations are900 m thick and consist of thin-bedded lime mudstone, oolite, and dolostone with restricted marine biota in four shallowing-upward packages that define depositional sequences. In the Chongzuo area the platform was terminated at the end of the EarlyTriassic by burial with up to 1600 m of felsic pyroclastic volcanics and lava flows. Transgression and shallow-marine carbonatesedimentation resumed early in the Middle Triassic followed by drowning in the Bithynian as indicated by a shift to deep-marinecarbonate and clastic deposition.

In the Pingguo area volcanic deposits are much thinner and only briefly interrupted carbonate accumulation. Here the platforminterior 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.

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Guohua Fm., up to 1700 m thick, continued to accumulate above the Early Triassic platform margin and were eventually drownedby 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 arecomposed almost entirely of Tubiphytes reinforced by large volumes of marine cement. These observations indicate that controlsbeyond 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 greatestdevastation in the marine sedimentary record, anaftermath characterized by the unusual abiotic carbonateprecipitates, and an exceptionally long biotic recovery

Fig. 1. Early Triassic paleogeographic map of the Nanpanjiang basin compBureau, 1987). Inset is a tectonic map of south China modified fromexposures used to constrain the distribution of platform and basin environmet al. (2005).

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

iled from regional geologic maps (Guangxi Bureau, 1985; GuizhouSun et al. (1989). For the detailed distribution of Lower Triassicents in this paleogeographic reconstruction see Fig. 3 in Lehrmann

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(CO2 poisoning), methane release, and hydrogen sulfidepoisoning linked to oceanic euxinia (e.g. Wignall andHallam, 1992; Knoll et al., 1996; Krull and Retallack,2000; Becker et al., 2004; Kump et al., 2005; amongmany others).

Many studies have focused on the facies, bioticchanges, geochemistry and geochronology of Perm-ian–Triassic boundary (PTB) sections from variouslocalities 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 directedtoward the Lower–Middle Triassic record of bioticrecovery and environmental conditions in the pro-longed aftermath following the extinction (e.g. Schu-bert and Bottjer, 1995; Twitchett, 1999; Woods et al.,1999; Lehrmann et al., 2001; Wignall and Twitchett,

Fig. 2. Geologic map of the Chongzuo area. Modified from Guangxi BureauDonghong (unpublished data). Sections BM, LJ, LY, and BN are presented

2002; Fraiser and Bottjer, 2004; Payne et al., 2004,2006b).

An obstacle in understanding the end-Permian ex-tinction has been the rarity of conformable PTB sectionsavailable for study. Even less common are the expanded,conformable and continuously exposed sections of theentire Upper Permian through Middle Triassic recordneeded to develop the high-resolution sedimentological,geochemical, and paleontological records required forunderstanding Permian–Triassic events. In previouspapers we have emphasized the importance of conform-able PTB sections and expanded Upper-Permianthrough Middle-Triassic sections of the Great Bank ofGuizhou (GBG) in the northern part of the Nanpanjiangbasin (Fig. 1; Lehrmann et al., 1998; Payne et al., 2004,2006b; Lehrmann et al., 2005). The purpose of thisstudy is to summarize the stratigraphic architecture,facies, and depositional environments from the PTB

(2000, geologic map 1:500,000), using field observations made by Peiin Fig. 3.

Fig. 3. Stratigraphic cross section of the Chongzuo area. Stratigraphic sections BM, LJ, and LYoccur 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 carbonateplatform, the Chongzuo-Pingguo platform (CPP), in thesouthern part of the basin. The CPP contains continu-ously exposed, relatively conformable and expandedPTB through Middle Triassic sections and it is dissectedby a fold that exposes a 2-D cross section revealingits architecture. Further study of this platform holdspromise for understanding the environmental conditionsof the extinction and biotic recovery.

2. Geological setting

The Nanpanjiang basin is a deep-marine embaymentin the southern margin of the Yangtze plate (Fig. 1,inset) which was located in the eastern Tethys sea-way near the equator during the Permian, progressivelymigrated northward and eventually docked with the

Fig. 4. Geologic map of the Pingguo area. Modified, using reconnaissance fieand Guangxi Bureau (unpublished geologic maps 1:200,000). Inset upper rigNM are presented in Fig. 5.

north China plate during the Late Triassic (Lehrmannet al., 1998). Several isolated platforms developedwithin the Nanpanjiang basin during the Triassicincluding the Great Bank of Guizhou (GBG) in southernGuizhou Province and the Chongzuo-Pingguo platform(CPP) in southern Guangxi (Fig. 1). Each of theplatforms is delineated by the regional distribution ofshallow-marine carbonate platform facies and deep-water basinal facies (Lehrmann et al., 2005). Theisolated platforms exhibit a pattern of greater longevityin the north, step-backed margins and pinnacle devel-opment in the south, and earlier drowning and burial bysiliciclastics in the south (Lehrmann et al., 2005). LowerTriassic volcanic horizons thicken dramatically south-ward indicating a southerly source. These differenceswere interpreted to have resulted from faster subsidencerates and volcanism in the southern part of the basin

ld mapping, from Guangxi Bureau (1987, 1:1000000; 2000, 1:500000)ht is a detailed map of the northern pinnacle platform. Sections TP and

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 ofthe northern pinnacle platform. See Fig. 4 for section locations.

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caused by tectonic convergence along the southernmargin of the south China plate (Lehrmann et al., 2005).

3. Stratigraphic architecture

Upper Permian strata in the CPP consist of the Heshanand Dalong formations. The Heshan Fm. consists of athin bauxitic mudstone at the base, and a thick upper partdominated by cherty limestone with lesser calcareousshale and coal seams (Guangxi Bureau, 1985). Theformation ranges up to 225 m thick in the Chongzuo areaand its upper part is dominated by cherty limestone withdiverse shallow-marine biota of brachiopods, foraminif-era, gastropods, dasycladacean algae, and corals. In thePingguo area the top of the unit contains several felsicvolcanic ash layers below a conformable PTB (Lehr-mann et al., 2003). The Dalong Fm. is composed of dark-colored spiculitic mudrock, chert, and cherty argilla-ceous limestone with volcanic ash layers and a deeperwater assemblage of ammonoids, bivalves and brachio-pods (Lehrmann et al., 2005). The unit ranges up to160 m thick (Guangxi Bureau, 1985).

In the Chongzuo area regional geologic mapscombine the Heshan and Dalong in one mapping unitacross much of the study area (Fig. 2). However, thePingxiang–Dongmen fault (P–D fault; Figs. 1, 2) wasactive during the Upper Permian affecting uplift andshallow-water deposition of the Heshan Fm. (e.g. LJsection; Figs. 2, 3) north of the fault and deeper-waterdeposition of the Dalong Fm. south of the fault (BMsection, Lianqiao; Fig. 2). Uplift and erosion haveremoved Upper Permian strata north of the fault west ofChongzuo (area west of LJ section) and in the areaaround Longzhou where Lower Permian strata areoverlain by Lower Triassic strata of the Majiaoling Fm.Regional geologic maps indicate that the Heshan Fm.has a widespread distribution in the Pingguo area thatextended well beyond the area of the Lower Triassicplatforms (Fig. 1; Guangxi Bureau, 2000). This changesuggests that southern Guangxi may have been the siteof vast shallow-marine carbonate, clastic and paraliccoal environments during the Late Permian prior to thedevelopment of smaller isolated carbonate platforms inthe Early Triassic.

Lower Triassic formations consist of shallow-marinecarbonates of the Majiaoling and Beisi formations andbasinal deposits of the Luolou Fm. The Majiaoling Fm.ranges from 100 to 130 m thick in the CPP and iscomposed of light to medium gray, thin-bedded limemudstone with minor oolite (Figs. 3, 5). The Beisi Fm.ranges up to 800 m thick in the CPP and is composed oflime mudstone with thick intervals of oolite, arranged

into shallowing-upward packages, and dolostone con-centrated in its upper part (Figs. 3, 5). The Luolou Fm.ranges from 40 to 538 m thick and consists of black todark-gray, laminated or horizontally burrowed limemudstone with shale intercalations and interbeds ofdebris-flow breccias (Guangxi Bureau, 1985).

The distribution of Lower Triassic formations fromregional geologic maps and ground observations deline-ates the extent of the CPP (Fig. 1; Guangxi Bureau, 1985;Lehrmann et al., 2005; Guangxi Bureau, 2000). Shallow-marine carbonates of the Majiaoling and Beisi forma-tions define the extent of the platform. In the Pingguoarea the platform is bounded to the north, west, and eastby basinal deposits of the Luolou Fm. (Fig. 4). Thesouthern margin of the platform in the Chongzuo area isdefined by the P–D fault which may have remainedactive or at least a bathymetric demarcation during theEarly Triassic as evidenced by the distribution ofshallow-marine Majiaoling and Beisi formations northof the fault and basinal deposits of the Luolou Fm. to thesouth of it (Figs. 1, 2). In the Chongzuo area the platformextends westward into Vietnam; to the east the margin isunconstrained owing to lack of outcrop (Fig. 1). Asignificant question is whether the Chongzuo-Pingguoarea is a single isolated platform or two separateplatforms. The problem arises because of the lack ofpreserved Triassic strata on the Nanning anticline thatseparates the areas (Fig. 1). Two lines of evidencesuggest that it developed as a single platform: (1)reconnaissance mapping revealed shallow-marine car-bonates with oolite extending to the southern extent ofexposures in the Pingguo area (Fig. 1) and (2) isolatedexposures 30 km west of Nanning also were interpretedto be shallow-marine carbonates of the Majiaoling Fm.(Guangxi Bureau, 2000; Pei Donghong, unpublished).

The Middle Triassic (Anisian) Guohua Fm. has itstype area in the Pingguo Area (Fig. 1). The Guohua Fm.consists primarily of thick-bedded massive and cyclicdolomite with restricted molluscan fauna and fenestrallaminites. The formation also contains minor butsignificant Tubiphytes boundstone and basin-marginfacies composed of pelagic limestones and interbeddeddebris-flow breccias. The stratigraphic architecture ofthe Pingguo area is revealed by a northwest trendingfold that exposes a 2-D cross section (Fig. 4). Thearchitecture is best visualized by viewing the stratadown structural dip on the northeast limb of the syncline(area of TP and NM sections, Fig. 4). The UpperPermian Heshan Fm. is conformably overlain byshallow-marine carbonate strata of the Lower TriassicMajiaoling and Beisi formations. Greater abundance ofoolite shoals near the northern and southern margins of


the platform in the Majiaoling and Beisi and the lack ofreefs indicate that the platform had a low-relief bankarchitecture during the Early Triassic. The platformcontinued to accumulate shallow-marine carbonate untilthe platform interior drowned and shifted to deep-marine siliciclastic deposition represented by the BannaFm. in the beginning of the Middle Triassic (Anisian,Bithynian; Figs. 4, 5; top of TP section). During theinterior drowning, shallow-marine carbonate depositionstepped back and continued as smaller pinnacle plat-forms (small isolated platforms analogous to pinnaclereefs but containing differentiated interior, reef, andslope facies) represented by the Guohua Fm. at thenorthern margin of the platform (NM section) andsouthern limit of the syncline northeast of Pingguo(Fig. 4). The pinnacle platforms of the Guohua Fm. are1700 m thick (NM section; Fig. 5) and contain peritidaldolomite in their interiors, Tubiphytes reefs at themargins, and slope carbonates that intertongue withsiliciclastic turbidites adjacent to the platforms (NMsection, Figs. 4, 5). The pinnacle platforms were finallydrowned and buried by siliciclastics of the Banna Fm.near the end of the Anisian, Pelsonian (Figs. 4, 5).

Numerous faults dissect the Chongzuo area. Inter-pretation of architecture in this area was constrained bythree measured stratigraphic sections in the platforminterior (BM, LJ, and LY sections; Figs. 2, 3) and onesection in the basin south of the P–D fault (BN section;Figs. 2, 3). Shallow-marine carbonate deposition wasinitiated in the Early Triassic during transgression overan unconformity above the Permian (Fig. 3). DuringEarly Triassic deposition of the Majiaoling and Beisiformations the platform is inferred to have developed alow-relief bank type architecture bordered to the southby oolite shoals at the platform margin and perhaps anabrupt escarpment along the P–D fault (Fig. 2). Thegreater abundance of oolite in LY section close tothe margin provides evidence for development ofmarginal oolite shoals (Figs. 2, 3). At the end of theEarly Triassic the platform terminated by burial beneath athick pile of felsic volcanics (Figs. 2, 3). After cessation ofvolcanics, shallow-marine carbonate deposition resumedbriefly (Banmo member) followed by drowning andburial by deep-marine siliciclastics of the Banna Fm. inthe Middle Anisian (BM section; Figs. 2, 3).

4. Lithofacies and depositional environments

4.1. Permian–Triassic boundary

The Permian–Triassic boundary in the Chongzuoand Pingguo areas has been described in detail by

Lehrmann et al. (2003). The boundary is conformablein the Pingguo area where cherty skeletal packstone ofthe upper Heshan Fm., with diverse open-marine faunaand volcanic ash interbeds, is overlain by calcimicro-bial framestone of the basal Majiaoling Fm. The upperHeshan Fm. contains the fusulinids Nankinella andSphaerulina indicating a Late Permian age. The end-Permian extinction is interpreted to have occurred atthe boundary between skeletal packstone containingdiverse Permian fossils and overlying calcimicrobialframestone lacking Permian macrofossils. The bio-stratigraphic PT boundary is currently placed 1 mabove the base of the calcimicrobial framestone on thebasis of Hindeodus latidentatus within the basal meterof the calcimicrobial framestone and the first occur-rence of H. parvus 1 m above the base. The skeletalpackstone and the calcimicrobial framestone areinterpreted to represent the continuation of shallow,open-marine environments during the end-Permianextinction; the abrupt facies change is interpreted toreflect a substantial decrease in the contribution ofskeletal animals and algae to carbonate accumulationand, perhaps, a change to oceanic chemistry intoler-able for much of the shallow-marine biota duringthe aftermath of the extinction (Lehrmann et al., 2003;Payne et al., 2006b). In TP section (Figs. 4, 5) thecalcimicrobial framestone unit is 4.6 m thick and iscomposed primarily of chambered globular frame-works similar to Renalcis surrounding irregularcavities (Figs. 4, 5). The very base of the microbialite,however, contains digitate fabrics constructed primar-ily by clusters of upward radiating aragonite fans(Fig. 6A, B).

The PTB is unconformable across much of theCPP in the Chongzuo area. At LY and LJ sections theMajiaoling Fm. unconformably overlies Upper Permianstrata of the Heshan Fm. (Figs. 2, 3). The unconformityis overlain at LY section by a thin conglomerate con-taining clasts of limestone, feldspar and coal derivedfrom erosion of underlying strata. West of LJ section andin the Longzhou area, the Majiaoling Fm. overliesLower Permian strata. Calcimicrobial framestone wasfound in the base of the Majiaoling Fm., overlying theHeshan Fm. in only one unnamed locality near thesouthern margin of the platform (Fig. 2; UN). Thebeveling of the strata into the Lower Permian indicatesthat the unconformity resulted from tectonic uplift. Theabsence of the calcimicrobial facies in other sectionssuggests that the basal Triassic record may be missingover much of the platform. It is uncertain whether thecalcimicrobial horizon is missing as the result of erosionor onlap of the Lower Triassic section along the

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


unconformity. However, the fact that the Lower Triassicis relatively similar in thickness between the LY and LJsections (Fig. 3) indicates that there was not substantialerosional truncation of Lower Triassic strata in theregion. Sections in the basin south of the P–D fault areprobably conformable where the Upper Permian DalongFm. 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 mediumgray, thin-bedded, platy limestone and minor oolite.Lime mudstone beds range from 5 to 40 cm thick and areseparated by argillaceous seams (Fig. 6C). The beds aretypically massive, homogenous or may have wavylamination or vague burrows. Fossils include very rare

gastropods, bivalves, and foraminifers, and one ammo-noid above the calcimicrobial framestone at TP section.Argillaceous seams result in a greater magnetic suscep-tibility and “spikey signature” in the lime mudstone ascompared with oolite intervals of the overlying Beisi Fm.(Figs. 3, 5). Oolite occurs in thin stringers commonlywith scoured bases and fining upward beds.

The homogenous massive beds suggest bioturbation;however, discrete burrows are rare. The low biodiversitymolluscan fauna are suggestive of restricted marinecirculation, but it may alternatively represent alteredabundance and distribution patterns of the marine biotaafter the end-Permian extinction. Oolites are interpretedto have been transported into the site of depositionduring storms. The predominance of fine-grainedsediment and presence of fining-upward oolite bedssuggests deposition in subtidal environments above


storm wave base. The lower part of the Majiaoling Fm.in the Chongzuo area is thicker bedded lime mudstonewith oolite stringers (Fig. 3; LY, LJ sections) in contrastto dominantly thin-bedded lower portion in the Pingguoarea (Fig. 5; TP section).

4.3. Lower Triassic Beisi Formation

The Beisi Fm. is 600–800 m thick and consistsprimarily of platy, thin-bedded lime mudstone andmassive to cross-bedded oolite arranged into meter-scale, shallowing-upward cycles and larger grainingupward depositional sequences 50 to 300 m thick. Theupper parts of depositional sequences commonlycontain evidence of subaerial exposure such as fenestrallaminates. The lower part of the formation containsshale in the Pingguo area and the upper part of theformation is dolomitized. Oolite apparently extendedacross the platform interior and thickens from LJ to LYsection southward toward the platform margin in theChongzuo area (Figs. 2, 3) and at the northern margin ofthe platform in the Pingguo area (northwest of NMsection; Fig. 4) indicating the presence of shoals at theplatform margin.

Thin-bedded lime mudstones are similar to thosedescribed from the Majiaoling Fm. They are massiveto wavy laminated and contain few burrows, thin-shelled pectinoid bivalves, foraminifers, and argilla-ceous seams between beds. The mudstones are in-terpreted to represent deeper subtidal transgressiveportions of depositional sequences. Thin fining upwardinterbeds of oolite with scoured bases are interpreted tobe storm deposits.

Oolites are spectacular cliff forming units and rangefrom individual beds a few meters thick to massiveamalgamated beds up to 25 m in thickness. The oolitesare massive to cross-bedded grainstone and coarsenupward in some sequences (Fig. 6D). Included are “giantooids” from 5 mm to 1 cm in diameter. Similar giantooids have been reported from the Lower Triassic inother platforms in the Nanpanjiang basin (Payne et al.,2006a) and in Germany (Weidlich, 2005). Compositecoated grains composed of grapestone-like aggregates ofooids, which are in turn enveloped in oolitic cortices,range up to 3 cm across. Additional grains includegastropods, bivalves, peloids, and intraclasts. The suiteof fabrics and grains indicates high-energy subtidal tointertidal shoals. The upper parts of some oolite intervalsare burrowed packstone indicating abandonment andstabilization of ooid shoals.

Evidence for subaerial exposure at cycle andsequence boundaries includes ribbon rock characterized

by flaser-laminated interlayering of lenticular and ripplecross-laminated fine peloidal grainstone with scouredbases and isopachous lime mud drapes. The ribbon rockalso contains microbial laminites and dewateringstructures that probably formed by desiccation. Theribbon rock is identical to Lower Triassic peritidal faciesdescribed from the Great Bank of Guizhou and is similarto that found in Lower Paleozoic tidal flat facies(Lehrmann et al., 2001). Subaerial exposure is alsoindicated by fenestral laminites with mud crackscapping sequences.

Facies changes within depositional sequences re-sult in a distinctive magnetic susceptibility signature(Figs. 3, 5). The lower transgressive argillaceous limemudstone yields a spikey high magnetic susceptibil-ity profile that changes upward into the regressiveoolite with a blocky, lower magnetic susceptibilityprofile resulting from lower argillaceous content inthe oolite. The presence of 4 or 5 depositional se-quences spanning the Lower Triassic (Figs. 3, 5) in-dicates that they are third or fourth order (each with aduration of approximately 1 my or somewhat less).There is some ambiguity in correlation of the depo-sitional sequences between the Chongzuo and Pingguoareas resulting from differences in facies distribution.The following describes our interpreted best correla-tion (sequences 1–4; Figs. 3, 5).

Sequence 1 The Beisi Fm. is differentiated from theunderlying Majiaoling Fm. by the greaterproportion of oolite and dolomitization inits upper part. The base of the Beisi Fm. isdominated by oolite representing the re-gressive phase of depositional sequence 1that began in the Majiaoling Fm. (Figs. 3,5). Oolite beds increase in frequency andthickness and magnetic susceptibility pro-gressively decreases upward (best devel-oped in LYand TP sections; Figs. 3, 5). Theupper oolite is cross-bedded and maximumbed thickness reaches 10 m. Giant ooidsand large composite coated grains arefound near the top of the sequence.Subaerial exposure is represented by rib-bon rocks, microbial laminites and waterescape structures capping the sequence atTP section.

Sequence 2 The transgressive portion of sequence2 begins in the Pingguo area with thedeposition of an interval of marine shale100 m thick followed by thin-beddedlime mudstone with thin oolite stringers


(Fig. 5). The regressive portion in Pingguoconsists of several shallowing upwardcycles 5–15 m thick. Each cycle coarsensupward from thin-bedded lime mudstonethrough oolite packstone and grainstoneand is capped with ribbon rock. In a fewcycles the oolite coarsens upward to giantooids and large composite coated grains inthe upper part. In the Chongzuo area,sequence 2 shallows upward from a limemudstone interval 25 m thick and coarsensupward to a package of oolite greater than100 m thick with massive or cross-beddedoolite in individual beds ranging from 5 to25 m thick (Fig. 3). Sequence 2 is cappedby fenestral laminite at Chongzuo. TheChongzuo area apparently contains anadditional sequence (2a) when comparedwith the Pingguo area (Figs. 3, 5). In theplatform interior section at LJ, sequence2a is characterized by a relatively thinoolite and negative magnetic susceptibilityexcursion within a thick interval dominat-ed by lime mudstone (Fig. 3). At LYsection, near the platform margin, se-quence 2a is dominated by oolite grain-stone separated from the underlyingsequence by fenestral laminite. It isimportant to note that sequences 2 and2A contain the greatest proportion ofoolite in the Beisi Fm. which is fol-lowed upward in both the Chongzuo andPingguo areas by greatly increased pro-portion of lime mudstone suggesting thatthis interval represents a second-ordermaximum regression.

Sequence 3 The transgressive portion consists of morethan 100 m of thin-bedded light tomedium gray lime mudstone with inten-sive bioturbation. The unit is burrowmottled and contains abundant horizon-tal burrows as well as bed penetrativebranched burrows with random orienta-tions. The lime mudstones contain peloids,bivalves, gastropods, ostracodes, and for-aminifers. Oolite stringers less than 0.5 mthick probably represent storm beds indeeper-water subtidal environment. Thetop of the sequence consists of oolite,peloidal, and skeletal packstone and grain-stone 20 m and 50 m thick in the Chong-zuo and Pingguo areas, respectively

(Figs. 3, 5). Sequence 3 is capped bymolluscan wackestone and fenestral lami-nites 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 andGlomospira and the conodont Chiosellagondolelloides indicating a late EarlyTriassic (Olenekian) or basal MiddleTriassic (Aegean) age. The fauna suggestsrestricted shallow-subtidal platform interi-or deposition. Sequence 4 is capped bymicrobial laminites and fenestral laminitesindicating the development of tidal flats inboth the Pingguo and Chongzuo areas. Inboth areas the dolostone is overlain byfelsic volcanics (Figs. 3, 5).

4.4. Lower Triassic Luolou Formation

The Luolou Fm. is widespread, spans the LowerTriassic, and is interpreted to be the basinal equivalentof the Majiaoling and Beisi formations (GuangxiBureau, 1985). The Luolou consists primarily oflaminated and horizontally burrowed, pyritic blacklime mudstone with shale interbeds, shale, and debris-flow breccias. Within the limestone/shale intercalatedfacies the limestone to shale proportions range from 0.5to 0.8 in sections measured south of Chongzuo. Fossilsare generally extremely low in abundance, and includethin-shelled pectinoid bivalves, ostracodes, and ammo-noids. Fossils become more conspicuous in the upperpart at BN section (Fig. 3) and contain crinoids. The unitis interpreted to represent deep-marine, dysoxic sedi-mentation. At BN section along the basin-margin slopesouth of Chongzuo (Figs. 2, 3), the Luolou containsslump folds and several polymict debris-flow breccias.Breccias are clast supported and contain pebble tocobble sized, rounded to angular clasts composed ofoolite packstone and grainstone, derived from theplatform margin, and black, tabular lime mudstone,derived from the slope. Oolite clasts commonly containleached ooids with dropped nuclei and rotated geopetalsindicating lithification and vadose dissolution at themargin prior to transport.

4.5. Volcanic member of Banna Formation

The volcanic member at the base of the Banna Fm. iscomposed of pyroclastics and lava flows of rhyolite todacite composition (Newkirk et al., 2002; Guangxi


Bureau, 2000). The unit is widespread in the Chongzuoarea (Fig. 2) with measured thickness of 740 m at LYsection (Fig. 3) and maximum thickness of 1640 mreported by the Guangxi Bureau (2000). In the Pingguoarea the unit is only 6 m thick and has been mappedacross the platform as a continuous horizon (GuangxiBureau, 1985; Fig. 5). Lava flows are interpreted fromthe Chongzuo area based on the presence of euhedralphenocrysts, plagioclase microlites, and spherulites anda lack of glass shards or shattered phenocrysts (Newkirket al., 2002). Pyroclastic breccias and tuff are alsoreported from Chongzuo (Guangxi Bureau, 2000). Thethinner volcanic horizon at Pingguo is vitric tuff withabundant glass shards and shattered crystals. It isinterpreted to be a water-lain ash on the basis oflamination and the fact that it is immediately underlainand overlain by marine carbonate.

Several lines of evidence support the correlation ofthe thick volcanic succession at Chongzuo with thethinner tuff at Pingguo. (1) Biostratigraphy indicates aposition near the Olenekian–Anisian (O–A) boundaryin both areas. This is supported by the occurrence of Cs.gondolelloides, an index fossil that straddles the upperOlenekian to the basal Anisian (Aegean-Bithynian),immediately below the volcanics at Chongzuo andabove the volcanics at Pingguo (Figs. 3, 5). Further-more, carbonates of the Banmo member overlying thevolcanics in both areas contain an assemblage of lowerMiddle Triassic (Bithynian) conodonts, foraminifera,and ammonoids (see section below). (2) Immobiletrace elements have a similar signature in both areasindicating a similar volcanic source (Newkirk et al.,2002). (3) A preliminary zircon age for the tuff atPingguo of ca. 247 ma corresponds to biostratigraphi-cally constrained tuffs above the O–A boundary on theGBG (Martin et al., 2001). (4) The volcanics occur at thesame lithostratigraphic position in both areas; underlainby dolostone of the Beisi Fm., and overlain by thedrowning succession of the Banmo member of theBanna Fm. (Fig. 3). (5) The ash horizon at the boundarybetween the Beisi Fm. and Banmo member is thethickest volcanic horizon in the Pingguo platform, thusit is probable that it is equivalent to the thick pile ofvolcanics at Chongzuo. In Chongzuo the volcanicsessentially terminated carbonate sedimentation andburied the platform. In contrast, the volcanics apparentlyinterrupted carbonate platform growth only briefly inthe Pingguo area. If our correlations are correct, theChongzuo area is proximal to the source of volcanic ashhorizons mapped across an enormous area of Guizhou,Guangxi, Yunnan, and southern Sichuan at the boundarybetween the Lower and Middle Triassic.

4.6. Banmo member of Banna Formation

The Banmo member overlies the volcanic memberand consists of a package of shallow-marine skeletal andoncolitic limestone that changes upward into deep-marine, black, nodular-bedded lime wackestones andlaminated marls marking the drowning and siliciclasticburial of a vast area of the CPP.

In the Chongzuo area the lower part of the Banmoconsists of rhythmic intercalation of dark gray to pinkishgray oncolitic skeletal packstone and argillaceous limemudstone and wackestone. Lime mudstone and wack-estone beds have wavy lamination and skeletal oncoliticbeds are nodular-bedded with numerous argillaceousstylolite seams. The interval contains a diverse open-marine fauna of bivalves, gastropods, dasycladaceanalgae, crinoids, and the foraminifers Meandrospira andPilammina (?). Oncoids have irregular cauliflowershapes, typically nucleate on bivalve or crinoidfragments, and are encrusted by cyanobacteria, forami-nifera, and unidentified skeletal encrusters (Fig. 7A).This unit is interpreted to represent resurgence incarbonate sedimentation following volcanic burial(Fig. 3). A normal “healthy” carbonate platform growthwas not re-established however, as evidenced byrelatively thin accumulation, the dark color, argillaceouscontent, and lack of restricted carbonate or shoal facies.Instead the Banmo member is interpreted to representdeeper water carbonate sedimentation within the photiczone and above storm wave base. Instead of represent-ing shallow restricted environments, the oncoids areinterpreted to represent deeper-water algal nodulesoccasionally turned during storms. Similar algal noduleshave been found in the termination sequence of otherplatforms in the Nanpanjiang basin (Lehrmann et al.,1998) and have been found in deep-water slopeenvironments of modern platforms (Enos and Perkins,1977). The oncolitic interval is overlain by black,nodular-bedded lime mudstone and wackestone. Thenodular-bedded interval contains thin-shelled bivalves,and conodonts characteristic of deep-marine biofacies.This unit is interpreted to represent drowning andtermination of carbonate sedimentation in the Chongzuoarea and is followed by burial by siliciclastics of theBanna Fm. (Fig. 3).

In the Pingguo area the Banna Fm. representsdrowning of a large area of the platform as the interiorwas flooded and shallow-marine carbonate depositionstepped back to form smaller pinnacle platforms of theGuohua Fm. (Fig. 4). In this area the lower part of theBanmo member consists of partly dolomitized car-bonate mudstones and wackestones with restricted

Fig. 7. Middle Triassic facies. (A) Polished slab of oncolitic lime packstone from the Banmo member of the Banna Fm. Note crinoids (C) andoncoids (O). Oncoids commonly have pustular morphology (upper left). Nearly all grains have coatings of cyanobacteria, foraminifera, andunidentified skeletal encrusters. From BM section, Chongzuo area. (B) Polished slab of hardgrounds in the Banmo member of the Banna Fm. atTP section. (C) Black, nodular-bedded lime wackestone of interior drowning succession of Pingguo area. Banmo member of the Banna Fm. atTP 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 iscomposed of marine cement.


molluscan fauna that apparently represent continuedplatform growth following deposition of volcanic ashacross the platform (Fig. 5). The presence of severalhard grounds in this interval (Figs. 5, 7B) reflectspauses in carbonate-sediment accumulation. Dolomi-tized carbonate mudstones are followed by black,nodular lime mudstones (Fig. 7C) containing crinoidsand Neogondolellid conodonts, indicating deposi-tion in a deep-marine environment. These are finallyoverlain by laminated ammonoid- and radiolarian-bearing marls (Fig. 7D). The succession providesrelatively unambiguous evidence for termination ofplatform sedimentation by drowning. The Banmomember is interpreted to be Middle Triassic (Bithy-

nian) in age on the basis of the foraminifera Mean-drospira and Pilammina (?), the conodonts Cs.gondolelloides and Neogondolella regalis, and theammonoids Protrachyceras, Bulogites, and Paracer-atites. The Banmo member is in turn overlain by theBanna shale member, a thick succession of shale andsandstone 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 threeisolated pinnacle platforms in the Pingguo area (Fig. 4).Two pinnacles occur on the northern margin of the CPP


in the town of Guohua and in the syncline northwest ofNM section; another occurs on the southeast end of thesyncline north of the town of Pingguo (Fig. 4). Thepinnacle platforms range from approximately 4 to 8 kmacross and a thickness of 1700 m was measured at NMsection. The best exposures occur in the syncline wheresteeply inclined strata preserve cross sections throughthe platform from the interior to basin margin (Fig. 4).Whereas over most of the Lower Triassic carbonateplatform deposition shifted to siliciclastic basinal faciesof the shale member of the Banna Fm., carbonatesedimentation continued in the areas of the pinnaclesinto the Middle Triassic Anisian. The isolated nature ofthe pinnacles is demonstrated by the pinchout of theBanna shale from TP section to the northeast andsoutheast against the Guohua Fm. (Fig. 4).

The detailed facies distribution of the northern NMpinnacle is illustrated in the inset map of Fig. 4. Theplatform interior facies is composed of thick-bedded,peritidal cyclic limestone and dolomite. Cycle bases areoncolitic, intraclastic, and peloidal packstone to wack-estone with few bivalves, gastropods, and dasyclada-cean algae. Cycle bases may contain flat-pebbleconglomerates and domal stromatolites or may beextensively bioturbated. Cycle caps contain fenestrallaminites. The lower part of the interior facies islimestone, the middle third is pervasively dolomitized,and the upper third consists of monotonous, massivethick-bedded peloidal and molluscan packstone withoutevidence of subaerial exposure. Massive Tubiphytesboundstone approximately 200 m thick occurs at thenorthern margin of the NM Pinnacle (Fig. 4; inset). Theboundstone is composed of branching Tubiphytesframeworks reinforced by encrusting fossils such asspongiostromate algae and Bacinella as well as micriticcement (Fig. 7E). The frameworks are further reinforcedby several generations of isopachous fibrous marinecement. Despite the fact that this facies apparently formsa robust platform-margin reef facing the open basinnorth of the CPP, the reef has extremely lowbiodiversity. Although in-place reefs were not foundon the southeast margin of the NM platform (Fig. 4;inset), the presence of Tubiphytes boundstone clasts inslope breccias at NM section (Fig. 5) indicates that therewere at least patch reefs developed along the southeastmargin. In-place Tubiphytes patch reefs crop out on thenortheast margin of the southerly pinnacle north ofPingguo (Fig. 4).

Basin-margin facies flank the pinnacles. The bestexposures, at NM section, are composed of a series ofintertonguing carbonate wedges and shale tongues(Figs. 4, 5). The carbonate intervals consist of black,

laminated lime mudstone with slump folds punctuated bypolymict debris-flow breccias. The lime mudstoneintervals are platy or nodular-bedded and are horizontallylaminated in part. They contain crinoids, thin-shelledbivalves, radiolarians, and conodonts. Ammonoids occurin shale intervals. Debris-flow breccia bedsmay be as thinas a fewmeters, but more typically are tens ofmeters thickand range up to 100 m. The breccias are dominantly clastsupported with pebble- to boulder-sized clasts composedof Tubiphytes boundstone, Tubiphytes fragment grain-stone, oncolite-intraclastic packstone, molluscan wack-estone and packstone, and black, tabular lime mudstone.The assortment of clasts indicates that erosion tapped intoplatform interior as well as platform margin and slopelithologies.

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. ispreserved in the southern pinnacle where the GuohuaFm. is overlain by shale in the Banna Fm. (north ofPingguo; Fig. 5). The final termination is marked by ashift to black, nodular-bedded, oncolitic wackestonefollowed by shale. The shift to a dark, nodular faciesindicates termination by drowning. Conodonts withinthis interval include Ng. bulgarica indicating that thefinal termination occurred during the Pelsonian. Termi-nation of the Guohua pinnacles was a significant eventas it represents the end of carbonate deposition in theNanpanjiang basin in Guangxi (Lehrmann et al., 2005).

5. Discussion: PT boundary and Lower Triassic

Expanded sections spanning shallow- and deep-water environments through the Lower and MiddleTriassic are essential for reaching an adequate under-standing of the environmental and biological causesand consequences of the end-Permian extinction, par-ticularly because the recovery interval was so protracted(cf. Hallam, 1991). Expanded sections provide anopportunity to distinguish facies-related artifacts fromchanges in the biota driven by regional or globalprocesses. Moreover, multiple sections across anenvironmental gradient are ideal sources for detailedgeochemical records that can be linked with the local(and global) paleontological record. Exposures ofexpanded sections in shallow-marine carbonate faciesspanning the entire interval from the Permian throughthe Middle Triassic are rare, making the CPP a valuableresource for improving understanding of the end-Permian extinction and its aftermath. The GBG containsperhaps the longest known Permian–Triassic record of


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 regionalgenerality of the paleontological and geochemicalrecords on the GBG, as well as data from more distantlocalities.

The biostratigraphically conformable contact be-tween diverse, fossiliferous Upper Permian (Changh-singian) packstone and grainstone and low-diversitycalcimicrobial boundstone in the Pingguo area isconsistent with evidence for a geologically rapid ex-tinction pulse (cf. Jin et al., 2000) and suggestsabrupt onset of microbialite deposition after extinction.The presence of microbialites overlying the extinc-tion horizon on all of the isolated platforms in theNanpanjiang basin (Lehrmann et al., 2003) and theoccurrence of stenohaline taxa such as echinoderms andarticulate brachiopods in grainstone lenses within themicrobialite indicate continued shallow, open-marinecarbonate deposition across the extinction horizon ratherthan any significant shift in physical depositionalenvironment (Lehrmann et al., 2003).

Lower Triassic platform interior strata of theMajiaoling and Beisi formations contain a low-diversityfauna dominated by mollusks. The low overall abun-dance of animal and algal skeletal grains in LowerTriassic platform interior, platform margin, and basinalsediments stands in contrast to higher skeletal abun-dance in Upper Permian and Middle Triassic strata fromthe same environments. Similarly low abundance ofskeletal grains was quantitatively demonstrated usingpoint-counts of samples on the GBG (Payne et al.,2006b). Qualitative observations of Lower Triassiccarbonate sections across Tethys are generally consis-tent 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 dominatedby lime mudstone and oolite to a much greater extentthan comparable Upper Permian or Middle Triassicstrata. Moreover, the presence of unusually large ooids(5–10 mm in diameter; Fig. 6D) (see Flügel, 1982, for acompilation Phanerozoic ooids by period) indicatesunusually rapid growth relative to abrasion (Swett andKnoll, 1989), likely induced by high CaCO3 saturationand/or inhibition of ooid nucleation (see Sumner andGrotzinger, 1993). Qualitative field and thin-sectionobservations indicate no significant change in theabundance of skeletal grains through the Early Triassic.The ichnofossil record, on the other hand, trends broadlyfrom low intensity of bioturbation and exclusively

horizontal burrowing near the base of the Lower Triassicto higher intensity of bioturbation and the appearance ofpenetrative burrow networks higher in the LowerTriassic. Platform-margin and basinal strata (LuolouFm.) likewise contain a low diversity biota with lowskeletal abundance. Each of these observations closelyparallels the record of Early Triassic biotic recovery andLower Triassic carbonate facies previously documentedon the GBG (Payne et al., 2006b) and elsewhere (e.g.Twitchett, 1999; but see Twitchett et al., 2004). Theparallels include an increase in the abundance of crinoidgrains high in the Lower Triassic on the basin margin inLuolou Fm. and the appearance of crinoids in the basalAnisian of the Banmo member in the platform interior.Increased abundance of crinoids in the Spathian hasbeen observed on the GBG (Payne et al., 2006b), thewestern USA (Schubert and Bottjer, 1995), and westernTethys (Ramovš, 1996). The only report of high relativeabundance of crinoids in the lower part of the LowerTriassic is from the Griesbachian in Oman (Twitchettet al., 2004).

Two end-member explanations have been hypothe-sized to account for the occurrence of microbialitesimmediately overlying the extinction horizon (cf. Schu-bert and Bottjer, 1992; Baud et al., 1997; Lehrmann,1999; Kershaw et al., 1999; Lehrmann et al., 2003; Baudet al., 2005): (1) microbialites represent a default mode ofcarbonate deposition in shallow-marine environmentsin the absence of significant skeletal production,metazoan grazing, and bioturbation, as a consequenceof the extinction. (2) The microbialites reflect a change inmarine chemistry governing patterns of carbonate depo-sition (e.g. carbonate saturation state and the concentra-tion of inhibitors to calcium-carbonate nucleation). Theydo not merely occur due to the removal of skeletal-carbonate sinks. Rather, they reflect conditions generatedby the same processes responsible for extinction.

The occurrence of abundant aragonite fans within thebase of the calcimicrobial boundstone on the CPP, manyof which grow upward in discrete horizons from thesubstrate (Fig. 6A, B), suggests that the PTB microbialitereflects controls beyond the mere removal of skeletalsinks for calcium carbonate. Similar aragonite fans arewidely distributed within the PTB microbialite unitoccurring in south China, Turkey (see Kershaw et al.,1999; Baud et al., 2005), and Japan (D. Lehrmann, H.Sano, and J. Payne, unpublished observations). Similarfans are absent fromyounger Lower Triassicmicrobialitesand are only known to occur locally in an outer shelf toslope setting (Woods et al., 1999). If the fans merelyreflected the removal of skeletal carbonate sinks, onewould expect them to continue in shallow water and in


association with microbialites throughmuch of the LowerTriassic given the continued low abundance of skeletalgrains (Payne et al., 2006b). Furthermore, the widespreadand synchronous deposition ofmicrobialites appears to beconfined to the lowermost Lower Triassic (H. parvuszone) (Lehrmann, 1999; Lehrmann et al., 2003). The fanscould reflect an interval of high carbonate saturation ofseawater, inhibition of other forms of carbonate deposi-tion (e.g. lime mud), or some combination of these andother aspects of seawater chemistry. For example, thepresence of metal-ion inhibitors of calcium-carbonateprecipitation (e.g. Fe2+, Mg2+, Mn2+) could decrease thenucleation rate of new CaCO3 crystals and favor growthof existing crystals preferentially, yielding aragonite fans(Sumner and Grotzinger, 1993). Ferrous iron andmanganese, in particular, are more soluble under lowoxygen conditions, and so inhibition of CaCO3 may havebeen especially pronounced immediately after theextinction if low oxygen conditions were widespread(e.g. Wignall and Hallam, 1992; Wignall and Twitchett,2002). Other possible drivers include the delivery ofalkalinity from upwelling deep water (Grotzinger andKnoll, 1995; Knoll et al., 1996; Woods et al., 1999) or aweathering pulse resulting from a large input of CO2 intothe atmosphere from Siberian Traps eruptions or theoxidation of large quantities of methane or organic matter(Krull and Retallack, 2000).

6. Discussion: Middle Triassic

Middle Triassic Tubiphytes reefs on the CPP areconstrained as Anisian (Bithynian-Pelsonian) in age byconodonts from the NM section (Fig. 5). The highabundance (55–60% by volume) of penecontempora-neous and early-diagenetic cements – peloidal micriticcement, brownish fibrous calcite cement, fibrous iso-pachous calcite cement, and aragonite botyroids (Fig. 7E;Christensen and Lehrmann, 2004) – is consistent withobservations of the Anisian reef on the GBG. In bothcases, Tubiphytes is the only volumetrically significantframework element, and both reefs contain only a lowdiversity biota of invertebrates (ostracodes, mollusks,crinoids), dasyclad algae, and encrusting microproblema-tica (Bacinella, Plexoramea) (Payne et al., 2006a). Theappearance of heavily cemented Anisian reef marginsfollowing Lower Triassic ramps with oolite on severalplatforms in the Nanpanjiang basin and worldwide(Flügel, 2002) suggests a global control on marginalreef development.

The Lower Triassic has long been cited as a gap inmetazoan reef development resulting from decimation ofbiota following the end-Permian extinction, with the

implication that the return of reefs in the Middle Triassicwas caused by biotic rediversification following theextinction (cf. Flügel and Stanley, 1984; Stanley, 1988).The near absence of framework-building metazoans inthese reefs and dominance of Tubiphytes and penecon-temporaneous cements in the reef framework indicatesthat the re-development of platform-margin reefs in theMiddle Triassic was governed primarily by factors otherthan the recovery of reef building animals.

The development of extensive oolite shoals contain-ing large ooids in the Lower Triassic and reefs in theMiddle Triassic composed largely of marine cementindicates that carbonate precipitation occurred readilyon the margins of platforms throughout this interval andthat the bulk of carbonate precipitation was non-skeletal(abiotic or biotically mediated). The shift in style ofprecipitation (ooids v. reef cements) as well as theconcurrent shift in architecture from oolite ramps toabrupt, steep reef margins may reflect increasedstabilization of platform-margin sediments by theproblematic organism Tubiphytes. The relatively deli-cate branching frameworks of Tubiphytes wereencrusted by marine cements forming relatively robust,rigid reef masses. The development of large, metazoanframework reefs dominated by scleractinian corals firstbegan in the Late Triassic (Stanley, 1988).

7. Discussion: controls on recovery

The CPP and the GBG record the end-Permianextinction and Lower–Middle Triassic recovery inunmatched detail. Strata from these platforms haveprovided and can provide multiple proxies for bioticrecovery and environmental change through this interval,including the abundance, diversity, and distribution ofskeletal grains, changing patterns of carbonate precipita-tion and deposition, and high-resolution geochemicalrecords. Each of these proxy records collected to date hasconfirmed the geological rapidity and wide-reachingeffects of the end-Permian extinction pulse. Together, theyalso indicate a protracted Early Triassic interval of carboncycle instability and only limited biological recovery.Accelerated Middle Triassic recovery occurred in asso-ciation with substantial change in carbonate depositionpatterns and carbon cycle stabilization. Although themechanisms linking biological recovery, carbon cyclestabilization, and the re-development of platform-marginreefs are currently poorly understood, a satisfactorymodelfor Permian–Triassic events will only be achieved withcoupled, high-resolution paleontological, sedimentary,and geochemical records similar to the ones beingdeveloped from the Chongzuo-Pingguo platform.


8. Conclusions

(1) The Chongzuo-Pingguo platform (CPP) is a vastisolated platform in the southern part of theNanpanjiang basin of south China. The architectureand platform evolution were reconstructed fromstratigraphic sections and mapping of a fold thatexposes a 2-dimensional cross section through theplatform. The platform evolved from a low-reliefbank with oolite shoals in the Early Triassic, stepback and drowning in the beginning of the MiddleTriassic (Bithynian), and development of smallerpinnacle platforms with Tubiphytes reefs in theMiddle Triassic (Bithynian to Pelsonian).

(2) The platform contains expanded, conformablePermian–Triassic boundary (PTB) sections thatclosely resemble other PTB sections in the Nan-panjiang basin, around Tethys, and outside theTethys. Diverse shallow-marine Upper Permianpackstone and grainstone units are overlain bypost-extinction calcimicrobial framestone with lowabundance and diversity of fossil grains despite alsobeing deposited in a shallow, open-marine setting.

(3) The presence of aragonite fans exclusively in thebase of the microbialite unit suggests a control ofseawater chemistry on microbialite depositionbeyond the mere loss of the skeletal biota. Changesin carbonate saturation of seawater or the concen-tration of inhibitors to CaCO3 precipitation couldaccount for the presence of these fans.

(4) The close resemblance of the Lower and MiddleTriassic rock and fossil record on the CPP and theGreat Bank of Guizhou (GBG), as well as moredistant platforms around Tethys, suggests globalcontrols on patterns of carbonate deposition.Lower Triassic carbonates are characterized bylow skeletal content and proportional dominanceby lime mud and ooids, including large ooids.

(5) The re-development of platform margin reefs inthe Middle Triassic was not a direct product ofmetazoan recovery. Instead, Middle Triassicreefs consist of heavily cemented Tubiphtyesframework.

Acknowledgements

This research is based upon work supported by theNational Science Foundation (EAR-9804835) and bythe Petroleum Research Fund of the American ChemicalSociety (ACS-40948-B2). This research has benefitedgreatly from the extensive involvement of undergradu-ate students as they completed independent research

studies at the University of Wisconsin Oshkosh. Wethank the University of Wisconsin Oshkosh forproviding funds in the form of undergraduate student–faculty research grants and support for students to attendmeetings. Steve Kershaw and Oliver Weidlich providedthoughtful reviews that improved this paper.

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