Palaeogeography, Palaeoclimatology, Palaeoecology · Simpliﬁed models in the literature predict that isolated platforms may initiate upon an antecedent topographic substrate produced

Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

PALAEO-05969; No of Pages 14

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Factors controlling carbonate platform asymmetry: Preliminary results from theGreat Bank of Guizhou, an isolated Permian–Triassic Platform in the NanpanjiangBasin, south China

Xiaowei Li a,⁎, Meiyi Yu a, Daniel J. Lehrmann b, Jonathan L. Payne c, Brian M. Kelley c, Marcello Minzoni d

a Department of Resources & Environmental Engineering, Guizhou University, Caijiaguan, Guiyang 550003, Guizhou Province, PR Chinab Geosciences Department, Trinity University, San Antonio, TX 78212, USAc Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305, USAd Shell International Exploration and Production Co., 3737 Bellaire Blvd., Houston, TX, 77025, USA

⁎ Corresponding author. Tel.: +86 187 4479 6212.E-mail address: [email protected] (X. Li).

0031-0182/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.palaeo.2011.11.023

Please cite this article as: Li, X., et al., Factoran isolated Permian–Triassic..., Palaeogeogr

a b s t r a c t
a r t i c l e i n f o
Article history:Received 19 May 2011Received in revised form 23 November 2011Accepted 25 November 2011Available online xxxx

Keywords:Asymmetric architectureCarbonate platformDepositional modelTriassicPermian

A well-exposed isolated carbonate platform, the Great Bank of Guizhou, in the Nanpanjiang Basin of southChina, developed from the latest Permian to the earliest Late Triassic. Platform strata are dissected by a faultedsyncline exposing a complete cross section through the interior, margins and flanks, enabling a detailed assess-ment of depositional controls. Previous studies portrayed the platform as having a relatively symmetrical archi-tecture even though much of the former work was focused on the platform interior and northern margin–basintransition. Our research reveals five aspects of the southern margin facies and stratigraphy that are significantlydifferent from those of the northern margin: (1) subaerial exposure and unconformity developed on top of theUpper Permian sponge boundstone and in the overlying Lower Triassic strata; (2) Permian and Triassic clastschaotically admixed within Early Triassic breccias; (3) Lower Triassic strata remarkably thinner on the southernmargin; (4) a much narrower Tubiphytes reef facies preserved along the southern margin in the Middle Triassic;and (5) large scallop shaped reentrants at the southern margin evident in satellite images.Three end-member models may explain the asymmetry: (1) antecedent topography of the underlying UpperPermian reef-rimmed margin coupled with eustatic sea level fluctuation; (2) differential tectonic uplift; and(3) large-scale submarine collapse of the platformmargin. Subaerial exposure and admixing of Permian andTriassicclasts observed at Yungan section is best explained by the tectonic uplift model. However, the submarine collapsemodel also explains several of the observations if it is associated with uplift(s) or sea level fall(s). Submarine col-lapse is supported by large concave erosional reentrants (scallops) visible in satellite images. Taken together, ourobservations suggest that a combination of tectonic uplift andmargin collapse contributed to platform asymmetry.Further work promises to further constrain the details and timing of processes that contributed to the asymmetry.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Nanpanjiang Basin of south China contains several spectacu-larly exposed isolated carbonate platforms of Triassic age as well asthe vast attached Yangtze Platform that surrounds the basin. Theseplatforms provide a natural laboratory for investigating the variousmechanisms controlling platform evolution (tectonic subsidence,sea level change, siliciclastic flux, etc.; Lehrmann et al., 1998, 2005,2007; Enos et al., 2006; Minzoni et al., 2010) as well as the environ-mental factors associated with the end-Permian mass extinction andbiotic recovery (Lehrmann et al., 2001, 2003, 2006; Adachi, 2004;Payne et al., 2004, 2006a, 2007, 2010, 2011; Algeo et al., 2007;

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s controlling carbonate platfo. Palaeoclimatol. Palaeoecol.

Kershaw et al., 2007; Tong et al., 2007; Collin et al., 2009;Brennecka et al., 2011; Meyer et al., 2011). Furthermore, Lehrmannet al. (1998) pointed out the importance of the Triassic carbonateplatforms of south China as another example to be compared withthe classic, intensively studied Triassic platforms of the Dolomitemountains of northern Italy.

The best exposed and most thoroughly studied of the isolated car-bonate platforms in the Nanpanjiang Basin is the Great Bank of Guizhou(GBG) in southern Guizhou province (Lehrmann et al., 1998). The GBGis dissected by a structural feature, the Bianyang syncline, which ex-poses a complete cross section through the platform interior andbasin margins.

Carbonate platforms are biochemical precipitates of calcium carbon-ate sensitive to awide range of environmental variables such as tectonicsubsidence, antecedent topography, sea-level fluctuation, climatechange, water quality and oceanographic conditions (Schlager, 2003,2005). Twomajor factors that affect the geometry of carbonate platform

rm asymmetry: Preliminary results from the Great Bank of Guizhou,(2011), doi:10.1016/j.palaeo.2011.11.023

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2 X. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

margins and the evolution of margin architecture over time are the rateof production of carbonate sediment and the degree of stability of theplatform margin and slope. There is an interplay between rates of in-situ carbonate sediment production and sediment stabilization (bybinding and marine cementation) that tend to cause up-building andsteepening of platform margins, versus factors that lead to significantoff platform sediment transport (such as granular unbound sediments,hydrodynamics, and slope failure) that tend to reduce up-buildingand steepening (Kenter, 1990; Schlager, 1992, 2005; Kenter et al.,2001, 2005; Pomar, 2001).

Spatial variation in the relative importance of factors tending toincrease and decrease steepness of the margin can lead to significantlateral variability in platform architecture and pronounced asymme-try. An especially important factor influencing carbonate platformasymmetry is the hydrodynamic windward–leeward effect, whichtends to produce abrupt, aggradingwindwardmargins versus progradingleewardmargins dominated by “highstand” shedding of sediment sweptoff the banktop (Eberli and Ginsburg, 1987; Schlager et al., 1994; Eberli etal., 1997; Bergman et al., 2010). Additional factors that may produce pro-nounced asymmetry include tectonic effects, antecedent topography orasymmetries in basin sediment fill patterns (Lehrmann et al., 2007).

Simplified models in the literature predict that isolated platformsmay initiate upon an antecedent topographic substrate produced byfault blocks during extension along a continental shelf (e.g.Bechennec et al., 1990; Dorobek, 1995; Bosence et al., 1998a; Wangand Shi, 2008), salt diapirs (e.g. Bosence et al., 1998b) or volcanic sea-mounts (e.g. Scott and Rotondo, 1983; Wheeler and Aharon, 1991;Grigg, 1997). The Great Bank of Guizhou (GBG) was initiated uponantecedent topography inherited from the retreat of the formerseaward-facing, reef-rimmed Permian margin of the underlyingYangtze Platform. Initiation of the GBG as an isolated platform oc-curred when a large area of the Yangtze Platform surrounding the in-cipient isolated platform drowned during the latest Permian andEarly Triassic (Lehrmann et al., 1998). Although inherited topographyfrom the former Permian reef margin appears to largely explain theinitial development of the GBG, tectonic movement at the edge ofthe former Permian margin (coinciding with the south margin ofthe GBG) may have also played a role.

This paper builds on the stratigraphic framework of the GBG estab-lished by Lehrmann (1993) and Lehrmann et al. (1998). Lehrmann'soriginal work on the GBG, albeit including work on the southern mar-gin, had the greatest control on margin architecture on the northernmargin in the Bianyang syncline (Lehrmann, 1993). As a consequence,

Fig. 1. Early to Middle Triassic paleogeography of the northern Nanpanjiang Basin and GreaYungan, and (B) Yutianao are also shown in Figs. 2B and 3.

Please cite this article as: Li, X., et al., Factors controlling carbonate platfoan isolated Permian–Triassic..., Palaeogeogr. Palaeoclimatol. Palaeoecol.

although simplified restored stratigraphic reconstructions of the GBGhave depicted the platform as having a relatively symmetrical architec-ture (e.g. Lehrmann et al., 1998), detailed geologicmaps of the Bianyangsyncline showed asymmetries such as amuch narrowerMiddle Triassicreef facies on the southern margin. Combined with significant gaps indata, this observation left to question the degree of asymmetry at differ-ent stages of platform evolution (Lehrmann, 1993; Lehrmann et al.,2005).

The goal of this study is to assess the degree and cause of asymmetryacross a continuous transect of the GBG during the Early and Middle Tri-assic. To achieve this goal, we conducted studies of the southern marginin the Bianyang syncline (Bangeng) and in the Yungan and Yutianaoarea east of the Bianyang syncline (Figs. 1 and 2). Significant asymmetryis evident from Lower Triassic sections and large erosional reentrants(scalloped margins sensu Mullins and Hine, 1989; Morsilli et al., 2002;Eberli et al., 2004) that developed on the southern margin. We haveused conceptual models to develop testable predictions that can beused to identify controls on asymmetry.

2. Geological setting

The Great Bank of Guizhou (GBG) is the northernmost of severalTriassic isolated carbonate platforms in the Nanpanjiang Basin,south China (Fig. 1). A regional transgression from the latest Permianto Early Triassic allowed the Nanpanjiang Basin to expand, while themargin of Yangtze Platform dramatically retreated from its formerposition near the Bangeng area to the Guiyang area (Fig. 1;Lehrmann et al., 1998). During this transgression, the GBG initiatednear the former southern margin of the Yangtze Platform as an isolat-ed area where shallow-marine carbonate sediments continued to ac-cumulate and the surrounding region of the Yangtze Platformdrowned.

The initial carbonate deposits on the incipient GBGwere uppermostPermian (Changhsingian) sponge patch reefs and shallow-marine bio-clastic packstone (Lehrmann et al., 1998). Lehrmann et al. (1998)reported no evidence for syndepositional faulting associated with theinitiation phase of the GBG, although their most detailed mapping wasconcentrated in the Bianyang syncline, along the northern margin ofthe GBG. The need for further work on the southern margin leavesopen the possibility of structural influences.

Sedimentation across the Permian–Triassic boundary (PTB) in theplatform interior continued without a significant time gap as deter-mined by biostratigraphy and carbon isotope stratigraphy (Lehrmann

t Bank of Guizhou (GBG), modified from Lehrmann et al. (1998). Section localities (A)



A

B

25o40

107o

A

B

\

Fig. 2. Satellite image and geologic map of field localities. (A) Landsat image of the Great Bank of Guizhou. The Bianyang syncline (area between Guandao and Bangeng displays acomplete, structurally uncomplicated cross section through the platform interior to platform-to-basin transitions. Study areas are illustrated with rectangles. The western rectanglerepresents Bangeng (also refer to Fig. 7); the eastern rectangle represents Yungan–Yutianao area (below). Amphitheater-shaped reentrants (scalloped margins) in the south marginare indicated by arrows. (B) Simplified geological map of Yungan–Yutianao area in the south margin of the GBG revised fromMoyang 1:50k geological map (Guizhou Bureau, 1997).

3X. Li et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

et al., 2003; Krull et al., 2004; Chen et al., 2009). However, the boundarycontains a sharp, ragged, undulatory truncation surface between upper-most Permian packstones and basal Triassic microbialites. Whether thiscontact is a submarine dissolution surface resulting from ocean chemis-try change associated with end-Permian mass extinction or simply asubaerial karst surface remains debated (Payne et al., 2007, 2009,2010; Collin et al., 2009;Wignall et al., 2009). Although the surface is ex-tensive in the platform interior, the maximum relief on the surface is15 cm, and no large scale karst features have been observed associatedwith it thus far (Lehrmann, Payne, Li, Kelley; personal communication).

In the Early Triassic, Scythian, the GBG developed a low-reliefbank stage of sedimentation with oolite shoal margins, followedeventually in the Middle Triassic Anisian and Ladinian by a progres-sively steepening platform with Tubiphytes reef-rimmed margins


and high-angle slopes. Finally in the Ladinian, the GBG developed anatoll topography in the platform interior with a high-relief erosionalescarpment at the margin until the demise of the platform and burialby siliciclastic turbidites at the beginning of Late Triassic (Carnian)(Lehrmann et al., 1998).

The GBG is dissected by a major structural feature, the Bianyang syn-cline,which is faultedon its east limb andexposes a complete, structurallyuncomplicated cross section through the interior of the platform andthrough both the northern and southern platform-to-basin transitions(Fig. 2A). This study focuses on asymmetry in platform development bycomparing the northern margin with new observations from the south-ern margin of the GBG in the Bianyang syncline (Bangeng area, Figs. 1and 2A) and east of the Bianyang syncline in the Yungan and Yutianaosections of the Dajing–Xiaojing area (Figs. 1 and 2B).




3. Stratigraphic architecture of the south margin

3.1. Facies distribution of the south margin

Regional geologic maps (1:200,000 and 1:500,000; GuizhouBureau, 1965, 1987) indicate that the southern edge of the GBG is un-derlain by the Upper Permian Wujiaping Formation, but either suggestthat the GBG developed above the former interior of the underlyingYangtze Platform (with the former Yangtze Platform margin existingmore than 10 km to the south) or are ambiguous, showing undifferen-tiated Upper Permian facies in the south of the GBG. Lehrmann et al.(1998) portrayed the GBG as having developed atop of a substrate ofUpper Permian platform interior facies consistent with the 1:500,000map (Guizhou Bureau, 1987).

More recent detailed mapping at the 1:50,000 scale by Yu Meiyiwith the Guizhou regional mapping team (Fig. 2B; Guizhou Bureau,1997) demonstrated that the southern edge of the GBG coincideswith the former margin of the Upper Permian Yangtze Platform. YuMeiyi (Guizhou Bureau, 1997) thus interpreted the Upper PermianYangtze Platform margin to have been an abrupt south-facing marginwith an east–west oriented barrier reef and deep-marine slope andbasin facies further south (Fig. 2B). According to this stratigraphic re-construction (Guizhou Bureau, 1997), the GBG developed atop thesouthern edge of the Yangtze Platform with the northern marginabove the platform interior facies and the southern margin of theGBG coinciding with the underlying abrupt reef-rimmed margin ofthe Yangtze Platform. This new reconstruction implies that the positionof theGBG's southernmarginwas determined by antecedent topographyinherited from the underlying Yangtze Platform whereas the northernmargin was developed over a flatter area of the former Yangtze platforminterior.

1

2

3a

3b

4a

4b

4c

4d

5

6

1

2

3

4

5

6

A

B

Fig. 3. Stratigraphic columns of Yungan (A) and Yutia


On the northern margin and interior, initiation of the GBG beganin the latest Permian with local patch reefs of sponge boundstoneand skeletal packstone of the uppermost Wujiaping Formation asthe surrounding Yangtze Platform drowned and shifted to the deep-water cherty pelagic facies of the Dalong Formation (Lehrmann etal., 1998). Although relationships are less clear on the southernGBG, presumably this initiation phase is represented by continuedshallow-water sponge reef accumulation of what was formerly thesouthern margin of the Yangtze Platform (Fig. 3, unit 1). In the plat-form interior, the facies succession consists of open-marine skeletalpackstones of the Wujiaping Formation, followed by a truncation sur-face marking the end-Permian extinction and microbialite in thebasal Griesbachian (as discussed above; Lehrmann, 1999; Lehrmannet al., 2003). This was followed by a thick succession of shallow-marine carbonates in the Lower Triassic (originally estimated to be~300 m: Lehrmannet al., 1998; revised on thebasis of chemostratigraphyand lithostratigraphy to include ~900 m: Kelly et al., 2011; Meyer et al.,2011). The Lower Triassic facies in the platform interior change upwardfrom thin-bedded limemudstone to dolo-oolite, followed by peritidal cy-clic limestone with microbialite, and then peritidal dolomite (Lehrmannet al., 1998, 2001).

In theBianyang syncline, the Lower Triassicmicrobialite, thin beddedlime mudstone, oolite and peritidal cyclic limestone facies graduallychange to massive dolomitized facies toward the southern margin,somewhat obscuring depositional fabrics and stratigraphic relation-ships. Thus we have chosen to focus our detailed facies studies of thesouthern margin at two sections east of the Bangeng area at Yunganand Yutianao (Figs. 1 and 2). Even though the southern margin in theBangeng area of the Bianyang syncline is dolomitized, large scale differ-ences in facies distribution from this area and from detailed mappingand sections at Yungan and Yutianao, where dolomitization is less

Wavy lamination Horizontal lamination

open-marine,shallow subtidal

(reef)

Subaerial exposure

Shallow to deep marine(slope?)

Subaqueous debrisflow breccia

(slope)

open-marine,shallow subtidal

(reef or platform)

Karst breccia (section A);Karst (?) and debris flow

breccia (section B)

Shallow marine (shoal)

Interpretedenvironments of

deposition

nao (B) sections. See Figs. 1 and 2B for locations.



Table1

Summaryon

symptom

sof

asym

metricstratigrap

hicarch

itecture

betw

eensouthan

dno

rthmargins

oftheGBG

from

thelatest

Perm

ianto

MiddleTriassic.

Symptom

sSo

uthmargin

North

margin

Late

Perm

ian

paleog

eograp

hicdifferen

ceOcean

-facingsp

onge

barrierreef

oftheYa

ngtzePlatform

(Fig.8

A)

Spon

gepa

tchreef

deve

lope

din

theplatform

interior

ofYa

ngtzeplatform

(Fig.8

A;Le

hrman

net

al.,19

98).

Retrea

ting

scaleof

platform

marginin

the

earliest

Triassic

~25

0m,stillcloseto

theform

ermarginof

Yang

tzeplatform

inEn

dPe

rmian.

~70

0m

(Leh

rman

net

al.,19

98).

Clasttype

swithinLo

wer

Triassic

breccia

EarlyTriassic

wav

ymicrobial

structures

andlow

dive

rsitybiotafacies

mixed

withlitho

clasts

ofPe

rmianfossiliferous

pack

ston

e(U

nit2in

Yung

ansection,

Fig.

3A);

Microbialitean

doo

idsmixed

withlitho

clasts

ofPe

rmianfossiliferous

pack

ston

e(U

nit5in

Yung

ansection,

Figs.3

Aan

d5B

);Ea

rlyTriassic

microbialitean

doo

lite

mixed

withgray

toblacklim

emud

ston

eto

wacke

ston

e(U

nit2in

Yutian

aosection,

Fig.

3B).

Clasts

areco

mpo

sedof

EarlyTriassic

oolite,

skeletal

pack

ston

ean

dlim

emud

ston

efrom

Gua

ndao

section(Leh

rman

net

al.,19

98).

Ooliteclasts

restricted

toGriesba

chianan

dDiene

rian

(Pay

neet

al.,20

06a).

Thickn

essof

Lower

Triassic

Less

than

70m

(Yutiana

osection,

Fig.

3B);

aslittleas

to0m

(MiddleTriassic

Tubiph

ytes

reef

prog

rada

tedto

directly

overlie

sCh

angh

sing

iansp

onge

reef

inthewestof

Yung

an;Fig.

2B)

Morethan

300m

atLalaicao

section(n

orthernmargin;

Mey

eret

al.,20

11),ev

enmorethan

900m

atDajiang

section

(platform

interior;Kelly

etal.,20

11),an

d~20

0m

atGua

ndao

section

(northernslop

e;Le

hrman

net

al.,19

98).

In-situMiddleTriassic

Tubiph

ytes

reef

size

andreef

interior

~20

0m

width

and20

0m

thickn

ess(B

ange

ngarea

).Gastrop

odan

dbiva

lvefrag

men

ts,T

ubiphy

tesbo

unds

tone

,grape

ston

e,cryp

talgal

laminae

,Tub

iphy

tesgrains

tone

interbed

ded

withon

colitepa

ckston

e(B

ange

ngarea

).

~1km

width

and80

0m

thickn

ess(P

ayne

etal.,20

06a).

Tubiph

ytes

grains

tone

andTu

biph

ytes

boun

dstone

withlocals

pong

ean

dscleractinianco

ral(

Gua

ndao

area

;Pa

yneet

al.,20

06a).

Along

strike

variab

ility

inthearch

itecture

ofplatform

margin

Amph

ithe

ater-sha

pedreen

tran

ts(scallo

pedmargins

)areev

iden

tin

thesouthe

rnmarginin

theBian

yang

sync

line(B

ange

ngarea

,Figs.

2Aan

d7)

.Noam

phithe

ater-sha

pedreen

tran

tsarein

theno

rthe

rnmargin

intheBian

yang

sync

line(G

uand

aoarea

,Fig.2

A).


pervasive, illuminate major differences in architecture between thesouthern and northern margins of the platform (Table 1).

3.2. Stratigraphy of Yungan and Yutianao sections

3.2.1. Yungan sectionYungan section is ~20 m thick spanning the Upper Permian through

Lower Triassic facies of the southernmargin of the GBG (Figs. 2B and 3A).The basal part of the section consists of Changhsingian sponge reef

facies, composed of boundstone with a diverse biota of rugose corals,crinoids, ostracodes, red algae, bryozoans, gastropods, bivalves, ben-thic foraminifera and sphinctozoan sponges which were commonlycoated with algal crusts. The top of the sponge boundstone of Unit 1is an irregular truncation surface, with up to 15 cm of relief, overlainby a polymict breccia with angular clasts in Unit 2 (Figs. 3A and 4A).

Internal voids within the reef framework of Unit 1 are most com-monly filledwith isopachous cements, sparry calcite or fossil fragmentsindicating marine phreatic diagenesis (Fig. 4B and C). Meniscus andpendant structures consistent with vadose diagenesis during subaerialexposure occur immediately below the truncation surface at the topof Unit 1 (Fig. 4D).

Early micritic envelopes accentuate the contact between the earlymeniscus and pendant cements from later penecontemporaneousisopachous cements and late blocky calcite cements (Fig. 4D). Althoughthe erosion surface between the sponge boundstone and the overlyingbreccia does not contain large scale, macroscopic evidence for karstingat this outcrop, the presence of meniscus and pendant cements withconsistent thickening on the undersides of grains and meniscus formsat grain contacts are consistent with vadose cementation (Fig. 4D).

Immediately above the erosion surface is a coarse grained polymictbreccia in Unit 2 (Fig. 3A). The breccia is composed of angular to sub-rounded, pebble to boulder size clasts with a reddish packstone matrix(Fig. 5A). Thematrix contains fragments of bivalves, foraminifera, dasy-cladaceans, and crinoids. Fossiliferous clasts within the breccia containPermian fossils, including Palaeofusulina and dasycladacean algae(Fig. 4C). Additional clasts contain clotted microbial structures andlowdiversity biotawith spirorbidworm tubes and thin-shelled bivalvesindistinguishable from those of Lower Triassic facies found in the GBGand elsewhere (Payne et al., 2006b). Some large fragments exhibit ir-regular outlines suggestive of dissolution (Fig. 5A). These features areassociated with subaerial exposure surfaces (paleokarst zone) and pro-duced by karst collapse (Esteban and Klappa, 1983; Scholle and Ulmer-Scholle, 2003). As the breccia contains amix ofUpper Permian and LowerTriassic clast lithologies it is interpreted to be Lower Triassic in age withUpper Permian clasts admixed during subaerial exposure. The “PTB” be-tween the Upper Permian strata and the Triassic is placed at the base ofthe breccia (Fig. 3A). However, it is important to note that there may bea significant time gap along the erosion surface at this section.

Above the breccia, the facies change to thin bedded (b30 cm) mol-lusk packstones of Unit 3a containing thin-shelled bivalves and smallgastropods, spirorbid worm tubes and peloids (Figs. 3A and 6A). Thisunit contains stylolites and is overlain by oolitic packstone in Unit 3b(Figs. 3A and 6B). These facies are interpreted to be Early Triassic inage because of their extremely low biodiversity, lack of diagnosticPermian fossils, and unmistakable similarity to Lower Triassic stratafrom the interior of the GBG (cf. Payne et al., 2006b).

Overlying strata (Unit 4a) consists of lime mudstone to wackes-tone containing small thin-shelled bivalves, small brachiopods, andspirorbid worm tubes and locally exhibiting micro-fenestrae fabricsindicative of subaerial exposure in a tidal flat environment (Figs. 3Aand 6C). The lime mudstone to wackestone is overlain by lime pack-stone (Unit 4b) containing gastropods, bivalves, peloids followed by athin oolite bed, 9 cm thick (Unit 4c, Fig. 3A). Unit 4d is also composedof lime wackestone with bivalve fragments and spirorbid worm tubessimilar to Unit 4a; however this unit also contains wavy bedding,microbial clots and microbial mats (Fig. 6D).

Please cite this article as: Li, X., et al., Factors controlling carbonate platform asymmetry: Preliminary results from the Great Bank of Guizhou,an isolated Permian–Triassic..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2011), doi:10.1016/j.palaeo.2011.11.023


Fig. 4. Upper Permian facies and Permian–Triassic boundary. (A) An irregular boundary separating the Changhsingian sponge boundstone from the paleokarst breccia (Units 1 and 2in Fig. 3A) at Yungan. Hammer for scale. (B) Outcrop photo of Changhsingian sponge boundstone (Unit 1 in Yungan section, Fig. 3A). Sphinctozoan sponges form conspicuousframework elements (S). Reef framework cavities filled with isopachous bladed marine cements (C). Scale bar is 2 cm. (C) Thin section photo of clast of Upper Permian skeletalpackstone within Lower Triassic breccia (Unit 2 in Yungan section, Fig. 3A) contains calcareous sponges (s), large benthic foraminifers (f) and isopachous marine cements (Is).Scale bar is 1 mm. (D) Vadose diagenetic fabrics near the top of Upper Permian reef facies (Unit 1, Yungan section). Meniscus (m) and pendant cements (p). Scale bar is 250 μm.


Unit 4d is overlain by a polymict breccia more than 6 m thick (Unit5, Fig. 3A). The breccia contains clasts up to 15 cm in diameter. Clastlithologies include both fossiliferous packstone clasts with the UpperPermian foraminifera Hemigordius and Reichelina, and microbialite

Fig. 5. Breccia fabrics. (A) Polished slab of Lower Triassic breccia (Unit 2, Yungan section). Treddish matrix (karst fabrics) with packstone matrix. Scale bar is 2 cm. (B) Polished slab illBreccia contains Triassic microbialite (M) and oolite (O), and fossiliferous packstone with Pewell as bladed isopachous marine cements (I) and equant spar (E). Scale bar is 2 cm.


clasts that are indistinguishable from the calcimicrobial facies fromthe Lower Triassic of the platform interior (Fig. 5B). Comparison withprevious detailed descriptions of GBG facies demonstrates that thesemicrobial clasts must have been derived from erosion of Lower Triassic

he irregular outline of clasts indicated by arrows is suggestive of vadose dissolution inustrating breccia fabric and clast lithologies in Lower Triassic (Unit 5, Yungan section).rmian fossils (P). Interstitial space contains intraclastic and skeletal packstone matrix as



Fig. 6. Lower Triassic facies at Yungan and Yutianao sections. (A) Mollusk packstone (Unit 3a, Yungan section) composed of gastropods (G) and thin-shelled bivalves (B), spirorbidworm tubes (W). Scale bar is 1 mm. (B) Dolomitized ooids (Unit 3b, Yungan section). Scale bar is 1 mm. (C) Micro-fenestrae (Unit 4a, Yungan section). Scale bar is 1 mm. (D) Limewackestone containing spirorbid worm tubes (W), microbial mats (M) and clots (C) (Unit 4d, Yungan section). Scale bar is 1 mm.


facies, although they could have been derived from either the basalGriesbachian microbialites or from the Dienerian peritidal cyclic lime-stone (Lehrmann et al., 1998, 2001; Payne et al., 2006b).

Clasts in the breccia are both angular and subrounded (Fig. 5B). Inter-stitial space contains intraclastic and skeletal packstonematrix as well asbladed isopachous marine cements and equant spar (Fig. 5B) indicatingthat they were likely deposited as submarine debris flows or rock falls.However, it is worth noting that Permian and Triassic clasts have notbeen found admixed in the Triassic submarine debris flow breccias onthe northern basin margin of the GBG (Lehrmann et al., 1998). The topof the section (Unit 6) is light gray limemud-wackestonewith horizontalwavy lamination.

3.2.2. Yutianao sectionYutianao section is divided into 6 units that span approximately

90 m in thickness and extends from the Upper Permian at the baseof the section to the Middle Triassic, Anisian at the top of the section(Figs. 2B and 3B). The lower units at Yutianao are comparable to thoseof Yungan (Fig. 3A).

Similar to the Yungan section, the base of the Yutianao section (Unit1) is the Upper Permian, Changhsingian sponge reef boundstone facies,overlain by polymict breccias of Unit 2 (Fig. 3B). However, unlike theYungan section, no obvious erosion surface was found separating theunderlying Changhsingian reef facies from the overlying breccias; in-stead there appears to be a gradual transition with clasts becomingmore abundant upward at the boundary between the two units. Thegradational contact between units 1 and 2may represent an upward in-crease of diagenetic brecciation from unit 1 to 2, although the fabrics atthe boundary are obscured by dolomitization (Fig. 3B).

Unit 2 consists of polymict breccia (Fig. 3B). The unit containssmall clasts of oolitic grainstone and large boulders (up to 3 m across)of microbialite in the lower and upper parts of the unit respectively.


Unit 2 also contains conspicuous subangular to angular clasts ofgray to black lime mudstone and wackestone. These clasts are sup-ported by matrix composed of light gray carbonate mud. The brecciaof Unit 2 is overlain by an oolite bed approximately 30 cm thick (Unit3) followed by mollusk packstone in Unit 4 (Fig. 3B).

A second breccia unit (Unit 5) greater than 35 m thick is preservedabove the lime packstone, again apparently with a gradational upwardchange to the breccia facies without a distinct bedding contact at theboundary with the underlying packstone (Fig. 3B). Clast lithologies arepredominantly gray and black lime mudstone to wackestone; a smallfraction of the clasts (~5%) are light gray laminated lime mudstone. Thematrix is light gray carbonate mud. In contrast with Yungan section, noclasts of Permian age were observed in either of the breccias in Yutianaosection. Approximately 4 m below the top of Unit 5 there is an interval ofcentimeter-scale contorted bedding consisting of several thin (3–4 cm)layers of marl interlayered with thin (3–5 cm thick) layers of intraclasticlime grainstone.

At the top of Yutianao section is massive Tubiphytes boundstone(Unit 6) that extends upward as an extremely thick facies unitabove the top of our measured section. This unit has been recognizedfrom previous mapping to be the Middle Triassic, Anisian platformmargin facies of the GBG (Guizhou Bureau, 1997; Lehrmann et al.,1998; Fig. 3B). At Yutianao section, the basal contact of the Tubiphytesboundstone is obscured by interspersed calcite veins, leaving fewwindows to observe the original lithological boundary. However, afewwindows into the fabric revealed lithoclasts of Tubiphytes grainstoneand boundstone in the uppermost part of the underlying breccia of Unit 5(Fig. 3B).

The total thickness of the Lower Triassic strata is less than 70 m atthis section. Furthermore, our reconnaissance observations along theroadwest of Yungan (Fig. 2B) indicate that the Lower Triassic is entirelymissing locally where Anisian, Tubiphytes boundstone immediately




overlies Upper Permian sponge boundstone. This observation is in con-trast with the platform interior and northern margin of the GBG in theBianyang syncline where Lower Triassic strata are up to 900 m thick(Kelly et al., 2011; Meyer et al., 2011).

3.2.3. Large erosional scallop features evident from satellite imagesLarge amphitheater-shaped reentrants that truncate the southern

margin are visible in satellite images (Figs. 2A and 7). The features areconcave southward away from the margin and mark an abrupt contactbetween carbonate strata of the platform and siliciclastic turbidites tothe south in the basin. The largest of these features is 4 kmwide at Ban-geng where it truncates the Anisian Tubiphytes reefs (Fig. 7; Lehrmannet al., 2005), smaller features 2–3 km across occur east of Bangeng(Fig. 2A). These features are identical in scale and morphology toscallop-shaped collapse features described from other ancient andmodern platforms (Mullins and Hine, 1989; George et al., 1995;Kenter et al., 2001; Morsilli et al., 2002; Eberli et al., 2004).

3.3. Contrast in the sedimentary architecture and facies succession betweenthe south and north margins of the GBG

Previous reconstructions of the GBG portrayed the platform ashaving a relatively symmetrical architecture (Lehrmann et al.,1998); however detailed faciesmaps of the area of the syncline indicatesignificant contrast such as a narrower Tubiphytes reef on the southernmargin (Lehrmann et al., 2005). In this study, we recognize several dif-ferences in architecture between the southern and northern marginsfrom detailed section description at Yungan and Yutianao, from recon-naissance observations, geologic maps and satellite imagery (Table 1;Figs. 2 and 3).

Major contrasts between the architecture of the southern andnorth-ern margins include the following: (1) evidence of subaerial exposurein the Upper Permian at the south margin; (2) admixed Triassic andPermian lithological clasts in Lower Triassic breccias at the south mar-gin; (3) significant variation in thickness of Lower Triassic strata; (4)preservedwidth of theMiddle Triassic Tubiphytes reef; and (5) presenceof large erosional reentrants (scalloped margins) observed in satelliteimages of the south margin. The differences observed indicate that acombination of subaerial exposure, erosional collapse and mixing ofPermian and Triassic clasts along the southern margin are mechanismsthat contributed to the asymmetry.

Fig. 7. Amphitheater-shaped reentrants (scalloped margins) evident in high resolutionsatellite image (World View image) of the Bangeng area of the south margin. Theimage illustrates the eastern half of the large concave southward truncation of the plat-form margin in the Bangeng area (see rectangle at Bengeng in Fig. 2A). The towerkarsts are platform carbonates north of the concave truncation, and the low lyingareas to the south are basin siliciclastic turbidites.


4. Models for evaluating controls on asymmetric architecture andfacies patterns at Yungan–Yutianao

Numerous studies suggest the geometry and evolution of carbonateplatform margins can be controlled by a wide variety of global or localmechanisms such as sea level change, climate, tectonism, antecedenttopography, carbonate producing biota, seawater chemistry, hydrologyand paleowind directions and windward–leeward effects (Schlager,2005; Lucasik and Simo, 2008, among many others).

In the following section we consider a series of models for gener-ating asymmetry in the GBG. For each scenario, we compare modelpredictions against the observed facies and stratigraphic relationshipsin the GBG to assess relative support for various models of platformevolution.

These geologic models are divided into three general categories.The first model invokes antecedent topography associated with theunderlying Upper Permian platform margin and eustatic sea levelfluctuation. A second set of models invokes local tectonic uplift. Athird set of models invokes large-scale submarine collapse of the plat-form margin.

4.1. Antecedent topography inherited from the Changhsingian reef margincoupled with sea level fall

In this model, the Upper Permian margin of the Yangtze Platformwas a barrier reef with significant elevation above its back reef lagoon(Fig. 8A). As a consequence the south margin of the GBG developedon significant antecedent topography formed by the elevated UpperPermian reef rim, whilst the northern margin of the GBG nucleatedonmuch smaller sponge patch reefs that occurred in theUpper Permianback reef lagoon (Fig. 8A and B).

In this configuration, the southern margin was much more proneto being emerged above sea level during sea level fluctuations thanthe north margin. In this model, one or more Early Triassic lowstandsin sea level resulted in the subaerial exposure surface recorded in theLower Triassic strata near the top of the underlying Upper Permiansponge reef and adjacent back-reef and platform-interior facies inthe Yungan and Yutianao sections (Fig. 8B). Erosion during exposureof this elevated region produced the observed mixed assemblage ofPermian and Triassic clasts in both the lower and upper brecciaunits. Furthermore, erosion and non-deposition resulted in signifi-cantly reduced thickness of the Lower Triassic strata along the south-ern margin of the platform. As sediments filled the accommodationspace in the former back-reef lagoon during the Early Triassic, theplatform developed a flat-topped geometry.

According to this model, because the Upper Permian margin of theYangtze Platform was an elevated sponge boundstone reef rim, sealevel fluctuation would have preferentially caused this area to be-come emergent, leading to the vadose diagenetic features and paleo-karst breccia observed in Units 1 and 2 of Yungan and Yutianaosections (Figs. 4D and 5A). Consistent with this model, Wu et al.(2010) reported subaerial exposure of a Changhsingian sponge reefalong the Yangtze Platform margin in the Ziyun area (Fig. 1) andinterpreted the exposure to have resulted from Late Permian sealevel fall. Wu et al.'s (2010) interpretation of a latest Changhsingianfall in sea level is in contrast to global data that indicate a rise in sealevel at this time (cf. Hallam and Wignall, 1999). Furthermore, Enoset al. (2006) have interpreted localized erosion of the Late Permiansuccession to have resulted from tectonic uplift in the Ziyun area.

Meniscus and pendant cements preserved near the top of theUpper Permian Unit 1 at Yungan section cannot be attributed un-equivocally to a latest Permian exposure event because a subaerialexposure event (or several episodes of exposure) in the Early Triassicrecorded by the breccia of Unit 2may have resulted in the developmentof vadose features in the underlying reef facies of Unit 1. If additionaldrops in sea level are invoked to explain the stratigraphic patterns on



Microbialite

Oolite

Peritidal cyclic

Subaerial unconformity

Breccia containing Permian and Lower Triassic clasts

(1)(1)

(2)

(2)(3)

5km

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Skeletal Grainstone (Wujiaping Fm.)

Silicious lutites (Drowned platform, Dalong Fm.)

Slope facies

NS

TP

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Permian-Triassic

Lower Triassic (Induan)

B

A

Fig. 8. Models involving sea level fall. (A) Interpreted paleobathymetric profile of Late Permian, Changhsingian Yangtze Platform in area of substrate of the GBG. (B) Model illus-trating unconformity on southern margin resulting from antecedent topography inherited from Upper Permian reef and sea-level fall. Subsidence and Upper Permian reef accumu-lation developed antecedent topographic high to the south (Stage 1) followed by Lower Triassic sediments filling in the accommodation space in the back reef, platform interior andnorth margin of the GBG because of substrate subsidence (Stage 2). Sea level drop caused deep dissection, erosion and redeposition of Upper Permian and Lower Triassic strata,resulting in significantly thinner Lower Triassic strata on the southern margin of the GBG (Stage 3).


the southernmargin, one possible explanation is that there would havebeen one ormore sea level falls associatedwith each of the breccia units(both lower and upper) in Yungan and Yutianao sections (Fig. 3). In thiscase, an Early Triassic fall in sea level would have had to cause the ero-sion of both Triassic strata, as well as downcut to cause renewed expo-sure and erosion of the underlying Upper Permian reef margin (Fig. 8B)in order to yield the Permian clasts that were then reworked into thebreccia.

With the model of two or more sea level falls and antecedent to-pography at the southern margin, it is also tempting to correlate thesubaerial exposure surface at the contact between the Upper Permianboundstone and Lower Triassic facies at Yungan and Yutianao sec-tions on the southern margin of the GBG (Figs. 3 and 4A) with theerosional truncation surface at the contact between the uppermostPermian packstone and the microbialite in PTB sections in the plat-form interior further north (Dawen and Langbai sections: Payneet al., 2007; Chen et al., 2009; Collin et al., 2009)). However, as dis-cussed above, it is still controversial whether the truncation surfaceat the PTB sections in the interior formed subaerially or in a subma-rine environment. Further, the biostratigraphy at the Langbai andDawen sections indicates the erosion took place before or withinthe basal Triassic H. parvus zone (Chen et al., 2009) whereas the brec-cia in Unit 2 of Yungan contains clasts such as microbialite and thin-shelled bivalve packstone that compare lithostratigraphically to strataabove the PTB truncation surface in the GBG interior. This observation


suggests that the subaerial exposure surface in Units 1 and 2 of Yunganis younger than the PTB erosion surface at Langbai.

To account for the observed differences in stratigraphic thicknessesand architectures, the substratum of the southern margin of the GBGwould have needed to develop significant antecedent topography, per-haps hundreds of meters above the level of the interior sections atDawen and Langbai. Thus, if the exposure surface was correlative be-tween the areas then one would expect deeply developed karst features(e.g. truncation and cave collapse extending at great depth below the ero-sion surface) on the southern margin. Thus far we have not found suchfeatures.

The extreme difference in thickness of the Early Triassic strata fromthe northern margin (several hundreds of meters thick) versus thesouthern margin (~70 m to as little as zero meters) is difficult to explainwith this model because it requires an unrealistic magnitude of anteced-ent topographic elevation of the underlying Permian margin (Fig. 8A) oran unrealisticmagnitude in fall of sea level (Fig. 8B).Moreover, the abun-dance of calcareous algae in the Upper Permian of theWujiaping Forma-tion at Dawen and Dajiang suggest these strata were deposited withinthe shallow photic zone and not beneath hundreds of meters of water.

The sequence stratigraphic framework of the GBG shows a majorback-step (~700 m) in the northern margin of the GBG from theUpper Permian margin at the sponge patch reefs to the Lower Triassicoolites of the middle ramp (Fig. 9; Lehrmann et al., 2007). Global sealevel curves indicate a rise in sea level across the Permian–Triassic



Lad2

Lad3

In1

In2

Ol1

Ol2Ol3

An1An2An3An4

Lad1

Car1

Indu

anO

lene

kian

Anisian

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Carnian

Gries.

Dien.

Smith.

Spath.

00.5 landward 00.5 landward

Haq et al.,1987, 2008

Haq et al.,2005

TethyanICS, 2011 S.China

SB-1

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Max F7

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TubiphytesBoundstone

Sponge reef mound

Siliclastic Turbidites

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L. Trias.

Anisian

Ladinian

Carnian

250 M

1 KM

Fig. 9. Sequence stratigraphic framework of the GBG (left, from Lehrmann et al., 2007) and compilation of eustatic sea level curved of the Permian–Triassic (right, modified fromHaq et al., 1987; Haq and Al-Qahtani, 2005; Haq and Schutter, 2008; ICS, 2011).


boundary (Fig. 9). These observations indicate that it is unlikely thatsubaerial exposure around the PTB of Yungan section was caused bya drop in sea level. Based on the above analysis, a model involvingonly antecedent topography of Changhsingian sponge barrier reefand sea level fluctuation cannot explain the combination of faciesand stratigraphic architectural differences between the southernand northern margin of the GBG; other geologic factors would be nec-essary as discussed below.

4.2. Syndepositional tectonic uplift

In contrast to the model based on antecedent topography coupledwith eustatic sea-level fall, models involving localized, syndeposi-tional tectonic uplift or tilting via faults (Fig. 10A) or folds (Fig. 10B)do not require antecedent topography of the Upper Permian barrierreef at the south margin to explain the observed facies patterns.

The tectonicmodels better explain themixing of Permian and Triassiclithoclasts in the breccia units (e.g. Unit 5 at Yungan section) because ofthe possibility that strata of different ages were elevated above sea levelby tectonic activity to be eroded and then mixed. Furthermore, localizedtectonic uplift would easily explain the large differences in thickness ofpreserved Lower Triassic strata between the northern margin and plat-form interior versus the southern margin (Table 1).

In this model, the area encompassing Yungan and Yutianao mayhave been subjected to a single or multiple pulses of tectonic uplift,subaerial exposure and erosion. In the case of a single uplift, a persis-tent high and repeated episodes of erosion would explain the repeat-ed shedding and deposition of breccias with reworked clasts. In thecase of multiple uplifts, the area would have been uplifted exposingthe Upper Permian reef and Lower Triassic strata thus forming the va-dose diagenetic fabrics in the Upper Permian strata of Unit 1, and thekarst breccia with admixed Permian and Lower Triassic clasts (in Unit2) on top of the Upper Permian sponge boundstone (Fig. 3). After thefirst uplift and a hiatus of unknown duration, the area again subsidedand received marine sedimentation during the Early Triassic (depos-iting Units 3–4 at Yungan and Yutianao; Fig. 3). A second tectonic


event uplifted the nearby section sufficiently to cause renewed expo-sure and erosion of the underlying Upper Permian strata shedding amix of both Lower Triassic and Upper Permian clasts to form the brec-cias of Unit 5 in Yungan and Yutianao (Fig. 3). In the tectonic upliftmodel, the persistence of an uplifted topographic high and significantthickness truncation may better explain the extreme differences inthickness between the north and southmargins (Fig. 10A and B). Finally,the area subsided again allowing for deposition of overlying strata, in-cluding the Middle Triassic, Anisian reef facies at Yutianao (Fig. 3B).

Differences between the tectonic models involving faulting versusfolding are the predicted aerial extent of the corresponding unconfor-mity and truncation of Lower Triassic strata. The fault uplift model(Fig. 10A) predicts relatively localized unconformity and truncation,which is most consistent with our observations of thick conformableLower Triassic strata nearby at Dawen (~5 km to the north), and extend-ing near to the southern margin at Bangeng (Lehrmann et al., 2005).

The Nanpanjiang basin contains several examples of asymmetry inbasin evolution and platform architecture that can be attributed totectonic affects such as the asymmetrical drowning and expansionof the eastern sector of the basin during the Permian–Triassic(Lehrmann et al., 1998), the asymmetric collapse of the Anisian reefof the Yangtze platform in the eastern part of the basin (Enos et al.,1997) and the Carnian drowning of the western portion of the Yang-tze Platform (Enos et al., 1998). There are also examples of syndepo-sitional faulting directly controlling the position and evolution of theChongzuo–Pingguo Platform margin (Lehrmann et al., 2007) and themargin of the western Yangtze Platform (Minzoni et al., 2010). Theregional geologic setting thus lends support to the likelihood that tec-tonics influenced stratigraphic patterns on the southernmargin in theYungan and Yutianao areas.

4.3. Submarine margin collapse

Carbonate platform margins commonly display indented and scal-loped outlines because of gravitational collapses. Such large scale de-struction of platform margins have been interpreted to result from



Microbialite

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Fig. 10. Models involving tectonic uplift. (A) Model illustrating fault uplift causing subaerial exposure along southern margin. Because of substrate subsidence, Early Triassic sed-iments deposited on the platform interior and margins (Stage 1). Syndepositional faults elevated Permian and Triassic strata near the southern margin (Stage 2). Strata of variousages (including Permian and Lower Triassic) were and eroded and causing redeposition of clasts (Stage 3). (B) Model illustrating fold uplift and subaerial exposure along southernmargin of the GBG.


syndepositional faulting or earthquake shocks, and erosional under-cutting (Mullins and Hine, 1989; Bosellini, 1998) or instability fromprogradation of cemented boundstone over compacting foreslope facies(Kenter et al., 2001; Morsilli et al., 2002; Eberli et al., 2004; Rush andKerans, 2010), or inherited topography (Weber et al., 2003; Collins etal., 2006). In a final model, we consider the possibility that platformasymmetry, unconformities and thickness patterns described fromYungan and Yutianao resulted from Early and/or Middle Triassic col-lapse of the margin (Fig. 11).

In this model, after Early Triassic sediments were deposited atop thePermian reef, submarine collapse was triggered by over-steepening ofthemargin over the antecedent topography inherited from the Permianreef, or by seismic shock (Mullins and Hine, 1989; Ellis et al., 1990).Margin collapse, during the Early or Middle Triassic would have notonly removed the Lower Triassic strata, but it also exhumed and erodedunderlying Upper Permian strata, explaining the mixture of LowerTriassic and Upper Permian clasts in the breccias units (Fig. 11A).Collapse may have also occurred in pulses and at different scales,explaining the occurrence of more than one interval of breccia and thefact that no Permian fossils have been found so far from Units 2 and 5at Yutianao section. This scenario may also explain the local relativelythin preservation of Lower Triassic strata due to wholesale removal bysubmarine collapse.

However, thismodel does not necessarily predict subaerial exposureand karsting. If submarine collapse of themargin occurred in the area, itwould have done so in concert with tectonic uplifts or sea level dropsthat would have independently generated the subaerial diagenesis, un-less the collapse was triggered by sea level fall or a tectonic event.


Large amphitheater-shaped reentrants visible in satellite imagesalong the southern platform edge (Fig. 7) and the truncation of themargin Tubiphytes reefs at Bangeng (Lehrmann et al., 2005) suggeststhat collapse played a significant role in margin evolution during theMiddle Triassic (Fig. 11B). A likely possibility is that the platformasymmetry and features along the southern margin resulted from acombination of local tectonic uplift and erosional truncation as wellas repeated margin collapse.

5. Discussion

Facies and stratigraphy at Yungan and Yutianao in the southernmargin of the GBG demonstrate significant differences from those ofthe northern margin, including a subaerial exposure surface andkarst breccia developed in the Lower Triassic, breccias containingadmixed Permian and Triassic clasts, and exceedingly thin Lower Triassicstrata. From comparison of observed features with conceptual deposi-tional models, a scenario involving tectonic uplift and margin collapseappears to best account for the observations.

The possibility that submarine collapse is responsible for the rela-tionships at Yungan and Yutianao is supported by evidence that thesouthern margin of the GBG suffered significant submarine collapseduring the Middle Triassic. Large amphitheater shaped reentrants(scallops) that truncated Middle Triassic strata are evident in satelliteimages west of the Yungan area (Figs. 2 and 7). In the area of the scallopat Bangeng,fieldmapping demonstrates that theMiddle Triassic, Anisian,Tubiphytes reef in this area is only about 200 mwide in contrast to ~1 kmwide at the north margin (Lehrmann et al., 2005). Furthermore our



(1)

(2)(3)

Microbialite

Oolite

Peritidal cyclic

Peritidal cyclic

Submarine unconformitycollapsed margin (scallop)

Breccia containing mix of plat-form margin Tubiphytes clastsand platform interior clasts(older clasts also possible).

T Tubiphytes boundstone

NS

T T T

5km

1kmApproximate

scale

L

I

I Lower Triassic (Induan)

Middle Triassic (Ladinian)L

A

B

Fig. 11.Models involving submarine unconformity and redeposition caused by collapse of margin. (A) illustrates Early Triassic collapse (B) illustrates Middle Triassic collapse. EarlyTriassic sediments deposited on the platform interior and margins (Stage 1). Over-steepening sediments in the south margin, lack of siliciclastic fill in the basin or seismic shockscaused collapse of south margin and deep erosion on Early Triassic or Upper Permian strata, which left amphitheater-shaped reentrants (scalloped margins; Stage 2). Erosion andredeposition produced submarine unconformity and mixture of Permian and Triassic lithoclasts (Stage 3).


reconnaissance observations indicate that it is similarly narrow alongother exposures of the south margin such as the Yungan–Yutianao area(Fig. 2B).

Submarine collapse of the Middle Triassic margin may have beentriggered largely by the steep slope of the inherited topography ofthe Upper Permian substrate, or tectonic movements along the formerPermian margin. In this scenario, tectonic movement may have takenplace along faults that had previously controlled the position of thePermian margin. In contrast, the northern margin was able to accom-plishmore extensive progradation because it advanced over a relativelyflat antecedent topography (and likely tectonically inactive surface)inherited from the underlying Upper Permian platform interior strata(which had drowned and shifted to deep basin deposition in the latestPermian).

A hybridmodel is also possible inwhich a combination of antecedenttopography, tectonic uplift, sea level drop and submarine slope collapseall conspired to yield the stratigraphic attributes. It is likely that mecha-nisms such as tectonics, antecedent topography and margin collapsewould be linked. However, as discussed earlier, the burden of the evi-dence suggests the region was tectonically active and that sea-level wasrising rather than falling in the Early Triassic making a model involvingtectonics and margin collapse more likely.

Consideration of the conceptual models suggests several areas forfurther work to better constrain the mechanisms and timing of eventsthat led to the asymmetry. For example, detailed mapping of the un-conformity surface is needed to further evaluate the geometry of thetruncation surface(s) responsible for the beveling of the Lower


Triassic strata. Detailed conodont biostratigraphic work is requiredto determine more precise ages of clasts and matrix in the brecciaunits at Yungan and Yutianao as well as other breccia intervalsalong the southern margin. Further analysis of the diagenesis alongthe unconformity surface(s) and clasts in the breccia will also help re-solve between models. Fault uplift, for example, predicts erosionaldowncutting that does not necessarily follow the margin, significantsubaerial diagenesis, and erosional “unroofing” patterns found inthe lithologies of breccias clasts. In contrast, the submarine collapsemodel predicts truncation confined to the margin area, localized re-moval of margin facies in vicinity of scalloped margins, submarinediagenesis, and marine onlap onto the erosion surface. If our interpre-tation is correct that both tectonic uplift and margin collapse played arole, then one would expect to find changes in facies and stratigraphicfeatures along the southern margin that vary depending upon whichmechanism had its greatest impact in a particular area of the margin.

6. Conclusions

This study demonstrates significant north-to-south asymmetry ofthe Great Bank of Guizhou. Five aspects of sedimentary facies and ar-chitecture from the southern margin are distinctively different fromthe northern margin: (1) Lower Triassic strata remarkably thinneron the southern margin than in the platform interior and northernmargin; (2) evidence for subaerial exposure and karst breccias devel-oped in the Upper Permian sponge boundstone and in the overlyingLower Triassic strata; (3) Triassic and Permian clasts chaotically




mixed together within breccias in the Lower Triassic strata; (4) amuch narrower Tubiphytes reef preserved along the southern marginin the Middle Triassic; and (5) large erosional reentrants (scallopedmargins) at the southern margin of the GBG evident in satelliteimages.

A model invoking an antecedent topographic high produced bythe Upper Permian reef margin coupled with sea-level fluctuationcan explain subaerial exposure of the southern margin but is inade-quate to explain the dramatically thinner Lower Triassic strata alongthe south margin.

Pulses of tectonic uplift near the southern margin, whether via fault-ing or folding could explain the subaerial unconformity, the mixture ofPermian and Lower Triassic clasts within breccia units, and truncationof Lower Triassic strata along the south margin. Localized uplift and un-conformity development and thickness patterns seem to bestfit the faultuplift model. Collapse of the Lower Triassic margin could explain manyof the observations, except localized subaerial exposure, which requiresrelative sea level fall or tectonic uplift.

The collection of observations at Yungan and Yutianao aswell as thelarge erosional collapse features visible in satellite images suggest that acombination of tectonic uplift and margin collapse likely explain thefeatures along the southern margin of the GBG and asymmetry of theplatform architecture.

The significant asymmetry in width of the Middle Triassic reef fa-cies and the coincidence of the Triassic platform margin with thesteep Permian margin of the Yangtze Platform suggests that large-scale collapse of the southern margin is best explained by the steepslope, and tectonic movements along a zone inherited from the un-derlying Permian reef margin of the Yangtze Platform. The northernmargin, in contrast, prograded extensively over a relatively flat andhorizontal foundation on the leeward side of the platform.

Although changes in biota and ocean chemistry strongly impactedthe evolution of the GBG during the end-Permian extinction and Triassicrecovery, this study shows that the mechanics of margin stability, tec-tonic uplift, and large scale margin collapse played a significant role inthe asymmetry of margin architecture.

Acknowledgments

The research was supported by National Science Foundation ofChina (40830212 to MY and 41002007 to GS), the United StatesNational Science Foundation (EAR-0807377 to JLP), the NationalGeographic Society (8102-06 to JLP), the Petroleum Research Fund ofthe American Chemical Society (45329-G8 to JLP, and 49341-UR8 toDJL) and Shell International Exploration and Production (46000572 toDJL). The authors wish to extend thanks to H. Fu, H. Yin, and H. Ma fortheir assistance in the field, to Y. Ye for her assistance in the laboratory,and to A. Bachan for his discussion onmicrofacies of carbonate rocks.Weacknowledge Mark Harris, Jeroen Kenter, andMitch Harris for thoroughreviews that significantly improved the manuscript.

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Palaeogeography, Palaeoclimatology, Palaeoecology · Simpliﬁed models in the literature predict that isolated platforms may initiate upon an antecedent topographic substrate produced