Carbonate platform evolution: from a bioconstructed ...

Carbonate platform evolution: from a bioconstructed platformmargin to a sand-shoal system (Devonian, Guilin, South China)

DAIZHAO CHEN*, MAURICE E. TUCKER� , JINGQUAN ZHU* and MAOSHENG JIANG**Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing 100029,China (E-mail: [email protected])�Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK(E-mail: [email protected])

ABSTRACT

The depositional organization and architecture of the middle–late Devonian

Yangdi rimmed carbonate platform margin in the Guilin area of South China

were related to oblique, extensional faulting in a strike-slip setting. The

platform margin shows two main stages of construction in the late Givetian to

Frasnian, with a bioconstructed margin evolving into a sand-shoal system. In

the late Givetian, the platform margin was rimmed with microbial buildups

composed mainly of cyanobacterial colonies (mostly Renalcis and Epiphyton).

These grew upwards and produced an aggradational (locally slightly

retrogradational) architecture with steep foreslope clinoforms. Three

depositional sequences (S3–S5) are recognized in the upper Givetian strata,

which are dominated by extensive microbialites. Metre-scale depositional

cyclicity occurs in most facies associations, except in the platform-margin

buildups and upper foreslope facies. In the latest Givetian (at the top of

sequence S5), relative platform uplift (± subaerial exposure) and associated

rapid basin subsidence (probably a block-tilting effect) caused large-scale

platform collapse and slope erosion to give local scalloped embayments along

the platform margin and the synchronous demise of microbial buildups.

Subsequently, sand shoals and banks composed of ooids and peloids and, a

little later, stromatoporoid buildups on the palaeohighs, developed along the

platform margin, from which abundant loose sediment was transported

downslope to form gravity-flow deposits. Another strong tectonic episode

caused further platform collapse in the early Frasnian (at the top of sequence

S6), leading to large-scale breccia release and the death of the stromatoporoid

buildups. Siliceous facies (banded cherts and siliceous shales) were then

deposited extensively in the basin centre as a result of the influx of

hydrothermal fluids. The platform-margin sand-shoal/bank system, possibly

with gullies on the slope, persisted into the latest Frasnian until the restoration

of microbial buildups. Four sequences (S6–S9), characterized by abundant

sand-shoal deposits on the margin and gravity-flow and hemipelagic deposits

on the slope, are distinguished in the Frasnian strata. Smaller-scale

depositional cyclicity is evident in all facies associations across the

platform–slope–basin transect. The distinctive depositional architecture and

evolution of this Yangdi Platform are interpreted as having been controlled

mainly by regional tectonics with contributions from eustasy, environmental

factors, oceanographic setting, biotic and sedimentary fabrics.

Keywords Carbonate platform, microbially rimmed platform margin, sand-shoal system, South China.

Sedimentology (2002) 49, 737–764

� 2002 International Association of Sedimentologists 737

INTRODUCTION

Numerous studies on the formation and evolutionof carbonate platforms within extensional tectonicsettings have been documented from the geologi-cal record (e.g. Burchette, 1988; Cocozza & Gandi,1990; Santantonio, 1993, 1994; Picard et al., 1994;Rosales et al., 1994; Bosence et al., 1998; Wilson,1999; Wilson et al., 2000). However, only a fewstudies have involved cases within a strike-slipsetting. Examples include the Lower Cretaceous of

northern Spain (Garcıa-Mondejar, 1989, 1990;Agirrezabala & Garcıa-Mondejar, 1992; Rosaleset al., 1994; Garcıa-Mondejar et al., 1996; Gomez-Perez et al., 1999; Rosales, 1999) and the Devo-nian of southern Hunan, South China (e.g. Jiang,1989, 1990).

In this paper, an example is presented of aplatform margin related to oblique, extensionalfaulting within a strike-slip fault zone in theGuilin region of South China (Fig. 1A; Chen et al.,2001a). The anatomy of the transition from

Fig. 1. (A) Sketch map showing the configuration of basins (shaded areas) and carbonate platforms (non-ornamentedareas + ornamented Yangdi Platform) in the Devonian of the Guilin region. The shaded spindle-shaped area is theYangshuo Basin. (B) Geological map of the study area, where deposits from platform margin (nearly horizontallystratified) to slope (SW-dipping) and basin crop out along the highway from Zhongnan to Fuhe and farther west.The platform margin extends roughly in a NNE–SSW direction in this segment. Giv., Givetian; Fr., Frasnian;Fa., Famennian.

738 D. Chen et al.

� 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764

platform margin to basin has been documentedthrough detailed field observations and analysisof depositional facies and facies associations. Awell-exposed across-strike transect from theplatform (the Yangdi Platform) to the basin (thepull-apart Yangshuo Basin, Fig. 1) allows anexamination of the original architecture andgeometry of the platform margin. The mechanismof platform-margin development and evolution insuch a tectonic setting is discussed, and specialemphasis is placed on the syntectonic sedimen-tation during the late Givetian to Frasnian. Thestratigraphic context for this case study is largelybased on the work of Zhong et al. (1992), modifiedby Chen et al. (2001a); a new stratigraphic unit isalso proposed in this study for convenience.

GEOLOGICAL SETTINGAND STRATIGRAPHY

The Yangdi Platform extended for about 25 km ina NW–SE direction and abutted the pull-apart

Yangshuo Basin to the south (Fig. 1A). Thesouthern margin was controlled by obliqueextensional faulting, which was induced by theactivity of a major strike-slip fault (cf. Chen et al.,2001a). A transect of about 2 km across theYangdi Platform and Yangshuo Basin is wellexposed along the highway from Fuhe to Zhong-nan, south-east of Guilin (Fig. 1B), where nearlyhorizontal strata at Zhongnan (back-marginfacies) gradually pass basinwards into inclinedslope strata (SW-dipping) around Wulibei (plat-form margin to slope facies). This exhumed stratalpattern probably represents the original deposi-tional profile across the platform margin andslope, as no post-depositional faulting hasoccurred at the platform edge (Fig. 1B). The strataof the basinal facies, although dipping very gently(�2–10�), may not represent the original deposi-tional declivity, in view of thrusting along strikeof the basin margin to the east of Fuhe (Fig. 1B),where basinal strata were uplifted and eroded.Three outcrop sections at Zhongnan, Wulibei andFuhe, representing the strata of back-margin,

Fig. 2. Nomenclature and distribution of lithostratigraphic units, depositional sequences (S1–S9) and platformphases (three phases) in the Guilin area and their correlation with the conodont stratigraphy. This study focuses onthe last two platform phases (S3–S9). Two conodont zonation schemes are listed for convenience, based on Zhonget al. (1992).

Carbonate platform evolution, Guilin, South China 739


platform margin-slope and basinal facies, respect-ively, were measured and logged in detail. Afurther section at Shihedong was also recorded(for location, see Fig. 1B).

The stratigraphic relationship between the plat-form and basin is illustrated in Fig. 2. The basalTangjiawan Formation, the first carbonate succes-sion overlying the clastic Xindu Formation, mainlymade of stromatoporoid biostromal facies repre-senting the initial stage of platform–basin evolu-tion, is not included in this study. Thus,the stratigraphy mainly involved in this studyincludes the Rongxian, Mintang, Gubi and Wuzhi-shan Formations and Zhongnan Limestone(Figs 1B and 2). The Rongxian Formation repre-sents platform margin-to-slope and back-marginfacies, ranging from the upper Givetian to Famen-nian; however, most back-margin deposits abovethe middle Frasnian have been truncated as a resultof Triassic uplift (Yanshanian orogenic stage). Themostly Upper Givetian Mintang Formation is char-acterized by alternating laminated microbialitesand nodular pelagic limestones. The ZhongnanLimestone, a new stratigraphic unit synchronouswith the Mintang Formation, is characterized bymicrobial buildups with minor breccias. The Liuji-ang Formation is characterized by starved basindeposits of siliceous rocks with minor tentaculitidcherty limestones. The Gubi Formation (mainlyFrasnian) is chiefly composed of gravity-flowdeposits at the platform margin and pelagic nod-ular limestones with minor gravity-flow beds nearthe basin centre. The Wuzhishan Formation repre-sents distal slope and basin deposits dominated bypelagic nodular limestones with minor calciturbi-dites, mainly of Famennian age (Fig. 2).

From middle Givetian to Frasnian times, threemain phases of platform–basin evolution havebeen distinguished, each characterized by a dis-tinctive stratigraphic succession and depositionalpackage (Chen et al., 2001a). The emphasis of thispaper is placed on the second and third phases(late Givetian and Frasnian respectively). In theupper Givetian successions, depositional pack-ages are characterized mainly by microbialites,with microbial buildups along the platform mar-gin (Zhongnan Limestone) grading into deep-water microbial laminites in the basin (MintangFormation). In contrast, the Frasnian depositionalpackages are characterized by shoal deposits(oolitic–peloidal grainstones–packstones) on themargin (Rongxian Formation), extensive gravity-flow and hemipelagic deposits on the slope (GubiFormation) and siliceous deposits in the basin(Liujiang Formation). The antecedent platform

margin was locally scalloped along the platformmargin (e.g. at Shihedong and south of Wulibei,Fig. 1B), with gullies developed downslopethrough which sediment was transported bygravity flows. The dramatic change in platformmorphology and architecture occurred close tothe Givetian–Frasnian boundary. Seven sequen-ces have been recognized in the whole studiedsuccession, three (S3–S5) in the upper Givetian(platform development phase 2) and four (S6–S9)in the Frasnian (platform development phase 3)(Fig. 2). Details of the sequence development andstratal patterns have been documented by Chenet al. (2001a,b).

BIOCONSTRUCTED PLATFORMSYSTEM: LATE GIVETIAN

From the late Givetian, the Yangdi Platform wasrimmed with microbial buildups (reefs) with steepmarginal slopes to basins, physiographically sim-ilar to the rimmed-platform category of Read (1982,1985). The well-exposed southern margin permitsa detailed description of the depositional architec-ture and facies changes across the margin. Based ondepositional features and facies associations, plat-form-interior, platform-margin (back-reef/reef-flat,fore-reef), slope (foreslope, lower slope) and basinenvironments have been distinguished within thebioconstructed rimmed-platform system (Fig. 3Aand B). Description of the platform interior facies isgiven elsewhere (Chen et al., 2001b) and will notbe considered in this paper. Only the anatomy ofthe platform–basin transect from Zhongnan viaWulibei to Fuhe is presented here (see Fig. 1 forlocation and Figs 4 and 5). Metre-scale cyclicitycan be discerned in most of the facies except themassive microbial buildups.

Platform-margin environment

Back-reef/reef-flat facies

Facies from the back-reef/reef-flat setting consistof stromatoporoid rudstone/floatstone (PM1),microbial rudstone/grainstone (PM2), Amphiporagrainstone/wackestone (PM3), skeletal wacke-stone/packstone (PM4) and minor microbial lam-inite (PM5) and fossil-poor mudstone (PM6) (seethe lithological logs of Zhongnan in Fig. 5;Table 1). They are mostly present in the RongxianFormation. These deposits are generally lightcoloured and thick to massive bedded (2–3 mcommon). Stromatoporoids and Amphipora are

740 D. Chen et al.


Fig. 3. Sketch diagram showing the depositional environments and facies changes from the platform to the basin.(A) Depositional environments and facies across the platform–slope–basin transect during the early microbiallyrimmed platform stage (S3, middle Givetian). The platform margin-to-slope rimmed with microbial buildupsaggraded vertically, and few sediments were exported downslope. (B) Depositional environments and facies acrossthe platform–slope–basin transect during the latest stage of the microbially rimmed platform system (S5, latestGivetian). The rimmed margin and slope were undercut with gullies, through which sediments from the platformwere transported downslope, leading to a less well-protected platform margin.

Fig. 4. A panoramic sketch from a photomosaic showing the transition from the horizontal platform strata (the hillright back in the distance) to the inclined foreslope strata of the upper Givetian. Note the aggradational architecture ofthe microbial buildup on the slope. Sequences (S2–S5) are separated by the thick lines. Box at the bottom of hill is3Æ5 m high. Dashed arrows represent measured sections shown in Figs 5 and 10. The Zhongnan section is measuredat the back of the hill (right back in the distance).



the most common fossils with subordinate ostra-cods and foraminifera. These biotic associationscharacterize semi-restricted, subtidal to peritidalback-reef environments of Devonian platforms

around the world (e.g. Read, 1973; Elrick, 1995;Chen et al., 2001b). Grains are usually abradedand more rounded than those in platform interiorlagoons (e.g. Chen et al., 2001b), suggesting

Fig. 5. Lithological logs and corre-lations of platform growth phase 2across the platform margin to thebasin from Zhongnan via Wulibei toFuhe. An aggradational microbialbuildup on the platform margin-to-slope is well illustrated andgranular sediments were funnelledthrough, inducing downslope grav-ity-flow deposits in the late stage ofbuildup construction. The microbialbuildup was finally terminated aftera significant collapse at the top ofthe Givetian. Thin lines on the rightof the logs from Fuhe and Zhongnanmark the high-frequency cycles(parasequences); arrows mark theboundaries of cycle sets. Refer toTables 1 and 2 for facies codes. SeeFigs 1B and 4 for section locations.LST, lowstand deposits; TST,transgressive deposits; HST, high-stand deposits. M, mudstone; W,wackestone; P, packstone; G, grain-stone; Co, conglomerate; B, breccia/megabreccia; Bs, boundstone; Bi,bindstone; Rs, rudstone; Fs, float-stone.

742 D. Chen et al.


Table

1.

Dep

osi

tion

al

facie

sof

the

bio

con

stru

cte

dp

latf

orm

syst

em

.

Dep

osi

tion

al

en

vir

on

men

tF

acie

sass

ocia

tion

/fa

cie

s(c

od

es)

Desc

rip

tion

Bio

taIn

terp

reta

tion

Pla

tform

marg

inB

ack

-ree

f/re

ef-fl

at

Str

om

ato

poro

idR

s/F

s(P

M1)

Th

ick

tom

ass

ively

bed

ded

(2–3

mcom

mon

);abra

ded

skele

tal

part

icle

s,m

icro

bia

lfr

agm

en

tsan

dp

elo

ids

com

mon

,m

inor

on

coid

s;bu

lbou

san

dd

om

al

stro

mato

poro

ids

con

stit

ute

Fs

Str

om

ato

poro

ids,

Sta

chyod

es,

Am

ph

ipora

,gast

rop

od

san

dost

racod

s;m

icro

bia

lcolo

nie

s

Mod

era

te-

toh

igh

-en

erg

yre

ef-

flat

Mic

robia

lR

s/G

s(P

M2)

Severa

lm

thic

k;

mic

robia

lfr

agm

en

ts(m

m-

tocm

-siz

ed

)d

om

inan

t,m

inor

on

coid

san

dp

elo

ids,

rare

skele

tal

gra

ins

Th

rom

boli

tic

mic

robes

cyan

obacte

rial

colo

nie

s(i

.e.

Ep

iph

yto

nan

dR

enalc

is),

gast

rop

od

san

dost

racod

s

Mod

era

te-

toh

igh

-en

erg

yback-r

eef

flat

Am

ph

ipora

Gs/

Ws

(PM

3)

10’s

cm

toa

few

mth

ick;

cu

rren

t-abra

ded

,p

ara

llel-

pro

ne

skele

tal

gra

ins

com

mon

,m

inor

pelo

ids,

on

coid

san

dm

icro

bia

lfr

agm

en

ts;

lam

init

ein

terc

ala

tion

slo

call

y

Am

ph

ipora

dom

inan

t,m

inor

Sta

chyod

es,

gast

rop

od

s,ost

racod

s;cyan

obacte

rial

colo

nie

s

Sem

i-re

stri

cte

d,

mod

er-

ate

-en

erg

y,

back-r

eef

sett

ing

Skele

tal

Ws/

Ps

(PM

4)

Th

in-

tom

ed

ium

-bed

ded

(10–50

cm

);re

lati

vely

dark

incolo

ur;

rest

ricte

dsk

ele

tal

gra

ins,

min

or

pelo

ids,

mic

riti

cm

atr

ix

Gast

rop

od

s,ost

racod

san

dA

mp

hip

ora

Rest

ricte

d,

low

-en

erg

ybeh

ind

marg

in(i

.e.

lagoon

)

Lam

init

e(P

M5)

Cm

-scale

lam

inati

on

,com

mon

lyin

dic

ate

dby

pelo

idal

Gs/

Ws,

mic

robia

lite

/pelo

idal

Gs

cou

ple

ts,

rare

lyby

inte

rgro

wn

mic

robia

lite

s;th

inA

mp

hip

ora

Gs

inte

rcala

tion

scom

mon

;fe

nest

ral

an

dd

isso

luti

on

cavit

ies

occu

rlo

call

y

Mic

robia

lcolo

nie

s;A

mp

hip

ora

Rest

ricte

d,

low

er

inte

r-ti

dal

tosh

all

ow

est

subti

dal

en

vir

on

men

t

Foss

il-p

oor

Ms

(PM

6)

Th

in-b

ed

ded

(<20

cm

);ra

rearg

illa

ceou

sse

am

s;ra

rere

stri

cte

dfa

un

aR

are

ost

racod

san

dgast

rop

od

sR

est

ricte

d,

qu

iet

shall

ow

subti

dal

Fore

-ree

fM

ass

ive

mic

robia

lB

s(F

R1)

Locall

yth

ick-b

ed

ded

(>50

cm

);m

icro

bia

lB

i/B

fd

om

inan

t,ra

reF

s,m

ain

lycon

sist

ing

of

cyan

obacte

rial

an

dth

rom

boli

tic

mic

robes

wit

hm

inor

skele

tal

fau

na;

Ren

alc

isd

om

inate

the

up

per

fore

-reef

an

dd

ecre

ase

inabu

nd

an

ce

dow

nsl

op

e;

stro

mata

cti

san

dcavit

ies

fill

ed

wit

hearl

ym

ari

ne

cem

en

tcom

mon

;aggra

dati

on

al

arc

hit

ectu

re

Mic

robia

lcolo

nie

s:R

enalc

is,

Ep

iph

yto

nan

dth

rom

boli

tic

mic

robes

com

mon

,m

inor

red

alg

ae;

skele

tal

fau

na:

att

ach

ed

bra

ch

iop

od

s,A

mp

hip

ora

,st

rom

ato

poro

ids;

rare

ost

racod

s,gast

rop

od

s

Mic

robia

lbu

ild

up

son

the

fore

-reef

an

du

pp

er-

most

fore

slop

e



Table

1.

Con

tin

ued

.

Dep

osi

tion

al

en

vir

on

men

tF

acie

sass

ocia

tion

/fa

cie

s(c

od

es)

Desc

rip

tion

Bio

taIn

terp

reta

tion

Slo

pe

Up

per

slop

e(f

ore

slop

e)B

ed

ded

mic

robia

lB

s(U

S1)

Gen

era

lly

1–5

mth

ick;

stro

mato

lite

san

dth

rom

boli

tes

com

mon

,esp

ecia

lly

the

form

er;

LL

H-

an

dS

H-t

yp

ela

min

ae

(up

tose

vera

l10’s

cm

hig

h)

com

mon

inth

efo

rmer,

ind

icate

dby

inte

rgro

wn

(mon

o-

or

hete

rosp

ecifi

c)

cyan

obacte

rial

colo

nie

s,ra

resk

ele

tal

fau

na;

the

latt

er,

thro

mboli

tic

mic

robes

dom

inan

t,m

inor

aggre

gate

sof

cyan

obacte

rial

colo

nie

s;st

rom

ata

cti

scom

mon

Mic

robia

lcolo

nie

s:E

pip

hyto

n(o

rA

ngu

loce

llu

lari

a),

Ren

alc

is,

thro

mboli

tic

mic

robes,

min

or

Wet

her

edel

laan

dR

oth

ple

tzel

la,

red

alg

ae;

rare

gast

rop

od

s,ost

racod

san

dA

mp

hip

ora

Mic

robia

lbio

herm

san

d/o

rbio

stro

mes

on

the

mid

-to

low

er

fore

slop

e

Low

ersl

op

e(t

oe-

of-

slop

e)F

ore

slop

ebre

ccia

/m

egabre

ccia

(LS

1)

5–15

mth

ick

an

dle

ns-

shap

ed

wit

hbasa

lero

sion

;com

pose

dm

ain

lyof

mic

robia

lB

sfr

om

fore

-reef

an

dfo

resl

op

e,

wit

hm

inor

mic

robia

lla

min

ites;

dis

org

an

ized

an

dcla

st-s

up

port

ed

bre

ccia

sw

ith

no

obvio

us

gra

din

g,

locall

ycalc

itu

rbid

ites

inte

rcala

ted

,p

art

icu

larl

yin

the

up

per

part

Mic

robia

lcolo

nie

ssi

mil

ar

toth

em

arg

inal

bu

ild

up

s,m

inor

skele

tal

fau

na

such

as

stro

mato

poro

ids,

bra

ch

iop

od

san

dgast

rop

od

s

Bre

ccia

talu

ssp

all

ed

from

the

bu

ild

up

an

dre

dep

osi

ted

on

the

low

er

slop

e(t

oe-o

f-sl

op

e)

Pebble

con

glo

mera

te(L

S2)

Lim

eM

sta

bu

lar

cla

sts

wit

hro

un

ded

en

ds,

usu

all

ym

ud

-su

pp

ort

ed

,p

ara

llel-

pro

ne

tobed

din

gan

du

psl

op

eim

bri

cate

dlo

call

y

Rare

Vis

cou

sd

ebri

sfl

ow

dep

osi

tsd

eri

ved

from

the

mid

-up

per

slop

eC

alc

itu

rbid

ite

(LS

3)

0Æ2

–3

mth

ick;

ooid

san

dp

elo

ids

com

mon

,m

inor

intr

acla

sts

an

dsk

ele

tal

gra

ins,

locall

yooid

sd

om

inan

t,fo

rmin

gooid

al

Gs;

norm

al

gra

din

gcom

mon

;coqu

ina

inte

rcala

tion

slo

call

y

Bra

ch

iop

od

s,gast

rop

od

s,calc

isp

here

s,ra

reost

racod

sT

urb

idit

yfl

ow

dep

osi

tson

the

low

er

slop

e

Coqu

ina

(LS

4)

Th

in-

toth

ick-b

ed

ded

(10–100

cm

);art

icu

late

dto

dis

art

icu

late

d,

abra

ded

shell

sd

isp

layin

gp

ara

llel-

pro

ne

ori

en

tati

on

an

dgeop

eta

lfa

bri

cs;

oth

er

gra

ins

inclu

de

mic

robia

laggre

gate

s,in

tracla

sts

Bra

ch

iop

od

sd

om

inan

t,m

inor

ost

racod

san

dgast

rop

od

s;m

icro

bia

lite

sas

lum

ps

or

aggre

gate

s

Sh

ell

sre

sed

imen

ted

tolo

wer

slop

eby

cu

rren

tsor

storm

s

Basi

nB

asi

nM

icro

bia

lla

min

ites

(B1)

Cm

-u

pto

50

cm

thic

k;

wavy

top

lan

ar

lam

inae

ind

icate

dby

inte

rgro

wn

(mon

o-

or

hete

rosp

ecifi

c)

cyan

obacte

rial

colo

nie

s,ben

thic

fau

na

abse

nt;

mic

rost

rom

ata

cti

scom

mon

;m

icro

mou

nd

s(5

–15

cm

hig

h)

locall

y

Ep

iph

yto

n(o

rA

ngu

loce

llu

lari

a)

an

dU

rsosc

op

ulu

scom

mon

,m

inor

Roth

ple

tzel

la

Deep

,qu

iet,

eu

trop

hic

con

dit

ion

sin

the

basi

nclo

seto

the

toe-o

f-sl

op

e

Pla

ty-

toth

in-b

ed

ded

lim

eM

s(B

2)

Lim

eM

sd

om

inan

t,ri

ch

inorg

an

icm

att

er,

wit

harg

illa

ceou

sp

art

ings;

coqu

ina

len

ses

inte

rcala

ted

locall

y;

fin

ela

min

ati

on

com

mon

;bio

turb

ati

on

weak,

pela

gic

toh

em

ipela

gic

fau

na

dom

inan

t,ra

reben

thic

fau

na

Ten

tacu

liti

ds,

fora

min

ifera

an

dgast

rop

od

s,ra

reS

trin

goce

ph

alu

sS

usp

en

sion

fall

-ou

tin

deep

er

wate

rsw

ell

belo

wst

orm

wave

base

Gs,

gra

inst

on

e;

Ps,

packst

on

e;

Ws,

wackest

on

e;

Ms,

mu

dst

on

e;

Bs,

bou

nd

ston

e;

Bi,

bin

dst

on

e;

Bf,

baffl

est

on

e;

Fs,

fram

est

on

e.

744 D. Chen et al.


relatively vigorous current activity. Overturnedand abraded stromatoporoids with peloidal grain-stone/packstone matrix characterize relativelyhigh-energy conditions, most probably in thereef-flat area, and microbial rudstones/grain-stones formed under similar conditions, butcloser to the buildups. The extensively distri-buted and abraded Amphipora suggest relativelystrong current activity close to the reef-flat in aplatformward direction. The skeletal wacke-stones/packstones reflect a decrease in currentactivity and were deposited in shallow to inter-mediate subtidal environments towards the plat-form interior. The fossil-poor mudstones are veryrare in the measured section and reflect restricted,quiet and shallow subtidal environments closer to

the platform interior (Elrick, 1995; Chen et al.,2001b). The microbial laminites commonly formthe caps to the subtidal facies and are thusconsidered as having been deposited under peri-tidal conditions.

Fore-reef facies

Organic buildups of the fore-reef (facies FR1) arecomposed of massive microbial boundstones inwhich cyanobacterial microbes, such as Renalcis,Epiphyton and thrombolites (Table 1), acted as theframebuilders with some attached brachiopods(mostly rhynchonellids). Generally, the upperfore-reef of the buildups is dominated by Renalcis(Fig. 6A and B), and those that are close to the

Fig. 6. (A) Polished slab of microbial framestone/rudstone composed overwhelmingly of monospecific microbialRenalcis (?), which is slightly fragmented. Cavity-filling calcite is extensively recrystallized. Scale bar is 3 cm long.(B) Polished slab of microbial bindstone/framestone composed mainly of Renalcis, with minor Epiphyton (1), redalgae (Solenopora) (2) and unidentified microproblematica. Upward-growing structures are preserved (apparently inthe mid–lower part). Early marine cements commonly rim the stromatactis (in the middle) and cavities. Scale bar is3 cm long. (C) Photomicrograph of deep-water microbial laminite, indicated by intergrown Epiphyton (or Angulo-cellularia) laminae. Scale bar is 1 mm long.



reef-flat updip are commonly fragmented(Fig. 6A), indicating relatively active currents.However, their abundance decreases downslope.In contrast, microbial Epiphyton increase volu-metrically basinwards. Cavities and stromatactis,extensively filled with early marine cements, arecommon (Fig. 6A and B), suggesting a constructiverole of early marine cementation in stabilizing themicrobial buildups. Such buildup margins aregenerally about 500 m across. The cyanobacterialmicrobes initiated on the steep platform marginand grew upwards to form an aggradational (sta-tionary) margin, with minor pulses of progradation(Figs 3–5) and local retrogradation in the finalstage of platform growth (e.g. at Shihedong, Chenet al., 2001a). The fore-reef clinoforms haveoriginal dips up to 45� but, in the early stage ofplatform construction, the dips were generallylower in the range 25–30� (Fig. 4).

This type of architecture suggests that sedimen-tation at the platform margin may have approxi-mately kept pace with the subsidence there,induced by deep-seated faulting. The verticalaccretion and stability of the margin were largelythe result of the microbial buildups, which boundand trapped most of the grains and lime mudexported from the platform, the early induration(calcification) of the microbialites themselves andearly marine cementation. These processeshelped to prevent the steep slopes from collapse.Similar situations to this have been documentedby Mountjoy & Riding (1981), Burchette (1988),Drachert & Dullo (1989), Kenter (1990), Purser &Plaziat (1998) and Bahamonde et al. (2000). Themicrobial binding may also have led to sedimentstarvation in the basinal area.

Marginal slope environment

Upper slope (foreslope) facies

The upper slope is morphologically composed ofclinoforms with original dips of 25–45� (e.g.Fig. 4). The values would be a little lower ifcompaction is taken into account. The values areconfirmed by the measurement of geopetal fabricsin the deposits. A decrease in the depositionaldip of the clinoforms occurs downslope, fromabout 45� to around 15� in the lower slope, andthe profile is concave-up especially in the lowersegment. Compositional changes in facies occurdownslope as well. The uppermost foreslope iscomposed of microbial bindstone/bafflestone(FR1) with extensive early marine cement(Fig. 7A), similar to that in the microbial buildups

on the fore-reef, but with a higher abundance ofEpiphyton (Figs 5 and 6A and B). The lowerforeslope mainly consists of thick-bedded stro-matolite boundstone (US1), locally with thick-bedded microbial boundstone (Wulibei section inFig. 5; Table 1), forming metre-scale, domal bio-herms and/or biostromes (Fig. 7B). Their featuresare similar to those reported in the Canning Basin(Playford et al., 1976; George, 1999). Similarly,cyanobacterial colonies played an important rolein the maintenance of the steep slope throughframebuilding, calcification and early marinecementation, as discussed above.

Facies associations indicate a low- to moderate-energy slope environment from the local photic(upper foreslope) to aphotic zone (lower fore-slope). The planar cross-strike distance from thefore-reef to the lower foreslope is in the range500–800 m (cf. Fig. 1B). The lowest value of 25�(a tangent of 0Æ47) is taken as the average clino-form dip in view of burial compaction and theconcave-up slope profile. In this case, the depo-sitional depth to the lower foreslope would havebeen in the range 230–370 m.

Lower slope (toe-of-slope) facies

The depositional dip on the lower slope decrea-ses from about 15� to 1–5�. Deposits in thisenvironment consist of foreslope breccias (LS1),pebble conglomerate (LS2), calciturbidites (LS3)and coquinas (LS4) (Fig. 3B; Table 1). At the baseof the Zhongnan Limestone (upper Givetian; S3),coquinas lie above microbial buildups (Figs 5 and7C). They colonized these areas or were transpor-ted from the platform margin and accumulated asshell banks in the front of the buildups andtowards the base of the clinoforms. During thisphase, calciturbidites were not developed owingto the relatively gentle slope during the initialacceleration of basin subsidence, but they dooccur in the upper Zhongnan Limestone. Grain-stones/packstones with ooids, peloids, minorintraclasts and skeletal grains, and pebble con-glomerates, mostly occur in the upper part of theZhongnan Limestone (i.e. S5, Figs 3B and 5).These represent more granular sediments shed offthe platform under relatively high-energy condi-tions, which may have induced turbidity currentsor debris flows on the now relatively steep slope.

Two breccia wedges occur in the upper part ofthe microbial succession and are mainly com-posed of clasts derived from fore-reef and fore-slope microbial buildups; they taper and pinchout both basinwards and platformwards over a

746 D. Chen et al.


short distance (typically 200–500 m) (Figs 5 and8). The lower one is intercalated with ooidalcalciturbidites and exhibits an upward-thicken-ing stratal pattern, suggesting an overproductivecarbonate factory on the platform, relative toaccommodation creation, and an unstable slope.This scenario reflects a less protected and stabil-ized microbial buildup system, on which gulliesand grooves may have been developed locally.The upper wedge of breccia contains rare calci-turbidite beds and exhibits an undulatory basalsurface, suggesting strong slope erosion duringthe onset of breccia formation.

Although most of the lower slope is notexposed, the planar cross-strike distance of thepresent lower slope is estimated to have been inthe range 500–800 m in view of the location ofexhumed basinal deposits (cf. Fig. 1B). Accord-ingly, the relief of the lower slope could havebeen in the range 45–75 m if the low dip value(5�, a tangent of 0Æ09) is taken as the averageestimate for the lower slope. Cumulatively, therelief of the overall slope from the platform edgeto the basin would have been in the range 275–450 m. This estimate for the relief of the steep,bioconstructed slope is broadly similar to the case

of the Devonian reef-flank in the Canning Basin,Western Australia, where the slope angle wasabout 25–30�, and slope relief was estimated tohave been in the range 300–400 m (Playford,1981).

Basinal environment

Basinal deposits consist mainly of deep-watermicrobial laminites (B1) and thin- to platy-bed-ded lime mudstones (B2) (Table 1). These aretypical of a deep, quiet and sediment-starvedenvironment with hemipelagic to pelagic fauna,weak bioturbation, fine lamination and highorganic content. The intergrowth of cyanobacte-rial Epiphyton (or Angulocellularia; Fig. 6C) (cf.Riding, 1991) and Ursoscopulus (cf. Weller, 1995;Chen et al., 2001a) in the laminites indicatesdeep-water conditions with no current agitationand sparse sediment influx. In general, anupward-deepening trend occurs in the upperGivetian, which is indicated by an upwardincrease in the abundance of clay and planktonicfauna and a decrease in storm intercalations. Thedepositional depth could have been deeper thanthat of the lower slope (275–450 m), as discussed

Fig. 7. (A) Irregular to networked cavities in themicrobial boundstone, which are filled with radiaxialfibrous cements on the cavity walls and equant calcitecrystals in the centre (big cavities common). Key forscale (4 cm). (B) Microbial boundstone showing domaltopography (bioherm) in the lower foreslope. Hammerfor scale (37 cm). (C) Abundant small brachiopods(Leiorhynchus sp. rhynchonellids mostly) concentrateto form coquinas. Articulated shells are common. Keyfor scale (4 cm).



above, particularly in the late stage of this micro-bially rimmed platform, based on the preservedplatform-to-basin profile. This would have beenwell below storm wave base.

SAND SHOAL-DOMINATED PLATFORMSYSTEM: FRASNIAN

From the earliest Frasnian, a pronounced changeoccurred in platform architecture as a result oflarge-scale collapse of the platform margin near theend of the Givetian. Under these circumstances,the bioconstructed platform margin was locallyscalloped in a platformward direction (i.e. atShihedong and to the south of Wulibei; Fig. 1B)and gullied on the slope. Bank-derived sedimentswere funnelled through the gullies and bypassedthe mid–upper slope to feed the lower slopeaprons, where many coarse gravity-flow depositswere deposited. Stacked breccia/megabrecciaunits occur in the lower Frasnian (S6 and S7) atDongcun (see Fig. 1A for location; Chen et al.,2001a; fig. 9). At this time, sand shoals dominatedthe platform margin in the absence of organicbarriers reducing wave action (Figs 3B and 9). Thissituation persisted throughout the Frasnian untilthe restoration of microbial buildups on the plat-form margin in the latest Frasnian (e.g. Wulibeilogs in Fig. 10). In the platform–basin transectfrom Zhongnan via Wulibei to Fuhe, platform-margin (back-margin, stromatoporoid buildup andsand shoal/bank), slope (upper slope, lower slope)and basin environments are recognized (Figs 9and 10), based on the depositional features, geom-etries and facies associations. Measurements indi-cate an increase in slope inclination (35–50�)during this platform growth phase, especially inthe early part (Fig. 8), probably in response to morerapid basin subsidence and some progradation ofthe platform margin.

Platform-margin environment

Back-margin facies

The deposits of this environment are only partlyexposed, and only the basal part of the Frasnianis seen. Facies are dominated by fenestral lime-stone (PM7) with minor Amphipora grainstone/wackestone (PM3) (Table 2). The fenestrallimestones are thick to massive bedded, butsmall-scale internal cycles ranging from severalcentimetres to tens of centimetres in thicknessare common and defined by the changes in

fenestral types, such as irregular to laminoid.Such deposits are widely distributed in plat-form-interior settings elsewhere in South Chinafrom the shallowest subtidal to intertidal envi-ronments (Chen et al., 2001b). Amphipora grain-stone/wackestone, similar to that describedearlier, is the common back-margin deposit.Only one subaerial exposure horizon with micro-karstic features such as reddening and vadose(meniscus and pendant) cements has beenrecognized (Fig. 10).

Stromatoporoid buildup facies

Stromatoporoid buildups (PM8) are not exposedbut are inferred to have been present at least in theupper part of S6 in the early Frasnian from theoccurrence of clasts in the slope megabreccias atthe base of S7 (Fig. 11A). The buildups weredominated by stromatoporoid bindstone/baffle-stone and would have been similar to thosedescribed elsewhere (e.g. Chen et al., 2001a).However, branching stromatoporoids and corals,such as Stachyodes and Thamnopora, are abun-dant, implying they were low wave-resistantstructures (Fig. 11B). Moreover, the coeval slopedeposits lack buildup clasts and breccias but aredominated by gravity-flow deposits (mainly calci-turbidites) (Fig. 10). These points suggest that thebuildups were limited in size and did not preventgranular sediments from being shed off the plat-form by binding and baffling. It is quite likely thatthe buildups were only developed on local pal-aeohighs along the platform margin that survivedthe intense platform collapse in the latest Givetian.This buildup episode is probably the youngesthorizon of this type occurring in Frasnian strata.

Sand-shoal and bank facies

Depositional facies of this environment com-monly consist of peloidal–ooidal grainstones/packstones (PM10) and peloidal–oncoidal rud-stones/packstones (PM9) (Table 2); they are themost common facies in the marginal shoal envi-ronment, although the majority of the upperFrasnian strata have been eroded at outcrop.These deposits are generally light coloured andthin to medium bedded. Compositionally, peloidsare abundant, commonly mixed with ooids, onc-oids and other grains (Fig. 10). These depositscharacterize a relatively shallow, moderate- tomoderately high-energy shoal/bank environment,with either peloidal–ooidal or peloidal–oncoidalsands (Fig. 9).

748 D. Chen et al.


Marginal slope environment

Upper slope facies

Autochthonous deposits of this environment aresparse, but gravity-driven contortion of previ-ously deposited, semi-consolidated sediments(slumps, US2) is extensive because of the steepslope. The sediments shed off the platform-mar-gin banks may have bypassed the upper slope andbeen transported farther downslope. The slumpscommonly comprise thin-bedded peloidal mud-stones/wackestones, which locally exhibit boud-inage bedding. Different degrees and scales ofsoft-sediment deformation, such as translationaland rotational slides, are present in the contortedhorizons (Fig. 11C and D), and local break-up ofthe overfolded part into clasts is observed. Thesefeatures are typical of small-scale creeps andglides down the slope.

Lower slope facies

The deposits of this environment are mainlycomposed of breccias/megabreccias (LS1),pebble conglomerates (LS2), calciturbidites (LS3)and muddy calciturbidites (hemipelagic peloidalmudstones/wackestones, LS5) (Fig. 11C; Table 2).

Breccias/megabreccias are usually wedge-shaped with basal erosion, ranging from severalmetres up to 20 m thick and less than 800 macross; they pinch out both platformwards and

basinwards. Four main breccia horizons occur inthe Frasnian strata, and the lower two horizons, atthe bases of S6 and S7, respectively, are relativelythick fore-reef breccias and megabreccias com-posed of clasts of microbial and stromatoporoidboundstones (Figs 5 and 10). These breccias aremost likely related to the erosion and/or collapseof the organic buildups. The rest of the brecciasare mostly slope-derived and, compositionally,are dominated by clasts of peloidal mudstone/wackestone, originally deposited in deep waterthrough suspension fall-out or muddy turbidityflow settling (cf. Chen et al., 2001a), locally withgrainstones of peloids and skeletal grains (e.g. thebase of S8). Similar Devonian examples have beendocumented in western Canada (e.g. Hopkins,1977) and southern Hunan (Jiang, 1989). Thesebreccias/megabreccias are usually interpreted asdebris-flow deposits formed through relativelylarge-scale slope failure and erosion, triggered byvarious mechanisms including tectonic-inducedseismic activity, tsunamis, storms and rapidrelative sea-level falls (cf. Cook & Taylor, 1977;Cook & Mullins, 1983; Spence & Tucker, 1997). Insuch circumstances, semi-consolidated masses onthe slope were forced to move and acceleratedownslope as soon as the shear strength of themasses was exceeded. These slides and slumpswere transformed into debris flows through theincreasing entrainment of water and mud in thecourse of downward movement. The remobilized

Fig. 8. Sketch from photomosaics showing the general depositional succession and geometry changing from amicrobially rimmed platform to sand-shoal platform system from the uppermost Givetian to the Frasnian. The twolower megabreccia/breccia units are composed of microbialites derived from the updip microbial buildups. The upperbreccia unit at the base of S7 is mainly composed of clasts of stromatoporoid buildup facies. See Fig. 10 for detailedstratigraphic logs (conducted along the dashed arrow). This diagram is the westward-extending part of Fig. 4 ap-proximately in the dip direction of the slope. Bush-like patterns mean heavy vegetation. Box at the bottom is 3Æ5 m high.



Table

2.

Dep

osi

tion

al

facie

sof

san

d-s

hoal

pla

tform

syst

em

.

Dep

osi

tion

al

en

vir

on

men

tF

acie

sass

ocia

tion

/fa

cie

s(c

od

e)

Desc

rip

tion

Bio

taIn

terp

reta

tion

Pla

tform

marg

inB

ack

-marg

in

Am

ph

ipora

Gs/

Ws

(PM

3)

See

Table

1S

ee

Table

1S

ee

Table

1

Fen

est

ral

lim

est

on

e(P

M7)

Th

ick-b

ed

ded

(>0Æ5

m),

cre

am

y-w

hit

eor

pin

kin

colo

ur;

com

posi

tion

all

yp

elo

ida

lM

s/P

sw

ith

min

or

ooid

s;fe

nest

ral

cavit

ies

vari

able

from

irre

gu

lar

tola

min

oid

insh

ap

e,

inte

rnal

sed

imen

tsin

cavit

ies

com

mon

Calc

isp

here

scom

mon

;ra

reost

racod

san

dgast

rop

od

s

Sem

i-re

stri

cte

d,

shall

ow

est

subti

dal

toin

tert

idal

en

vir

on

men

ts

Str

om

ato

poro

idbu

ild

up

Str

om

ato

poro

idB

s(P

M8)

Main

lym

ad

eu

pof

Bi/

Bf

of

mass

ive,

sph

eri

cal,

bu

lbou

s(b

ind

ers

)an

dbra

nch

ing

(baffl

ers

)st

rom

ato

poro

ids,

min

or

cora

lsan

dbra

ch

iop

od

s,base

don

the

succeed

ing

bre

ccia

hori

zon

on

the

toe-o

f-sl

op

ean

dback-r

eef

dep

osi

ts(P

M3)

Mass

ive,

sph

eri

cal

an

dbu

lbou

sst

rom

ato

poro

ids

an

dbra

nch

ing

on

es

(i.e

.S

tach

yod

esan

dA

mp

hip

ora

);cora

ls,

bra

ch

iop

od

san

doth

ers

Str

om

ato

poro

idbu

ild

up

sd

evelo

ped

on

pala

eoh

igh

sw

hic

hsu

rviv

ed

the

pla

tform

coll

ap

seS

an

dsh

oal/

ban

kP

elo

idal–

on

coid

al

Rs/

Ps

(PM

9)

Main

lycom

pose

dof

oval

an

dobla

teon

coid

san

dp

elo

ids

wit

hm

inor

ooid

san

dsh

ell

s;on

coid

s,m

mto

10’s

cm

insi

ze,

wit

hcon

cen

tric

an

dcon

ical

lam

inae

an

dn

ucle

us;

sharp

inte

rnal

ero

sion

surf

aces

locall

y

Ort

on

ella

?p

roble

mati

cm

icro

bes;

ost

racod

shell

sM

od

era

te-e

nerg

y,

marg

inal

shoals

an

dban

ks

infl

uen

ced

by

waves

an

dst

orm

s

Pelo

idal–

ooid

al

Gs/

Ps

(PM

10)

Ooid

san

dp

elo

ids

com

mon

,on

coid

ssu

bord

inate

;m

inor

ero

sion

surf

aces

com

mon

,bu

tcu

rren

t-re

late

dst

ructu

res

abse

nt,

fen

est

ral

fabri

cs

locall

y

Rare

Mod

era

tely

hig

h-e

nerg

ysa

nd

ysh

oals

an

dban

ks

Slo

pe

Up

per

slop

eS

lum

p(U

S2)

Con

tort

ed

hori

zon

sof

sem

i-li

thifi

ed

,h

em

ipela

gic

dep

osi

ts,

locall

ygra

din

gin

tobre

ccia

sbasi

nw

ard

s;vari

able

degre

es

an

dsc

ale

sof

defo

rmati

on

stru

ctu

res

Ori

gin

all

yp

lan

kto

nic

fau

na,

i.e.

ten

tacu

liti

ds,

of

pre

vio

us

un

con

soli

date

dm

ass

es

Sm

all

-scale

slid

es/

gli

des

on

up

per

slop

ebefo

resl

op

efa

ilu

re

Low

ersl

op

eB

reccia

/megabre

ccia

(LS

1)

Fore

slop

ebre

ccia

s:st

rom

ato

poro

idB

scla

sts

up

toa

few

min

size

(oli

stoli

ths)

;cla

st-s

up

port

ed

ingen

era

l,p

oorl

yso

rted

wit

hn

oobvio

us

gra

din

g,

an

dw

ed

ge-s

hap

ed

Str

om

ato

poro

ids

as

bu

ild

up

con

stru

cto

r;S

tach

yod

es,

Am

ph

ipora

,cora

ls,

bra

ch

iop

od

san

doth

ers

Bre

ccia

sor

talu

ssp

all

ed

from

the

up

slop

ebu

ild

-u

ps

an

dre

dep

osi

ted

at

toe-o

f-sl

op

e

750 D. Chen et al.


Table

2.

Con

tin

ued

.

Dep

osi

tion

al

en

vir

on

men

tF

acie

sass

ocia

tion

/fa

cie

s(c

od

e)

Desc

rip

tion

Bio

taIn

terp

reta

tion

Slo

pe

bre

ccia

s:cm

-to

m-s

ized

cla

sts;

cla

sts

of

lim

eM

s/W

sd

om

inan

t,m

inor

pelo

idal

Gs/

Ps,

rare

mic

robia

lB

san

dsk

ele

tal

Gs;

dis

org

an

ized

,m

ud

-to

cla

st-s

up

port

ed

,cru

de

gra

din

gof

cla

sts

locall

y

Rare

cora

ls,

bra

ch

iop

od

san

dcri

noid

s;h

em

ipela

gic

top

ela

gic

fau

na

dom

inan

tin

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parts would settle down in places where the slopebecame gentle enough (2–5�), although the possi-bility of coeval deposition on the mid–upperslope also existed.

Pebble conglomerates are commonly present inthe successions below horizons of breccia andmegabreccia and are relatively thin (commonly<3 m). Locally, they grade upwards into calcitur-bidites. They are also interpreted as debris-flowdeposits (e.g. Cook & Taylor, 1977; Cook &Mullins, 1983). Their occurrences suggest anincreasing energy level, probably as a result ofthe acceleration of accommodation loss on theplatform (relative sea-level fall) or slope instabil-ity (Chen et al., 2001a).

Calciturbidites are widely distributed in thelower slope successions, and generally increasein volume and coarsen upwards towards thebreccia/megabreccia horizons. Their composi-tion, dominated by peloids and ooids, suggestsderivation mostly from the platform. Deposits atthe distal toe-of-slope fringing the basin aremainly muddy calciturbidites composed of fine-grained peloidal mudstones/wackestones (chertynodules intercalated locally), and they gradebasinwards into cherty lime mudstones, bandedcherts/siliceous shales and nodular limestones(Fig. 10). They onlap the upper slope facies asrelative sea-level rose rapidly. In the succeedingsea-level fall, these unconsolidated, fine-grainedhemipelagic deposits were readily subjected tosoft-sediment deformation and slumping owing

to the increasing energy level and slope instabil-ity (see above).

Basinal environment

In the lower Frasnian, basinal deposits are mainlymade up of cherty lime mudstones (B3) andbanded cherts/siliceous shales (B4) (Fig. 10;Table 2). Salient features are the extensive occur-rence of the siliceous facies, abundant planktonicfauna and organic matter and weak bioturbation,all characteristics of a deep, oxygen-depleted,sediment-starved environment with poor circula-tion. It is probable that biogenic silica only madea minor contribution to the silica source, and thatmost of the silica was supplied by syndepositional

Fig. 9. Sketch showing deposi-tional environments and faciesacross the platform–slope–basinprofile during the stage of the sand-shoal platform system in the Fras-nian. Abundant sediments wereshed offbank and transporteddownslope as gravity flows, becauseof a lack of any protection at theplatform margin.

Fig. 10. Lithological logs and correlations of platformphase 3 from the platform margin to the basin fromZhongnan via Wulibei to Fuhe. See Fig. 1B for loca-tions. The section of the platform margin at Zhongnanwas mostly eroded and is exaggerated on the right.Thin lines on the right of the logs for Zhongnan markthe high-frequency cycles (parasequences). Refer toTables 1 and 2 for facies codes. Conodonts: 1, Meso-taxis asymmetricus; 2, Pa. punctata; 3, Pa. jamieae; 4,Pa. gigas gigas; 5, Pa. triangularis; 6, Polygnathus sp.; 7,Pa. punctata; 8, Pa. triangularis. LST, lowstanddeposits; TST, transgressive deposits; HST, highstanddeposits. M, mudstone; W, wackestone; P, packstone;G, grainstone; Co, conglomerate; B, breccia/megabrec-cia; Bs, boundstone; Rs, rudstone.

752 D. Chen et al.


hydrothermal fluids rising up through deep-sea-ted fault zones (Zhou, 1990; Wang & Chen, 1996;Chen et al., 2001a). The bathymetric level of thebasin floor could have been deeper than that ofthe earlier phase of platform growth in view ofaccelerated basin subsidence supported by faciesevidence and increased slope declivity (docu-mented earlier), but the magnitude is difficult todetermine.

In the upper Frasnian, basinal deposits aredominated by nodular limestones (B5, Table 2)and are commonly interfingered with calciturbi-

dites close to the toe-of-slope. Locally, brecciasoccur in the basinal succession (see Fuhe logs inFig. 10). In the upper Frasnian strata, siliceousmaterial disappears rapidly along with the plank-tonic fauna and organic matter. In contrast,bioturbation intensifies locally, and clay contentincreases a little. These features suggest that astriking environmental change occurred on thebasin floor, such as the attenuation of volcano-genic hydrothermal activity and an increase inthe oxygen content of the water column, fromincreased water circulation.

Fig. 11. (A) Megabreccia at the base of S7, which overlies platy calciturbidites and hemipelagic carbonates (at thelower right corner). Hammer for scale (37 cm long). Gubi Formation, Wulibei. (B) Details of the megabreccia in (A).The clast is mainly composed of stromatoporoids (Stachyodes common, see arrows), Amphipora, ostracods andbrachiopods. The matrix is mainly lime mudstone and wackestone. Scale bar is 3 cm long. (C) Platy to boudinage-bedded, hemipelagic cherty limestones. Cherty bands with relatively high relief above the weathered surface. Thin-bedded calciturbidites are intercalated (see the lower right and top left corners), which constitute the upper part of anupward-thickening cycle. Slight slump folds occur here. Arrows point to the top of cycles. Hammer for scale (37 cmlong). Gubi Formation, Wulibei. (D) Strongly deformed syndepositional tight slump folds in the hemipelagic lime-stones, which are intercalated with thin-bedded calciturbidites (see the top of the hammer). Hammer for scale (37 cmlong). Gubi Formation, Shihedong (see Fig. 1B for location).

754 D. Chen et al.


DEPOSITIONAL SEQUENCESAND PLATFORM EVOLUTION

Bioconstructed platform system (sequencesS3–S5)

During this late Givetian platform developmentphase (actually phase 2 in the development of theplatform as a whole, see above), microbialiteswere deposited from platform margin to slope tobasin floor, resulting in a distinctive architecture.Three sequences (S3–S5) are identified in thesuccession. In the back-reef/reef-flat deposits,upward-shallowing, metre-scale cycles (com-monly 1–5 m thick) are extensive (see Zhongnanlog in Fig. 5), but the cyclicity is commonlyobscured by the light-coloured appearance andweak stratification. Typically, these cyclesconsist of upward-shallowing units fromfossil-poor lime mudstone (PM6) to skeletalwackestone/packstone (PM4) to stromatoporoidrudstone/floatstone (PM1) to Amphipora grain-stone/wackestone (PM3) and laminites (PM5) atthe top. The cycles reflect aggradation of subtidalfacies up to near sea level. More commonly,however, not all of these facies are present in eachcycle; common cycles are PM1–PM3, PM1–PM5,PM3–PM5, PM6–PM4 (or PM3); minor ones arePM4–PM1–PM3 (or PM5). Some are dominantlysubtidal cycles (Fig. 5). Similar Devonian cycleshave been extensively described by many authors(Read, 1973; Elrick, 1995, 1996; Lamaskin &Elrick, 1997; Chen et al., 2001b). Locally, cyclesof microbial rudstone/grainstone (PM2) or micro-bial boundstone (FR1) to laminites (PM5) arerecognized.

The metre-scale cycles can be grouped intolarger scale cycle sets, and depositional sequences(S3–S5) are identified within the back-reef/reef-flatsuccession based on the vertical stacking patternsof the metre-scale cycles (five cycle sets in S3–S4and four cycle sets in S5). These can be correlatedwell with cycles in the basinal succession at Fuhe(Fig. 5). In general, more subtidal cycles arepresent in the lower parts of the sequences andmore peritidal cycles in the upper parts, wherethey display increasing-upward vadose cementa-tion. Nevertheless, persistent subaerial exposure isonly found at the top of S5 in the form ofmicrokarstic features (i.e. reddening and vadosecementation) (Fig. 5). This subaerial exposuresurface is widely recognizable in platform succes-sions across South China (Chen et al., 2001b).

The stratal patterns from fore-reef to foreslopedisplay an aggradational architecture, with minor

pulses of small-scale progradation, but metre-scale cycles are generally not recognized in themicrobial buildups, especially in the fore-reefzone (Figs 4 and 5). Each episode of microbialupbuilding was interrupted by a rapid facieschange, commonly involving a deeper water facies(e.g. coquinas, platy hemipelagic peloidal wacke-stones/mudstones) or non-deposition (commonon the upper foreslope and bankwards) (Fig. 5).These surfaces are interpreted as the disconform-able boundaries between the constructional epi-sodes of microbial buildup growth, and they arephysically linked updip to the surfaces of exten-sive platform shallowing (± exposure); thus, theycorrespond to depositional sequence boundaries.During the later growth phase of the microbialbuildups, increases in gravity-flow deposits andslope declivity of the lower foreslope deposits,especially in the upper part of S4 and S5 (Figs 5and 10), indicate an enhancement of energy level,slope instability and tectonic activity on deep-seated faults or a relative sea-level fall (cf. Mullinset al., 1986, 1991; Bosellini, 1989; Bosellini et al.,1993).

Small-scale erosive grooves and/or gullies mayhave been formed on the slope, down whicheroded sediments were transported gravitational-ly to the toe-of-slope. During phases of non-deposition, carbonate productivity on the platformand early cementation of the microbial buildupswere depressed. These processes would have ledto the instability of oversteepened clinoforms andgiven rise to local scalloped embayments of theplatform margin or collapse of the platform mar-gin itself (Fig. 1B), as seen in the late stage ofbioconstruction. This scenario would result in theshedding of large olistoliths (or megabreccias) tothe lower slope, as relative sea-level fell, possiblyenhanced by contemporaneous tectonic activity.

The coquinas near the base of the ZhongnanLimestone represent shell banks in front of theforeslope (toe-of-slope), and they are consideredas transgressive deposits. There are two horizonsof buildup-derived breccia/megabreccia wedgeson foreslope (Figs 5 and 8). The lower one isintercalated with ooidal grainstone/packstone(turbidites), particularly in the upper part(Fig. 5), and the breccia beds themselves displayan upward-thickening pattern, indicating a grad-ual increase in energy level, slope instability andoff-platform shedding of sediment. This is reinter-preted here as having been formed throughhighstand shedding (cf. Chen et al., 2001a). Theupper breccia wedge on the other hand is inter-preted as a lowstand deposit in view of its strong



basal erosion surface, rare tapering updip, calci-turbidite intercalations and normal grading in theupper part of the breccia. This wedge was prob-ably a response to a rapid loss of accommodationspace on the platform margin. The undulatorybasal surface of this wedge-shaped breccia unit isthus interpreted as the sequence boundary, and itcan be traced updip to join the subaerial exposuresurface (Fig. 5). The two breccia wedges areoverlain by thin-bedded peloidal mudstone/wackestone (LS5), representing the consequenceof a rapid sea-level rise (transgression).

In the basinal succession, the thin-bedded limemudstones (B2) generally grade upwards intodeep-water microbial laminites (B1), constitutingupward-shallowing cycles (Fig. 5; Chen et al.,2001b), although they are not a real reflection ofchanging water depth. Vertical stacking of thesecycles gives lower order cycle sets, in which thelower components of the cycles thin upwards,whereas the upper parts thicken upwards. Thestacking patterns of these cycle sets provide thebasis for sequence identification and for theircorrelation with the back-reef succession. The topof these cycle sets, which possess the largestamount of microbial laminite (B1), is consideredas the sequence boundary, and this is overlain bya cycle set commencing with a thick unit of thin-bedded lime mudstone (B2) (Fig. 5). Three sequ-ences are identified in the Mintang Formation,and they can be correlated well with the back-reefsequences on the basis of cycle sets.

Sand-shoal/bank system (sequences S6–S9)

After the large-scale platform collapse and funda-mental environmental change at the end of theGivetian, the bioconstructed platform systemcame to an end, and a new sand-shoal systemwas established in the Frasnian (Fig. 8); this wasphase 3 in the history of the platform as a whole.Stromatoporoid buildups and microbial biohermscolonized the platform margin and upper slope inthe early and latest Frasnian respectively. Fourdepositional sequences are recognized in thisplatform phase (Fig. 10), representing four stagesof platform construction.

Only a small part of the platform-margin/back-margin deposits is preserved from this platformphase at outcrop. In S6, the lower part is domin-ated by shoal deposits mainly comprising peloi-dal–oncoidal rudstone/packstone, and the upperpart is dominated by back-margin deposits, mainlyAmphipora packstone/wackestone. Within thissuccession, metre-scale upward-shallowing

cycles [PM9–PM3 (PM5), PM3–PM5, minorPM10–PM5] are extensive and can be furthergrouped into larger scale cycle sets (Fig. 10).Upwards, the sequence is dominated by sand-shoal deposits comprising peloidal–ooidal pack-stone/grainstone. Metre-scale, thickening- andcoarsening-upward cycles, indicating the upward-increasing energy, are widely developed, and theyare also grouped into larger-scale cycle sets.

Four breccia units, including the basal onedocumented earlier, are present in the lowerslope area of the platform. These breccias/megab-reccias are considered here as lowstand deposits,and their basal surfaces are thus the sequenceboundaries in view of their erosive contact withunderlying horizons, rare calciturbidite beds andupward-thinning stratal patterns in response torapid decrease in accommodation space on theplatform (cf. Sarg, 1988; Garcıa-Mondejar &Fernandez-Mendiola, 1993; Spence & Tucker,1997; Gomez-Perez et al., 1999; Chen et al.,2001a). Four sequences are recognized in theFrasnian strata and, ideally, each starts with abreccia horizon with a basal erosion surface in theslope setting and locally grades upwards intocalciturbidites; overlying hemipelagic depositsare interpreted as transgressive deposits (Figs 5and 10).

Within the lower slope facies association,upward-coarsening cycles commencing withhemipelagic peloidal mudstone/wackestonepassing up into calciturbidites and/or pebbleconglomerates are common (Figs 10 and 11C).These cycles may further group into larger-scalecycle sets, in which the turbidite/debris bedscommonly increase in volume and frequencyupwards, reflecting increasing energy and slopeinstability. Thus, the basal cycles usually pos-sess the thickest fine-grained, hemipelagic depos-its and are interpreted as transgressive deposits.The overlying upward-coarsening cycle sets areconsidered as the highstand deposits in view oftheir occurrence and composition, as proposedby some authors (e.g. Schlager et al., 1994).During times of relative sea-level highstand,more sediment is exported off the platform andtransported downslope, forming the turbidityflows or debris flows, as the speed of accommo-dation creation is slower than the rate of car-bonate production.

In the basinal deposits, the sequences aredominated by hemipelagic to pelagic depositswith minor calciturbidites and breccias(Fig. 10). It is noted that the deepest basinalfacies associations in S6 and S7 correspond to

756 D. Chen et al.


the thickest slope facies (including brecciaunits), particularly in the latter, suggesting arapid subsidence of the basin floor at the onsetof their formation. In contrast, an obviousshallowing and/or subaerial exposure trend inthe platform succession was recorded beforeand at the beginning of the formation of thesetwo sequences (Fig. 10). Similar trends duringthis time interval have been reported in otherplatforms elsewhere in south China (Chen et al.,2001a,b).

CONTROLS ON THE ARCHITECTUREAND EVOLUTION OF THEPLATFORM–SLOPE–BASIN SYSTEM

Tectonism

During Devonian times in South China, tecton-ism exerted a fundamental control on thedevelopment and evolution of carbonate plat-forms and interplatform basins (e.g. Wu et al.,1987; Zhong et al., 1992). A sinistral transten-sional tectonic setting was considered to beresponsible for their origin (Shen et al., 1987;Jiang, 1990; Liu et al., 1993; Liu, 1998; Chenet al., 2001a). The sinistral strike-slip faultingwas initially reactivated along the rigid intra-continental antecedent basement fault zones,and this induced further secondary strike-slipfaulting within the blocks. The Yangshuo Basinwas formed in such a secondary synthetic(sinistral) strike-slip fault zone; a spindle- torhomboidal-shaped basin was created throughthe pull-apart process between the strike-slipfaults (cf. Fig, 12; Chen et al., 2001a). Thisconfined basin geometry restricted and dam-pened the current flows and favoured thegrowth of cyanobacterial microbes in the basinalrealm. The Yangdi Platform was tectonicallylocated on the northern side of the Yangshuopull-apart basin, along one of the oblique,extensional faults (Chen et al., 2001a). Thisfault delineated the rectilinear alignment ofthe southern margin of the Yangdi Platform.

From the late Givetian to Frasnian, the YangdiPlatform changed from a microbially rimmedplatform system to a sand-shoal system. Thelong-term evolution of carbonate platform–basinarchitecture is attributed to local and regionaltectonism. The intensification of extensional tec-tonic activity, induced by the strike-slip faultingfrom the late Givetian, accelerated the differentialsubsidence between platform and basin. The

acceleration of basinal subsidence resulted indifferential carbonate production across the plat-form–slope–basin transect; this led to steepeningof the slope and sediment starvation there owingto low carbonate production below the photiczone. This situation would also have allowed thebioconstructors to aggrade upwards rather thanprograde at the platform margin. Cyanobacterialmicrobes (mainly Renalcis and Epiphyton) on themargin-to-slope acted as the framebuildersthrough binding, trapping and baffling sedimentexported from the platform, and prevented thesediment being shed to the basin floor; thus,calciturbidites were rarely deposited on the slopeat this time (Fig. 3A).

The aggradational architecture of the micro-bial buildups reflects a crude balance betweenthe rate of accommodation space increase con-trolled by relative sea-level rise and the rate ofsediment supply controlled by environmentalfactors (e.g. Gomez-Perez et al., 1999), partic-ularly in the early stages of platform growth.With time, however, this balance became vul-nerable as the platform margin thickened, andthe relief between the platform and basinincreased through progressive acceleration ofbasin subsidence and steepening of the slope.During the latest Givetian (top of S5), large-scaleplatform collapse occurred synchronous withsignificant uplift (subaerial exposure) and scal-loping of the platform margin (backstepping ofthe platform margin locally, e.g. at Shihedong)and demise of the microbialites (Fig. 5). Afterthis, the platform evolved into the sand-shoal-dominated platform system as a result of col-lapse and slope erosion; organic buildups madeof stromatoporoids and some other biota colon-ized the palaeohighs surviving the collapse ofthe margin.

A subsequent large-scale platform collapse inthe early Frasnian (top of S6) occurred in relationto the following significant uplift (and/or plat-form shallowing). Huge masses of megabrecciawere released to the toe-of-slope in these cata-strophic events. Simultaneously, the jerky subsi-dence of the basin led to rapid deepening andpoor circulation, encouraging the deposition ofchert in the basin (Fischer & Arthur, 1977) as seenat outcrop (Figs 5 and 10). This type of tectonicdynamics and associated depositional responsebetween the platform and basin generally charac-terizes extensional tectonic settings (e.g. Barr,1987; Leeder & Gawthorpe, 1987; Jackson et al.,1988; Yielding, 1990; Ward, 1999), fitting in well



Fig. 12. Palaeogeography and palaeotectonic setting of the Yangdi Platform and Yangshuo Basin in Devonian times.(A) Palaeogeography of Frasnian time in eastern Guangxi–Hunan, South China (modified from Chen et al., 2001a)with platforms alternating with interplatform basins. Inset box is the area of (C). (B) Interpreted syndepositionalstructural styles of the platform–slope–basin system in (A). A large-scale sinistral transtensional movement (F1, F2)was interpreted as the primary mechanism responsible for the formation of the platform–basin system in (A), whichinduced a series of secondary fault systems (f1–f5; Chen et al., 2001a). (C) Exaggeration of the area of the inset box in(A). The spindle- to rhomb-shaped Yangshuo Basin was developed in the north of Yangshuo, south-eastern Guilin,and abutted the Yangdi Platform to the north. Two dashed circles show the location of subdeeps in the basin.Anatomy of cross-strike transition from platform–slope–basin is carried out for this study (see the shaded bar). (D)Structural style of the Yangshuo Basin and Yangdi Platform, which was formed as a result of the secondary, syntheticsinistral strike-slip faulting (f3 and f4). The northern margin of the Yangshuo Basin was controlled by the extensionalfaulting induced by f3 and f4, and delineated the orientation of the platform margin. Compare (B) and (D) for thestructural interpretation.

758 D. Chen et al.


with the proposed tectonic setting of the YangdiPlatform.

Contemporaneous intensification of volcanicactivity, indicated by the occurrence of intrusivesills in the Givetian platform successions andpillow lavas and tuffs in the Frasnian basinalsuccessions farther to the south in Guangxi (Wuet al., 1987; Zhong et al., 1992), also suggestsenhanced tectonism from the latest Givetian tothe early Frasnian. Under these circumstances, alarge quantity of volcanogenic hydrothermalfluids rich in siliceous and toxic matter wasintroduced into the basins through the deep-seated fault zones, leading to the deposition ofsignificant chert in the basin in the early Fras-nian (e.g. Fig. 10). Furthermore, hydrothermalfluids rising up through deep-seated faults,which commonly delineated the orientation ofplatform margins, may also have affected thestability of margin-to-slope through corrosionand hydrofracturing.

The strong episode of tectonism and associatedenvironmental changes across the Givetian/Fras-nian transition finally led to the decline of thebioconstructed platform system and the onset ofthe sand-shoal/bank system, which persisted intothe latest Frasnian. During this stage, large quan-tities of granular and muddy sediments wereexported off the platform to induce turbidityflows and suspension clouds (Figs 9 and 10).After this, the restoration of microbial buildupsallowed a bioconstructed platform margin todevelop again in the latest Frasnian (see Wulibeilogs in Fig. 10).

Relative sea-level changes

Depositional sequences and their internal organ-ization are produced by fluctuations in relativesea level, with contributions from variations incarbonate production and sediment input, andenvironmental factors (e.g. Sarg, 1988). Seventhird-order sequences have been recognized inthe Devonian strata discussed in this paper (Chenet al., 2001a,b), with three sequences (S3–S5) inthe upper Givetian and four sequences (S6–S9) inthe Frasnian. The two sequence sets represent thebioconstructed platform and the sand-shoal-dom-inated platform systems respectively. Although itseems reasonable that regional tectonism playedan important role in generating the sequences inthe latest Givetian to early Frasnian (see above),the role of relative sea-level change in sequenceformation is still uncertain. Regardless of theirorigin, the relative sea-level change history

deduced from the sequences is not consistentwith the proposed eustatic curve of Johnson et al.(1985). In addition, the pattern of accommodationspace changes recorded on platform successionselsewhere in South China is different from theeustatic curve (cf. Chen et al., 2001b), probablyreflecting a regional tectonic influence. Further-more, recent work on the Devonian stratigraphyaround the world indicates that the pattern ofrelative seal-level change is variable (House &Ziegler, 1997).

Taking into account all the information,regional tectonism probably played a major rolein relative changes in sea-level through theGivetian–Frasnian, driving the development ofthe depositional sequences and probably over-riding the eustatic signals. An intraplate stresstectonic model for vertical crustal movementsproposed by Cloetingh et al. (1985) may explainthe short- to long-term (105)107 years) relativechanges in sea level in the basin and on theplatform, as proposed for other carbonate se-quences developed in active tectonic settings(Gomez-Perez et al., 1999; Rosales, 1999). Theaccumulation of the transtensional stress wouldinduce periods of relative sea-level rise, whereasthe sudden stress release would result in normalfault movement at depth for each tectonic pulse,with uplift of the footwall and downthrow of thehangingwall. This process reached a climax inthe early Frasnian.

Thus, it is considered that the depositionalsequences were formed largely through the effectof regional tectonic activity, which may havebeen superimposed on a long-term eustaticcycle. The higher frequency (metre-scale) depo-sitional cyclicity on the platform is more likelyto have been driven by Milankovitch orbitalforcing, as postulated by numerous authors (e.g.Goodwin & Anderson, 1985; Goldhammer et al.,1990; Balog et al., 1997; Chen et al., 2001b). Thiscyclicity, however, is commonly modulated orinterrupted by lower frequency eustatic andtectonic signals and, in such a tectonically activebasin, jerky subsidence could also be a drivingmechanism.

Environmental factors and oceanographicsetting

Environmental factors, such as current circulation,nutrient levels, oxygen content and oceanographicsetting (windward vs. leeward), are also veryimportant controls on the architecture and evolu-tion of carbonate platforms. On the Yangdi



Platform, the extensive microbialites (microbialbuildups on the margin-to-slope, microbial lami-nites in the basin) in the upper Givetian reflect asupersaturated environment with poor circula-tion and oxygen-deficient and eutrophic bottomwaters. Such an environment would have favouredblooming and early calcification of cyanobacterialcolonies, but would have excluded normal-marinebenthic organisms through the serious environ-mental stress (e.g. Feldmann & McKenzie, 1997;Neuweiler et al., 1999; Riding, 2000).

The occurrence of siliceous facies in thelower Frasnian suggests a more euxinic basinalmilieu and more serious environmental stressstill (Vogt, 1989). The cherts are unlikely tohave been formed in depths of several thousandmetres like their modern analogues in view ofthe geometry of the platform-to-basin profile.The long residence time of oxygen-depletedbottom waters with poor circulation and raresediment influx may have exerted a profoundcontrol on the precipitation of silica (cf. Fischer& Arthur, 1977). The upwelling of anoxic,nutrient- and silica-rich volcanogenic fluidsfrom deep-seated fault zones would have accel-erated the expansion of oxygen-depleted waters(Vogt, 1989), favouring substantial accumulationof silica for chert. Simultaneously, they wouldhave increased the environmental stress, poi-soned the environment and led to the demise ofthe microbialites in the basin realm, in spite oftheir high tolerance to eutrophication. Similarly,such an environmental setting could also havecaused the termination of the stromatoporoidbuildups in the early Frasnian, which werereplaced by the sand-shoal system.

Palaeogeographically, the southern margin ofthe Yangdi Platform was situated in a wind-ward location to the southern margin. Theprevailing winds from the west and south-west(cf. Wu et al., 1987; Zhong et al., 1992) werealso significant to platform construction andwould have provided a continuous energyflux, favouring prolific growth of microbialbuildups and development of carbonate sandshoals on the platform margin. They may alsohave inhibited the progradation of the plat-form, which is usually more pronounced onleeward margins (e.g. McLean & Mountjoy,1993).

Biotic constituents and sediment fabrics

Biotic constituents can influence carbonate plat-form morphology, stratal patterns, slope declivity

and sediment redistribution to the basin. Thecyanobacterial microbes colonized the platformmargin as framebuilders through binding andbaffling sediment being exported from the plat-form interior. In this case, little sediment wasshed off the platform to the basin, leading to theprogressive steepening of the slope and increasein relief between the platform and the basin. Onthe other hand, the growth of cyanobacterialcolonies on the slope would have helped tomaintain the steep slope and protect it fromcollapse through encrusting and early lithifica-tion (Mountjoy & Riding, 1981; Burchette, 1988;Drachert & Dullo, 1989; Kenter, 1990; Purser &Plaziat, 1998; Bahamonde et al., 2000). In the lateGivetian, the aggradational (locally slightly retro-gradational in the late stage of bioconstruction)architecture of the platform margin was thereforegenerated under circumstances in which thevertical growth of the microbial buildups wasapproximately keeping pace with the creation ofnew accommodation space controlled by relativesea-level changes.

Sediment fabrics and grain size mainly affectslope angles (Kenter, 1990; Drzewiecki & Simo,2000). Generally, mud-supported deposits buildslope angles lower than those constructed bygrain-supported sediments. More commonly,fine-grained sediments are unstable on slopesgreater than about 5� (e.g. Kenter, 1990). In theFrasnian strata, most of the slope facies are fine-grained, micrite-rich hemipelagic deposits (e.g.peloidal mudstone/wackestone). Thus, deforma-tion structures from creeps, rotational–transla-tional slides to slumps on all scales, are commonin the slope deposits.

CONCLUSIONS

1 From late Givetian to Frasnian, a carbonateplatform was developed on a palaeohigh locatednorth of the Yangshuo pull-apart basin. Itssouthern margin was delimited by one of theoblique, extensional faults trending NW–SE in asinistral strike-slip fault zone, and this exerted afundamental control on platform developmentand evolution.

2 In the late Givetian, the platform was rimmedwith microbial buildups consisting mainly ofcyanobacterial colonies (Renalcis and Epiphytondominantly), which grew upwards, generating anaggradational to slightly retrogradational archi-tecture with a steep foreslope (up to 45� in dip).This reflected the rough balance between the

760 D. Chen et al.


growth rate of the microbial frame constructorsand the rate of accommodation creation con-trolled by relative changes in sea level. Platform-margin, slope and basinal environments areidentified across the platform–slope–basin pro-file, and each is characterized by distinctivedepositional features and facies associations.Three sequences (S3–S5) are distinguished in themicrobialite-dominated successions of the upperGivetian, representing three stages of biocon-structed platform evolution.

3 From the latest Givetian to early Frasnian,two phases of extensional tectonism caused large-scale collapse of the platform margin and slopeerosion (at the top of both sequences S5 and S6) attimes of significant platform uplift (± subaerialexposure) and basin subsidence. The two col-lapse events brought about the demise of theorganic buildups and created scalloped embay-ments along the platform margin. This led tosubstantial release of megabreccia downslope andthe subsequent sedimentation of siliceousdeposits within the basin in response to blocktilting and the introduction of significant hydro-thermal fluids into the basin. Afterwards, a plat-form system dominated by sand shoals on theplatform margin was established, characterizedby grainstone shoals on the margin and extensivegravity-flow and hemipelagic deposits on theslope. This persisted into the latest Frasnianuntil the restoration of microbial buildups on theplatform margin. In the Frasnian, platform-mar-gin, slope and basin environments, characterizedby different depositional features and faciesassociations, are distinguished across the plat-form–basin transect, and four sequences (S6–S9)are recognized.

4 The onset of the transtensional extensionalmovements from the late Givetian acceleratedthe differential subsidence between the platformand basin. A confined basin displaying a spin-dle- to rhomboidal-shaped configuration wasformed, which gave rise to slope steepening,sediment starvation and poorer circulation ofwaters in the basinal realm. This favoured thecolonization of the periplatform zone by cyano-bacterial microbes, and their existence alsohelped to maintain steep slopes by processes offrame-building, encrusting and early lithifica-tion. The windward position of the platformmargin was also a factor influencing platformconstruction. Nevertheless, the energy of theprevailing winds may have been dampened bythe stagnant waters in the confined basin. Rapidexpansion of oxygen-depleted waters, coupled

with rare sediment influx and abundant supplyof nutrient- and silica-rich hydrothermal fluidsupwelling from deep-seated faults, increased theenvironmental stresses and poisoned the eco-logical conditions. This also caused the demiseof the benthic organisms (i.e. cyanobacterialcolonies, stromatoporoids and corals) in theperiplatform area, and simultaneously promotedthe extensive precipitation of silica in the basin.The less protected platform margin of sandshoals and banks provided more sediment foroff-bank downslope export by turbidity currentsand debris flows. The fine-grained, micrite-richdeposits on the slope were easily subjected tosynsedimentary deformation.

5 Depositional sequences were formed byfluctuations in relative sea level. It is likely thatsynsedimentary tectonic activity made animportant contribution to the changes, although asuperimposed eustatic signal cannot be ruled out.The tectonic activity was related to the exten-sional faulting induced by the strike-slip tectonicactivity.

ACKNOWLEDGEMENTS

This work was supported financially by theNational Natural Science Foundation of Chinathrough grant 49872043 to C.D.Z. Partial supportfrom the Foundation for Returned OverseasScholars, Chinese Academy of Sciences, is grate-fully acknowledged. We are much indebted tosenior geologists Zhengliang Li and Bao’an Yin,who introduced several outcrops, from theRegional Geological Survey and Research ofGuangxi. Thanks also go to Hongjin Lu for hisassistance in the field and identification of con-odonts. Support from the K. C. Wong Fellowshipof the Royal Society of London for the seniorauthor’s stay at Durham University is gratefullyacknowledged, as is a grant from the RoyalSociety for M.E.T.’s fieldwork in China.

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