Waulsortian Type Bioherm Development and Response to Sea Level Fluctuations Upper Visean of Bechar Basin, Western Algeria

8/3/2019 Waulsortian Type Bioherm Development and Response to Sea Level Fluctuations Upper Visean of Bechar Basin, We

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W A U L S O R T I A N - T Y P E B I O H E R M D E V E L O P M E N T A N D R E S P O N S E T O S E A - L E V E L F L U C r U A T I O N S :U P P E R V I S E A N O F B E C H A R B A S I N , W E S T E R N A L G E R I A

H E R R E -A N D P R E B O U R Q U E x , A C H O U R M A D I , ANDB E R N A R D L . M A M E T 2' Dt~partement de G ~ologie , Universi t ( Laval , QuEbec, Qc G IK 7P4, Canada

2 D @ anem ent de G t~ologi e, U nive rs iM de MontMal , Mont rEaL Q c H 3C 3J7 , Canada

ASSTWCr: An except ionally w el l -developed and w el l-preserved upper Vi -scan success ion r ich in sponge-bryozoan-crinoid mounds in the forelandCarboni ferous ]~ch ar Bas in (northwestern Algerian Sahara) i s describedand di scussed. The success ion i s com posed of recurrent facies mosaicsforming indiv idual superposed members . An idea l m ember i s ma de up oftwo di s t inct facies assemblages . Th e lower assemblage form s the actualmounds , and i s com posed of sponge baf l l es tone-wackes toue at the base,overlain by massive sponge-fenestel l id bafOestone-wackestone,and cappedby mass ive crinoid wackes tone wi th bedded f lanks of l i thoclas ti c wacke-s tone. In contras t to m ass iveness o f the mound s , the u pper facies assem-blage i s composed of wel l -bedded crinoid packstone-gra ins tone and oo idgra iastone , wi th loca l rngose cora l and a lga i - foramini fe ra l bank s . On th ebas i s o f benthic assemblages and nature of substrate , seven Imthymetr iezones are def ined, and indicate that each m em ber of the bioherm-richformations i s a shal lowing-upward parasequeuee contro ll ed by an asym-metrical transgressive-regre ssive cycle. T he local curve of relative sea levelfor the la te Visean shows 13 cycles interpretedas fourth-ordereastatic sea-level changes , each cycle averaging hal f a mi l l ion years in d urat ion. Third-order cycles could not be identified.

The platform model proposed is a di s ta l ly s teepened ramp, 15-20 kmwide, that developed in a rapidly subsiding foreland basin. Lateral distri-but ion of facies on the r m p depended s trongly on loca l tectonic set t ing ,whereas vert ical development was contro l l ed by sea- level f luctuation. Thedeep-water, m ud-rich, sponge-bryozoan-crinoidmounds developed in 1 00-150 m water depths during phases o f sea- level highstand, whereas depo-s i tion of shal low-water facies occurred on top of the m ounds , in l es s than70-80 m water depths during regress ive phases .

The l~ch ar mounds share s imi lari t ies wi th the c lass ica l Low er Carbon-i ferous Wauisort ian m ounds , but they di f fer in two aspects : the abu ndanceof large sponges , which i s unique, and their vert ica l zonat lon, wherein basa lfacies cons i s t o f sponge-dominated assemblages .

Th e p roposed model can serve as a too l in developing explorat ion s trat-egies in the search for hydro carbon reservoirs in the I~ char Bas in, becausethe pecul iar architecture of the platform m ay provide suitable plumb ingsys tems for f luid migrat ion, and the depos i t ioanl s ignature of the m oundsand associated facies is possibly recognizable on seismic profi les.

INTRODUCTIONThe C arboni fe rous i s a per iod tha t exper ienced dras t ic dec l ine o f bio-const ruc ted b ui ldup s (Wilson 1975; James and Bourque 1992) , and par -

t icula r ly , the disappearance of the la rge- framed s t rom atopo roid-co ra l r ee f stha t bu i l t r ee f t r ac ts r immin g she lves and pla t form s dur ing the Si lur ian-Dev onian (e .g . , P layford 1 980; Burche tte 1981; Bourque e t a l . 1986; Bo-u r que 1988 ; M oor e 1988 ; S ha ve r a nd S unde rma n 1989 ; C oppe r a nd B r un-ton 1991) . Par t icula r ly , in Lower Carboni fe rous sec t ions , bui ldu ps a remainly r epresented by mud- r ich, low-energy, deep-wate r mounds wi th alow-d iver s i ty f auna inc luding de l ica te organisms such as f enes t ra te bry -oz oa ns, s ponge s, a nd c r i no i ds . T he s e mou nds a r e know n f rom ma ny l o -ca l i t ies in the wor ld and a re of ten re fe r red to as Wa ulsor t ian m oun ds orr e ef s (W i l s on 1975) . T he a bs enc e o f m e t a z oa n c om muni t i e s c a pa b l e o fe rec ting r igid and pe rman ent s t ruc tures in rough-wate r envi ron me nts , andthere fore cont rol ling deve lopm ent of r imm ed pla t forms or she lves , hasJOU~tNALOFSEDIMENTARY E S~ R~, VO L.B 65 , No. 1, FESRUARV, 99 5, P. 80 .-95Copyright 1995,SEPM Society or SedimentaryGeology) 1073-1318/95/0B65-80/$03.00

signi f icant impl ica t ions for the prof i le of ca rbona te pla t forms tha t werema i n l y o f r a mp s t yl e (e .g . , A h r 1989 ; W r i gh t a nd F a u l kne r 1990) du r ingthe Carboni fe rous . D espi te year s of descr ipt io ns and discuss ions , the prob -l e m o f de e p - w a t e r mou nds i s no t f u l ly unde rs t ood . T h i s pa p e r de a l s w i t ha n e xa mpl e w he r e s uc h m ounds a r e e xc e pt iona ll y w e l l de ve l ope d a nd w e l lp r e s e r ve d , a nd w h i c h m a y w e l l c on t r ibu t e t o ou r unde r st a ndi ng o f t hep r ob l e m. O ur pu r pos e i s ( 1 ) t o p r e s e n t a d e t a i l e d de s c r i p t ion o f t he upp e rVisean fac ies of the B~char Basin, and o f the i r spa t ia l and temp ora l dis-t r ibut ion ( f ac ies a rchi tec ture ) , and (2) to inte rpre t the evolut ion of thesuccess ion through t im e in l ight of sequence ana lys is in ord er to dec ipherf a c t o r s t ha t c on t r o l de ve l opme n t o f de e p - w a t e r mou nds a nd t o unde r s t a nds e d i me n t a r y dyna m i c s a nd e vo l u t i on o f t h i s L ow e r C a rbon i fe r ous p l a t -f o r m.Spec tacula r bioherms exposed in the Carboni fe rous B&har Basin(nor thweste rn Alger ian Sahara ) were r ecognized as ea r ly as 1930 by M en-chiko f f (1930) , and were la te r descr ibed by Pareyn (1959, 1961 ) , whorefe r red to them as r ee f s ; hence the so-ca lled "B~char r ee f s" o f the loca loil industry. Recently, SONATRACH,wi th oth er fore ign par tner s, has inten-s i f ied explo ra t ion in the B 6char Basin, wi th pa r t icula r a t tent ion to thehioherms. This pa per i s a cont r ibu t ion to this projec t. De ta i l s on m icro-fac ies , benthic assemblages , and diagenesis and poro si ty evolut ion wi l l begiven in for thcoming papers .

GEOLOGICAL U ITINGT he B / c ha r B a s in i s l oc a t e d be tw e e n t he h i s b l y de f o r me d V a r is c anOrogenic Be l t to the nor th and the a lmost undeformed Saharan Pla t form

to th e sou th (Fig. 1A) . I t i s pa r t ofa n cas t -west - tr ending, e longa ted fur rowtha t extends f rom the eas te rn t ip o f the M oroccan A nt i -At las to Tunis ia ,a l ong t he p r e s e n t - da y A t l a s moun t a i ns ( A l p i ne O r oge ny) ( K a z i - T a n i e t a l .1991) . I t is borde red to the west by the int r ap la te Ougar ta folded be l t , tot he e a s t by t he H a s s i R 'me l H i gh ( F ig . 1B ) , t o t he no r t h b y t he S ou t h -At las Fau l t , and to the south by a se r ies of basem ent highs separat ing theB 6c ha r B a si n a nd t he o i l - a nd ga s -p r oduci ng T i m mi m oun B a s in . M os t o fthe C arboni ferous rocks in this a rea a re in the subsur face, except for ase r ies of rock mass i fs tha t pie rce through the younger Mesozoic and C e-noz o i c c ove r e a s t o f t he M or oc c an A n t i - A t l a s .T he C a mbr i a n t o D e von i a n o f t he l ~c ha r B a s in i s c ompos e d e xc lus i ve l yof s i l ic ic lasf ic rocks , whereas the Carboni fe rous i s ca rbona te -dom ina ted(Fig. 2) . Par t o f the Lower Carboni fe rous i s miss ing in the s tudy a rea , sot ha t t he up pe r V i s e an l i e s unc onf o r ma b l y on t he U p pe r D e vo n i a n f i ne -g r a i ne d s f li c ic l as t ic s . M ound- s ha pe d b i ohe r ms a r e know n f r om t he uppe rV i s ca n a nd t he N a m ur i a n , bu t t he y a r e be t t e r de ve l ope d i n t he f o r me r.The s tudy a rea i s only par t of the l~ch ar B asin outc rops . I t l ie s southeas tof the town o f B~char , on the nor thweste rn margin o f the M eharez High(Fig. 1B) . The upp er Visean bioh erm s and th e i r associa ted fac ies exposedin the mass i f s sur rounded by dunes of the Grand Erg Occ id enta l (Weste rnGrand Sand Dunes; F ig. 3) were s tudied. In addi t ion, success ions in f iveboreholes were considered. There , beds f rom the Djebe l Ioucha to thenor th form a hom ocl ina l sequence dipping gent ly to the nor th, whereast he T a oudr a r a be ds d i p ge n tl y t o t he s ou t h . T w o s e d i me n t a r y be l ts a r er e cogn i ze d : a b i ohe r ma l be l t w i t h s e ve ra l t e ns o f ma s s i ve m ound- s ha pe dbioherms bur ied under wel l -bedded de t r i ta l f ac ies , and a de t f i ta l be l t ,v i r t ua l l y de vo i d o f b i ohe r ms bu t i n s t e a d c ons is ti ng o f de t f i t a l l i me s t one


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WA ULSORTL4N- TYPE BIOHERMS, ALGERM 81

S P A I N 5 ~ F0 500 km ~ F,~ A Ig iec-s

/ ~ ~ ,-,._,.-,--,~r'-ruN,s=. \o C a ~ , . . ~ - " . . . ': ~ ' - / / L . ~ ~ . . _ _ /

F,o. I - Location maps. A) The Brchar Basin outcrop area (dark shaded) inNorlh Africa . B) Subsurface Paleozoic lopography and location of B~char Basin.White areas are outcrops of B&har Basin Carboniferous stra ta . Box indicatesstudy area (Fig. 3) a t northweslern margin o f Meharez High . Oil and gas f ie ldsshown produ ce from Devonian silic ic lastics. (Modif ied from Alley e t a l . 19 71.)

wi th a f e w low - re l ie f c ora l th ic ke ts a nd s i l ic ic la s t ic s . Th e ba s in w e s t o f thec a rbo na te be l t s c ons is t s o f f ine -gra ine d s i lic ic la st ics w i th a f e w de t r i ta ll i m e s t o n e b e d s , a s i n d i c a t e d i n b o r e h o l e s I c 1 a n d M r 1 (F i g . 3 ).

STRATIGRAPHYAND FACIESARCHITECTURET h e s t r a ti g r a p h i c f r a m e w o r k o f t h e C a r b o n i f e r o u s s u c c e s s i o n o f t h e

G r a n d E r g O c c i d en t a l w a s i n i t i a ll y p r o p o s e d b y P a r e y n ( 1 9 61 ) a n d l a t e rr e v i s e d b y L e m o s q u e t a n d P a r e y n ( 1 9 8 2 ) . T h e f o r m e r s h o w e d t h a t t h es u c c e s si o n c o m p r i s e s t h r e e d i s t i n c t p h a s es o f b i o h e r m a l d e v e l o p m e n t ,se pa ra te d by de t f i ta l pha se s. How e ve r , in de ta il s , som e of Pa re y n ' s s t r a t i -g ra ph ic bounda r ie s a r e d i f f ic u l t to r e c ogniz e . We propose a s l igh t ly mod-i f ie d l i th o s t r a t i g ra p h i c f r a m e w o r k i n w h i c h l o w e r a n d u p p e r l i m i t s o f t h et h r e e b i o h e r m - f i c h f o r m a t i o n s ( I o u c h a , K e b i r , a n d O u b e u r ) a r e t r a c e d a tt h e b e g i n n i n g a n d t h e e n d o f b i o h e r m a l p h a se s , r e s p e c t i v e ly ( F ig . 4 ).

B e s i d e s t h e v a r i o u s s e c t io n s w e m e a s u r e d a n d s t u d i e d i n t h e f i el d , fo u rb o r e h o l e s ( N e k - 3 , U t - 2 , I c - 1 , M r - 2 ; F i g . 3 ) w e r e u s e d t o c o n s t r u c t t h es t r a t ig r a p h i e s e c t i o n o f F i g u r e 4. M a x i m u m t o t a l t h i c k n e s s o f t h e u p p e rV i s e a n s u c c e s s io n i n t h e b i o h e r m a l b e l t i s a b o u t 1 5 0 0 m . A l l f o r m a t i o n s ,

QUATERNARY

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8 2 P I E R R E - A N D R E B O U R Q U E , A C H O U R M A D I , A N D B E R N A R D L . M A M E T

~' I ~ ~ o e ' r a r r A LMe ha rez ~.I~'.:,..>~*.~,, , ~ B E L T

C A R B O N A T E R A M P " ~ ~ i t , ~ t~

Ic 1

. . . . ~

GRAND ERG OCCIDENTAL

0I I I

F~;. 3.-Geologic map of the study area showing bioherrnaland d etrflal belts,and terrigenousbasin (modified rom Pareyn 1961). Location of m ap is on Fig-ure I.

except the Kh iam, are div ided into mem bers based on recurrence of faciesassemblages and sedimentary breaks .The Ioucha-Oubeur success ion has been dated by m ean of fomm ini fers(Fig . 4; Mam et et a l . 1994) . Up per Visean Archaediscidae, Endothyridae,Palaeotextulaf iidae, and Tetratax idae assemblage zone s 15 , 16 i , and 16s(Mamet 1974) are ident i f ied, a l though the lowermost beds o f the s tudiedsequence could not be dated wi th preci s ion.

T~,BLi~ .- S u m m ar y of acies characteristics n upper Visean succession of BE-char Basin, Algerian SaharaField Observations: Facies Microfacies

Sponge bafBestone-wackestonMassive dark-colored calcilu tite , locally very richin sponge bodies, forming a distinctive horizon

Massive sponge-fenestellid baJflestone-wackestoneMassive. light gray, finely crystalline limestone,rich in sponge bodies and fenestrate bryozoansheets , with local s tromatactis-llke w hite sparmasses representing centripetal cementation o fcentimeter-size cavities

Mamive crinoid waekestone-packstoneMassive crinoid calcilu tite /fine-grained calcarenite

Bedded itkociaslJwlckestome-paeksloneDecimeter- to m eter-thick bedded calcilu tite and

calcarenite

Coral-microbial frrunestomRugose coral colonies (Lithoslrotion) cncrusted by

the eyanobacteria ~4phralv$ia, nfiltrated andsurrounded by caleilu titeCrinoid packstmc-grtinstone

Well-bedded ealcarenite,~calcirudite n beds 20-813em thick

Algabforaminiferal gmiastoaMassive to bedded bioclastic calcarenite

O o i d I r a i ~ t o ~Well-bedded and cross-laminated, decimeter to

meter-thick bedded calcarenite/calcimditeProductid limestom~

Distinctive dark ~icil u tite , locally very rich inlarge productid brachiopods (G igantoproduc t~ l

Limestone brecciaBreccia made up of centimeter to a few lens ofmeter-size clasts composed o[" sponge andsponge-fenestellid bafllestone-waekestone Faciesembedded in a erinoidal calcarenite matrix likethat of crinoid w ackestone-packstone Facies.

Units are roughly graded, the large meter-sizeclasts at base

Balllestone-wackestone rich in sponge clasts andsponge-spicule& with a few fenestrate bryozoanfiagments

Sponge and fantestrate bryozoan ba llkstone, infiltrat-ed by fine-geamed biodastic w ackestone, contain-ing brachiopods, ostracodes, fomminifers , and, inupper pan of facies, red algae

Crinoid , bryozoan and intraclast wackestone, withaccessory brachiopods, ostracodes, tr ilobites, fom -minifers , and red algae

Wackestone-packstone composed of fenestrate bryo-man, sponge, and crinoid clasts in a sponge-spic-ule-rich lime mud, with accessory brachio!~ds.gastropods, tr ilobites, ostracodes, foraminifers ,and red algae

Coral frame infiltra ted by an oncolite wackestonewith accessory crinoids, hrachiopods, and bryozo-an s

Large crinoid and broken bryozoan packstone-Brain-stone, with accessory intraclasts, miccritized bio-clasts , gaslropods, ostracodes, b ivalves, brachio-pods, mlobites, foraminifers , and red algae

Crinoid , foraminifes, and green alga grainstone andminor packstone, with sam e accessory fauna as incrinoid packstone-grainstone Facies

Ooid grainstone, with common foraminifers andgreen algae

Mudstone with ubiquitous sponge-splcales

F a c i es S p e c t r u m a n d D i s t r i b u ti o nTen distinctive, recurrent imestone facies were recognized on the basis

of f i e ld and petrographic characters . Their m ain attr ibutes are summarizedin Tab le 1. Th ere are also si l iciclastics in the section (Fig. 4), but the y arevolumetrically unimportant. The following section discusses the distri-but ion o f facies with respect to each formation in the success ion.

I o nc ha F o r m a t i o n . - The I o uc ha F o r m a t i on l i e s unc o nfo r m a bl y o n U p-per Dev on ian fine-grained si l iciclastics. A north-south cross section (Fig.5) i l lustrates the spatial distribution of facies. Two distinctive plane sur-faces (Figs . 5 , 6 ) are readi ly observed in c l i f f sect ions in the biohermalbelt, and separa te the thre e memb ers w ithin the for mation: IA, In, and Ic.

Mem ber [A i s composed a lmost exclus ively o f oo id gra ins tone facies ,but overly ing m embers IB and Ic are each comp osed of nine facies (Fig.5). Together, sponge bafflestone-wackestone, massive sponge-fenestel l idbaf l ]estone-wackcs tonc and m ass ive crinoid wackes tone-packstone facies(Table 1 ) form mass ive , m ound-shaped, m ud-rich, sponge-bryozoan-cri -no id bioherms that are loca l ly up to 120 m high. The sponge-bryozoancore of the bioherms accounts for an average of as much as 80% of atypica l bioherm. On ly the uppermost part o f the bioherms has bedded


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WA ULSORTIAN- TYPE BIOHERMS, ALGERIA 83

St a ge Fors m,Z o n e sN A M . 1 7

1 6 s

Z"~ 1 6 iuJ>D2IL l(3_

F o r m a t i o n sM e m b e r s

A O U I D

1 5

O U B E U R

OA

S BS A M E H

KI~BIR

K H I A M

I O U C H A

?U P P E R D E V O N I A N

SA

Biohe rma l Bel t Det r i ta l Be l t, i , t

Ne k 3

g

\\

El (KA

I M r 2

Ic

' 1IA

Ic 1

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1 0 0

Ut 2

Bedded Limestone~ -~ Ool it ic L imes toneF~ :~ Muddy L imes tone~ ] Do lomi te~ SandstoneI Mudstone& Siltstone

$ponge-Bryozoan MoundsCrinoid Banks

F IG . 4 . - C o l u m n a r s e c t i o n s a n d c o r r e l a t io n sb e t w e e n b i o h e rm a l a n d d e t r i t a l b e l t s i n t h es t u d y a r e a ( se e F i g u re 3 fo r l o c a t i o n o f se c -t i o n s l , l c 1 , Mr 2 , U t 2 , a n d N e k 3 a r e b o re -h o l e s .


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84 P I E R R E - A N D R E B O U R Q U E , A C H O U R M A D I , A N D B E R N A R D L . , ~ 4 M E T

B i o h e r m a l B e l t ~ ~,~ D e t r i ta l B e l t - -NORTHERN tt SO UT HE RNNORTH T AO U DR AR A It T A O U D R A R A SOUTHDJEBEL OUCHA

,Northwesternedge o fi E Me mD er CMEH ARE S .,%.:.':-':.":..":" Me mD er B

Member A

, m o , , o oo

.. .. . "%,:'.?.i'/ 9 v"Sv' - [ ~ A lga l- fo ramin ife ra l ra ins tonounconformity Crinoid packstone-grainstone

100 m ~ Rugosecoral-microbial ramestoneI ~ Bedded i thoclasticwackestone-packstoneL -0 Crinoid wackestone-packstone 1pproximate Sponge -fenestellid Mas sivelimestoneverticalscale boundstone-wackestone (mound)Spong e bafflestone-wackestone FIr. L-Fa cies distribution nd architectureof the Ioucha Formation.Plane surfaces e-ferred to in tex t are those that separate hethree membersof the formation. Boxed areaindicates field of Figure 6.

flanks, because the lithoclastic limestone facies is coeval only w ith theuppermost p an o f the sponge-bryozoan core and w ith the entire massivecrinoid wackestone-packstone facies. Well-bedded crinoid packstone-grainstone and ooid grainstone facies onlap the bioherms. The algal-fo-raminiferal grainstone facies was observed only locally, and forms small,massive bodies lying disconformably on bioherm facies. Limestone brec-cias have been observed at tw o localities, in northwestern Djebel Iouchaand at western end o f Meharez es Seghir (Fig. 3).

The two plane surfaces h at separate the memb ers of the formation showthe same geom etry with respect to und erlying and overlying beds. In thecore of Djebel Ioucha (Fig. 3), where mo und s are closely packed together,these surfaces parallel underlying beds and are m arked by ab rupt changesfrom well-bedded calcarenites and calcirudites to massive mound lime-stone. At the western end of Djebel loucha, surfaces also parallel theunder lying beds , but are covered in a dow nlapping man ner by beddedcalcarenites and calcirudites of m oun d flanks (Fig. 6). No erosional featureswere found to be associated with the surfaces.Sponge-bryozoan-cfinoid mounds are present in northern Taoudrara(Figs . 3 , 5) but are absent in southern Taoudram, w here the do mina ntlithologies are m eter-thick c oral-microbial bioherms cove red by crinoidpackstone-grainstone and ooid g rainstone facies.

Khiam Fo rma tion. -T he K hiam Formation is a s i l iciclast ic-dominatedunit. Within the biohermal belt (Borehole Mr-2), it is a homogeneousterrigenous mu ddy sequen ce I 15 m thick. In the detrital belt (El KhiamCuestas area, Fig. 3), the formation is poorly exposed. Nevertheless, red-dish sandstones w ith ball-and-pillow structures, interbedded with greenishmudstones, are ob served at the base of the section (Fig. 4). In BoreboleUt-2, the formation c onsists of alternating bioelastic limestone and fine-grained siliciclastie units.

Kebir For ma tion . - The K ebir Formation, near ly 350 m thick, cons t i-tutes the second biohermal phase. Its base is not exposed (Fig. 7). Fourmem bers (KA to KD) are easily delineated. The three b oundary planes thatseparate these memb ers are as follows. The lower one (the contact between

Mem bers KA and K a) corresponds to the contact between a sandstonemarker bed 2 m thick that overlies ooid limestone and a dark sponge-spicule-rich limestone. This contact w as observed only in two sma ll out-crops, whe re it is conformable. The middle bo und ary plane (the contactbetween M embers IO and Kc) is a beveled surface readily observed at thewestern en d of M eharez el Ke bir (Fig. 8), but actual erosional truncationis not observed. The upper bou nda ry plane (the contact between Mem bersKc and KD) is a w ell-exposed erosional surface that is easily traced overa distance o fa t least 4 km (Fig. 8). Un like the two plane surfaces of theIoucha F ormation, this surface exposes features that suggesterosion duringsubaerial exposure, such as bed truncation (Fig. 9), karst topography, an dfresh-water diagenesis as evidenced by stalactitic cements and stable Cand O isotope data (Madi 1994) . The contact between the K ebir and Samehformations is conformable between crinoid packstone-~ainstone faciesand dark fenestellid-fich calcihitites (Fig. 7).The facies architecture of the Kebir Formation (Fig. 7) is similar to thatof the Ioucha F ormation, but w ith two notable exceptions: (1) The sponge-fenestell id mounds of the three lower members are capped by up to 40 mof coral-microbial fmmestone facies. Rugose coral colonies are comm onlyin g row th position, although in several pla ces they are ove rturned. (2)Upper Member K9 has extens ive development of mass ive cr inoidalwackestones-packstones that form imbricated banks tens o f meters highand a few hundred m eters wide (Fig. 10). The base of this me mber iscomposed of the distinctive dark gigantoproductid-fich limestones thatveneer the erosional surface between Mem bers Kc and K n. Only two low-relief sponge-fenestellid mo und s were observed in this mem ber. In easternpar t o f Meharez el Kebir , at the hinge between the biohermal an d thedetrital belts, a wedge o f greenish fine-grained siliciclastics with a few thinlimestone beds rich in crinoid stems corresponds in time to the darklimestone horizon that covers the surface between Members Kc and(Fig. 7).

Farther east, in the detfital belt, the same fine-grained sfliciclasticsonlapa ba nk com posed of cn no id packstone-grainstone facies (Fig. 7). Except


6/16

WAULSOR TIAN-TYPE BIOHERM S, ALGERIA 85

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7/16

86 PIERRE-ANDRE BOURQUE, ACHOUR MADL AND BERNARD L. MAMET

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- 6 k m ) ~ , , I ( - 3 k m -i i i ! i i i ! i i>Fnc. Z- Fa ci es distr ibution and architectureof the Kebir Formation. Symbolsare as in Figure 5. Box ed area indicates f ield of Figure 8.fo r th is c f ino ida l bank and a few mete r - th ick rugnse cora l -micro b ia l b ioh-erms, the Keb ir Format ion in the de t f i ta l be l t i s composed o f dc t f i ta ll imestones, main ly wel l -bedded cr ino id and oo id g ra instones. A d is t inc t ivehum mock y cross-bedded sandstone res ts on Mem ber KA.Sa m e h Fo r m a t i o n . - T h e Sa m e h F o r m a t i o n i s a m i x e d s il ic i cl a st ic - li m e -stone de t r i ta l un i t . I t i s near ly com ple te ly exposed in the de t f i ta l be l t (Fig .4 ) , and four m embers a re recogn ized (SA o So , upward) . I t i s poor ly knownin the b iohe rmal be l t , where on ly i t s upper qu ar te r (Sc and So) i s exposed ,wi th no s ign if icant change com pared to the de t f i ta l be l t . We canno t as -cer ta in tha t the fo rmat ion is en t i re ly de t f i ta l in the b iohe rmal be l t .

M emb er SA is dom inated by wel l -bedded cr ino id a l packstones-g ra in-s tones , wi th basa l rugnse cora l co lon ies over la in by cr ino id wackestone-packstone banks. M emb er SB is most ly s i l icic last ic , wi th th ick-bedd ed redsandstones wi th ba l l -and -p i l low st ructu res in the lower pa r t and g reen ishm a ss i v e m u d s t o n e s i n t h e u p p e r p a r t . M e m b e r Sc i s c o m p o se d o f a d i s -t inc t ive , ledge- fo rming cher ty l imestone composed o f oo id g ra instonesand rugnse coral horizons. M em ber SD is siliciclastic, and co nsists ofin terbedd ed g reen ish mudstones and b io tu rba ted sandstones, over la in bypara l le l - lam inated and cro ss- laminated sandstones ven eered by an oo idgra instone 30 cm th ick tha t caps the fo rm at ion (Fig . 11) .Large-scale synsedimentary deformation features are well exposed in ac l i f fa t C habet e l O ubeur (Fig . 3 ) . The ledge-fo rming l imestone un i t Sc o fthe Sameh Fo rmat io n , 30 m th ick, shows a 25 m downwarp tha t develope dpr io r to d eposi t ion o f he ove r ly ing s i l ic ic las t ie Mem ber So (Fig . 11) . No r thof th is f lexure and because o f i t , a wedge o f dark l imestone , reach ingth ickness o f 20 m, i s p resen t be tween the s i l ic ic las ties o f Me mbe r SD andthe basa l ledge-fo rming l imestone un i t o f the over lying Oubeur F orma t ion .O u b c u r Fo r m a t i o n . - T h e O u b e u r Fo r m a t i o n i s k n ow n o n l y i n t h e b i o h -ermal b e l t . The fac ies a rch i tec tu re o f the fo rm at ion (Fig . 11) i s s imi lar tot h a t o f t h e K e b i r Fo r m a t i o n . M e m b e r s O A a n d O B a r e s i m i l a r t o M e m b e r sKA, KB, and Kc o f the Keb ir Format ion , wi th la rge sponge-bryozoanh ioherms iden t i f ied a t the base o f Member OB. Green ish , f ine-g ra inedsi l ic ic las t ics l ike those in M em ber Kc o f the K eb ir Form at ion are observed

i n M e m b e r O A . A s i n M e m b e r K o o f t h e K e b i r Fo r m a t i o n , u p p e r M e m b e rOc has ex tensive cf ino ida l banks (Fig . 11) .T h e b o u n d a r y p l a n e s th a t s e p a r a t e t h e t h r e e m e m b e r s o f t h e O u b e u rForm at ion (Fig . 11) a re apparen t ly conform able contac ts . T he top o f theOub eur Form at ion is mark ed by a th in , dark sponge-spicu le- r ich l imestoneover ly ing the ma ssive cr ino id wackestone-packstone banks. The c on tac tbe tween the Oubeur and the over ly ing Aouid ja Format ion is marked byonl ap o f greenish, f ine-grained siliciclastics.

The T ifafines m assifs (Fig. 3) are a series of aligned rock hillocks pro-trudin g through the plain. These hillocks are olistoliths composed of sponge-feneste l l id -cfino id b ioherm al l imestones tha t m ay reach up to a few hun-dreds o f meters in s ize . Bedd ing o f ind iv idual o l i s to l i ths range f rom ho r -izon tal to vertical. In places, large slabs of bed ded limestone are folded.Assoc ia ted sed imentary b recc ias and s lum ps are a lso observed ; over tu rnedgeopetals a re comm on. These o l is to l i ths and assoc ia ted l ime stones l ike lybelong to the sam e o l is tost rom e. The o l is to l i ths a re da ted as be long ing tofo ramin ifera l Zone 16s, therefo re suggesting they are der ived f rom theO u b e u r Fo r m a t i o n .

Facies Architecture in Me Biokermai BeltThree ba sic types o f facies a rch i tec tu re const i tu te m embe rs o f f he b ioh-erm-f ich fo rm at ions (Fig . 12). Type I architecture characterizes MembersIB a n d I c o f t h e I o u c h a Fo r m a t i o n a n d M e m b e r s O A a n d O B o f th e O u b e u rFormat ion , and inc ludes two b road assemblages o f fac ies . The lowestassemblage is com posed o f mas sive l imestones fo rming the ac tua l moun ds(Fig . 13) , inc lud ing the fo l lowing ver t ical succession : sponge ba Ses tone-

wackestone, overlain b y sponge-bryozoan bafllestone-wackestone, and thencr ino id wackestone-packstone . In a dd i t ion to these th ree fac ies , beddedf lanks o f l i thoc last ic wackestone-packstone are la te ra l ly equ iva len t on lyt o t h e u p p e r m o s t p a r t o f t h e m a ss i v e m o u n d . T h i s a s se m b la g e o f f o urfacies therefore constitutes sponge-bryozoan-cfinoid mo unds that deve l-oped bedded f lanks on ly dur ing the i r la te s tage o f deposi t ion . The upper


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W A U L S O R T I A N - T Y P E B I O H E R M S , A L G E R I A 8 7

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88 P I E R R E - A NDR E B O UR QUE , A CHOU R MA DI , A ND B E R NA R D L . MA ME T

FIG. 9.-Be d truncation long he unconformitybetweenMem bers Kc an d K.Dof K ebir Formationon the south face of Meharez el Keb ir. In the foregroundar emassive imestonesofs~ng e-feneste llid afllestone-wackestone nd coral-micro-bial frame stone facies, and well-beddedonlapping rinoid packstone-grainstonefacies, both trun cated and ov erlain by dee ~r-wa ter, dark, sponge-spicule-richproductid limestoneof Mem ber KD of the K ebir Formation, n turn ov erlainbysponge-fenestellid balflestone-wackestone a cies. Perso n for scale, stan ding ontruncation surface, center of photograph.

assemblage of facies is composed of well-bedded crinoid packstone-grain-s tone, and ooid grains tones that on lap the f lanks and tops of the m ounds(Fig. 12). Locally, small lenses (up to 20 m high ) composed of massivecores of algal-foraminiferal grainstones with bedded flanks lie unconform -ably on f lanks of mounds .Type H characterizes the three lowest mem bers of the K ebir Formation,and is a slight variation of Type I. Again, sponge-bryozoan-crinoid mo und sdeveloped bedd ed flanks in their late stage only (Fig. 12). Un like moun dsof Type I architecture, these mou nds arc overlain by rugose-coral-micm-bial framestones, and then, b y well-bedded crinoid packstones-grainstonesand ooid g rainstones as in Type I.Type l I l character izes the upper members o f both the Keb ir and Oube urformations. Although this facies architecture includes fa cies similar toTypes I and I f , i t m ainly sho ws extensive development of cr inoidwackestone-packstone banks with m assive cores and bedded flanks (Figs.

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FIG. 11.- -Faciesdistribution nd architectureof he OubeurFormation; ymbolsas in Figure5.

10, 12). The b anks pile up o n top of, or o n the flanks of, eac h other. Onthe flanks of the pile, wedges of dark sponge-spicule-rich limestone arelocally present between individ ual mo und s (Fig. 12).

F i G . 10.-M assive c rinoid wackestone-packstonehank with bedded flanks nMember Ko of K ebir Formation,north face of Meharez el Kebir. White ine sthe unconformity etween MembersKc and KD. Height ofcliffis 200 m ; camelsfor scale in low er ight.

S E Q U E N C E A N A L Y S I S

SequenceStratigraphyOn the basis of depositional texture and w ater-depth indicators (seebelow), we interpret that the mem bers w ithin the three bioherm-rich Io-ucha, Kebir, and Oubeur F ormations are separated by sedimentary breaks

expressed by strong contrast be tween shallow-water (e.g., ooid limestone)an d d eep-water (mud-rich bafliestone) facies. Each break marks a n abruptincrease in water depth, an d is therefore a marin e flooding surface. Fur-thermore, eac h me mb er is interpreted as a shallowing-upward sequence.In terms of sequence stratigraphy, the members are therefore parasequ-ences separated b y marine flooding surfaces. At least one, and po ssiblytwo, of these surfaces (the contacts between Members Kc and KD andbetween Mem bers KB and Kc, respectively; Figs. 7, 8) are uncon formities(sensu Van Wagoner et al. 1988). The other surfaces should be called"conformities" (semu Van Wago ner et al. 1988), although for some (e.g. ,like the contacts between Members IA and IB and between Members IBan d Ic; F ig. 6), the downlapping o f overlying strata indicates that thesurfaces correspond to at least condensed intervals, if not hiatuses, equiv-alent in time to most mound accretion. Onlapping of terrigenous facieson top of Member Oc of the Ou beur Forma tion also indicates condensed


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W A U L S O R T I A N - T Y P E B I O H E R M S , A L G E R I A 89

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FxG. 13.-- Massive core ofa sponge-bryozoan mound of Member K c of he KebirFormation, with bedded lithoclastie wackestone-packstone f lanks and onlappingwell-bedded crinoid packstone-grainstone. Mound is about 125 m high.

z one , whe re the re i s l i t t l e l igh t , whe re a s g re e n a lga e ( e .g . , l s s ine l la , Kon-inckopora , Pa laeoberese l la ) i n d i c a t e t h e e u p h o t i c z o n e . S p o n g e s a n d f e -n e s t r a t e b r y o z o a n s p r o l i f e r a t e d o n l y b e l o w s t o r m w a v e b a s e , i n m u d - r i c he n v i r o n m e n t s . T h e f i r st e v i d e n c e o f s e d i m e n t r e w o r k i n g b y w a v e s i s r e -

IIF~6. 12 .-T hre e basic types of facies architecture in m embers of upper Viseanbioherrnal-rich formations in the study area . Symb ols are as in Figure 5. AF, a lgal-foraminiferal grainstone: P, sponge-spicule-r ich productid limestone.

d e p o s i t i o n o r a h i a t u s a t t h e O u b e u r - A o u i d j a c o n t a c t . A t t h e c o n t a c t s o fa l l o t h e r m e m b e r s , d e e p - w a t e r m o u n d f a c ie s c o n f o r m a b l y o v e r l i e s h a ll o w -w a t e r f a c i e s w i t h o u t e v i d e n c e o f e r o s i o n o r h i a t u s .

Water-Deptk IndicatorsA r e l i ab l e s e q u e n c e a n a l y s is d e p e n d s u p o n r e l i ab i l i t y o f w a t e r -d e p t h

ind ic a tor s . Be s ide s phys ic a l p rope r t ie s o f the f a c ies , a s de sc r ibe d a bov e ,t h e f a u n a l a n d f lo r al c o n t e n t o f t h e r o c k s a n d t h e r e c u r r e n c e o f d i s t i n c t i v eb i o t i c a s s em b l a g e s a l l o w r e c o g n i t i o n o f s ev e n d e p t h - d e p e n d e n t b e n t h i ca s s e m b l a g e s ( M a d i 1 9 9 4 ). L i t h o l o g i c a n d b i o u c d a t a w e r e u s e d t o g e t h e rt o i n f e r s e v e n b a t h y m e t f i c z o n e s w h o s e d i s t r i b u t i o n a l o n g a p r o f i le i ss h o w n i n F i g u r e 1 4 . A l g ae w e r e u s e d a s i n d i c a t o r s o f l ig h t p e n e t r a t i o n .T h e b a s e o f p h o t i c z o n e i s c o n s i d e r e d t o b e a t t h e f i rs t o c c u r r e n c e o f a lg a e:r e d a lga e ( e .g . , Fascie l la , U ngdarel la , S tacheineae) d e f i n e t h e d y s p b o t i c

M O U N D " " "~ , ~ 7

BATHYMETRIC BENTHICCOMMUNITY S U B S T R A T EZONE

1234567

CALCAREOUS ALGAE,crinoids, green and red algae,foraminifersCRINOIDS-RAMOSEBRYOZOANS, green and red algae,foraminifersLARGE PRODUCTIDBRACHIOPODS,sponges, coralsCORALS-MICROBIALENCRUSTERS,large productids, red algaeCRINOIDS, FENESTRATEBRYOZOANS, red algae,spongesSPONGES, FENESTRATEBRYOZOANS, cdnoidsSPONGES

Shifting sands

Shifting sands

Muddy sand bosom

Rocky bottom (?) &mudsMuddy sands

Muddy bottomMuddy bottom

F~c. 14.- Dis tr ibu tion ofbathymetr ic zones along a ramp profile , deduced frombenthic assemblages and substmte nature . Light-dependent limits are based ondistr ibution of calcareous a lgae. FW B, fa ir-weather wave base; SWB , storm w avebase.


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90 PIERRE-ANDRE BOURQUE, ACHOUR MAD L AND BERNARD L . MAM ETT R A N S G R E S S I O N

-F a i r -w e a th e r W a v e B a s e-B a s e O f Euphoric Z on e . . . . . . . . . - - - m --S to rm W a v e B a s e f V ~ - -

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BREGRESSION L O W S T A N D S E A L E V E L

Fi rst Gree n~?'i AlgaeFiG. 15.--Evolution of a parasequencecon-trolled by a single ransgressive-regressive yclein the biohermal belt, using type 1 parasequ-enee as an exam ple. Mou nd growth and sedi-mentation kept pace with very rapid subsi-

dence.corded in upper pa r t o f Zone 5 (Fig . 14) where fenestra te b ryozoan s werebroken , and in the rngose-cora l -microb ial Zone 4 where cora l co lon ieswere loca l ly overtu rned . The fac t tha t Zone 7 i s dom inated by spongespossib ly ind ica tes low leve ls o f oxygen in th is zone , by analogy w i th som emod ern sponges tha t a re known to to lera te poor ly oxygenated cond i t ions(Byers 1977).

Evolutlon o f a P a r a s e qu e n c eW e in terp re t tha t d eposi t ion o f an idea l sha l lowing-upward parasequ-ence results from a com plete transgressive-regressive cycle. Obviously, the

deep-w ater base o f the moun ds (Zones 7 o r 6 , Fig . 14) , r es ting on sha l low-water, green-alga-rich, algal-foraminiferal banks and ooid sands (Zones 1or 2 ) , tes t i f ies to a marked increase in water dep th . Mater ia l o f a lga l -fo ramin ifera l banks and oo id sands (a lga l - fo rarnin i fera l g ra instone fac iesand o oid grainstone facies, respectively) is autochthunous shallow-watermater ia l , and no t a l loeh thonous, fo r two reasons: (1 ) abund an t unbrokendasycla d a lgae o f the a lga l -fo raminifera l banks, wi th which od id sandsinterfinger, are too fragile to indicate significant transport; and (2) ma ssivecores o f a lga l- fo ramin ifera l banks w i th bed ded f lanks ind ica te in situ a c-cumula t ions. The development o f the en tke sha l lowing sequence subse-quen t to t ransgression can hard ly be ex p la ined by p rogressive f i ll ing o fthe av a i lab le space be tween the sea f loor and a s tand ing sea su r face . Indeed ,the fac t tha t sha l low-water c r ino id sands a nd g ravels (c r ino id packstone-grainstone facies) , ooid sands, and the algal-foraminiferal banks locallyon lap the f lanks and tops o f the deep-water sponge-bryozoan-cr ino idmo unds (Fig . 12) impl ies tha t the sha l low-water fac ies develope d no t on lyabove the de ep .wate r mounds, bu t a lso a t topograph ic leve ls be low the i rtops . Sea- leve l fa l l i s needed to de velop , s ide by s ide , deep-w ater moundfac ies and sha l low-water sand and bank fac ies dur ing the deposi t ion o f asequence.

Consequen tly , evo lu t ion o f an Upp er Visean parasequence sho u ld beanalyzed according to a transgressive-regressive cycle. Inferred develop-ment o f Type I f ac ies a rch i tec tu re (parasequence) i l lus t ra tes th is concep t(Fig . 15) . Transgression resu l ted in o o id sands o f the p reced ing parase-quence being drowned, which resulted in sponges being able to colonizea p ossib ly poor ly oxygenated , sea f loor (Fig . 15A ) . Dur ing ensu ing sea-leve l r i se and u l t imate h ighstand , sponges and b ryozoa ns bu i l t mud- r ichm o u n d s i n l o w - en e rg y e n v i r o n m e n t s b e l o w s t o r m w a v e b a se a n d i n a p h o -t ic zones. The p resence o f red a lgae in up per pa r ts o f sponge- feneste l lid

baf l les tone-wackestone fac ies ind ica tes tha t mou nd su r faces bu i l t in to thedysp ho t ic zone (Fig . 15B) . Al te rna t ive ly , the mou nd su r face was reachedby the dys pho t ic zone d ur ing ear ly reg ression. Lower ing o f sea leve l dur ingthe ear ly reg ressive phase exposed m ound tops to s to rm waves (Fig . 15C),causing ep isod ic rework ing o f sea- f loor sed iments, which in h ib i ted con-st ruc t ion by f rag i le b ryozoans and sponges and a l lowed cr ino ids to dom -i n a t e t h e b i o t i c c o m m u n i t i e s . W e l l - b e d d e d f la n k s d e v e l o p e d a t t h a t t i m e .Dur in g la te r reg ression , c r ino id - r ich sands (c r ino id packstone-grainstonefac ies) f i lled depressions in low areas on and be tween the m ounds. Thesea f loor was then in the euphor ic zone (Fig . 15D) , a llowing developme ntof loca l a lga l- fo ramin ifera l banks in low-energy areas o f th is zone . Oo idsand shee ts f ina lly leve led he sea f loor dur ing the la tes t phase o f reg ressionand the ensu ing phase o f iowstand (Fig . 15D) .

Evo lu t ion o f Type I I parasequences does no t d i f fe r s ign i f ican t ly f romthose o f Type I . The main d i f fe rence l ies in tha t dur ing the regressivephase , ruguse cora l -microb ia l f ac ies developed on the s ides and top s o fsponge-bryozoan moun ds, in the zone af fected by s to rm w aves. Com paredto T ypes I a nd I I parasequenees , the amp l i tude o f t r ansgressive- regressivecycles was weaker in T ype I I I parasequences , as ind ica ted by poo r ly de-vdo ped sponge-bryozoan m ound fac ies , a t the base , and the g rea ter de-v e l o p m e n t o f c r i n o i d b a n k f a ci e s.

PaleobatkymetryBy using the ideas regard ing sequence develop men t d iscussed above , apa leoba thym etf ic cu rve can be const ruc ted fo r the s tud ied succession .Transgressive- regressive cyc les a re o bv ious fo r the th ree b ioherm -f ieh

fo rmat ions ( Ioucha , Keb ir , and Oubeur ) , where they cor respond to fo r -mat io n mem bers , whereas they are no t so obv ious fo r the mix ed carbonate-s i l i c i d a s t i e Sa m e h a n d K h i a m f o r m a t i o n s . We h a v e o u t l i n e d th r e e c y cl e sin the Sameh , on the basis o f the na tu re o f l imestone fac ies on ly , becausethe paleoenvironmental significance of the siliciclastic facies is uncertain.There is no correspondence between these three cycles (parasequences)a n d f o r m a t i o n m e m b e r s i n t h i s u n i t .Coun t ing the th ree cyc les in the Sameh Format ion , which are ques-t ionab le , the s tud ied succession co mpr ises th i r teen t ransgressive-regres-sive cycles (Fig. 16). These cycles are prob ably asymmetrical. Indeed, inmo st o f them, the deepest-water fac ies ab rup t ly over l ie the sha l low-waterfac ies wi thou t a ny in terven ing fac ies o f in te rmedia te com posi t ion . Sucha rap id supe rposi t ion o f fac ies ind ica tes tha t sea leve l had a l ready reached


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WA ULSOR TIAN- TYPE BtOHERMS, ALGERIA 91highstand wh en m oun d accretion began, or at least during earliest phaseof mo und a ccretion, which indicates rapid transgression. On the c ontrary,superposition of facies with intermediate composition expressing a pro-gressive decrease of water depth (e.g. , the vertical succession of massivecrinoid wackestone-packstone, coral-microbial framestone, bedded cri-noid packstone-grainstone and o oid grainstone; Type II facies architecture,Figure 12) su~ ests relatively slow regression.

Providing reliable estimates o f water depth for ancient facies is not easy.Wave s in mo dern ocean s facing open shelves are known to rework sedi-me nts in the depth range 140--180m (e.g., James et al. 1992; Draper 1967).At 20 latitude, wh ich roughly corresponds to that o f the study area durin gVisean t ime (Scotese and McKerrow 1990) , calculated maxim um waterdepth at which the sea bottom was reached by waves, using Ekman'sequation (Pond a nd Pickard 1983), is about 200 m. Taking into accountthat ind ividu al mo und s we described attained thicknesses in excess of 100m before their tops were reworked by waves, a water depth up to about300 m for the sea floor around the sponge-bryozoan moun ds during sea-level highstand w ould seem reasonable. Howe ver, this estimate does no tconsider subsidence, the rate of which probably paralleled the rate ofsedimentation (see below). Moreover, the estimates given above for mod -em settings apply to ocean-facingopen shelves, and are probably excessivefor a foreland basin, which is the c ase for the B&har Basin setting (Karl-Tani et al. 1991).

For better estimates, one can possibly rely on compa rative depth dis-tributio ns of living algae and their presumed ancient counterparts. Max -imu m depths at which mo dem green algae proliferate is 70--80m, althoughsome are found at depths of 1 0 m (James and Ginsburg 1979) . Modernred algae are known down to 250 m, but their proliferation stops belowabout 100 m (Wray 1977). However, uncertain biological relationshipsbetween living and ancient algae comm and caution (Wray 1 977). There-fore, taking into account th e foreland basin setting of the B& har Basin,the high rate o f subsidence, and the distribution of algae, we suggest thatwater depth at which the mo und s were initiated a nd grew was more likelyin the range of 100-150 m rather than 3 00 m. In contrast, most of thedetrital belt (Fig. 3) is made up o f coral-microbial framestone, crinoidpackstone-grainstone, and ooid grainstone facies (Fig. 4), thus oscillatingbetween bathymetfic zon es 4 and 1 (Fig. 14, upper dysphotic to eup horiczones). Using the present-day distribution of abu nda nt green algae, max-imum water depth probably did no t exceed 70-80 m in this belt .

PLATFORMMODELAND EVOLUTIONThe profile of the studied p latform segm ent is a distally steepened ramp

(sensu Read 1982) with a rather limited width on the order of 15-20 k in(Figs. 3, 17). There is a slight break on the ram p itself at th e hinge of thedetfital and bioherm al belts. Indeed, in a distance of less tha n 2 km , facieschange laterally from shallow-w ater coral thickets (70-80 m deep) to deep-water sponge-bryozoan mou nds (100-150 m deep). The thickness of thesuccession in the b iohermal belt is greater than in the detrital belt (Fig.4), implying that the rate of subsidence was higher in the former. Thehinge line between both belts possibly corresponded to synsedimentaryfaults, for which, however, we found no field evidence.The distal edge of the ra mp (Fig. 3) is probably fault-controlled. Sup..porting evidence includes: (1) presence at the edg e of the ram p of imestonebreccia in the loucha F orm ation (Table I; Fig. 5), compo sed of clasts ofsponge an d sponge-ffenestellidbafilestone-wackestone facies in a crinoidwackestone-packstone matrix, implying that breccia deposition occurredafter cementation (or at least partial cementation) o f sponge and sponge-fenestellid baft]estone-wackestone acies but d uring deposition o f crinoidwackestone-packstone. This situation sugg ests that the breccia was syn-sedimentary and fault-related; (2) presence of a synsedimentary flexureand sediment wedges in upper Sameh Formation at the wes tern t ip of

Foram. FormationsZ o n e s M e m b e r s1 7 A O U I D J A

U p p e r1 6 O U B E U R

, ' , " " " " I 2 0 0 m

I l o oO B

. . . . 0

L o w e r1 6

S A M E H

s ES c . ' , 'o, . ' . , : . ,1' l ' i ' , FiSB

SAI . . . . .

K D

K I ~ B I R K c

K B

K H I A M

1 5 , ; , . ~ ,

I O U C H A . , - ;, iIB

? f , * . " , * . , ,, " 7 " , " 7 lI A ' . ' . ' . ~ ', , , ,

I IP P E R D E V O N I A N

- - ~ g >oI l a o o

ffl

! ~ ' , i i

j i, i

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FIG. 16.-Pale obath yme tric u rve for the upp er Visean sequence n th e studyarea. Num berson the c urve refer to bathym etfic zones of Figure 14. Cyclesarewall defined or the three bioherm -rich orm ationsand correspond o formationmembers. They are less well defined n the Sam eh Formation,and do not cor-respond to formationmembers.Chabet el Oub eur (Fig. 11); and (3) presence of the aligned Tifarinesolistoliths west of the ramp edge.Therefore, the lateral distributio n of facies in the study area depend edstrongly on local tectonic setting, whereas the vertical developmen t of theplatform was controlled by sea-level fluctuations. Figure 17 sh ow s the


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9 2 P I E R R E - A N D R E B O U R Q U E , A C H O U R M A D I , A N D B E R N A R D L . M A M E T

NORTHWEST SOUTHEAST- - B I O H E R M A L B E L T -~ = D E TR IT A L B E LT

T r a n s g r e s s i v e S t a g eF i rs t Rugo s e Co r a l -Eu~2 : ne t i C M ic r ob ia l T h i c k e t s . f

Co lon i s a t i onb y S p o n g e s _ _ ~ = , . ,~ : . ~ F ~ " - / _ , f " .Time )

H i g h s t a n d S t a g eR u g o s e C o r a l -E u p h o t i c S p o n g e -B ry .o z oa n / M i c r o b ia l T h i c k e ts ~ "z o n e Mo una s ~ # . . . . .. .. . .

. . . . . . . . . . . " i . ~ ~ ' '' ' : ' . .. . . . . . . . .. . ' .. . . : . - . . .

E a r l y R e g r e s s i v e S t a g eZone / ~ ' - . : ' .......

A l - a I - F e r a m . . . . . W a v e A b r a s i o nL a t e R e g r e s s i v e S t a g e B ~ k S " oo . a ~no a l s Z on e , , ...:. . . . . . ~ . . . : . . . .- ' - C r nnO d~nd O o id ~ - - / ~ ~ " ~ ~ y " ~ " > : ~

. ,. . . : . . H . : . ' . , v

S u b a e n a l E x p o s u r eL o w s t a n d S t a - e W a v e A b r a s i o n Z o n e ~. . , . . . . : . : - . . . : . : ~ . . . . . . . . . - . . . : . . : . . . . . . . : . . . . . . . . . . . . . . . . - . . . . . . . . . . ' . . . -. ~ ......~...::...% '.;::....'...~.. . .........:.. ::...:.... ..

0 o " . . . . . . . . . . . . :

5 km 0I I I I Iapproximatehorizontal caleF iG . 1 7 . - E v o l u t i o n o f d i s t al l y s t e e p e n e d c a r -b o n a t e r a m p i n t h e s t u d y a r e a , s h o w n a t v a r i -o u s s t a g e s o f a c o m p l e t e , a s y m m e t r i c a l t r a n s -

g re ss i v e - r e g re ss i v e cy c l e. V e r t i c a l sc a l e i ss t ro n g l y e x a g g e ra t e d .


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WAULSORTIAN- TYPE BIOHERMS, ALGERIA 93evo lu t ion o f the ramp and the var ious fac ies tha t developed dur ing acom plet e cycle, at the following stages: transgressive, highstand, earl y re-g ressive , la te reg ressive , and lowstand . T h is mo del i s based on a Iou cha-type cycle bu t i s la rge ly app l icab le to the K eb ir and O ubeur fo rmat ions.The amp l i tude o f sea- leve l f luc tua t ions was g rea t enough to cause consid -erab le la te ra l sh i ft ing o f ba thym etf ic zones, so tha t the sha l lowest fac ies(algal-foraminiferal ban ks and associated o oid grainstones) ove rlie thedeep-w ater mou nds d ur ing la te s tage o f reg ressions (Fig . 17) .The bu i ldup o f the sea f loor on the w estern par t o f the ram p in the waveabrasion zone during sea-level Iowstands (Fig. 17) is responsible for thedevelop men t o f h e p lane su rfaces tha t end parasequences in the b ioherm albe l t ( e .g . , Fig . 6 ) . Th is s i tua t ion suggests tha t the p la t fo rm in the de t f i ta lbe l t may ha ve been expo sed to subaer ia l d iagenesis , a s i tua t ion tha t wasrepeated severa l t imes dur ing deposi t ion o f the upper Visean sequenceand m ay have had a n ef fect on hydrocarbon res ervo ir developm ent .

DISCUSSIONEustasy and Tectonics

The re la t ive sea- leve l cu rve o f Figure 16 is the expression o f the in ter -ac t ion be tween ra tes o f eustasy , subsidence , and s ed ime nt supp ly a t ag iven p lace fo r a g iven t ime in terva l . I t does n o t necessar i ly co r respo ndto a eustatic curve. The extent to which the transgressive-regressive cyclesare re la ted to loca l tec ton ics o r eustasy , o r bo th , i s a mat te r tha t needsfurther discussion.In the b ioherm al be l t o f the s tudy area , 15 00 m o f sed iments w asdeposi ted du r ing 6 - .-8 m.y . (Sando 1990) , g iv ing a m in imu m average sed-imen ta t ion ra te o f near ly 0 .2 m/Icy . Th is f igure i s s ix t imes tha t o f anc ien tcarbonate rock successions, bu t one- f i f th o f tha t p roposed fo r ra tes o fHolocene carbonate sed im enta t ion (abou t I m /ky ; Wilson 1975 ; Sch lager1981 ; Sarg 1988 ; Tucker and W righ t 1990) . However , i t inc ludes per iodsof nondeposi t ion and erosion , an d does no t a l low fo r mechan ica l a ndchemica l compact ion , so tha t the ra te i s cer ta in ly h igher than the c i ted0 .2 m /ky and p robab ly much c loser to Holocene f igures . For com par ison ,ma ximu m accumula t ion ra te on Ear ly Carbon iferous carbon ate ramps are0 .05-0 .09 m/k y (Wrigh t and F au lkner 1990) .The Carboniferous Brchar Basin was a subsiding foreland basin (Kazi-Tan i e t a l . 1991) . No do ub t synsed imentary tec ton ism occur red dur ingthe U pper Visean , a t least a t the d is ta l edge o f the s tud ied ramp . How ever ,it is doubtful that rep etitiv e collapses and uplif ts were responsible for thecycl ic i ty observed in the th ree b iob erm-f ich fo rmat ions. Tec ton ism a lonewould have p roduced a m ore er ra t ic and var iab le ver t ica l succession w i th incycles than the recur ren t s imi lar sequence o f fac ies f rom cycle to cyc le .Therefo re , a m ore un iform and constan t mechan ism is needed to p roducesuch a repe t i t ive facies mosaic , and more l ike ly , the ten cyc les recordedby the b ioher rn- f ich fo rmat ion s ( Ioucha , Ke b ir , and Oubeur ) a re re la tedto eustatic sea-level f luctuations, which constitute a more uniform andconsis ten t repe t i t ive mechan ism. As fo r the th ree cyc les o f the mix edcarbonate- s il ic ic las tic Sameh Form at ion , i t i s no t c lear whether they werere lated to eustasy o r tec ton ics . The te r rigenous in f luxes o f bo th un i ts ma ybe re la ted to loca l change in con t inen ta l base leve l , and i t i s possib le tha tthe poor ly def ined Sameh cycles resu lted f rom th is ins tab i l i ty .Ross and Ross (1988) have p ropose d tha t th ree _+ 2 m.y . ( th ird -order )eustatic cycles occurred during the Up per V isean tim e (V~a to V~ c; fora-min ifera l Zones 15 and 16 o f Ma met 's 1974 ) . Our unconformity a t thecon tac t be tween Members Kc and Ko cor responds to the l imi t be tweenZones 15 and 16 i (Figs . 4 , 7 , 8 ). I t i s cer ta in ly the mo st imp or tan t b reakin our sequence , and p robab ly cor re la tes wi th the th i rd -o rder l imi t p ro -posed by Ross and R oss (1988) a t th is zone boundary . However , excep tfo r the con tac t be tween Me mbers K.B and Kc, which possib ly i s e rosional,the o ther su r faces separa t ing th e parasequences a re "c onform it ies" . Noneseems more impor tan t than the o ther , even a t Zones 16 i-16s o r 16s-17

boundar ies , which cons t i tu te the two o ther th i rd -o rd er boundar ies o f Rossand Ross (1988) . Therefo re , i t i s no t possib le wi th our da ta to re l iab lyiden t i fy Ross and Ross 's (1988) upper Visean th i rd -o rde r cyc les . Assuminga dura t ion o f 6 -8 m .y . fo r the U pper Visean , our parasequences averagehalf a mi l l ion years each , which represen ts four th -o rder eusta t ic cyc l ic i ty .Asy mm etry o f cyc les (Fig . 17) i s suggest ive o fg lac io -eu sta t ic con tro l (W il -l iams 1988 ; Mia l l 1990) .Recur rence o f the same fac ies a t the end o f every cyc le impl ies tha t thera te o f subsidence was equal to the ra te o f sed ime nta t ion ( inc lud ing g rowthra te o f mounds). Th is ind icates a very ac t ive keep-up o f the m ound g rowthre la t ive to subsidence dur ing sea- leve l h ighstands. Because o f the h igh ra teof subsidence in the b ioherm al be l t , most fac ies o f eusta t ic cyc les wereexcep tionally wel l developed and p reserved .

C o m p a r i s o nThe mound be l t we descr ibe here canno t escape compar ison wi th thec lass ic W aulsor t ian mo und be l ts typ ica l o f the Lower Carbon iferous (Mis-s iss ipp ian) e .g ., Lees e t a l . , 1985 ; Lees and Mil le r 1985 ; Ahr 1989) . Indeed,

the B rchar Basin mounds were ca l led W auisor t ian by Lemosquet andPareyn (1982) , an ass ignment la te r questioned by Lees (1988) . To d iscussi n d e t a i l w h a t sh o u l d o r sh o u l d n o t b e c a l l ed Wa u l so r t ia n m o u n d s i s n o twi th in the scope o f th is pape r and is le f t fo r a fu ture con tr ibu t ion . However ,a few comments seem appropr ia te . The B~char mounds share severa lfea tu res wi th the c lassica l Waulsor t ian mounds, bu t they apparen t ly d i f fe rf rom them in two respec ts .(1) Their r ichness in large sponges is unique. To our knowledge, thel ~c h a r m o u n d s a r e t h e o n l y o n e s a m o n g t h e Ca r bo n if e ro u s Wa u l so r t i a n -type mounds to con ta in so many p reserved sponges. Al though spongesp icu les a re com mon ly repor ted as s ign i f ican t compone n ts o f some fac iesof Wau lsor t ian moun ds (e .g ., Lees and M il le r 1985 ; Br idges and C hapma n1988), sponges were no t co nsidered imp or tan t bu i lders . However , Bourqueand Gignac (1983 , 1986) and Bourque and Boulva in (1993) argued tha t ,in the case o f Si lu rian and D evon ian deep -water mounds, the absence o fsponge body fossi l s i s no t an abso lu te c r i te r ion on which to deny thesign if icant ro le o f sponges in m ound accre tion .

(2 ) The ver t ica l water -dep th - re la ted zonat ion o f the B rchar moun ds(Fig . 14) i s un l ike tha t p ropo sed by Lees e t a t . (1985) and Lees and M il le r(1985) fo r the m id-Din an t ian W aulsor t ian . The deepest sponge facies o fthe B~char mounds, and a lso o f the Silu r ian -Devonian mud - r ich deep-wate r moun ds (Bourque and Boulva in , 1993) , i s apparen t ly missing inm a n y o r m o s t d a s s i c Wa n l so r t i a n m o u n d s .Lees e t a l . 098 5) p roposed a ram p prof ile fo r the Wau lsor t ian mound-b e a r i n g p l a tf o r m o f t h e m i d - D i n a n t i a n o f Be l gi u m a n d so u th e rn G r e a tBr i ta in , wi th the mounds occupy ing the deepest -water posi t ions on theramp. Th is model a lso ex tends to Europe and Nor th Amer ica (Lees andM il le r 1985) and app l ies to p la t fo rm s tens o f k i lometers wide (up to 110km accord ing to Lees e t a t . 1985 ; the i r f ig . 1 ), on which the m ound be l ti s separa ted f rom th e sha l low-water fac ies o f the ramp by a t least 30 k in .The mode l we p ropose here i s a t a much smal le r scale , wi th a ramp wid thof 10-15 k in . I t app l ies to ra ther smal l p la t fo rms tha t developed wi th ina rap id ly subsid ing basin w here ac t ive fau l ts de l imi ted b locks on to w hichc a r b o n a t e p l a t f o r m s d e v e l o p e d , p ro v i d e d t h a t p r o p e r w a t e r d e p t h s e x i s t e dfo r mou nd accre tion .

R e g i o n a l m p l i c a t i o n sThe m ~ el we propose can serve as a tool in developing explo~tions t ra teg ies in the search fo r po ten t ia l hydrocarb on reservo irs in the B ~charBasin (Fig. 1) , in two ways: (1) the peculiar architecture of this small

carbona te p la t fo rm, together wi th i t s posi t ion on the to ps o f fau lted b locks,p rov id e a s ignatu re tha t shou ld be recognizab le on se ismic p ro f iles ; (2 )recognition o f two superposed facies assemblages fo r each cycle ( i .e., low


15/16

94 PIERRE-ANDRI~ BOURQUE, ACHOUR MADI, AND BERNARD L. MAMETpr imary porosi ty , mud-r ich carbonates a t the base , and h igh pr imaryporosi ty , g ra in - rich ca rbonate a t the top) can serve as a base to m odelp lumb ing systems o f flu id migra t ion dur ing bur ia l, p rov ided tha t p r imaryporosi ty was kep t ope n fo r a whi le ( th is aspect i s curren t ly being inves-tigated).

C O N C L U S I O N SThis p aper descr ibes a nd d iscusses an exccp t ionaUy wel l-developed a nd

wel l -preserved upp er Visean sponge-bryozoan-crinoid moun d-rich suc-cess ion, which m ay wel l contribute to the c larif i cation of the W aulsortianmou nd ques t ion and to the unders tanding of the deep-water mou nd pbe-nom cnon in gcncral , particularly as to the factors that contro l their ini -t ia tion and development . The success ion, in the foreland B6char Bas in,Algeria , is comp osed of recurrent facies mosaics forming indiv idual su-perposed members . Th e m ost s tr iking feature i s the repet i tion ofbioge nicmound-rich phases a t l eas t ten t imes during 6-8 m.y . An idea l memberi s made up of two di s t inct facies assemblages . The lower assemblage,forming the actual mounds , i s composed of sponge baff i estone-wackes toneat the base, overla in by ma ss ive sponge-fenes tell id baf l les tone-wackes tone.There are no f lanks , except in uppermost parts o f the la t ter facies . Themound i s capped by m ass ive crinoid wackes tone wi th bedded f lanks ofl i thodast ic wackes tone. In contras t wi th the m ass iveness o f the m ounds ,the u ppe r assemblage i s wel l -bedded crinoid packstone-gra ins tone andenid gra ins tone, w i th loca l rugose cora l and a iga l- foramini fera l banks .

Each mem ber i s interpreted as a shal lowing-upward parasequence con-tro l led by an asymmetrica l transgress ive-regress ive cycle . The curve o flocal relative sea level for the up per Visean su ccession show s 13 cycles,of which at least 10 are interpreted as the result of fourth-order ustaticsea- level f luctuat ions , each cycle averaging hal fa mi l l ion years . We fa i ledto recognize c learly the three third-order cycles o f Ross and Ross ' s (1988)eus lat ic curve in our fourth-order cycle success ion. H owever, we can cor-relate third-order sea-level low stands that l im it foraminiferal Zon es 15-16 i , 16 i -16s , and 16s -17 wi th our Kc-KD member, Sameh-Oubeur, andOubeur-Aouidja con tacts.

The sponge-bryozoan-crinoid moun ds developed on a di s tal ly steepenedramp 15-20 km wide that evo lved in a rapidly subs iding foreland bas in.Latera l di s tribut ion of facies on the ram p depended s trongly on loca ltectonic setting, whereas vertical development was controlled by sea-levelf luctuat ions . The deep-water mud-rich biogenic mounds developtd in100-1 50 m water depth during phases o f sea- level highstand, whereasshal low-water facies were depos i ted on top of the m ounds in water depthsless than 70 . . -80 m, during regressive phases . The Brchar mou nds shares imi lari t ies wi th the c lass ic Lower C arboni ferous Waulsort ian mounds ,but they di f fer in two respects : ' their r ichness in large sponge bodies , w hichis uniqu e, and their vertical zonation, w herein basal facies consist o f sponge-dominated assemblages .

Th e proposed platform model can serve as a too l in developing explo-rat ion s trateg ies in the search for hydrocarbon reservoirs in th e BrcharBas in, in v iew of the pecul iar archi tecture of the platform, which canprovide a sui table plumbing sys tem for f luid migrat ion and a s ignaturethat is possibly recognizable on seismic p rofi les.

A C I I N O W L E D G M E N T SW e t h a n k A l a i n P r r a t ( U n i v e r s i t 6 L i b r e d e B r u x e l l e s ) f o r c o m m e n t s o n a n e a r l i e r

d r a f t o f th i s p a p e r, a n d t h e t w o j o u r n a l r e v i e w e r s , W . C . D a w s o n { T e x a c o) a n dS , J. M a z z u l l o ( W i c h i t a S t a t e U n i v e r s i t y ) , f or t h o r o u g h r e v i e w a n d c o n s t r u c t i v ec o m m e n t s a n d s u g g e s t i o n s , w h i c h c o n t r i b u t e d g r e a t l y t o w a r d i m p r o v e m e n t o f t h i sm a n u s c r i p t . W e a l s o t h a n k J o u r n a l e d i t o r J o h n B . S o u t h a r d a n d a s s o c i a t e e d i t o rS J . M a z z u l l o f or g re a t i m p r o v e m e n t o f o u r E n g l i s h . P A B i s m u c h i n d e b t e d t o P .D a n s e r e a u f o r a s s is t a n c e in f i gu r e a n d m a n u s c r i p t p r e p a r a t i o n . W e a r e i n d e b t e dt o t h e D r p a r t e m e n t d ' l ~ tu d e s e t S y n t h r s e s o f t h e E x p l o r a t i o n D i v i s i o n o f S o s a t ~ c a ,A l g e r i a , f o r f ie l d - w o r k s u p p o r t. T h e w o r k h a s a l s o b e e n s u p p o r t e d b y i n d i v i d u a l

r e s e a r c h g r a n t s t o P A B a n d B L M f r o m N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c hC o u n c i l o f C a n a d a .

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