Ecological succession, palaeoenvironmental change, and depositional sequences of Barremian–Aptian shallow-water carbonates in northern Oman

Ecological succession, palaeoenvironmental change,and depositional sequences of BarremianAptianshallow-water carbonates in northern Oman

BERNARD PITTET*1, FRANS S. P. VAN BUCHEM*, HEIKO HILLGARTNER*2,PHILIPPE RAZIN , JURGEN GROTSCH3 and HENK DROSTE4

*Institut Francais du Petrole, 92506 Rueil-Malmaison Cedex, France(E-mail: [email protected])Centre EGID, Bordeaux University III, F-33407, Pessac Cedex, FranceAbu Dhabi Company for Onshore Operations (ADCO), PO Box 270, Abu Dhabi, UAEPetroleum Development Oman (PDO), PO Box 81, 113 Muscat, Sultanate of Oman

ABSTRACT

Barremian and Aptian shallow-water carbonate facies (uppermost Lekhwair,

Kharaib and Shuaiba Formations) are described from outcrops in northern

Oman. Based on facies analysis and bedding pattern, three orders of

depositional sequences are defined (third to fifth order) and correlated

between sections. Over the course of three third-order sequences, covering

the Barremian to Lower Aptian, a third-order depositional pattern is

documented that consists of a succession of three distinct faunal

assemblages: discoidal orbitolinids and calcareous algae were deposited

during early transgression; microbialites and microencrusters dominate the

late transgressive to early highstand facies; and a rudist- and miliolid-

dominated facies is typical of the highstand. This ecological succession was

controlled largely by palaeoenvironmental changes, such as trophic level and

clay influx, rather than sedimentological factors controlled by variations

in accommodation space. Orbitolinid beds and carbonates formed by

microbialites and microencrusters seem to be the shallow-water carbonate

response to global changes affecting Late Barremian to Aptian palaeoclimate

and palaeoceanography.

Keywords BarremianAptian, cyclostratigraphy, high-resolution sequencestratigraphy, Oman, orbitolinids, palaeoclimate, rudists, trophic level.

INTRODUCTION

In shallow-water carbonate systems, carbonate-producing organisms are influenced by both thephysico-chemical changes in the sea water (tem-perature, salinity, light, nutrients) and the

dynamics of sea-level changes (e.g. Lees, 1975;Wilson, 1975; Kendall & Schlager, 1981; Neuman& Macintyre, 1985; Schlager, 1991; Strasser, 1991;Tipper, 1997; Dupraz & Strasser, 1999; Pittet et al.,2000). This paper addresses the question of whathappens to the carbonate-producing platformfauna and the stratigraphic architecture duringtimes of strong climatic change. Ecological suc-cessions at the scale of third-order sequenceshave been postulated by Homewood (1996), andevidence for these intrashelf basinal environ-ments was given by Van Buchem et al. (2002a).However, no good documentation of this phe-nomenon exists as yet for shallow-water carbon-ate environments.

Present addresses: 1Universite Claude Bernard Lyon 1,UFR des Sciences de la Terre, La Doua, F-69622, Ville-urbanne Cedex, France.2Free University, De Boelelaan 1105, 1081 HV Amster-dam, The Netherlands.3Shell Gas Abu Dhabi BV, PO Box 46807, Abu Dhabi.4Carbonate Research Center, Sultan Qaboos University,Muscat, Sultanate of Oman.

Sedimentology (2002) 49, 555581

2002 International Association of Sedimentologists 555

The chosen time interval of this study is theBarremianAptian, which is known for its strongclimatic changes (Weissert, 1989; Ruffel & Batten,1990; Erba, 1994; Mutterlose & Bockel, 1998;Norris & Wilson, 1998). Shallow-water carbonatedeposits of this age have been studied in excellentoutcrops in northern Oman. These are character-ized by the presence of the benthic foraminiferaOrbitolina and abundant rudists, which aretypical of platform facies in the Neo-Tethys realmduring the BarremianAptian. The rudist-domin-ated facies is also known as the Urgonian Facies(e.g. in France, Arnaud-Vanneau & Arnaud, 1990;Hunt & Tucker, 1993; in Spain, Garca-Mondejar,1990; Vennin & Aurell, 2001), whereas orbitoli-nid-dominated facies types are commonly namedOrbitolina beds in France and Switzerland (e.g.Arnaud-Vanneau & Arnaud, 1990; Funk et al.,1993).

The present study is concerned with the plat-form deposits outcropping in northern Oman; thisresearch is part of a larger project dealing with theBarremianAptian sediments along a 600 km longtransect in the subsurface and outcrops of theUnited Arab Emirates and northern Oman. Hill-gartner et al. (2002) have documented the passivemargin of the Indian Ocean exposed in the Nakhlarea of northern Oman, and Van Buchem et al.(2002b) have presented the subsurface data inOman and Abu Dhabi for this project.

In this paper, a refined sedimentological modeland sequence stratigraphic interpretation is pro-posed for the BarremianAptian shallow-plat-form sediments of northern Oman. First, aninterpretation of the depositional environmentof the observed facies is presented; second, ahigh-resolution sequence stratigraphic frameworkis established, based on the recognition of threeorders of depositional sequences and their stack-ing pattern; finally, a dynamic sedimentologicalpalaeoecological model is proposed, including adiscussion of specific controlling factors.

GEOLOGICAL SETTING

Lower Cretaceous sediments are well exposed inthe Jebel Akhdar and the Adam Foothills ofnorthern Oman, in the autochthonous portion ofthe Arabian plate (Fig. 1; Glennie et al., 1974).The Lower Cretaceous Lekhwair, Kharaib andShuaiba Formations (HauterivianAptian) consistof shallow-water limestones typically dominatedby benthic foraminifera and rudists (Simmons &Hart, 1987; Simmons, 1994; Witt & Gokdag, 1994;

Vahrenkamp, 1996; Masse et al., 1997; Sharlandet al., 2001). The lithostratigraphic term Hawarmember is here informally introduced for theoutcrop sections to define an interval of decime-tre-scale, bedded, orbitolinid-rich, argillaceouslimestones forming the upper part of the KharaibFormation (Fig. 1c). This unit can be readilyidentified in all studied outcrops. In the sub-surface, a slightly different usage of this term isapplied (Hughes-Clarke, 1988; see also VanBuchem et al., 2002b).

During the Early Aptian, the Bab intrashelfbasin developed, more or less at the present-dayposition of the Arabian Gulf and extending intoAbu Dhabi (Fig. 1a; Murris, 1980; Vahrenkamp,1996). The position of the palaeocoastline on theIndian Ocean side corresponds roughly to thepresent-day coastline (Fig. 1a).

METHODOLOGY

Four outcrop sections were measured in northernOman. Three of them were logged in Jebel Akhdar(Wadi Bani Kharoos, Wadi Nahr and WadiMuaydin) and one at Jebel Madar (Adam Foot-hills; Fig. 1a and b). Palaeogeographically, theJebel Madar section represents slightly moreproximal platform environments, whereas theWadi Bani Kharoos section (Jebel Akhdar) isclose to the platform margin (Nakhl area; Fig.1b). The Wadi Muaydin and the Wadi Nahrsections were situated in an intermediate positionon the platform. The sections were described,including their weathering profiles, bedding andsedimentary surfaces, and facies and microfaciescompositions. Semi-quantitative analyses of 350thin sections were performed to define differentmicrofacies types. The relative abundance of eachcomponent seen in thin section was independ-ently estimated (see Table 1): an abundance indexof 3 means that the component is always presentin the field of view under the microscope (mag-nification is adapted to the grain size of thecomponents); an index of 2 indicates that thecomponent is common (many are found inthe thin section); an index of 1 indicates a rarepresence.

The working method consisted of (1) envi-ronmental interpretation based on bedding pat-tern and facies (fauna and lithology) analysis;(2) definition of different orders of depositionalsequences, their hierarchy and stacking pattern;(3) correlation of the sequences between thesections. The hierarchical sequence stacking

556 B. Pittet et al.

2002 International Association of Sedimentologists, Sedimentology, 49, 555581

pattern is formed by small-, medium- and large-scale depositional sequences: small-scale (deci-metres to metres) sequences are the buildingblocks of medium-scale (metres to tens of metres)sequences which, in turn, compose large-scalesequences (tens to hundreds of metres, e.g. VanWagoner et al., 1990; Strasser et al., 1999).Sequences commonly express changes in relativesea level, changes from open to protected palaeo-environments and/or palaeoecological changesreflected by changes in the biotic composition ofthe sediments. The high-resolution correlationwas based on facies interpretation, sedimentaryevolution, bedding and facies stacking patterns,where the number of small-, medium- and large-scale sequences as well as their hierarchicalrelationships have to be consistent betweenthe sections in order to trace time-lines. This

approach follows the methodology of previousworkers (e.g. Strasser, 1994; Van Buchem et al.,1996; DArgenio et al., 1997; Pasquier & Strasser,1997; Pittet & Strasser, 1998a,b; Strasser &Hillgartner, 1998; Hillgartner, 1999) and allowsthe differentiation between autocyclic andallocyclic processes that control sedimentaryevolution and sequence formation (Strasser,1991).

FACIES CLASSIFICATION, BIOTICASSEMBLAGES AND ENVIRONMENTALINTERPRETATION

The facies classification is shown in Table 1. Eachfacies type is defined by its texture, diagnosticcomposition (relative abundance of characteristic

Ibra

Afar

Adam

Sanaw

Nizwa Izki

MUSCAT

JEBEL MADAR

100 km

Jebel AkhdarJ.

Nakh

lW. BANI

KHAROOSNakhl

W. MUAYDINW. NAHR

Rustaq

Adam Foothills

Gulf of Oman

b

Albian p.p.

Upper Aptian

Lower Aptian

Upper Barremian

Lower Barremian

Hauterivian p.p.

Nahr Umr Fm.

HIATUS

Lower Shuaiba Mb.Exposure

Hawar UpperKharaib Mb.

ShuaibaFormation

KharaibFormation

Lower Kharaib Mb.

Lekhwair Fm.

?

?

?

?

c

SaudiArabia

United ArabEmirates

Yemen

OMAN

Iran

54

18

24

Muscat

AbuDhabi

Bab basin

a

22

58

Fig. 1. (a) Location map of the study area. (b) Location map of the studied outcrops. (c) Stratigraphic frameworkbased on Simmons & Hart (1987), Hughes-Clarke (1988), Simmons (1994), Witt & Gokdag (1994), Vahrenkamp (1996),Masse et al. (1997, 1998), Immenhauser et al. (1999) and Sharland et al. (2001). Note the uncertainty in the bio-stratigraphical dating of the LekhwairKharaib, Upper KharaibHawar and ShuaibaNahr Umr boundaries.

Palaeoenvironments and sequences of the BarremianAptian of Oman 557


Table

1.

Facie

scla

ssifi

cati

on

an

dse

dim

en

tolo

gic

al

inte

rpre

tati

on

of

the

facie

s.



components), complementary (common) micro-facies and sedimentary features (where present).The described facies is then attributed to aspecific depositional environment and integrated(with other facies) in a more general depositionalsetting on the carbonate platform [proximal vs.(more) distal position; open vs. restricted envi-ronments; subtidal vs. intertidal vs. supratidalenvironments; low energy vs. high energy].Figure 2 illustrates some common sedimentarystructures, and Figs 3 and 4 show the mainmicrofacies types encountered in the studiedsections.

The 17 facies (Table 1) are grouped into fourgeneral depositional settings that are character-ized by the following biotic assemblages: (1) anassociation of discoidal orbitolinids, calcareousalgae (and echinoderms) characterizes the low-energy, muddy, platform top; (2) an association ofrudist debris, miliolids and mono- and biserialforaminifera characterizes the high-energy, grainyplatform top; (3) microbialites and microencrus-ters, commonly associated with rudists in lifeposition, characterize the bioconstructed plat-form; and (4) a mixed, diverse biota associationcharacterizes the open lagoon. These four envi-ronments differ with respect to one or severalof the following conditions: bathymetry, hydro-dynamic energy and trophic level.

Assemblage 1: Discoidal orbitolinids,calcareous algae and echinoderms

Discoidal orbitolinids and calcareous algae arethe most common skeletal components of theupper part of the Lower Kharaib and of the Hawarmembers where they can form packstones. Cal-careous algae are Permocalculus, classified asGymnocodiaceae, a group of red algae. Theseresemble green algae and are present in thestudied sections as fragments of various size andpreservation (Fig. 4c and d). Orbitolinids andcalcareous algae are commonly associated withsmall echinoid fragments.

Assemblage 1 is typically found in thin inter-beds of argillaceous limestone (Fig. 2a and b).These sediments are commonly intensively bio-turbated (Thalassinoides burrows are common),suggesting that they were deposited under con-ditions of relatively low accumulation rate. Inorbitolinid wackestone-to-packstone intervals,different degrees of preservation of the orbitoli-nids are observed: (a) completely micritized;(b) partly micritized; and (c) unaltered orbitoli-nids. The same features have been observed for

the calcareous algae. This suggests reworking oforbitolinids and calcareous algae by bioturbationand/or during periods of higher energy conditions(storms, spring tides). The intense reworking thatcharacterizes the orbitolinid-rich sediments alsopoints to relatively low accumulation rates.

Large foraminifera of recent shallow-marineenvironments commonly bear symbiotic algae(Hallock, 1985). Large fossil foraminifera such asorbitolinids are also generally considered ashaving been symbiont-bearing and, thus, light-dependent organisms (Hottinger, 1982, 1996,1997; Banner & Simmons, 1994; Immenhauseret al., 1999; Simmons et al., 2000). These authorsrelated orbitolinid morphology (namely widthheight ratio) to depth of depositional environ-ments. High widthheight ratios (i.e. the mostdiscoidal orbitolinids) were related to deeperenvironments, whereas conical forms (lowwidthheight ratios) were related to shallowerenvironments. Banner & Simmons (1994) esti-mated that orbitolinids lived at depths between10 and 50 m. Hallock (1985) demonstrated thatforaminifera growth to large sizes is only advan-tageous under stable environmental conditionswhere food resources are limited. Thus, largesymbiont-bearing foraminifera are generally typi-cal of oligotrophic marine conditions (Hallock,1985; Hottinger, 1997).

The size and morphology of the Kharaib andShuaiba orbitolinids were analysed in the field.Changes in size and abundance of the orbitolinidsare observed at a decimetre scale in the upper partof the Lower Kharaib and in the Hawar membersof the four studied sections: the highest abun-dance and greatest size of discoidal orbitolinidsoccur in argillaceous limestones, whereas theyare generally smaller in more calcareous sedi-ments, where they are associated with abundantmiliolids and mono- and biserial foraminifera.This suggests a relationship between detritalinflux, orbitolinid abundance and the functionalmorphology of orbitolinid foraminifera. Terrigen-ous run-off could have increased nutrient supplyfavouring fast-growing organisms with asexualreproduction (Birkeland , 1988), as proposedby Vilas et al. (1995) for large foraminifera suchas orbitolinids. This hypothesis could be analternative to the symbiotic model of Hottinger(1982), which implies low nutrient levels(Hallock, 1985).

In proximal areas (Jebel Madar; Fig. 1b), orbit-olinid-rich levels alternate at a centimetre todecimetre scale with intertidal microbial lami-nites (Fig. 2gi), suggesting that these discoidal



orbitolinids were living in shallow subtidal tointertidal environments. Also, in the uppermostLower Kharaib and Hawar members, orbitolinid-

rich levels alternate at a decimetre scale withmiliolid-rich sediments typical of slightly hyper-saline shallow environments. This suggests rapid

a b

d f

g

h

i

c

e

dc

m

m

m

t

t

rr



palaeoenvironmental changes rather than merelya high relative sea-level stand as indicated by thepresence of large discoidal orbitolinids (Hottin-ger, 1982; Simmons et al., 2000). Rapid, high-amplitude sea-level changes also seem unlikely toexplain these alternations.

The association of discoidal orbitolinids withcalcareous algae and echinoderms suggests ratherhigh trophic conditions (mesotrophic?) or, alter-natively, periods of rapid, episodic nutrientinflux accompanying detrital input, which con-trol the development of low-diversity biotic com-munities (such as the one observed). Variations indetrital (and nutrient) input may explain why thelarger discoidal orbitolinids, which were prob-ably symbiont-bearing and thus light-dependentforaminifera (Hottinger, 1982), occur in greaterabundance in clay-rich deposits, because increas-ing clay input would imply increasing waterturbidity. Alternatively, changes in orbitolinidabundance and morphology with detrital influxcould be explained by high nutrient inputfavouring fast-growing organisms (Birkeland ,1988; Vilas et al., 1995). High nutrient inputduring orbitolinid episodes would also imply lowcarbonate production rates (Hallock & Schlager,1986), and thus explain the low accumulationrates that characterize the uppermost LowerKharaib and Hawar members.

The close association of calcareous algae anddiscoidal orbitolinids might suggest an epiphyticmode of life for these foraminifera (Arnaud-

Vanneau, 1976). If this is true, the biotic associ-ation might characterize algal meadows in ashallow-platform top environment. Very intensebioturbation with abundant Thalassinoides, com-mon reworking of orbitolinids and calcareousalgae by currents and/or bioturbation and lowaccumulation rates are features of such environ-ments. Thus, Assemblage 1 is interpreted as beingcharacteristic of intertidal to shallow-subtidalenvironments influenced by detrital and nutrientinput.

Assemblage 2: Microbialites, microencrustersand rudists in life position

Microbialites are microscopic encrusting struc-tures formed by microbial organisms such ascyanobacteria and other bacteria, fungi and greenalgae filaments. Microbialites form very denseand indurated rocks, generally lighter colouredthan the surrounding allomicrite, commonly ex-hibiting a milky, vitreous shine caused by arelatively high original porosity later cementedby sparite. They may involve different symbioticorganisms, building together complex and micro-scopic features variable in size and form. Theygenerally occur in association with rudists, coralsand/or stromatoporoids. They can also form low-relief mudmounds and are associated with othermicroencrusters: foraminifera, bryozoans (Beren-icea), red algae, encrusting sponges, serpulids,Bacinella and Lithocodium.

In the lower part of the Lower Shuaiba member,microbialites and microencrusters commonlybuild low-relief biostromes or form large oncoids(commonly 36 cm in diameter). In some inter-vals, and mainly in the middleupper part of theLower Shuaiba member, they are associated withrudists (Fig. 2d) to form build-ups. In such cases,a BacinellaLithocodium association may domin-ate the microbial assemblage (Fig. 4a). In domi-nantly microbial intervals, leiolitic crusts are veryabundant (Fig. 4b), and thrombolitic crusts arepresent, whereas rudist contents are variable butgenerally low.

Some microbial and microencrusting associa-tions can be used to constrain the environmentalsetting. Although they generally favour meso-trophic conditions related to siliciclastic input(Leinfelder et al., 1993), they do not seem to likedirect clay input (Schmid, 1996). Abundantmicrobial/microencrusting activity thus com-monly relates to high, but not excessive, nutrientconditions (Leinfelder, 1992). However, differentmicrobial and microencrusting associations can

Fig. 2. Sedimentary structures and macrofacies. Scalebars 5 cm. (a) Firmground: Thalassinoides burrowsinfilled by clayey, bioturbated orbitolinid facies, top ofthe Lower Kharaib, Wadi Muaydin. (b) Typical argil-laceous, bioturbated orbitolinid (black particles) lime-stone facies of the Hawar member, Wadi Muaydin. (c)Depositional sequences at the base of the Lower Kha-raib: a marly, partly dolomitized interval at the base ofthe sequences is overlaid by a pure carbonate interval,capped by a firmground (level of hammerhead), WadiMuaydin. (d) Rudist (caprinid)microbialite bound-stone in the Lower Shuaiba member, Wadi Muaydin.(e) Rudist floatstone showing low-angle cross-beddingin the upper part of the Lower Kharaib, Wadi Muaydin.(f) Channel infill by a rudist (caprinid) floatstone,Lower Shuaiba member, Wadi Muaydin. (g) Intertidalfacies in Jebel Madar, Hawar member: alternation oflaminites (m, microbial mats) showing desiccationcracks (dc) and high-energy deposits (t, tempestites)containing microbial mats rip-up clasts (r). (h) Alter-nations of desiccated microbial mats (m) and tempest-itic layers (t) in the Hawar member of Jebel Madar. (i)Desiccation polygons in the Hawar member of JebelMadar.



be distinguished that have different palaeoeco-logical requirements (Leinfelder et al., 1996;Schmid, 1996; Dupraz & Strasser, 1999). ABacinellaLithocodium association correspondsto well-oxygenated, very shallow-water environ-ments and normal marine salinity; leiolitic andthrombolitic crusts may point to higher nutrientlevels (Dupraz & Strasser, 1999). Consequently,the observed association of rudists with dominantBacinellaLithocodium microencrusters is inter-preted to represent very shallow, well-oxygenated

and relatively oligotrophic environments,whereas intervals built by leiolite and thrombo-lite with or without associated rudists reflecthigher trophic level (mesotrophic?) and possiblyslightly deeper environments.

Assemblage 3: Rudist debris, miliolids,and mono- and biserial foraminifera

Rudist debris is much more abundant thanrudists in life position in the Kharaib and

a b

c ed

Fig. 3. Microfacies of the uppermost Lekhwair, Kharaib and Shuaiba Formations. Scale bars 05 mm. (a) Facies 1:miliolid grainstone showing, in the upper third of the picture, a thin micropeloidal layer of probable microbial origin(arrow); such layers may be reworked as irregular lithoclasts, such as those shown in the lower third of this picture,Lower Shuaiba member, Wadi Muaydin. (b) Facies 1: in the upper part of the photograph, a grainstone lithoclast hasbeen microbially cemented before being reworking into a miliolid grainstone containing oncoids, Lower Shuaibamember, Wadi Muaydin. (c) Facies 2: peloidal grainstone; peloids are either pellets or, more commonly, micritizedbioclasts or microfossils (e.g. miliolids), Lower Shuaiba member, Wadi Muaydin. (d) Facies 11: Rudistmicrobialiteboundstone; the matrix typically shows a micropeloidal structure and contains some bioclasts and microfossils;details of the matrix are shown in (e). (e) Miliolids and calcareous algae fragments are trapped within a micropeloidalstructure, Lower Shuaiba member, Jebel Madar.



Shuaiba formations (Fig. 2e and f). Rudist-richstratigraphic intervals are present in the fourstudied sections at the base of the LowerKharaib and in the middle part of the Upper

Kharaib members as well as in the uppermostLower Shuaiba member. These intervals aredominantly formed by high-energy depositssuggesting large platform areas influenced by

c d

f

b

e

a

p

p

tr

p

p

Fig. 4. Microfacies of the uppermost Lekhwair, Kharaib and Shuaiba Formations. Scale bars 05 mm. (a) Facies 9:BacinellaLithocodium boundstone, Lower Shuaiba member, Wadi Muaydin. (b) Transition between facies 8 andfacies 9: bound oncoids tending to form a microbialite boundstone; between these bound oncoids, micropeloidalmaterial has trapped some bioclasts or microfossils (tr); note the residual porosity (p) in the boundstone resultingfrom incomplete growth of microbial encrustation, and downward growth of microbialite in the pore on the left of thepicture (arrow). (c) Facies 14: Orbitolinid wackestone; other bioclasts are dominantly calcareous algae fragmentsalthough some echinoid fragments occur; note the twin orbitolinid in the centre of the picture, Hawar member,Wadi Nahr. (d) Facies 14: alternation of orbitolinid layers and bioclastic layers containing calcareous algae, echinoidand rudist fragments, Hawar member, Wadi Bani Kharoos. (e) Facies 17: conical orbitolinids in Thalassinoidesburrows, Upper Kharaib member, Wadi Muaydin; note that the sediment in the burrow is strongly dolomitized.(f) Facies 17: dominantly conical orbitolinids in a Thalassinoides burrow; peloids, miliolids and agglutinatedforaminifera also occur, Upper Kharaib member, Wadi Nahr.



waves or tides. Reworked rudist material pro-duced floatstones and rudstones and forms thedominant sediment fraction in beach, tidal barand channel deposits. This suggests that rudistsinhabited wave- or tide-agitated areas (e.g. inter-bar depressions) and explains why rudists inlife position are only seen in thin levels.

Rudists are suspension feeders (Scott, 1995). Inhigh-energy conditions, food may be transportedin suspension and dispersed over large platformareas, thus favouring rudist development. Suchhigh-energy conditions correspond to well-oxy-genated environments and, generally, to efficientrecycling of organic matter to nutrients. A com-mon association of rudists with miliolids (andalso diversified mono- and biserial foraminifera)is observed only in purely calcareous intervalsand implies shallow-water environments, poss-ibly with a raised sea-water salinity. This suggestslow-nutrient input, and evaporation probablyslightly exceeding recharge by rainfall, rivers oropen ocean waters. Consequently, high-energyrudist facies probably corresponded to periods ofoligotrophy during which organic matter pro-duced on the platform was efficiently recycledand redistributed.

Assemblage 4: Mixed, diverse biota

In the lower part of the Upper Kharaib memberor, locally, in the uppermost Lekhwair Forma-tion and Lower Shuaiba members, the sedi-ments are characterized by mixed, diverse biotaincluding various foraminifera, echinoderms,bivalves (including some rudists) and gastro-pods. Miliolids are rare; mono- and biserialforaminifera are common; orbitolinids (mainlyconical forms) are common; and Choffatellaplus some lenticulinids are present. The sedi-ments are mostly mudstones and wackestones,suggesting low-energy conditions. Locally, dec-imetre-scale rudist floatstones and rudstonestruncate the low-energy deposits. These levelsrepresent possible tempestites. Bedding is rarein these deposits, and the facies are quitehomogeneous. Local bioturbated levels occur,within which the burrow systems (Thalassino-ides) are infilled by dolomitized sediment (Fig.4e).

The diverse biota, the scarcity of miliolids andthe succession of homogeneous facies point tonormal, stable marine conditions. The few sedi-mentary structures that have been observed indi-cate storm influence, and the texture of thesediments (mainly mudstones and wackestones

truncated by coarse-grained sediments) suggestsrelatively deep, lagoonal environments abovestorm wave base.

SEQUENCE STRATIGRAPHICINTERPRETATION

The Wadi Muaydin section (Figs 5 and 6) isrepresentative of the sedimentary evolution of theuppermost Lekhwair, Kharaib and Shuaiba For-mations recognizable in the four sections inves-tigated. This section is interpreted in terms of itsdepositional sequences. The sequence analysis ofthe section serves as an example of the analyticalprocedure followed for all the other outcropsections.

The Wadi Muaydin section shows a systematicorganization of the bedding pattern with analternation of fine, decimetre- to metre-scalebedded intervals and thicker, massive intervals(Fig. 6). Thin bedding occurs in the upper parts ofthe Lekhwair and Lower Kharaib members andin the Hawar member. This feature has beenobserved in all the studied outcrop sections.

Large-scale depositional sequences

The vertical facies evolution in the uppermostLekhwair, Kharaib and Shuaiba Formations ofthe Wadi Muaydin section shows a systematicorganization (Fig. 6), with a typical faciestrend from: (a) orbitolinid and/or miliolidwackestonepackstone, low-energy platform topenvironment; to (b) a mixed-fauna mudstonewackestone open lagoon environment; followedby (c) a rudist/miliolid wackestonepackstonegrainstoneframestone high-energy, shallowplatform top environment. The environmentalinterpretation of this trend shows an evolutionfrom a very shallow/mesotrophic to deeper/oligo-mesotrophic to again very shallow butnow more oligotrophic conditions. In terms ofrelative changes in sea level, these environmentsmight correspond to early transgressive, latetransgressive/early highstand and late highstanddeposits respectively. This general pattern isrepeated three times and corresponds to theuppermost Lekhwair Formationlower part ofthe Lower Kharaib member, the upper part ofthe Lower Kharaiblower part of the UpperKharaib member and the HawarLower Shuaibamembers (Fig. 6).

These large-scale (tens to 100 m) sequences arebounded by distinct exposure surfaces, which



mark a dramatic change in facies in all threecases. The rudistmiliolid facies types below andthe orbitolinidcalcareous algae above thesesurfaces are both interpreted as having beendeposited in shallow-water environments. Thus,the drastic facies changes at the boundarybetween the lithological units do not correspondto important variations in bathymetry, butmainly to changes in palaeoenvironmental con-ditions (trophic level). However, evidence ofexposure at the base of the large-scale sequencesindicates that these surfaces also correspond torelative sea-level falls and can be interpreted assequence boundaries. At the base of Sequence I(upper part of the Lekhwair Formation) in JebelMadar and Wadi Muaydin, mudcracks werefound. Truncated coral heads in Wadi BaniKharoos provide evidence for subaerial exposureat the base of Sequence II (middle part of theLower Kharaib). At the base of Sequence III(Hawar member), a major exposure eventoccurred, testified by the presence of root traceson the platform top (observed in core material inthe United Arab Emirates; Van Buchem et al.,2002b), and lowstand deposits along the oceanmargin (Hillgartner et al., 2002). In the topmostLower Shuaiba member, reworked microbialmats in centimetre-scale tempestites in WadiMuaydin and miliolid-rich beach sediments inWadi Bani Kharoos, Wadi Nahr and at Jebel

Madar indicate that the top of the Lower Shuaibamember also shows a shallowing-upward trend.The exposure surface is covered in all sectionsby an iron-crusted hardground, which corres-ponds to a hiatus in the order of several millionsof years (Harris et al., 1984; Immenhauser et al.,1999). This exposure surface can be correlatedover the entire Arabian plate and corresponds toa regionally registered drop in relative sea level(Sharland et al., 2001).

Based on the above criteria, three large-scaledepositional sequences are defined in the WadiMuaydin section (Fig. 6): Sequence I, whichcorresponds to the uppermost Lekhwair Forma-tionlower part of the Lower Kharaib member;Sequence II, which corresponds to the upperpart of the Lower Kharaib and lower part of theUpper Kharaib; and Sequence III, which corres-ponds to the Hawar and the Lower Shuaibamembers. Although these three sequences showstriking similarities in their bedding pattern andfacies evolution, there are also a number ofsignificant differences: (1) they vary dramatic-ally in thickness; Sequence III is twice as thickas Sequence II, which in turn is twice as thickas Sequence I (Fig. 6); (2) the increase inthickness from Sequence I to Sequence II occursnotably in the deeper water, open lagoon de-posits. The rudistmiliolid unit in the upperpart of Sequence II is comparable in thickness to

TEXTURE

m: marlsM: MudstoneW: WackestoneP: PackstoneG: GrainstoneF: FloatstoneR: RudstoneB: Boundstone

Desiccation cracks

Bioturbation(dominantly Thalassinoides)

Hardground

Cross-bedding

Totally dolomitized interval(in burrows - Thalassinoides trace fossils)

SEDIMENTARY STRUCTURES

Peloids

Oncoids

Microbialite/micro-encrusterfragments

Microbialites/micro-encrusters

Rudist fragments

Rudists (in life position, or onlyslightly transported)

Corals and stromatoporoids

Coral/stromatoporoid fragments

Miliolids

Orbitolinids

Calcareous algae (mainly Permocalculus)

Gastropods

Echinoderms(mainly echinoids)

Bivalves

Other foraminifera(mainly mono- and biseriate)

CALCAREOUS GRAINS

Fig. 5. Legend to Figs 6, 10 and 12.



mMW GP BF R

90

95

5

15

10

20

25

30

35

40

45

50

55

60

65

70

100

105

110

115

120

125

130

135

140

155

160

165

170

175

180

145

150

75

80

85

BE

DD

ING

mMW GP BF R

TE

XT

UR

E

SA

MP

LES

met

res

LOW

ER

KH

AR

AIB

ME

MB

ER

LEK

HW

AIR

Fm

.

UP

PE

R K

HA

RA

IB -

HA

WA

R M

EM

BE

RLO

WE

R S

HU

AIB

A M

EM

BE

R

UP

PE

R K

HA

RA

IB M

EM

BE

R

Upp

er B

arre

mia

nB

arre

mia

n p.

p.

Upp

er B

arre

mia

n

Low

er A

ptia

n

Low

er A

ptia

nS

imm

ons

&H

art (

1987

)

Mas

se e

t al.

(199

7)

Protectedlagoon

Protectedlagoon(slightly

argillaceous)

Open lagoon

Open lagoon

Intertidaland

subtidalrudist shoals

Protectedlagoon

Intertidaland subtidalrudist shoals

GENERALDEPOSITIONALENVIRONMENT

Intertidalshoal

(beach)

Intertidal to subtidal

(rudist) shoals

Subtidal(rudist) shoals

andrudist-

microbialitebiostromes

Open toprotected

lagoon

Protectedlagoon

Openlagoon

Protectedlagoon

Openlagoon

Protectedlagoon

Protectedlagoon

Microbialitebiostromesin protected

lagoon

Openlagoon

Protected toopen lagoon -algal meadows

(slightlyargillaceous)

Intertidalto subtidal

shoals

Protectedlagoon- algal

meadows(slightly

argillaceous)

Seq

uenc

e I

I.1I.2

II.1

II.2

II.3

II.4

II.5

III.1

III.2

III.3

III.4

III.5

III.6

III.7

III.8

III.9

Seq

uenc

e II

Seq

uenc

e III

d

c

b

b

b

b

b

b

b

b

b

a

a

a

a

a

a

a

a

a

c

c

c

c

c

c

c

c

d

d

d

d

d

d

d

b

b

b

b

b

b

a

a

a

a

a

a

c

c

c

c

c

c

d

d

d

d

d

d

?

?

?

?

??

??

?

?

?

DEPOSITIONALSEQUENCES

Sm

all-s

cale

Med

ium

-sca

le

Larg

e-sc

ale

13

17

17-5

4

2

131

2

1

13

16

14-15

14

17

14

17

1614

17

13

17

13-3

2

431

FAC

IES

3

1

2

13

13

1

2

4

172

2

2

11

11

11

11

2-4

4

414

145

5

5

14

14

14

14

14

14

14

17

179

9

9-8

13

17

17

17

14

14

14161-3

15

14 -15 -16

2

4

17

13

5

1413

13-9

7-14

7-14

17

17

11

13

13

1

4

1

2

Fig

.6.

Desc

rip

tion

an

dd

ep

osi

tion

al

sequ

en

ces

of

the

Wad

iM

uayd

inse

cti

on

.L

ocati

on

inF

ig.

1;

facie

sd

esc

rip

tion

sin

Table

1;

legen

din

Fig

.5.



Low

er

Shuaib

a

Mb.

Nahr

Um

r

Form

ation

Natih

Form

ation

Haw

ar

Upper

Kharaib

Mb.

Lower Kha

raib Mb.

Lekhw

air

Fm

.

II.3

II.4

II.4

II.5

II.5

II.2

II.3

- II.2 -

II.1

III.1

III.1

III.2

III.3

III.2

III.3

45

km

38

km

Wa

di N

ahr

Wa

di M

ua

yd

inW

ad

i B

an

i K

ha

roo

s

SE

NW

Lower Shuaiba

Member

Upper Kharaib Member

Hawar

Low

er

Khara

ib M

em

ber

Natih F

m.

Nahr

Um

r F

m.

Fig

.7.

Com

pari

son

betw

een

the

Wad

iN

ah

r,W

ad

iB

an

iK

haro

os

an

dW

ad

iM

uayd

inse

cti

on

s.S

cale

bars

10

m.

Th

eu

pp

er

part

of

the

Lekh

wair

Form

ati

on

isfo

rmed

by

an

alt

ern

ati

on

of

mass

ive

lim

est

on

eu

nit

s(>

10

m)

an

dbed

ded

inte

rvals

.T

he

up

perm

ost

part

of

the

Lekh

wair

iscap

ped

by

the

rud

ist

hig

h-e

nerg

yfa

cie

sof

the

low

erm

ost

Low

er

Kh

ara

ibth

at

form

am

ass

ive

un

it.

Above

this

un

it,

the

bed

ded

up

per

part

of

the

Low

er

Kh

ara

ib,

the

mass

ive

base

of

the

Up

per

Kh

ara

ib,th

ebed

ded

Haw

ar

an

dth

em

ass

ive

Low

er

Sh

uaib

am

em

bers

can

easi

lybe

corr

ela

ted

from

secti

on

tose

cti

on

.N

ote

that

the

bed

ded

inte

rvals

at

the

top

of

the

Low

er

Kh

ara

iban

din

the

Haw

ar

mem

bers

are

traced

here

over

38

an

d45

km

resp

ecti

vely

.A

bed

din

gst

ackin

gp

att

ern

of

small

-scale

(th

efo

ur

un

its

inS

equ

en

ce

II.3

)or

med

ium

-scale

dep

osi

tion

al

sequ

en

ces

(Sequ

en

ces

II.1

toII

.5an

dII

I.1

toII

I.3)

can

be

obse

rved

.



the same unit in Sequence I, whereas insequence III (Lower Shuaiba), the increase inthickness is of equal size in the three environ-

ments. (3) In Sequence III, the open lagoonalfacies, deposited during the late transgression(increased rate of sea-level rise?), is typically

LekhwairFm.

LowerKharaib

Mb.

Upp

er K

hara

ib M

b. Haw

ar

LowerShuaiba

Mb.

W. BANIKHAROOS J. MADARW. NAHR W. MUAYDIN

SENW

Seq

. IS

eque

nce

IIS

eque

nce

III

100 km45 km38 km

Latehighstand

Latehighstand

Highstand

Early transgression

Early transgression

Transgression

Latetransgression

Early highstand

Earlyhighstand

Latetransgression

Miliolid high-energy facies12345

6789

1011

13

17

High-energyshallow platform top

Rudist fragment high-energy facies (F-R / P-G)Rudist fragment low-energy facies (F-R / MW-B)

Laminites and mudstones (tidal flat)Oncoid faciesBound oncoid faciesMicrobialite boundstoneMicrobialite boundstone with some rudistsRudist-microbialite boundstone

Miliolid low-energy facies

16 Miliolid-orbitolinid faciesDiverse fauna facies

15 Orbitolinid high-energy facies14 Orbitolinid low-energy facies

High-energy faciesRudist-miliolid facies

Bioconstructedplatform

Open lagoon

Argillaceous low-energyshallow platform top

Carbonate low-energyshallow platform top

FACIESENVIRONMENTSTIMELINES

Tidal flat

Large-scale sequence boundary

Medium-scale sequence boundary

Small-scale sequence boundary

Uncertainty of cycle boundary

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

10

20

30

40

50

60

70

80

90

100

110

120

130

mM

W GP BF

R

mM

W GP BF

Rm

MW G

P BFR

mM

W GP BF

R

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

a

Fig. 8. (a) Correlation of the studied sections (locations in Fig. 1). The distribution of facies and their generaldepositional environments are indicated. (b) General facies and palaeoenvironmental trends recorded in theuppermost Lekhwair and in the Kharaib and Shuaiba Formations, and interpretation of this evolution in terms oftrophic level, energy, accommodation, relative sea-level changes and large-scale sequences.



dominated by microbial encrusting organisms.This occurred at the same time as the formationof the Bab Basin intrashelf, and the bioticresponse (high microbial activity) may be aresult of higher primary productivity at thattime, as shown by organic matter accumulationin the Bab Basin (Witt & Gokdag, 1994; VanBuchem et al., 2002b).

The sedimentary succession observed in theWadi Muaydin section (Fig. 6) was also observedin Wadi Nahr, Wadi Bani Kharoos and at JebelMadar; (Fig. 1b). Figure 7 shows the entirestudied interval in Wadi Muaydin, Wadi BaniKharoos and Wadi Nahr. The same bedding styleand sequence organization is observed as iden-tified in Wadi Muaydin (Fig. 6). The large-scaledepositional sequences represented by theuppermost Lekhwairlower part of the LowerKharaib member, the upper part of the LowerKharaiblower part of the Upper Kharaibmembers and the Hawar and Lower Shuaibamembers can be correlated between the foursections (Fig. 8a). Along a transect of more than100 km, they show the same facies evolution ofthe three major depositional phases described in

Wadi Muaydin (Fig. 6). Phase 1 is composed ofslightly clayey sediments deposited in shallowsubtidal to intertidal environments with orbitol-inids (upper part of the Lower Kharaib and theHawar members) or without orbitolinids (upper-most Lekhwair Formation) and some miliolids.Phase 2 exhibits open lagoon sediments withfrequent Thalassinoides burrows and related

dolomitization. In the Shuaiba Formation, thisinterval is replaced by microbialite boundstonesand orbitolinid wackestones. Phase 3 is formedby rudistpeloidalmiliolid grainstones, float-stones and rudstones and some rudistmicrobia-lite boundstones.

Thus, the three large-scale depositionalsequences defined in Wadi Muaydin (sequencesI, II and III in Fig. 6) can readily be correlated,based on the clear similarities of the faciesevolution and the sequence boundaries (Fig. 8).The different phases in the building of thesesequences are interpreted as early transgressive(Phase 1), late transgressiveearly highstand(Phase 2) and late highstand (Phase 3) phasesthat reflect long-term fluctuations in relative sealevel and palaeoenvironmental conditions (Fig. 8b).

Low

er

Exp

osur

e

Inte

rtid

al

Sha

llow

sub

tidal

"Dee

p" s

ubtid

al

For

mat

ions

Orb

itolin

ids

Cal

care

ous

alga

e

Mic

robi

alite

s/m

icro

encr

uste

rs

Rud

ists

in li

fe-p

ositi

on

Rud

ist f

ragm

ents

Mili

olid

s

Principalfacies components

Hig

her

Bab

bas

in in

itiat

ion

and

orga

nic

mat

ter

accu

mul

atio

n

Cla

y in

flux

Ene

rgy

Relativebathymetry

Acc

omm

odat

ion

Seq

uenc

es

LekhwairFm.

LowerKharaib

Mb.

Upp

er K

hara

ib M

b.H

awar

LowerShuaiba

Mb.

Late highstand

Late highstand

Highstand

Early transgression

Early transgression

Transgression

Latetransgression

Early highstand

Early highstand

Latetransgression

Seq

. IS

eque

nce

IIS

eque

nce

III

Trophiclevel

Low

er

Exp

osur

e

Inte

rtid

al

Sha

llow

sub

tidal

"Dee

p" s

ubtid

al

For

mat

ions

Orb

itolin

ids

Cal

care

ous

alga

e

Mic

robi

alite

s/m

icro

encr

uste

rs

Rud

ists

in li

fe-p

ositi

on

Rud

ist f

ragm

ents

Mili

olid

s

Principalfacies components

Hig

her

Bab

bas

in in

itiat

ion

and

orga

nic

mat

ter

accu

mul

atio

n

Cla

y in

flux

Ene

rgy

Relativebathymetry

Acc

omm

odat

ion

Seq

uenc

es

LekhwairFm.

LowerKharaib

Mb.

Upp

er K

hara

ib M

b.H

awar

LowerShuaiba

Mb.

Late highstand

Late highstand

Highstand

Early transgression

Early transgression

Transgression

Latetransgression

Early highstand

Early highstand

Latetransgression

Seq

. IS

eque

nce

IIS

eque

nce

III

Trophiclevel

b

Fig. 8. Continued.



In this correlation, no evidence for the develop-ment of progradational and/or retrogradationalgeometries has been found. Considering that theorientation of the studied transect is roughlyparallel to the palaeocoastline (which more orless follows the present-day coastline of Oman;Fig. 1), an aggradational trend can indeed beexpected.

Small- and medium-scale depositionalsequences

The example of the Wadi Muaydin section (Fig. 6)shows that large-scale (tens to 100 m) deposi-tional sequences are composed of medium-scalesequences (metres to tens of metres), which inturn are formed by small-scale sequences (deci-metres to metres). The stacking pattern and faciesorganization of the small- and medium-scalesequences vary as a function of their positionwithin the large-scale sequences (early transgres-sive, late transgressive/early highstand and latehighstand). The early transgressive phase ischaracterized by sedimentation of decimetre- tometre-scale beds, clearly bounded by bioturbated/clayey levels. The facies is rather uniform withalternating wackestones and packstones. Thefaunal content is generally dominated by orbitol-inids and/or miliolids and by abundant calcar-eous algae. Notably at the top of the LowerKharaib and in the Hawar member, four of thesebeds may form a bundle (Fig. 6). During the latetransgression and early highstand, bed surfacesare very rare, probably because of more rapidlyincreasing accommodation space. The small-scalesequences have a metre-scale thickness and areidentified by their characteristic facies trends.Locally, some dolomitic, bioturbated horizonsoccur. They may represent the maximum deep-ening and/or flooding of the large-scale sequence.Floatstonerudstone levels that have a randomdistribution are interpreted as storm deposits.During the late highstand, accommodation spacedecreased again, and the thickness of the small-scale sequences was reduced to the decimetre tometre scale. The environment was characterizedby much higher energy deposits, with rudist barsand tidal channels. The laminated and miliolidgrainstone deposits commonly represent beachenvironments. The higher energy conditionsresult in small-scale depositional sequences beingmore difficult to identify (autocyclic control onfacies succession).

Figure 9 shows the upper part of the LowerKharaiblower part of the Upper Kharaib mem-

bers in Wadi Muaydin. Some lithological bodiesmay easily be identified, and the bedding stackingpattern allows the definition of beds, packages ofbeds and/or massive units that correspond tofacies packages (Fig. 6). This stacking pattern ofbeds and packages of beds shows a hierarchicalorganization that is very well expressed in theupper part of the Lower Kharaib and recognizablein other sections (Figs 7 and 10). Figure 10 showsthree successive medium-scale depositionalsequences at the base of the large-scale SequenceII. The first one exhibits three to four alternationsof greyyellow marly limestones and marls.These alternations are interpreted as small-scalesequences. The second one is formed by four darklimestone beds intensively bioturbated at theirclayey base, which appears greyish. The thirdmedium-scale sequence exhibits four small-scalesequences formed by dark massive limestoneswith more clayey and bioturbated then dolomi-tized light grey intervals at their base and top. Thesmall-scale depositional sequences that comprisethe medium-scale sequences illustrated in Figs 9and 10 commonly exhibit facies changes during ashort-term cycle of relative sea-level change:protected lagoon orbitolinid or orbitolinidmilio-lid facies occur at their base and top, whereasopen lagoon facies occur in their middle part.

Commonly, four small-scale sequences buildone medium-scale sequence. These small-scalesequences generally show a weathering profile,which can be followed regionally. Figures 11 and12 illustrate the youngest medium-scale sequenc-es of the Hawar (Sequence III.3). The four small-scale sequences composing medium-scaleSequence III.3 have an architecture that can becorrelated over long distances (180 km; seeFig. 12). At their base, the small-scale sequencesexhibit one to three alternations of intensivelybioturbated marls and marly limestones overlaidby one or two pure massive, less bioturbatedlimestone beds (Figs 12 and 11b and c). The top ofthe last relatively thick (0515 m) limestone bedof each small-scale sequence is capped by afirmground allowing good preservation of originalThalassinoides burrows (Sequence III.3b in Fig.11c). Above this surface, a nodular thin limestonebed is recorded. Figure 12 shows that these small-scale sequences are somewhat different in theJebel Madar section, probably as a result of lessaccommodation space. However, the generalweathering profile (carbonate/clay ratio) is com-parable, and a similar position of the intervals ofmaximum bioturbation within the small-scalesequences is observed.



The correlation of small- and medium-scaledepositional sequences is presented in Fig. 8aand helps to refine the correlation scheme basedon large-scale depositional sequences. Medium-scale sequences, which are essentially bundlesof small-scale sequences, can be correlatedfrom section to section with reasonable certainty.The correlation of individual small-scale sequen-ces is more hazardous, on account of localnon-deposition, erosion or amalgamation of thebeds (e.g. in the Lower Shuaiba member).

Evolution in thickness of medium-scalesequences in a large-scale sequence is interpretedas reflecting changes in accommodation space.During early transgression and late highstand, no

marked changes in thickness of the medium-scalesequences occur between the sections but, at themaximum of space creation during late transgres-sion, a thickening-upward trend of the beds isobserved (e.g. in the Upper Kharaib member,Sequences II.3 and II.4; Figs 6, 8 and 9).

The general correlation scheme shows thatthere is an average of four small-scale sequencesfor each medium-scale sequence (Fig. 8). Basedon the high-resolution correlation, a hiatus maybe present in the upper part of the Lower Kharaib(Sequence II.1) in Jebel Madar and in WadiMuaydin, because the number of small-scalesequences is less here than in the sections inWadi Bani Kharoos and Wadi Nahr.

Hawar

Up

pe

r K

ha

raib

Me

mb

er

Lo

we

r K

ha

raib

Me

mb

er

Sequen

ce II.5

Sequen

ce II.2.b

Sequen

ce II.2.c

Sequen

ce II.2.d

Sequen

ce II.4

Sequen

ce II.3

Sequ

ence

II.2

Sequ

ence

II.1

a

b

Sequen

ce II.2.a

50

cm

10

m

Fig. 9. (a) General view of the Lower and Upper Kharaib members in Wadi Muaydin; the depositional sequences aredescribed in Fig. 6; note that medium-scale Sequences II.2 and II.3 are formed by four small-scale sequences.(b) Detail of the small-scale depositional sequences at the top of the Lower Kharaib in Wadi Muaydin; note theirslightly argillaceous, bioturbated, light grey bases and darker, and purely carbonate tops, which are commonlycapped by a firmground.



I.2II.1II.2II.3II.4

Sequence II Seq. I

bb aaa ccc ddd

? ?

DE

PO

SIT

ION

AL

SE

QU

EN

CE

S

Wa

di

Mu

ayd

in

?

?

Small-scale

metres

Medium-scale

Large-scale

45

km

38

km

510

15

20

25

BE

DD

ING

Kharaib Member

LowerUpper

510

15

20

25

Wa

di N

ah

r

mM

WG

PB

FR

mM

WG

PB

FR

mM

WG

PB

FR

510

15

20

Wa

di

Ba

ni K

ha

roo

s

TE

XT

UR

E

Fig

.10.

Corr

ela

tion

of

small

-an

dm

ed

ium

-scale

sequ

en

ces

docu

men

ted

at

the

top

of

the

Low

er

Kh

ara

ibm

em

ber.

Note

that

the

sam

ebed

din

gst

ackin

gp

att

ern

,w

ith

alt

ern

ati

on

sof

ligh

tgre

yan

dd

ark

gre

yin

terv

als

,m

ay

be

corr

ela

ted

over

lon

gd

ista

nces.

Locati

on

of

the

secti

on

sin

Fig

.1.

Legen

din

Fig

.5

an

dse

qu

en

ce

nom

en

cla

ture

inF

ig.

6.

See

Fig

.9a

for

ou

tcro

poverv

iew

of

this

inte

rval

inW

ad

iM

uayd

in.



Lo

we

r S

hu

aib

a

Me

mb

er

Lo

we

r S

hu

aib

a

Me

mb

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ORIGIN AND DURATIONOF DEPOSITIONAL SEQUENCES

Origin of sequences

Changes in facies composition within the large-scale sequences not only reflect relative sea levelbut also palaeoenvironmental changes indicatedby changes in biotic assemblages. Assemblage 1(discoidal orbitolinids, calcareous algae andechinoderms), Assemblage 2 (microbialites, micro-encrusters and in situ rudists) and Assemblage 3(rudist fragments, miliolids, mono- and biserialforaminifera) are also interpreted in terms oftrophic level (see above): the change from Assem-blage 1 to 2 to 3 during a complete long-term cycleof sea-level change (forming large-scale sequenc-es) expresses a change from higher to lowertrophic levels, probably resulting from a gradualdecrease in the influx of clays (Fig. 8b). In theLower Shuaiba member, Assemblage 3 may cor-respond to a relatively high (or intermediate)trophic level, but the absence of clays in this casesuggest that this development may be related tochemical changes in the ocean water.

This study suggests that changes in long-termrelative sea level are accompanied by changes introphic levels and clay input affecting the plat-form environment, possibly through a couplingwith palaeoclimatic changes. Consequently,large-scale depositional sequences may have adominantly eustaticclimatic rather than a tec-tonic origin. This is also supported by the pres-ence of only subtle differences in thickness, andthus probably in subsidence rates, between thefour sections (Fig. 8a), as well as the lateralcontinuity of at least the medium-scale sequencesover more than 100 km. This suggests thatspasmodic tectonic events did not play a signifi-cant role during sedimentation of the studiedsuccession.

Similar to large-scale depositional sequences,changes in facies composition and biotic associ-ations within the small- and medium-scalesequences also reflect, albeit shorter term, chan-ges in palaeoenvironmental conditions. Rapid

Fig. 12. Correlation of the small-scale sequences com-prising the third medium-scale sequences of the Hawarmember (Sequence III.3). The stacking pattern observedin Wadi Muaydin (Fig. 11) is also present in Wadi BaniKharoos and Wadi Nahr. Only in Jebel Madar is thisstacking pattern not well expressed. However, thealternations between more clayey and more carbonateintervals exhibit a comparable trend to that in the otherthree sections.

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changes from Assemblage 1 to Assemblage 3commonly occur in early transgressive phasesand coincide with changes in the bedding pattern(Figs 612). Changes in nutrient level and clayinput may imply that palaeoclimatic changeswere also coupled to short-term changes inrelative sea level to form the small- and me-dium-scale depositional sequences.

Average duration of sequences

The uppermost Lekhwair Formation is of EarlyBarremian age, and the top of the Lower ShuaibaFormation in the studied area corresponds to theLowerUpper Aptian boundary (e.g. Simmons &Hart, 1987; Hughes-Clarke, 1988; Sharland et al.,2001). Based on the time-scale of Gradstein et al.(1995), the mean durations of the maximumnumber of medium- (16) and small-scale sequen-ces (64) was calculated for the uppermost Lekhw-air, the Kharaib and Shuaiba Formations. In theabsence of a precise biostratigraphic framework,only rough approximations of the mean durationswere made. Because of uncertainties related tothe biostratigraphical framework, two options areconsidered to estimate sequence duration: (1) thebase of the studied interval corresponds to thebase of the Barremian; and (2) the base ofthe studied interval corresponds to the LowerUpper Barremian boundary. In the first case,Gradstein et al. (1995) suggested a duration of

about 9 Myr from the beginning of the Barremianto the end of the Early Aptian. Thus, the averageduration of the medium-scale sequences is 562kyr, and the average duration of the small-scalesequences in the order of 140 kyr. In the secondcase, Gradstein et al. (1995) calculated about78 Myr for the Late Barremian and Early Aptian.Thus, the average duration of the medium-scaleand small-scale sequences would correspond to488 kyr and 122 kyr respectively. Hiatuses occurat the base of the Hawar and also in the middlepart of the Lower Kharaib. Assuming durations of05, 1 and 2 Myr of non-deposition for hiatuseswithin the studied interval, the following averagedurations of medium-scale sequences may becalculated: in the first case, 531, 500 and 438kyr and, in the second case, 456, 425 and 362 kyr.Thus, the calculated duration of the medium-scale sequences varies between 362 and 562 kyr,with a mean duration of 462 100 kyr. With aratio of four small-scale sequences for onemedium-scale sequence, the mean duration of asmall-scale sequence is estimated at 116 25 kyr.

The hierarchical relationship between thesmall- and medium-scale sequences and theirestimated duration suggest that they might repre-sent the Earths two orbital eccentricity cyclesthat have periodicities of about 100 and 400 kyrrespectively (Berger et al., 1989). These twocycles may also have controlled the cyclic pal-aeoenvironmental changes indicated by the facies

Fig. 13. BarremianAptian geological history and the stratigraphic distribution of orbitolinid-rich beds. Stratigra-phy, carbon isotope curve and the stratigraphic positions of oceanic anoxic events OAE1a and 1b are based onWeissert et al. (1998), Erba (1994), and Mutterlose & Bockel (1998). The stratigraphic position of orbitolinid beds inFrance (Arnaud-Vanneau & Arnaud, 1990), Switzerland (Helvetic realm; Funk et al., 1993), Spain (Ruiz-Ortiz &Castro, 1998), and Oman (Masse et al., 1998; Immenhauser et al., 1999; Hillgartner et al., in press; Van Buchem et al.,2002a). Episodes of Tethyan platform drowning (Follmi et al., 1994), plume activity (Larson, 1991), biota turnover(Mutterlose & Bockel, 1998) and the position of nannoconid crisis (Erba, 1994) are also shown.



associations. Assuming this interpretation to becorrect, different durations for the three succes-sive large-scale depositional sequences result(Fig. 8): the upper Lekhwairlower Lower Kharaibsequence would have been deposited in 800 kyr,the upper Lower Kharaiblower Upper Kharaibsequence in 2 Myr, and the HawarLower Shuaibasequence in 4 Myr (Fig. 8). Thus, the majorincrease in thickness observed in the successivelarge-scale sequences (each time almost doublingin size) might reflect doubling in their durationrather than doubling in accommodation spacecreated per time unit.

DISCUSSION

The third-order depositional sequences (sensuVail et al., 1991) distinguished in the Barremianand Aptian sediments of northern Oman (upper-most Lekhwair, Kharaib and Shuaiba Formations)are characterized by a repeated succession ofthree faunal assemblages: a discoidal orbitolinidassemblage during early transgression; a mixedassemblage with diverse biota/microbialites,microencrusters and rudists in life position dur-ing late transgression and early highstand; and arudist, miliolid, mono- and biserial foraminiferaassemblage during late highstand. These changesin faunal content suggest that sea-level changes ofprobable eustatic origin controlled not only sedi-mentation patterns on the platform, but alsopalaeoenvironmental conditions that were notdirectly related to bathymetry, such as the trophiclevel and clay input.

The orbitolinids, which in the studied area arecommonly associated with abundant calcareousalgae and echinoderms in slightly argillaceouslimestones, are interpreted to have been depos-ited in relatively high trophic conditions (meso-trophic). Conversely, the grainy, rudist- andmiliolid-dominated facies indicate lower trophicconditions (oligotrophic). Changes in trophiclevel and clay input might be related to variablehumidity patterns during sea-level rise and fall.During (early) transgression, both increasedweathering rates induced by stronger rainfalland clay reworking during flooding of theexposed platforms may have contributed to thecreation of mesotrophic conditions favourable forthe development of orbitolinids. Indeed, fromseveral locations around the Neo-Tethys, theyhave been reported to occur during the firstepisode of transgression (Arnaud-Vanneau &Arnaud, 1990; Funk et al., 1993; Raspini, 1998).

Figure 13 illustrates the stratigraphic distributionof some of these orbitolinid-rich intervals fromthe western Neo-Tethys. In particular, during theEarly Aptian, orbitolinid-rich beds may correlateover thousands of kilometres, suggesting a link toglobal changes. This also implies that orbitolinid-rich facies types cannot be directly related to aspecific palaeoenvironmental setting on carbon-ate platforms. Their occurrence is widespread,and they occupied various platform environ-ments at different stratigraphical intervals. Inthe studied sections, orbitolinid-rich facies wereessentially deposited in very shallow-marineenvironments, as on the Vercors platform inFrance (Arnaud-Vanneau & Arnaud, 1990),whereas other orbitolinid-rich facies have beenrecognized in deeper, sand-dominated settings(Vilas et al., 1995). In addition, orbitolinid-richfacies occur generally contemporaneous frominner to outer platforms and in intrashelf basins(Vilas et al., 1995; Van Buchem et al., 2002b),which also suggests a palaeoclimatic control ontheir occurrence. In contrast, the rudist-domin-ated facies, also known as the Urgonian facies,recognized all around the Neo-Tethys, are anexpression of more oligotrophic conditions, poss-ibly caused by warmer and dryer climates duringperiods of eustatic highstand.

The time of deposition of the Kharaib andShuaiba Formations (Barremian, Aptian) corres-ponds to a transitional period in Earths history(Fig. 13). This transition is characterized by amajor biotic turnover that is marked by changes inthe nektonic, planktonic and benthic micro- andmacrobiota (Mutterlose, 1998). Disappearancesand new appearances of species, genera andfamilies are recorded in many different groups(e.g. belemnites, ammonites, benthic foramini-fera, calcareous nannofossils, calcispheres; Mut-terlose, 1992, 1998; Erba, 1994; Mutterlose &Bockel, 1998). Diversification of planktonic fora-minifera occurred (a major speciation event;Coccioni et al., 1992; Mutterlose & Bockel,1998). Such biotic turnovers were probablyinduced by changes in the palaeoceanographicdynamics (Bralower et al., 1994) and configur-ation (such as the opening of new seaways;Mutterlose & Bockel, 1998). The development oforbitolinid-rich deposits also seems to be relatedto this biotic turnover. From the Late Barremianto the Albian, deposits rich in orbitolinidsbecame more and more frequent on the Arabianplatform. A gradual opening of orbitolinid nichesis recorded (Figs 8 and 13), with the first episode,still limited in extension, occurring during the



deposition of the Lower Kharaib member (LateBarremian), the second, more extended episodecorresponding to the Early Aptian Hawar mem-ber, whereas the (mainly) Albian Nahr UmrFormation marks a third maximum developmentof orbitolinid facies (Fig. 13).

The first two Cretaceous oceanic anoxic events(OAE1a, OAE1b) occurred in the earliest andlatest Aptian. Black shales, most likely linked tothese events, are recorded in many marine realmsworldwide (Bralower et al., 1994; Weissert et al.,1998). Significant concentrations of organic mat-ter have also been observed in the intrashelfArabian Bab basin (Fig. 1c; Witt & Gokdag, 1994;Van Buchem et al., 2002b). Such palaeoceano-graphic events were probably characterized by astrong stratification of the oceans and subsequentdecrease in oceanic circulation. Drowning ofcarbonate platforms also occurred, suggestingperturbations in the carbon cycle (Funk et al.,1993; Follmi et al., 1994). Phosphatic deposits arecommonly recorded in basinal areas and arerelated to changes in primary production cycles(Follmi, 1995). Global perturbation in the carboncycle in the Aptian is also testified by importantcarbon isotope excursions (e.g. Follmi et al., 1994;Grotsch et al., 1998). The Hawar orbitolinidepisode (Fig. 13) is recorded at the moment ofmaximum changes in the biota and in the oceanicgeochemistry and records the clear shift to lowercarbon isotope values that preceded OAE1a (Vah-renkamp, 1996; Van Buchem et al., 2002b). Thus,the orbitolinid-rich Hawar member is possibly thelithofacies expression of a global change.

A comparison of the two first sequences (I andII) with the HawarLower Shuaiba sequence (III)shows the presence of a particular microbialfacies in the transgressive phase of the latter. Animportant development of microbialites andmicroencrusters occurred in the lagoonal settingforming microbial build-ups or large oncoids(Fig. 8). The middle part of the Lower Shuaibaplatform can thus be considered as having beenbioconstructed. Laterally to the microbial facies,orbitolinid-bearing sediments were deposited.Such important microbial developments gener-ally reflect relatively mesotrophic conditionson the shallow platform (Leinfelder et al., 1993).This microbial development is contemporaneousto the global positive shift in carbon isotope andto the deposition of organic-rich layers in the BabBasin, which most probably reflects OAE1a (Vah-renkamp, 1996; Van Buchem et al., 2002b). Thissuggests periods of dysoxia or anoxia at the seabottom and higher trophic conditions (increased

primary productivity) in the basin and on theplatform. Development of microbial build-ups is acommon feature in the Aptian of Oman (Hillgart-ner et al., in press), but examples of thick Aptianmudmounds are also described in other Tethyanareas (e.g. France; Lenoble & Canerot, 1993).Microbial build-ups of the Lower Shuaiba mem-ber may thus have been caused by generallyhigher trophic levels in the ocean waters.

Volcanism may be responsible for the biological,geochemical and sedimentary changes recordedfrom the end of the Barremian to the AptianAlbian. High global spreading rates (high ocean-floor basalt production) are recognized during theAptian and might result from increased mantleplume activity (Larson, 1991). Such volcanicactivity might have generated stratification ofoceanic waters and sluggish water circulation(Bralower et al., 1994). Enhanced CO2 in theatmosphere might have favoured greenhouse cli-mate, temperature elevation (mainly in mid- andhigh latitude; Barron & Moore, 1994), with oxygendepletion in deep marine waters favouring or-ganic matter preservation (Arthur et al., 1985;Caldeira & Rampino, 1991; Erba, 1994; Mutter-lose, 1998; Mutterlose & Bockel, 1998). Warmerclimates would have favoured evaporation andincreased rainfall, resulting in more humid con-ditions globally (Follmi et al., 1994). Such humidconditions might have favoured organisms livingin platform environments that are specialized (ortolerant) to relatively high trophic conditions(orbitolinids, microbial organisms).

The Barremian is interpreted as a period ofwarm and arid climate; the Aptian marked thebeginning of a still warmer and more humidclimate that characterizes the mid-Cretaceous(Weissert, 1989; Ruffel & Batten, 1990; Erba,1994; Mutterlose & Bockel, 1998; Norris & Wilson,1998), while the Upper Aptian was probably acooling phase (e.g. Hochuli et al., 1999). Theobserved sedimentation pattern may thus berelated to volcanic episodes inducing changes inpalaeoceanographic dynamics and palaeoclimate.The sequence stratigraphic model proposed herefor Oman, with its typical facies evolution, maythus well be applicable to other parts of theArabian Plate, and possibly to other areas aroundthe Neo-Tethys.

CONCLUSIONS

In the BarremianAptian shallow-water carbon-ates of the Arabian platform in northern Oman,



three orders (third to fifth) of depositional seq-uences have been distinguished based on bioticassociations, facies evolution and bedding stack-ing patterns.

Over the course of three third-order sequences,a pattern develops of a succession of three faunalassemblages: (1) a discoidal orbitolinidscalcar-eous algaeechinoderms assemblage characteri-zes the early transgression; (2) a mixed diversebiota assemblage or an association of microbia-lites/microencrustersrudists in life positiondeveloped during times of maximum sea-levelrise; and (3) a rudistmiliolidmono- and biserialforaminifera assemblage predominated duringhighstands. This suggests that the facies succes-sion in the third-order depositional sequenceswas not only controlled by relative sea-levelchanges (accommodation) but also, and to a largeextent, by climatically driven palaeoenvironmen-tal changes (clay influx, trophic level). Discoidalorbitolinidcalcareous algaeechinoderm faciesmay have been deposited in higher trophic envi-ronments in the presence of clays, whereas faciesdominated by microbialites (with or withoutrudists in life position) were deposited in highertrophic environments in the absence of clays. Therudistmiliolid facies is interpreted as indicatingoligotrophic platform environments.

The main factor controlling this systematicsedimentation pattern at the scale of third-ordersequences on the platform is probably increasedvolcanism, influencing palaeoclimate and palae-oceanography worldwide. At the shorter timescale, control by the Earths two orbital eccentri-city cycles on the formation of small- andmedium-scale depositional sequences is pro-posed: their facies evolution reflects short-termsea-level oscillations and also climatic changesthat possibly induced changes in rainfall pat-terns, and subsequent changes in clay and nutri-ent inputs into the sedimentary environments.These short-term climatic changes modulated thelong-term climatic trend that took place globallyin the BarremianAptian.

ACKNOWLEDGEMENTS

This IFP study was initiated by the Abu DhabiCompany for Onshore Oil Operations (ADCO)and financed by both ADCO and PetroleumDevelopment Oman (PDO). Publication of thispaper is by kind permission of ADNOC andADCO, PDO and the Ministry of Oil and Gas ofthe Sultanate of Oman. We would like to thank

Omar Al-Jeelani, Abdullah Al Mansouri andKhalil Al-Mohsen from ADCO, and AbdullahAl-Habshy, Ali Al-Jahadmi, Magda Al-Kharusi,Mohamed Al-Mamary and Hisham Al-Siyabifrom PDO, and Guy Desaubliaux and OlivierLerat from IFP for their help with the field datacollection, and Heiko Oterdoom and Ian Billingfrom PDO for sharing with us their knowledge ofthe Kharaib and Shuaiba geology and stimulatingdiscussions. We would like to thank reviewersAdrian Immenhauser and Mike Simmons for theircomments and suggestions, which greatlyimproved the quality of this paper. Mike Sim-mons is also acknowledged for his contribution tomicropalaeontological determinations.

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Documents

Ecological succession, palaeoenvironmental change, and depositional sequences of Barremian–Aptian shallow-water carbonates in northern Oman