Facies distribution of the Lower Cambrian cryptic microbialand epibenthic archaeocyathan-microbial communities, westernAnti-Atlas, Morocco

J . JAVIER ALVARO*� , SEBASTIEN CLAUSEN� , ABDERRAZZAK EL ALBANI�and EL HASSANE CHELLAI§*Departamento Ciencias de la Tierra, Universidad de Zaragoza, Zaragoza, Spain (E-mail:jose-javier.alvaro@univ-lille1.fr)�LP3, UMR 8014 CNRS, UFR Sciences de la Terre, Universite des Sciences et Technologies de Lille,59655-Villeneuve d’Ascq, France�Laboratoire HIDRASA, UMR 6532 CNRS, UFR SFA, Universite de Poitiers, 40 av. du Recteur Pineau,86022-Poitiers, France§Departement de Geologie, Faculte des Sciences-Semlalia, Universite de Cadi Ayyad, Marrakech,Morocco

ABSTRACT

Marine microbial communities recorded in the Moroccan Anti-Atlas were

unaffected across the Neoproterozoic–Cambrian transition. A stromatolite-

dominated consortium was replaced at the beginning of the Atdabanian

(ca 20 Myr after the Neoproterozoic–Cambrian boundary) by shelly metazoan

and thromboid consortia, which contain the oldest biostratigraphically

significant fossils of the Moroccan Cambrian. The associated collapse of

microbial mat (stromatolitic) growth appears to coincide with a change from

pre-Atdabanian shallow-water restricted conditions into Atdabanian deeper,

open-sea conditions. It is postulated that this environmental change led to an

episode of improved water circulation over carbonate platform interiors,

promoting shelly metazoan immigration into the region. The Tiout/Amouslek

lithostratigraphic contact in the early Atdabanian marks the end of an

episodically unstable seafloor as suggested by the abundance of slumping

and sliding structures, and synsedimentary microfaults and cracks recorded in

the underlying Tiout Member. Concurrent with the transition is the occurrence

of a network of cryptic fissures and cavities that provided habitats for a

coelobiontic chemosynthetic–heterotrophic microbial community composed

of stromatolitic crusts, Renalcis–Epiphyton–Girvanella intergrowths, and

Kundatia thalli. In the overlying Amouslek Formation, archaeocyathan–

thromboid reefs were constrained by substrate stability, water depth and

subsidence rate. Four reef geometries are distinguished: (i) patch reefs

surrounded by shales, (ii) bioherms in which flank beds intercalate laterally

with carbonate and shale inter-reef sediments, (iii) biostromes or low-relief

structures formed as a result of lateral accretion of patch reefs, and (iv)

kalyptrate complexes that nucleated because of a marked tendency for

aggregation, and in which patch reefs and bioherms occur stacked together

bounded by clay–marl–silt seams.

Keywords Anti-Atlas, benthic replacement, cryptic cavities, Lower Cam-brian, microbialites, reefs.

Sedimentology (2006) 53, 35–53 doi: 10.1111/j.1365-3091.2005.00752.x

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INTRODUCTION

The pattern of metazoan diversification that tookplace in the late Neoproterozoic (Ediacaran) andearliest Cambrian was recorded differently invarious marine platforms depending upon theirgeodynamic, environmental and taphonomic con-ditions (see an updated revision in Zhuravlev &Riding, 2001). However, some geographical bar-riers on some platforms allowed the microbial(stromatolite–thrombolite) consortium to survivein isolation until middle Early Cambrian times.

One such sample exists in the Moroccan Anti-Atlas, where the marine microbial communitiesof the Souss Basin (Geyer, 1989) were unaffectedacross the Neoproterozoic–Cambrian transition.This microbial consortium was maintainedthrough this transition as preserved in the Adou-dou, Lie-de-vin and lower member of the Igou-dine formations (Schmitt, 1979). In other basins,new groups of algae, heterotrophic protistans andmetazoans evolved, and microbial communitiesdeclined dramatically in diversity and abundancebecause of their retreat from many of the subtidalenvironments that had been most favourable forthem. A relic microbial community, however,appears to have been ‘protected’ in the westernSouss platform until the Early Atdabanian,ca 20 Myr after the Neoproterozoic–Cambrianboundary. The stromatolite-dominated faciesassemblage on the western Souss platform wassucceeded in the Early Atdabanian by an abruptintrusion of shelly facies (Sdzuy, 1978; Monnin-ger, 1979). As a result, the location of theNeoproterozoic–Cambrian boundary (defined by the

first appearance of the ichnospecies Treptichnuspedum; Narbonne et al., 1987) remains problem-atical in Morocco because it lies at the base orwithin a thick carbonate-dominated succession(Adoudou, Lie-de-vin and lower member of theIgoudine formations) that is extremely poor inshelly metazoans and ichnofossils. The boundaryis tentatively correlated with radiometric ages andcarbon-isotope signatures (Kirshvink et al., 1991;Magaritz et al., 1991; Fig. 1).

The aim of this paper is to better understand thedepositional processes that controlled the EarlyAtdabanian change from restricted to somewhatnormal marine conditions and the associatedabrupt immigration of shelly metazoans into thewestern Anti-Atlas. Both events allowed micro-bial nucleation in cryptic cavities and fissurescrosscutting episodically unstable rigid sub-strates, and development of epibenthic microbialand archaeocyathan–microbial reefs. The lattercontain the oldest biostratigraphically significantfossils of the Lower Cambrian sedimentary rocksin Morocco.

GEOLOGICAL SETTING ANDSTRATIGRAPHY

The Neoproterozoic(?)–Cambrian successions ofthe Moroccan Atlas are located in the Anti-Atlasand central High-Atlas (Fig. 2), although somedisconnected outcrops occur in the Jbilet andRehamna regions, and the Meseta plateau. Theoldest Cambrian shelly metazoans of the SoussBasin occur in limestones and dolostones of the

Fig. 1. Stratigraphic framework of the Neoproterozoic–Cambrian transition in the Moroccan Atlas with the chro-nologic succession of event stratigraphy (summarized after Houzay, 1979; Leblanc & Lancelot, 1980; Clauer et al.,1982; Destombes et al., 1985; Tucker, 1986; Latham & Riding, 1990; Kirshvink et al., 1991; Magaritz et al., 1991;Debrenne & Debrenne, 1995; Geyer et al., 1995; and references therein).

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upper member of the Igoudine Formation in theTiout area and at the bottom of the overlyingAmouslek Formation in other outcrops (Geyeret al., 1995; Fig. 3). Both units formed an EarlyCambrian carbonate platform that bordered partof the western Gondwana margin (Alvaro et al.,2003).

The Calcaires superieures (Choubert, 1952) orIgoudine Formation (Geyer, 1989), up to 400 mthick, are composed of massive limestones,largely dolomitized at the lower part, locallyinterbedded with shales and sandstones.An important lithofacies change identified in

its stratotype (Tiout section) allows subdivisioninto two members: (i) an unnamed lower member(ca 185 m thick), composed of black, commonlylaminated, lime mudstones and dolostones, and(ii) an upper member named the Tiout Member(110 m thick in its stratotype) subdivided byMonninger (1979) and Schmitt (1979) into a‘black oolitic limestone facies’ overlain by an‘archaeocyathid–bioherm facies’. As Hupe (1953)identified the top of the Calcaires superieures atthe occurrence of shales and archaeocyathanbioherms, it is proposed here to amend theTiout–Amouslek lithostratigraphic contact by

Fig. 2. Geological sketch of (A) the Neoproterozoic(?)–Cambrian outcrops of the High Atlas and Anti-Atlas (modifiedfrom Geyer et al., 1995), and (B) setting of the studied sections: Ti, Tiout; Ta, Tazemmourt; and Am, Amouslek(modified from Boudda et al., 1979).

Fig. 3. Neoproterozoic(?)–LowerCambrian lithostratigraphic andbiostratigraphic units of the Amou-slek–Tiout area (modified fromGeyer et al., 1995), and correlationwith the Siberian scale basedon archaeocyathans (Debrenne &Debrenne, 1995).

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introducing the ‘archaeocyathid–bioherm facies’within the Amouslek Formation. This takes intoaccount the sharp contact between the lowerblack, bedded limestones of the ‘oolitic limestonefacies’ and the nodular aspect of the overlyingbrownish patch reefs embedded in green shalesshown by both the ‘archaeocyathid–bioherm’facies and the overlying Amouslek Formation.This boundary is easily mappable and currentlyused in the geological maps of the Anti-Atlas (seeFig. 3 for lithostratigraphic and biostratigraphicrelationships). An Atdabanian age (according tothe Siberian chart; Spizharski et al., 1986) of theTiout Member is based on the occurrence ofthe oldest Moroccan trilobites (Eofallotaspis tri-lobite zone; Sdzuy, 1978; Geyer, 1990) andarchaeocyaths (Erismacocisnus fasciolus–Reteco-scinus minutus archaeocyathan zone; Debrenne &Debrenne, 1995), and on the carbon-isotopestratigraphy (Latham & Riding, 1990).

The overlying Serie schisto-calcaire (Choubert,1952) or Amouslek Formation (Geyer, 1989) is a40–220 m thick succession of variegated shaleswith interbedded limestones rich inarchaeocyathan–microbial reefs. The formationlocally contains abundant trilobites (Hupe, 1953)and archaeocyaths (Debrenne & Debrenne, 1995)that allow subdivision into several Lower Cam-brian, archaeocyathan and trilobite zones (Fig. 3).

Three key sections containing the Tiout/Amou-slek lithostratigraphic transition, which are thefocus of this paper (Fig. 2B), are considered bythe International Subcommission on CambrianStratigraphy as the reference sections of thelowermost part of the Issendalenian Stage (Geyeret al., 1995). The Tiout section [global positioningsystem (GPS) coordinates 30�23¢N/08�41¢W of theTaroudannt map sheet] is one of the best-studiedCambrian outcrops in the Anti-Atlas includingdepositional environments (Monninger, 1979),microbial structures (Schmitt, 1979), trilobites(Sdzuy, 1978), archaeocyaths (last revision inDebrenne & Debrenne, 1995), carbon-isotopecurves (Tucker, 1986; Magaritz et al., 1991),magnetostratigraphy (Kirshvink et al., 1991) andtrace fossils (Latham & Riding, 1990). TheTazemmourt section (GPS coordinates 30�23¢N/08�48¢W of the Taroudannt map sheet) is anotherkey section. Abadie (log published in Hupe, 1953)was the first to report its stratigraphy in detail,Hupe (1953) described its trilobites and Debrenne(last revision in Debrenne & Debrenne, 1995) itsarchaeocyaths. Choubert (1953) was the first topoint out the tectonically disturbed aspect ofsome of the outcrops at the Tazemmourt section.

Finally, the Amouslek section (GPS coordinates30�11¢N/09�02¢W of the Aıt Baha map sheet) isone of the most complete stratigraphic referencesof the Moroccan Lower Cambrian. Its lithostrati-graphy and biostratigraphy have been succes-sively improved from Hupe’s (1953) pioneer workby Destombes et al. (1985), Geyer (1989, 1990)and Geyer et al. (1995), among others.

FACIES ASSOCIATIONS OF THE TIOUTMEMBER

The facies associations described below crop outup to 100 m below the amended Tiout/Amousleklithostratigraphic contact. Figure 4 shows thestratigraphic logs, whereas the small-scale seq-uence types or parasequences (sensu VanWagoner et al., 1988) are shown in Fig. 5.

Stromatolitic reefs

This facies association was previously reported as‘type 4 of reefal growth framework’ by Monninger(1979) and ‘cumulate stromatolites’ by Geyeret al. (1995), and is only recognizable directlyunderlying the ‘black oolitic limestone facies’ atthe Tiout section. Monninger (1979) described thefacies association as ‘very irregular to bulbous,centimetre-sized heads, irregularly laminated,Nimbophyton-type (Schmitt, 1979) stromatolites,with finely crystalline, impure dolostone (locallylimestone) as host rocks’. Some parasequences(0Æ3–1 m thick) have, at their base, slumped,contorted, brecciated and faulted stromatoliticand peloidal-rich laminae (Fig. 6A and B). Crin-kled stromatolites comprise the bulk of the para-sequences, which accrete vertically to producedome-shaped stromatolites, up to 1 m in dia-meter. Interdomal areas are either filled with fine-grained peloidal packstones to wackestones, orhost smaller domal and crinkled stromatolitesthat are gradually enveloped by the larger accret-ing domes. Finally, synoptic relief diminishesupwards, where domes are covered by dolo-mudstones displaying desiccation cracks, tepeestructures and scalloped surfaces (see detaileddescriptions in Schmitt, 1979).

The superposition of parasequences is inter-preted as repeated asymmetric shallowing-up-ward events. Each parasequence commences withan initial phase of rapid deepening and isfollowed by gradual stromatolite accretion, fromcrinkled to dome-shaped morphologies. Theuppermost part of some parasequences provides

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Fig. 4. Stratigraphic sections of Tiout, Tazemmourt and Amouslek (see text for more details). The facies of thecryptic fissures found in the Tiout Member at Tazemmourt are not described in the figure.

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evidence for subaerial exposure although thestromatolite reefs appear to have commonlydeveloped in shallow-water, peritidal environ-ments. The tops of the parasequences are markedby erosive surfaces associated with subsequenttransgression (Fig. 5) that was related to episodesof seafloor instability that affected both consolid-ated and unconsolidated biomats.

Peloidal rhythmites

The rhythmites define well-bedded asymmetricparasequences (0Æ1–1Æ2 m thick) recognized astabular units traceable over several tens of metresin outcrop. From bottom to top, the parasequencescontain an intraclastic lag, micropeloidal grain-

stone alternations (Fig. 6C), and peloidal grain-stones. The intraclastic lag (up to 0Æ1 m thick),which occurs overlying scoured bases with flamestructures, are clast-supported and typically dis-organized. Clasts are randomly oriented, angularin shape, and commonly comprise grainstonesof peloids and peloidal aggregates. They arelaterally associated with centimetre-thick, intra-formational slumps, synsedimentary beddingdistortion, and microfaulting rich in truncationand onlapping surfaces. The overlying milli-metre-thick alternations (up to 1Æ1 m thick) showundulatory- and planar-laminar bedding, rarelow-angle cross-laminae, irregular laminoid(fenestral) fabrics, and numerous low-angle ero-sive surfaces (up to 4 cm deep) directly onlapped

Fig. 5. Basic small-scale sequences or parasequences described in the text, and summary of major depositionalsystems.

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by rhythmites. The dark laminae (< 2 mm thick)are essentially micropeloidal packstones to wacke-stones, whereas the light laminae (up to 8 mmthick) consist of micropeloidal grainstones. Theyare composed of 70–80% micropeloids (200–400 lm in diameter), 5–10% subangular intra-clasts (up to 600 lm in diameter), and 5–10%quartz silt, along with several generations of fine,equant to bladed calcite cements occluding inter-particle pores. These alternations pass gradually

upwards into alternations (1–10 cm thick) ofmedium-sorted, peloidal grainstones and pack-stones, with angular to subangular peloids that areup to 3 mm in diameter.

The sorting, roundness, grading, alternatinglaminae and basal erosive surfaces of the para-sequences suggest intermittent reworking ofmicritic to peloidal sediments in a medium- tohigh-energy subtidal environment. The intraclas-tic lags appear to have formed by disruption and

Fig. 6. Field and photomicrographs of representative facies. (A) Slumped stromatolite and peloidal-rich layers at thebottom of the stromatolitic parasequences at Tiout; (B) synsedimentary fracture affecting consolidated stromatolitesat the bottom of the stromatolitic parasequences at Tiout; (C) micropeloidal to peloidal grainstones and packstones(top on the left) at Tazemmourt; (D) aggregate-rich grainstone of the mottled limestones at Amouslek; (E) microbial(Proaulopora) grainstones in the mottled limestones at Amouslek; (F) centimetre-thick, microbial patch overlying theooidal shoals.

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erosion of the coherent thin rhythmic layers,which were accumulating cemented detritusderived from the peloidal grainstones. The tab-ular shape of the rhythmites and the lack of cross-stratified structures (except local low-anglefeatures) suggest deposition as tabular blankets.The abundant synsedimentary slumping andmicrofaulting in the lowermost parts of paraseq-uences support bedding distortion and formationof some of the lowermost erosive bases (Fig. 5).The peloidal, coarsening- and thickening-upwardtrends of parasequences record medium-energy,subtidal conditions, shallowing upwards intohigher energy, shallow-water inner-platform set-tings (likely foreshore), and lack evidence ofsubaerial exposures. The lack of skeletal materialsuggests that conditions were unfavourable forshelly fauna.

Carbonate sand shoals (mottled limestones)

Asymmetric parasequences (0Æ5–4Æ6 m thick),which are laterally continuous at a scale of tensof metres, coexist with slumped, contorted lamin-ated beds, rare microfaulting, and intraclasticbreccia (0Æ2–1Æ4 m thick) at their bases that areseparated by erosive surfaces. These surfaces areoverlain by coarsening-upward grainstone bedsand lenses (ca 0Æ2–0Æ8 m thick), arranged intotabular to planar and trough cross-stratified, ooid-peloidal sets up to 4Æ6 m thick. These sets displayparallel, low-angle and rippled lamination at thetop. Both the contorted and bedded grainstonesconsist predominantly of medium-sorted peloidsand ooids, with lesser amounts of coated grains,lumps, aggregates, oncoids and bioclasts (Fig. 6D).The peloids, lumps and aggregates are subangularto subrounded in shape, and up to 4 mm indiameter at the top of the parasequences. In well-preserved samples, some peloidal-like texturesconsist of Proaulopora debris, which occurs ascomplete or broken filaments dispersed in peloidalgrainstones. Filaments are solitary, straight toslightly curved tubes, sometimes bifurcated inlongitudinal section. Micritic tubes are well sortedand fragmented, and are 40–80 lm in diameter andup to 500 lm in length (Fig. 6E). The ooids (up to1 mm in diameter) display radial, concentric orcomposite cortices; nuclei typically consist ofmicrite, micritic aggregates and intraclasts. Theoncoids (up to 4 mm in size) become abundanttowards the top of the parasequences; they haveconcentric, millimetre-thick laminae, oftenarranged asymmetrically, around nuclei of intra-clasts and skeletal material. The fossil record

occurs as nuclei of some oncoids or as uncoated,disarticulated and broken archaeocyaths andtrilobite sclerites. In addition, very finely dis-persed quartz and K-feldspar silt occur, increasingin abundance upwards. Some grainstone layershave isopachous cement around allochems(formed by fibrous crystals ca 40–50 ·400–500 lm) followed by equant spar (< 200 lmin size).

At the Tiout section, the topmost parts of someparasequences exhibit conical microbial patches(< 40 cm in diameter and up to 60 cm high) thatare easily recognizable because the contortedbeds lateral to these (which belong to the over-lying parasequence) are commonly absent due toselective weathering (Fig. 6F). Microbial patchescontain Renalcis, Epiphyton and other undeter-minable thromboid textures (Fig. 7A), andnumerous allochems such as oncoids and ooidslocally arranged into low-angle laminae. Archaeo-cyathans have not been observed.

The grainstones record high-energy water con-ditions probably above fair-weather wave base.The parasequences are interpreted as having formedas prograding shoals on an open-marine platformsuggesting deposition by storms and waves fromupper shoreface to foreshore that reworked ooids,pellets, oncolites, intraclasts and fossils. The sandshoals are arranged in asymmetric, coarsening-and thickening-upward parasequences, withscoured and erosive bases, and reworking ofunconsolidated to partly consolidated sedimentindicating the unstable character of the substrate.At the Amouslek section, the lowermost part ofparasequences lack distortion structures, and dis-play mudstones, < 10 cm thick that record calm-water conditions in offshore settings. The fossilrecord is unique in that it provides evidence forthe colonization of archaeocyathan and trilobitefauna. The setting of conical microbial patches atthe uppermost part of parasequences indicatesepisodes in which the shoal bedforms ceased tomigrate and were locally stabilized by microbialthromboids. Hence, episodes of decreasing energyin migrating shoals being subsequently floodedare indicated by the presence of epibenthicthromboid patches. Proaulopora debris did notform grain-supported fabrics within the shoalsbut constituted microbial bioaccumulations inhigh-energy environments. The possible bindingrole of Proaulopora was constrained by high-energy conditions, and the lack of self-supportedstructures and microbial biofilms capable ofstabilizing the sediment as pointed out in otherLower Cambrian deposits (Alvaro et al., 2000).

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Cryptic fissures

The lowermost part of parasequences describedin the peloidal rhythmites is locally crosscut by

several generations of high-angle fissures, micro-faults and cavities, forming an irregular networkthat stands out in contrast as irregular masses ofwhite and reddish limestone encased in brownish

Fig. 7. (A) Microbial boundstone rich in Epiphyton (e) and Renalcis (r) of the patch shown in Fig. 6F displaying aradial arrangement of thromboids. (B–I) Cryptic cavities of Tazemmourt; (B) erosive contact of breccia (right) with thebedded peloidal rhythmites; (C) detail of fractured contact with contorted and fractured stromatolites (s), fractures (f)and cemented fissures (cf); (D) stromatolitic infill of one fissure; (E) aspect of the brecciated infill with some fissures;(F) detail of previous fissure with centripetal infill composed of undetermined thromboids (t), Kundatia thalli (k),and central mosaic of calcite cements (c); (G) laminated oncoidal packstones and grainstones, bounded by laminatedbiofilms (arrowed); (H, I) transverse sections of Kundatia thalli.

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to black limestones. Individual fissures and cav-ities are irregular in shape and up to 1 m wide,whereas the microfault walls are straight and upto 1 cm wide (Fig. 7B–D). Most fissures andmicrofaults are steeply inclined, may bifurcateor anastomose, and commonly deviate signifi-cantly towards the stratification plane. Theycontain chaotic breccia reworked from the fissureand microfault walls, in some cases cementedand rebrecciated (Fig. 7C). The opening andbrecciation of fissures were sometimes followedby growth of cavity-dwelling microbialites on thesides of the fractures, and microspar precipita-tion. At least two distinct filling patterns arerecognized in the more complex cavities, andvary according to cavity width. The first patternrepresents complete infill of open fissures andcavities > 10 cm wide, whereas the second fillingpattern occurs in the thinnest microfaults. Theepisodic character of fracturing is evidenced byoverlapping and truncation patterns displayed bythe following four facies:

1 Stromatolitic crusts, up to 1 cm thick, areplanar to crinkled in shape, in some cases, abruptlychanging laterally into clotted-like textures. Theydirectly encrust fissures walls (Fig. 7D).2 Thromboid masses directly overlie the earli-

est generation of stromatolites. Included here aredistinct thromboid structures (such as Epiphyton,Girvanella, and Renalcis), and other unclassified,poorly preserved, microbial remains. Epiphytonand Renalcis are common in the first millimetresas pendent fringes and masses on roofs and walls,and are arranged as rounded to irregular patches,locally displaying a somewhat laminated struc-ture. They pass gradually centripetally into acomplex framework of undetermined thromboids.Open space within this framework is filled bymicropeloids (150 lm in size), quartz-rich coatedaggregates (up to 3 mm in size), and micrite.Some transverse and longitudinal sections ofKundatia tubes (Korde, 1971) occur dispersed inthe thromboid texture. These were interpreted byBuggisch & Flugel (1988) as encrusting red algaewith chain-like cells arranged along the axis ofthe thalli. Its distinct transverse sections are cir-cular, up to 300 lm in diameter, filled with sparrycalcite and surrounded by concentric micritichaloes including very small spar-filled cells. Ob-lique and transverse sections exhibit peripheralspine-like processes with < 20 lm inner dia-meters (Fig. 7E, F, H, I).3 Laminated peloidal/oncoidal packstones and

grainstones fill the remaining voids of the larger

decimetre-wide fissures and cavities. They havealternating fine- and coarse-grained laminaeoccurring as light to dark grey, millimetre-thickalternations (in which the angle of laminaebecomes greater with proximity to the cavitywalls), and common erosive contacts (Fig. 7G).The light laminae are peloidal-dominated grain-stones and packstones, and contain reworkedoncoids, coated intraclasts, lumps, terrigenousagglomerates, and uncoated dolomite wall-rockclasts, interbedded with isolated Girvanella andstratiform sheets. Relic wall particles are easilydistinguished by their rusty appearance, rangingfrom cobble to fine-grained sand. The most obviousterrigenous fraction is angular, very fine sand tosilt-size grains of quartz and feldspar, occurringboth as dispersed grains (comprising 10–30% ofthe matrix) and as subrounded agglomerates. Incontrast, the dark laminae (< 1 mm thick) areessentially stromatolitic crusts with subordinatepeloids and quartz silts cemented by microsparite.Dark laminae bind the oncolitic packstone andgrainstone sets and directly overlie irregular sur-faces. Rounding and moderate sorting of oncoidsand coated allochems filling the thicker fissuresindicate influence by traction currents that reflectthe hydrodynamic regime and carbonate pro-ductivity of the overlying seafloor. Evidence forepisodic microbial activity occurs as variablydistributed thromboid textures composed ofpeloids of clotted micrite, stromatolite crusts, andcoated allochems. The input of allochems wasepisodically interrupted as indicated by nuclea-tion of stromatolitic crusts on erosive cicatrices.4 Sparry cement linings are sometimes present

filling the remaining porosity, in some cases as aresult of rebrecciation. Sparry cements usuallyoccupy < 10% in volume. Crystals are fibrous andbladed, up to 100 · 250 lm in size.

Evidence of polyphase brecciation indicatesthat the fracturing process may have extendedover a considerable time span. Fissures andfractures crosscut fully rigid sediments that dis-play in situ brecciation. There is no evidence ofkarstic structures.

FACIES ASSOCIATIONS OF THE LOWERPART OF THE AMOUSLEK FORMATION

Calcareous quartz siltstones

Siltstones occur at the uppermost part (5–60 cmthick) of shale-dominated parasequences (up to

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20 m thick). Tops are composed of interbeddedsiltstones/very fine-grained sandstones and clay-stones, which are rich in quartz and feldspar, andsecondary pellets and peloidal aggregates. Sand-stones occur as 5–30 cm thick beds and lenses,some of which are channelized. Bedforms includetrough, parallel- and low-angle lamination, andwave-rippled laminae (mainly on bed tops). Thelaminae are commonly eroded by abundantinclined-to-subhorizontal truncation intra-sur-faces dipping at 5–15�. The truncation angle isgreater near the top of the beds. The contact withthe underlying and interbedded shales is abruptand erosive, and contains numerous ichnofossilssuch as the biostratigraphically significant ichno-species Psammichnites gigas (Fig. 8A; see Alvaro& Vizcaıno, 1999, for biostratigraphic precisions).Palaeocurrents taken from foresets are 110–130�N, whereas axis channels are 25–30�N.Shales are structureless except for the presenceof local centimetre-thick convolute beds.

The presence of wave-induced structures,small-scale sets of cross and parallel lamination,and channel features suggest episodes of migra-ting bedforms in a shoreface environment. Low-angle truncation surfaces represent reactivationsurfaces resulting from reworking of bedformsby currents. The lack of mud-draped laminaeand herringbone structures indicates theabsence of tidal influences. On the contrary, thepalaeocurrent pattern suggests the migration ofsuperimposed bedforms in an open-sea shoreface-dominated environment. The sedimentary fea-tures of the lower shale-dominated part of theseparasequences are subsequently described indetail in the open-shelf facies association.

Archaeocyathan-microbial reefs and peri-reefsediments

The term ‘reef’ is used in the following descrip-tion because the facies association is dominatedby peloidal and clotted fabrics rather than homo-geneous mud fabrics. The fabrics correspond to‘Frame Reefs’ (sensu Riding, 2002), in which in-place skeletons (including calcified microbes) arein contact and microbial clotted–peloidal fabricsare considered as microframe structures. Fourgeometries are distinguished: (i) patches (James &Kobluk, 1978) or isolated pillow-shaped massesranging in size from 0Æ2 to 2 m and 0Æ2 to 1 mhigh, surrounded by well-bedded to massiveshales; (ii) bioherms or dome- to lens-like massesof greater dimensions (but < 3Æ0 m thick), inwhich flank beds are well developed, and their

margins commonly intercalate with lithologicallyvariable inter-reef sediments; (iii) biostrome orlow-relief structures (up to 1Æ2 m thick) compris-ing tabular or sheet-like beds, more than 10 mwide; and (iv) patch-reef complexes, kalyptraecomplexes (sensu Rowland & Gangloff, 1988;Rowland & Shapiro, 2002), in which patchesand bioherms occur stacked together (> 10 mthick) bounded by clay–marl–silt, millimetre-to centimetre-thick seams and discontinuities(Fig. 8B–E). The term biostrome is used herealthough in Cumings’s (1932) original definition,a biostrome consists mainly or exclusively of theremains of organisms, and in the Kershaw’s(1994) concept of ‘parabiostrome’ the construct-ing organisms are < 20% in place.

Reef-core faciesReefs stand out in contrast as lenticular masses ofwhite, resistant limestones encased in thinlybedded limestones and massive green shales.Reef spacing is best observed along the Tioutsection, where modern differential erosion hasstipped away the soft inter-reef sediments result-ing in exposure of individual patches (Fig. 8D).Patches occur: (i) as single units (or individualreefs) surrounded by skeletal lime sand or homo-geneous shale, (ii) arranged into stacked patchesbecoming aggregated into bioherms and bio-stromes upwards, and (iii) a complicated arrayof patch reefs or a kalyptrate complex caused bynucleation of numerous patches in close proxim-ity. Where reefs are directly overlain by limesands, the contact is commonly erosive withsands mantling a surface of truncated and disori-ented archaeocyaths and surrounding lime mud.

Texturally, the reef-core facies consists ofarchaeocyathan–microbial boundstones and float-stones with local development of lenticularhomogeneous micrite. Archaeocyaths are con-spicuous contributors (see systematics in Debr-enne, 1964; Debrenne & Debrenne, 1995), but theyvary in abundance and some patch reefs appar-ently lack them. The calcified microbial commu-nity comprises Epihyton, Girvanella, Renalcis,and other unclassified thromboid textures(Fig. 8F–H). Their relative abundance varies fromreef to reef. Other fossil remains include trilo-bites, chancelloriids, hyoliths, hyolithelids,sponge spicules, tannuolinids and torellellids.Peloids are major contributors to core reef facies,particularly observed when accompanied bystructureless micrite. Peloidal textures consist ofpoorly defined, irregular silt- to sand-sizedpeloids of microcrystalline calcite set in a

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groundmass of calcite microspar. Peloidal matrixtextures grade into clotted micrite where peloidpacking increases. Homogeneous micrite occursin lenses (0Æ5–4 mm in diameter) or thin wavy,discontinuous laminae often inclined at variousangles to the inferred depositional bedding. Thelaminae are less fossiliferous than the envelopingpeloidal and clotted micrite fabrics. The mostobvious extraclasts are angular silts of quartz andfeldspars (the latter, in some cases, secondarilyreplaced by calcite), which make up 10–15% ofthe matrix. Clay minerals are present as flakes oras finely dispersed material between microsparcrystals.

Flank faciesFlank beds form the margin of bioherms and, lessso, the steep margins of patch reefs. They aregenerally thinly bedded (0Æ1–0Æ2 m) limestonesthat show depositional slopes. Two types of flankbeds are recognized over a distance of 1–10 m: (i)beds of reef facies that thin away from the core,passing laterally into debris beds or flat beddedinter-reef sediments; and (ii) debris beds formingapron deposits around the margin of individualbioherms and patches. Debris textures containcentimetre-sized clasts of reef-core facies com-mon in proximal positions. The skeletons andintraclasts are chaotically arranged and com-monly set in a matrix of poorly sorted packstoneand grainstone (with blocky cements up to 10 lmin size), the latter bearing dispersed Girvanellabundles and biodictyon (sensu Krumbein et al.,2003; Fig. 8I, J). Low-angle scour surfaces withinflank sediments attest to the episodic erosion ofthe reef surface that was responsible for gener-ating the intraclasts present in debris beds andproximal inter-reef sediments.

The dip of the flanks varies from 5� to 40�, sothat flanking-slope grainstones and packstonesattest to development of significant syndeposi-tional relief on some bioherms. As steep (up to40�) depositional slopes were developed, muchdebris was shed during intermittent erosive epi-sodes and deposition on flanks. Geopetal struc-tures indicate that some skeletons weresecondarily reworked.

Inter- and off-reef sedimentsInter-reef sediments are dominated by beddedcalcarenites that can extend out from the reefsand grade over < 3 m into nodular limestones,shales bearing centimetre-thick limestone nod-ules paralleling stratification, and ultimatelyclaystones. In the absence of calcarenites, thepatches and bioherms are directly surrounded byoff-reef green shales with isolated centimetre-thick carbonate nodules. Inter-reef calcarenitesand nodular limestones show local packstone andbioturbated wackestone textures. They contain adiverse fauna comprising reworked archaeo-cyaths, trilobites, brachiopods, hyoliths andchancelloriid sclerites. The nature of inter-reefsediments in each reef complex provides insightinto the prevailing water conditions. The domin-ant wackestones, nodules and thin beds interbed-ded with shales suggest deposition in quieterand/or deeper water environments with a minorsiliciclastic supply. Inter- and off-reef sedimentsare indicative of low-energy water environments,although minor scours at the base of the wacke-stones together with reef-related intraclasts (up to2 cm in size) suggest episodic reworking.

Size, spacing and distribution of reefs appear tohave controlled the nature of inter-reef sediments,which vary from wakestones, packstones to clay-stones. The variability in inter-reef sedimentrecords a wide range of hydrodynamic conditionsduring deposition. Individual reefs influencedthe local hydrodynamic regime, and were suffi-ciently coherent to withstand fairly turbulentconditions. However, they were subject to epi-sodic erosion as indicated by the abundance oflithified intraclasts in adjacent flank and inter-reef facies and erosive surfaces within the reefs.The sharp contact between reefs and their sur-rounding sediments, the high-angle nature ofmany of the flanks and their microfacies indicatean important early diagenetic consolidation. Thisrigidity is important because it permitted develop-ment of steep slopes and preserved the lenti-cular shape of patches and bioherms. In addition,the lack of bioturbation in the reef-core and flankfacies suggests that microbially controlled sub-strate consistency was a major control on the

Fig. 8. (A) Psammichnites gigas preserved as convex epireliefs in calcareous quartz siltstones at Tazemmourt; (B)slumped patches (right) at Tazemmourt; (C) isolated archaeocyathan–microbial patch surrounded by laminatedalternations of limestones and shales at Tiout; (D) bedding surface at Tiout showing the lateral distribution ofarchaeocyathan–microbial patches; (E) kalyptrate complex at Tiout with vertical arrangement of patches and bio-herms; (F) Epiphyton thromboid (right) encrusting an archaeocyathan wall; (G) Epiphyton (e) bushes and Renalcisclots (r); (H) Girvanella (g) and Renalcis thromboids; (I) alternations of bioclastic grainstone and eroded mudstone-wackestone, biohermal flank; (J) Girvanella-like filaments in a grainstone flank.

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composition of the reef biota. Trapping andbinding of sediment on the surface of the reefcores resulted in steep depositional slopes andflanks. The cores supported steep-sided erosivescours on their tops indicating that they couldwithstand deposition in a predominantly high-energy environment. The abundance of microbialactivity (recorded as thromboid textures, biodict-yon, and micritized grains and bioclasts), sheltercements and lack of bioturbation indicate abun-dant early lithification.

The reef and peri-reef sediments are character-istically rhythmic displaying individual para-sequences that, in some cases, can only becorrelated laterally to up to 50 m. Each shallow-ing-upward parasequence (5–20 m thick) compri-ses a basal shale unit that contains carbonatenodules that increase in number upwards. Thisfacies is overlain by nodular limestones, anduppermost patch reefs passing upwards intobioherms, biostromes and kalyptrae. Influx ofquartz and feldspar silt reflects exposure of theinterior platform source region. Slump structuresthat incorporate individual patches indicate theexistence of steep-sided reef morphologies andunstable, soft clay substrates (Fig. 8B). The lack oflateral continuity of the parasequences indicatesthat conditions for reef nucleation were notwidely developed probably due to differences inaccommodation space.

Open-shelf sediments

Green shales form the bulk of the unit and aredominated by fine quartz and angular feldspar silt(the latter locally up to 60% in volume), sericite,illite, and muscovite flakes. Fossil fauna is spor-adically abundant, preserved as articulated, dis-articulated and broken debris, and comprisestrilobites, echinoderm ossicles, brachiopods andhyoliths. Despite the massive aspect of the shales,their stratification is suggested by millimetre-thick, finely crinkled, silty laminae and thelayering of centimetre-thick carbonate nodules.Centimetre-thick, grey siltstone intercalationsexhibit erosive bases and grading. Centimetre-thick, limestone nodules contain bioclastic pack-stones–wackestones (changing upwards fromskeleton- to claystone-supported) that rest uponsharp erosive bases, which display commongutter marks. The fossil content is similar to thatdescribed in shales, but is predominantly frag-mented and highly abraded. Finally, scatteredsoft-sediment deformation structures also occurin many beds in the form of intraformational

slumping and sliding structures, up to 0Æ6 mthick, and lithologically similar to the underlyingbeds.

This facies association was deposited on off-shore substrates, under calm-water conditions asevidenced by the dominance of siliciclastic mudand episodic preservation of a partially arti-culated shelly fauna. The interbedded siltstonesand limestones were probably introduced as aresult of storm surges, as indicated by thepresence of erosive bases, grading and fragmen-ted skeletons. Thus, this is interpreted as anenvironment characterized by continuous deposi-tion below fair-weather conditions that waspunctuated by storm-induced episodes. Slump-ing and synsedimentary bedding distortion ofthe substrate resulted in local sliding and plasticsediment masses.

DEPOSITIONAL TRENDS

According to a proximal-to-distal facies distribu-tion and the Cambrian palaeogeographic recon-structions proposed by Destombes et al. (1985)and Geyer et al. (1995) for the Souss Basin,relatively proximal facies occur at the Tioutsection, whereas distal sediments are distributedalong a transect between the Tazemmourt and theAmouslek sections. The pattern of depositionacross the Tiout/Amouslek lithostratigraphictransition is punctuated by two major stratigraphicdiscontinuities (D1 to D2 in Fig. 4).

Discontinuity D1 is located at the top of theabove-reported peloidal rhythmites and stroma-tolitic reefs (Tiout Member). At the Tiout section,this sharp erosive contact marks the base of theEofallotaspis trilobite zone. This contrast repre-sents a distinct facies shift from tabular peloidalrhtyhmites changing laterally into coeval peri-tidal stromatolitic reefs to an intra-shelf rampwith ooid/bioclastic shoals. Discontinuity D2 isplaced at the Tiout/Amouslek lithostratigraphiccontact (according to the amended lithostrati-graphy), and appears as a rectilinear surface oferosion with small-scale irregularities. D2 marksthe bottom of the Eofallotaspis zone outside theTiout section, and is overlain by the transgressiveAmouslek Formation, characterized by siliciclas-tic input and open-sea offshore conditions. Bothdiscontinuities show a widespread distributionin the whole Anti-Atlas, reflected by the commoninclusion of the Tiout Member in recent geologi-cal maps (Chalot-Prat et al., 2001; Gasquet et al.,2001).

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The facies distribution of the Tiout/Amousleklithostratigraphic transition throughout the wholewestern Anti-Atlas can be related to three dis-continuity-bounded facies associations that arearranged in a broad retrogradational (transgres-sive) trend. This succession can be subdividedinto vertically stacked, asymmetrical para-sequences exhibiting either conformable orerosive contacts. The peloidal–stromatolite sedi-mentary system is composed of successive shal-lowing-upward parasequences (0Æ1–1Æ2 m thick),which include the aforementioned stromatoliticreefs and peloidal rythmites. The bases of theparasequences are flooding surfaces, whereastheir tops are bounded by minor discontinuities(tp in Fig. 4). Discontinuities are directly overlainby reworked intraclasts, microfaulting and minorslumps. Discontinuity D1 marks a sharp sedimen-tologic change, as it marks the end of a stroma-tolite-dominated system and the establishment ofopen-sea conditions related to the immigration ofshelly fossils. Parasequences (0Æ5–4Æ6 m thick) inthe stratigraphic level are dominated by ooid-dominated grainstones (shoreface shoals) thatcontain the first biostratigraphically significantshelly fossils (Sdzuy, 1978). D2 marks the begin-ning of a broad transgression of the platform,recorded by shallowing-upward parasequences(up to 20 m thick) of the Amouslek Formation.These, in turn, are topped by either calcareousquart siltstones or archaeocyathan–microbial reefsystems. The rhythmic sedimentation of theAmouslek Formation represents a gradual deep-ening of the platform culminating with floodingand blanketing of the whole platform by fine-grained terrigenous material.

One important implication of this analysis is thecharacter of the base of the Eofallotaspis zone.This interval is properly identifiable at the Tioutsection overlying discontinuity D1, but it is dia-chronously correlatable in other outcrops over-lying discontinuity D2 as a result of sampling biases.

FACTORS CONTROLLINGREPLACEMENT OF REEF-BUILDINGCONSORTIA

The aforementioned facies spatial patterns arerelated to the successive establishment of threemain reef-building benthic communities (Fig. 9).The oldest reef-building community is the frame-building stromatolite-dominated system. Theyoungest stromatolite-thrombolite consortiumthat dominated the western Souss platform

across the Neoproterozoic–Cambrian transition(Adoudou, Lie-de-vin and lower member of theIgoudine formations) is recorded in the studiedarea by stromatolite reefs flourishing on a peri-tidal platform that record repeated substrateinstability. A distinct microbial community epi-sodically developed within cryptic fissures,cracks and microfaults located at the lowermostpart of the parasequences reported in the TioutMember. Fissures provided habitats for pioneercoelobiontic organisms that directly encrustedcavity walls and roofs, even after their reactiva-tion by synsedimentary fracturing. The coelo-biontic community was relatively diverse andconsisted of encrusting, self-supported chemo-synthetic–heterotrophic microbial communities,including stromatolitic crusts, Renalcis–Epiphy-ton intergrowths, and Kundatia algae(?) (the algalaffinity of the latter is open to discussion due toits presence in cryptic fissures). The multiplegenerations of high-angle fractures indicate abrittle deformation at the onset of cavity fillings,suggesting that the polyphase filling of cavitieswas controlled by displacements on nearbymicrofaults. The aforementioned penecontempo-raneous fissures, breccias, crackage and micro-faults can be developed in response to overalldifferential subsidence. Their setting seems to befacies- and parasequence-controlled, and doesnot require any regional tectonism. One distinctfactor that controlled the infill of larger cavitieswas local hydrodynamic conditions determinedin part by surrounding submarine topography.The main phase of hydrodynamic filling isassociated with reworked ‘oncolitized’ extra-and intraclasts washed in from an overlying,unpreserved substrate. Crucial prerequisites formicrobial colonization within the thinnest(< 1 cm wide) and deepest fissures include thepresence of water, and the potential of thehabitats to support microbial growth and proli-feration related to chemoautotrophic microbialmats. These mats have yielded a rich supply oforganic compounds to nourish possible hetero-trophs. The microbial coelobiontic communitiesdescribed above are relatively diversified ifcompared with other Lower Cambrian meta-zoan-rich coelobiontic communities (Kobluk,1988; Vennin et al., 2003).

The second stage of reef-building benthic com-munities was the frame-building pioneer thrombo-lite–archaeocyathan consortium. Overlying D1, ashoal belt dominated by cross-bedded grainstonesdeveloped locally proximal to the transect of theAmouslek–Tiout–Tazemmourt sections. These

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shoals were adjacent to a shelly benthic commu-nity (probably reefal in character) represented byreworked (allochthonous) archaeocyathans andtrilobites. This biotic assemblage represents theoldest biostratigraphically significant Early Cam-brian shelly fossils from Morocco. Thromboid-rich patches occur on the top of ooid-dominatedparasequences, reflecting episodes of decreasinghydrodynamic energy over migrating shoals, andepibenthic thromboid colonization of relativestable substrates. By contrast, the shoals areevidence of development of ineffectual-builder

biofilm relics (oncoids), deposition of microbial(Proaulopora) bioaccumulations and reworkedarchaeocyaths. Archaeocyaths, not preserved inlife position, could colonize either the conicalthromboid patches or laterally equivalent, back-shoal, trombolite–archaeocyathan reefs.

The final stage of frame-building benthic com-munities is the thrombolite–archaeocyathan con-sortium. Subsequent transgression resulted indeposition of fine-grained siliciclastic materialand the immigration of open-sea shelly faunas inthe whole western Souss platform. The change in

Fig. 9. Evolution of reef-building consortia during Early Atdabanian times in the western Souss platform (not toscale).

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habitat seems to have supported a broad migra-tion of shelly fauna onto the whole western Soussplatform, and the final collapse of flat-laminatedmicrobial mat growth. The fine peloidal andclotted nature of the reef micrite is apparently atvariance with the prevailing moderate to highenergies indicated by well-sorted, cross-bedded,inter-reef lithologies, suggesting some stabilizingbiotic influence, as well as early cementation. Themicrobial communities probably formed a matacross the surface of both reef cores that trappedand bound the fine skeletal debris washed ontothe surface, and their flanks. Rapid cementation,microbial binding and hence internal stabiliza-tion of reef fabrics would have prevented com-paction and promoted steep depositional dips.The reported reef parasequences of the AmouslekFormation indicate shallowing-upward trendsreaching fair-weather base, as suggested by theassociated intraclasts and sedimentary structures.

The vertical architecture of the overlyingAmouslek Formation is the result of variationsin relative sea level leading to the establishment ofnucleation sites for the succeeding benthic com-munity composed of archaeocyathan–microbialreefs. The carbonate platform recorded develop-ment of archaeocyathan–microbial reefs that cov-ered, with their detritus, increasingly expandingsectors of the seafloor. The associated transgres-sive trend accompanied dramatic biotic radiationby increasing habitat size, and allowing shellymetazoans (including frame-building organisms)to invade platform interiors.

Reef geometries were influenced by substratestability, water depth, subsidence rate, and localhydrodynamic conditions. In areas of relativelyshallow water and slow subsidence rates, reefsgrew by lateral accretion and amalgamationdeveloping low-relief biostromes (i.e. patchesare always overlain by biostromes). In areas ofhigher subsidence, vertical growth of reefs wasless constrained, and they developed predomin-antly by vertical accretion as bioherms. Becauseof a marked tendency for aggregation and conse-quent formation of patch complexes or kalyptraein the Amouslek Formation, the dimensions oftheir individual patches and bioherms are oftendifficult to determine.

CONCLUSIONS

The Tiout–Amouslek lithostratigraphic transition(early Atdabanian in age) of the western Anti-Atlasrecords one of the most significant Lower

Cambrian benthic communities related to theCambrian explosion. When shelly metazoansarrived in the western Souss endemic platform,they proliferated to the detriment of the stromato-lite-dominated frame-building community thatflourished across the underlying Adoudou, Lie-de-vin formations and the lower member of theIgoudine Formation. The Early Atdabanian trans-gression led to habitat destruction and environ-mental perturbation. The invasion of shell taxa canbe viewed as a primary result of this transgressionleading to the establishment of a reef fringe onthis part of the western Gondwana margin. Threestages of reef-building microbial-dominatedbenthic communities are distinguished, from stro-matolite-dominated to thrombolite–archaeocya-than consortia. The latter records the abruptcollapse of stromatolite growth.

An archaeocyathan–thrombolite consortiumdisplaced the stromatolite-frame-building assem-blages in the Siberian platform-reef settings at thebase of the Tommotian. Although this metazoan–calcimicrobe reef-building consortium did notrange beyond the Siberian platform during theTommotian (Zhuravlev, 1986), during the Atda-banian and Botoman the archaeocyathan–throm-boid reef-building consortium spread from theSiberian platform to sites along the Gondwanamargin. Across the Atdabanian-Botomian trans-ition the archaeocyathan–thromboid reef-build-ing consortium had spread to North America andAustralia (Rowland & Shapiro, 2002). In thewestern Souss platform, the immigration of Edi-acaran and pre-Atdabanian early shelly fossils isnot recorded due to ecological restricted condi-tions that disappeared associated with the estab-lishment of the Atdabanian stratigraphicdiscontinuity D1.

In summary, the stratigraphically abruptappearance of the earliest Cambrian shelly faunawithin the Tiout shoals reflects changes in thesedimentary patterns of the western Souss plat-form. The previous western Souss platformprobably had poor circulation and restrictedexchange with surrounding basinal areas,whereas the Tiout shoals record a depositionalsetting open to oceanic water circulation, prob-ably adjacent to an oceanic basin. Differentialsubsidence of the carbonate platform and trans-gression may have enabled improved water cir-culation over carbonate platform interiors in theAtdabanian. Conditions over the carbonate plat-form may therefore have been more open marineduring the Early Atdabanian allowing the suc-ceeding microbial and archaeocyathan–microbial

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reefs to colonize shelf-interior settings. Episodicinstability of the seafloor, which may be tecton-ically induced, favoured the growth of coelobi-ontic communities below the Tiout/Amousleklithostratigraphic contact and archaeocyathan–microbial reefs in the overlying AmouslekFormation.

ACKNOWLEDGEMENTS

The authors warmly acknowledge the construct-ive criticism made by I. Montanez, R. Riding andan anonymous referee, who have greatly im-proved the ideas expressed in the manuscript,and the advice and comments made by J. Des-tombes, J. M. Rouchy and E. Vennin. Theoutcrops were visited by the ISCS in 1995.Research has been supported by ECLIPSE project‘Evolution of Biogeochemical Cycles from Arche-an to Recent Environments’, and is a contributionto IGCP project 485 ‘Cratons, Metacratons andMobile Belts: Keys from the West African CratonBoundaries, Eburnian versus Pan-African Signa-ture, Magmatic, Tectonic and Metallogenic Impli-cations’.

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