Facies distribution and coral-microbialite reef development on a low-energy carbonate ramp (Chay Peninsula, Kimmeridgian, western France)

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Sedimentary Geology 205 (2008) 14 – 33 www.elsevier.com/locate/sedgeo

Facies distribution and coral-microbialite reef development on a low-energy carbonate ramp (Chay Peninsula, Kimmeridgian, western France) Nicolas Olivier a,⁎, Bernard Pittet a , Winfried Werner b , Pierre Hantzpergue a , Christian Gaillard a a

UMR 5125 PEPS, CNRS, France; Université de Lyon, Université Lyon 1, Campus de la DOUA, Bâtiment Géode, 69622 Villeurbanne Cedex, France b Bayerische Staatssammlung fürPaläontologie und Geologie and GeoBio-CenterLMU, Richard-Wagner-Straße 10, D-80333 München, Germany Received 20 June 2007; received in revised form 17 December 2007; accepted 20 December 2007

Abstract The Chay Peninsula (western France) succession corresponds to a Kimmeridgian shallow and low-energy ramp system that developed in the northeast of the Aquitaine Basin. The sedimentation shows a transition from a carbonate-dominated to a mixed carbonate–siliciclastic depositional environment that takes place at the base of a major, third-order sea-level transgression. Facies partitioning and coral-microbialite reef levels (i.e., reef windows) suggest higher-frequency (fourth-order) sea-level oscillations. Reef-growth phases (i.e., high-frequency reef windows) are interpreted to have been controlled by fifth-order sea-level oscillations. In the mixed siliciclastic–carbonate system, successive prograding units of shoreface sediments pass into thick marl–limestone alternations over a distance of tens of metres. Such remarkable short-distance lateral facies variations suggest a complex sedimentary system made of juxtaposed deposits that rapidly shift through time. Terrigenous fluxes, in modifying the light intensity and the nutrient level in the water column strongly controlled the facies distribution and notably the reef composition and growth. Identified coral genera suggest mixed photo-heterotrophic mode of nutrition and low-mesotrophic conditions in marine waters. For each reefgrowth phase, nutrient increase favoured large amounts of microbialites to develop, forming a crust that entirely capped the coral reef surface. Thus, high-frequency fluctuations in nutrient input in tune with fifth- (or higher) order sea-level fluctuations controlled the reef development at a millennial scale. The Chay Peninsula reef distribution and development provide insight into understanding the response of reef communities under climatically-induced environmental changes. © 2008 Elsevier B.V. All rights reserved. Keywords: Ramp; Carbonates; Siliciclastics; Coral-microbialite reefs; Sea-level changes; Nutrient; Kimmeridgian

1. Introduction Shallow shelves were widespread during the Late Jurassic along the northern Tethyan shelf (Leinfelder, 1993; Kiessling, 2001). In these depositional settings, reefs exhibit a broad range of dimension, structure and composition (Leinfelder, 2001). These bioconstructions are particularly sensitive archives for palaeoecological and palaeoenvironmental reconstructions (e.g., Bertling and Insalaco, 1998; Helm and Schülke, 1998; Olivier et al., 2004). In mixed carbonate–siliciclastic ramp systems, the nature of carbonates and their lateral and stratigraphic extents are related to sea-level fluctuations, climate and tectonics, which in

⁎ Corresponding author. E-mail address: [email protected] (N. Olivier). 0037-0738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2007.12.011

turn controlled water depth, incidence of light, wave energy, sedimentation rate, siliciclastic and nutrient inputs. Geometric and sedimentary evolution of reef systems have been already documented across large-scale (pluri-kilometric) Upper Jurassic carbonate ramps (e.g., Bádenas et al., 2005; Benito and Mas, 2006). However, examples of sedimentary reef systems subject to important lateral and stratigraphical facies changes over few tens to hundreds of metres are rare (e.g., Samankassou et al., 2003). Additional examples are necessary to deepening understanding of the response of reef communities under high-frequency palaeoenvironmental change (Hallock, 2005). During the Early Kimmeridgian, the northeastern part of the Aquitaine Basin is marked by the development of the shallow La Rochelle and Angoumois platforms (Fig. 1). Along these platforms, local tectonic movements related to the opening of the young Atlantic Ocean individualized gently southwestern-

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Fig. 1. (A, B) Palaeogeography of the study area during the Upper Kimmeridgian (after Thierry, 2000; Hantzpergue and Maire, 1981). (C) Correlation of the Lower Kimmeridgian La Rochelle platform sections along a proximal-distal transect. La Rochelle and St Jean d'Angely sections are redrawn after Hantzpergue (1988). Rochefort and Jonsac sections were obtained from core drilling (Hantzpergue, unpublished data). Sequence stratigraphic interpretation based on Jacquin et al. (1998) and regional discontinuities (D1–D9) on Hantzpergue (1985b).

sloping epicontinental ramps upon which shallow coral reefs developed (Hantzpergue, 1985a). On the La Rochelle shelf, the present-day Chay Peninsula cliffs reveal a coral-microbialite reef system that developed on a low-energy ramp, either in a carbonate-dominated or a mixed carbonate–siliciclastic setting.

Excellent outcrop conditions across a km-scaled profile allow the study of lateral and vertical facies variations. A depositional model of facies is proposed to explain the rapid lateral facies variations observed in the study area. High-frequency sea-level oscillations and terrigenous and nutrient inputs emerge as major

16 N. Olivier et al. / Sedimentary Geology 205 (2008) 14–33 Fig. 2. Cross-section of the Lower Kimmeridgian succession of the Chay Peninsula showing variations of facies and interpretation of facies and geometries in term of third- and fourth-order depositional sequences.

Table 1 Description of the main facies types observed in the Chay Peninsula succession and their interpretation in term of depositional environment Description

Fabric (Dunham, 1962)

Main components (this work; Soutoul, 1971; Hantzpergue, 1991; David, 1998; Vadet et al., 2002)

Sedimentary structures

F1

Well-bedded wackestones

W (−N P)

Organisms: endobenthic (e.g. Pholadomya, Ceratomya), epibyssate (e.g. pectinids, limids) and cementing (oysters) bivalves; gastropods (Natica rupellensis), rare regular echinoids (e.g. Rolliericidaris, Paracidaris), bryozoans, polychaetes (serpulids, Terebella), agglutinating foraminifera (Alveosepta, Lenticulina), dasycladalean (Goniolina); and rare nautiloids

Well-bedded stratification (beds ≤40 cm-thick) Distal tempestites (small HCS)

F2

Argillaceous mudstones

M (−N W)

Common angular small skeletal components; organisms: bivalves (e.g. oysters), gastropods, serpulids, few regular echinoids (e.g. Nenoticidaris, Pedina), rare crinoids (Millericrinus, Angulocrinus), and rare ammonites (Ardescia pseudolictor) and nautiloid (Paracenoceras giganteum)

Well-bedded stratification (beds ≤ 20 cm-thick) Distal tempestites (small HCS)

F3

Oo-biomicrite wackestones

W

Subangular to rounded skeletal components; organisms: bivalves (e.g. oysters), gastropods, regular echinoids (Paracidaris, Pseudodiadema), microencrusters (e.g. Bacinella, Cayeuxia); and ooids of radial structure (type 3 of Strasser, 1986)

F4

Bio-oomicrite packstones

P (−N G)

Skeletal components, frequently fragmented, reworked and bioeroded; organisms: regular echinoids (Paracidaris, Pseudodiadema), bivalves, gastropods, soltary corals (Montlivaltia), algae (e.g. Solenopora), micorencrusters (Bacinella), peloids, and micritic ooids with a thin radial lamina (type 3 of Strasser, 1986)

F5

Brachiopod-rich packstones

P (−N G)

Well-preserved (or angular fragments) skeletal components; organisms: terebratulid (Terabratula subsella, T. equestris) and rhynchonellid (Rhynchonella inconstans) brachiopods, epibenthic cementing bivalves (e.g. Lopha, Nanogyra), diverse regular (e.g. Holectypus, Hemicidaris) and few irregular echinoids (e.g. Polydiadema), crinoids (Angulocrinus, Apiocrinus), serpulids, fixo-sessile foraminifera, green algae, and ammonites (e.g. Ardescia pseudolictor, Ataxioceras sp.)

Low-angle stratification (to the South) Topped-lap surface, encrusted and boeroded

F6

Trichites-rich packstones

P (−N W/G)

Well-preserved (or angular fragments) skeletal components; organisms: abundance of semi-infaunal (Trichites saussurei, frequently articulated) and epibenthic cementing (Nanogyra, Lopha) bivalves, some epibyssate (e.g. Arcomytilus) and infaunal (e.g. Ceratomya) bivalves, few corals (branched Calamophylliopsis, reworked and bioeroded massive Microsolena), highly diverse echinoid fauna (regular and irregular forms, e.g. Nenoticidaris, Pedina, Holectypus), crinoids (only in the upper unit: Angulocrinus, Apiocrinus, Millericrinus), calcareous sponges, serpulids, and agglutinated foraminifera

Low-angle stratification (to the South) Local current ripples

F7

Bioclastic packstones

P (−N G)

Subangular to rounded skeletal components; organisms: epibenthic fixo-sessile and rare endobenthic bivalves, terebratulid and rhynchonellid brachiopods, few gastropods, frequent crinoids (Angulocrinus, Millericrinus, Isocrinus), highly diverse echinoid fauna (only regular forms; e.g. Balanocidaris, Pseudocidaris, Acrocidaris); and coral fragments encrusted by microbialites

Poorly-bedded limestones

F8

Marl–limestone alternations

m/W (−N P)

Limestones: echinoids (e.g. Balanocidaris, Paracidaris), crinoids (Angulocrinus, Millericrinus, Guettardicrinus), bivalves (e.g. oysters), gastropods, corals (Calamophylliopsis), and Thalassinoides Marls: shallow and burrowing bivalves (e.g. Protocardia, Pholadomya, Ceratomya); and crinoids (e.g. Guettardicrinus, Apiocrinus, Millericrinus, Isocrinus)

Distal tempestites (small HCS)

F9

Coral-microbialite reefs

Fr

Branched and massive corals, high amount of microbialites, highly diverse encruster assemblage (e.g. Lithocodium, Bacinella, Troglotella, Thaumatoporella; Terebella, Tubiphytes, nubeculariids, serpulids, Bullopora), epibenthic and boring bivalves, reef dwelling regular echinoids, and few gastropods

Well-stratified bioherms (i.e. with numerous surfaces of reef-growth interruption)

Depositional environment

Laterally reef levels

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Facies type

Texture is after Dunham (1962) and Embry and Klovan (1971).

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controlling factors onto facies evolution and development of coral-microbialite reefs. 2. Geological setting The Kimmeridgian series of the northern Aquitaine Basin suggest the existence of a shallow carbonate shelf that joined the Armorican Massif in the northeast and connected southeast to the Tethys (Fig. 1A; Enay et al., 1980). Late Jurassic palaeogeography was probably controlled by reactivated Hercynian south-Armorican faults, which permitted the installation of shallow coral reefs on local topographic highs (Fig. 1B; Hantzpergue, 1985a). The study area located about 10 km south of La Rochelle (Charente-Maritime, France) corresponds to two adjacent outcrops in the west (Barbette, Chay, and Belette Points) and south (Chirats Point) of the Chay Peninsula (Fig. 2). These outcrops form two continuous cliffs, about 1 km long and 3 to 9 m high, where coral reefs and adjacent facies are freshly exposed due to continuous marine erosion. The Chay Peninsula section was first illustrated by d'Orbigny (1852) for his stratotype of the Corallian. This section and its three reef levels are now dated as Lower Kimmeridgian in age (Cymodoce Zone; Hantzpergue, 1991). The Kimmeridgian was a period of rising global sea level, and the Chay Peninsula section is included in the Boreal secondorder transgressive cycle T9a of Western Europe Basins (Jacquin et al., 1998). The second reef level is truncated by a major discontinuity surface (D4 of Hantzpergue, 1985b), which corresponds to the third-order transgressive surface between sequence boundaries K3 and K4 (Fig. 1C; Hantzpergue, 1995; Jacquin et al., 1998). 3. Methods The present work consists of a detailed analysis of reefs in relation to the nature and architecture of laterally equivalent deposits. The lateral and vertical facies evolution was directly followed along cliff outcrops. The whole section cannot be observed in one part of the cliffs, but a gentle dip (less than 5°) to the southwest and two faults (f1 and f2) allow the observation of a stratigraphic succession of about 18 m thick (Fig. 2). Nine closely spaced sections were logged along the Chay and Chirats cliffs in order to illustrate the lateral variations in thickness and facies. Facies associations were distinguished on the basis of facies characteristics and geometries. They were correlated from section to section following key stratigraphic surfaces. Stratal geometry observed along differently oriented outcrops allowed determination of the palaeoslope. Facies analysis, sedimentary structures and bedding orientation were studied in the field. Microfacies analysis was performed on 73 thin sections. Coral-microbialite reefs of the Chay Peninsula have been already studied in a previous paper that focused on microbialite morpho-structures and their constructional role (Olivier et al., 2003). For the three reef levels, mapping of bioconstructions was carried out to determine their morphologies and structures (i.e., stratigraphic growth phases). A point-counting method using a 20 cm × 20 cm grid of 100 points was used on randomly

scattered reef surfaces in order to establish the percentage covered by the various components (i.e., microbialites, corals, sediments, and others). Coral and microbialite samples collected in the reef levels were located in the field. Details of the relationship between primary framebuilders (i.e., corals) and the encrusting succession (i.e., microbialites and microencrusters) were studied in the field and in laboratory using polished-glazed slabs and thin sections. As large amounts of microbialite crusts make direct coral identification in the field difficult, 358 samples were analyzed in the laboratory with transverse and longitudinal polished slabs and thin sections. Semi-quantitative data on microencruster composition was obtained by the analysis of 79 thin sections. 4. General geometries The nine sections are distributed along an approximately N–S transect, more or less parallel to the coastline (Fig. 2). The 18 m thick succession is subdivided into a lower carbonate-dominated and an upper mixed carbonate–siliciclastic interval. Nine facies associations have been recognized and are briefly described and interpreted below. Facies 1 to 8 correspond to non-framebuilding deposits and facies 9 corresponds to coral-microbialite reefs (Table 1). General geometries of the Chay Peninsula deposits are based on key stratigraphic surfaces that correspond to downlap, toplap and erosional surfaces due either to subaerial exposure or ravinement during transgression (Fig. 2). The encrusted and bioeroded surface S1 has possibly eroded the top of bioconstructions of the first reef level (RL1), which clearly shows a progradational pattern in its upper part (Fig. 3A and B). The surface S2 truncates the second reef level (RL2) and the contemporaneous brachiopod-rich limestone unit (Fig. 3E and F). This unit shows sparse low-angle stratification with toplap and downlap surfaces, suggesting first progradation on the previously deposited argillaceous limestones and then erosion of its upper part. The surface S3 marks the transition between a first limestone unit rich in bivalves of the genus Trichites and marl–limestone deposits. This surface overlies an additional erosional and ferruginous surface S3b that truncated branched corals within the first limestone unit rich in Trichites (Fig. 2). Although clear low-angle stratification is barely visible, this unit becomes progressively thinner toward the south, pinching out into marl-rich deposits (Fig. 4). A second Trichites-rich limestone unit also progressively becomes thinner to the south over a very short distance (Fig. 5). In its lower part, this second unit laterally passes over a few metres into marl–limestone alternations (Fig. 4). In its upper part, this second Trichites-rich limestone unit reveals well-marked downlaps on marl–limestone alternations and is capped by a toplap surface, suggesting that the surface S4 was erosional (Fig. 5). All these observations emphasize a prograding geometry towards the south for the first and second limestone units rich in Trichites. On the other hand, marl–limestone alternations display a retrograding geometry on the surfaces S2 and S3 (Fig. 2). Concerning the bioclastic limestones contemporaneous to the third reef level (RL3), they are more than 3 m thick between the Chay and Belette Points, whereas they are only 70 cm-thick along the Chirats

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Fig. 3. (A, B) Main bioconstruction of the first reef level. This coral-microbialite reef is about 50 m wide and 5–6 m high. Note the prograding pattern to the southeast of the reef onto the well-bedded wackestone facies (F1). (C, D) Close-up of the bioconstruction shown in (A). Reef architecture can be subdivided into successive reefgrowth phases delimited by surfaces of reef-growth interruption. Note the large (dm- to m-scaled) intra-reef cavities. S1: erosional surface. (E, F) Bioconstruction of the second reef level. This bioconstruction is about 10 m wide and less than 2 m thick and shows a prograding trend to the southeast. It is capped by a horizontal erosional surface (S2).

cliffs. Even if no clear low-angle stratification could have been observed, such thinning toward the south emphasizes a prograding trend on marl–limestone alternations comparable to that of the Trichites-rich limestone units. The surface S5 is clearly visible

along the Chirats cliffs, where it corresponds to a thick crust with abundant oysters covering reefs of the third reef level. This possible transgressive surface becomes less marked between the Belette and Chay Points where it is included within the

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Fig. 4. (A, B) Panorama of the Trichites-rich facies (F6) at the north of the fault f2. This facies can be subdivided into two main units separated by a marlier (marl– limestone alternation facies F8). Note the rapid (few tens of metres) lateral facies variation between the lower part of the Trichites-rich upper unit and marl–limestone alternations (F8). This lower part of the Trichites-rich upper unit also becomes progressively thinner toward the south (black arrows). S2–4: erosional surfaces.

bioconstructions of the third reef level. At the Chirat Point, the top of the section corresponds to marl–limestone alternations suggesting a new retrograding pattern of this facies toward the north. 5. Facies associations 5.1. Non-framebuilt facies 5.1.1. Facies 1: well-bedded wackestones This facies consists of light-to-medium grey, well-bedded mudstones to wackestones, with local packstones (Figs. 3 and 6A).

Stratal geometry is subtabular with beds up to 40 cm-thick, some of which alternate with thin marly intervals. Skeletal fragments are generally moderately rounded (Table 1). These sediments are interstratified with cm- to dm-thick, fine-grained packstones showing hummocky cross stratification (HCS; Fig. 7A). The facies exhibits a moderately diverse benthic fossil assemblage. The bivalve fauna consists mainly of endobenthic (e.g., Pholadomya, Ceratomya, and Trigonia), byssate (e.g., pectinids and limids), and cementing oyster taxa reflecting a muddy, but relatively consolidated and stable substrate. Biogenic skeletal elements were overgrown by a diverse encrusting fauna of oysters (e.g.,

Fig. 5. (A, B) Panorama of the Trichites-rich facies (F6) at the south of the fault f2. The upper unit of the Trichites-rich facies (F6) clearly shows a prograding geometry to the southeast onto marl–limestone alternations (F8). The upper part of this unit pinches out in few tens of metres onto the downlap surface (S4b) and is capped by the erosional surface S5. Note that reef bodies largely perforate the underlying sediments. The northeastern part of this panorama is visible in the Fig. 4. S3: flooding surface (see also Fig. 2).

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Fig. 6. Main facies types. (A) Facies 1: well-bedded wackestones; (B) Facies 3: oo-biomicritic wackestones; (C) Facies 4: bio-oomicritic packstones; (D) Facies 5: brachiopod-rich packstones; (E) Facies 6: Trichites-rich packstones; (F) Facies 7: bioclastic packstones; (G) Facies 8: calcareous bed of a marl–limestone alternations; (H) Facies 2: argillaceous mudstones.

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Fig. 7. Hummocky-cross stratification in (A) the well-bedded wackestone facies F1, laterally to the first reef level; in (B) a calcareous bed of a marl–limestone alternation (F8), above the third reef level along the Chirats cliffs.

Nanogyra), bryozoans, polychaetes, and agglutinating foraminifers. Vagile epibenthos is represented by a few gastropods (Natica rupellensis) and echinoids. Remarkable is the occurrence of the dasycladacean green alga Goniolina which points to shallow water conditions. Lateral to this facies bioconstructions of the first reef level occur (Figs. 2 and 3). This facies type is interpreted to have been deposited in the upper offshore environments of a mid ramp setting, located below the fair weather wave base (Fig. 8A). The preservation of articulated fossils does not imply strong physical abrasion, reflecting an autochthonous to parautochthonous assemblage. Only short high-energy events interrupted the generally lowenergy conditions, allowing mud deposition and preservation.

5.1.2. Facies 2: argillaceous mudstones Light-grey to blue argillaceous mudstones, to locally wackestones, constitute this facies (Fig. 6H). Stratal geometry is subtabular with thin beds (up to 20 cm-thick). Skeletal components (e.g., bivalves and echinoids) are common and fragmented with angular sections (Table 1). Common cm-thick and fine-grained storm deposits showing HCS occur. Bioturbation is represented by large Thalassinoides. These deposits laterally correspond to the lower part of the bioconstructions of the second reef level (Fig. 3E and F). Similarly to Facies 1, Facies 2 represents an upper offshore environment of a mid ramp setting, located above the storm wave base (Fig. 8A).

Fig. 8. Facies models for (A) depositional sequences 1 and 2 and (B) depositional sequence 3. Bioconstructions (F9) of the third reef level only occur in the top of the depositional sequence 3, contemporaneously to the bioclastic packstones (F7).

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5.1.3. Facies 3: oo-biomicrite wackestones Nodular and cm-thick beds of oo-biomicrite wackestones characterize this facies (Fig. 6B). Abundant ooids with a radial structure (type 3 of Strasser, 1986) are associated with various skeletal components (e.g., bivalves, gastropods, and echinoids) that are moderately rounded (Table 1). Bioturbation is well developed. Stratigraphically, this facies gradually passes into facies 2. Facies 3 is interpreted to have been deposited in an intermittently agitated inner ramp (Fig. 8A). The proximity of an oolitic bar is suggested by abundant ooids that were transported into a quiet-water upper offshore environment, where the deposition of mud together with allochems was possible. 5.1.4. Facies 4: bio-oomicrite packstones This facies corresponds to bio-oomicritic packstones (Fig. 6C). These deposits are organized in subtabular beds (50–60 cm-thick) which are strongly bioturbated. Skeletal components are commonly fragmented and bioeroded (Table 1). Peloids are frequent and irregular micritic ooids with a fineradial lamina are common. The upper part of the first reef level laterally passes into this facies (Fig. 3A and B). Light-dependent organisms (e.g., Bacinella) suggest clear and oligotrophic waters. The ooids (type 3 of Strasser, 1986) are defined as occurring in quiet-water environments with intermittent high energy. Well-rounded skeletal fragments of variable size give further support to a certain degree of energy, suggesting an inner-ramp setting located above fair weather wave base (shoreface; Fig. 8A). 5.1.5. Facies 5: brachiopod-rich packstones This facies consists of a dm-thick level of packstones (locally grainstones), extremely rich in terebratulid and rhynchonellid brachiopods (“biofaciès à térébratules”; Lafuste, 1953; Hantzpergue, 1991; Fig. 6D). Other skeletal elements are notably represented by spines of regular echinoids, crinoid ossicles, cementing bivalves (e.g., Lopha and Nanogyra) and few gastropods (e.g., Amauropsis and apporhaid indet.). Vadet et al. (2002) report from this facies a diverse echinoid fauna consisting of regular and, more rarely, irregular taxa (Table 1). The biogenic elements are encrusted by a diverse fauna of some serpulid species and foraminifera (e.g., Placopsilina). Shells are generally well-preserved, but some are fragments. These skeletal elements are generally embedded into a fine-grained matrix. Locally, low-angle parallel stratifications occur, sloping towards the southeast. These deposits continue laterally to the upper part of the second reef level, and are capped by a flattopped bioeroded surface, strongly encrusted by oysters (e.g., Nanogyra and ?Liostrea; Figs. 3E, F and 5). The texture is generally grain-supported, but the local presence of micrite and the good fossil preservation suggest a parautochthonous assemblage of brachiopods and a moderateto low-energy inner-ramp setting (Fig. 8A). The dasycladacean green alga Goniolina points to very shallow water. Low-angle stratifications form 100 m-scaled clinoforms that record progradation of the ramp system (Fig. 2). This low-angle stratified facies is capped by the toplap surface S2 that implies

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an erosional event. Facies 5 likely corresponds to shoreface deposits. Erosional processes probably prevented true foreshore deposits from being recorded. 5.1.6. Facies 6: trichites-rich packstones Yellowish to light-grey packstones with locally subordinate grainstones and wackestones belong to this facies (Fig. 6E). The facies occurs in two main stratigraphic units and a cm-thick basal unit that only crops out at the north of the fault f2 (Fig. 2). This basal unit is composed of abundant oncoids, brachiopod and bivalve fragments, rare nautiloids (Paracenoceras giganteum), and foraminifera (Lenticulina). In the two main units, the most spectacular biogenic element is the large, thick-shelled pinnacean bivalve Trichites saussurei which is generally represented by articulated specimens, but fragments are also frequent (Fig. 9). The shells of Trichites generally serve as secondary hard substrates for a moderately diverse but densely settling encruster fauna consisting of oysters (e.g., Nanogyra and Lopha), bryozoans (cf. Berenicea and Stomatopora), and serpulids (Table 1). In articulated specimens encrustation is almost limited to the posterior end of Trichites shells. This clearly points to a semi-infaunal life habit of Trichites with the shell sticking obliquely in a muddy, but stable substrate (Werner, 1986; Fürsich and Werner, 1986), contrarily to an epibenthic life position as proposed by Seilacher (1984, 1990). Oysters and serpulids often form small pavements and clusters on the bioclastic sediment. Some serpulids are embedded in a thin, dense micritic crust. Additional sparse faunal elements are

Fig. 9. (A) Trichites-rich packstone facies (F6). The marly level at the middleupper part of the picture separates the lower and upper parts of this unit. Note the rounded and bioeroded massive microsolenid corals (white arrows). (B) A specimen of the bivalve Trichites saussurei with articulated valves.

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epibyssate (Arcomytilus and Plagiostoma) and infaunal (e.g., Ceratomya) bivalves, gastropods, and crinoids. Vadet et al. (2002) list a high diversity echinoid fauna (27 species) composed of a mixture of taxa characteristic of low energy (e.g., Nenoticidaris, Pedina, and Pseudodiadema) and high energy (Pseudocidaris and Psephechinus) environments. In contrast to other facies types it comprises also irregular, nonburrowing (e.g., Pygaster and Holectypus) and burrowing (Taphropygus) species indicating a muddy but stable bioclastic substrate. Within the lower Trichites-rich unit, scarce branching corals (Calamophylliospsis) preserved in life position are truncated by the reddish surface S3b. Rare massive domeshaped corals (Microsolena) show evidence of reworking (rounded morphology, perforations by boring organisms; Fig. 9). On the whole, the faunal composition corresponds well to the Nanogyra nana/Lopha (A.) solitaria association of Heinze (1991). Beds of cm to dm thickness show parallel or oblique stratification (Fig. 4). The lower unit progressively becomes thinner to the south and laterally transforms into a cmthick grain-supported bed that shows a dm-scale, asymmetrical wavelength upper surface (Lafuste, 1956; Fig. 2). The upper unit also becomes progressively thinner to the south (Fig. 5). To the north of the fault f2, the lower part of this second unit gradually passes into marl–limestone alternations over a few tens of metres (Fig. 4). To the south of the fault f2, this upper unit shows less abundant skeletal components to the southeast and laterally passes over a few tens of metres into dm-thick wackestone beds (Figs. 2 and 5). In its upper part, this unit displays clear low-angle stratification. Grain composition and texture of this facies suggest an innerramp setting near the shoreline that prograded to the southeast. Low-angle stratifications have a clinoform geometry that testifies to the migration of the ramp system, whereas the wavy, grain-supported beds correspond to a more distal southeasterly setting influenced by waves. Heinze (1991) assumes for this faunal association a consolidated substrate (mixture of fine matrix and bioclasts) within a normally calm environment below fair weather base which was occasionally subjected to reworking by storms. Hallam (1976) suggests for Trichites an environment in the proximity of a reef where this bivalve could tolerate higher energy conditions with its heavy shell. A similar peri-reefal environment was proposed by Vadet et al. (2002) for the echinoid fauna. From the present study, the good fossil preservation and the micritic matrix suggest generally moderately to gently agitated shoreface to upper offshore settings, allowing mud deposition (Fig. 8B). Periodically, reworking caused fragmentation of Trichites shells, the input of bioclasts from neighbouring facies, and regular occurrences of mortality events in oysters before they became adult. The latter is indicated by a high percentage of juvenile shells of Nanogyra within the sediment. On the other hand, the frequent preservation of articulated Nanogyra specimens on Trichites shells reflects rapid burial by mud after storm events and may reflect in-situ reworking of sediment rather than import of mud from other environments. The truncation of phaceloid Calamophylliopsis corals preserved in life position emphasizes a period of erosion, and possible subaerial exposure. Although not recorded

along the cliffs, the proximity of foreshore deposits is suggested by the well-rounded and bioeroded massive corals that could correspond to sparse pebbles coming from a beach. 5.1.7. Facies 7: bioclastic packstones Reddish to yellowish, poorly bedded packstones comprise this facies (Fig. 6F). Skeletal fragments are mostly weakly to moderately rounded (Table 1). The fossil assemblage is composed mainly of vagile (some cidaroid echinoids, and few gastropods) and epibenthic sessile organisms (e.g., cementing and epibyssate bivalves, terebratulid and rhynchonellid brachiopods, crinoids). David (1998) reports particularly abundant crinoids referable to Isocrinus rupellensis at the surface S5 along the Chirats cliffs, whereas it is extremely rare between the Chay and Belette Points. Endobenthic faunal elements are rare (e.g., astartid bivalves). Bioturbation is locally important. Intraclasts are abundant (e.g., corals and microbialite fragments) and were derived from the neighbouring coral-microbialite patch reefs of the third reef level. A grain-supported texture of this facies and the presence of a matrix and weakly rounded skeletal components support moderate-energy conditions on the inner ramp (shoreface to upper offshore; Fig. 8B). 5.1.8. Facies 8: marl–limestone alternations This facies is represented by white to light-grey mudstones and wackestones, with locally subordinate packstones (Fig. 6G). They form thin, nodular beds, and include sparse and relatively wellpreserved skeletal components (Table 1). From the fault f2 towards the south, cm-scale bioclasts in limestone beds progressively die out over a distance of approximately 30 m (Fig. 5). Limestone beds frequently record cm-scale HCS (Fig. 7B). These limestones alternate with blue-grey to dark marl levels (Fig. 2). These marls are characterized by a sparse fauna of mainly burrowing bivalves (e.g., Protocardia, Pholadomya, and Ceratomya), indicating a relatively soft, muddy substrate. At the Chirats Point, marly levels that capped the bioconstructions of the third reef level show a diverse crinoid fauna with notably frequent I. rupellensis (David, 1998). The features of this facies suggest mid- to outer-ramp environments (Fig. 8B). Relatively calm waters with only episodic influences of storms (upper offshore) are suggested by a reduced transport of biogenic elements and small-scale HCS (Burchette and Wright, 1992). 5.2. Coral-microbialite reefs 5.2.1. Geometry and general composition The first reef level (RL1), at Barbette Point is about 50 m long and up to 8 m high (Fig. 2). This reef consists of a massive and complex bioconstruction made of several, more or less coalescent reefs (Fig. 3C and D). At the south of the fault f1, this first reef level appears as a few domes on the present foreshore. The entire reef complex can be subdivided into eight stratigraphic units or reef-growth phases, delimited by surfaces of reef-growth interruption that can laterally be followed through the bioconstruction. The upper part of the reef clearly shows a

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Fig. 10. Relative abundance of the different reef components of the three reef levels. Composition of the third reef level corresponds to bioconstructions located along the cliff between the Chay and Belette Points. S: analysed surface.

progradation pattern to the southeast, forming an asymmetrical reef geometry (Fig. 3A and B). Intra-reef palaeocaves, formed by roofing-over processes or resulting from the tilting of reef bodies, are common and can reach up to 1 to 2 m high and wide and to 3 to 4 m long (Olivier et al., 2003). Today, the bioconstruction presents only two preserved paleocaves because of marine erosion that caused the roof collapse of the four others. This first reef level is composed mainly of microbialites (57% of the reef volume) and corals (37%; Fig. 10). Other components (oysters, brachiopods, serpulids, sponges, and bryozoans) constitute only 3% of the reef. Intra-reef sediments (3%) are rare.

The second reef level (RL2) crops out as a small promontory on the modern foreshore, and has a length and width of 7 to 8 m, and a thickness of 2 m (Fig. 3E and F). However, the exact extent of this reef body is unknown. Another small m-scale isolated patch reef was also observed locally in the cliff. The main bioconstruction can be subdivided into 7 stratigraphic units, delimited by laterally continuous surfaces of reef-growth interruption. These growth phases are of limited vertical dimensions (less than 50 cm-thick) and display numerous intra-reef cavities that do not exceed a few centimetres in size. This reef level shows a flat-topped surface (S2) that is strongly encrusted and perforated (Fig. 2). The reef is composed of microbialites (69%), corals

Fig. 11. (A, B) Bioconstruction of the third reef level at the Chay Peninsula at mid-distance between the Chay and the Belette Points. (C, D) Small metre-scale lens-like bioconstruction of the third reef level along the Chirats cliffs. Cr: thick crust with abundant oysters covering the reefs of the third reef level. S5: transgressive surface.

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(22%), other organic components (oysters, brachiopods, serpulids, sponges, and bryozoans; 2%), and sediments (7%; Fig. 10). The third reef level (RL3) consists of abundant patch reefs, which crop out in section in the cliff or as domes on the recent tide area. Between the Chay and the Belette Points (Fig. 2), the patch reefs are generally spaced every few tens of metres along the cliff. These reefs exhibit reduced dimensions, and are at most 7 to 8 m long and 3 to 5 m thick (Fig. 11). Many have a lens shape. Larger reefs cut the underlying layers by up to 1.6 m, squeezing up sediments into invaginations between adjacent reef bodies by compaction (Figs. 5, 11A and B). Along the Chirats cliffs, patch reefs have more circular shapes with smaller dimensions (generally 3 to 4 m long and a thickness around 2 m; Fig. 11C and D). In this third reef level, bioconstructions are characterized by surfaces of reef-growth interruption that delimit 5 main stratigraphic units in the Chay cliff and 3 in the Chirats cliff (Fig. 11). These reef-growth phases generally did not exceed a thickness of 50 cm, and reveal small dm-scale intra-reef cavities. Chirats patch reefs have a thick dm-scale top encrusted surface (S5) covered by oysters (Fig. 11C and D). The equivalent surface is less visible and developed within the Chay reefs, between the fourth- and fifth-reef growth phases. The Chay reefs are essentially composed of corals (56%; mainly branching) and microbialites (38%). Intra-reef sediment and other components form 4% and 2% of the reef, respectively (Fig. 10).

5.2.2. Corals A total of 19 coral taxa was identified along the Chay Peninsula section (Fig. 12). The corals are restricted mainly to the reefs of the three reef levels, with only a few isolated colonies of the phaceloid Calamophylliopsis and massive reworked Microsolena occurring in the facies type 6 (i.e., Trichites-rich packstones; Fig. 9). The determination of the corals is given here at the genus level to allow a better comparison between the different reef levels. Another simplification concerns the corals with a microsolenid structure. In order to reduce the risk of errors of determination, genera of the microsolenid family (e.g., Microsolena, Comoseris, and Meandraraea) have been grouped together, although Microsolena appears to be the most common genus. In the first reef level, 13 taxa have been identified. Coral forms are diverse, phaceloid, ramose, lamellar, dome-shaped and massive with irregular outlines. The dominant corals are ramose Thamnasteria, stylinids (Stylina, Pseudocoenia, and Stylosmilia), phaceloid Calamophylliopsis, and massive colonies of microsolenids that constitute 26.8%, 22%, 20.6%, and 16.7% of the coral fauna, respectively (Fig. 12). The basal part of the reef shows massive colonies of Stylina and large colonies of the ramose Thamnasteria and of the phaceloid Calamophylliopsis. Above this basal part, the successive main reef-growth phases display similar coral successions: (i) large colonies of

Fig. 12. Distribution of the coral fauna in the three reef levels. Distinction has been made between the reefs of the third reef level of the Chay and Chirat cliffs. N: number of specimens.

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Fig. 13. Schematic sketch of a coral-microbialite reef-growth phase. The main building role is assumed by corals during the primary-framework growth, whereas microbialites become the main constructor after a palaeoenvironmental change. Mu: mammilated microbialites of reef underside; Mf: mammilated microbialites of reef flanks; Ps: pseudostalactitic microbialites. See text for further explanations.

ramose Thamnasteria (commonly greater than 60 cm in diameter and up to 1.2 m high), particularly abundant lateral to the intra-reef cavities; and (ii) lamellar and encrusting forms (microsolenids) in the upper reef part. Twelve taxa have been identified in the second carbonatedominated reef level (Fig. 12). They are mainly represented by massive Thamnasteria that constitutes 43.1% of the coral fauna. Other massive colonies are stylinids (Stylina 15.6% and Pseudocoenia 5.9%), microsolenids (11.8%) and Ovalastrea (9.8%). Dimensions of coral colonies are generally small, except in the basal part of the reef where large colonies of Isastrea and Thamnasteria exceed 30 cm in diameter. Branching colonies are only represented by rare colonies of Calamophylliopsis, Stylosmilia, and Enallhelia. The third reef level displays an important variation in its coral composition between the Chay and Chirats cliffs. Compared with the latter, the Chay reefs exhibit a significantly higher diversity coral fauna with 11 taxa (Fig. 12). The coral assemblage is dominated by large colonies (up to 1.2 m in diameter) of the phaceloid Calamophylliopsis (40% of the coral fauna). Other phaceloid corals are Stylosmilia (14%) and rare Cladophyllia (2%). Massive colonies are mainly represented by microsolenids (14%), Thamnasteria (12%), and stylinids (Stylina 6% and Pseudocoenia 4%). Conversely, the Chirats reefs reveal a very low diversity coral fauna with only 5 taxa (Fig. 12). Colonies display very small dimensions and are mainly represented by massive microsolenids and Calamophylliopsis (48.9% and 40.4% of the coral fauna, respectively). Other taxa are some stylinids (Stylosmilia 6.4% and Stylina 2.1%) and rare Thamnasteria (2.1%). 5.2.3. Microbialites Though first described by Taylor and Palmer (1994), a detailed description of microbialites can be found in Olivier et al. (2003), and thus only a brief summary will be given here. Microbialites are abundant in the three reef levels, forming 57%, 69%, and 38% of reef volumes, respectively (Fig. 10). Microbialites form thick crusts (up to 10 cm) on corals or various encrusters (e.g., ostreid bivalves), and constitute the

outer surfaces of reef bodies (Fig. 13). The typical fabric of Chay Peninsula microbialites is thrombolitic, but can be locally structureless (leiolitic). Microscopically, microbialites are made of dense, clotted and peloidal micrites, some of which are locally laminated. Each reef-growth phase is characterized by various microbialite morphologies that differ according to the position within the reef structure (Fig. 13): pseudostalactitic microbialites at the top of intra-reef palaeocaves; mammilated microbialites at the flanks or underside of the reef body; reticular microbialites at the periphery or in the area between two reef bodies (Olivier et al., 2003). Microbial crusts are generally two-layered, with a first thin (mm- to cm-thick) and dense inner layer. The second cm-thick layer generally corresponds to thrombolitic columns (up to 5 cm long). The structure of microbialite crusts is locally disrupted by the participation of microencrusters that records periods of microbialite growth interruptions (Olivier et al., 2004). Chirats microbialites (third reef level) form particularly massive crusts devoid of internal columnar structure. Their outer surface represents intense bioerosion and is frequently encrusted by abundant serpulids and oysters. 5.2.4. Microencrusters Abundant and diverse encrusting organisms occur in the Chay peninsula reefs (Fig. 14). Microencrusters are generally interlayered with microbialites, but are also observed directly on corals or at the microbialite surface. Main microencrusters are polychaetes (Terebella lapilloides and serpulids), foraminifera (nubeculariids, Bullopora, and Tolypammina), a few bryozoans (cf. Berenicea), and some problematica. The latter comprise Tubiphytes morronensis, interpreted as a foraminifer encrusted by a calcified microbial mat (Pratt, 1995) or a foraminifer with symbiotic algae forming an outer test (Schmid, 1996); Koskinobulina interpreted as a red algae (Cherchi and Schroeder, 1985) or a foraminifer (Schmid, 1996); Thaumatoporella attributed to red algae (Flügel, 1979) or alternatively to green algae (De Castro, 1991); Lithocodium aggregatum related to a foraminifer (Luftusiacea; Schmid and Leinfelder, 1996) or to cyanobacteria (Cherchi and Schroeder, 2006); Bacinella irregularis interpreted as a cyanobacterium (Camoin and Maurin, 1988).

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Fig. 14. Distribution of the microencrusters in the three-successive reef-building events. Relative abundance scale: 0, not observed; 1, rare; 2, present; 3, common; 4, abundant. N: number of thin sections analysed.

According to the reef level, microencruster composition significantly changed. The first reef level is mainly composed of Terebella, Tubiphytes, nubeculariids, and some bryozoans (Fig. 14), but its third reef-growth phase reveals two characteristic levels mainly consisting of Lithocodium. This encruster forms laterally continuous crusts up to 30 cm-thick with an iron oxidized upper surface. Associated with Lithocodium are some Bacinella, Troglotella and Thaumatoporella. The second reef level is mainly composed by Terebella, Tubiphytes, nubeculariids, serpulids and Bullopora, and to a lesser extent by some bryozoans (cf. Berenicea) and Koskinobulina. The distribution of these microencrusters remains constant all along the second reef level. The third reef level still shows a major development of Terebella, Tubiphytes, nubeculariids, serpulids and Bullopora, associated with rare bryozoans. Koskinobulina is only present locally in patch reefs between the Chay and Chirats Points, and is not observed in the Chirats reefs (Fig. 14). In the encrusting succession, a preferential distribution of microencrusters can be highlighted. Lithocodium associated with some Bacinella and Thaumatoporella were always observed directly fixed onto a coral substrate. Nubeculariids and Tubiphytes are generally associated with microbialites. Berenicea and Koskinobulina are either present directly on a skeletal support (e.g., corals or oysters) or on microbialites. Terebella preferentially occurs on the outer surfaces of the second microbialite layer. 5.2.5. Associated fauna Apart from the corals, other macrofaunal elements are rarely recorded within the reefal facies (Table1). All of them have an

epibenthic life habit with the exception of boring bivalves (e.g., lithophagids) found in corals and microbialite crusts. Byssally attached bivalves (e.g., Chlamys, Camptonectes, and Plagiostoma) as well as taxa cementing on corals and microbialites (oysters and calcareous sponges) occur sparsely in all reef levels. Taylor and Palmer (1994) observed that particularly abundant thecideidinid brachiopods (up to 10 individuals per cm2) were present at the surface of pseudostalactitic microbialites within intra-reef cavities of the first reef level. In a few growth phases of the first reef level and at the Chirats, terebratulid and rhynchonellid brachiopods occur. David (1998) shows that the crinoid fauna seems to be absent from the first reef level. Conversely, crinoids are particularly diverse lateral to the third reef level with notably abundant Angulocrinus simplex within the reef bodies. The vagile epibenthos is in our study represented by few gastropod genera (e.g., Ampullospira). Based on rich material of private collectors, Vadet et al. (2002) record from this facies a comprehensive list of regular echinoids. They show remarkable differences in species composition and diversity between the first and third (Chirat) reef levels. According to these authors a low diversity echinoid fauna (five species; e.g., Paracidaris florigemma and Acrocidaris nobilis) adapted to low-energy conditions prevailed in the first reef level. In contrast, the echinoid fauna at the Chirats is more diverse (20 species) and characterized both by species adapted to high-energy biotopes within coral thickets (e.g., Balanocidaris marginata and Pseudocidaris mammosa) and species of lowenergy reefal environments (e.g., Acrocidaris nobilis). The frequency of the echinoids and the rarity of organisms with aragonitic shells show that the present-day low diversity and rarity of the reef-associated fauna does not reflect the former reefal communities but is mainly due to dissolution processes (only primary calcitic elements such as echinoids are preserved). In addition, the low diversity may also be due to the scarcity of intra-reef sediments (only 2.7% to 6.7% of reef volumes) in which the fauna could be embedded. After death, most byssate and vagile faunal elements (e.g., gastropods and bivalves) were possibly exported from the reefs by currents. 5.3. Depositional sequences The Chay Peninsula succession shows a major surface of discontinuity (S2) that separates carbonate-dominated lowstand deposits from mixed carbonate–siliciclastic transgressive deposits (Fig. 2). The distribution of the nine facies types (F1 to F9) reveals a stacking pattern of depositional sequences that can be correlated to medium-scale sequences as defined by Strasser et al. (1999) or fourth-order sequences sensu Vail et al. (1991). The first depositional sequence corresponds to well-bedded wackestones and bio-oomicrite packstones (F1 and F4, respectively), including bioconstructions (F9) of the first reef level (Fig. 2 and Table 1). Deposits of the base of the transgressive trend do not crop out. Following the maximum flooding interval, the regressive trend is marked by the coalescence of different reef bodies and their prograding geometry (Fig. 3A–D). The depositional environment evolved from a mid ramp frequently subjected to storm events (upper offshore; Facies F1) to an inner

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ramp under constant wave agitation (shoreface; Facies F4). The encrusted and bioeroded surface S1 at the top of the first reef level possibly corresponds to the sequence boundary. The second depositional sequence consists of oo-biomicrite wackestones (F3), argillaceous wackestones (F2), brachiopodrich packstones (F5), and reefs (F9) of the second reef level (Figs. 2 and 4; Table 1). A transgressive trend is suggested by the transition between a relatively agitated inner ramp (shoreface; Facies F3) towards an upper offshore setting on the mid ramp temporarily subjected to storm influence, which reflects a maximum of depth (Facies F2; Fig. 8A). The prograding pattern of the second reef level (Fig. 3E and F) and the brachiopod-rich packstones (F5) might correspond to a regressive trend. These facies are finally truncated by the strongly encrusted and bioeroded erosional surface S2 that likely corresponds to a sequence boundary. The third depositional sequence corresponds to the Trichitesrich packstones (F6) and distally (toward the south) to marl– limestone alternations (F8; Fig. 2 and Table 1). The transgressive part of this sequence is marked by the thin basal oncoidrich level of F6 north of the fault f2, and to the south by the possible retrogradation of upper offshore deposits (i.e., marl– limestone alternations; F8) on the surface S2. The maximum paleo-depth is marked by the northward limit of retrograding marl–limestone alternations on the surface S3. The surface S5 is interpreted to represent a sequence boundary. This surface caps reefs (F9) of the third reef level along the Chirats cliffs and marks the maximum of progradation trend of the bioclastic packstones (F7). Within this third depositional sequence, higher-frequency sea-level oscillations are also recorded. They correspond to rapid facies belt migrations as observed for marl– limestone alternations (F8) on surfaces S3 and within the two units of Trichites-rich packstones (F6) on downlap (S4b) or erosional (S3b) surfaces (Fig. 2). Only the transgressive part of the fourth-depositional sequence can be observed along the Chirats cliffs. Directly above the surface S5, first deposits of this sequence are marl– limestone alternations (F8) that capped reefs (F9) of the third reef level along the Chirats cliffs. A small recurrence of the bioclastic packstones (F7) and contemporaneous small reefs (F9) might correspond to the regressive deposits at a highfrequency of sea-level fluctuations. The maximum paleo-depth possibly occurs into marly levels (F8) capping the last reef occurrence of the third reef level along the Chirats cliffs (Fig. 2). 6. Discussion Along a reduced transect of about 1 km, and recording a relatively short period of time (shorter than the duration of the Cymodoce ammonite Zone; Hantzpergue, 1995), the Chay Peninsula section shows a remarkable facies belt migration along a carbonate-dominated to mixed carbonate–siliciclastic ramp (Fig. 2). This highlights the interplay of various factors such as sea-level fluctuations, terrigenous and nutrient inputs, which probably controlled the nature and the distribution of sediments (i.e., both coral-microbialite build-ups and their lateral deposits) at high-frequency temporal scales.

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6.1. Ramp energy regime The low-angle stratifications in the Trichites-rich (F6) and brachiopod-rich (F5) packstone facies emphasize a prograding pattern onto deeper-water deposits toward the south (Figs. 2 and 5). Moreover, the transition between Trichites-rich packstones (F6) and marl–limestone alternations (F8) reveals remarkable metre-scale lateral facies changes over short distances (Fig. 4). Such facies shifts, in response to sea-level fluctuations, demonstrate the ramp-type geometry of the shelf (e.g., Burchette and Wright, 1992). In this ramp system, the fact that the different facies types never reached a grainstone texture implies periods of low-energy conditions, allowing the possibility for mud to be deposited (Table 1). The occasional occurrence of ooids (type 3 of Strasser, 1986), observed in facies F3 and F4, also emphasizes quiet-water settings with only intermittent high-energy conditions. Deposits of mid- to outer-ramp, such as well-bedded or argillaceous wackestones (F1 and F2) laterally to the first two reef levels and marl–limestone alternations (F8) record modest tempestites indicating distal fluctuating hydrodynamic conditions of upper offshore settings (Figs. 7 and 8). In more proximal inner-ramp environments, Trichites-rich packstones (F6) record physical abrasion (i.e., surface S3b; Fig. 2). This suggests that these deposits might belong to an upper shoreface to foreshore zone (Figs. 2 and 8B). However, abundant Trichites preserved with both valves and the high amount of mud highlight only limited, periodic reworking or winnowing. Moreover, tidally-influenced sedimentary structures are missing. Thus, the sedimentation observed along the Chay Peninsula section reflects a general low-energy, wavedominated regime (Fig. 8). The apparent paradox between a lowenergy regime and a shallow setting could possibly be explained by the peculiar palaeogeographical position of the Chay Peninsula ramp system at the margin of a basin of relatively small size (Aquitaine Basin), which did not permit the development of high amplitude waves (Fig. 1). Alternatively, the study area might have been protected from strong winds in the Kimmeridgian, either due to a leeward setting of this area or by being located within an embayment. 6.2. Sea-level fluctuations Although recording a global, long-term sea-level rise (Jacquin et al., 1998), the Lower Kimmeridgian Chay Peninsula section reveals numerous relative sea-level fluctuations conditioning facies partitioning and reef development. The erosional surface S2 is recorded along the Aquitaine Basin margin (surface D4 of Hantzpergue and Maire, 1981; Fig. 1), but also in the Paris Basin and the northern Jura (Hantzpergue, 1985b). This surface is interpreted as a third-order transgressive surface (Jacquin et al., 1998). Thus, the lower part of the Chay Peninsula succession corresponds to third-order lowstand deposits, whereas the upper part possibly belongs to a transgressive systems tract. Such a sequence stratigraphic model is confirmed by the different depositional geometries and migration of the facies belts (Fig. 2). At the base of the section, the transition between well-bedded wackestones (F1) and bio-ooclastic packstones (F4) marks a

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gradual shallowing-upward trend. This tendency is corroborated by bioconstructions of the first two reef levels that display progradational patterns to the southeast (Fig. 3). Above the surface S2, the retrograding pattern of marl–limestone alternations (F8) testifies to the general third-order transgressive trend in the upper part of the section. Along such a third-order relative sealevel oscillation, the transition from a carbonate-dominated to a mixed carbonate–siliciclastic context reflects an increase in the terrigenous content above the transgressive surface S2 (Fig. 2). Bádenas and Aurell (2001) observed a similar sedimentological evolution in the Iberian Basin during the transgressive systems tracts of sequence J3.5 that includes the Lower Kimmeridgian. These authors suggest that this clastic supply may be related to tectonic activity that began at the Oxfordian–Kimmeridgian transition. The Iberian and Ebro Massifs and the Armorican Massif could have been subjected to tectonic uplift accompanying a phase of rifting in the northern Atlantic graben systems (Leinfelder, 2001). Indeed, the Lower Kimmeridgian recorded a change in the sedimentation regime towards more clay-rich deposits along the entire northern shelf of the Tethys, but its development shows spatial and temporal heterogeneities (Leinfelder, 1993). Thus, these siliciclastic inputs are probably governed by regional North-Atlantic and West European tectonics. In that context, a tectonic control accompanying climatic thirdorder sea-level changes cannot be excluded to explain the transition from carbonate-dominated to mixed carbonate–silicilastic regimes recorded in the Chay Peninsula succession. Jurassic reefs are generally considered to occur within thirdorder sea-level rises (Leinfelder, 2001). At the Chay Peninsula, reefs developed both during regressive and transgressive trends, but they did not grow across an entire third-order cycle. The stratigraphic extents of the first and second reef levels are included within fourth-order sequences (Fig. 2). Although more difficult to identify, reef architectures also reveal possible fifth- (or sixth-)

order sea-level fluctuations. Fig. 15 presents a theoretical reefgrowth model where reef architecture is mainly triggered by fifthorder sea-level oscillations. This model suggests that reef windows (i.e., reef levels) are confined between two main relative sea-level thresholds. A lower threshold is related to potential emergence or to low accommodation that does not allow the growth of reef builders. An upper threshold relates to a maximum paleo-water depth below which the reef builders would be drowned. Two main scenarios can be proposed according to the position within a third-order sequence. The first two reef levels occurred during a third-order regressive trend. In that case, reefgrowth phases (i.e., high-frequency reef windows) are delimited by periods of potential emergence or low accommodation that corresponded to fifth-order lowstands of sea level (Fig. 15). On the other hand, the third reef level occurred during a third-order sea-level transgressive trend. Here, each phase of reef-growth interruption can be correlated to drowning events during fifthorder rising sea level. Considering that the third reef level ocõcurred in a mixed siliciclastic–carbonate setting, an increase in turbidity due to higher terrigenous fluxes probably resulted in lowering the drowning threshold. This model highlights the possible prominent role played by high-frequency (fifth-order) sea-level oscillations, whereas lower frequency (fourth- and thirdorder) sea-level fluctuations only defined the time intervals in which paleobathymetry was favourable for reef development. Thus, the frequency and the stacking pattern of the successive phases of reef growth (i.e., high-frequency reef windows) within the three reef levels emphasize fifth- (or higher) order sea-level fluctuations (Fig. 15). Within each phase of reef growth, the development of microbialites at the reef front followed the primary coral framework formation (Fig. 13). Thus, along with fifth-order sea-level oscillations, variations in the nutrient content probably reflect periods of more humid conditions during millennial time-scale climatic fluctuations.

Fig. 15. Impact of third- to fifth-order sea-level changes on the potential for coral-microbialite reefs to develop. Within a third-order regressive or transgressive trend, fourth-order sea-level changes opened reef windows corresponding to the three reef levels. Fifth-order sea-level changes opened the high-frequency reef windows corresponding to the successive reef-growth phases observed within bioconstructions of the three reef levels. Changes in siliciclastic inputs, modifying turbidity and light conditions, might also lead to displacement of reef window thresholds.

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Above the surface S2, sedimentation reflects the establishment of a mixed siliciclastic–carbonate ramp system (Fig. 2). In that context, the amount of terrigenous material might have strongly affected facies partitioning and reef development (e.g., Sanders and Baron-Szabo, 2005). The Trichites-rich (F6) and bioclastic (F7) packstones facies are mainly interpreted to record the shoreface zone (Fig. 8B). Limestone units seem to be confined to shallower-water settings where light was sufficient to allow the growth of carbonate producers. A control by light penetration could explain the transition towards the more distal facies represented by marl–limestone alternations (F8) over very short distances (Figs. 2 and 4). When corals were able to develop real frameworks such as contemporaneously to the bioclastic packstones (F7), an influence of terrigenous inputs can also be invoked. In a carbonate-dominated system, the first reef level reveals thick crusts of Bacinella and Lithocodium that emphasize periods of well-illuminated waters, particularly favourable to the growth of these phototactic and oligotrophic microencrusters (Schmid, 1996; Dupraz and Strasser, 2002; Shiraishi and Kano, 2004; Helm and Schülke, 2006). These latter are absent in bioconstructions of the third reef level, suggesting more turbid waters. Moreover, environmental stress caused by a reduced light penetration can also be proposed for the time interval in which the bioconstructions of the third reef level formed. Compared to the reefs observed between the Chay and Belette Points, reefs along the Chirats cliffs are smaller and show a drastically reduced coral diversity of only 5 taxa (Fig. 12). Microbialites observed in these reefs also show numerous surfaces of growth interruption that are testified by heavily bored and encrusted surfaces. Low light conditions could be responsible for such microbialite growth interruptions (Olivier et al., 2007). The presence of a specific crinoid fauna (i.e., Millericrinina and Isocrinus) laterally to the top of the reefs along the Chirats cliffs leads to the possibility of a relatively deep environment (N40 m; cf. David, 1998). Thus, a more distal position along the ramp suggests that the deeper setting (and associated lower light conditions) of the Chirats reefs can explain their reduced dimensions (Fig. 8B). However, the small distance between the Chirats and Chay reefs (less than 1 km) and the low dip of the ramp does not support an substantial bathymetric change from one locality to the other (Fig. 2). A control by turbid waters accompanying relatively high-terrigenous fluxes better explains differences in reef composition and development between the Chay and the Chirats than differences in paleobathymetry from one site to the other.

feeding microsolenids form a moderate portion of the coral assemblages, almost in similar proportion to stylinids that correspond to a group characteristic of nutrient-poor waters (Dupraz and Strasser, 2002; Olivier et al., 2004). The relatively high diversity coral fauna (11–13 taxa) in the three reef levels suggests low-mesotrophic conditions that were favourable to corals (Leinfelder et al., 1996; Dupraz and Strasser, 2002). Thus, whatever the depositional context (i.e., carbonate-dominated to mixed carbonate–siliciclastic) the environmental conditions that occurred during the establishment of the primary coral framework show certain similarities. Only the coral assemblage of the small bioconstructions of the third reef level along the Chirats cliffs is more characteristic of a heterotrophic mode of nutrition with a high proportion of microsolenids (48.1% of the coral fauna; Fig. 12). However, as discussed above, nutrients are probably not a dominant factor in controlling coral distribution compared to the turbidity of waters. A detailed analysis of the secondary framework (i.e., microbialites and microencrusters) highlights the influence of nutrient-rich waters. Phases of relatively high nutrient content in marine waters have been invoked in the formation of microbialites in coral reefs (e.g., Dupraz and Strasser, 1999; Olivier et al., 2003; Camoin et al., 2006). More mesotrophic conditions probably allowed blooms of benthic microbial communities at the primary-framework surface made of corals (Sprachta et al., 2001; Fig. 13). Thus, the complex structure of these bioconstructions mainly made of corals and microbialites emphasizes high-frequency palaeoenvironmental fluctuations that are not recorded in contemporaneous non-framework facies (Olivier et al., 2007). Common heterotrophic microencrusters are also associated with microbialites, but their assemblages do not significantly differ in the analysed bioconstructions of the three reef levels (Fig. 14). Only a higher amount of serpulids in the bioconstructions of the third reef level suggests nutrient-richer water during the formation of the secondary framework. In that case, the lowest amount of microbialites in bioconstructions of the third reef level is probably due to a shorter period favourable to their formation. The environmental stress (i.e., nutrient-richer waters) that affected coral growth and allowed the formation of microbialites could be explained by the presence of upwelling (Leinfelder, 2001; Cabioch et al., 2006; Camoin et al., 2006). However, neither sedimentary nor palaeontological arguments confirm such a hypothesis for the Chay section. More likely, a nutrient source associated with terrigenous fluxes can explain the observed patterns. Rare Earth Element signatures of the Chay Peninsula microbialites emphasize a high-siliciclastic contamination during their formation (Olivier and Boyet, 2006).

6.4. Nutrient inputs

7. Conclusions

Nutrients are commonly associated with terrigenous inputs and can also influence carbonate production and reef growth (e.g., Mutti and Hallock, 2003; Dupraz and Strasser, 2002). Whatever the considered depositional context (carbonate-dominated or mixed carbonate–siliciclastic), the coral fauna is typically mixed photo-heterotrophic. Colonies of Thamnasteria are able to survive in highly variable environments (Bertling, 1993), and frequently dominate the assemblage (Fig. 12). Pennular filter-

The Kimmeridgian Chay Peninsula succession formed in a shallow and low-energy, gently sloping ramp-type shelf allowing rapid migration of facies belts and short-distance lateral facies variations. A major sedimentological change occurs after a thirdorder transgressive surface. Below this surface, two reef levels occurred within a carbonate-dominated setting. Above, bioconstructions of a third reef level developed in a mixed carbonate– siliciclastic environment. The analysis of the distribution and

6.3. Siliciclastics and turbidity

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mode of growth of these reefs gives information on the response of ancient reef communities to environmental parameters associated with global changes. Several orders of relative sea-level oscillations controlled facies distribution and reef development. Facies belts and reef levels (i.e., low-frequency reef windows) are mainly controlled by fourth-order sea-level fluctuations. Phases of reef growth (i.e., high-frequency reef windows) seem controlled by fifth-order sealevel oscillations. During a fifth-order sea-level fluctuation, surfaces of reef-growth interruption have different significance according to the position within a third-order sequence. In a thirdorder regressive interval, surfaces of reef-growth interruption possibly reflect periods of subaerial exposure during low sea levels. In a third-order transgressive interval, they probably emphasize drowning events during sea level rises. Thus, the impact of fifth-order sea-level oscillations is directly recorded in the reef architecture, whereas fourth- and third-order sea-level oscillations only controlled the accommodation potential for reefs to develop. The three reef levels correspond to coral-microbialite bioconstructions made of successive reef-growth phases where a primary coral framework serves as support to a thick microbialite crust. Reef compositions and development seem to be dependent on terrigenous inputs in modifying light intensity and the amount of nutrients. Whatever the depositional context, corals are relatively well diversified and characteristic of a mixed photo-heterotrophic fauna, suggesting general low-mesotrophic conditions. The presence of microbialites at the surface of the primary framework is correlated to periods of relatively nutrient-rich marine waters associated with increased terrigenous fluxes. Highfrequency stacking patterns of reef-growth phases highlight (fifth- or higher-order) sea-level and nutrient variations that are related to millennial time-scale climate fluctuations. Acknowledgments This work has been funded by the National Science Research Council of France (UMR CNRS 5125 “Paléoenvironnements et paléobiosphère”, Lyon). Contribution UMR5125-07.049. We are particularly grateful to B. Pratt and an anonymous reviewer for their constructive reviews that greatly improved this manuscript. Authors are also thankful to C. Fielding for his critical comments and valuable suggestions. References Bádenas, B., Aurell, M., 2001. Proximal-distal facies relationships and sedimentary processes in a storm dominated carbonate ramp (Kimmeridgian, northwest of the Iberian Ranges, Spain). Sed. Geol. 139, 319–340. Bádenas, B., Aurell, M., Gröcke, D.R., 2005. Facies analysis and correlation of high-order sequences in middle–outer ramp successions: variations in exported carbonate on basin-wide δ13Ccarb (Kimmeridgian, NE Spain). Sedimentology 52, 1253–1275. Benito, M.I., Mas, R., 2006. Sedimentary evolution of the Torrecilla Reef Complex in response to tectonically forced regression (Early Kimmeridgian, Northern Spain). Sed. Geol. 183, 31–49. Bertling, M., 1993. Ecology and distribution of the Late Jurassic Scleractinian Thamnasteria concinna (Goldfuss) in Europe. Palaeogeogr. Palaeoclimatol. Palaeoecol. 105, 311–335.

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