Coral-microbialite reefs in pure carbonate versus mixed carbonate-siliciclastic depositional environments: the example of the Pagny-sur-Meuse section (Upper Jurassic,northeastern France)

Middle to Upper Oxfordian reefs of a shallow marine carbonate platform located in northeastern France show important facies changes in conjunction with terrigeneous contents. The Pagny-sur-Meuse section shows coral-microbialite reefs that developed both in pure carbonate limestones and in mixed carbonate-siliciclastic deposits. Phototrophic coral associations dominated in pure carbonate environments, whereas a mixed phototrophic/heterotrophic coral fauna occurred in more siliciclastic settings. Microbialites occur in pure carbonate facies but are more abundant in mixed carbonate-siliciclastic settings. Reefs seem to have lived through periods favourable for intense coral growth that was contemporaneous with a first microbialitic layer and periods more favourable for large microbialitic development (second microbialitic layer). The first microbialitic crust probably developed within the reef body and thus appears to be controlled by autogenic factors. The second generation of microbialites tended to develop over the entire reef surface and was probably mainly controlled by allogenic factors. Variations in terrigeneous input and nutrient content, rather related to climatic conditions than to water depth and accumulation rate, were major factors controlling development of reefs and their taxonomic composition.     

Reefal and mud mound facies development in the Lower Devonian La Vid Group at the Colle outcrops (León province, Cantabrian Zone, NW Spain)

In the locality of Colle (Cantabrian Zone, NW Spain), the upper part of the Valporquero Shale Formation (Emsian, La Vid Group) contains an interval of shales and marlstones (barren, greenish-grey shales and fossiliferous, greenish-grey or reddish shales/marlstones) with beds and packages of homogeneous and cross-bedded skeletal limestones. Metre-scale mud mounds and coral biostromes occur encased in the fossiliferous reddish and greenish-grey shale/marlstones, respectively, with the coral biostromes overlying conspicuous skeletal limestone bodies. These rocks were deposited on a carbonate ramp, ranging from above storm wave base for the cross-bedded skeletal limestones to below the storm wave base for the remaining deposits, organic buildups included. The vertical stacking of these facies and the occurrence of the two types of buildups are interpreted to reflect the interplay among several (possibly 4th and 5th) orders of relative sea-level variations, during a 3rd-order highstand. Coral biostromes occur in early 5th-order transgressive system tracts developed within late 4th-order highstand, and are interpreted to have thrived on a stable granular substrate (skeletal limestones) in non-turbid waters, being later aborted by the onset of muddy sedimentation. Biostrome features suggest that they developed under environmental conditions essentially different from those related to the sedimentation of their granular substrate. Mud mounds occur in 5th-order transgressive and early highstand system tracts tied to early 4th-order sea-level rise. Field relationships suggest that mud mounds grew coevally with muddy sedimentation, with high-frequency variations in carbonate vs. terrigenous mud sedimentation influencing their development.An erratum to this article can be found at     

Continental, brackish and marine carbonates from the Lower Cretaceous of Kolone–Barbariga (Istria, Croatia): stratigraphy, sedimentology and geochemistry

During the Early Cretaceous, wide areas of the Dinaric–Adriatic Carbonate Platform emerged for long periods. The Hauterivian–Barremian carbonates from Kolone–Barbariga show a few typical examples of lacustrine facies with dinosaur bones and brackish/palustrine facies. The sequence of the platform is made for the most part by subtidal and intertidal limestones. The bone levels are located in a large depression few meters deep in the uppermost Hauterivian marine limestones. The filling facies of this depression are made by oncolitic rudstones and algal boundstones, which represent marginal lacustrine facies, and by laminated limestones, thin stromatolitic levels and distal fringes of rudstones which represent relatively open lacustrine facies. The fossil content is characterized by rare charophyte stems, ostracods, gastropods and plant remains, while typical marine fauna is absent. At the Hauterivian–Barremian boundary a major emersion event has been observed, then a slow transgressive phase occurred. The transgressive facies are primarily made by mudstones with ostracods, charophytes and Spirillina (brackish and probably freshwater facies), wackestones with Ophtalmidiidae and rare dasyclad algae, storm layers with gastropods and miliolids and breccia-like dinoturbated beds. Wackstones, packstones and very rich in dasyclad grainstones outcrop at the top of the section, representing the maximum of the transgression. Trace elements content, carbon and oxygen stable isotope analyses have been performed to aid the palaeoenvironmental interpretation. In this geological setting, Barium seems to discriminate between brackish and freshwater facies. The isotopic values of the marine carbonates appear to depend on early diagenetic processes, meanwhile lacustrine facies seem to show a weak signal of the depositional environment.     

Controls on Devonian hemi-pelagic limestone deposition analyzed on cephalopod ridge to slope sections,Eastern Anti-Atlas,Morocco

The hemi-pelagic Tafilalt Ridge separating the Maider Basin from the Tafilalt Basin developed progressively from an Early Devonian homoclinal ramp, through a Middle Devonian ramp-slope stage of moderate topography, to a mature cephalopod ridge during the Late Devonian and formed a spur-like element extending from the southern shallow-water Maider Platform to the broader northern hemi-pelagic Tafilalt Platform. During the Middle Devonian, thick lowstand aprons were shed onto the ridge-slope from an active mid-ramp carbonate factory in the south (Maider Platform). The shallow-water derived sediments first by-passed the central paleohigh, but then onlapped the ridge during start of sea-level rise. The allodapic limestones (storm-induced turbidites, tempestites, and debrites) of the aprons consist of large parts or entirely of reworked lithic peloids. The lithic peloids are interpreted as the main foundation of all micritic lithofacies types in the distal ramp locations, including those on the hemi-pelagic cephalopod ridge. Lateral facies transition of allodapic limestones into nodular cephalopod limestones on the ridge suggests the latter originate from allodapic beds and were subsequently transformed into nodular limestones by bioturbation, early diagenesis, and the faunal input from a hemi-pelagic community. Iron-rich hardgrounds formed on the ridge during major regressive phases as a result of increased winnowing by the wave-base and correlate with siliciclastic turbidite deposition on the slope. The hardgrounds can be correlated with hiatuses on the Tafilalt Platform, which formed a broader hemi-pelagic swell to the north. During the Late Devonian, the southern Maider Basin carbonate factory became unproductive or disappeared, and a ridge facies of truly hemi-pelagic autochthonous limestones developed. Allodapic deposits on the slope were shed from the ridge itself at this time but did not form well-defined aprons.     

Coral zonation of an Oxfordian reef tract in the northern French Jura

During the Middle Oxfordian, numerous coral reefs flourished on the northern margin of the Tethys Ocean. The outcrop of Bonnevaux-le-Prieuré (northern French Jura mountains) provides a rare opportunity to observe a nearly complete section allowing the installation, evolution and demise of this global carbonate reef rich event to be studied. Quantitative data on coral assemblages together with sedimentological and palaeoecological observations lead to the reconstruction of a reef tract coral zonation. Starting from the outer slope, Dimorpharaea, Microsolena, Dendraraea, Comoseris, and Stylina ecozones are recognized. This new facies model implies a central position for an oolitic shoal in the highest energy zone, within the Comoseris ecozone. Applying this facies model to the sequence stratigraphic interpretation of the vertical succession results in recognising a third-order relative sea-level fluctuation, which can be correlated at least with Lorraine (France) and Switzerland.     

Anthracoporella mounds in the Late Carboniferous Auernig Group,Carnic Alps (Austria)

Summary A heretofore undocumented example of skeletal mounds formed by the dasycladacean algaAnthracoporella spectabilis is described from mixed carbonate-clastic cycles (Auernig cyclothems) of the Late Carboniferous (Gzhelian) Auernig Group of the central Carnic Alps in southern Austria. The massive mound facies forms biostromal reef mounds that are up to several m thick and extend laterally over more than 100 m. The mound facies is developed in the middle of bedded limestones, which are up to 16 m thick. These limestones formed during relative sea-level highstands when clastic influx was near zero. The mound facies is characterized by well developed baffler and binder guilds and does not show any horizontal or vertical zonation. Within the massive mound faciesAnthracoporella is frequently found in growth position forming bafflestones and wackestones composed of abundantAnthracoporella skeletons which toppled in situ or drifted slightly.Anthracoporella grew in such profusion that it dominated the available sea bottom living space, forming ‘algal meadows’ which acted as efficient sediment producers and bafflers. BecauseAnthracoporella could not provide a substantial reef framework, and could not withstand high water turbulence, the biostromal skeletal mounds accumulated in shallow, quiet water below the active wave base in water depths less than 30 m. The massive mound facies is under- and overlain by, and laterally grades into bedded, fossiliferous limestones of the intermound facies, composed mainly of different types of wackestones and packstones. Individual beds containAnthracoporella andArchaeolithophyllum missouriense in growth position, forming “micromounds’. Two stages of mound formation are recognized: (1) the stabilization stage when bioclastic wackestones accumulated, and (2) the skeletal mound stage when the sea-bottom was colonized byAnthracoporella and other members of the baffler and binder guilds, formingAnthracoporella bafflestones and wackestones of the mound facies. A slight drop in sea-level led to the termination of the mound growth and accumulation of organic debris, particularly calcareous algae, fusulinids, crinoids and bryozoans, forming well bedded limestones, which overlie the mound facies     

An early cretaceous section in the Kircaova area (Berdiga Limestone,NE-Turkey) and its correlation with platform carbonates in W-Slovenia

Summary In the Kale (Gümüshane) area in the North Eastern Turkey, platform carbonates of the Berdiga Limestone were deposited during Late Jurassic-Early Cretaceous time in environments varying from intertidal to fore reef. The sequence shows extensive lateral and vertical alterations and interfingering of different facies types. In the upper part of the Berdiga Limestone in the Kircaova area a bituminous thin-bedded to platy limestone and shale 5 to 6 m thick occurs at the Early/Late Aptian boundary. It is underlain by limestones rich in silica nodules of up to 10 cm size. A facies analysis of a section about 70 m thick including the bituminous interval was carried out in 1994/95 at the SW border of the Kircaova area close to the road from L?rikas to Kale. The limestones consist mainly of packstones and grainstones locally rich in calcareaous algae and forminifera. Fragments of molluscs and echinoids as well as some ostracods and calcispheres occur. Some sponges, corals, and beds rich in molluscs occur in minor amounts in the middle part of the section which is characterized by intertidal to shallow subtidal facies. Algae and foraminifera indicate a Barremian-Early Aptian age of the lower part and Late Aptian age of the upper part of the section (e.g.Salpingoporellamuehlbergii, Salpingoporella aff.melitae, Clypeina solkani, Novalesia producta), divided by the bituminous limestones. In West Slovenia (close to the Italian border) a complete Cretaceous section occurs at Sabotin mountain containing Aptian beds with comparable faunal composition. In contrast to the Berdiga Limestone, in Slovenia at the rim of the dinaric platform a patch reef about 50 m in thickness is developed which is also covered by a bituminous limestones (black shale) marking the Early/Late Aptian boundary. Faunal elements in Slovenia arePalorbitolina lenticularis, Cuneolina laurentii, Orbitolina (Mesorbitolina) texana andSalpingopoprella dinarica. The bituminous limestone appears to be a marker horizon. At both locations it is locally rich in characeans probably indicating a regressive maximum before another transgression began in the Late Aptian/Albian as world-wide drowning event. Possibly the occurrence of the bituminous limestone (black shale) is associated with volcanic activity during the Aptian. If so it could be used as a chronostratigraphic marker horizon in both areas analyzed.     

The schulterkofel section in the Carnic Alps,Austria: Implications for the carboniferous-permian boundary

Summary The upper part of the LowerPseudoschwagerina Limestone (Rattendorf Group), outcropping on the northwestern flank of Schulterkofel Mountain, Carnic Alps (Austria) is described with special emphasis on fusulinid microfossils and facies. This fusulinid-rich section offers an ideal opportunity for biostratigraphy in defining the Permo-Carboniferous boundary in this region. The LowerPseudoschwagerina Limestone is composed of shallow-marine limestones with intercalated thin siltstone and sandstone beds. Fusulinid limestones are represented by two types of wackestones, both containing large quantities of smaller foraminifers. Fusulinid grainstones are rare. Limestones rich in fusulinids were found only within the bedded limestone facies in beds both below and especially above siliciclastic intercalations. This may indicate that the best living conditions for fusulinids existed immediately before and especially after the climax of a regressive phase (sea-level lowstand). The fusulinid limestones were deposited within a protected, shallow-marine shelf environment with normal salinity. Pseudoschwagerinid fusulinids appear in the upper part of the LowerPseudoschwagerina Limestone, in samples SK 107d (undeterminable species) and SK 108, i.e. between 92 m and 93 m above the base of the section within a bedded limestone immediately above the uppermost clastic intercalation. The fusulinid fauna is represented by about 30 species belonging to only a few genera. Species ofTriticites andRugosofusulina dominate, whereas those ofDaixina, Rugosochusenella andPseudofusulina are rare. A characteristic feature of the fauna is the strong similarity with fusulinid faunas described from Russia as well as from Middle and East Asia. Some of the described fusulinids are new for the Carnic Alps. The first appearance ofPseudoschwagerina andOccidentoschwagerina (Occidentoschwagerina alpina Zone) in the upper part of the LowerPseudoschwagerina Limestone in the Schulterkofel section defines the position of the Carboniferous-Permian boundary.     

Applications of nummulitids and other larger benthic foraminifera in depositional environment and sequence stratigraphy: an example from the Eocene deposits in Zagros Basin,SW Iran

The Tale-Zang Formation in Zagros Mountains (south-west Iran) is a Lower to Middle Eocene carbonate sequence. Carbonate sequences of the Tale-Zang Formation consist mainly of large benthic foraminifera (e.g. Nummulites and Alveolina), along with other skeletal and non-skeletal components. Water depth during deposition of the formation was determined based on the variation and types of benthic foraminifera, and other components in different facies. Microfacies analysis led to the recognition of ten microfacies that are related to four facies belts such as tidal flat, lagoon, shoal and open marine. An absence of turbidite deposits, reefal facies, gradual facies changes and widespread tidal flat deposits indicate that the Tale-Zang Formation was deposited in a carbonate ramp environment. Due to the great diversity and abundance of larger benthic foraminifera, this carbonate ramp is referred to as a “foraminifera-dominated carbonate ramp system”. Based on the field observations, microfacies analysis and sequence stratigraphic studies, three third-order sequences in the Langar type section and one third-order sequence in the Kialo section were identified. These depositional sequences have been separated by both type-1 and type-2 sequence boundaries. The transgressive systems tracts of sequences show a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts of sequences contain predominantly imperforate foraminifera.     

Reduced carbon emissions and fishing pressure are both necessary for equatorial coral reefs to keep up with rising seas

A rapid increase in sea-level rise is generating vertical accommodation space on modern coral reefs. Yet increases in sea-surface temperatures (SSTs) are reducing the capacity of coral reefs to keep up with sea-level rise. We use ensemble species distribution models of four coral species (Porites rus, Porites lobata, Acropora hyacinthus and Acropora digitifera) to gauge potential geographic differences in gross carbonate production. Net carbonate production was estimated by considering erosional rates of ocean acidification, increasing cyclone intensity, local pollution, fishing pressure and the projected burdens of increases in SSTs (under Representative Concentration Pathways (RCPs) 4.5, 6.0 and 8.5) through to the year 2100. Our models predict that only 4 ± 0.1% (~60 000 km²) of Indo-Pacific coral reefs are projected to keep up with sea-level rise by the year 2100 under RCP 8.5 – most of which will be located near the Equator. However, with drastic reductions in emissions (under RCPs 4.5 and 6.0 Wm⁻²), we predict that 15 ± 0.3% (~250 000 km²) (under RCP 4.5 Wm⁻²) and 12 ± 0.7% (~200 000 km²) (under RCP 6.0 Wm⁻²) of Indo-Pacific coral reefs, have the potential to keep up with sea-level rise by the year 2100. Yet the burdens of fishing pressure and its cascading effects are projected to be responsible for substantial reef erosion, nearly halving the number of reefs able to keep up with sea-level rise. If action is taken immediately and emissions are drastically reduced to RCPs 4.5 or 6.0 Wm⁻², and reef management reduces the burdens of local pollution and fishing pressure, then our model predicts that 21–27% (~350 000–470 000 km²) of Indo-Pacific coral reefs – most of which will be located near the Equator – would have the potential to keep up with sea-level rise by the year 2100.     

Facies and diagenetic evaluation of the Permian–Triassic boundary interval and basal Triassic carbonates: shallow and deep ramp sections,Hungary

The Permian–Triassic boundary and basal Triassic shallow-marine successions were studied and correlated in sections of two structural units in Hungary (Transdanubian Range and Bükk units). Core sections in the Transdanubian Range unit recovered inner ramp deposits whereas outcrops in the Bükk unit expose deposits of the deeper ramp area of the western Tethys. The inner ramp section (studied ca. 10 m in thickness) is characterized by a succession of dolomites overlain by bioclastic limestones, peloidal grainstones (which recorded the biotic decline) and oolites with finely crystalline limestone interlayers. The deeper ramp section (studied ca. 15 m in thickness) is characterized by a succession of bioclastic limestones and marlstones, mudstone beds (recording the first biotic decline), the ‘boundary shales’ (recording the second biotic decline and the stable carbon isotope marker), mudstones with wackestone laminae, and stromatolite boundstones. Accordingly, oolite formation and microbial micrite precipitation represent carbonate sedimentary responses of end-Permian mass extinction on the carbonate shelf. In both successions, mudstones predominate the upsection, suggesting a relative sea-level rise. The succession of the deep ramp area exhibits a continuous sediment accumulation and the diagenesis here was influenced by marine and marine-derived pore water. The δ¹³C curve shows a continuous change towards more negative values, starting in bioclastic limestones, followed by a sharp symmetric negative peak at the second biotic decline that is a chemostratigraphic marker of the boundary event. Facies and microfacies trend of the inner ramp carbonates in the Transdanubian Range unit exhibits close similarities to that found in many South Alpine sections. Relict peloidal deposits, formed cemented submarine hardground substrate, indicate the extinction level. Sedimentary and diagenetic features of the overlying oolite bedset revealed slightly different depositional environments in the two studied Transdanubian Range unit sections. Petrography of the oolites highlighted shallow burial diagenetic alterations which includes marine cementation, marine-burial replacement and dolomitization. A lack of the specific negative peak in the δ¹³C values is most likely due to the multiple redeposition events of the sedimentary grains. This led to the conclusion that the deeper ramp deposits (e.g., in Bükk unit) have greater potential for recognizing trends in processes, affecting the marine environments and related to the end-Permian mass extinction, at the western Tethys.     

Growth models of Bajocian coral-microbialite reefs of Chargey-lès-Port (eastern France): palaeoenvironmental interpretations

Very large amount of microbialites, up to 70% of the reef volume takes part in the edification of Lower Bajocian coral reefs in the Chargey-lès-Port quarry (Haute-Saône, France). Such high amounts of microbialites were unknown within bioconstructions of Middle Jurassic age. Along the 16 m-thick section, seven successive biohermal or biostromal units developed on a shallow platform. Bioconstructions display a first coral growth phase with either constratal or superstratal growth fabrics. Coral fauna is relatively poorly diversified and is dominated by massive forms (Isastrea, Thamnasteria, and Periseris) or branched phaceloid (Cladophyllia) and ramose (Dendraraea) colonies. Corals can be heavily encrusted by microbialites of diverse forms and fabrics (leiolitic, thrombolitic, and stromatolitic). According to the coral growth fabrics, microbialite crusts developed on top of or at the underside of coral colonies, forming a coral-microbialite elementary unit. Microbialites show a multiphase development: (i) directly at the coral surface, a first and mm-scale microbialite layer locally developed; (ii) a second, cm-scale microbialite layer (up to 8 cm thick) covered the entire coral reef framework and assumed the main building role; and (iii) a third, mm- to cm-scale, laminated microbialite layer may also be observed onlapping previous reef structures, before having been progressively buried under sediments. Contemporaneously to the coral growth phase, the first microbialite layer developed on dead portions of coral colonies. The transition between coral growth and microbialite development (i.e., second layer of microbialites) is interpreted as a result of a coral reef crisis, probably reflecting more nutrient-rich conditions. The passage to a stromatolitic (third) layer suggests a control of the accumulation rate. Composition and architecture of coral-microbialite reef units of Chargey-lès-Port highlight the relations between high-frequency fluctuating environmental factors (mainly accumulation rate and trophic conditions) and reef development.     

Depositional environment and biofacies characterization of the Upper Pennsylvanian–Lower Permian deposits of the San Salvador Patlanoaya section (Puebla, Mexico)

The San Salvador Patlanoaya section (Puebla State of Mexico) has been subdivided into seven informal members labeled A–F. Members C–F have been dated as Missourian to Leonardian (equivalent to Kasimovian to Kungurian, i.e. lower Upper Pennsylvanian to upper Lower Permian). Members C–E display a shallowing-upward trend as does member F. The biodiversity of these carbonate deposits outcropping in the San Salvador Patlanoaya section is relatively low, although multiple microfossils (algae, small foraminifera, fusulinids and fish remains) are represented. Member C consists of calcarenitic limestone interpreted as distal tempestites interbedded with shaly limestone facies. These storm beds are composed mainly of silt, sand-sized quartz grains and bioclasts. Member D corresponds to coarse bioclastic limestones represented by calcareous tempestites and channel beds with erosional bases. Member E is composed of green shales and cross-bedded sandstones and gravelly conglomerates. Member F corresponds to a condensed package of limestones and interbedded siliciclastics and is Cisuralian (Early Permian) in age. Member G consists of condensed black shales and limestone nodules. The results of the study have significant implications for recognition of climatic and/or sea-level fluctuations in bioclastic–siliciclastic facies during the Late Pennsylvanian–Early Permian. The skeletal limestones and channel sandstones, common throughout the Pennsylvanian–Permian section, provide a constraint on palaeobathymetry, with the water depth fluctuating frequently around a position below, but near, the storm wave base.     

Early Triassic peritidal carbonate sedimentation on a Panthalassan seamount: the Jesmond succession,Cache Creek Terrane,British Columbia,Canada

The Jesmond succession of the Cache Creek Terrane in southern British Columbia records late Early Triassic peritidal carbonate sedimentation on a mudflat of a buildup resting upon a Panthalassan seamount. Conodont and foraminiferal biostratigraphy dates the succession as the uppermost Smithian to mid-Spathian. The study section (ca. 91 m thick) is dominated by fine-grained carbonates and organized into at least 12 shallowing-upwards cycles, each consisting of shallow subtidal facies and overlying intertidal facies. The former includes peloidal and skeletal limestones, flat-pebble conglomerates, stromatolitic bindstones, and oolitic grainstone, whereas the latter consists mainly of dolomicrite. The scarcity of skeletal debris, prevalence of microbialite, and intermittent intercalation of flat-pebble conglomerate facies imply environmentally harsh conditions in the mudflat. The study section also records a rapid sea-level fall near the Smithian-Spathian boundary followed by a gradual sea-level rise in the early to mid-Spathian.     

Fossil analogues of present-day Cladocora caespitosa coral banks: Sedimentary setting, dwelling community, and taphonomy (Late Pliocene, W Mediterranean)

 Cladocora caespitosa is a common zooxanthellate, ahermatypic, constructional scleractinian coral in shallow waters of the present-day Mediterranean. Extensive coral banks in Upper Pliocene shallow marine deposits of the Almería-Níjar Basin (SE Spain) contain the same species. These banks occur on debris-flow conglomerates deposited in a fan delta, or on bioclastic accumulations interpreted as storm deposits. Direct relationships of coral beds with coastal facies indicate that C. caespitosa colonized shallow settings near the paleocoast, probably not deeper than 20–30 m. Low turbulence allowed corals to colonize substrates, which remained stable for long periods. Activity of organisms in the coral community, storms, and detritic discharges from the fan delta were the most significant mechanisms disturbing the coral development. The hard substrata provided by coral banks promoted colonization by cemented and epibyssate organisms. Coral banks marked maximum flooding surfaces at the end of transgressive systems tracks. They were suddenly buried by sediment input into the basin. Taphonomic signatures measured on components of the coral bank communities indicate a low turbulence environment, probably a bay. The low hydraulic energy further inhibited post-burial reworking, thus promoting the in situ preservation of a great part of the organisms inhabiting the bioconstructions. Accepted: 2 December 1997     

Microfossil assemblages and relative sea-level fluctuations in a lagoon at the Oxfordian/Kimmeridgian boundary (Upper Jurassic) in the eastern part of the Paris Basin

The Late Oxfordian–Early Kimmeridgian interval of the eastern part of the Paris Basin is characterized by a carbonate succession deposited in shallow-marine platform environments. The Gudmont-Villiers section is represented by deposits ranging from barrier to typical lagoonal environments often poor in macrofossils. Previously unpublished calcareous microfossils are more abundant and provide alternative paleoenvironmental indicators. They also provide a biostratigraphical framework across the Oxfordian–Kimmeridgian boundary. The evolution of microfossil associations (algae and benthic foraminifera) in the lower part of the section, based on statistical analyses, is correlated to the sea-level variations. The first highly diversified association composed of small agglutinated and calcitic foraminifera (miliolids, textulariids, Spirillina, Trocholina, Molherina basiliensis etc.) characterizes high sea-level deposits; a second association richer in large agglutinated foraminifera (Alveosepta jaccardi, Everticyclammina, Nautiloculina oolithica) is significantly abundant in low sea-level deposits. A third association characterizes beds with a significant occurrence of encrusting microorganisms and algae (Lithocodium aggregatum, Troglotella incrustans, Cayeuxia piae, dasycladaceans). The upper part of the section is marked by more argillaceous beds and by the occurrence of one opportunist taxon (Lenticulina). This study shows that the microfauna-flora evolution in an internal carbonate platform environment constitute an efficient tool to determine variations in the relative sea level.     

Holocene sea level changes and reef development in the southwestern Indian Ocean

 The sedimentological and chronological study of Holocene reef sequences recovered in drill cores through modern reefs of Mauritius, Réunion Island and Mayotte allows the reconstruction of sea level changes and reef growth patterns during the Holocene. The branching-coral facies systematically predominates over coral head facies throughout the Holocene reef sequences, and Acropora is the main frame builder among the branching forms. The reconstructed sea level curves, based both on identification of coral assemblages and on radiometric U/Th ages, are characterized by a rapid rise between 10 and 7.5 ky BP, followed by a clear inflection between 7.5 and 7 ky BP. The stabilization of sea level at its present level occurred between 2000 and 3000 years ago, probably without a higher sea level stand. Rates of vertical reef accretion range between 0.9 and 7 mm. y^-1. In Mauritius, and also probably in Réunion Island, the reef first tracked, then caught-up to sea level to reach an equilibrium position (“catch-up” growth), while the barrier reef margin off Mayotte has been able to keep pace with rising sea level (“keep-up” growth). Accepted: 1 March 1997     

Drowning of a Lower Jurassic carbonate platform: Jbel Bou Dahar, High Atlas, Morocco

Summary The high-plateau of the Jbel Bou Dahar, situated in the Central and Eastern High Atlas of Morocco, represents a Lower Jurassic carbonate platform that drowned at the beginning of the Toarcian. Three phases of platform evolution can be distinguished: During thepre-drowning phase (upper Sinemurian— upper Pliensbachian) the platform interior facies reflects a restricted-marine lagoonal environment, protected by scattered buildups and cemented debris at the platform margin. Upper and mid-slope are dominated by coarse-grained, poorly sorted limestones, deposited through debris flows during sea-level lowstands. Sea-level highstand deposits occur at the toe of slope and are formed by an alternation of fine-grained litho- and bioclastic pack- to grainstones (turbidites), marls and mud- to wackestones (hemipelagic oozes). A condensed section, reflecting an abrupt and fundamental environmental change along the entire platform, characterises thedrowning phase (upper Pliensbachian— lower Toarcian). Within the platform interior densely packed biosparites represent the switch to high-energy environments, causing erosion of the former pre-drowning lagoonal sediments. These erosional products were redeposited on the platform slope, leading to the formation of coarse-grained non-skeletal sparites and micrites. Both platform interior and slope successions show a series of cyclic variations in sediment composition that could have been triggered by small-scale sea-level fluctuations. In contrast to the abrupt facies change at the pre-drowning —drowning boundary, the transition to thepost-drowning phase (lower Toarcian—Aalenian) is gradual. During this phase, biopelmicrites and pure micrites were deposited in all platform sections, followed by the deposition of calcistiltites. The facies point to quiet-water conditions below storm-wave base and display a uniform deep-marine sedimentation. This analysis shows that the drowning of the Jbel Bou Dahar carbonate platform was caused by abrupt and fundamental changes in the shallow-water realm. After exposure of the platform, these changes prevented the carbonate factory from re-establishing itself and made it impossible for the platform to keep up with the subsequent rise in sea level. These local changes were probably triggered by high-frequency sealevel variations in combination with regional or even worldwide changes in ocean circulation patterns.     

Disappearance of <Emphasis Type="Italic">Acropora</Emphasis> from the Marquesas (French Polynesia) during the last deglacial period

The major reef-building coral genus Acropora has never been recorded, living or fossil, from the Marquesas Islands in the central Pacific Ocean, which are characterized by limited modern reef formations. During the “Musorstom 9” cruise in 1997, investigations of marine platforms representing drowned reef systems revealed for the first time the presence of two Acropora species as fossils at seven Marquesas islands. The predominant species was Acropora valida, which was widespread in the archipelago and dated between 7.4 and 48.6 ka, providing evidence of an earlier Pacific distribution pattern broader than previously observed. It is proposed that disappearance of Acropora after 7.4 ka was linked to climatic events probably ENSO events controlling the distribution of corals and coral reefs in the eastern Pacific without excluding alternatively the effects of an increase in sea-level rise.