the bonarelli level and other black shales in the cenomanian-turonian of the northeastern dolomites...

Cretaceous Research (1999) 20, 135–167Article No. cres.1999.0146, available online at http://www.idealibrary.com on

The Bonarelli Level and other black shales in theCenomanian-Turonian of the northeasternDolomites (Italy): calcareous nannofossil andforaminiferal data

*Valeria Luciani and †Miriam Cobianchi

*Dipartimento di Scienze Geologiche e Paleontologiche, Universita degli Studi di Ferrara, Corso Ercole I) d’Este, 32,44100 Ferrara, Italy†Dipartimento di Scienze della Terra, Universita degli Studi di Pavia, via Ferrata, 1, 27100 Pavia, Italy

Revised manuscript accepted 16 October 1998

The Cenomanian-Turonian Antruiles section, in the northeastern Dolomites, northern Italy, consists of a cyclicallimestone-marl succession characterized by several black intervals. Two formations have been recognized: the ScagliaVariegata and the Scaglia Rossa. Integrated calcareous nannofossil and planktonic foraminiferal biostratigraphic data enableus to assign the Scaglia Variegata Formation to the Cenomanian (from the nannofossil CC9c to the CC10a Subzones p.p. andfrom the foraminiferal Rotalipora brotzeni to the Whiteinella archaeocretacea Zones p.p.), while the Scaglia Rossa Formationcorrelates with the Early-Middle Turonian (from the nannofossil CC10a Subzone to the CC12 Zone p.p., and fromthe foraminiferal W. archaeocretacea to the Helvetoglobotruncana helvetica Zones p.p.). The 1-m-thick black shale separating theScaglia Variegata from the overlying Scaglia Rossa has been identified as the Bonarelli Level, and is considered to be theexpression of the global OAE2 (Oceanic Anoxic Event). In the section described in this paper, more anoxic/dysoxic episodes,predating and postdating the Bonarelli Level, have been documented. The Total Organic Carbon (TOC) values and themicrofossil distribution patterns are discussed for these horizons, contributing to a better understanding of the extent ofdysoxic versus anoxic conditions. The TOC content reaches a high value only in the Bonarelli Level, which is devoid of bothplanktonic and benthonic faunas, while the other black levels contain less than 1% TOC. Corresponding to these levels, bothplanktonic foraminiferal and nannofossil assemblages are generally well preserved and diverse without any dominance ofeutrophic indicators; on the contrary, an increase of oligotrophic forms (rotaliporids and marginotrucanids) is recorded.Eutrophic radiolaria and calcispheres are very rare throughout the section. These data suggest that bottom redox cyclesprobably prevailed over productivity cycles, among the mechanisms leading to the formation of the Cenomanian-Turonianblack shales recognized in the Antruiles section.

Benthonic foraminiferal fauna is absent from the deposits where the TOC ranges from 0.5 to 1%, suggesting anoxicconditions on the sea floor. Where the TOC is less than 0.5%, benthonic forms are present, indicating dysaerobic conditionsof variable intensity. ? 1999 Academic Press

K W: calcareous plankton; integrated biostratigraphy; Cenomanian-Turonian; black shales; Bonarelli Level;OAE2.

1. IntroductionIn the northeastern Dolomites, near Cortinad’Ampezzo (Southern Alps, northern Italy), a cyclicalCenomanian-Turonian limestone-marl succession iswell exposed in the Antruiles section. It is character-ized by numerous black intervals containing a variablepercentage of organic matter, including the BonarelliLevel (OAE2 of Schlanger & Jenkyns, 1976).

The continuous occurrence and high abundance ofcalcareous nannofossil and planktonic foraminiferalassemblages throughout the section allowed a de-tailed, integrated study to be undertaken. This con-

0195–6671/99/020135+33 $30.00/0

tributes to a better understanding of the Cenomanian-Turonian interval, in which correlation of bioevents ofthe two fossil groups investigated is still being debated.

A further result of this study is the chronologicalattribution through biostratigraphy of the numerousblack intervals which occur below and above theBonarelli Level. Although more detailed results of acyclostratigraphic and isotopic study will be furnishedin a separate paper, the quantitative analyses of thefossil assemblages related to the lithology suggestsimplications for regional, palaeobiological andpalaeoceanographic events.

? 1999 Academic Press

136 Valeria Luciani and Miriam Cobianchi

Figure 1. Simplified map showing the palaeogeography of the northeastern part of the Southern Alps during the LateCretaceous and location of the section studied (modified from Bosellini et al., 1981).

2. Geological setting and stratigraphicframework

The Antruiles section crops out in a gorge cut by theRouibes de Inze torrent, 10 km northwest of Cortinad’Ampezzo, in the northeastern part of the Dolomiteregion (Figure 1). The name Antruiles (reported asAntruilles in Stock, 1994, and Bini et al., 1995),according to the local toponym, indicates the area ofpasture surrounding the valley.

The Dolomites belong to the northeastern SouthernAlps, which are interpreted as a south-verging thrustbelt of Neogene age, separated by the PeriadriaticLine from the north-verging Northern Alps (e.g.,Castellarin, 1979; Doglioni & Castellarin, 1985;Doglioni & Bosellini, 1987). In the Antruiles area inparticular, an intensive tectonic history, partiallyPaleogene and mainly Neogene in age, has distruptedthe stratigraphical succession (Bini et al., 1995). Thishas led to an isolation and lowering of theCenomanian-Turonian terrains with respect to theUpper Triassic and Jurassic carbonate-platformformations, which now constitute the peakssurrounding the valley. As a consequence of both thevegetative (tree and grass) cover and the tectonic

effects, the stratigraphic boundaries with the under-lying and overlying Cretaceous rocks are notvisible.

Cretaceous sediments are sporadic and of limitedextension throughout the whole Dolomite region. Thediscontinuity of outcrops reduces the possibility ofdelineating a detailed palaeogeography for thissector during the Cretaceous Period. However, theLower Cretaceous, plankton-rich, micritic sediments(Biancone Formation) suggest the occurrence of adeep basin which received some platform-deriveddebris. Dilution by a strong, probably cyclical (Claps& Masetti, 1994), input of clay in the pelagic sedimen-tation is documented by the Marne del PuezFormation of Neocomian age, which also containsrich ammonoid and bivalve faunas (Baccelle & LucchiGaravello, 1967; Leonardi, 1967). As a result of theerosion of the Austro-Alpine sector, deformed bythe Eoalpine phase, further terrigenous input isdocumented by silty and arenaceous sediments ofAptian-Albian age (e.g., Flysch di Ra Stua: ScudelerBaccelle & Semenza, 1974). The Late Cretaceousdeposits are enriched in resedimented calcarenitescontaining rudist fragments.

The Bonarelli Level and other black shales 137

The Cretaceous succession of the Dolomite regionwas thus deposited in a deep basin, receiving varioustypes of resedimented deposits. This basin was locatedin the northeastern part of the Trento Plateau, apalaeogeographic element identified as a consequenceof the drowning of the Lower Jurassic carbonateTrento Platform, which is roughly oriented north-south and bordered eastwards by the Belluno Trough,and westwards by the Lombardian Basin (Aubouin,1964; Castellarin, 1972; Winterer & Bosellini, 1981).During the Cretaceous, the Lombardian Basin re-ceived a huge amount of flysch sediment with respectto the eastern basin, distinguished for this reason asa distinct domain. A shallow-water platform with arudist reef margin, the Friuli Platform, was active inthe southwestern sector (Figure 1).

Lithostratigraphy

On the Italian geological map (Sheet 12, Pieve diCadore of the ‘‘Carta Geologica delle Tre Venezie’’and ‘‘Carta Geologica 1:1000.000’’), the Antruilessuccession was attributed to the Lower Cretaceous.More recent studies refer the succession to a Ceno-manian (Stock, 1994) or Cenomanian-Turonian age(Bini et al., 1995). Neither of these studies, however,presents a detailed biostratigraphic analysis, and theydo not show the occurrence of the Bonarelli Level andthe other black intervals. A controversy exists withrespect to the attribution of a Cretaceous age for thesesediments. Stock (1994) attributed almost the entiresuccession to the Scaglia Variegata, because of itssimilarity to the mostly marly unit which crops outextensively in the Southern Alps (Figure 2). In theupper part, where the amount of terrigenous inputincreases, the Antruiles Formation was distinguished(20 m, at the top of the Cenomanian). Conversely,according to Bini et al. (1995), the Cenomanian-Turonian succession is assignable to the Rouibes deInze Formation. These authors determined its thick-ness to be 60 m. Stock (1994) estimated it to be150 m. According to our measurements it is 132.5 mthick.

A further complication exists. Although the simi-larity of the Late Cretaceous sediments to the ScagliaRossa Formation (the well-known pelagic unit thatcrops out in many Mediterranean areas) is admitted inboth of the previously-mentioned papers, Stock(1994) named the succession the Rouibes Formationwhereas Bini et al. (1995) attributed it to the AntruilesFormation (Figure 2).

The lithologies of the section analyzed in our paperconsist of alternations of grey-greenish to reddishmarls or silty marls with micritic marly limestones or

siltstones, and include a number of black sediments ofvariable thickness (see below for a more detailedlithostratigraphic description). A major change fromthe predominantly grey-greenish marly-silty sedimentsto red-coloured silty marls and limestones occursimmediately above the Bonarelli Level (a black shale,1 m thick), 37.2 m below the top. It is beyond theaim of this paper to solve the lithostratigraphicalquestion. In this context, in order to avoid theintroduction of further lithostratigraphical names andthe disorder introduced by the previous studies, weconsider the lower part of the Antruiles section tobelong to the Scaglia Variegata Formation, and theupper part, above the Bonarelli Level, to the ScagliaRossa Formation. We take into account the mainsimilarities between the succession studied and theabove-mentioned, widespread stratigraphic units,rather than the differences in the local lithofacies(Figure 2).

Figure 2. Proposed lithostratigraphy for the Late Creta-ceous succession of the Cortina sector, northeasternDolomites. The columns show the attribution to differ-ent formations of the same sediments by variousauthors. For discussion, see text.

3. Lithostratigraphy of the section

Two main units can be recognized within the succes-sion (Figure 3). The lower unit is 95.3 m thick, andbelongs to the Scaglia Variegata Formation (base notexposed). High-frequency sediment cycles occurthroughout the unit. These consist of a succession ofthree types of lithology: burrowed marly limestone(or sometimes siltstone), marlstone, and calcareousshale which are usually black. In addition to this,


Figure 3. Stratigraphic column of the Antruiles section with sample numbers alongside, and biostratigraphic data fromcalcareous nannofossils (underlined italic) and planktonic foraminifera. Solid arrows indicate the main biostratigraphicevents; hollow arrows indicate subsidiary events. Planktonic foraminiferal zones are according to Robaszynski & Caron
.


(1995); the R. brotzeni Zone is equivalent of the R. globotruncanoides Zone, as the latter species is retained as a juniorsynonym of R. brotzeni. The subdivision of the R. cushmani Zone into two subzones is according to Sliter (1989). Thenannofossil zones are according to Sissingh (1977) and Perch-Niesen (1985).


there is a cyclical variation in colour, fossils andichnofacies.

The black levels are more-or-less laminated on a‘varve’-scale. The marls are greenish-grey or reddishwith frequent black Chondrites burrows, especiallynear the black intervals. The marly limestones aregreenish-grey or pinkish, and contain large burrows(Zoophycos). Planktonic foraminifera occur through-out the rock types, while the calcium carbonatecontent reflects nannofossil abundance.

In the Scaglia Variegata Formation, seven lithologi-cal units, A–G, can be distinguished, from bottom totop as follows (Figure 3):

A (samples 1–5; 6.2 m thick). Greenish-grey marlsand thinly-bedded marly limestones with conspicu-ous, large black burrows.

B (samples 6–11; 8.9 m thick). Dark grey, fissilemarlstones.

C (samples 12–22; 13.3 m thick). Cyclical alter-nation of red to grey marly limestones and marls. Thethickness of the couplets is c. 50 cm (Figure 4).

D (samples 23–30; 6.9 m thick). Cyclically-arranged couplets of greenish-grey to pinkish, biotur-bated marly limestones, containing Zoophycos, andgreenish-grey to reddish marls. In this lithologicalunit, three redox cycles on a bed scale are recorded.They consist of grey marl-laminated, black, calcareousshale couplets.

E (samples 31–38; 1 m thick). Cyclical alternationof greenish-grey to reddish calcareous marls andgreen-grey, sometimes pinkish, limestones.

F (samples 39–69; 46.6 m thick). Laminated blackshales on a bed-scale, green-grey to reddish marl-stones, and finally green-grey to pinkish limestones.These sequences show thickening-upwards trends.Thirty-one redox cycles are recorded in this interval.

G (samples 70–72a; 5.5 m thick). Couplets ofgreenish-grey limestone and marlstone, sometimesdark red in colour.

The Scaglia Variegata Formation is topped by theBonarelli Level (sample 73; 1 m thick). This consistsof black, bituminous shales laminated on a ‘varve’-scale.

The upper unit is 37.2 m thick, and belongs to theScaglia Rossa Formation (unit H; samples 73a–91). Itis characterized by a cyclical alternation of brick-redmarl and pink-red pelagic limestone (Figure 5).Bedding thickness ranges from 20 to 30 cm. Stylolitesare common. Towards the top of the section, threerhythmically-bedded units are identified, consisting ofalternating black-laminated clay and greenish-greymarls. The highest measured limestone bed containsblack cherty nodules and layers.

4. Calcareous nannofossils

Figure 4. Cyclical alternations of marls and marly lime-stones of the Scaglia Variegata Formation, Antruilessection.

Figure 5. Marl and limestone couplets in the Scaglia RossaFormation, Antruiles section.

4.1. Materials and methods

One hundred and nine samples from all the rock-types(marly limestone, marl, calcareous shale and blackshale) were analyzed. Smear-slide preparation waskept simple in order to retain the original compositionof the nannofossil assemblages, and was made fromthe same samples as used for the foraminiferal study.

Estimates of calcareous nannofossil totals andspecies abundance were carried out using a polarizinglight microscope at a magnification of 1250#. Foreach smear-slide, 300 fields of view were observed inrandom traverses. Calcareous nannofossil totals andspecies abundance were semiquantitatively evaluatedas follows:

Total abundance: A=abundant: 10-15 specimens perfield of view (FOV); C=common: 1–9 specimens per


FOV; F=few: 1 specimen per 1–10 FOV; R=rare: 1specimen per 11–300 FOV; B=barren.Species abundance: A=abundant: >1 specimen perFOV; C=common: 1 specimen per 1–10 FOV;F=few: 1 specimen per 11–30 FOV; R=rare: 1 speci-men per >30 FOV.

The preservation estimate was recorded after closeinspection of dissolution and overgrowth, andwas coded as follows: M=moderate; PM=poor tomoderate; P=poor.

Figure 6. Comparison between calcareous nannofossil biozonations for the Cenomanian-Turonian interval.

4.2. Calcareous nannofossil biostratigraphy

Many nannofossil biostratigraphic zonations for theLate Cretaceous have been proposed (Cepek & Hay,1969; Thierstein, 1976; Manivit et al., 1977; Sissingh,1977; Verbeek, 1977; Roth, 1978; Crux, 1982;Perch-Nielsen, 1985). However, these schemesshow certain discrepancies, for example in theCenomanian-Turonian interval, probably related tothe reliability of datums (Figure 6). As pointed out byBralower (1988), this interval is characterized by

variable facies patterns. For example, a widespreadhiatus is documented in most of the northwestEuropean sections and, in the Tethys area, carbonatedissolution occurred in bottom waters or during earlydiagenesis, hence affecting biostratigraphic data.Nevertheless, the Cenomanian-Turonian interval hasbeen characterized by marked changes in the palaeo-environmental and biological features, as well as in theburial of large amounts of marine organic matter.

Although the calcareous nannofossil biostratigraphyfor the Cenomanian-Turonian interval has beenimproved in the last decade, resulting in increasedbiostratigraphic resolution (Bralower, 1988; Gartnerin Robaszynski et al., 1990; Ghisletti & Erba inPremoli Silva & Sliter, 1995; Bralower et al., 1995;Burnett, 1996; Burnett, in press), there is some dis-agreement about the sequence of the biostratigraphicevents and their relative chronostratigraphic cali-bration, as well as the nannofossil definition ofthe Cenomanian/Turonian boundary. In the SecondInternational Symposium on Cretaceous StageBoundaries in Brussels (1995), the Global Boundary


Stratotype Section and Point for the Cenomanian/Turonian boundary has been placed at the base of Bed86 in a section at Rock Canyon Anticline (Colorado,USA), coincident with the first occurrence of theammonite Watinoceras devonense Wright & Kennedy,1981. The nannofossil Quadrum gartneri and theplanktonic foraminifer Helvetoglobotruncana helveticafirst occur less than 1 m above the base of Bed 86.

The main aim of this study is to provide newbiostratigraphic data, thus contributing to the data-base on the stratigraphy and palaeoceanographic andsedimentologic events of this time interval. Among the109 samples analyzed, five are devoid of calcareousnannofossils. Figure 7 summarizes the total abun-dance, preservation and species distribution of thenannofloras in the section. Several species are shownin Figures 8 and 9.

Preservation worsens and abundance decreasesfrom the bottom of the section upwards. The cyclicalalternations of limestones, marlstones and blackshales, belonging to the Scaglia Variegata Formation,contain well-preserved and diverse nannofloras show-ing high total abundance. The Bonarelli Level iscompletely barren of calcareous nannofossils. Thecalcareous nannofossil content of the Scaglia RossaFormation has been strongly altered by diageneticprocesses. The assemblages show the effects ofdissolution and calcite overgrowth.

In the section studied, the major calcareous nanno-fossil events recognized are as follows (in stratigraphi-cal order): the first occurrence (FO) of Lithraphiditesacutus (sample 33, 42 m from the bottom of thesection); the FO of Gartnerago segmentatum (sample70, 89.5 m); the last occurrence (LO) of Axopodorhab-dus albianus (sample 72a, 94 m); the LOs ofCorollithion kennedyi and Microstaurus chiastus (sample74, 98 m); the FO of Quadrum gartneri (sample 74b,101 m); the FO of Eiffellithus eximius (sample 86,127 m). Subsidiary biohorizons recorded in theinvestigated interval are discussed below.

From the bottom of the section upwards fourbiozones and three subzones have been determined.

CC9c Zone of the Standard Zonation. Section: 0–41 m.Age: Cenomanian (Perch-Nielsen, 1985). This zonewas defined by Thierstein (1971) and emended bySissingh (1977). The FO of E. turriseiffelii has beenused by all authors to define the lower boundary of thezone, while the definition of the upper boundary variesin the different biostratigraphic schemes. Apparently,the most reliable event is the FO of Lithraphiditesacutus proposed by Manivit et al. (1977). Accordingto these authors, this falls within the CC9 Zone ofSissingh (1977), while Perch-Nielsen (1985) places

this event at the base of CC10. This interval, based onthe LO of Hayesites albiensis and the FO of Corollithionkennedyi, was divided by Perch-Nielsen (1985) intothree subzones, named CC9a–c.

The lowest 41 m of the section (samples 1–32) canbe correlated with the CC9c Subzone of Perch-Nielsen (1985) which in turn equates to SubzoneNC10B of Bralower et al. (1995). The nannofossilassemblages are highly diversified and moderately wellpreserved. C. kennedyi is recorded from the bottom ofthe section upwards. In sample 4, Watznaueriabritannica disappears (LO); in sample 5 the FO ofHelicolithus trabeculatus is recorded.

Species abundant in this interval include Biscutumconstans, Cretarhabdus crenulatus, Cribrosphaerellaehrenbergii, Eiffellithus turriseiffelii, Lithraphidites carnio-lensis, Prediscosphaera columnata, Rhagodiscus achlyos-taurion, R. embergeri, and Zeugrhabdotus diplogrammus.Rare nannoconids are represented by Nannoconustruttii and N. elongatus.

CC10 Zone of the Standard Zonation. Section: 41–101 m. Age: Late Cenomanian (Perch-Nielsen, 1985).Manivit et al. (1977) defined the Lithraphidites acutusZone as the interval from the FO of L. acutus to theFO of Quadrum gartneri, and thus this zone coincideswith the CC10 Zone of Sissingh (1977). Manivit et al.(1977) and Perch-Nielsen (1985) divided this intervalinto two subzones (CC10a and CC10b) on the basisof the LO of Microstaurus chiastius.

The stratigraphic interval from sample 33 to sample74 (65 m) can be correlated with the CC10a Subzoneof the Standard Zonation (Sissingh, 1977; Perch-Nielsen, 1985). In this interval the Bonarelli Level(sample 73), completely devoid of calcareous nanno-fossils, was recognized. The stratigraphic interval fromsample 74 to sample 74b (3 m) is correlatable with theCC10b Subzone, which in turn equates to Zone UC6of Burnett (in press).

In the section studied, and in most other sections,the FO of Q. gartneri is recorded above the LO ofM. chiastius. According to many authors, however,(e.g., Manivit et al., 1977; Crux, 1982; Gartner inRobaszynski et al., 1990; Ghisletti & Erba in PremoliSilva & Sliter, 1995) the FO of Q. gartneri predates theLO of M. chiastus, while according to Bralower (1988)this species appears slightly below the first occurrenceof Gartnerago segmentatum.

Probably because of preservational processes,the nannofossil assemblages of the Scaglia RossaFormation, above the Bonarelli Level, are depletedof some species found by other authors across theCenomanian-Turonian boundary. These includeRhagodiscus asper, whose LO has been used by


Bralower (1988) to define the upper boundary of hisR. asper Zone. However Burnett (in press) also docu-ments the LO of R. asper below the Cenomanian/Turonian boundary.

Other nannofossil events occur in this interval,including the LO of A. albianus (sample 72a), the LOsof M. chiastius and C. kennedyi (sample 74) and theFO of Quadrum intermedium (sample 74a). The LOsof A. albianus and M. chiastius are consistently re-corded in this order, and have high reliability indices(Bralower, 1988; Burnett, in press). The LO of C.kennedyi is always documented below that of A.albianus and/or M. chiastus, and below the BonarelliLevel (Ghisletti & Erba in Premoli Silva & Sliter,1995). In the Antruiles section C. kennedyi seems todisappear later, above the LO of A. albianus and theBonarelli level. For this species a younger extinctionwas also documented for North Africa by Gartner inRobaszynski et al. (1990). However this younger ex-tinction in the Antruiles section can be explained byreworking at this level. Finally, in the section studied(and elsewhere) the genus Quadrum occurs for thefirst time below the first appearance of Q. gartneri withthe species Quadrum intermedium.

CC11 Zone of the Standard Zonation. Section: 101–126.9 m. Age: Early and Middle Turonian (Perch-Nielsen, 1985). This zone was defined by Cepek &Hay (1969) as the interval from the FO of Quadrumgartneri to the FO of Eiffellithus eximius. Sissingh(1977) defined his Q. gartneri Zone as the intervalfrom the FO of Q. gartneri to the FO of Lucianor-habdus maleformis. Perch-Nielsen (1985) equated theFOs of E. eximius and L. maleformis, but recognisingE. eximius as more widely correlatable.

The stratigraphic interval from samples 74b to86 can be correlated with the CC11 Zone of theStandard Zonation. The assemblages are dominatedby the genus Watznaueria and Cretarhabdus crenulatus,Eiffellithus turriseiffelii, Eprolithus floralis, Lithraphiditescarniolensis and Zeugrhabdotus erectus.

In this interval, the FOs of Eprolithus octopetalus(Varol, 1992; sample 79) and Eprolithus eptapetalus(Varol, 1992; sample 81) are recorded. These twoevents slightly predate the first occurrence of thegenus Lucianorhabdus (sample 82).

CC12 Zone of the Standard Zonation. Section: 126.9–132.5 m. Age: Late Turonian to early Early Coniacian(Perch-Nielsen, 1985). This zone was defined bySissingh (1977) as the interval from the FO of Lucian-orhabdus maleformis (E. eximius, Perch-Niesen, 1985)to the FO of Marthasterites furcatus. As the FO ofE. eximius was observed in sample 86, the uppermost

part of the section (5.6 m) can be correlated with thelower portion of the CC12 Zone (M. furcatus isabsent). The assemblages record a decrease inabundance and diversity.

Figure 10 shows the comparison between the eventssequence around the Cenomanian/Turonian bound-ary highlighted by Burnett (in press) from a globalperspective and the events sequence recorded in thiswork. Several events show the same order and may becorrelatable in different basins, such us: the FO ofC. kennedyi; the LO of W. britannica; the FO and LOof L. acutus; the LO of A. albianus; the LO of M.chiastius, and the FOs of Q. gartneri and E. eximius. Anumber of events are not correlatable (e.g., the FO ofG. segmentatum; the LOs of C. striatus and C. kennedyi;the FO of Q. intermedium). For shedding light on whythey cannot be correlatable, it is neccessary to studyother sections in this basin or in adjacent basins.However, this comparison might provide someinformation on the local nannofossil biogeographicsituation around this time.

5. Planktonic foraminifera

5.1. Materials and methods

Foraminiferal analysis was carried out on 108 sampleswhich were collected every 1.5 m, with a closersampling rate through the black intervals,depending on the condition of the outcrops. Indu-rated lithologies (limestones and some marlylimestones) were analyzed in thin section; washedresidues were obtained from marls, silty marls andblack shales by disaggregation using desogen andsieves of 38 ìm and 63 ìm mesh. In some cases,ultrasonic treatment was necessary to clean encrustedspecimens.

Quantitative analysis was applied to the planktonicassemblages by counting numbers of each species in apopulation of at least 300 specimens, both in thinsection and washed residues (on populations >63 ìm)being carried out. Percentages are reported in therange charts (Figure 11). Washed residues (includingthe smallest fraction) and thin sections werefurther observed to ascertain the occurrence ofvery rare species. The state of preservation variesfrom poor (encrusted, infilled and recrystallized speci-mens) to moderate (clearly recognizable specimens,although mainly recrystallized). Some species,including the zonal markers, are illustrated inFigures 12–14.

5.2. Planktonic foraminiferal biostratigraphy

Several schemes have been proposed in the last fewdecades for Cretaceous planktonic foraminiferal bio-


stratigraphy (e.g., Bolli, 1959, 1966; Moullade, 1966;Sigal, 1977; Robaszynski & Caron, 1979, 1995; Sliter,1989, 1992). Although problems still exist in theapplication of planktonic foraminiferal zones, particu-larly in the Lower Cretaceous, biozones for theCenomanian-Turonian interval are quite well estab-

lished and widely applicable, at least at low-temperatelatitudes.

Planktonic foraminifera are mostly abundant andwell preserved throughout the section, allowing recog-nition of the bioevents on which the zones and sub-zones of this interval are based. The zonal schemes of

Figure 7. Calcareous nannofossil species distribution in the Antruiles section plotted against age and biozones.


Figure 7. Continued.


Figure 8. Cross-polarized light (XPL) micrographs of calcareous nannofossil species from the Antruiles section. Magnifi-cation 3200#. a, Biscutum constans; sample 97. b, Chiastozygus litterarius; sample 65. c, d, Corollithion kennedyi; sample65. e, Cretarhabdus conicus; sample 97. f, Cretarhabdus crenulatus; sample 97. g, Cyclagelosphaera margerelii; sample 7.h, i, Eiffellithus eximius; sample 91. j, Helicolithus trabeculatus; sample 98. k, Eiffellithus turriseiffelii; sample 15. l, m,Eprolithus eptapetalus; sample 88. n, Eprolithus floralis; sample 74b. o, Eprolithus octopetalus; sample 86. p, Manivitellapemmatoidea; sample 28. q, Haqius circumradiatus; sample 100. r–t, Microstaurus chiastius; sample 71.


Figure 9. XPL micrographs of calcareous nannofossil species from the Antruiles section. Magnification 3200#. a, Microrhab-dulus decoratus; sample 82. b, Prediscosphaera columnata; sample 72. c, Quadrum gartneri; sample 86. d, Quadrum intermedium;sample 74a. e, f, Rhagodiscus angustus; sample 33. g, R. angustus; sample 97. h, R. asper; sample 15. i, Rhagodiscus splendens;sample 70. j, Watznaueria barnesae; sample 72. k, Zeugrhabdotus embergeri; sample 72. l, Z. erectus; sample 97.

Sliter (1989, 1992) and Robaszynski & Caron (1995)are applied. Deposition of black levels throughout thesection did not generally influence the planktonicassemblages (Figure 11), except for the BonarelliLevel (see discussion below). The zones and subzonesrecognized are as follows (from oldest to youngest):

Rotalipora brotzeni Zone p.p.

Interval zone from the FO of R. brotzeni to the FO ofR. reicheli. (samples 1–14, 0–25.6 m). The R. brotzeniZone corresponds to the R. globotruncanoides Zoneof Robaszynski & Caron (1995); this species isconsidered to be a junior synonym of R. brotzeni.

The base of the zone has not been recognized,R. brotzeni already occurring in the basal sample. Thezonal marker is, however, rare or very rare, particu-larly in the lower part of the section. The scarcity of R.brotzeni, which makes it difficult to identify precisely

the base of the zone, is a common characteristic ofmost Cenomanian sections. This fact induced someauthors (e.g., Leckie, 1984) to define a Rotaliporagandolfii Zone, containing the zonal marker, from theLO of Planomalina buxtorfi to the FO of R. reicheli. Inthe Antruiles section, R. gandolfi is generally abundantthroughout this interval, whereas P. buxtorfi is absent.

The top of the zone, coinciding with the FO ofR. reicheli, has been reliably recognized in sample 15,despite the scarcity of the species.

Rare to common specimens attributed to R. green-hornensis have been observed among rotaliporids.With respect to the typical, more evolved represen-tative of the species (occurring in the Dicarinellaalgeriana Subzone), they are smaller and have aslightly more biconvex profile instead of the typical,almost plano-convex, side view. This species is gener-ally reported later in the R. cushmani or R. reicheliZones (e.g., Robaszynski & Caron, 1979; Caron,


1985; Sliter, 1989; Sliter & Premoli Silva 1990;Premoli Silva & Sliter, 1995). An earlier occurrenceof Rotalipora greenhornensis, represented by both bi-convex and umbilico-convex specimens, has also beenrecorded by Robaszynski et al. (1993) from theCenomanian of Tunisia, where the species occursbelow the FO of R. reicheli, in agreement withour data.

Hedbergellids and globigerinelloids constitute thebulk of assemblages of the Rotalipora ticinensisZone. Among smaller, unkeeled forms, shackoinids(Shackoina cenomana, S. moliniensis, S. multispinata),Guembelitria cenomana and Heterohelix moremani areconsistently present, but generally very rare.

Among the larger forms, rotaliporids (particularlyR. appenninica) are more abundant than praeglo-botruncanids. Rotalipora montsalvensis first occurs withrare specimens 10 m below the top of the zone.

Rotalipora reicheli Zone

Interval from the FO of R. reicheli to the FO of R.cushmani (samples 15–19, from 25.6 m to 30 m abovethe base).

This zone was originally defined as a total rangezone by Bolli (1966), and is generally used with thismeaning in the standard low-latitude schemes (e.g.,Caron, 1985; Sliter, 1989). However, the LO of R.reicheli has not proved to be exactly coincident with

the FO of R. cushmani, because the ranges of thesespecies are, at times, partially overlapping. In theirrevision of the Cretaceous zonation, Robaszynski &Caron (1995) take into account this problem, definingthe Rotalipora reicheli Zone as an interval zone. In theAntruiles section, R. cushmani appears 14 m below theLO of R. reicheli; however, R. reicheli is generally rare.Rotalipora appenninica is still the dominant formamong rotaliporids; R. gandolfi and R. micheli are alsoabundant. Praeglobotruncana stephani, constitutes animportant part of the assemblages and shows morpho-logical variability with respect to the height of thespire; in this interval the high-spired morphotypes areabundant. No additional bioevents were recognized inthis short zone.

Rotalipora cushmani Zone

Interval corresponding to the total range of the species(sample 19–72b, 30–62.5 m). The top of the zonecoincides with the extinction of the genus Rotalipora,below the Bonarelli Level (OAE2). The Rotaliporagreenhornensis and Dicarinella algeriana Subzones havebeen distinguished on the basis of the FO of D.algeriana (Sliter, 1989, 1992; Bralower et al., 1995).

Rotalipora greenhornensis Subzone

Interval from the FO of R. cushmani to the FO of D.algeriana (samples 19–32A; 30–41.6 m). Among thelarger forms, rotaliporids are more abundant withrespect to praeglobotruncanids. Praeglobotruncanagibba first occurs slightly above the base of the sub-zone. R. reicheli overlaps the range of R. cushmani foran unusually wide interval. In fact, the LO of theformer species is in the lower part of the followingsubzone.

The planktonic assemblage is similar to that of theprevious zone, except for the disappearance of R.gandolfi and the FO of P. gibba near the base of thesubzone. Hedbergellids and globigerinelloids consti-tute the bulk of the smaller unkeeled group. Althoughconsistently present, heterohelicids and shackoinidsare never a major component of the assemblages.

Figure 10. Comparison between the stratigraphic sequenceof the calcareous nannofossil events throughout theCenomanian-Turonian interval indicated by Burnett(in press) and in this work.

Dicarinella algeriana Subzone

The zonal marker first occurs in sample 32A (41.6 mabove the base of the section). This subzone is alsocharacterized by the FO of the genus Whiteinella,small specimens of W. baltica and W. brittonensisoccurring in the basal part. Whiteinellids, however,represent a quantitatively minor component of the


assemblages. They increase in abundance, size anddiversity in the upper part of the subzone.

Globigerinelloids and hedbergellids still consitutethe bulk of the smaller fraction, which also containsshackoinids and heterohelicids. Heterohelix reussi firstoccurs, although inconsistently, towards the base ofthe subzone (sample 38) as small, very rare specimens,whereas H. moremani increases in size from the middlepart of the subzone.

Dicarinellids (Dicarinella imbricata, D. algeriana) areconsistently present and almost as abundant as rotali-porids. Towards the top of the subzone D. hagni andD. canaliculata also occur for the first time. Praeglo-botruncanids are a constant component of theassemblages, but less common than dicarinellids.

Rotaliporids reach their greatest diversity in thelower part of the subzone, as the Late Albian-EarlyCenomanian species coexist here with the moreevolved forms. Towards the top of the subzone, therotaliporids decrease remarkably in abundance. Glo-bigerinelloides bentonensis occurs for the last time at thetop of the subzone, below the Bonarelli Level. Thelast occurrence of this species has also been recordedbelow the Bonarelli Level in the Umbria MarcheBasin (Premoli Silva & Sliter, 1995).

Whiteinella archaeocretacea Zone

Interval from the LO of the rotaliporids to the FO ofHelvetoglobotruncana helvetica (samples 73–74a). Thebase of this interval includes the Bonarelli Level,which is devoid of foraminifera. A poor, very smallplanktonic fauna occurs in the first sample above it.It contains mainly hedbergellids, schackoinids andwhiteinellids, as well as extremely rare dicarinellidsand praeglobotruncanids. Larger planktonic assem-blages, dominated by whiteinellids, characterize theupper part of the zone.

Helvetoglobotruncana helvetica Zone

Interval corresponding to the total range of the zonalmarker. In the Antruiles section, this zone is onlypartially represented, as the LO of H. helvetica was notobserved (samples 74b–91). The first representativesof the zonal marker are quite small and rare; theybecome more abundant and highly variable morpho-logically towards the top of the section. In this inter-val, the FO of the genus Marginotruncana (with thespecies M. renzi, M. sigali, and M. schneegansi) isrecorded; the FO of the more evolved M. pseudolin-neiana is recognized higher up, in sample 85. Thespecies Helvetoglobotruncana praehelvetica, which ap-pears below the Bonarelli Level, is still present at thetop of the section.

5.3. Evolutionary changes in the planktonic foraminiferalassemblages

The evolution and diversification of planktonicforaminifera are generally associated with times ofoceanic stability, favouring a water-column stratifi-cation which presents a trophic structure with differ-ent ecological niches (e.g., Hart, 1980; Caron &Homewood, 1983; Leckie, 1987, 1989). This conceptis based in part on the similarity of these forms withthe modern species, which display greater diversity atlow latitudes where the ecological niches are numer-ous as a consequence of stable oceanic conditions anda deep thermocline. Complex morphotypes have ahigh specific diversity, occupy deeper oligotrophicenvironments, and have adopted a k-mode-specializedlife-strategy. Simple morphotypes inhabit shallowerwater, which is characterized by unstable, mostlyeutrophic conditions. These forms predominate at highlatitudes and in upwelling areas, displaying an opportun-istic, r-mode life-strategy. Between these two extremes,mesotrophic niches are occupied by intermediate mor-photypes (e.g., Be, 1977; Fairbanks & Wiebe, 1980;Fairbanks et al., 1982; Hemleben et al., 1989).

Although extinct, the life strategies of Cretaceousplanktonic foraminifera are believed to have beensimilar. This has been indicated by the palaeoeco-logical, biogeographic and isotopic studies of manyauthors (e.g., Eicher & Worstell, 1970; Douglas,1972; Sliter, 1972; Wonders, 1980; Boersma &Shackleton, 1981; Caron & Homewood, 1983;Leckie, 1989; Corfield et al., 1990). For theCenomanian-Turonian interval, the r-mode, oppor-tunist, eutrophic forms have been identified as thesmall, thin-walled Hedbergella, Heterohelix andGuembelitria, whereas the specialized, oligotrophic,k-strategist group includes the more complex, thicker-walled, larger-keeled forms, such as Rotalipora andMarginotruncana. Mesotrophic environments wereinhabited by intermediate morphotypes such as Cos-talligerina, Globigerinelloides, Schackoina, Whiteinella,Dicarinellida, Praeglobotruncana, Falsotruncana andHelvetoglobotruncana (Leckie, 1987, 1989; PremoliSilva & Sliter, 1995). Fluctuations in abundance anddiversity of the various groups can contribute tounderstanding palaeoceanographic changes.

For the low to middle-latitude planktonic foramin-ifera, the Cenomanian-Turonian interval represents aperiod of diversification, mainly expressed by enrich-ment in the oligo-mesotrophic groups, and inter-rupted by the major crisis corresponding to theCenomanian/Turonian Boundary Event (CTBE ofArthur et al., 1987). This general trend is alsodocumented in the Antruiles section, where the


Figure 11. Distribution of planktonic foraminiferal species recovered from the Cenomanian and Cenomanian-TuronianAntruiles section plotted against age and biozones. Sample numbers in italic indicate thin sections; shading highlights theblack levels. In the chart, only those samples that contain abundant planktonic assemblages, and which are preserved wellenough to have allowed quantitative analysis, are plotted. Subdivision of planktonic foraminifera into eu-, meso- andoligotrophic groups is tentative, and based on analogies of Cenomanian-Turonian forms with the modern species, in part

corroborated by palaeoecological, biogeographic and isotopic studies of many authors (see discussion in the text).Benthic forms represent a minor component of the foraminiferal assemblages throughout the section. Distribution ofother fossil groups, such as radiolaria, calcispheres and ostracods, are discontinuous and generally too scarce to beincluded; radiolaria are very abundant only in sample 90.


Figure 12. Photomicrographs of selected Cenomanian-Turonian planktonic foraminifera from the Antruiles section,including most of the zonal markers. Magnification 80#. a, Rotalipora brotzeni; sample 32A. b, Rotalipora reicheli; sample32A. c, Rotalipora cushmani; sample 40. d, Helvetoglobotruncana helvetica; sample 87. e, Rotalipora micheli; sample 27. f,Rotalipora montsalvensis; sample 19. g, Dicarinella algeriana; sample 99. h, Dicarnella canaliculata; sample 84. i, Dicarinellaimbricata; sample 84. j, Whiteinella brittonensis, primitive specimen, sample 44. k, Whiteinella baltica; sample 118.l, Whiteinella paradubia; sample 84A. m, Costalligerina lybica; sample 19. n, Praeglobotruncana stephani; sample 28.o, Hedbergella simplex; sample 92.

FOs and diversification of the meso-and oligotrophicgenera Whiteinella, Dicarinella and Marginotruncanahave been observed. Some forms belonging to theoligotrophic and eu-mesotrophic groups are shownrespectively in Figures 13 and 14. Diversity (simplyexpressed by number of species) increases from 20 to

29 from the base to the top. The rate of increase is lowin the R. brotzeni and R. reicheli Zones and in the R.greenhornesis Subzone. Diversity reaches a total of 26in the lower part of the D. algeriana Subzone, wheredicarinellids and whiteinellids first occur, and themore primitive rotaliporids (R. appenninica, R. micheli,


Figure 13. Scanning electron micrographs of some Cenomanian-Turonian species from the Antruiles section, belonging tothe more specialized, oligotrophic groups. Magnification 80#. a–c, Rotalipora greenhornensis; sample 53. d, Rotaliporabrotzeni; sample 17. e–g, Rotalipora cushmani; sample 55. h, i, Marginotruncana pseudolinneiana; sample 91.j, Marginotruncana schneegansi; sample 91. k, l, Rotalipora reicheli; samples 15 and 22, respectively.

R. brotzeni) coexist with the evolved forms (R. cush-mani, R. reicheli, R. greenhornensis, R. montsalvensis).The disappearance of primitive rotaliporids causes areduction in diversity (22) through a short section(10 m) in the middle part of the Dicarinella algerianaSubzone. This is followed in the upper part of the

same subzone by an increase in diversity (up to 27),related to the diversification of the intermediate forms(dicarinellids and whiteinellids). The assemblage inthe basal part of the W. archaeocretacea Zone, justabove the Bonarelli Level, is less diverse (23 species)owing to the disappearance of rotaliporids and


Figure 14. Scanning electron micrographs of some Cenomanian-Turonian species from the Antruiles section, belonging tothe eu-mesotrophic groups. a, b, Helvetoglobotruncana helvetica; sample 86, #80. c, Helvetoglobotruncana praehelvetica;sample 86, #80. d, Heterohelix moremani; sample 106, #200. e, Whiteinella archaeocretacea; sample 85, #80. f,Whiteinella paradubia; sample 78, #90. g, Dicarinella canaliculata; sample 91, #100. h, i, Dicarinella hagni, sample 91,#80. j, Praeglobotruncana gibba; sample 52, #90. k, Praeglobotrunaca stephani; sample 14, #100. l, m, Praeglobotruncanaaumalensis; sample 52, #90. n, Hedbergella delrioensis; sample 85, #100. o, Hedbergella simplex; sample 85, #100.p, Globigerinelloides bentonensis; sample 52, #100. q, Shackoina cenomana; sample 24, #250.

G. bentonensis. The disappearance of the oligotrophic,k-strategist specialized groups has been widely docu-mented at low to middle latitudes, and correlated withthe CTBE.

Diversity increases (up to 29) at the top of thesection, in the H. helvetica Zone, in relation to are-establishment of stable conditions after the crisis,which led to the FO of marginotruncanids.


Figure 15. Integrated calcareous nannofossil and planktonic foraminiferal biostratigraphy of the Cenomanian-Turoniansuccession in the northeastern Dolomites, and correlation with formations and ages.

6. Calcareous nannofossil and planktonicforaminiferal biostratigraphy and relativechronostratigraphy

The two fossil groups investigated are evenly distrib-uted throughout the section studied, and are abun-dant enough to provide a continuous record from thebase upwards. Our data can therefore contributetowards improving the integrated biostratigraphicscheme based on these fossil groups (e.g., Bralower,1988; Robaszynski et al., 1990; Bralower et al., 1995;Premoli Silva & Sliter, 1995; Bellanca et al., 1996;Lamolda et al., 1997). The correlation between themain calcareous nannofossil and planktonic foraminif-eral biostratigraphic events and biozones is indicatedin Figure 15. Analogies and differences with theprevious schemes can be better appreciated by acomparison with Figure 6.

In the lower part of the Antruiles section (ScagliaVariegata), the FO of the nannofossil species L. acutusfalls within the R. cushmani Zone although it occursearlier (top of the R. brotzeni Zone) according toBralower et al. (1995). However, this event can beonly tentatively placed in our section because L. acutus

occurs sporadically. In central Italy this species hasnot been recorded (Ghisletti & Erba in Premoli Silva& Sliter, 1995). The LO of M. chiastius, occurringslightly above the Bonarelli Level and within the W.archaeocretacea Zone, is in accordance with data ofBralower et al. (1995).

The correspondence of the FOs of Q. gartneriand H. helvetica was recorded in the upper portionof the section (Scaglia Rossa), according to oursampling: thus the lower boundaries of the H. helveticaZone and the nannofossil CC11 Zones coincide.Ghisletti & Erba in Premoli Silva & Sliter (1995;Umbria-Marche Basin) and Lamolda et al. (1997;Spain) found that the FO of Q. gartneri notablyprecedes the FO of H. helvetica. On the other hand,the FO of the two species also coincides accordingthe data of Gartner in Robaszynski et al. (1990;Tunisia). It is, therefore, difficult to establish if anerosional hiatus occurs in this interval in the Antruilessection.

Finally, the correspondence with the lower part ofthe CC12 Zone (based on FO of the nannofossilspecies E. eximius) with the upper part of the


H. helvetica Zone, correlates well with the previousintegrated zonations.

The correlation between the biostratigraphy andchronostratigraphy of the Cenomanian-Turonian isbased on the more widely followed schemes (e.g.,Leckie, 1985; Bralower, 1988; Robaszynski, 1989;Robaszynski et al., 1990; Robaszynski & Caron, 1995;Bralower et al., 1995; Lamolda et al., 1997; Burnett,in press). The Scaglia Variegata corresponds to almostthe entire Cenomanian, and the Scaglia Rossa isEarly-Middle Turonian in age, on the basis of directcorrelation to the planktonic foraminiferal-ammoniteevents reported by Robaszynski & Caron (1995) andnannofossil-ammonite events reported by Burnett (inpress).

As pointed out earlier, the Global Boundary Strato-type Section and Point for the Cenomanian/Turonianboundary has been placed in a section at RockCanyon Anticline (Colorado, USA) at the base of Bed86. The boundary coincides with the first occurrenceof the ammonite Watinoceras devonense Wright &Kennedy, 1981. The nannofossil Quadrum gartneriand the planktonic foraminifer Helvetoglobotruncanahelvetica first occur less than 1 m above the base ofBed 86. According to these data, the Cenomanian-Turonian boundary was placed in the Antruiles sec-tion within the W. archaeocretacea Zone, immediatelyabove the Bonarelli level, and below the FO of Q.gartneri and H. helvetica. Moreover, the Cenomanian/Turonian boundary slightly postdates the LO of M.chiastius since Burnett (in press) documents thisevent just above the appearance of the ammoniteWatinoceras devonense.

7. Cyclical pelagic sedimentation anddistribution of biogenic material

The pelagic sediments of the Antruiles section show acyclical pattern. Pelagic oozes containing variableamounts of plankton-derived skeletons and largelycurrent-transported clay and silt, are interbedded withblack shales. Bands and layers of chert are absent orvery rare throughout the section, while levels contain-ing variable amounts of organic carbon are frequent inthe Cenomanian interval, and decrease in numberin the Turonian.

Fluctuation in abundance of plankton is extremelysensitive to temperature, chemistry, nutrient supplyand light penetration, influencing the sedimentaryregimes. Variation of the oxygen supply to the seafloor affects benthonic faunas and bioturbation, whilecarbonate preservation is influenced by variationsin lysocline depth. As postulated by many authors,cyclical patterns can arise from productivity rhythms

in the plankton, redox variations on the sea floor,fluctuations in the supply of terrigenous sedimentsdiluting the carbonate, and dissolution in deeperfacies. Identification of these various processes anddifferentiation of their effects is not always possible,and is beyond the scope of our paper. The follow-ing discussion presents additional biological andsedimentological features, together with a generalconsideration about the depositional dynamics ofthe succession we have examined. A more-detailedanalysis of the anoxic sediments will be treated in aseparate paper.

Figures 16 and 17 summarize the main featuresof the observed cyclical patterns. The Cenomaniancouplets are characterised by: (1) greenish-grey toreddish limestones with large, deep burrows, such asZoophycos, and green-grey marls with large, shallowerChondrites; (2) bioturbated limestones and laminatedblack calcareous shales; (3) marls and black shales.These couplets are generally attributed to bottom-water redox cycles; the limestone represents an aero-bic phase, the laminated black interval an anaerobicphase with preservation of organic matter, and themarl with large, shallow burrows owing to the anoxicconditions in the sediments indicates a dysaerobicphase.

The Turonian couplets are constituted by: (1)thinly interbedded limestones and marls with evidenceof extensive dissolution (stylolite surfaces, iron oxideenrichments); (2) limestones and black marls (lessfrequently).

7.1. Accumulation rates

Accumulation rates for the Antruiles section can onlybe roughly estimated because the position of theAlbian/Cenomanian and Turonian/Coniacian stageboundaries is lacking. Using the values derived fromGradstein et al. (1994), the rate of sediment accumu-lation for the Cenomanian Scaglia Variegata was c.17.6 m per my, while for the Early-Middle TuronianScaglia Rossa it was c. 12.7 m per my.

These accumulation rates are notably faster thanthe values estimated for the Cenomanian-Turonianpart of the Scaglia Group in the Umbrian successionsof central Italy (Premoli Silva & Sliter, 1995), and forthe Cenomanian sediments from the Cismon sectionof the Southern Alps (Bellanca et al., 1996). Ourhigher rate of accumulation is probably a reflection oflocal input of large amounts of terrigenous sedimentsinto the Dolomites area, and perhaps of the redistri-bution of peri-platform ooze from the adjacentcarbonate shallow-water sectors.


Figure 16. Simplified model of the northeastern Dolomite Basin showing a possible mechanism for deposition of theCenomanian limestone-marl or limestone-black shale couplets, whose characteristics are summarized at the top ofthe figure. a, deposition of limestones; b, deposition of marls and black intervals (modified after Einsele & Ricken, 1991).For discussion, see text.

7.2. Palaeobathymetry

One method for approximately defining palaeo-bathymetry of the basins and evaluating the distancefrom the shore of sedimentary environments is tocalculate the percentage of planktonic foraminiferawith respect to benthonic forms (e.g., Murray, 1976,1991).

The percentages of the two groups have beenobtained by counting the plankton-benthos ratio for

populations of at least 300 specimens. Planktonicforaminifers prove to be much more abundant (from85 to 97%) in comparison with benthonic foraminifersthroughout the section, with no appreciable changesin percentage for the Scaglia Variegata and ScagliaRossa Formations. The lower percentages of benthoswere found in the more calcareous lithotypes,probably owing to a dilution of assemblages by theintroduction of larger amounts of sediment (mainlycomprising nannoplankton). In the residues derived


from some black shales, planktonic forms can consti-tute 100% of the entire foraminiferal fauna. The sharpdecrease or absence of benthonic foraminifera wasevidently caused by dysaerobic/anaerobic conditionswhich reduced or prohibited life on the sea floor.Some lower percentages of planktonic forms inselected samples appear in the upper part of the sec-tion (samples 81, 91). These can probably be ascribedto resedimented episodes instead of shallowing, as alsosuggested by the composition of washed residueswhich contain a conspicuous fraction of terrigenousquartz.

In conclusion, the values of the plankton/benthosratio indicate that the Antruiles section was depositedin a deep basin, the depth probably correspondingto a middle-lower bathyal environment. Generacharacterizing benthonic assemblages (Gyroidinoides,Gavelinella, Dorothia, pleurostomellids, and small,thin nodosarids) are consistent with thesebathymetries (e.g., Sikora & Olsson, 1991; Lamolda& Peryt, 1995; Premoli Silva & Sliter, 1995).

7.3. Plankton distribution patterns

On the basis of the quantitative data available, avariation in percentages of eu-, meso- and oligo-trophic planktonic foraminifera in relation to litholo-gies is apparent. Unlike the Bonarelli Level, theaccumulation of variable amounts of organic matterin the numerous black horizons did not affectplanktonic assemblages. In fact, the various groupsof planktonic foraminifers do not generally show sig-nificant fluctuations in abundance correspond-ing with these levels. The lowest percentages ofoligotrophic forms (rotaliporids and marginotrunca-nids), accompanied by an increase in eutrophic/mesotrophic indicators (hedbergellids, heterohelicids,globigerinelloids), occur in the limestones whichalso contain the largest amount of nannofossils. Thesedata suggest that the deposition of limestonesoccurred during times of enhanced surface-waterproductivity.

The best indicators of eutrophic conditions areconsidered to be radiolaria and calcispheres (e.g.,Caron & Homewood, 1983; Roth & Krumbach,1986; Hallock, 1987; Jarvis et al., 1988; Robaszynskiet al., 1990, 1994). Although they constitute a micro-fossil group of uncertain taxonomic position, calci-spheres have been related to calcareous dinoflagellatecysts because their morphology is similar to that of thecalcitic dinoflagellate Thoracosphaera. In some cases,a dinoflagellate-like tabulation inside the calcispheretests has been observed (Willems, 1985, 1988, 1990,

1992). In his analysis of Late Cenomanian distri-butions, Hart (1991) noted that calcispheres wereabundant in intervals where dinoflagellate cysts weresharply reduced in numbers or missing altogether. Heconcluded that calcispheres may represent particulardinoflagellates associated with stressed conditions orforms able to suddenly colonize niches left vacant bynon-calcareous dinoflagellates. In modern oceans,dinoflagellates are second only to siliceous plankton aseutrophic indicators (e.g., Hallock et al., 1991). In thesection studied, both radiolaria and calcispheres aregenerally present only sporadically and in very lowpercentages.

The Cenomanian-Turonian interval in theAntruiles section appears, therefore, to have been aperiod of relatively low productivity, with stablewater-column conditions favouring the occurrenceand diversification of oligo-mesotrophic groups. Thispattern reflects the situation documented elsewhere,and the long-lasting stable conditions have beenrelated to the widespread transgression characterizingthis period (e.g., Leckie, 1987, 1989; Caron &Homewood, 1983). Low productivity during theCenomanian-Turonian interval has also been docu-mented by other authors (e.g., Bralower & Thierstein,1984). Within this general long-term context, thedeposition of limestone probably corresponds to high-frequency periods of good ventilation induced byhigher rates of oceanic circulation, increasingnutrient recycling and consequent enhanced produc-tivity (Figures 16a, 17a). During these intervals,however, well-oxygenated bottom environments pre-vented the preservation of organic matter. An episodeof particularly high productivity is indicated by a40-cm-thick black limestone at the top of the Antru-iles section, in the H. helvetica-CC12 Zones. This highproductivity is suggested by the almost exclusiveoccurrence of eutrophic forms in the planktonicforaminiferal fauna (particularly heterohelicids), witha minor component of mesotrophic indicators and anegligible fraction of oligotrophic fauna. Moreover,a very high percentage of radiolaria is found at thislevel.

The deposition of marls and black levels corre-sponded to times of relatively lower productivity incomparison with the limestone (Figures 16b, 17b). Amore stable, stratified water-column favoured thespecialized oligotrophic keeled forms in the plank-tonic foraminiferal associations. Poor ventilation,related to a sluggish current circulation, may haveinduced dysaerobic/anaerobic conditions at thewater-sediment interface (as indicated by benthonicassemblages; see below), in some cases prevent-ing the oxidation of the organic matter, which has


been preserved in variable amounts in the blackintervals.

In recent years, the role of orbital forcing in thedeposition of Cretaceous cyclical successions has beenwidely investigated. The cyclical mechanism that ledto the rhythmic sedimentation of the Antruiles sectionis likely to have been influenced by this phenomenon;similar patterns have been reported previously for theCenomanian-Turonian interval (e.g., by Cottle, 1989;Leary et al., 1989; Leary & Hart, 1992; Bellanca et al.,1996).

8. Dysoxic and anoxic events

Figure 17. Simplified model of the northeastern Dolomite Basin showing a possible mechanism for deposition of thesediment now represented by the Turonian limestone-marlstone or limestone-black shale couplets, whose characteristicsare summarized at the top of the figure. a, deposition of limestones (probably included the black calcareous level §);b, deposition of marls and black levels (modified after Einsele & Ricken, 1991). For discussion, see text.

8.1. Benthonic foraminifera as indicators of aerobic/anaerobic conditions

Analyses of benthonic foraminiferal assemblageshave been widely applied as an aid to interpretingpalaeoenvironments. For the Cretaceous successions,characterized by deposition of large amounts of or-ganic matter related to extensive anoxic episodes,benthonic foraminifera represent a very powerful


means of detecting conditions related to dysoxic-anoxic events. Several studies deal with benthonicassemblages and anoxic deposits (e.g., Koutsoukoset al., 1990; Koutsoukos & Hart, 1990; Coccioni &Galeotti, 1991, 1993; Lamolda & Peryt, 1995; Hart,1996).

Recent oxygen-minimum zone faunas are charac-terized by high abundance, low diversity and smallsize. Relatively high organic carbon contents areassociated with assemblages dominated by infaunalspecies. Infaunal deposit-feeders, living within thesediment, are normally adapted to less oxygenatedenvironments with respect to epifaunal forms, andthey may survive under dysaerobic conditions or evenproliferate, benefiting from an abundance of food.The mode of life and trophic strategies are roughlyrelated to the test morphology. The epifaunal mor-phogroup is characterized by rounded, planoconvexor biconvex trochospiral forms, whereas the infaunalgroup is generally associated with planispiral,flattened-ovoid, elongated morphotypes. Some taxamay, however, change their microhabitats dependingon more or less favourable conditions and food supply(e.g., Corliss, 1985; Bernhard, 1986).

Cretaceous aerobic and dysaerobic faunas observedin different areas are comparable (e.g., Central Italy:Coccioni & Galeotti, 1991, 1993; Brazil: Koutsoukos& Hart, 1990; Koutsoukos et al., 1990; Spain:Lamolda & Peryt, 1995). Epifaunal, plano-convextrochospiral forms, such as certain species of Gyroidi-noides, conorbinids, and rosalinids, are considered tobe aerobic indicators. Low-trochospiral, elongated,uniserial and flattened globular forms (e.g., low-spiredGavelinella, Vaginulinidae, pleurostomellids, andnodosarids) are considered to be epifaunal to shallowinfaunal deposit feeders, whereas tapered, elongatedforms represent the most dysaerobic group (mainlyagglutinant species; e.g., Clavulinoides, some species ofDorothia, Eggerella, and Marssonella)

The benthonic foraminiferal assemblages of theAntruiles section contain both epifaunal-shallowinfaunal and infaunal groups in variable proportions.The more aerobic forms are represented mainly byGyroidinoides (2–3 species) and, in some cases, theconical-truncated Oristhostella. Hyaline epifaunal toshallow infaunal forms are generally the most abun-dant and diversified group; they are represented bygavelinellids, Astacolus, Dentalinoides, Lenticulina,Nodosaria, Oolina, Pleurostomella, Saracenaria, Tristix,and Vaginulina. The genus Gavelinella, characterizedby a low trochospire, has been considered as bothepifaunal and shallow-infaunal (Koutsoukos & Hart,1990; Coccioni & Galeotti, 1993; Lamolda & Peryt,1995). Its scarcity or absence from some black levels

of the Antruiles section indicates aerobic/dysaerobicconditions. Infaunal deposit-feeders are representedby agglutinated forms, such as the rounded-tapered and elongated-tapered Clavulinoides, Dorothia,Eggerella, Marssonella, Spiroplectammina, Textularia,Triplasia and Verneulina. These generally comprise aminor component of the assemblages.

8.2. The Bonarelli Level and the other black shales: TOCand microfossil distribution

The thickest of the numerous black levels in theAntruiles section is the Bonarelli Level, whichseparates the Scaglia Variegata from the overlyingScaglia Rossa. It consists of laminated, black,bituminous shales (CaCO3=4.66%; Figure 18),which are devoid of both benthonic and planktonicfossils.

The other black intervals, occurring both below andabove the Bonarelli Level, have a higher calciumcarbonate content which varies between 46.99% and66.62%. The Total Organic Carbon (TOC) is lessthan 1% by weight for these levels, while the TOC ofthe Bonarelli is 5.82% (Figure 18).

The oldest black level occurs in the R. greenhornensisSubzone-CC9c Subzone (sample 23). An increasein dysaerobic conditions at the sediment-water inter-face is documented by the absence of aerobic formsin the benthonic assemblages (e.g., Gavelinella andGyroidinoides); the reduced size of the benthos,and the abundance of tapered hyaline forms display-ing unornamented tests (Dentalinoides, Lenticulina,Nodosaria, Vaginulina); and a significant increasein abundance and size of the agglutinated infaunaldeposit feeders (Ammobaculites, Clavulinoides, Gaud-ryina, Marssonella, Textularia). This assemblageresembles the latest Cenomanian assemblages ofKoutsoukos & Hart (1990), which are dominated byagglutinated infaunal forms, and interpreted toindicate dysaerobic, almost anaerobic, conditions. Bycontrast, the composition of the planktonic foramin-ifera at the same level does not vary. The LO ofR. gandolfi below this level is unlikely to be relatedto a perturbation of its ecological niches owing tothe expansion of the anoxic conditions; in fact, thepercentage of rotaliporids increases slightly in sample23 (Figure 11). The nannoflora is moderatelydiversified without any particular dominance of thefertility indices (as identified by Erba, 1992), such asBiscutum constans, Zygodiscus erectus and Zeugzhabdotusdiplogrammus.

More persistent anaerobic conditions on the seafloor prevented the existence of benthonic foramin-ifera, which are absent from sample 30. In this case, a


Figure 18. Wt% TOC and wt% CaCO3 of the black levels recorded in the Antruiles section.

probable perturbation in deeper pelagic niches mightbe indicated by the reduced size of the rotaliporids.

Numerous closely-spaced dark levels occur inthe Dicarinella algeriana Subzone-CC10a Subzone(Figure 3). Anaerobic conditions at the water-sediment interface correspond to the deposition ofthe beds represented by samples 49 and 54, whichare devoid of benthonic fauna. The oligotrophic,specialized rotaliporids record a slight increase inabundance.

The set of thin, dark levels occurring from samples94 to 116 contains the lowest percentage of TOC.Benthonic associations generally do not show anygreat differences from those of the light marlysamples. In some cases, more dysaerobic con-ditions are suggested by the reduction in size ofepifaunal aerobic forms, and an increase in hyaline,unornamented, elongated-tapered, shallow infaunalforms (e.g., Dentalinoides, Pleurostomella, Tristix),whereas agglutinated infaunal deposit feeders arerare. Planktonic assemblages are diverse, with bothmesotrophic and oligotrophic forms well represented.Calcareous nannofossil assemblages are relativelydiverse, with the number of eutrophic species varyingslightly.

In the upper part of the Cenomanian, the 20-m-thick stratigraphic interval below the Bonarelli Levelcontains 12 thin, black levels (Figure 3). Theseindicate variable dysaerobic conditions. In the blacksamples, epifaunal forms are rare or absent, andthe percentage of shallow-infaunal hyaline forms

increases; anaerobic conditions are expressed insample 60 by the absence of benthonic foraminifera.

About 6 m below the Bonarelli Level (sample 69,black), the benthonic assemblage consists almostentirely of shallow infaunal forms, although the nor-mal size of the fauna might indicate less severedysaerobic conditions. The increase in dysaerobicconditions is indicated 2 m below the Bonarelli Level,where the benthos is represented only by sporadicoccurrences of small Dentalinoides and Textularia. Itappears to have been partly renewed in the EarlyTuronian by comparison with the Cenomanianassemblages. New species of Gavelinella, Gyroidinoidesand Lingulogavelinella occur, and some agglutinantforms disappear.

In the calcareous interval (about 5.5 m thick) belowthe Bonarelli Level, the eutrophic (hedbergellidsand heterohelicids) and mesotrophic (whiteinellids,globigerinelloids) forms increase in the planktonicforaminiferal population, whereas rotaliporids simul-taneously decrease in abundance. This variation inplanktonic assemblage could indicate an increase ineutrophic conditions; the nannofossil fertility speciesalso show a modest increase. The best indicators ofeutrophic conditions, radiolaria and calcispheres, are,however, absent.

The species Rotalipora cushmani, R. deeckei, R.greenhornensis, and Globigerinelloides bentonensis dis-appear below the Bonarelli Level. Moreover, thecalcareous nannofossils clearly experienced a crisis, asindicated by the extinction of some species.


In the Scaglia Rossa Formation above the BonarelliLevel, in the black level represented by sample 83, novariations in the planktonic foraminiferal populationhave been observed, whereas dysaerobic conditions onthe sea floor are indicated by the almost total absenceof benthos (represented by only very rare, small unor-namented Dentalinoides, Lenticulina and Marginulina)

An indication of enhanced surface-water produc-tivity is documented in sample 90, from a blackcalcareous level at the top of the section, which is richin radiolaria and contains a planktonic fauna thatconsists almost exclusively of eutrophic forms(particularly heterohelicids).

In the Antruiles section, increasingly dysaerobic/anaerobic conditions are testified by the followingchanges in the benthonic assemblages: (1) reductionin size and abundance, or total absence of epifaunalforms; (2) increase in the abundance of shallow-infaunal hyaline forms which are smaller, andhave unornamented, flattened, elongated tests; (3)increase of elongated-tapered, infaunal, agglutinantforms, which are present with tests of normal oreven larger size; (3) low abundance and diversityor absence of benthonic foraminifera.

Low numbers of benthonic forms in some Albianblack shales deposited under a low-productivityregime have been attributed to oxygen depletionunder stratified water conditions (Coccioni &Galeotti, 1991). A higher surface-volume ratio inmodern species provides a greater area for mitochon-drial oxygen uptake, and may be achieved both by areduction in size and an elongated-tapered mor-phology (Leutenegger & Hansen, 1979; Bernhard,1986). Morover, elongated tests move upwards moreeasily to the sediment-surface where the quantity ofdissolved oxygen is greater (Coccioni & Galeotti,1991).

There is a correlation between our geochemicaldata and the benthonic content. Where the TOC isless than 0.5%, the occurrence of benthonic faunaindicates dysaerobic conditions on the sea floor; wherethe TOC it is greater than this, the absence of benthosindicates anaerobic conditions.

9. Summary and conclusions

This paper presents, for the first time, a detailedintegrated biostratigraphic analysis of a Cenomanian-Turonian pelagic succession in the Dolomites. Acyclical variation in rock-types (marly limestone, silt-sone, marlstone and black calcareous shale), colour,fossil content and ichnofacies has been documented.

Two stratigraphic units have been recognized instratigraphical order: the Scaglia Variegata and the

Scaglia Rossa Formations. The nannofossil andforaminiferal biostratigraphical study has enabled theScaglia Variegata Formation to be assigned in itsentirety to the Cenomanian interval (from the nanno-fossil Subzones CC9c to the CC10a p.p. and theforaminiferal R. brotzeni to W. archaeocretacea Zonesp.p.). The Scaglia Rossa Formation of the Antruilessection is Early-Middle Turonian in age (from thenannofossil CC10a Subzone to the CC12 Zone p.p.,and from the foraminiferal W. archaeocretacea to theH. helvetica Zones p.p.). The good outcrop conditionsand continuous fossil record allow the recognition ofseveral nannofossil and foraminiferal bioevents.

Another important result of this paper is the firstdocumentation of the Bonarelli Level and otherCenomanian-Turonian black intervals in the Creta-ceous of the Dolomites. The oldest anoxic episodesoccurred in the Cenomanian R. cushmani-CC9c andCC10a Zones; the biostratigraphic data assigns theBonarelli Level to the W. archaeocretacea-CC10aZones, while the younger time interval of anoxia isTuronian in age (H. helvetica-CC11 and CC12Zones).

The Bonarelli Level has been recognized in variousItalian successions in the Umbria-Marche Basin,Belluno Basin and Southern Apennines (Arthur &Premoli Silva, 1982; Cresta et al., 1989; MarcucciPasserini et al., 1991; Premoli Silva & Sliter, 1995;Bellanca et al., 1996; Gallicchio et al., 1996). Itsstratigraphic position suggests that it is the sedimen-tary expression of the global OAE2 (Schlanger &Jenkyns, 1976). In the Antruiles section it showspeculiar characteristics with respect to the other blacklevels identified here, such as an abrupt lithologicalchange, a marked increase of organic carbon content,and the absence of both benthonic and planktonicorganisms. High-productivity indicators (radiolaria,calcispheres) are absent. On the contrary, in theUmbria-Marche Basin, the Bonarelli Level is associ-ated with radiolarian sands, thereby suggesting that itcan be related to an upwelling, high-productivityepisode (Arthur & Premoli Silva, 1982; de Boer,1982; Premoli Silva & Sliter, 1995). However, marineproductivity is not the only factor controlling organicrichness in the sediments. In fact, the positive carbon-isotope peak recorded in pelagic carbonates at theCenomanian/Turonian boundary is conventionallyinterpreted as the response of the oceans to the burialof a huge amount of organic carbon (e.g., Jenkyns,1980; Arthur et al., 1987; Gale et al., 1993; Jenkynset al., 1994; Jenkyns & Clayton, 1997).

Dysoxic-anoxic episodes preceding deposition ofthe Bonarelli Level have also been documented in theUmbria-Marche Basin (Beaudoin et al., 1995) where


40 thin black shales occur at about 20 cm intervalsbelow this level. Moreover, Erbacher et al. (1996)tentatively proposed the occurrence of a separate OAEin the R. cushmani Zone. Turonian anoxic episodeshave not been recognized previously in other basins.Therefore, while the Cenomanian black shales can berelated to more widespread anoxic events involvingglobal causes, the Turonian black intervals of theAntruiles section are probably linked to local oxygendeficient episodes.

The distribution of benthos and plankton, and theTOC percentages in the black shales, contribute to abetter understanding of the extent of dysoxic versusanoxic conditions throughout the section investigated.The content of TOC reaches a relatively high value onlyin the Bonarelli Level (5.82%), which is devoid of bothplanktonic and benthonic faunas, thus documentinganoxic conditions for the entire water column. Theother Cenomanian and Turonian black levels containless than 1% TOC. Both planktonic foraminiferal andnannofossil assemblages are well preserved and diverse.Eutrophic indicators dominate only in the black level atthe top of the section. In the deposit where the TOCranges from 0.5 to 1% the benthonic foraminiferalfauna is absent, indicating anoxic conditions on thesea floor. On the other hand, where the TOC is lessthan 0.5%, benthonic forms are present, indicatingdysaerobic conditions of variable intensity.

Examples of pelagic sediments in which the contentof carbonate and organic matter changes rhythmicallyare common in Cretaceous successions. Continuousvariations in productivity induced by orbital forcinghave been postulated for these rhythms by manyauthors. The major indicators of high productivityand eutrophic conditions are radiolarian and calci-spheres. These fossil groups are rare or absent fromthe Antruiles section. Moreover, the planktonic for-aminiferal assemblages document meso-oligotrophicconditions throughout the succession. In this generalscenario, the limestones correspond to times of en-hanced surface water productivity whereas the marlsand black shales are related to more oligotrophicconditions. In the Cenomanian and Turonian blacklevels, recognized in the Antruiles section, bottomredox cycles probably prevailed over productivitycycles among the mechanisms leading to theformation of black shales.

Acknowledgements

We thank Prof. A. Bosellini who proposed this study,helped us in the field, and contributed to the focussingof our work. We are indebted to Dr H. Jenkyns forhelpful discussions and the TOC analyses, which

represent preliminary data of a detailed chemo-isotopic stratigraphy in progress for the succession.The first version of the paper benefited from therevision of Dr G. Villa. We are very grateful to Prof.D. J. Batten, Dr J. Burnett and to an anonymousreviewer for having greatly improved this paper withtheir suggestions and comments. The Centro diMicroscopia Elettronica of the University of Ferrara isacknowledged for SEM micrographs. This work isfinancially supported by MURST (ex 40% program,C. Loriga; ex 60% program, V. Luciani), FAR (G.Brambilla) and CNR (A. Bosellini) grants.

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Appendix

Nannofossil speciesAuthor attributions, dates and a few accompanying remarks areprovided for the numerous calcareous nannofossil species identifiedin this work.

Actinozygus geometricus (Gorka, 1957) Rood et al. 1971Axopodorhabdus albianus (Black, 1967) Wind & Wise in Wise &Wind, 1977Biscutum coronum Wind & Wise in Wise & Wind, 1977: this isshaped like B. constans but the coccolith size is greater than 8 ìm;moreover, the central area is very bright.B. constans (Gorka, 1957) Black, 1959Braarudosphaera africana Stradner, 1961Broinsonia enormis (Shumenko, 1968) Manivit, 1971Chiastozygus litterarius (Gorka, 1957) Manivit, 1971Corollithion exiguum Stradner, 1961C. kennedyi Crux, 1981Cretarhabdus angustiforatus (Black, 1971) Bukry, 1973C. conicus Bramlette & Martini, 1964: a species of Cretarhabduscharacterized by a conical central area with concentric cycles ofperforations and a distinct cross aligned along the axes of the ellipsesupporting a solid stem.C. crenulatus Bramlette & Martini, 1964C. striatus (Stradner, 1963) Black, 1973C. surirellus (Deflandre, 1954) Reinhardt, 1970Cribrosphaerella ehrenbergii (Arkhangelsky, 1912) Deflandre inPiveteau, 1952Crucibiscutum salebrosum (Black, 1971) Jakubowski, 1986

Cyclagelosphaera margerelii Noel, 1965Discorhabdus ignotus (Bukry, 1969) Perch-Nielsen, 1968Eiffellithus eximius (Gorka, 1957) Perch-Nielsen, 1968Eiffellithus turriseiffelii (Deflandre, 1954) Reinhardt, 1965Eprolithus eptapetalus Varol, 1992E. floralis (Stradner, 1962) Stover, 1966E. octopetalus Varol, 1992Flabellites oblongus (Thierstein, 1973) Crux, 1982Gartnerago segmentatum Thierstein, 1974Haqius circumradiatus (Stover, 1966) Roth, 1978Helicolithus trabeculatus (Gorka, 1957) Verbeek, 1977Lithraphidites acutus Manivit et al., 1977L. carniolensis Deflandre, 1963L. pseudoquadratus Crux, 1981Lucianorhabdus sp.: the oldest specimens of Lucianorhabdus cannotbe identified because of heavy overgrowth. However the lowestoccurrence of the genus at this stratigraphic level remains a remark-able event.Manivitella pemmatoidea (Deflandre in Manivit, 1965) Thierstein,1971Microstaurus chiastus (Worsley, 1971) Grun in Grun & Allemann,1975Microrhabdulus decoratus Deflandre, 1959Nannoconus elongatus Bronnimann, 1955N. truitti Bronnimann, 1955Nannoconus sp.Prediscosphaera cretacea (Arkhangelsky, 1912) Gartner, 1968P. columnata (Stover, 1966) Manivit, 1971P. spinosa (Bramlette & Martini, 1964) Gartner, 1968Quadrum gartneri Prins & Perch-Nielsen in Manivit et al., 1977Q. intermedium Varol, 1992Rhagodiscus achlyostaurion (Hill, 1976) Aguado, 1994R. angustus (Stradner, 1963) Reinhardt, 1971R. asper (Stradner, 1963) Reinhardt, 1967R. splendens (Deflandre, 1953) Verbeek, 1977Tranolithus gabalus Stover, 1966T. orionatus (Reinhardt, 1966) Perch-Nielsen, 1968T. salillum (Noel, 1965) Crux, 1981Watznaueria barnesae (Black, 1959) Perch-Nielsen, 1968W. biporta Bukry, 1969W. britannica (Stradner, 1963) Reinhardt, 1964W. communis Reinhardt, 1964Watnaueria aff. W. manivitae Bukry, 1973: a species very similar toW. manivitae but smaller than 9 ìm. It is an elliptical Watznaueriawith a wide central area filled by irregular calcite elements.Watznaueria aff. W. manivitae is a synonym of both Watznaueria sp.3 in Cobianchi (1992) and Cobianchi et al. (1992) and Watznaueriasp. 5 in Erba (1990).W. ovata Bukry, 1969Zeugrhabdotus diplogrammus (Deflandre in Deflandre & Fert, 1954)Gartner, 1968Z. embergeri (Noel, 1958) Perch-Nielsen, 1984Zygodiscus erectus (Deflandre, 1954) Reinhardt, 1965

Classification of planktonic foraminiferal genera and speciesThe classification of planktonic foraminiferal genera and speciesfollowed in this paper is generally that of Robaszynski & Caron(1979) and Loeblich & Tappan (1988). Some specifications andadditional considerations are given below.

Genus Dicarinella Porthault, 1970According to Loeblich & Tappan (1988, pp. 698, 699) this genusshould be replaced by Concavatotruncana Korchagin, 1982 becausethe holotype Globotruncana indica Jacob & Sastri is lost and topotypematerial is not available. However, Dicarinella is generally retainedin the literature in order to maintain taxonomic stability (see alsodiscussion in Premoli Silva & Sliter, 1995). The species D. algeri-ana, D. canaliculata, D. hagni and D. imbricata have been identifiedin the Antruiles section. The first representatives of D. algeriana aresmaller; this species can be confused with Praeglobotruncanastephani, from which it differs, however, in having two true keels (at


least in the first chambers of the last whorl), that are more widelyspaced than the rows of pustules on P. stephani.

Genus Globigerinelloides Cushman & ten Dam 1948The species G. bentonensis, G. caseyi and G. ultramicrus are distin-guished, in agreement with Premoli Silva & Sliter (1995), althoughLeckie (1984) considered G. caseyi to be synonymous with G.ultramicrus whereas G. bentonensis is regarded as a senior synonym ofG. caseyi by several authors (e.g., Caron, 1985). G. ultramicrus wasidentified as a small form with a large umbilicus and 7–8 globularchambers that gradually increase in size in the last whorl. Itfluctuates in abundance throughout the section, and decreases innumber through the Upper Cenomanian-Lower Turonian part. G.bentonensis has a larger test, a smaller umbilicus, and 6–8 chamberswith the last one of the final whorl much larger and more inflatedthan the first. It disappears just below the Bonarelli Level. The 7–9chambers in the last whorl of G. caseyi are more radially elongatedand slightly compressed in comparison with those of G. bentonensis,thus giving it a thinner profile.

Genus Costalligerina Petters, El Nakhal & Cifelli, 1985The species Costalligerina lybica, previously attributed to the genusHedbergella, shows the characteristics of this genus except for theoccurrence of poorly-defined costellae (resulting from fusion ofpustules) that resemble those of Rugoglobigerina (from which itdiffers in lacking a tegilla). For this reason it is here attributed to thegenus Costalligerina, in agreement with Robaszynski et al. (1990)and Premoli Silva & Sliter (1995). This species occurs for the lasttime up-section in the Dicarinella algeriana Subzone. It generallycomprises less than 10% of the total planktonic foraminiferalassemblages.

Genus Helvetoglobotruncana Reiss, 1957The generic attribution of the species helvetica and praehelveticais uncertain because of their morphological resemblance to bothPraeglobotruncana and Whiteinella. Robaszynski et al. (1990)included ‘‘praehelvetica-helvetica’’ in the evolutionary group ofwhiteinellids, with ‘‘Whiteinella’’ praehelvetica considered to be anintermediate form between W. baltica and H. helvetica. Taking intoaccount these difficulties, the genus Helvetoglobotruncana, alsoretained by Loeblich & Tappan (1988), is here adopted toaccomodate the species.

Genus Marginotruncana Hofker, 1956Species referred to the genus Marginotruncana by Robaszynski et al.(1979) have been separated in more recent studies into differentgenera or subgenera. These have been proposed, in particular, forthe first representatives of this group, such as M. sigali. Robaszynskiet al. (1990) proposed various new phylogenetic subgenera(Carpathoglobotruncana Jon, 1983; Falsomarginotruncana Salaj,1987; Rosalinella Marie, 1941; Sigalitruncana Korchagin, 1982).The genus Sigalitruncana was separated from Marginotruncana by

Loebich & Tappan (1988) on the basis of the lack of true keels inthe final chamber, and by Robaszynski et al. (1990) on account ofthe absence of two widely separated keels, but it is considered hereto be synonym of Marginotruncana. The morphotypes mentioned bythese authors simply show the characteristics of the primitive formsof the group. Moreover, a paratype of Globotruncana sigali Reichel,the type-species of the genus Sigalitruncana, clearly shows twodistinct keels in the inner whorls (Reichel, 1950).

Alphabetical list of foraminiferal species and authorsCostalligerina lybica (Barr, 1972)Dicarinella algeriana (Caron, 1966)D. canaliculata (Reuss, 1854)D. hagni (Scheibnerova, 1962)D. imbricata (Mornod, 1950)Falsotruncana maslakovae Caron, 1981Globigerinelloides bentonensis (Morrow, 1934)G. caseyi (Bolli, Loeblich & Tappan, 1957)G. ultramicrus (Subbotina, 1949)Hedbergella delrioensis (Carsey, 1926)H. hoelzi (Hagn & Zeil,1954)H. planisira (Tappan, 1940)H. simplex (Morrow, 1934)Helvetoglobotruncana helvetica (Bolli, 1945)H. praehelvetica (Trujillo, 1960)Heteroelix moremani (Cushman, 1938)H. reussi (Cushman, 1938)Marginotruncana pseudolinneiana Pessagno, 1967M. renzi (Gandolfi, 1942)M. schneegansi (Sigal, 1952)M. sigali (Reichel, 1950)Praeglobotruncana aumalensis (Sigal, 1952)P. delrioensis (Plummmer, 1931)P. gibba Klaus, 1960P. stephani (Gandolfi, 1942)Rotalipora appenninica (Renz, 1936)R. brotzeni (Sigal, 1948)R. cushmani (Morrow, 1934)R. deeckei (Franke, 1952)R. gandolfi Luterbacher & Premoli Silva, 1962R. greenhornensis (Morrow, 1934)R. micheli (Sacal & Debourle, 1957)R. montsalvensis Mornod, 1950R. reicheli Mornod, 1950Schackoina cenomana (Schacko, 1897)S. moliniensis Reichel, 1948S. multispinata (Cushman & Wickenden, 1930)Whiteinella aprica (Loeblich & Tappan, 1961)W. archaeocretacea Pessagno, 1967W. baltica Douglas & Rankin, 1969W. brittonensis (Loeblich & Tappan, 1961)W. paradubia (Sigal, 1952)