Cenomanian events in the deep western Basque

Basin: the Leioa section

*Julio Rodriguez-LaÂzaro, *Ana Pascual and {Javier Elorza

* Departamento de EstratigrafõÂa y PaleontologõÂa, Facultad de Ciencias, Universidad del PaõÂs Vasco/EuskalHerriko Unibertsitatea, Apartado 644, 48080 Bilbao, Spain{Departamento de MineralogõÂa y PetrologõÂa, Facultad de Ciencias, Universidad del PaõÂs Vasco/EuskalHerriko Unibertsitatea, Apartado 644, 48080 Bilbao, Spain

Revised manuscript accepted 8 January 1998

Analysis of the microfaunas (foraminifera, ostracods) and the stable isotope values (d13C, d18O) of theLeioa section, as representative of the deep Basque Basin, has allowed us to propose a detailedpalaeoenvironmental reconstruction of this region during the Cenomanian, as well as to register globalchronostratigraphic reference levels to facilitate interregional correlations. During the Cenomanian,part of the basin, the Plentzia Trough, was occupied by intermediate water masses, as deduced by therelative percentages of planktonic (Rotalipora) and benthonic foraminifera. A noticeable change isobserved at the middle-late Cenomanian transition: the replacement of the dominance of keeled (rota-liporids) by incipiently-keeled (dicarinellids, praeglobotruncanids) planktonic foraminifera, indicativeof the new in¯uence of the upper intermediate waters. The temporary effect of shallow waters isdeduced in one interval of the latest early Cenomanian and two more of the middle Cenomanian, asindicated by the dominance of globular planktonic foraminifera (hedbergellids). These water masseswere moderately to strongly hypoxic (<4 to <2 ml/l of dissolved oxygen) after the ostracod platycopidsignal and benthonic foraminiferal hypoxic indicators. The dysaerobia may have been particularlystrong (almost anoxia?) during part of the middle Cenomanian. Micronutrient availability was alsorestricted during several intervals of the middle Cenomanian, as indicated by the sudden decrease inthe species diversity of the calcitic benthonics during the period when increased trends of the d13Cisotopic signal are observed. The combination of both hypoxia and nutrient depletion produced dras-tic changes in the microfaunal assemblages, with emigrations and local extinctions, showing benthonicperturbations from the time indicated by the base of the Rotalipora reicheli Zone onwards. From thebeginning until the end of the middle Cenomanian, eleven of these perturbations are recorded asregional bioevents, using as bioevent-markers, intervals where microfauna was absent (includingbenthic-free intervals, B-FI; benthonic calcitic-free intervals, BC-FI; and ostracod-free intervals, O-FI). These changes led to the renewal of the microfaunas: benthonic foraminifera renewed theirspeci®c stocks during the early to early middle Cenomanian, with planktonic foraminifera and ostra-cods undergoing renewal at the end of the middle Cenomanian. Isotope values of d18O and d13C areconsistent with the palaeoenvironmental changes detected by the microfaunas; their maximum andminimum shifts coincide with the bioevents. The double-peaked positive shift of d13C for the mid-Cenomanian of northwest Europe (Jenkyns et al., 1994; Paul et al., 1994a) has been recognized inthis series of the Basque Basin. The palaeoenvironmental perturbations deduced in the Cenomanianof the Leioa section are attributed essentially to palaeoceanographic changes, where intermediatewater masses profoundly in¯uenced the planktonic and benthonic ecosystems. The in¯uence of otherlocal causes, such as volcanic activity at that time, or tectonics between the Iberian and Europeanplates, are more dif®cult to prove. Several of the bioevents de®ned in the middle Cenomanian of thisbasin could probably be global in nature, and thus may be useful for establishing interregional corre-lations. # 1998 Academic Press Limited

KEY WORDS: foraminifera; ostracods; stable isotopes; palaeoecology; palaeoceanography;Cenomanian; Basque Basin.

1. Introduction

From the pioneer papers of Jefferies (1962, 1963) and Jeans (1968), who

studied the macro and microfaunal changes of the Plenus Marls (latest

Cretaceous Research (1998) 19, 673±700 Article No. cr980125

0195±6671/98/060673 + 28 $30.00/0 # 1998 Academic Press

Cenomanian, Anglo-Paris Basin), many researchers have contributed to our

knowledge of the Cenomanian-Turonian Boundary Event (CTBE). In the

programmes of the Deep See Drilling Project (DSDP) anomalous carbonaceous

sediment concentrations (black shales) were detected in several oceanic basins of

the Cretaceous, while in the shelf facies, limestones display anomalous stable

carbon isotope compositions.

In Normandy the CTBE was registered by Pomerol (1976) as a geochemical

event, with a strong enrichment in Mn. Subsequent work in the Anglo-Paris

Basin con®rmed a positive shift of d13C and Mn, and was compared with the

classic section of Pueblo (Colorado) and Ponca State Park (Nebraska) by Pratt etal. (1991). Pomerol & Mortimore (1993) extended these studies in an attempt to

improve the correlation of the numerous sections studied in the Anglo-Paris Basin

with sections in the Western Interior USA. They con®rmed the existence of

geochemical shifts (manganese and 13C) in the CTBE which, given its widespread

occurrence, can be considered as a global event, though slightly diachronous

(Bralower, 1988).

The CTBE is characterized fundamentally by a widespread deposition of

organic carbon (Herbin et al., 1986; Jenkyns, 1980; Schlanger et al., 1987); an

important positive excursion of d13C (Arthur et al., 1987; Gale et al., 1993;

Jenkyns, 1985; Schlanger et al., 1987; Scholle & Arthur, 1980); a minimum

planktonic/benthonic (P/B) ratio; and minima in the diversity and abundance of

foraminiferal assemblages (Peryt & Wyrwicka, 1991) which can even be

interpreted as a marine mass extinction (Sepkoski, 1986). The causes and effects

of this important event are controversial and different points of view abound (see

Jenkyns et al., 1994; Paul et al., 1994a).

In contrast to the meticulous study of the CTBE, little attention have been paid

to other secondary events produced during the Cenomanian. Certain bioevents

were speci®ed by Paul et al. (1994b) in six sections of the mid-Cenomanian of

northwest Europe (Anglo-Paris Basin and Cleveland Basin, UK). Thus, in

addition to these bioevents, the temporary absence of benthonic foraminifera

(Tritaxia macfadyeni), the brief appearance of planktonics (Favusella washitensis),and a reduction in the abundance of the dino¯agellates, a double-peaked d13C

excursion was also detected, resulting from the burial of organic carbon during

transgressions following sea-level falls. This double-peaked d13C was compared to

the late Cenomanian carbon excursion (Paul et al., 1994a). Among the

coincidences, both excursions (a) follow a sea-level fall; (b) present signi®cant,

but relatively short-lived positive carbon excursions; (c) are associated with the

occurrence of ``similar pulse faunas'', especially belemnites, bivalves and some

brachiopod species and (d) involve faunal turnover at least partially related to

changes in water masses.

With respect to the differences between both bioevents, the following can be

considered: (a) the sea-level drop at the base of the CTBE seems to have been

unusual and of great magnitude, to judge by the changes of the facies, while the

mid-Cenomanian event seems to have been `stepped'; (b) the mid-Cenomanian

carbon excursion starts from a lower background level (about 1.7-) and is less

extreme (maximum difference about 1-) whereas the CTBE excursion starts

from a background level as high as the peak of the mid-Cenomanian excursion

and has a greater maximum difference (about 2-); (c) the mid-Cenomanian

event apparently had only temporary effects on the fauna and ¯ora and is not

associated with any known extinction, whereas the CTBE is associated with

674 J. Rodriguez-LaÂzaro et al.

signi®cant extinctions of both macro- and microbiota (Gale et al., 1993; Jarvis et

al., 1988; Jefferies, 1962; Lamolda et al., 1994). Biostratigraphic characterization

of the upper Albian-lower Cenomanian boundary in the Vocontian Basin

(France) has been provided by Gale et al. (1996).

Within the Basque-Cantabrian Domain, the CTBE has been studied in the

Navarro-Cantabrian Ramp by Paul et al. (1994a). Seven characteristic bioevents

and a curve of the d13C with a modest maximum, within a context of oceanic

productivity in decline, have been detected. Upon comparing their results with

those of other sections studied at Dover (UK), these authors considered that the

variations suggest a probable global control. Reitner et al. (1995) mentioned the

substitution of the coralline sponge community by stromatolites with encrusting

foraminifera (Miniacina type) during the late Cenomanian (upper Rotalipora

cushmani Zone), recorded in a deep water hardground succession at Liencres

(Santander, northern Spain), within the Navarro-Cantabrian distal Ramp (see

Figure 1).

Peryt & Lamolda (1996), in their analysis of the CTBE in the Navarro-

Cantabrian Ramp and of the benthonic foraminifera, detected a mass extinction

process with survival intervals. This extinction was interpreted as having been

produced by a decrease in the oxygenation of the sea-bottom water, at the end

deposition of the R. cushmani Zone, and by the persistence of unfavourable

conditions during Whiteinella archaeocretacea Zone times. On the other hand

Floquet et al. (1996) correlated the CTBE in three different areas of the Basque-

Cantabrian Domain, deducing biological events, such as the disappearance of R.

cushmani in the Basque Basin and the `bloom' of Pithonella and Heterohelix

together with the disappearance of some benthonic foraminifera and rudists, in

the Navarro-Cantabrian and Nord-Castilian Ramps. These events, which contain

correlative anomalies in the curves of d13C and d18O as well as in the trace-

element values, occur at the same time (Zone of W. archaeocretacea) in all of the

passive margin of the Basque-Cantabrian Domain. They were the result of a

strong marine transgression that produced anoxia in the deep Basque Basin and

hypoxia in the shallower Navarro-Cantabrian and Nord-Castilian Ramps.

Lamolda et al. (1997) studied the ammonoids, inoceramids, foraminifera and

calcareous nannofossils from the upper Cenomanian to lower Turonian

succession at Ganuza, located within the outer platform in the eastern sector of

the Basque Basin.

Babinot et al. (in press) correlated the main sedimentary discontinuities with

the biological crises of the ostracods of southwest Europe (Basque Basin,

Navarro-Cantabrian and Castilian Ramps, Provence, southern Alpine Chains)

during the Cenomanian. They deduced two great crises in the assemblages of

these organisms; one in the middle/late Cenomanian transition and the other

during the terminal Cenomanian [base of Neocardioceras juddii (ammonite) Zone],

without ®nding either sedimentary discontinuity or evident crises in the

populations of ostracods at the Cenomanian/Turonian boundary throughout the

European basins studied. Both crises coincide with greater discontinuities which

separate second and third order sedimentary cycles. Therefore the authors suggest

that such crises were caused by the alteration of the habitats in shallow platform

areas because of eustatic rises that produced at the same time hypoxia in the

platforms and anoxia in the deep basins, in a manner similar to that mentioned

previously.

Cenomanian events in the deep western Basque Basin 675

However, in comparison to the CTBE, there are fewer publications dealing

with earlier events in the Cenomanian of the Basque-Cantabrian Domain. Only

Rodriguez-LaÂzaro et al. (1996) and Rodriguez-LaÂzaro & Pascual (1997) have

presented preliminary descriptions of some biotic and geochemical signals of the

Cenomanian of the deep Basque Basin, where certain events can be related to

palaeoceanographic modi®cations.

The aim of this paper is to complete previous studies and propose a detailed

regional palaeoenvironmental reconstruction of the Cenomanian in deep facies of

the western sector of the Basque Basin, based on biotic (planktonic and

benthonic foraminifera, ostracods) and isotopic (d13C, d18O in whole rock)

markers. The detailed analysis of such signals will permit the recognition of

regional bioevents and chemical events, some of which may be globally relevant

and therefore useful as reference levels in eventual interregional correlations.

2. Geological setting

Up to the beginning of the Late Cretaceous, the Bay of Biscay was subjected to a

phase of extension owing to the tectonics among the European and Iberian plates.

A wide sedimentation area was created in this way (Basque-Cantabrian Domain)

formed by platforms, basins and subsiding troughs, as a result of active basement

dynamics, with a progressive deepening from the southwest toward the northeast.

The Navarro-Cantabrian Ramp, the Plentzia Trough, the `Bizkaia-Gipuzkoa

Shallows', and the Saint-Jean-de-Luz Trough represent a tectonic type-pro®le for

the Cenomanian of this region (Figure 1A, B). The presence of deep tectonic

structures (Calamua and Bilbao faults), together with important submarine

alkaline volcanic activity, seems to mark the limit among these plates (Mathey,

1986, 1988).

The stratigraphy of the Cenomanian in the Basque Basin is relatively poorly

known, owing to the problem of facies recognition because of the virtual absence

of macrofaunal biostratigraphic references and strong tectonization of these levels,

mostly in the south ¯ank of the Bizkaia Synclinorium. Mathey (1982) de®ned

several lithologic formations in the Cenomanian of this area. The Durango

Formation (see Figure 2) consists of sandy-silty lithologies, representing deltaic

marine environments of the late Albian-earliest Cenomanian. The Elgeta

Formation comprises marls and clayey or silty limestones, with some volcanic

levels that in this area are replaced in the middle Cenomanian by the calcareous-

marly ¯ysch of the Plentzia Formation. This ¯ysch was deposited in a small

trough (Plentzia Trough, see Figure 1) that progressively opened toward the

southeast during the Late Cretaceous (Mathey, 1988). The presence in this basin

of important submarine volcanic activity that began at the end of the Albian and

persisted until the Coniacian or Santonian in some areas of the Bizkaia

Synclinorium (Lamolda et al., 1983; Mathey, 1986) is characteristic.

Five sections of the Cenomanian of the western Basque Basin have been

studied (locations given in Figure 1). The Arbacegi and Gerrika sections, located

on the north ¯ank of the Bizkaia Synclinorium, contain very little microfauna,

partly because the sediments accumulated in an adverse environment of active

submarine volcanism. In the sections of the south ¯ank (Leioa, Elgeta,

Amorebieta), located within the Plentzia Trough and apparently far from direct

volcanic in¯uence (see Figure 1B), a microfaunal content more adequate for

micropaleontological analysis has been found. The Leioa section, proposed as

676 J. Rodriguez-LaÂzaro et al.

representative of the deep Cenomanian in this region, is situated in the village of

the same name, at the eastern exit of the highway from Asua to La Avanzada,

some 13 km northwest of Bilbao. It comprises the Elgeta Formation and the

lower part of the Plentzia Formation. In contrast to the other units studied,

important volcanic levels are not apparent in the Leioa section.

The succession (Figure 3) comprises 63 m of grey to brown marls, with dark

grey to black marl intervals and some silty levels. The entry of the ®rst levels of

turbidites (31 m above the base; packstone with echinoderm fragments, sponges

and brachiopods, calcispheres, radiolarians and diatoms), is considered to be the

base of the Plentzia Formation. This unit also contains a thick dark grey marl

interval, which becomes more calcareous (marlstone/limestone; mudstone/

wackestone) towards the top. Isolated prisms of inoceramids, which appear

towards the middle of the unit (top of the Elgeta Formation), are the only

remains of macrofauna evident. In the lowest sample of the upper Cenomanian

(sca-10-4; 56.5 m above the base; Figure 3) there is a glauconitic concentration

containing a mixed microfauna, which could indicate a condensation level. The

Figure 1. A, B. Palaeogeographic map of the Basque Basin during the Cenomanian, indicating thelocation of the studied sections; modi®ed from Mathey (1988). The outcrops of the Bizkaia Syn-clinorium approximately include the western area of the Bizkaia-Gipuzkoa Shallows and thePlentzia Trough.

Cenomanian events in the deep western Basque Basin 677

upper Cenomanian/Turonian transition seems to be faulted, and the presence of a

gap is deduced from the microfaunal content. This would affect the boundary

between the stages and part of the early Turonian, which is dated here on the

basis of the presence of Helvetoglobotruncana helvetica (see faunal reference list in

the Appendix).

3. Material and methods

In order to complete previous samplings, a total of 63 samples has been collected

for micropaleontological and isotopic studies (Figure 3). The micro-

paleontological analysis was carried out using the classic techniques of

washing, with two fractions (0.63-0.25 mm; 0.25-0.10 mm) from which the

foraminifera and ostracods have been separated, ®rst counting 300 individuals

(foraminifera and ostracods) per sample, to establish their relative percentages.

This permits the determination of several indices: simple diversity (number of

Figure 2. Summary of the Cenomanian stratigraphy of northwest Europe, with the ammonite bio-zonation (Gale, 1995), planktonic foraminiferal zones (Robaszynski et al., 1983; Caron, 1985),sequence stratigraphy (Haq et al., 1987), isotope stratigraphy (Jenkyns et al., 1994) and radio-metric data (Obradovich in Gale, 1995) for reference. The lithologic formations of the area arealso indicated (shaded means interval of non-deposition in the Leioa section). The isotope strati-graphy shows three positive shifts in the �13C value. The second (2) is double (2-1, 2-2) andoccurs within the R. reicheli biozone. The third (3) is the most important, and marks the begin-ning of the Oceanic Anoxic Event of the Cenomanian/Turonian boundary (OEA 2).

678 J. Rodriguez-LaÂzaro et al.

Figure 3. Lithological log of the Leioa section, and samples studied. Chronostratigraphy and bios-tratigraphy are based on the distribution of planktonic foraminiferal zone indicators. The lastappearance datum (LAD) and ®rst appearance datum (FAD) of several planktonic foraminiferalspecies are also indicated.

Cenomanian events in the deep western Basque Basin 679

Figure 4. Microfaunal indicators of the Cenomanian in the Leioa section. Planktonic foraminifera(in percentages) have been separated into keeled (Rotalipora), incipiently keeled (Dicarinella, Hel-vetoglobotruncana, Praeglobotruncana) and globular or non-keeled (Hedbergella, heterohelicids), thathave been related to deep-water, intermediate-water and shallow-water, respectively (Jarvis et al.,1988). Three of the curves are based on benthonic microfaunal occurrences: percentages of tex-tulariids (shaded, >30% of total benthonic foraminifera), and simple diversity (no. spp. � numberof species per sample) of benthonic foraminifera (BF) and ostracods. The heterohelicid graph isonly approximately to scale. Shaded areas in the benthonic foraminiferal and ostracod diversitygraphics (no. spp.), indicate intervals of very low diversity, probably caused by palaeoenviron-mental perturbations.

species, no. spp.); oceanicity index (percentage of planktonic foraminifera/total

foraminifera, PF/F); percentage of ostracods/total studied microfauna; percentage

of keeled planktonic foraminifera, incipiently keeled and globulars/total planktonic

foraminifera; type of tests of benthonic foraminifera (agglutinated, hyalines; see

Figure 4).

Sixty three whole rock analyses of the stable isotopes (d18O/16O and d13C/12C)

of marls and marly-limestones were performed using a VG SIRA-9 mass

spectrometer at the University of Salamanca (Spain). Extraction of CO2 from

carbonates was carried out according to the method described by McCrea (1950).

The results are expressed in `d' notation in - relative to the Pee Dee Belemnite

(PDB) standard, from a rock of Cretaceous age in Carolina (USA).

4. Biostratigraphy

In Figure 2 the stratigraphic context of the Cenomanian in southwest Europe is

illustrated, including radiometric dates from Obradovich (in Gale, 1995), a

reference scale of ammonites (Gale, 1995), a biostratigraphic scale based on

planktonic foraminifera (Caron, 1985; Robaszynski et al., 1983), a sequence

stratigraphy (Haq et al., 1987) and an isotope stratigraphy (Jenkyns et al., 1994).

The planktonic foraminifera have been used to date the succession because

these are very abundant and allow interregional correlation. In the studied

interval the three Cenomanian zones have been recognized (Figure 3). The

Rotalipora brotzeni Zone (11 m, lower part of the section), is characterized by the

presence of the index species in addition to Rotalipora montsalvensis, which is

dominant, Hedbergella delrioensis, H. simplex, Praeglobotruncana gibba, P. stephani,and R. greenhornensis. The Rotalipora reicheli Zone (33 m) contains, in addition to

the nominate species, the same species of Rotalipora and Hebergella listed for the

preceding zone. The Rotalipora cushmani Zone (18 m) includes the assemblage H.delrioensis, R. cushmani, R. greenhornensis, and successively appearing

Helvetoglobotruncana praehelvetica, Rotalipora deeckei, Whiteinella brittonensis,Dicarinella hagni and others towards the upper part of this zone.

To establish the limits between the substages of the Cenomanian, the

distribution of the planktonic foraminiferal zones mentioned above have ®rst to

be taken into account. The middle/upper Cenomanian boundary is drawn in this

section above the middle of the R. reicheli Zone, at the LAD (last appearance

datum) of R. montsalvensis and the FAD (®rst appearance datum) of H.praehelvetica and R. deeckei, which is coincident with the Rotalipora decline

bioevent and glauconitic concentration in this section (Figure 3). The greatest fall

in d18O in this succession (d18O boundary intervals II-III, Figure 5) and the

beginning of the OAE 2 after the increasing values of d13C (d13C boundary

intervals III-IV, Figure 5) represent the geochemical signals of this boundary. The

transition between lower and middle Cenomanian has been placed in this

succession in the middle of the R. reicheli Zone, coincident with the d13C positive

shift 2-1 and the d18O boundary intervals I-II (Figure 5), the LAD of P. stephani,and the ®rst major and permanent fall in the diversity of the benthonics (starting

with the ®rst benthic-free interval, B-FI, see Figures 4, 7).

Among the planktonic foraminifera, Rotalipora is the most representative genus,

with a change in the dominance of its species through the Cenomanian (R.montsalvensis, R. reicheli, R. greenhornesis, R. cushmani; Rodriguez-LaÂzaro &

Pascual, 1997). In the upper Cenomanian, H. praehelvetica replaces Rotalipora (R.

Cenomanian events in the deep western Basque Basin 681

cushmani, R. greenhornensis; Rotalipora decline; see Figure 3) as dominant species;

R. cushmani continues up to the highest samples of the Cenomanian. In the

Ganuza section (Navarra; outer shelf palaeoenvironment), where sedimentation

was continuous during the Cenomanian-Turonian boundary interval, R.

greenhornensis becomes extinct locally 9.3 m below R. cushmani, based on LADs

(Lamolda & Peryt, 1995). In Leioa, however, the LADs of R. greenhornensis and

R. cushmani are separated by only 0.5 m, which is more closely comparable to

those of the type-section of south England (Dover, 0.8 m; Jarvis et al., 1988).

Figure 5. Analysis of stable isotopes in 63 samples from the Leioa section. Three intervals are dis-tinguished for the �18O values and four for the �13C curves on the basis of their relative trends.Shifts marked 2-1 and 2-2 are equivalent to those of the Cenomanian isotope stratigraphy ofFigure 3. OAE � Oceanic Anoxic Event.

682 J. Rodriguez-LaÂzaro et al.

Owing to the incompleteness of the Leioa succession, the LAD of Rotaliporaoccurs in the sample sca-10-9 (61.5 m above the base; see Figure 3), probably

indicating its disappearance a short time before the general extinction of this

genus (Rotalipora event, 93.8 Ma; Kauffman & Hart, 1996). The three

Cenomanian Zones of planktonic foraminifera appear in the thick succession of

the Cenomanian of Kalaat Senan (Tunisia; 655 m; Robaszynski et al., 1993), as

well as in the Leioa section. In Tunisia, the LAD of R. greenhornensis is located

some 8-10 m below the LAD of R. cushmani, which at the same time occurs some

20 m below the CTB (Cenomanian/Turonian Boundary). On the other hand,

Robaszynski et al. (1993) found a glauconitic concentration level high in the R.cushmani Zone, some 6-7 m below the CTB. The same occurs in the Leioa

section, where the glauconitic level appears some 6 m below the apparent top of

the Cenomanian.

5. Microfaunal analysis

A preliminary study of the microfauna of the Leioa unit was made by Rodriguez-

LaÂzaro & Pascual (1997), where some characteristics of the foraminiferal and

ostracod assemblages are described.

5.1. Planktonic foraminiferaA total of 15 360 specimens of planktonic foraminifera belonging to 20 species

have been analyzed in this study. All the samples contain a high percentage of

planktonic foraminifera, generally constituting more than 80% of the whole

microfauna (Figure 4). The heterohelicids are present only in very low

percentages in the lower part of the R. reicheli Zone, reappearing suddenly in this

section, from the upper Cenomanian upwards. Keeled (Rotalipora) and

incipiently keeled (Dicarinella, Helvetoglobotruncana, Praeglobotruncana) planktonic

foraminifera are dominant in this section, but within two intervals of the R.reicheli Zone and another at the base of the R. cushmani Zone, globulars

(Hedbergella, Whiteinella, heterohelicids) dominate with percentages around 80%

(see Figure 4).

5.2. Benthonic microfaunaThe benthonic microfauna were studied alongside the foraminifera and ostracods.

Agglutinated (Textulariina, 1790 individuals) and hyalines (Rotaliina, 851

individuals) have been identi®ed in the ®rst group; no porcellanous (Miliolina)

have been found. The speci®c diversities of the 112 species of benthonic

foraminifera studied average some 10 species per sample, with six intervals in

which diversity decreases considerably (Figure 4). The numbers of these

benthonic microfossils are variable throughout the succession, the ostracods being

numerically more important, except during the middle Cenomanian where they

are very scarce, practically disappearing in some intervals in which the textulariids

persist. The percentages of agglutinated foraminifera are, as a rule, greater than

30% of the total of the benthonic foraminifera (BF), and can, in some samples,

constitute more than 90% of the total population (Figure 4). The intervals with

maximum values of these foraminifera coincide with minimal percentage values

for calcitics (hyaline foraminifera and ostracods).

We have differentiated six benthonic foraminiferal assemblages in the

Cenomanian and one more in the lower Turonian (Figures 6, 7). Assemblage BF-

Cenomanian events in the deep western Basque Basin 683

Figure 6. Benthonic microfaunal indicators of water masses and hypoxia compared with benthonicforaminifer and ostracod assemblages in the Cenomanian of the Leioa section. Two of thecurves show the percentage distribution of benthonic foraminiferal species typical of shallow,intermediate and deep water masses, plotted against samples from this section. Evidence ofhypoxia is shown in the other three curves, with the distribution of `hypoxic' benthic foramini-fera, and the percentages of platycopids and Cypridacea (Ostracoda) that were able to thrive indysaerobic environments (Whatley, 1991, 1995). The benthonic foraminiferal and ostracodassemblages are described in the text.

684 J. Rodriguez-LaÂzaro et al.

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Cenomanian events in the deep western Basque Basin 685

1 occurs through the entire R. brotzeni Zone, and is characterized by the

dominance of Textularia subconica, accompanied by Ammobaculites impexus,Ammodiscus cretaceus, Glomospira corona and Marssonella kummi. Agglutinated tests

dominate this assemblage, representing 62-76% of all benthonics. The number of

species oscillates from 7-19, while the planktonics/benthonics proportion reaches

55-93%.

Three faunules are recorded in the R. reicheli Zone: in Assemblage BF-2

(15 m), located at the base of this Zone. Clavulina gaultina is the dominant

species, in addition to Glomospira charoides and Glomospirella irregularis, whereas

other species, such as Dorothia ®liformis and Pseudotextulariella cretosa, have

disappeared. The percentage of agglutinates decreases slightly (28-73%), while

that of planktonics oscillates between 69-89%, with speci®c diversity increasing

(13-25 species). Assemblage BF-3 (8 m), appears to be limited by two intervals

without benthonics (B-FIs, benthic-free intervals, see Figure 7; equivalent to

those described from the Livello-Bonarelli unit of Gubbio, Italy; Coccioni et al.,1995). Glomospira charoides dominates, with A. cretaceus, Glomospirella irregularisand M. kummi as subordinate species. The relative percentages of agglutinated

foraminifera (63-79%) and planktonics (80-100%) increase, while the number of

species falls to fewer than 18. A. impexus disappears in this interval. Assemblage

BF-4 is located in the highest 10 m of the zone where Clavulina gaultinadominates, and Gavelinella balthica, Glomospirella irregularis and M. kummi also

appear. Agglutinated tests form 63-88% of the assemblage; towards the top of

this interval Glomospira charoides and Glomospirella irregularis disappear. The

speci®c diversity increases slightly to <22, and the percentage of planktonics falls

(75-89%).

In the R. cushmani Zone two assemblages are differentiated: Assemblage BF-5

(12 m), with abundant specimens of Textularia chapmani along with Clavulinagaultina, Lenticulina rotulata and Marssonella trochus. The proportion of

agglutinates is similar to that of the previous assemblage except in the highest

metre of sediments where the proportion decreases to 38%. A fall in the speci®c

diversity is registered (no. spp. < 14), as well as an increase in the number of

planktonics (83-98%) and the disappearance of Glomospira corona and M. kummi.Assemblage BF-6 occupies the top 6 m of this zone and consists almost

exclusively of L. rotulata and M. trochus. In this interval the species that were very

abundant at lower levels, such as A. cretaceus, C. gaultina, G. balthica, T.chapmani and Tritaxia pyramidata, become extinct. The agglutinated foraminifera

do not exceed 50%. The speci®c diversity is low (no. spp. < 11), with planktonic

foraminifera predominating (78-95%). Assemblage BF-7 represents the highest

metre of the succession (lower Turonian). It is de®ned by the dominance of

Gyroidinoides nitida, accompanied by the presence of L. rotulata.

A total of 3160 individuals and 56 species of ostracods have been identi®ed in

this section. Their presence is very variable and oscillates between 0-63.5% of the

studied microfauna. The most characteristic genera are Cytherella and

Pontocyprella, constituting representative assemblages of the deep Basque Basin

during the Cenomanian (Rodriguez-LaÂzaro & GarcõÂa-Zarraga, 1992a). Their

record through the Leioa unit includes intervals when the speci®c diversity is

almost zero (shading in Figure 4). Within these intervals the persistence of

individuals of the genera Cytherella and Platella (Suborder Platycopa) suggests

conditions of hypoxia (Platycopid signal sensu Whatley, 1991, 1995). They can be

categorized into seven assemblages of ostracods, characteristic of the early

686 J. Rodriguez-LaÂzaro et al.

Cenomanian to early Turonian interval, taking into consideration the

characteristic species, the speci®c diversity trend, and the presence of levels where

they disappear (ostracod-free intervals, O-FIs; Figures 6, 7).

Assemblage O-1 occupies the ®rst 7 m of the series, within the R. brotzeniZone. The speci®c diversity has a decreasing trend (see Figure 4) and ends with a

calcitic benthic-free interval (CB-FI, Figure 7). It contains a relatively rich

assemblage consisting of the species Acuticytheretta sp. 1, Cardobairdiacenomanensis, Neocythere kayei, Praephacorhabdotus aff. semiplicatus and Xestoleberissp 2. The O-2 Assemblage extends through the overlying 22 m, up to the middle

of the R. reicheli Zone. The speci®c diversity is variable, though a generally

decreasing trend is recognisable upwards through the succession; it is limited

upwards by a B-FI. The most typical species are Bythoceratina sp. 2, Bythocyprissp. 3, Oertliella sp. 3, Paracypris acuta and Pontocyprella rara. In this interval the

LADs of P. aff. semiplicatus and Acuticytheretta sp. 1 occur. Assemblage O-3 is

located in the overlying 9 m within the same zone. It is a very poor association, as

much in individuals (<5 per sample) as in species (<5 spp./sample), formed

exclusively by Platella sp. 1 and Pontocyprella sp. 5. The LAD of Oertliella sp. 3

occurs at the base, and a thick B-FI/O-FI occurs at the top of this level (Figure 7).

The O-4 Assemblage (6 m) occurs up to the boundary of the R. reicheli/R.cushmani Zones. The speci®c diversity continues to be very low (no. spp. < 5 spp./

sample), but is more consistent than within the preceding association. Typical

species are P. acuta, Pontocyprella sp. 5, Rehacythereis sopeirensis and R.sp. 5. The assemblage ends at an O-FI. Assemblage O-5 extends 8 m into the

lower part of the R. cushmani Zone. The trend of decreasing diversity noted for

the preceding assemblage continues, with a CB-FI towards the base and

culminating in another O-FI at the top. Cytherella sp. 1, which disappears at the

base of the O-3 zone, reappears in this assemblage, while other species apparently

become extinct at these levels (LADs of Rimacytheropteron sp. 1 and Schulerideajonesiana).

The O-6 assemblage characterizes the highest 4 m of the middle Cenomanian

of this succession, and is limited below and above by O-FIs. The speci®c diversity

at the base shows a maximum of autochthonous species (16 spp./sample;

Figure 4), even though the apparent diversity is greater (20 spp./sample), owing

to the presence of other specimens transported from the outer platform (including

Curfsina sp., Cytherelloidea sp., Limburgina seuvensis, Platycythereis sp.; `shelf' in

Figure 7). In the upper part of this section the regional extinctions of six species

(LADs of Rehacythereis huescaensis, P. rara, N. kayei, Cytheropteron bispinosa,Praephacorhabdotus sp. 2, Pontocyprella sp. 5) occur, allowing for an important

renewal of the ostracod assemblages. The O-7 assemblage appears in the top 7 m

of the succession, in the upper Cenomanian to lower Turonian interval. Speci®c

diversity tends to increase, reaching moderate values at the top of the section (16

spp./sample). In the lower and middle levels, several species of Turonian af®nity

(Pterygocythere pulvinata, Imhotepia marssoni, Bairdoppilata cuvillieri, Mauritsinababinoti) successively appear.

6. Stable isotopes

The analyses of the stable isotopes of carbon and oxygen (d13C, d18O PDB)

obtained through 63 m of the succession can be grouped according to their

tendency to increase or decrease. The d13C index presents, from base to top, four

Cenomanian events in the deep western Basque Basin 687

intervals with different trends (Figure 5). Within interval I (basal 12 m; R.brotzeni Zone), the trend is towards lighter values, with a toothed form that

suggests some cyclicity. Thus, it begins with a value of 1.06- PDB (sample LJ-4)

and reaches negative values (ÿ0.43-, sca-1-4; ÿ 0.53-, sca-2-2; see Figure 5).

It is just at this last minimal value where Rodriguez-LaÂzaro et al. (1996)

established their ®rst Bioevent, at the junction of the R. brotzeni and R. reicheliZones.

Interval II, ranging over the next 24 m, begins from the minimum mentioned

above and is developed throughout the R. reicheli Zone. It is characterized by a

trend towards increasing d13C values. Four maxima are apparent (0.95-, sca-4-

2; 1.57-, LJ-2-3; 2.09-, sca-7-3 and 1.98-, sca-8-2), with positive values being

of longer duration. There are also four minima (ÿ0.12-, sca-3-1; 0.02-, sca-

4-4; 0.36-, sca-6-2 and 1.02-, sca-8), with sharp falls toward negative values.

In interval III, the curve recovers quickly towards higher values (1.83-), a

situation that is steadily maintained (mean value: 1.67-) over more than 15 m,

from the highest part of the R. reicheli Zone to the upper half of that of R.cushmani, within the middle Cenomanian. From the base of the upper

Cenomanian a continuous positive trend is observed in interval IV. This does not

show strong variations and reaches maximum values of 2.31- (sca-10-7). The

boundary between intervals III and IV corresponds to the fourth bioevent

described by Rodriguez-LaÂzaro et al. (1996) and Babinot et al. (in press). It can

be considered to coincide with the beginning of the Oceanic Anoxic Event 2

(OAE-2) in this basin.

The d18O curve is characterized by light values, from ÿ3.5- to ÿ5.72- PDB

(Figure 5). Three intervals can be distinguished according to trends in d18O

values. Within the lower Cenomanian of this section (interval I), the values of

d18O remain relatively stable, and lie within a negative band (mean value

approximately ÿ5-). In the middle Cenomanian (interval II), these values

¯uctuate much more, with negative minima (ÿ5.55-, sca-8-1; ÿ5.72-, sca-9-1

and ÿ5.59-, sca-10-4). This last minimum indicates the beginning of the fourth

bioevent noted above. An increasing trend in the high part of this interval,

practically from the base of the R. cushmani Zone is apparent. The highest

interval recognized (interval III) is present at upper Cenomanian levels and is

characterized by oscillations in the d18O value around a value of ÿ4- (see

Figure 5).

As can be seen in Figure 5, the d18O and d13C values do not vary together; the

d18O values remain stable while d13C values increase and, in contrast, the former

increase when the values of the d13C remain stable. Both signals vary

simultaneously only at the base of the upper Cenomanian, where a marked fall in

d18O values and the beginning of a strongly increasing trend in d13C values mark

the beginning of OAE 2.

7. Discussion

In the following discussion, microfauna are used as indicators of the

characteristics of marine water masses and as detectors of hypoxia. The analysis

of the variation in the speci®c diversity of the microfauna, measured as

appearances/emigrations/extinctions of species, permits a de®nition of biomarker

levels which, together with isotope values, makes it feasible to propose a

688 J. Rodriguez-LaÂzaro et al.

palaeoenvironmental reconstruction of the Cenomanian in this region of the

Basque Basin.

7.1. Microfauna and water massesSome aspects of the microfaunal analysis provide indications of the characteristics

of the water masses where such fauna developed, the foraminifera and ostracods

being interesting proxies of these masses (Murray, 1995). Thus, an index of

oceanicity (PF/PF + BF) greater than 50% corresponds to bathyal depths (200-

4000 m; Seyve, 1990). Furthermore, a high content of planktonic keeled

foraminifera may be used to con®rm the depth of the environment of deposition

(Tur, 1996). In the greater part of the Leioa succession, the oceanicity index

reaches percentages greater than 80% (Figure 4), which suggests a

palaeoenvironment of a mesobathyal to infrabathyal type (800-4000 m). In

Figure 4 the percentages of textulariid foraminifera are shown to be greater than

30%, which also indicates a depth that is consistent with cold bathyal conditions

(Murray, 1991). The absence of porcellanous foraminifera in this succession is a

consequence of the fact that the depth of the basin was, in any case, located

below the outer margin of the platform (Boltovskoy, 1965).

The keeled planktonic foraminifera that dominate in our section (Figure 4, and

microfaunal analysis above), indicate the major presence in this basin of

intermediate oceanic waters (Bellier, 1989). It is also possible to distinguish

lower-intermediate and upper-intermediate waters as a function of the relative

percentages of both keeled and incipiently keeled planktonic foraminifera,

respectively (Jarvis et al., 1988; see also Figure 4). On the other hand, the three

intervals in which we encountered a maximum of globular planktonic foraminifera

is typical of more super®cial waters (Jarvis et al., 1988; CrumieÁre, 1989).

In order to con®rm these observations, as well as to register other changes in

the characteristics of the water masses (e.g., oxygenation, depth, thermal

character), the ecological limitations of some species of benthonic foraminifera

and ostracods have been considered. In Figure 6 the benthonic foraminifera that

characterize shallow, intermediate and deep water masses have been grouped (see

Appendix). It is observed that the highest percentages correspond to intermediate

water masses, which would have been typical of this part of the basin during the

Cenomanian. This supports the interpretation of the keeled planktonic

foraminifera (Figure 4). The in¯uence of deep water masses was generally minor,

being represented by fewer than 10% of the species, with an abrupt rise in the

middle Cenomanian (lower part of the R. cushmani Zone), and another important

rise during the early Turonian. The in¯uence of shallow water masses is

insigni®cant in this series, being evident only in intervals where the presence of

the intermediate water masses decreases (Figure 6).

The biotic indices of hypoxia have also been considered. The heterohelicids are

characteristic of waters in areas of extensive development of a Minimum Oxygen

Zone (MOZ; Sliter & Premoli-Silva, 1990). In the Leioa section, they are more

abundant from the upper Cenomanian upwards (Figure 4). It seems, therefore,

that during the deposition of part of the succession this hypoxia affected the

upper-intermediate waters inhabited by these planktonic foraminifera.

The species of benthonic foraminifera that are resistant to conditions of

hypoxia are relatively important, with intervals in which they represent more than

40% of the benthonic foraminifera assemblage, and a greater presence within the

upper Cenomanian, precisely when the heterohelicids are abundant. They include

Cenomanian events in the deep western Basque Basin 689

Ammobaculites impexus, Eggerellina brevis, Gavelinella berthelini, Globulina prisca,Glomospira charoides, Glomospirella irregularis, Lenticulina rotulata, Lingulogavelinellaglobosa, Marssonella kummi, M. trochus, Textularia chapmani, and T. subconica(Jarvis et al., 1988; Koutsoukos et al., 1990; Kuhnt, 1992; Peryt & Wyrwicka,

1991), and are referred to on Figure 6 as BF `hypoxic' spp.

The platycopid and Cypridacea `signals' provide evidence for hypoxia in the

case of ostracods (Whatley, 1991). Horne & Rosenfeld (in Jarvis et al., 1988) and

Horne et al. (1990), have shown that across the Cenomanian-Turonian boundary

at Dover (UK), the podocopid ostracods, which dominate the marine

assemblages in `normal' conditions, were replaced by platycopids during the

hypoxic interval of the OAE 2. The platycopid genera Cytherella and Platella, as

well as the cypridacean Argilloecia, Macrocypris, Paracypris and Pontocyprella,

possess characteristics that allow them to resist low levels of dissolved oxygen.

Hence, it is possible to calculate approximately the oxygen content in these waters

(Whatley, 1995).

The levels of the Leioa succession with marked increases in percentages of

these platycopids are shown in Figure 6. These indicate the presence of intervals

with an oxygen content below about 5 ml/l. It is, therefore, deduced that during

the Cenomanian there may have been times when the oxygen content was 2-3 ml/

l (Figure 7). The virtual absence of platycopids through part of the middle

Cenomanian does not permit use of these percentages as signs of hypoxia, but it

is precisely their virtual absence which indicates that the conditions were so

strongly hypoxic (near anoxia?) that they were unable to live, a fact which is

supported by the presence of certain benthonic foraminifera, such as Textulariachapmani, which could survive in practically anaerobic conditions (Jarvis et al.,1988). The Cypridacea signal is minor in this succession (Figure 6), with maxima

(>30%) in three middle and upper Cenomanian intervals. These occur at levels

which precede the maximum development of the platycopids. This suggests a

precursor character of the Cypridacea signal which was produced during the ®rst

phase of environmental alteration. The persistence of this environmental

modi®cation could have led to the disappearance of the Cypridacea and the

appearance of the platycopid signal which replaced it. Therefore the presence of

hypoxia in the benthos during the greater part of the succession can be deduced.

It was particularly intensive during the deposition of the middle Cenomanian

sediments.

7.2. Origin and extinction bioeventsThe description of species appearance/extinction bioevents in a sedimentary series

is a very useful tool in high resolution sequence stratigraphy. These bioevents are

de®ned as rapid responses of evolution, ecology or biogeography of the biotas

because of abrupt environmental changes. They can involve an increase or a

reduction in diversity. Diversi®cation bioevents may indicate favourable but rapid

environmental changes, such as an increase in water temperature, greater

availability of nutrients, light, and/or dissolved oxygen and, as a rule, greater

stability of habitat, among other factors. Bioevents characterized by a reduction in

diversity may re¯ect environmental disturbances owing to abrupt climatic changes

and/or oceanographic, sedimentologic, tectonic and volcanic or biological

disturbances, or events of an extraterrestrial nature (Kauffman & Hart, 1996).

During these phases of environmental alteration, some characteristic speci®c

records are produced, including ecological generalist species, opportunists,

690 J. Rodriguez-LaÂzaro et al.

disaster species, preadapted surviving, crises progenitors and refuge species

(Lazarus), that permit a detailed characterization of extinction episodes and the

subsequent recovery of ecosystems, which constitute regional and/or global

bioevents (Kauffman & Harries, 1996).

For each sample, we have recorded (a) the species that enter the record, (b)

those that disappear, presumably as a result of emigration (L, Lazarus species),

(c) those that become extinct locally and (d) those that persist. We followed the

methodology described by Rodriguez-LaÂzaro & GarcõÂa-Zarraga (1992b), the aim

being to measure the impact of the ecological changes on the microfaunas

studied.

Planktonic foraminifera are present at several levels with appearances

(Figures 3, 7) at (1) the base of the section, where Praeglobotruncana stephani and

P. gibba enter the record; (2) the boundary between the R. brotzeni and R. reicheli

Zones, where the latter species appears; (3) the boundary between R. reicheli and

R. cushmani Zones, where large numbers of Rotalipora cushmani appear and (4)

the middle/late Cenomanian transition, at the point of entry of

Helvetoglobotruncana praehelvetica, Rotalipora deeckei and Whiteinella paradubia

(Figure 3). This level of Rotalipora decline which, in this section, concerns R.

cushmani and R. greenhornensis, is located at Dover (UK) at the base of bed 2 of

the Plenus Marl (Metoicoceras geslinianum Zone), corresponding to the uppermost

R. cushmani Zone, and just above the R. greenhornensis LAD (Jarvis et al., 1988).

The regional extinction levels (LADs) of the planktonic foraminiferal species

are: (1) the middle part of the R. reicheli Zone, where P. stephani disappears; (2)

the lower part of the R. cushmani Zone, with the successive disappearance of R.

brotzeni and R. reicheli; (3) the top of the middle Cenomanian, at the regional

extinction of R. montsalvensis, and (4) at higher levels of the upper Cenomanian

part of this section, with LADs of R. greenhornensis and R. deeckei.

Concerning the benthonic foraminifera, up to seven appearance levels of

species occur, the most important being located within the R. reicheli Zone (up to

22 spp., sample sca-7-2; see Figure 7). These entrances may signify a temporary

recovery of the benthonic environment, since most of the species are Lazarus

taxa. In the R. cushmani Zone there are fewer appearances, with two maxima of

10 species in each. Concerning the emigrations/regional extinctions, two major

episodes can be distinguished. During the ®rst, in basal part of the R. reicheli

Zone, there is an emigration peak which is accompanied by a regional extinction

(5 species), and ends with an LAD of Dorothia ®liformis. Above, four new

emigration peaks (up to 16 Lazarus species) occur. In the upper maximum there

is the LAD of Arenobulimina advena. Toward the top of the R. reicheli Zone a

regional extinction peak (5 species) is registered with few `emigrations' (7

species). Above this level, previously abundant species disappear (LADs of

Glomospira charoides and Glomospirella irregularis). The base of the R. cushmani

Zone is characterized by a double emigration maximum (8 spp./sample), which

may indicate a progressive environmental alteration. A continuous background

extinction is registered through the rest of this zone, without large emigrations.

This produces a very important change in the composition of the assemblages of

benthonic foraminifera with the LADs, from base to top, of Eggerellina mariae,

Glomospira corona, Marsonella kummi, Allomorphina trochoides-E. brevis-Textularia

chapmani, C. gaultina, G. balthica and T. pyramidata. The extinction of the last

two of these species marks the end of the late Cenomanian in central Poland

Cenomanian events in the deep western Basque Basin 691

(Peryt, 1991). Hence, we think that, in the Leioa section, the highest levels of the

Cenomanian are represented.

With reference to the ostracods, we recorded up to nine appearance levels in

the Cenomanian (Figure 7): four in the R. reicheli Zone and another ®ve in the R.cushmani Zone. One of the latter, which is more important, is located in the lower

part of the zone, and occurs following an interval containing many emigrations.

The appearance of 17 species of ostracods in samples at the base of the R.cushmani Zone (sca-10, sca-10-1) is owing to the fact that many of them are

derived from the platform (see section 5.2). They cannot, therefore, be included

in this analysis, even though their presence at this level indicates the effect of a

possible eustatic lowering of sea level (marked `shelf' in Figure 7).

On the other hand we have detected four large emigration peaks within the

lower Cenomanian (Figure 7), suggesting that up to 10 species per level must

have emigrated. Thus a strong oscillation in the character of the benthos is seen

within this temporary interval. The subsequent entry in the record of these and

other species partly compensates for the emigrations, at least up to the middle

part of the succession (middle Cenomanian, Figure 7), where a greater number of

emigrations than appearances are apparent. Ostracod assemblages are virtually

absent from these upper levels of the R. reicheli Zone. Up to seven levels with

local extinctions of these crustaceans have been registered (see Figure 7), even

though the number of species that became extinct is relatively low (1-2/sample),

and in a stepwise manner. This suggests that, as a rule, there were strong

background stepwise extinctions. The abrupt extinction of Oertliella sp. 3 is

particularly signi®cant (sample sca-6-1; middle Cenomanian, R. reicheli Zone),

because up to that level they are abundantly present (about 500 individuals in

total). Similarly, within a relatively short interval (top of middle Cenomanian)

there are three levels recording local extinctions of six species of ostracods (LADs

of Cytheropteron cf. bispinosa, Neocythere kayei, Pontocyprella rara, Pontocyprella sp.

5, Praephacorhabdotus sp 2, Rehacythereis huescaensis).

7.3. Palaeoenvironmental evolutionDetailed analysis of the biotic signals (planktonic and benthonic foraminifera and

ostracods), the isotopic signals of the d13C and d18O, and the occurrence of

intervals without benthonic microfauna (bioevent markers: benthic-free intervals,

calcitic benthic-free intervals, ostracod-free intervals) permits consideration of

seven episodes in the palaeoenvironmental evolution of the Cenomanian part of

this succession (Figure 7). In Episode 1, the planktonic microfaunas are

dominated by R. montsalvensis and the benthonics are composed of the

assemblages BF-1 and O-1. Towards the top of this interval a marked increase in

H. delrioensis is observed which parallels a decrease of diversity in the benthonics,

as much in foraminifera as in ostracods, until an interval without calcitic

benthonics (BC-FI) is apparent. The d13C trend is diminishing in the lower half,

increasing thereafter up to the top of the interval where it falls suddenly. The

d18O values remain stable. Since the increase in the d13C signal can be interpreted

as a reduction in the amount of nutrients available, particularly in the levels of

phosphates and nitrates (Berger & Vincent, 1986; Brasier, 1995), the negative

trend recorded in the diversities of the benthonics, could be a result of a shortage

of nutrients.

Episode 2 is characterized by impoverished benthonic assemblages (BF-1, O-2

basal). The d13C isotope signal is variable, descending at the top of the episode to

692 J. Rodriguez-LaÂzaro et al.

the lowest value of the entire Cenomanian part of the Leioa section. This

decrease may be related to an increase in the available nutrients (see above). This

would explain the subsequent increase in the diversity of the ostracods and

benthonic foraminifera.

In the R. reicheli Zone, three episodes are distinguished. The ®rst (Episode 3)

encompasses the assemblages BF-2 and O-2 (upper). Rotalipora dominates the

planktonics, except in an interval where the hedbergellids are predominant

(`globular' in Figure 4), which may indicate the substitution of intermediate by

super®cial waters (Jarvis et al., 1988; Leary et al., 1989). The d13C trend is to

increase through this episode, de®ning two cycles with a maximum (sample LJ-2-

3; 1.57- PDB, Figure 5) that is comparable to maximum 2-1 of the R. reicheliZone of Jenkyns et al. (1994). This episode ends with the disappearance of the

benthonics (2 levels B-FI; Figure 7). The virtual disappearance of ostracods at

the end of the early Cenomanian coupled with the total disappearance of the

benthonics at the end of the episode, points to environmental degradation

probably owing to the development of intensive hypoxia (< 1 ml/l; almost

anoxia?), as well as to a decrease in nutrient levels.

The lower and upper limits of Episode 4 are delimited by B-FI levels. The

assemblages of this episode are taxonomically impoverished (BF-3, O-3). The

platycopid signal is notable, being indicative of hypoxia, as previously mentioned.

Textularia chapmani is known to have disappeared when the oxygen concentration

decreased (Lamolda & Peryt, 1995). The absence of this species supports the

hypothesis of decreased oxygen content. On the other hand, species that tolerated

marked environmental changes, such as Ammodiscus cretaceus, Glomospira charoidesand Glomospirella irregularis (Kuhnt, 1992) persist and even predominate. The

d13C trend is variable, a small maximum of d18O being de®ned during this

interval. The change in the water-type indicated by an increase in the globulars in

the planktonic assemblages (see Figure 4) could have also produced alterations in

the benthonic environment that led to faunal impoverishment. The positive signal

of the carbon in the upper part of the interval could be interpreted to indicate a

decrease of nutrients in the environment. In a manner similar to that which

occurred during the previous episode, the speci®c diversity of the benthonic

calcareous fauna coincides with the beginning of intervals of increasing d13C

values. A comparable change, where the more important faunal modi®cations

occur at the beginning of the d13C cycles, has been observed in echinoid

assemblages in the Cenomanian of Charentes (France; Neraudeau et al., 1997).

Episode 5 occupies the uppermost section of the R. reicheli Zone. The

rotaliporas return to become dominant among the planktonics. A recovery of the

benthonic faunal assemblages (BF-4) is observed, but the ostracods (O-4) only

partially recovered in the middle part of the episode, their stratigraphic

occurrence being limited by two O-FI (ostracod-free intervals) levels above and

below. The isotope signals are variable, with a marked positive shift of the carbon

values (sample sca-7-3; 2.09- PDB) which is comparable to the 2-2 shift of

Jenkyns et al. (1994). All these data suggest an altered environment that is a

continuation of the previous one, but with a higher level of hypoxia in the

benthos.

Episode 6 encompasses a large part of the R. cushmani Zone. Even though the

rotaliporas generally dominate, the hedbergellids are more important in its lower

part (`globular', Figure 4), while at the same time the ostracods and practically all

the hyaline foraminifera disappear (bioevent marker CBF-FI, Figure 7). The

Cenomanian events in the deep western Basque Basin 693

assemblages (BF-5 and O-5, O-6) are of low diversity. Several regional extinction

levels and subsequent appearances of ostracods are evident which indicate revival

of the Cenomanian species, chie¯y towards the end of the episode (O-6). The

carbon isotope values are maintained at a stable level, while those of oxygen

clearly show an increasing trend, ending with a pronounced fall at the upper

boundary. The environment continued to change, according to the shortage of

calcareous benthonics, except during deposition of the upper-middle part of the

episode when a recovery of the benthonics occurred. This may have been induced

by the better oxygenation of the water (5 ml/l?; see Figure 7) detected within this

interval. Towards the upper part of the episode, the abundance of allochthonous

benthonic individuals originating from the platform indicate a tectonic and/or

eustatic in¯uence on these levels (possible eustatic fall of 94 Ma; `shelf' in

Figure 7).

Episode 7 occurs in the upper Cenomanian part of this section, beginning with

a drastic change in the composition of the planktonic foraminifera, consisting of

the substitution in dominance of the genus Rotalipora by Helvetoglobotruncana(Rotalipora decline event). The heterohelicids occur only rarely up to this level. At

the base of the episode the benthonic assemblages (BF-6, O-7) show a recovery

of calcareous forms at the expense of agglutinated species. The assemblage BF-6

seems to indicate a less oxygenated environment, since Lenticulina rotulata and

Marssonella trochus, species which tolerated decreased oxygenation levels,

dominate (Koutsoukos et al., 1990). The isotope signals indicate a strongly

increasing trend in d13C values (up to 2.35- PDB, see Figure 5), which might

indicate the beginning of Oceanic Anoxic Event 2 (OAE 2). A change within the

intermediate waters from lower-intermediate to upper-intermediate, possibly less

oxygenated, water (see Figure 4) is suggested by the composition of the

foraminiferal assemblages, and in particular by the presence of heterohelicids

(Sliter & Premoli Silva, 1990).

Above the stratigraphic gap that re¯ects the Cenomanian/Turonian boundary,

there is an interval characterized by planktonic species, such as

Helvetoglobotruncana helvetica, that are typical of the early Turonian.

On the right of Figure 7, several European, North American and global events

are shown, for comparison with those recognised in the Basque Basin. The

temporary coincidence of the beginning of the bioevents in the deep Basque

Basin, and the timing of the global bioevents of the earliest middle Cenomanian

(Kauffman & Hart, 1996), support the hypothesis that global causes may have

produced the changes in the Basque Basin.

8. Conclusions

Assemblages of benthonic foraminifera and ostracods indicate a marked alteration

in the sea ¯oor conditions which coincides with the base of the R. reicheli Zone

(late early Cenomanian) and lasted, with a varying intensity, until the end of the

Cenomanian. The biotic markers indicate that the greatest alteration occurred

during the middle Cenomanian.

The observed faunal changes (appearances, emigrations, regional extinctions)

are not contemporaneous for the different groups: while the benthonic

foraminifera were renewed fundamentally during the early and early middle

Cenomanian, the composition of the planktonic foraminiferal and ostracod

assemblages changed most markedly at the end of the middle Cenomanian.

694 J. Rodriguez-LaÂzaro et al.

The d18O and d13C isotope values obtained correlate with the

palaeoenvironmental changes indicated by the faunal assemblages. Minima and

maxima shifts denote precisely the established biovents. The increasing trend of

d13C coincides with a reduction in nutrients and a long-term sea-level rise. The

two positive shifts of d13C (2-1 and 2-2) established by Jenkyns et al. (1994) and

Paul et al. (1994a) for the mid-Cenomanian of northwest Europe are clearly

recognisable. However, differences have been detected because we have observed

some lower values (1.57- and 2.09- respectively) (>1- difference) and abrupt

peaks in contrast to the mostly rounded curves for the northwest European

sections.

The last regional appearance of Rotalipora in this section provides a

chronostratigraphic reference, the upper limit of which is the global Rotaliporaextinction event (93.8 Ma; Kauffman & Hart, 1996). Equally, the eustatic fall

detected in the upper part of the middle Cenomanian of the Leioa section

possibly marks the chronostratigraphic level of 94 Ma which separates the

sequences of 2nd order UZA 1 and UZA 2 (Haq et al., 1987).

The Basque Basin, and more speci®cally the Plentzia Trough, was occupied

during the Cenomanian by intermediate water masses 500-3000 m in depth. It is

possible to distinguish some lower-intermediate waters (early Cenomanian-end of

the middle Cenomanian) and upper-intermediate waters (end of middle

Cenomanian-late Cenomanian). The in¯uence of super®cial water masses is

deduced to have occurred during an interval at the end of the early Cenomanian

and in two other intervals in the middle Cenomanian.

These water masses were variably hypoxic (moderate to strong; some 4-2 ml/l?)

during the greater part of the Cenomanian. This is indicative of the effect of the

OMZ of the waters that in¯uenced the benthos more directly. The dysaerobia

was probably especially important (<1 ml/l, posibly anoxia?) during part of the

middle Cenomanian. These anomalous oxygen levels caused important

modi®cations in the benthonic microfauna, with a great number of emigrations

and several regional extinctions, and subsequent revitalisation of the microfauna.

In the planktonic realm, hypoxia has been detected in only the upper

Cenomanian part of the succession.

The nutrient content of the water could be another important factor that

controlled the microfauna. From the latest early Cenomanian onwards through all

of the middle Cenomanian, the percentages of nutrients available to the

benthonic micro-organisms were very reduced. Thus the benthonic populations

suffered from considerable alteration of their ecosystems, registered by major

reductions in their speci®c diversity. These came about preferentially at the

beginning of the sedimentary cycles at levels where d13C isotope values have been

shown to increase, con®rming the hypothesis that such increases imply a decrease

in available nutrients.

The palaeoenvironmental alterations deduced for the Cenomanian sediments of

the Leioa section are basically attributable to palaeoceanographic changes, the

water masses having controlled the characteristics of the planktonic and

benthonic environments in the Plentzia Trough. The effects of other local causes,

such as the strong volcanic activity at this time and tectonic movement associated

with the European and Iberian plates, are more dif®cult to demonstrate. Several

of the bioevents determined in the middle Cenomanian of this basin probably had

global origins. This makes them valuable as reference markers for eventual

interregional correlations.

Cenomanian events in the deep western Basque Basin 695

Acknowledgements

This work has been partially funded by the Projects UPV 130.310-EB059/93 and

UPV 130.310-EB177/96 of the University of the Basque Country. Dr F. GarcõÂa-

Garmilla is thanked for help in the ®eld, E. GarcõÂa-Zarraga for some microfaunal

data from the Leioa section and Dr D. Horne for critical reading and English

correction of the original manuscript. Prof. D.J. Batten is especially thanked for

valuable comments and improving our English.

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Appendix

1. Faunal reference list with author attributions

Planktonic Foraminifera (Globigerinacea)Dicarinella hagni (Scheibnerova)Favusella washitensis (Carsey)Hedbergella delrioensis (Carsey)

698 J. Rodriguez-LaÂzaro et al.

Hedbergella simplex (Morrow)Helvetoglobotruncana helvetica (Bolli)Helvetoglobotruncana praehelvetica (Trujillo)Praeglobotruncana gibba KlausPraeglobotruncana stephani (Gandol®)Rotalipora brotzeni (Sigal)Rotalipora cushmani (Morrow)Rotalipora deeckei (Franke)Rotalipora greenhornensis (Morrow)Rotalipora montsalvensis MornodRotalipora reicheli MornodWhiteinella archaeocretacea PessagnoWhiteinella brittonensis (Loeblich & Tappan)Whiteinella paradubia (Sigal)

Benthonic Foraminifera (Textulariina, Rotaliina)Allomorphina trochoides (Reuss)Ammobaculites impexus EicherAmmodiscus cretaceus (Reuss)Arenobulimina advena (Cushman)Clavulina gaultina MorozovaDorothia ®liformis (Berthelin)Eggerellina brevis (d'Orbigny)Eggerellina mariae Ten DamGavelinella balthica BrotzenGavelinella berthelini (Keller)Globulina prisca ReussGlomospira charoides (Jones & Parker)Glomospira corona Cushman & JarvisGlomospirella irregularis (Reuss)Gyroidinoides nitida (Reuss)Lenticulina rotulata LamarckLingulogavelinella globosa (Brotzen)Marssonella kummi ZedlerMarssonella trochus (d'Orbigny)Pseudotextulariella cretosa (Cushman)Textularia chapmani LalickerTextularia subconica FrankeTritaxia macfadyeni (Cushman)Tritaxia pyramidata Reuss

Ostracoda (Podocopa, Platycopa)Acuticytheretta sp. 1Bairdoppilata cuvillieri (Damotte)Bythoceratina sp. 2Bythocypris sp. 3Cardobairdia cenomanensis NuytsCurfsina sp.Cytherella sp.1Cytherelloidea sp.Cytheropteron bispinosa DingleImhotepia marssoni (Bonnema)Limburgina seuvensis AndreuMauritsina babinoti Colin, Lamolda & Rodriguez-LaÂzaroNeocythere kayei WeaverOertliella sp. 3Paracypris acuta (Cornuel)Platella sp.1Platycythereis sp.Pontocyprella rara KayePontocyprella sp. 5Praephacorhabdotus aff. semiplicatus (Reuss)Praephacorhabdotus sp. 2Pterygocythere pulvinata DamotteRehacythereis huescaensis AndreuRehacythereis sopeirensis AndreuRehacythereis sp. 5

Cenomanian events in the deep western Basque Basin 699

Rimacytheropteron sp. 1Schuleridea jonesiana (Bosquet)Xestoleberis sp. 2

2. Benthic foraminifera used to characterize water masses(After Burnaby, 1961; Kaiho et al., 1993; Paul et al., 1994b; Mitchell, 1996).

Benthonic foraminiferal species of shallow water massesArenobulimina advena (Cushman)Gavelinella berthelini (Keller)

Benthonic foraminiferal species of intermediate water massesAmmodiscus cretaceus (Reuss)Dentalina annloomisae McLeanDentalina basiplanata CushmanDentalina catenula ReussDentalina gracilis d'OrbignyDentalina solvata CushmanDentalina variata Magniez-JanninDentalina spp.Frondicularia ungeri ReussLenticulina gaultina (Berthelin)Lenticulina rotulata (Lamarck)Lenticulina secans (Reuss)Marssonella kummi ZedlerMarssonella ozawai CushmanMarssonella trochus (d'Orbigny)Nodosaria bistegia (Olszewski)Nodosaria cylindrica (Reuss)Nodosaria sp.Pseudotextulariella cretosa (Cushman)Textularia chapmani LalickerTextularia obtusangula RoemerTextularia subconica Franke

Benthonic foraminiferal species of deep water massesBathysiphon sp.Gyroidinoides nitida (Reuss)Gyroidinoides praestans Magniez-JanninGyroidinoides sp.Tritaxia macfadyeni (Cushman)Tritaxia pyramidata ReussTritaxia tricarinata (Reuss)

700 J. Rodriguez-LaÂzaro et al.