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Lower Aptian carbon isotope stratigraphy from a distal carbonate shelf setting: the Cau section, Prebetic zone, SE Spain
Palaeogeography, Palaeoclimatology, Palaeoecology 200 (2003) 207^219 www.elsevier.com/locate/palaeo
Lower Aptian carbon isotope stratigraphy from a distal carbonate shelf setting: the Cau section, Prebetic zone, SE Spain G.A. de Gea a; , J.M. Castro a , R. Aguado b , P.A. Ruiz-Ortiz a , M. Company c a
Departamento de Geolog|¤a, Universidad de Jae¤n, Facultad de Ciencias Experimentales, Campus Universitario Las Lagunillas, 23071 Jae¤n, Spain b Departamento de Geolog|¤a, Universidad de Jae¤n, Escuela Universitaria Polite¤cnica de Linares, Alfonso X El Sabio 28, 23700 Linares, Spain c Departamento de Estratigraf|¤a y Paleontolog|¤a, Universidad de Granada, Facultad de Ciencias, 18002 Granada, Spain Received 26 July 2001; accepted 21 March 2003
Abstract A N13 C curve is reported for the latest Barremian to Early Aptian at a section located in the Prebetic zone (Cau section, SE Spain). The studied section records a hemipelagic succession of dark shales, deposited on a distal carbonate ramp with a high subsidence rate, adjacent to shallow carbonate environments. The integrated biostratigraphy of the section is based on ammonites, planktonic foraminifera and calcareous nannofossils, and it has allowed an accurate dating of the succession. The N13 C curve presented shows a distinctive evolution, leading to the recognition of three major excursions, as well as a subdivision into eight segments, which represents an improvement of the current biostratigraphic resolution. The correlation, both isotopic and biostratigraphical, with other well resolved sections is very accurate even at the higher resolution attained. Correlation with sections with lowresolution biostratigraphic characterisation from shallow platform limestones also gives good results, which supports the effectiveness of carbon isotope stratigraphy as a correlation tool. 3 2003 Elsevier B.V. All rights reserved. Keywords: Carbon isotopes; Aptian; biostratigraphy; Prebetic zone; SE Spain
1. Introduction Recent developments in correlating marine carbonates include the analysis of the carbon isotope
* Corresponding author. Tel.: +34-53-012030; Fax: +34-53-012141. E-mail address: [email protected] (G.A. de Gea).
record. This method has been successfully applied to Cretaceous marine carbonate sediments (Jenkyns, 1995; Weissert et al., 1998; Moullade et al., 1998). The best results have been obtained from pelagic successions, although recently these techniques have been applied also to shallow platform limestones (Jenkyns, 1995; Ferreri et al., 1997; Gro«tsch et al., 1998, among others), with very interesting results, also checked with biostratigraphic data (Masse et al., 1999; Erba et al.,
0031-0182 / 03 / $ ^ see front matter 3 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00451-6
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1999) and with magnetostratigraphy (Lini et al., 1992; Hennig et al., 1999). The Lower Cretaceous has been one of the main objectives of the study of the carbon isotope record, revealing important excursions within the curves, with a high chronostratigraphic potential. The Early Aptian was marked by a very well recognised excursion of the N13 C curve, linked to the Oceanic Anoxic Event (OAE) 1a (Coccioni et al., 1987; Menegatti et al., 1998). Other notable palaeogeographical, palaeoceanographical and palaeoecological events have been registered within this interval. This emphasises the importance of global high-resolution correlation for this age, in order to determine cause^e¡ect relationships between the di¡erent events recorded (Menegatti et al., 1998). In this study we present the high-resolution carbon isotope curve for the Lower Aptian of the Cau section, located in the Betic Range of SE Spain (Fig. 1). The ¢rst objective of this paper is the detailed analysis of the isotopic record of the studied section ; achieving this will lead to the correlation between the isotopic and biostratigraphical data of the succession. Finally, the results obtained from the studied section will be compared with other isotopic curves provided from the same time interval in other basins.
2. Geological and palaeogeographic setting of the Prebetic During the Cretaceous, the Prebetic was a wide epeiric carbonate platform, situated on the passive southern Iberian continental margin to the north attached to a Hercynian hinterland (the Iberian Massif) (Fig. 1). This platform was located on the northern margin of the Tethys, within the Tethys^Atlantic seaway. This margin was a¡ected during the Early Cretaceous by extensional tectonics, in relation to the opening of the North Atlantic (Vilas et al., 1993). This extensional episode was re£ected in a rifting of the margin, with high subsidence throughout most of the Early Cretaceous, mainly developed in the distal areas of the margin. The subsidence pattern was controlled by synsedimentary listric
faults, leading to the development of tilted blocks (Arias et al., 1994; Vilas and Querol, 1999) with notable lateral changes in the subsidence rate (Vilas et al., 2000; Castro and Ruiz-Ortiz, 2000). Nevertheless, the widespread greenhouse and transgressive context of the Early Cretaceous resulted in a balance between accommodation space and sedimentation rate, and the main expressions of this rifting are the notable lateral changes in thickness, as well as local to regional discontinuities located in the uplifted tops of the tilted blocks (Vilas et al., 1993). During the Early Cretaceous, carbonate ramps developed on the Prebetic platform, which graded from shallow carbonate shelf environments with rudists and corals in the north to distal hemipelagic environments to the south, with a very gentle slope. The Alicante region represents a distal part of the Prebetic platform, with ramp morphology and a high subsidence rate, representing the transition between shallow shelf and hemipelagic environments. The relative changes in sea level in this region were re£ected in rapid lateral shifts of the sedimentary environments, which resulted in a vertical alternation of hemipelagic and shallow carbonate shelf units (Company et al., 1982; Castro, 1998; Ruiz-Ortiz and Castro, 1998). The Aptian sediments of the Prebetic of the Alicante region record a second-order transgressive^regressive sequence bound by two important, tectonically enhanced, sequence boundaries. The studied Lower Aptian deposits were deposited during the transgressive general trend corresponding to the lower part of the sequence (Fig. 2) (Castro, 1998; Ruiz-Ortiz and Castro, 1998). This transgressive trend has been registered worldwide (Haq et al., 1988; Hallam, 1992), and led to the submergence of broad continental areas, and also, in relation to tectonic pulses, to signi¢cant processes of backstepping and drowning of carbonate platforms in the Alpine areas (Vilas et al., 1993; Funk et al., 1993; Castro and Ruiz-Ortiz, 1995).
3. The Cau section The Cau section is located in the NE of Ali-
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Fig. 1. Geographical and geological location of the studied section.
cante Province, SE Spain (Fig. 1). It was ¢rst studied by Gignoux and Fallot (1926, in Darder, 1945), and later by Darder (1945) and R|¤os et al. (1961), focusing mainly on the presence of ammonites. More work has been carried out in the region during recent years, dealing with the lithostratigraphy, biostratigraphy, sedimentology, and sequence stratigraphy (Company et al., 1982; Castro and Ruiz-Ortiz, 1994, 1995; Castro, 1996, 1998; Ruiz-Ortiz and Castro, 1998; Aguado et al., 1999; Castro and Ruiz-Ortiz, 2000; Castro et al., 2001; Garc|¤a-Herna¤ndez et al., 2001). 3.1. Lithostratigraphy The studied section comprises a relatively complete sequence from the Upper Barremian to the Aptian (about 220 m thick; see Fig. 3) comprising rhythmic alternations of marls and marly limestones with ammonites. These deposits belong to the Almadich Formation (Castro, 1998), which, in this outcrop, overlies the hemipelagic marls and marly limestones of the Los Villares Formation (Ruiz-Ortiz, 1980; Aguado et al., 1996) and
underlies the shallow platform limestones of the Seguil|¤ Formation, of Late Aptian to earliest Albian age (Castro, 1998; Ruiz-Ortiz and Castro, 1998). Towards the north, the lower and upper parts of the Almadich Formation shift, by lateral changes of facies, to the shallow platform limestones of the Llopis and Seguil|¤ formations, respectively (Castro, 1998; Ruiz-Ortiz and Castro, 1998). Fig. 2 summarises the lithostratigraphy of the Aptian successions of the Prebetic in Alicante, showing the relationship between the Almadich Formation and the other lithostratigraphic units (after Ruiz-Ortiz and Castro, 1998). The Almadich Formation has been divided into three members. (1) The lower member (55 m thick) is composed of a rhythmic alternation of marly limestones and light-grey marls containing abundant ammonites and nannofossils, and rare to common planktonic foraminifera. Cyclic marls/marly limestones (1 m thick on average) are present, sometimes with iron oxide on their tops. Bed 11c (Fig. 3) shows a hardground at its top, which corresponds to a discontinuity omitting the Barremian^Aptian
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Fig. 2. Synthetic lithostratigraphy of the Upper Barremian^Aptian of the Prebetic of Alicante.
boundary (see below) and marking the basal boundary of the Aptian second-order stratigraphic sequence (Ruiz-Ortiz and Castro, 1998). In the upper part of this member, there is a bed (14.6 in Fig. 3) of 35 cm of black marls with pyrite nodules. This member correlates laterally with a shallow carbonate ramp unit (Llopis formation; Fig. 2), that crops out a few kilometres north of the location of the Cau section (Castro, 1998). (2) The middle member (27 m thick), is mainly composed of black to dark-blue shales and marly sediments, organised in decimetre-thick cycles, with black lutites at the base (TOC content about 0.8%). They are followed by grey, more marly and silty sediments, and the cycles end with a 1^2-cmthick bed of light-grey marlstones and ochre to yellow marls (TOC content about 0.5%), sometimes capped by a slightly reddish surface. In the neighbouring sections located northwards, this member is represented by outer shelf marls and marlstones with large ammonites, accompanied by planktonic and benthonic fauna, and further north are shallow platform carbonates of equivalent age (Vilas et al., 1993). This con¢gura-
tion correlates to a strong retrogradation of the platforms that took place in response to an important transgressive pulse, at the onset of the deposition of this member. (3) The upper member (127 m thick) is characterised by alternating marly limestones and grey marls forming 1.5-m-thick cycles. They contain very scarce ammonites and common to abundant planktonic foraminifera and nannofossils. Some beds of marly limestones of this upper member present a ferruginous top and, in the uppermost part of the unit, the marly limestone layers become slightly sandy. Only the upper part of the upper member has a shallow platform equivalent in adjacent areas (Seguil|¤ Formation; Fig. 2) (Castro, 1998), which was deposited during the ¢nal, regressive part of the second-order Aptian stratigraphic sequence (Ruiz-Ortiz and Castro, 1998) (Fig. 2). The sediments of the Cau section were deposited under hemipelagic conditions, palaeogeographically close to the deposition site of shallow platform carbonates, at a shallow depth of probably only few tens of metres, (Company, 1987;
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Castro, 1998; Ruiz-Ortiz and Castro, 1998). The tectonic context of the depositional area of the selected section corresponded to a rapidly subsiding zone, £anked distally by low subsident areas, which were emergent during the earliest Cretaceous (Sierra Helada and Cabezo¤n de Oro sections, described in Granier, 1987 and Castro, 1998). This tectonic setting was probably related to the tilting of a block, uplifted distally, with a higher subsidence rate at the position of the Cau section. 3.2. Biostratigraphy The integrated biostratigraphic analysis of the studied interval (samples 11^24; Fig. 3), with ammonites, planktonic foraminifera and calcareous nannofossils, has led to the recognition of most of the biostratigraphic units based on these three fossil groups, and to the correlation among them (Fig. 3 ; see Aguado et al., 1999 for more details). Study of the ammonites has led to the identi¢cation of the Martelites sarasini, Deshayesites weissi, Deshayesites deshayesi and Dufrenoyia furcata zones. The study of the nannofossil assemblages, of markedly Tethyan character, led to the identi¢cation of the Micrantholithus hoschulzii, Hayesites irregularis and Rhagodiscus angustus zones. The relative abundance and good preservation of planktonic foraminifera through most of the studied section, except for the basal part, has allowed the identi¢cation of the Blowiella blowi, Schackoina cabri, Globigerinelloides ferreolensis and Globigerinelloides algerianus zones. According to these data, we can state that the studied interval comprises the uppermost Barremian, the Lower Aptian, and the lower part of the Upper Aptian (Aguado et al., 1999). The absence of the Deshayesites tuarkyricus ammonite zone, in addition to the presence of nannofossils such as Nannoconus truittii and Braarudosphaera africana in the lower beds of the Hayesites irregularis zone indicate the existence of a stratigraphic discontinuity a¡ecting the Barremian^Aptian boundary (Aguado et al., 1997, 1999). This discontinuity would comprise, at minimum, the whole D. tuarkyricus and even a part of the Deshayesites weissi zones of ammonites. Be-
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tween beds 13b and 20.4, including the ‘dark shales’ interval (Fig. 3), the ammonite associations are composed exclusively of forms with minor biostratigraphic signi¢cance (juvenile desmoceratids and fragments of heteromorphs or cheloniceratids). The quantitative analysis of the nannofossil abundance has allowed us to identify the ‘nannoconid crisis’ (Erba, 1994; Aguado et al., 1999), which starts in level 13.2 (Fig. 3), clearly preceding the ‘dark shales’ interval, in which, however, the e¡ects of the crisis are still present. Within the ‘dark shales’ interval three parts can be distinguished with respect to the nannoconid abundance: the lower part, about 2 m thick, characterised by a great scarcity ( 6 10%); the intermediate, of about 15 m thick, that records a noticeable recovery (15^30%); and the upper part, about 10 m thick, that is characterised by a new decline in the nannoconid abundance. The same evolution has been observed by Premoli-Silva et al. (1999), from the results obtained in the Cismon log APTICORE (northern Italy), where the ‘dark shale’ interval corresponds to the ‘Selli level’ and reaches only a total thickness of 5 m. The good to moderate degree of preservation of the planktonic foraminifera has encouraged us to use the most recent taxonomic classi¢cation (Banner and Desai, 1988; BouDagher-Fadel et al., 1997), based mainly on shell texture and morphology. Approximately at the Lower^Upper Aptian boundary, within the Globigerinelloides ferreolensis zone, a notable change in the composition of the assemblages is recorded ; they shift from a dominance of taxa belonging to the family Praehedbergellidae to an assemblage mainly composed by Hedbergellidae (Aguado et al., 1999). From level 14.6 (Fig. 3), the earliest trochospiral forms with slightly elongated chambers (Lilliputianella) are observed, but only from level 15.2 onwards the elongation of chambers increases, and planispiral forms with elongate chambers (Claviblowiella) appear. The ¢rst occurrence of Schackoina cabri occurs in sample 15.5, at the basal part of the ‘dark shales’ interval (Fig. 3), correlating with the uppermost part of the Deshayesites weissi zone, or with the lower part of the D. deshayesi zone of ammonites, and with the Hayesites irreg-
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Fig. 3. Detailed Cau section, with isotopic data and integrated biostratigraphy.
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ularis zone of nannofossils (Aguado et al., 1999). In the Cau section we have not found Leupoldina nor Schackoina below the ‘dark shales’ interval (Aguado et al., 1999). Nevertheless, Magniez-Jannin et al. (1997) found rare specimens of Schackoina cabri below the base of the ‘Niveau Goguel’, within the uppermost beds assigned to the D. weissi zone, in the Vocontian Trough. PremoliSilva et al. (1999) and Erba et al. (1999), also refer to the presence of S. cabri (very rare) below the base of the ‘Selli level’ in the Cismon APTICORE, in northern Italy.
4. Methods The section was sampled for isotope analysis with all samples calibrated to the detailed integrated biostratigraphy. Twenty-¢ve samples were taken for laboratory study. All of the samples were analysed twice at least, using for each analysis 1^5 mg. The analysis has been performed in the Stable Isotope Laboratory (SIDI) of the Universidad Auto¤noma of Madrid. For the determination of N13 Ccarb and N18 O, the samples were treated with H3 PO4 (103%), at 90‡C in a VG Isocarb system and the liberated CO2 was measured with a stable isotope ratio mass spectrometer VG Model Prism-II, calibrated with reference to the National Bureau of Standards and Technology (NBS 18 and 19). The results obtained are expressed in the standard PDB (Belemnitella americana of the Pee Dee Formation). The standard deviations for the mass spectrometer used are of S 0.1x for carbon and S 0.2x for oxygen. For the analysis of N13 Corg , the samples were treated previously with HCl (5%) during 24^36 h to completely remove carbonates. After this, samples were treated by combustion within an elemental analyser Carlo Erba CHNS 1108, at 1020‡C and the CO2 obtained from combustion was analysed in continuous £ux (CF-SIRMS) within a spectrometer Micromass model Isochrom, calibrated against SOCROSA ANU, GRAFITO (USGS 24) and PEF1 (reproducibility: S 0.2x). The results are expressed following standard PDB.
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5. Results The isotopic curve of N13 Ccarb (Fig. 3) ranges between values of 0.3x and 3.74x. The analysis of the curve has been made at two di¡erent scales. At a broader scale, three clearly expressed excursions are shown (one negative anomaly and two positive ones), easily correlatable with the isotope events observed by Moullade et al. (1998) in the Gare de Cassis section (SE France). On the other hand, at a high-resolution scale, a more detailed analysis reveals eight segments, in accordance with the ones observed by Menegatti et al. (1998) in the Swiss and Italian Alps, and also by Erba et al. (1999) in the Italian Alps. The lowest part of the curve is poorly documented, as it is represented only by two analyses, and is capped by the discontinuity at the Barremian^Aptian boundary. It is recorded within the Martelites sarasini ammonite zone, in the Upper Barremian, with a lowest value of 1.45x, and a positive trend. In spite of the scarcity of data, the trend shown, in addition to the accurate dating, suggests that this positive evolution can be probably equivalent to the same trend observed by Moullade et al. (1998) and Masse et al. (1999) on top of the negative excursion they observed in the upper part of the M. sarasini zone. The ¢rst positive excursion, with a maximum value of 2.66x, is recorded within the Weissi zone (see Fig. 3), predating the onset of the ‘nannoconid crisis’. This excursion is correlated with the upper part of the excursion observed in La Be¤doule (Moullade et al., 1998), where it comprises also the Tuarkyricus ammonite zone, absent here because of the Barremian^Aptian boundary discontinuity in our study section. The subsequent negative excursion attains a minimum value of 0.38x, and is recorded within the uppermost part of the Blowiella blowi biozone, the upper boundary of which is also the top of the excursion. Our data indicate that this excursion would lie within the transition beds between Deshayesites weissi and D. deshayesi zones (Fig. 3); however, following Moullade et al. (1998), it would lie within the D. deshayesi ammonite zone. The uppermost positive excursion is the best developed one, with a maximum value of
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3.74x, and a notable thickness of about 50 m. This excursion lies within the Deshayesites deshayesi and Dufrenoyia furcata ammonite zones (Fig. 3). This positive excursion is globally recorded, and is also related to OAE 1a, which is just antecedent or associated with the onset of the excursion (Weissert et al., 1985; Follmi et al., 1994 among others in the Alpine region; Vahrenkamp, 1996 in the Middle East; Jenkyns, 1995 in Paci¢c guyots). When analyzing the isotope curve in more detail, most of the intervals de¢ned by Menegatti et al. (1998) and recognised by Erba et al. (1999), can be documented (Fig. 3). Segment 1 is only partially represented, because of the discontinuity detected by means of ammonites in the Cau section, which comprises the Barremian^Aptian boundary. The N13 Corg curve (Fig. 3) is rather parallel to the N13 Ccarb curve, although the excursions are less clear, because there is in general a lower contrast between the values obtained. A geochemical study of the organic matter (de Gea et al., 2001) has shown an important contribution of terrestrial plants in the organic matter present in the studied sediments. Following Grocke et al. (1999), terrestrial plants record global long-term evolution of N13 C during the Aptian, but the two curves usually do not coincide, and they need not necessarily be in phase. The N18 O curve does not show an analogous evolution to that of N13 C, as the oxygen isotope content in carbonates is more dependent on factors such as diagenesis, and its composition is more variable (Menegatti et al., 1998; Vera, 1994, among others). Nevertheless, the evolution of the N18 O within the studied section show interesting trends. Within the Blowiella blowi biozone, the N18 O values are decreasing by 0.6% (marked as O1 in Fig. 3), and the opposite trend is recorded in the lower part of the Schackoina cabri biozone (indicated as O2 in Fig. 3). Both trends can be correlated with the segments di¡erentiated in the N13 C, C2^C3 and C4^C6, respectively. The Fig. 4. Correlation of the Cau section with selected other sections.
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same trends in the N18 O curve have been recognised by Menegatti et al. (1998) in two di¡erent sections in the Alps, and they also can be observed in other sections referred to by the same authors.
6. Discussion 6.1. Correlation of the isotope curves The correlation of the curve from the Cau section with those of the Roter Sattel section (Switzerland), based on data from Menegatti et al. (1998), of the Cismon core (Italy), studied by Erba et al. (1999), of the historical section of La Be¤doule in southern France, studied by Moullade et al. (1998), and of a core in the Resolution Guyot in the Mid-Paci¢c, published by Jenkyns (1995), is summarised in Fig. 4. Correlation of the various curves of N13 C shows a very good agreement among the di¡erent sections, even at the scale of the segments (C1^C8), ¢rst de¢ned by Menegatti et al. (1998), and recognised in the section we studied (Figs. 3 and 4). The main di¡erences between the Cau curve and the curves from the other sections presented in Fig. 4 are due to two reasons: ¢rstly, the thickness of our section is higher than that of the others (except for the shallow carbonates of the Resolution Guyot), because of a di¡erent palaeogeographic setting, which gave rise to a very high sedimentation rate. Secondly, there is a di¡erence in the biostratigraphic attribution of the ‘dark shales’ from the Cau section and the corresponding black shales of the ‘Selli level’ in Rotel Sattel according to Menegatti et al. (1998). Nevertheless, the biostratigraphic attribution presented in the more recent paper by Erba et al. (1999) (which studies a core close to the section studied by Menegatti et al., 1998), coincides with our own data. This probably indicates new biostratigraphic details from the Cismon area (Erba et al., 1999), that can also probably be applied to the Roter Sattel section (Menegatti et al., 1998). If this is correct, the biostratigraphic and isotopic correlations with the Cau section would be very accurate. The correlation with the historical section of La Be¤doule is
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also very good, with a negative excursion at the top of the Blowiella blowi biozone. The apparent poorer correspondence with the Resolution Guyot curve (from Jenkyns, 1995) can be explained by a low accuracy of the age attribution, because the section is composed of shallow platform limestones (without planktonic fauna), and the published age has been deduced indirectly. Ignoring the biostratigraphic di¡erences, there is a good correlation between this curve and the other referred to, with a strongly negative excursion followed by a notable positive one, clearly corresponding to the C4^C7 segments (Figs. 3 and 4). In this sense, the data presented from the isotope record in this work could be applied to re¢ne the dating of shallow platform sections, in which the pelagic fauna is absent. 6.2. Sedimentary environments The sediments of the Cau section were deposited under hemipelagic conditions, palaeogeographically close to the depositional site of shallow platform carbonates, at a shallow depth of probably only few tens of metres, (Company, 1987; Castro, 1998; Ruiz-Ortiz and Castro, 1998). The tectonic context of the depositional area of the selected section corresponded to a rapidly subsiding zone, £anked distally by low subsident areas, which were emergent during the earliest Cretaceous (Sierra Helada and Cabezo¤n de Oro sections, described in Granier, 1987 and Castro, 1998). This tectonic setting was probably related to the tilting of a block, uplifted distally, with a higher subsidence rate at the position of the Cau section. In this particular palaeogeographic setting, the sedimentary evolution was closely related to that of more proximal environments, and the hemipelagic sedimentation was sensitive to the relative sea-level £uctuations, well documented in the Alicante and adjacent areas of the Prebetic zone (Vilas et al., 1993; Castro and Ruiz-Ortiz, 1995; Castro, 1998; Ruiz-Ortiz and Castro, 1998). Several third-order sequences have been deduced from the Aptian record of the Prebetic (Vilas et al., 1993; Ruiz-Ortiz and Castro, 1998), which in any case represent rapid lateral shifts of the depositional
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environments within the shelf, resulting in drastic vertical and lateral facies changes (Fig. 2). The most relevant event in this evolution was the strong transgressive pulse that led to the drowning of the shallow platform (Llopis Formation) at the onset of the deposition of the middle member of the Almadich Formation. In this sense, several authors have recently pointed out a relationship between rapid rises in sea level, platform drowning and deposition of black shales (Weissert et al., 1998; Grotsch et al., 1998; Davey and Jenkyns, 1999). The dark lutites of this middle member can be correlated with other Lower Aptian black shale deposits from central and northern Italy (‘Selli level’), SE France (‘Niveau Goguel’) (Bre¤he¤ret, 1988) and, probably, from NW Germany and the North Sea area (‘Fischschiefer’) (Mutterlose and Bo«ckel, 1998). Moreover, this member of the Almadich Formation should represent the local record of OAE 1a (Arthur et al., 1990; Larson et al., 1993; Bralower et al., 1994). Most sections from SE France and northern and central Italy showing anoxic facies equivalent to that of the Cau section are barren of planktonic foraminifera, or they are of little signi¢cance (Bre¤he¤ret and Delamette, 1989; Tornaghy et al., 1989; Coccioni et al., 1992), with the exception of the Cismon APTICORE (Erba et al., 1999; Premoli-Silva et al., 1999). In the ‘dark shales’ interval of the Cau section, the planktonic foraminifer assemblages are composed of common to abundant forms with elongated chambers and/or tubulospines (Aguado et al., 1999). The elongation of the chambers in the planktonic foraminifera seems to have been a response to low dissolved oxygen conditions (BouDagher-Fadel et al., 1997; Magniez-Jannin, 1998; Aguado et al., 1999; Premoli-Silva et al., 1999). The di¡erence in thickness between this member and the ‘Selli level’ can be explained by a di¡erent palaeogeographic context, the Cau section corresponding to a distal ramp, with a very high subsidence rate and moderate terrigenous input from the continent. This anomalous high thickness can also account for the lower content in organic matter (about 0.8%), in relation to other coeval sections (with TOC contents about 2^
5%), due to terrigenous dilution. In this sense, we use the term ‘dark shales’ for this facies in the Cau section instead of ‘black shales’, because the last is usually reserved for TOC contents higher than 1% (see, for example, de Graciansky et al., 1987; Vera, 1994). Another interesting peculiarity of the Cau section is related to the fact that the dark shales extend into the positive N13 Ccarb excursion, whereas in other sections the black shales only occur at the beginning of the isotope anomaly. This can be explained within the palaeogeographic context, with a rapid subsident area bounded by shallower sectors. This context could have led to a slight stagnation of the waters, with the permanence of disoxic conditions after the global event.
7. Conclusions The carbon isotope curve of the uppermost Barremian to the base of the Upper Aptian of a hemipelagic succession deposited in the northern margin of the Tethys has been obtained and analysed. The selected succession was deposited on a distal hemipelagic area of a shallow epeiric platform with signi¢cant in£uence both of shallow platform and adjacent continental areas. Moreover, the sedimentation rate of the studied section was abnormally high, due to the palaeogeographic and palaeotectonic setting, leading to a good record of the analysed isotope events. The N13 Ccarb and N13 Corg curves show a clear and distinctive vertical evolution during the latest Barremian^Early Aptian, which permits the detailed interpretation of a high-resolution stratigraphic scale. The N18 O curve is less signi¢cant, but also shows recognisable trends. The studied section is biostratigraphically very well characterised by means of ammonites, planktonic foraminifera and calcareous nannofossils. All the biozones from the uppermost Barremian to the base of the Upper Aptian have been identi¢ed and characterised. A discontinuity a¡ecting the Aptian^Barremian boundary has been detected mainly from the study of the ammonite and calcareous nannofossil record. At a medium scale, two positive excursions and
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a negative one are clearly identi¢ed. At a highresolution scale, eight segments (one below the discontinuity and seven on top of it) can be di¡erentiated. These eight segments have been recognised within a minimum of two biozones (calcareous nannoplankton), and a maximum of four biozones (for ammonites and planktonic foraminifera), indicating the potential of C-isotope stratigraphy as a high-resolution tool, beyond the scope of biostratigraphy. Moreover, this resolution is noteworthy in that the part of the section with a higher isotopic resolution (¢ve segments) is the one poorly characterised by ammonites. Analyses performed at di¡erent scales ¢t fairly well with the previously published data at a global scale, and especially with those curves elaborated at a higher biostratigraphic resolution, mainly from Alpine sections. This good correlation points to a weak in£uence of local factors such as diagenesis or provenance of organic matter. In other words, the correlation potential of the carbon isotopic record is emphasised, and the calibration of the isotopic curve with integrated biostratigraphy is con¢rmed as an extremely useful tool for stratigraphic correlation and dating.
Acknowledgements This study was co-¢nanced by Projects BTE2000-1151 and PB-960429 of DGICYT of the Spanish government and Research Groups RNM200 and 4064 of the Junta de Andaluc|¤a. Sample preparation was completed by A. Carrillo at the Geology Department, EU Polite¤cnica de Linares. Isotopic analysis were performed by Dr. R. Redondo, SIDI, Universidad Auto¤noma de Madrid, and TOC by Dr. J.F. Llamas, Universidad Polite¤cnica de Madrid. Peter Skelton kindly reviewed the English text of an earlier version of this paper which bene¢ted from very interesting comments and suggestions by J.P. Masse and H. Weissert.
References Aguado, R., de Gea, G.A., Ruiz-Ortiz, P.A., 1996. Datos bio-
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mentolog|¤a y IV Congreso del Creta¤cico de Espan‹a Gu|¤a de Excursiones, Jae¤n, pp. 63^91. Gea de, G.A., Gonza¤lez-Vila, F.J., Polvillo, O., Castro, J.M., Molina, J.M., Aguado, R., Ruiz-Ortiz, P.A., 2001. Uso de biomarcadores en el estudio de materiales del Aptiense inferior de las Zonas Externas de las Cordilleras Be¤ticas. Geotemas 3 (1), 175^179. Granier, B., 1987. Le Cre¤tace¤ infe¤rieur de la Costa Blanca entre Busot et Altea, Alicante, (Espagne): Biostratigraphie, Se¤dimentologie, Evolution tectono-se¤dimentaire. Ph.D. Thesis, Univ. of Paris, 279 pp. Grocke, D.R., Hesselbo, S.P., Jenkyns, H.C., 1999. Carbonisotope composition of Lower Cretaceous fossil wood ^ Ocean-atmosphere chemistry and relation to sea-level change. Geology 27, 155^158. Gro«tsch, J., Billing, I., Vahrenkamp, V., 1998. Carbon-isotope stratigraphy in shallow-water carbonates ^ Implications for Cretaceous black-shale deposition. Sedimentology 45, 623^ 634. Hallam, A., 1992. Phanerozoic Sea-Level Changes. Columbia University Press, New York, 266 pp. Haq, B.U., Hardenbol, J., Vail, P.R., 1988. Mesozoic and Cenozoic chronostratigraphy and eustatic cycles. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.S., Posamentier, H., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes. An Integrated Approach. Soc. Econ. Paleontol. Mineral. Spec. Publ. 42, pp. 71^108. Hennig, S., Weissert, H., Bulot, L., 1999. C-isotope stratigraphy, a calibration tool between ammonite and magnetostratigraphy: The Valanginian^Hauterivian transition. Geol. Carpathica 50, 91^96. Jenkyns, H.C., 1995. Carbon-isotope stratigraphy and paleoceanographic signi¢cance of the Lower Cretaceous shallowwater carbonates of resolution Guyot, Mid-Paci¢c Mountains. Proc. Ocean Drill. Program, Sci. Results, pp. 99^104. Larson, R.L., Fischer, A.G., Erba, E., Premoli-Silva, I., 1993. APTICORE-ALBICORE: A Workshop Report on Global Events and Rhythms of the Mid-Cretaceous, Perugia, 56 pp. Lini, A., Weissert, H., Erba, E., 1992. The Valanginian carbon isotope event: A ¢rst episode of greenhouse climate conditions during the Cretaceous. Terra Nova 4, 374^384. Magniez-Jannin, F., 1998. L’e¤longation des loges chez les foraminife'res planctoniques du Cre¤tace¤ infe¤rieur: Une adaptation a' la sous-oxyge¤nation des eaux? C. R. Acad. Sci. Paris IIA 326, 207^213. Magniez-Jannin, F., Bre¤he¤ret, J.G., Delanoy, G., 1997. Un exemple de spe¤ciation lie¤ a' l’eustatisme: l’apparition pre¤coce de Schakoina cabri (foraminife're planctonique me¤soge¤en). C. R. Acad. Sci. Paris IIA 325, 225^230. Masse, J.P., El Albani, A., Erlenkeuser, H., 1999. Isotope stratigraphy (Delta-C-13) of the Lower Aptian from Provence (SE France) ^ Application to platform/basin correlations. Eclogae Geol. Helv. 92, 259^263. Menegatti, A.P., Weissert, H., Brown, R.S., Tyson, R.V., Farrimond, P., Strasser, A., Caron, M., 1998. High-resolution N-C-13 stratigraphy through the Early Aptian Livello Selli of the Alpine Tethys. Paleoceanography 13, 530^545.
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the Early Cretaceous evolution of the Arabian Gulf region. AAPG 80, 647^662. Vera, J.A., 1994. Estratigraf|¤a. Principios y Me¤todos. Editorial Rueda, Madrid, 806 pp. Vilas, L., Querol, R., 1999. El l|¤mite septentrional de la extensio¤n prebe¤tica en el sector de Murcia, Libro Homenaje a Jose¤ Ram|¤rez del Pozo. AGGEP, pp. 219^226. Vilas, L., Masse, J.P., Arias, C., 1993. Aptian mixed terrigenous and carbonate platforms from Iberic and Prebetic regions, Spain. In: Simo, T., Scott, R.W., Masse, J.P., (Eds.), Atlas of Cretaceous Carbonate Platforms. AAPG Mem. 56, pp. 243^253. Vilas, L., Arias, C., Mart|¤n-Chivelet, J., 2000. Quanti¢cation of subsidence in the Cretaceous carbonate platforms of the Prebetic Domain (Spain), Quantitative Models on Cretaceous Carbonates and the Eastern Margin of the Apulia Platform. CRER WG4, Vieste, pp. 31. Weissert, H., Mc Kenzie, J.A., Channell, J.E.T., 1985. Natural variations in the carbon cycle during the Early Cretaceous. In: Sundsquist, E.T., Broecker, W.E. (Eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. Geophys. Monogr. Ser. 32, American Geophysical Union, Washington, DC, pp. 531^545. Weissert, H., Lini, A., Fo«llmi, K.B., Kuhn, O., 1998. Correlation of Early Cretaceous carbon isotope stratigraphy and platform drowning events: A possible link? Palaeogeogr. Palaeoclimatol. Palaeoecol. 137, 189^203.
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Lower Aptian carbon isotope stratigraphy from a distal carbonate shelf setting: the Cau section, Prebetic zone, SE Spain G.A. de Gea a; , J.M. Castro a , R. Aguado b , P.A. Ruiz-Ortiz a , M. Company c a
Departamento de Geolog|¤a, Universidad de Jae¤n, Facultad de Ciencias Experimentales, Campus Universitario Las Lagunillas, 23071 Jae¤n, Spain b Departamento de Geolog|¤a, Universidad de Jae¤n, Escuela Universitaria Polite¤cnica de Linares, Alfonso X El Sabio 28, 23700 Linares, Spain c Departamento de Estratigraf|¤a y Paleontolog|¤a, Universidad de Granada, Facultad de Ciencias, 18002 Granada, Spain Received 26 July 2001; accepted 21 March 2003
Abstract A N13 C curve is reported for the latest Barremian to Early Aptian at a section located in the Prebetic zone (Cau section, SE Spain). The studied section records a hemipelagic succession of dark shales, deposited on a distal carbonate ramp with a high subsidence rate, adjacent to shallow carbonate environments. The integrated biostratigraphy of the section is based on ammonites, planktonic foraminifera and calcareous nannofossils, and it has allowed an accurate dating of the succession. The N13 C curve presented shows a distinctive evolution, leading to the recognition of three major excursions, as well as a subdivision into eight segments, which represents an improvement of the current biostratigraphic resolution. The correlation, both isotopic and biostratigraphical, with other well resolved sections is very accurate even at the higher resolution attained. Correlation with sections with lowresolution biostratigraphic characterisation from shallow platform limestones also gives good results, which supports the effectiveness of carbon isotope stratigraphy as a correlation tool. 3 2003 Elsevier B.V. All rights reserved. Keywords: Carbon isotopes; Aptian; biostratigraphy; Prebetic zone; SE Spain
1. Introduction Recent developments in correlating marine carbonates include the analysis of the carbon isotope
* Corresponding author. Tel.: +34-53-012030; Fax: +34-53-012141. E-mail address: [email protected] (G.A. de Gea).
record. This method has been successfully applied to Cretaceous marine carbonate sediments (Jenkyns, 1995; Weissert et al., 1998; Moullade et al., 1998). The best results have been obtained from pelagic successions, although recently these techniques have been applied also to shallow platform limestones (Jenkyns, 1995; Ferreri et al., 1997; Gro«tsch et al., 1998, among others), with very interesting results, also checked with biostratigraphic data (Masse et al., 1999; Erba et al.,
0031-0182 / 03 / $ ^ see front matter 3 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00451-6
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1999) and with magnetostratigraphy (Lini et al., 1992; Hennig et al., 1999). The Lower Cretaceous has been one of the main objectives of the study of the carbon isotope record, revealing important excursions within the curves, with a high chronostratigraphic potential. The Early Aptian was marked by a very well recognised excursion of the N13 C curve, linked to the Oceanic Anoxic Event (OAE) 1a (Coccioni et al., 1987; Menegatti et al., 1998). Other notable palaeogeographical, palaeoceanographical and palaeoecological events have been registered within this interval. This emphasises the importance of global high-resolution correlation for this age, in order to determine cause^e¡ect relationships between the di¡erent events recorded (Menegatti et al., 1998). In this study we present the high-resolution carbon isotope curve for the Lower Aptian of the Cau section, located in the Betic Range of SE Spain (Fig. 1). The ¢rst objective of this paper is the detailed analysis of the isotopic record of the studied section ; achieving this will lead to the correlation between the isotopic and biostratigraphical data of the succession. Finally, the results obtained from the studied section will be compared with other isotopic curves provided from the same time interval in other basins.
2. Geological and palaeogeographic setting of the Prebetic During the Cretaceous, the Prebetic was a wide epeiric carbonate platform, situated on the passive southern Iberian continental margin to the north attached to a Hercynian hinterland (the Iberian Massif) (Fig. 1). This platform was located on the northern margin of the Tethys, within the Tethys^Atlantic seaway. This margin was a¡ected during the Early Cretaceous by extensional tectonics, in relation to the opening of the North Atlantic (Vilas et al., 1993). This extensional episode was re£ected in a rifting of the margin, with high subsidence throughout most of the Early Cretaceous, mainly developed in the distal areas of the margin. The subsidence pattern was controlled by synsedimentary listric
faults, leading to the development of tilted blocks (Arias et al., 1994; Vilas and Querol, 1999) with notable lateral changes in the subsidence rate (Vilas et al., 2000; Castro and Ruiz-Ortiz, 2000). Nevertheless, the widespread greenhouse and transgressive context of the Early Cretaceous resulted in a balance between accommodation space and sedimentation rate, and the main expressions of this rifting are the notable lateral changes in thickness, as well as local to regional discontinuities located in the uplifted tops of the tilted blocks (Vilas et al., 1993). During the Early Cretaceous, carbonate ramps developed on the Prebetic platform, which graded from shallow carbonate shelf environments with rudists and corals in the north to distal hemipelagic environments to the south, with a very gentle slope. The Alicante region represents a distal part of the Prebetic platform, with ramp morphology and a high subsidence rate, representing the transition between shallow shelf and hemipelagic environments. The relative changes in sea level in this region were re£ected in rapid lateral shifts of the sedimentary environments, which resulted in a vertical alternation of hemipelagic and shallow carbonate shelf units (Company et al., 1982; Castro, 1998; Ruiz-Ortiz and Castro, 1998). The Aptian sediments of the Prebetic of the Alicante region record a second-order transgressive^regressive sequence bound by two important, tectonically enhanced, sequence boundaries. The studied Lower Aptian deposits were deposited during the transgressive general trend corresponding to the lower part of the sequence (Fig. 2) (Castro, 1998; Ruiz-Ortiz and Castro, 1998). This transgressive trend has been registered worldwide (Haq et al., 1988; Hallam, 1992), and led to the submergence of broad continental areas, and also, in relation to tectonic pulses, to signi¢cant processes of backstepping and drowning of carbonate platforms in the Alpine areas (Vilas et al., 1993; Funk et al., 1993; Castro and Ruiz-Ortiz, 1995).
3. The Cau section The Cau section is located in the NE of Ali-
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Fig. 1. Geographical and geological location of the studied section.
cante Province, SE Spain (Fig. 1). It was ¢rst studied by Gignoux and Fallot (1926, in Darder, 1945), and later by Darder (1945) and R|¤os et al. (1961), focusing mainly on the presence of ammonites. More work has been carried out in the region during recent years, dealing with the lithostratigraphy, biostratigraphy, sedimentology, and sequence stratigraphy (Company et al., 1982; Castro and Ruiz-Ortiz, 1994, 1995; Castro, 1996, 1998; Ruiz-Ortiz and Castro, 1998; Aguado et al., 1999; Castro and Ruiz-Ortiz, 2000; Castro et al., 2001; Garc|¤a-Herna¤ndez et al., 2001). 3.1. Lithostratigraphy The studied section comprises a relatively complete sequence from the Upper Barremian to the Aptian (about 220 m thick; see Fig. 3) comprising rhythmic alternations of marls and marly limestones with ammonites. These deposits belong to the Almadich Formation (Castro, 1998), which, in this outcrop, overlies the hemipelagic marls and marly limestones of the Los Villares Formation (Ruiz-Ortiz, 1980; Aguado et al., 1996) and
underlies the shallow platform limestones of the Seguil|¤ Formation, of Late Aptian to earliest Albian age (Castro, 1998; Ruiz-Ortiz and Castro, 1998). Towards the north, the lower and upper parts of the Almadich Formation shift, by lateral changes of facies, to the shallow platform limestones of the Llopis and Seguil|¤ formations, respectively (Castro, 1998; Ruiz-Ortiz and Castro, 1998). Fig. 2 summarises the lithostratigraphy of the Aptian successions of the Prebetic in Alicante, showing the relationship between the Almadich Formation and the other lithostratigraphic units (after Ruiz-Ortiz and Castro, 1998). The Almadich Formation has been divided into three members. (1) The lower member (55 m thick) is composed of a rhythmic alternation of marly limestones and light-grey marls containing abundant ammonites and nannofossils, and rare to common planktonic foraminifera. Cyclic marls/marly limestones (1 m thick on average) are present, sometimes with iron oxide on their tops. Bed 11c (Fig. 3) shows a hardground at its top, which corresponds to a discontinuity omitting the Barremian^Aptian
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Fig. 2. Synthetic lithostratigraphy of the Upper Barremian^Aptian of the Prebetic of Alicante.
boundary (see below) and marking the basal boundary of the Aptian second-order stratigraphic sequence (Ruiz-Ortiz and Castro, 1998). In the upper part of this member, there is a bed (14.6 in Fig. 3) of 35 cm of black marls with pyrite nodules. This member correlates laterally with a shallow carbonate ramp unit (Llopis formation; Fig. 2), that crops out a few kilometres north of the location of the Cau section (Castro, 1998). (2) The middle member (27 m thick), is mainly composed of black to dark-blue shales and marly sediments, organised in decimetre-thick cycles, with black lutites at the base (TOC content about 0.8%). They are followed by grey, more marly and silty sediments, and the cycles end with a 1^2-cmthick bed of light-grey marlstones and ochre to yellow marls (TOC content about 0.5%), sometimes capped by a slightly reddish surface. In the neighbouring sections located northwards, this member is represented by outer shelf marls and marlstones with large ammonites, accompanied by planktonic and benthonic fauna, and further north are shallow platform carbonates of equivalent age (Vilas et al., 1993). This con¢gura-
tion correlates to a strong retrogradation of the platforms that took place in response to an important transgressive pulse, at the onset of the deposition of this member. (3) The upper member (127 m thick) is characterised by alternating marly limestones and grey marls forming 1.5-m-thick cycles. They contain very scarce ammonites and common to abundant planktonic foraminifera and nannofossils. Some beds of marly limestones of this upper member present a ferruginous top and, in the uppermost part of the unit, the marly limestone layers become slightly sandy. Only the upper part of the upper member has a shallow platform equivalent in adjacent areas (Seguil|¤ Formation; Fig. 2) (Castro, 1998), which was deposited during the ¢nal, regressive part of the second-order Aptian stratigraphic sequence (Ruiz-Ortiz and Castro, 1998) (Fig. 2). The sediments of the Cau section were deposited under hemipelagic conditions, palaeogeographically close to the deposition site of shallow platform carbonates, at a shallow depth of probably only few tens of metres, (Company, 1987;
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Castro, 1998; Ruiz-Ortiz and Castro, 1998). The tectonic context of the depositional area of the selected section corresponded to a rapidly subsiding zone, £anked distally by low subsident areas, which were emergent during the earliest Cretaceous (Sierra Helada and Cabezo¤n de Oro sections, described in Granier, 1987 and Castro, 1998). This tectonic setting was probably related to the tilting of a block, uplifted distally, with a higher subsidence rate at the position of the Cau section. 3.2. Biostratigraphy The integrated biostratigraphic analysis of the studied interval (samples 11^24; Fig. 3), with ammonites, planktonic foraminifera and calcareous nannofossils, has led to the recognition of most of the biostratigraphic units based on these three fossil groups, and to the correlation among them (Fig. 3 ; see Aguado et al., 1999 for more details). Study of the ammonites has led to the identi¢cation of the Martelites sarasini, Deshayesites weissi, Deshayesites deshayesi and Dufrenoyia furcata zones. The study of the nannofossil assemblages, of markedly Tethyan character, led to the identi¢cation of the Micrantholithus hoschulzii, Hayesites irregularis and Rhagodiscus angustus zones. The relative abundance and good preservation of planktonic foraminifera through most of the studied section, except for the basal part, has allowed the identi¢cation of the Blowiella blowi, Schackoina cabri, Globigerinelloides ferreolensis and Globigerinelloides algerianus zones. According to these data, we can state that the studied interval comprises the uppermost Barremian, the Lower Aptian, and the lower part of the Upper Aptian (Aguado et al., 1999). The absence of the Deshayesites tuarkyricus ammonite zone, in addition to the presence of nannofossils such as Nannoconus truittii and Braarudosphaera africana in the lower beds of the Hayesites irregularis zone indicate the existence of a stratigraphic discontinuity a¡ecting the Barremian^Aptian boundary (Aguado et al., 1997, 1999). This discontinuity would comprise, at minimum, the whole D. tuarkyricus and even a part of the Deshayesites weissi zones of ammonites. Be-
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tween beds 13b and 20.4, including the ‘dark shales’ interval (Fig. 3), the ammonite associations are composed exclusively of forms with minor biostratigraphic signi¢cance (juvenile desmoceratids and fragments of heteromorphs or cheloniceratids). The quantitative analysis of the nannofossil abundance has allowed us to identify the ‘nannoconid crisis’ (Erba, 1994; Aguado et al., 1999), which starts in level 13.2 (Fig. 3), clearly preceding the ‘dark shales’ interval, in which, however, the e¡ects of the crisis are still present. Within the ‘dark shales’ interval three parts can be distinguished with respect to the nannoconid abundance: the lower part, about 2 m thick, characterised by a great scarcity ( 6 10%); the intermediate, of about 15 m thick, that records a noticeable recovery (15^30%); and the upper part, about 10 m thick, that is characterised by a new decline in the nannoconid abundance. The same evolution has been observed by Premoli-Silva et al. (1999), from the results obtained in the Cismon log APTICORE (northern Italy), where the ‘dark shale’ interval corresponds to the ‘Selli level’ and reaches only a total thickness of 5 m. The good to moderate degree of preservation of the planktonic foraminifera has encouraged us to use the most recent taxonomic classi¢cation (Banner and Desai, 1988; BouDagher-Fadel et al., 1997), based mainly on shell texture and morphology. Approximately at the Lower^Upper Aptian boundary, within the Globigerinelloides ferreolensis zone, a notable change in the composition of the assemblages is recorded ; they shift from a dominance of taxa belonging to the family Praehedbergellidae to an assemblage mainly composed by Hedbergellidae (Aguado et al., 1999). From level 14.6 (Fig. 3), the earliest trochospiral forms with slightly elongated chambers (Lilliputianella) are observed, but only from level 15.2 onwards the elongation of chambers increases, and planispiral forms with elongate chambers (Claviblowiella) appear. The ¢rst occurrence of Schackoina cabri occurs in sample 15.5, at the basal part of the ‘dark shales’ interval (Fig. 3), correlating with the uppermost part of the Deshayesites weissi zone, or with the lower part of the D. deshayesi zone of ammonites, and with the Hayesites irreg-
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Fig. 3. Detailed Cau section, with isotopic data and integrated biostratigraphy.
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ularis zone of nannofossils (Aguado et al., 1999). In the Cau section we have not found Leupoldina nor Schackoina below the ‘dark shales’ interval (Aguado et al., 1999). Nevertheless, Magniez-Jannin et al. (1997) found rare specimens of Schackoina cabri below the base of the ‘Niveau Goguel’, within the uppermost beds assigned to the D. weissi zone, in the Vocontian Trough. PremoliSilva et al. (1999) and Erba et al. (1999), also refer to the presence of S. cabri (very rare) below the base of the ‘Selli level’ in the Cismon APTICORE, in northern Italy.
4. Methods The section was sampled for isotope analysis with all samples calibrated to the detailed integrated biostratigraphy. Twenty-¢ve samples were taken for laboratory study. All of the samples were analysed twice at least, using for each analysis 1^5 mg. The analysis has been performed in the Stable Isotope Laboratory (SIDI) of the Universidad Auto¤noma of Madrid. For the determination of N13 Ccarb and N18 O, the samples were treated with H3 PO4 (103%), at 90‡C in a VG Isocarb system and the liberated CO2 was measured with a stable isotope ratio mass spectrometer VG Model Prism-II, calibrated with reference to the National Bureau of Standards and Technology (NBS 18 and 19). The results obtained are expressed in the standard PDB (Belemnitella americana of the Pee Dee Formation). The standard deviations for the mass spectrometer used are of S 0.1x for carbon and S 0.2x for oxygen. For the analysis of N13 Corg , the samples were treated previously with HCl (5%) during 24^36 h to completely remove carbonates. After this, samples were treated by combustion within an elemental analyser Carlo Erba CHNS 1108, at 1020‡C and the CO2 obtained from combustion was analysed in continuous £ux (CF-SIRMS) within a spectrometer Micromass model Isochrom, calibrated against SOCROSA ANU, GRAFITO (USGS 24) and PEF1 (reproducibility: S 0.2x). The results are expressed following standard PDB.
213
5. Results The isotopic curve of N13 Ccarb (Fig. 3) ranges between values of 0.3x and 3.74x. The analysis of the curve has been made at two di¡erent scales. At a broader scale, three clearly expressed excursions are shown (one negative anomaly and two positive ones), easily correlatable with the isotope events observed by Moullade et al. (1998) in the Gare de Cassis section (SE France). On the other hand, at a high-resolution scale, a more detailed analysis reveals eight segments, in accordance with the ones observed by Menegatti et al. (1998) in the Swiss and Italian Alps, and also by Erba et al. (1999) in the Italian Alps. The lowest part of the curve is poorly documented, as it is represented only by two analyses, and is capped by the discontinuity at the Barremian^Aptian boundary. It is recorded within the Martelites sarasini ammonite zone, in the Upper Barremian, with a lowest value of 1.45x, and a positive trend. In spite of the scarcity of data, the trend shown, in addition to the accurate dating, suggests that this positive evolution can be probably equivalent to the same trend observed by Moullade et al. (1998) and Masse et al. (1999) on top of the negative excursion they observed in the upper part of the M. sarasini zone. The ¢rst positive excursion, with a maximum value of 2.66x, is recorded within the Weissi zone (see Fig. 3), predating the onset of the ‘nannoconid crisis’. This excursion is correlated with the upper part of the excursion observed in La Be¤doule (Moullade et al., 1998), where it comprises also the Tuarkyricus ammonite zone, absent here because of the Barremian^Aptian boundary discontinuity in our study section. The subsequent negative excursion attains a minimum value of 0.38x, and is recorded within the uppermost part of the Blowiella blowi biozone, the upper boundary of which is also the top of the excursion. Our data indicate that this excursion would lie within the transition beds between Deshayesites weissi and D. deshayesi zones (Fig. 3); however, following Moullade et al. (1998), it would lie within the D. deshayesi ammonite zone. The uppermost positive excursion is the best developed one, with a maximum value of
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3.74x, and a notable thickness of about 50 m. This excursion lies within the Deshayesites deshayesi and Dufrenoyia furcata ammonite zones (Fig. 3). This positive excursion is globally recorded, and is also related to OAE 1a, which is just antecedent or associated with the onset of the excursion (Weissert et al., 1985; Follmi et al., 1994 among others in the Alpine region; Vahrenkamp, 1996 in the Middle East; Jenkyns, 1995 in Paci¢c guyots). When analyzing the isotope curve in more detail, most of the intervals de¢ned by Menegatti et al. (1998) and recognised by Erba et al. (1999), can be documented (Fig. 3). Segment 1 is only partially represented, because of the discontinuity detected by means of ammonites in the Cau section, which comprises the Barremian^Aptian boundary. The N13 Corg curve (Fig. 3) is rather parallel to the N13 Ccarb curve, although the excursions are less clear, because there is in general a lower contrast between the values obtained. A geochemical study of the organic matter (de Gea et al., 2001) has shown an important contribution of terrestrial plants in the organic matter present in the studied sediments. Following Grocke et al. (1999), terrestrial plants record global long-term evolution of N13 C during the Aptian, but the two curves usually do not coincide, and they need not necessarily be in phase. The N18 O curve does not show an analogous evolution to that of N13 C, as the oxygen isotope content in carbonates is more dependent on factors such as diagenesis, and its composition is more variable (Menegatti et al., 1998; Vera, 1994, among others). Nevertheless, the evolution of the N18 O within the studied section show interesting trends. Within the Blowiella blowi biozone, the N18 O values are decreasing by 0.6% (marked as O1 in Fig. 3), and the opposite trend is recorded in the lower part of the Schackoina cabri biozone (indicated as O2 in Fig. 3). Both trends can be correlated with the segments di¡erentiated in the N13 C, C2^C3 and C4^C6, respectively. The Fig. 4. Correlation of the Cau section with selected other sections.
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same trends in the N18 O curve have been recognised by Menegatti et al. (1998) in two di¡erent sections in the Alps, and they also can be observed in other sections referred to by the same authors.
6. Discussion 6.1. Correlation of the isotope curves The correlation of the curve from the Cau section with those of the Roter Sattel section (Switzerland), based on data from Menegatti et al. (1998), of the Cismon core (Italy), studied by Erba et al. (1999), of the historical section of La Be¤doule in southern France, studied by Moullade et al. (1998), and of a core in the Resolution Guyot in the Mid-Paci¢c, published by Jenkyns (1995), is summarised in Fig. 4. Correlation of the various curves of N13 C shows a very good agreement among the di¡erent sections, even at the scale of the segments (C1^C8), ¢rst de¢ned by Menegatti et al. (1998), and recognised in the section we studied (Figs. 3 and 4). The main di¡erences between the Cau curve and the curves from the other sections presented in Fig. 4 are due to two reasons: ¢rstly, the thickness of our section is higher than that of the others (except for the shallow carbonates of the Resolution Guyot), because of a di¡erent palaeogeographic setting, which gave rise to a very high sedimentation rate. Secondly, there is a di¡erence in the biostratigraphic attribution of the ‘dark shales’ from the Cau section and the corresponding black shales of the ‘Selli level’ in Rotel Sattel according to Menegatti et al. (1998). Nevertheless, the biostratigraphic attribution presented in the more recent paper by Erba et al. (1999) (which studies a core close to the section studied by Menegatti et al., 1998), coincides with our own data. This probably indicates new biostratigraphic details from the Cismon area (Erba et al., 1999), that can also probably be applied to the Roter Sattel section (Menegatti et al., 1998). If this is correct, the biostratigraphic and isotopic correlations with the Cau section would be very accurate. The correlation with the historical section of La Be¤doule is
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also very good, with a negative excursion at the top of the Blowiella blowi biozone. The apparent poorer correspondence with the Resolution Guyot curve (from Jenkyns, 1995) can be explained by a low accuracy of the age attribution, because the section is composed of shallow platform limestones (without planktonic fauna), and the published age has been deduced indirectly. Ignoring the biostratigraphic di¡erences, there is a good correlation between this curve and the other referred to, with a strongly negative excursion followed by a notable positive one, clearly corresponding to the C4^C7 segments (Figs. 3 and 4). In this sense, the data presented from the isotope record in this work could be applied to re¢ne the dating of shallow platform sections, in which the pelagic fauna is absent. 6.2. Sedimentary environments The sediments of the Cau section were deposited under hemipelagic conditions, palaeogeographically close to the depositional site of shallow platform carbonates, at a shallow depth of probably only few tens of metres, (Company, 1987; Castro, 1998; Ruiz-Ortiz and Castro, 1998). The tectonic context of the depositional area of the selected section corresponded to a rapidly subsiding zone, £anked distally by low subsident areas, which were emergent during the earliest Cretaceous (Sierra Helada and Cabezo¤n de Oro sections, described in Granier, 1987 and Castro, 1998). This tectonic setting was probably related to the tilting of a block, uplifted distally, with a higher subsidence rate at the position of the Cau section. In this particular palaeogeographic setting, the sedimentary evolution was closely related to that of more proximal environments, and the hemipelagic sedimentation was sensitive to the relative sea-level £uctuations, well documented in the Alicante and adjacent areas of the Prebetic zone (Vilas et al., 1993; Castro and Ruiz-Ortiz, 1995; Castro, 1998; Ruiz-Ortiz and Castro, 1998). Several third-order sequences have been deduced from the Aptian record of the Prebetic (Vilas et al., 1993; Ruiz-Ortiz and Castro, 1998), which in any case represent rapid lateral shifts of the depositional
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environments within the shelf, resulting in drastic vertical and lateral facies changes (Fig. 2). The most relevant event in this evolution was the strong transgressive pulse that led to the drowning of the shallow platform (Llopis Formation) at the onset of the deposition of the middle member of the Almadich Formation. In this sense, several authors have recently pointed out a relationship between rapid rises in sea level, platform drowning and deposition of black shales (Weissert et al., 1998; Grotsch et al., 1998; Davey and Jenkyns, 1999). The dark lutites of this middle member can be correlated with other Lower Aptian black shale deposits from central and northern Italy (‘Selli level’), SE France (‘Niveau Goguel’) (Bre¤he¤ret, 1988) and, probably, from NW Germany and the North Sea area (‘Fischschiefer’) (Mutterlose and Bo«ckel, 1998). Moreover, this member of the Almadich Formation should represent the local record of OAE 1a (Arthur et al., 1990; Larson et al., 1993; Bralower et al., 1994). Most sections from SE France and northern and central Italy showing anoxic facies equivalent to that of the Cau section are barren of planktonic foraminifera, or they are of little signi¢cance (Bre¤he¤ret and Delamette, 1989; Tornaghy et al., 1989; Coccioni et al., 1992), with the exception of the Cismon APTICORE (Erba et al., 1999; Premoli-Silva et al., 1999). In the ‘dark shales’ interval of the Cau section, the planktonic foraminifer assemblages are composed of common to abundant forms with elongated chambers and/or tubulospines (Aguado et al., 1999). The elongation of the chambers in the planktonic foraminifera seems to have been a response to low dissolved oxygen conditions (BouDagher-Fadel et al., 1997; Magniez-Jannin, 1998; Aguado et al., 1999; Premoli-Silva et al., 1999). The di¡erence in thickness between this member and the ‘Selli level’ can be explained by a di¡erent palaeogeographic context, the Cau section corresponding to a distal ramp, with a very high subsidence rate and moderate terrigenous input from the continent. This anomalous high thickness can also account for the lower content in organic matter (about 0.8%), in relation to other coeval sections (with TOC contents about 2^
5%), due to terrigenous dilution. In this sense, we use the term ‘dark shales’ for this facies in the Cau section instead of ‘black shales’, because the last is usually reserved for TOC contents higher than 1% (see, for example, de Graciansky et al., 1987; Vera, 1994). Another interesting peculiarity of the Cau section is related to the fact that the dark shales extend into the positive N13 Ccarb excursion, whereas in other sections the black shales only occur at the beginning of the isotope anomaly. This can be explained within the palaeogeographic context, with a rapid subsident area bounded by shallower sectors. This context could have led to a slight stagnation of the waters, with the permanence of disoxic conditions after the global event.
7. Conclusions The carbon isotope curve of the uppermost Barremian to the base of the Upper Aptian of a hemipelagic succession deposited in the northern margin of the Tethys has been obtained and analysed. The selected succession was deposited on a distal hemipelagic area of a shallow epeiric platform with signi¢cant in£uence both of shallow platform and adjacent continental areas. Moreover, the sedimentation rate of the studied section was abnormally high, due to the palaeogeographic and palaeotectonic setting, leading to a good record of the analysed isotope events. The N13 Ccarb and N13 Corg curves show a clear and distinctive vertical evolution during the latest Barremian^Early Aptian, which permits the detailed interpretation of a high-resolution stratigraphic scale. The N18 O curve is less signi¢cant, but also shows recognisable trends. The studied section is biostratigraphically very well characterised by means of ammonites, planktonic foraminifera and calcareous nannofossils. All the biozones from the uppermost Barremian to the base of the Upper Aptian have been identi¢ed and characterised. A discontinuity a¡ecting the Aptian^Barremian boundary has been detected mainly from the study of the ammonite and calcareous nannofossil record. At a medium scale, two positive excursions and
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a negative one are clearly identi¢ed. At a highresolution scale, eight segments (one below the discontinuity and seven on top of it) can be di¡erentiated. These eight segments have been recognised within a minimum of two biozones (calcareous nannoplankton), and a maximum of four biozones (for ammonites and planktonic foraminifera), indicating the potential of C-isotope stratigraphy as a high-resolution tool, beyond the scope of biostratigraphy. Moreover, this resolution is noteworthy in that the part of the section with a higher isotopic resolution (¢ve segments) is the one poorly characterised by ammonites. Analyses performed at di¡erent scales ¢t fairly well with the previously published data at a global scale, and especially with those curves elaborated at a higher biostratigraphic resolution, mainly from Alpine sections. This good correlation points to a weak in£uence of local factors such as diagenesis or provenance of organic matter. In other words, the correlation potential of the carbon isotopic record is emphasised, and the calibration of the isotopic curve with integrated biostratigraphy is con¢rmed as an extremely useful tool for stratigraphic correlation and dating.
Acknowledgements This study was co-¢nanced by Projects BTE2000-1151 and PB-960429 of DGICYT of the Spanish government and Research Groups RNM200 and 4064 of the Junta de Andaluc|¤a. Sample preparation was completed by A. Carrillo at the Geology Department, EU Polite¤cnica de Linares. Isotopic analysis were performed by Dr. R. Redondo, SIDI, Universidad Auto¤noma de Madrid, and TOC by Dr. J.F. Llamas, Universidad Polite¤cnica de Madrid. Peter Skelton kindly reviewed the English text of an earlier version of this paper which bene¢ted from very interesting comments and suggestions by J.P. Masse and H. Weissert.
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