Rendiconti online Soc. Geol. It., Vol. 2 (2008), 65-70
Carbon-isotope stratigraphy of upper Barremian−lower Albian shallow-water carbonates of the southern Apennines (Italy): highresolution correlation with deep-water reference sections MATTEO DI LUCIA & MARIANO PARENTE (*)
RIASSUNTO Stratigrafia con gli isotopi del Carbonio nei carbonati di mare basso del Barremiano superiore-Albiano inferiore dell’Appennino meridionale: correlazione di alta risoluzione con le sezioni bacinali di riferimento Negli ultimi 20 anni molti lavori hanno dimostrato come la stratigrafia con gli isotopi del carbonio possa essere utilizzata con successo per migliorare la risoluzione stratigrafica in successioni carbonatiche di mare basso e per stabilire precise correlazioni con le successioni bacinali (FERRERI et alii, 1997; GRÖTSCH et alii, 1998; PARENTE et alii, 2007, e riferimenti bibliografici in esso contenuti). In questo lavoro viene presentato uno studio chemostratigrafico condotto su due successioni di piattaforma carbonatica del Cretacico inferiore affioranti in Appennino centro-meridionale. Per ciascuna successione è stata ottenuta una curva del δ13C di alta risoluzione microcampionando preferenzialmente mudstone o matrice micritica di wackestone e floatstone, evitando vene, cavità e grandi bioclasti. Come punto di datazione indipendente, per ancorare le curve del carbonio delle successioni studiate e facilitarne la correlazione con la curva composita di riferimento (FÖLLMI et alii, 2006), è stato utilizzato il livello ad Archaeoalveolina reicheli, datato come Gargasiano medio (FOURCADE & RAOULT, 1973). La correlazione chemostratigrafica ha permesso di identificare nelle successioni studiate le escursioni isotopiche positive del δ13C relative all’Evento Anossico Oceanico 1a (OAE1a) dell’Aptiano inferiore (livello Selli) e all’ OAE1b del limite Aptiano-Albiano. La correlazione di alta risoluzione con la curva di riferimento di FÖLLMI et alii (2006) apre nuove ed interessanti direzioni di ricerca, quali ad esempio la possibilità di correlare i principali eventi biostratigrafici delle successioni di mare basso con le scale standard ad ammoniti ed a foraminiferi planctonici ed, attraverso queste, con la scala cronostratigrafica.
Key words: carbon isotope stratigraphy, shallow-water carbonates, lower Cretaceous, OAE1, southern Apennines.
know on these events was derived from the study of geochemical proxies and biotic change in deep-water sequences. Comparatively much less is known from shallowwater carbonate systems. The full exploitation of the information contained in these archives is often hampered by the diagenetic processes, the low biostratigraphic resolution and the lack of well-defined correlations with biochronological standard schemes based on ammonites and calcareous plankton and nannoplankton. Secular changes in the δ13C composition of sea-water have been well documented in Lower Cretaceous pelagic and hemipelagic sequences from different localities (HERRLE et alii, 2004, and references therein) and during the last 20 years C-isotope stratigraphy has been increasingly used to correlate shallow-water carbonate with deep-water sequences. This tool has been recently applied also to the southern Apennines carbonate platform (D’ARGENIO et alii, 2004; PARENTE et alii, 2007). The successful application of the δ13C record as a stratigraphic tool in shallow-water carbonate sections requires that the degree of preservation/alteration of the original isotopic signal is correctly evaluated and that independent tie points are established to facilitate correlation with the reference curve. In this study we present high-resolution δ13C curves of two Barremian-Albian shallow-water carbonate sections of the central-southern Apennines (Italy) (fig. 1): Monte Croce (north of Itri, Lazio) and Monte Motola (south of Salerno, Campania).
GEOLOGICAL SETTINGS INTRODUCTION The middle Cretaceous was a period of repeated perturbations of the global carbon cycle. High atmospheric pCO2 has been invoked as the main cause of middle Cretaceous super greenhouse, Oceanic Anoxic Events (OAEs) and biocalcification crises recorded by calcareous nannoplankton (WEISSERT & ERBA, 2004). Most of what we _________________________ (*) Dipartimento di Scienze della Terra, Università di Napoli “Federico II” Largo San Marcellino,10 – 80138, Napoli (Italy). Corresponding author:
[email protected]
The thick carbonate successions that are widely exposed in the southern Apennines were deposited during the Mesozoic in shallow-water areas on the African margin of the Ligurian– Piedmont Ocean. During the Neogene, thrusting and folding affected the Mesozoic passive margin sedimentary cover, producing a complex imbricate of tectonic units with duplex structures, dismembered by low-angle and high-angle normal faults (BUTLER et alii, 2004). Palaeogeographical reconstructions for the Mesozoic units of the southern Apennines show the occurrence of a more or less complex pattern of carbonate platforms separated by deep basins (D’ARGENIO et alii, 1973; MOSTARDINI, et alii, 1986; SGROSSO, 1988). The Upper Triassic to Lower Cretaceous shallow-water carbonates of the southern Apennines are
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RESULTS
Fig. 1 – Location of the studied sections.
generally referred to flat-topped, tropical carbonate platforms dominated by chloralgal or chlorozoan associations (D’ARGENIO, 1974). The Monte Croce section is located in the Aurunci mountains, in the southern part of the Latium-Abruzzi sector of the Apenninic carbonate platform. The Monte Motola section is part of the Campania-Lucania sector of the Apenninic carbonate platform and can be referred to the Alburno-Cervati Unit.
MATERIALS AND METHODS The two studied sections have been logged at centimetric to decimetric scale and sampled at decimetric to metric scale, depending on the quality of the exposure. Field observations have been integrated with the microfacies analysis of about 500 thin sections in order to individuate the lithofacies associations, the diagenetic features and the main biostratigraphic events. In the attempt to produce reliable highresolution δ13C curves we decided to sample, as a first choice, micrite, occurring either as lime mudstone or as the matrix of bioclastic facies (foraminiferal wackestones and bioclastic floatstone). Several studies have demonstrated that, although this material can be partly the product of diagenesis (DICKSON & COLEMAN, 1980), under favourable conditions it can preserve if not the absolute value at least the major trends and excursions of the pristine marine signal (JOACHIMSKY, 1994; IMMENHAUSER et alii, 2002). About 5 mg of powder were obtained for each sample by microdrilling a polished slab under the binocular microscope, taking care to avoid cement filled veins and cavities and larger bioclasts. A 0.8 mm Ø bit was used for lime mudstones whereas a 0.5 mm Ø bit was employed for the micritic matrix of bioclastic facies. Stable isotope analyses were performed at the Isotopen-labor of the Institut für Geologie, Mineralogie und Geophysik at the Ruhr University (Bochum) and at the laboratory of the “Istituto per l’ Ambiente Marino Costiero”, Geomare, National Research Council, Napoli. The results are reported as δ values with reference to the PDB standard. In order to facilitate chemostratigraphic correlation and to smooth high frequency fluctuations, the δ13C curves of the studied sections were built by applying a five points moving average to the original data.
The Monte Croce section is 241 m thick. It consists of well -bedded shallow-water carbonates with a few thin levels of marls and marly limestones. 250 samples have been analyzed from this section, with an average spacing of about 1 m. Five chemostratigraphic intervals have been identified on the basis of the main isotopic trends and peaks, and correlated with the reference composite section of FÖLLMI et alii (2006) (fig. 2). Interval A (8.4–55 m) shows a marked rising trend from -1‰ to about 2‰ followed to a more gradual decrease to a minimum of about 1‰. Interval B (55–76.7 m) is characterized by a sharp positive excursion with δ13C values reaching a maximum at about 3‰ and then a rapidly returning to the pre-excursion value of about 1‰. Interval C (76.7–121.9 m) is marked by another positive excursion reaching a maximum of 3.8‰, followed to a return to pre-excursion values (1.2‰). Interval D (121.9–174.3 m) starts with a rising trend (121.9−134.5 m) peaking at about 2‰, followed by a gradual decrease with superimposed minor fluctuations (134.5−159.4 m) and by a more sharp decrease reaching a minimum of about 0‰. Interval E (174.3−241 m) starts with a very sharp positive shift of about 2‰, followed by a general decreasing trend with superimposed high-frequency fluctuations of up to 1‰. The Monte Motola section is 205 m thick. From this section 165 samples have been analyzed, with an average spacing of 1.2 m. Four chemostratigraphic intervals have been identified on the basis of the main isotopic trends and peaks, and correlated with the Monte Croce section and with the reference composite section of FÖLLMI et alii (2006) (fig. 2). Interval A (21.6–85.3 m) starts with a general rising trend, reaching a peak at about 2‰. This positive trend is interrupted by two minor negative excursions. The upper part of this interval is characterized by a gradual decrease to a minimum of about 1‰. Interval B starts with a small positive shift followed by a more pronounced decrease to a minimum of about 0.6‰. Interval C (98–137.7 m) is characterized by a prominent positive excursion with a rise to maximum of about 3‰ and a symmetrical decrease to pre-excursion values. Interval D (137.7–205 m) starts with a positive shift of about 0.8‰ followed by a very gradual decrease with superimposed minor fluctuations. The interval is closed by a more sharp decrease, reaching values as low as -0.5‰ at the end of the section.
DISCUSSION The original stable-isotope signature of marine carbonates can be strongly modified by interaction with diagenetic fluids (BRAND & VEIZER, 1981; MARSHALL, 1992). Therefore before using shallow-water carbon-isotope curves as a dating and correlation tool, their stratigraphic reliability must be carefully evaluated.The studied sections show generally a repetition of subtidal to peritidal shallowing-upward cycles.
CARBON-ISOTOPE STRATIGRAPHY OF LOWER CRETACEOUS SHALLOW-WATER CARBONATES (SOUTHERN APENNINES)
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Fig. 2 – Carbon-isotope stratigraphy of the Monte Croce and Monte Motola sections and correlation with the composite reference curve of FÖLLMI et alii (2006). The δ13C curve of the studied sections have been obtained by smoothing the original data with a five points moving average. Five main chemostratigraphic intervals (A−E) have been identified and correlated with the reference curve. Dotted lines represents chemostratigraphic correlation of minor events between the Monte Croce and the Monte Motola sections. These correlations have been further constrained by lithostratigraphic and biostratigraphic correlation.
The Monte Croce section is characterized by more open marine facies, with rare evidence of meteoric diagenesis and\or subaerial exposure surfaces. At Monte Motola, where more restricted facies occur, evidence of subaerial exposure is more frequent. However a close inspection of the δ13C curves showed that the most prominent shifts are independent of facies variation and cannotbe interpreted in terms of facies and diagenesis. This would suggest that at least the major carbonisotope excursions record the global marine signal. In the attempt to anchor the δ13C curves of the studied sections to the chronostratigraphic scale and to facilitate their correlation with the composite reference curve of FÖLLMI et alii (2006), we used the Archaeoalveolina reicheli level (fig. 3), dated as middle Gargasian by FOURCADE & RAOULT (1973). Carbon
isotope stratigraphy indicates that both at Monte Croce and at Monte Motola this marker occurs a few meters below the end of the very prominent positive excursion of the interval C. Therefore we correlate this excursion with the positive excursion related to the OAE1a Selli event. This allows to tie our δ13C curves to the composite reference curve of FÖLLMI et alii (2006) and to establish the chemostratigraphic correlation shown in figure 2. In particular we suggest that the positive excursion of interval B at Monte Croce corresponds to the positive excursion preceding the Selli event in the reference section and that the sharp positive shift at the beginning of interval E correlates with the positive shift marking the onset of OAE1b. Our chemostratigraphic correlation of figure 2 shows that
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Fig. 3 – Archaeoalveolina reicheli.
Fig. 4 – Plot of average sedimentation rate for the Monte Croce and Monte Motola sections. Duration of intervals A-D is constrained by correlation to the composite reference curve of FÖLLMI et alii (2006).
the Monte Motola carbon isotope curve misses the prominent positive excursion preceding the Selli event. This is due to the presence of a stratigraphic gap, confirmed by three lines of evidence: 1) a prominent subaerial exposure surface, with microkarstic cavities filled by greenish marls, occurs in this interval; 2) the “Orbitolina level”, a well known lithostratigraphic marker of central-southern Apennines, occurs at the very top of interval B at Monte Croce but is lacking at Monte Motola; 3) a plot of accumulation rate shows that interval B at Monte Motola has the lowest slope while at Monte Croce there is no significant difference between interval B and C (fig. 4). Assuming constant sedimentation rate across these two intervals also for Monte Motola allows estimating a maximum duration of about 800 ky for the stratigraphic gap.
correlation with time-equivalent deep-water sections will allow looking in detail at the response of the Apenninic carbonate platforms to middle Cretaceous anoxic events and biocalcification crises.
CONCLUSION
CARANNANTE G., CHERCHI A. & SIMONE L. (1995) Chlorozoan versus foramol lithofacies in Late Cretaceous rudist limestones. Palaeogeogr., Palaeoclimat., Palaeoecol., 119, 137–154.
The carbon-isotope curves of two sections of shallow-water carbonates from the upper Barremian–lower Albian of the central-southern Apennines show some prominent positive excursions that are believed to record the global paleoceanographic signal of marine water. The middle Gargasian A. reicheli level has been used as an independent tie-point to anchor the shallow-water curves and to facilitate their correlation with the reference curve of FÖLLMI et alii (2006). The positive excursion of the OAE1a Selli event is clearly recorded in both the studied sections and the positive shift at the onset of OAE1b is a prominent feature of the Monte Croce carbon isotope curve. Integration of carbon-isotope stratigraphy and biostratigraphy results in a dramatic increase of stratigraphic resolution and correlation ability in the studied shallow-water carbonate sections. This opens up some very promising research avenues. Biostratigraphic events of carbonate platforms can now be correlated to ammonite zones and assigned precise chronostratigraphic age. Precise
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