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Introduction to thematic issue, “Spatial patterns of change in Aptian carbonate platforms and related events”
Cretaceous Research 39 (2013) 1–5
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Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes
Editorial
Introduction to thematic issue, “Spatial patterns of change in Aptian carbonate platforms and related events”
The Aptian Stage serves as a microcosm of the Cretaceous world, showing evidence for all the latter’s distinctive features, such as massive plume-related volcanism, strong climatic fluctuations within a broadly ‘greenhouse’ regime, and major perturbations of the global carbon cycle involving both oceanic anoxic events (most notably, OAE 1a) and crises of biocalcification, including the episodic demise of vast carbonate platforms (e.g., Ando et al., 2008; Blättler et al., 2011; Dumitrescu et al., 2006; Erba et al., 2010; Keller et al., 2011; Kuhnt et al., 2011; Méhay et al., 2009; Tejada et al., 2009; Skelton and Gili, 2012, from an extensive literature). Causal linkages between these various phenomena seem likely, but identifying exactly how they interacted requires (at least) precise documentation of the relative temporal and spatial patterns of the physical and biotic events involved. Pioneering studies in the last century initially held out the hope of a relatively simple scenario for the suppression of biocalcification based on excess nutrient flux (e.g., Hallock and Schlager, 1986; Schlager and Philip, 1990; Föllmi et al., 1994; Weissert et al., 1998). Subsequent detailed chemo-, and biostratigraphical analyses of Aptian successions over the last decade and a half, however, have confounded that earlier optimism, suggesting that the ‘Aptian story’ was more complicated than originally supposed. The following issues, over which consensus has proved elusive, especially merit further critical attention: (1) Correlation. Problems of biostratigraphical correlation between different basin and platform successions, reliant upon an assortment of different, non-ubiquitous taxa (e.g., planktonic foraminifers, calcareous nanofossils, ammonites, benthic foraminifers, especially orbitolinids, calcareous algae and rudist bivalves) have led to disagreements over the detailed chronostratigraphy of geochemical changes, particularly the carbon isotopic signature for OAE 1a (see Moreno-Bedmar et al., 2009, Fig. 1; Moullade et al., 2011a, Fig. 1). (2) Spatial variation of effects. Chemo-, and biostratigraphical studies around the northwestern (Franco-Swiss) Tethyan margin dominated the narrative in the earlier literature on the demise of carbonate platforms in the Early Aptian. But in order to assess any possible modulating effects of climate/ocean interactions, a more palaeogeographically inclusive survey of regional variations in the development of carbonate platform and pelagic facies, and associated biotic turnover, is necessary (Skelton and Gili, 2012). (3) Climatic and eustatic fluctuations. The amplitudes of climatic fluctuations assessed from different proxies, including postulated short-term ‘cold snaps’, are currently debated (see, for example, Jenkyns et al., 2012), while Masse and Fenerci-Masse (2011) have 0195-6671/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cretres.2012.09.001
proposed a stepped series of platform ‘drownings’ in the Early Aptian, rather than a single main event linked with OAE 1a. (4) Causal factors. Given the continuing lack of agreement on so many aspects of the Aptian record (as in 1–3, above), it is not surprising that a consensus is still lacking on the relative importance of various possible influences on the biota such as nutrient flux, seawater acidification and temperature in bringing about the changes seen in the marine geological record (cf., for example, Föllmi, 2012, and Skelton and Gili, 2012). These issues were variously addressed at the 4th French Congress on Stratigraphy held in Paris, France, in September, 2010 (‘STRATI2010’), especially in a thematic session that focused on the second issue mentioned above (Session 8: ‘Spatial patterns of changes in the Aptian’). Nine of the eleven papers in this volume are based on presentations that were given at various sessions in that conference and the remaining two (those by Graziano and by Masse and Fenerci-Masse) were subsequently submitted as additional contributions relevant to the theme. The papers herein are organized according to palaeogeography (Fig. 1), both in keeping with the main emphasis of Session 8 at STRATI2010 and also because several of them touch on more than one, if not all of the issues listed above, rendering an alternative, purely thematic ordering impractical. Our tour of the Aptian record thus begins with two studies from the traditional French heartland of Aptian stratigraphy, Provence (Moullade et al., 1998, 2009, 2011b). Lorenzen et al., (Fig. 1, #1) report on the geochemistry and foraminiferal biostratigraphy of three new drill cores from the historical Lower Aptian stratotype at Roquefort-La Bédoule. In agreement with previous outcrop studies in the area, they demonstrate an expanded record of the negative d13C excursion (C3 stage of Menegatti et al., 1998) that heralds OAE 1a extending up to the blowi/cabri foraminiferal zonal boundary situated within the Upper Bedoulian deshayesi ammonite zone, and calculate a duration for it of >200 kyr from estimated sedimentation rates, and >300 kyr for the subsequent main positive (C4) isotopic shift. A little further north, Masse and FenerciMasse (Fig. 1, #2) contrast the rudist-bearing limestones at Orgon – the type area for d’Orbigny’s ‘Urgonian Stage’, now defunct as a stratigraphical term, but still widely deployed in reference to such facies – with those in the Monts de Vaucluse–Apt region. Whereas the former belong to the uppermost Barremian, the latter are mostly confined to the Lower Aptian, though with an uppermost Barremian (‘U1’) equivalent of the Orgon limestones preserved in the western part of the Monts de Vaucluse, beneath a Palorbitolina lenticularis–Heteraster oblongus (‘Pa 1’) guide level.
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Editorial / Cretaceous Research 39 (2013) 1–5
Fig. 1. Tethyan palaeogeography for the Early Aptian. Numbered stars refer to studies in this volume: 1, Lorenzen et al.; 2, Masse and Fenerci-Masse; 3, Clavel et al.; 4, Ivanov and Idakieva; 5, Cherchi and Schroeder; 6, Graziano; 7, Ruberti et al.; 8, Elkhazri et al.; 9, Peybernes et al.; 10, Granier and Busnardo; 11, Wilmsen et al. Map generated from ODSN Plate Tectonic Reconstruction Service (Hay et al., 1999).
The succeeding caprinid rudist-rich (‘U2’) Urgonian limestones are in turn terminated by a mid-Bedoulian discontinuity, overlain by a second (‘Pa 2’) Palorbitolina horizon, followed by bioclastic or coral (‘U3’) limestones, then marls with marly limestones. The demise of the rudist-rich U2 Urgonian limestones in this region thus pre-dated the d13C signature for OAE 1a as recognized at La Bédoule (see also Moullade et al., 1998; Masse and Fenerci-Masse, 2011). The third paper, by Clavel et al., extends the survey of platform stratigraphy northwards around the margins of the Vocontian Basin to the Swiss Jura (Fig. 1, #3). With a series of detailed palaeogeographical maps of successive late highstand facies distributions, dated by ammonites together with a wide range of other biota, these authors illustrate a step-wise centripetal progradation of carbonate platforms into the basin from the early Late Hauterivian. Platform development became most restricted in early Late Barremian times, coinciding with a marked turnover of orbitolinids. Subsequent transgressive re-establishment of the platform was then terminated in the Early Aptian, in late weissi Zone times, similarly to the U2 rudist limestones of the Monts de Vaucluse described in the previous paper – though in this case the discontinuity was directly overlain by hemipelagic facies. It is hypothesized that this striking change in sedimentation may reflect an increased connection with cooler northern waters accompanying the relative rise in sea-level. The next paper, by Ivanov and Idakieva takes us eastwards along the northern Tethyan margin to the uppermost Barremian to Lower Aptian succession in Bulgaria (Fig. 1, #4), focussing upon its ammonite biostratigraphy. Here they recognize, inter alia, a Roloboceras hambrovi Subzone in the upper part of the Lower Bedoulian Deshayesites forbesi Zone (replacing the former
Paradeshayesites weissi Zone; Reboulet et al., 2011), in agreement with Moreno-Bedmar et al. (2009) working in the Maestrat Basin of eastern Spain, though in contrast to its placement in the Upper Bedoulian Deshayesites deshayesi Zone at La Bédoule (Ropolo et al., 1998, 2008) (Fig. 2). The reliability of Roloboceras as a chronostratigraphical index thus remains open to question, as previously suggested by Ropolo et al. (2008). Interestingly, in this context, Ivanov and Idakieva also attribute an interval of thinly laminated, organic-rich clays in their (Lower Bedoulian) R. hambrovi Subzone to OAE 1a. The paper by Cherchi and Schroeder shifts attention to the central and southern Tethyan carbonate platforms (Fig. 1, #5), focussing upon orbitolinid biostratigraphy. Their recognition of a Praeorbitolina/Palorbitolinoides association characterizing the uppermost part of the Bedoulian in the Apulian, Adriatic and eastern Arabian platforms provides a valuable tool for refining the dating of such platforms. Next, Graziano synthesizes the Lower Aptian record of the Apulian carbonate platform margin from detailed studies of local exposures in the Gargano Promontory of SE Italy (Fig. 1, #6). Here, caprinid rudist-rich platform margin facies are shown to extend up into the lowermost Upper Bedoulian, where they are succeeded by a thin ‘crisis interval’, characterized by a distinctive brachiopod/microbial association attributed to a ‘hothouse’ regime and marking the inception of drowning of the platform margin. Physical correlation with isotopically calibrated pelagic successions of the Ionian Basin links this crisis and subsequent drowning interval with the negative d13C excursion heralding OAE 1a. The crisis also marks the eventual replacement here of an aragonite-dominated, ‘Urgonian-type’ platform association of caprinid rudists, corals and green algae by one dominated by
Editorial / Cretaceous Research 39 (2013) 1–5 Fig. 2. Two recent examples of divergent proposals for latest Barremian to earliest Albian chronostratigraphic subdivision and ammonite zonation: left, that from the ‘Kilian Group’ (Reboulet et al., 2011); right, that from Moullade et al. (2011b).
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more calcite-rich benthos such as chondrodontid bivalves, chaetetid sponges, bryozoans, echinoids and orbitolinids. A similar change in biota is recorded from the southern Apennine platform by Ruberti et al., (Fig. 1, #7), though at a higher stratigraphical level – around the Bedoulian/Gargasian boundary – and in a somewhat more internal setting affected by a phase of incipient tectonic restructuring. In this case the biotic changeover is mediated by an interval of thin paleosol-capped metric cycles containing laminites and oncoids with an impoverished biota that includes orbitolinids and small cerithiids, shortly followed by a horizon attributed to the Lower Gargasian Salpingoporella dinarica Abundance Zone. Again, the subsequent Upper Aptian to Albian platform benthos is dominated by calcite-rich rudists, including radiolitids, together with large chondrodontids. The next two papers concern North African successions. Elkhazri et al., report on a geochemical (including carbonisotopic) and microfaunal study of a section in northeastern Tunisia (Fig. 1, #8) in which they have identified the signature of OAE 1a in a 25 m interval of pelagic black limestones and marly limestones. They report the first appearance, followed by expansion to an acme of Schakoina cabri, accompanied by a radiolarian bloom, within the latter interval, which they ascribe to the C4–C7 stages of Menegatti et al. (1998), in broad agreement with the findings at La Bédoule reported in the first paper of the present volume. Further west, Peybernes et al., have studied a deepening-upward ramp succession of somewhat younger age (latest Early Aptian to Early Albian) in the Essaouira-Agadir Basin of southern Morocco (Fig. 1, #9). Here, a decrease both in calcium carbonate content and in Nannoconus abundance are noted at the Aptian–Albian transition, attributed either to climatic cooling or to an associated increased input of terrigenous sediments and nutrients. In one section (Tamzergout), an 8 m interval of dark marl in the Lower Albian that shows a marked decrease in calcium carbonate content and nanofossil productivity is correlated with OAE 1b. However, nanofossil fluxes here are considerably lower than those recorded in the Vocontian Basin of southeastern France, because, the authors suggest, of more arid conditions prevailing along the southern Tethyan margin. The last two papers focus on the Middle East. Combining bio-, litho-, and sequence stratigraphy with well log data and d13C-based chemostratigraphy, Granier and Busnardo review the Upper Barremian to Aptian stratigraphy of the eastern Arabian craton, using a reference core from offshore Abu Dhabi that covers the entire stratigraphical interval (Fig. 1, #10). They conclude that the Barremian/Aptian boundary falls within the third of four third-order transgressive–regressive cycles making up the Kharaib Formation. The succeeding Hawar Formation constitutes a single cycle, overlying a locally karstified surface regarded as time-equivalent to that terminating the Franco-Swiss Urgonian platform. OAE 1a most likely coincided with the forced regression that terminated the Hawar cycle, with a Bacinella proliferation occurring yet later, during the transgressive phase of the succeeding, single Shu’aiba cycle. Ammonites in condensed basinal facies at the top of the latter cycle show it to extend into the Deshayesites furcata Zone (¼ basal Gargasian instead of topmost Bedoulian in the convention followed by these authors; see Moullade et al., 2011b and Fig. 2 herein). The base of the succeeding Bab cycle is considered to correspond to that of the Kazhdumi intra-shelf deposits of the Zagros Mountains in western Iran. The final paper takes us to Central Iran (Fig. 1, #11), where Wilmsen et al., have also studied orbitolinid-, and rudistrich platform limestones of latest Barremian to Early Aptian age (Shah Kuh Formation), overlain by basinal deposits of the Bazyab Formation of (ammonite-dated) Late Aptian to Albian age. The Shah Kuh apparently developed from a narrow, high energy shelf to a broad, flat-topped rudist platform. Along with other such
regionally important ‘Orbitolina limestones’, it forms part of a widespread transgressive depositional megacycle. Exact dating of the demise of this Early Aptian platform remains to be established, though its rudist fauna bears some interesting resemblances to that of the Shu’aiba Formation in eastern Arabia, despite their wide palaeogeographical separation either side of the Neo-Tethys Ocean (Fig. 1, #10 and 11). So what conclusions can be drawn from this collection of papers in relation to the four interrelated issues cited at the start of this Introduction? With respect to correlation, divergences of opinion evidently persist over the exact dating of OAE 1a (cf., Lorenzen et al., and Ivanov and Idakieva, for example). But as Elkhazri et al., correctly observe, ‘In fact, for the moment no method demonstrates its isochroneity with certainty’. As a speculative alternative to this stratigraphical impasse, the (for some, heretical!) question could indeed be posed as to whether the iconic isotopic signals really were globally synchronous, as widely assumed; or could they have been regionally modulated by the isotopic feedback effects of independently occurring changes in platform carbonate production, along the lines proposed by Swart (2008)? Likewise, one might envisage distinct, asynchronous ‘Roloboceras bio-events’ possibly related to transient local OAE-related conditions. Hence neither carbon isotopic chemostratigraphy nor biostratigraphy alone can be considered infallible as chronostratigraphical tools without critical reference to each other as well as the implied context of relative sea-level change, as demonstrated in a recent critical study by Raspini (2012). Moreover, from both stratigraphical and palaeogeographical considerations, it is surely even clearer than it was before that the demise of Early Aptian Tethyan carbonate platforms cannot be interpreted as a single, OAE 1arelated event. While the termination of the northern, FrancoSwiss rudist-dominated platform facies can be dated to the midBedoulian (Masse and Fenerci-Masse; Clavel et al.), that of the Gargano margin seems to have followed somewhat later, in the early part of the Late Bedoulian (Graziano), while both the southern Apennine, and eastern Arabian (Shu’aiba) platforms persisted through the Late Bedoulian (Ruberti et al.; Cherchi and Schroeder; Granier and Busnardo). Nevertheless, the analysis of the Arabian depositional cycles by Granier and Busnardo suggests that the emersion of the earlier Kharaib platform was synchronous with that of the Franco-Swiss Urgonian platform, some time before the OAE 1a and microbial Bacinella proliferation events. The crucial difference is that the central and southern Tethyan platforms largely recovered in the Late Bedoulian, in contrast to the northern platforms (with the exception of those on the Iberian promontory), as previously stressed by Skelton and Gili (2012). It will be interesting to see how the Shah Kuh platform of Wilmsen et al., eventually compares with this pattern, given its palaeogeographical proximity to the northern margin, though in an even more southerly palaeolatitude than the Iberian promontory (Fig. 1). Unsurprisingly, the relative influences on such a complex history remain unresolved. Some authors emphasize variations in nutrient fluxes related to arid versus humid conditions to explain observed biotic contrasts (e.g., Peybernes et al.), while others additionally contemplate a role for CO2-driven changes in seawater chemistry and temperature, particularly noting apparent effects on the balance of aragonite and calcite in the platform biota (Graziano; Ruberti et al.). Also remarked upon is the influence of changing accommodation space on the shallow tops of platforms, whether in response to tectonic activity (Ruberti et al.) or to presumed eustatic fluctuations (Clavel et al.). Of course, these possible factors are not mutually exclusive, and they could, and probably did all have a part to play. So, despite some progress on revealing the stratigraphical complexities of the Aptian world, it seems that we still have some way to go to understand its workings.
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References Ando, A., Kaiho, K., Kawahata, H., Kakegawa, T., 2008. Timing and magnitude of early Aptian extreme warming: unraveling primary d18O variation in indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central Pacific Ocean. Palaeogeography, Palaeoclimatology, Palaeoecology 260, 463–476. Blättler, C.L., Jenkyns, H.C., Reynard, L.M., Henderson, G.M., 2011. Significant increases in global weathering during Oceanic Anoxic Events 1a and 2 indicated by calcium isotopes. Earth and Planetary Science Letters 309, 77–88. Dumitrescu, M., Brassell, S.C., Schouten, S., Hopmans, E.C., Sinninghe Damsté, J.S., 2006. Instability in tropical Pacific sea-surface temperatures during the early Aptian. Geology 34, 833–836. Erba, E., Bottini, C., Weissert, H.J., Keller, C.E., 2010. Calcareous nannoplankton response to surface-water acidification around Oceanic Anoxic Event 1a. Science 329, 428–432. Föllmi, K.B., 2012. Early Cretaceous life, climate and anoxia. Cretaceous Research 35, 230–257. Föllmi, K.B., Weissert, H., Bisping, M., Funk, H., 1994. Phosphogenesis, carbon-isotope stratigraphy, and carbonate-platform evolution along the Lower Cretaceous northern Tethyan margin. Geological Society of America Bulletin 106, 729–746. Hallock, P., Schlager, W., 1986. Nutrient excess and the demise of coral reefs and carbonate platforms. Palaios 1, 389–398. Hay, W.W., DeConto, R., Wold, C.N., Wilson, K.M., Voigt, S., Schulz, M., WoldRossby, A., Dullo, W.-C., Ronov, A.B., Balukhovsky, A.N., Soeding, E., 1999. Alternative Global Cretaceous paleogeography. In: Barrera, E., Johnson, C. (Eds.), The Evolution of Cretaceous Ocean/Climate Systems. Geological Society of America Special Paper, 332, pp. 1–47. http://www.odsn.de/odsn/services/paleomap/ paleomap.html (accessed 12.09.12.). Jenkyns, H.C., Schouten-Huibers, L., Schouten, S., Sinninghe Damsté, J.S., 2012. Warm Middle Jurassic–Early Cretaceous high-latitude sea-surface temperatures from the Southern Ocean. Climates of the Past 8, 215–226. http://dx.doi.org/10. 5194/cp-8-215-2012. Keller, C.E., Hochuli, P.A., Weissert, H., Bernasconi, S.M., Giorgioni, M., Garcia, T.I., 2011. A volcanically induced climate warming and floral change preceded the onset of OAE1a (Early Cretaceous). Palaeogeography, Palaeoclimatology, Palaeoecology 305, 43–49. Kuhnt, W., Holbourn, A., Moullade, M., 2011. Transient global cooling at the onset of early Aptian oceanic anoxic event (OAE) 1a. Geology 39, 323–326. Masse, J.-P., Fenerci-Masse, M., 2011. Drowning discontinuities and stratigraphic correlation in platform carbonates. The late Barremian–early Aptian record of southeast France. Cretaceous Research 32, 659–684. Méhay, S., Keller, C.E., Bernasconi, S.M., Weissert, H., Erba, E., Botín, C., Hochuli, P.A., 2009. A volcanic CO2 pulse triggered the Cretaceous Oceanic Anoxic Event 1a and a biocalcification crisis. Geology 37, 819–822. Menegatti, A.P., Weissert, H., Brown, R.S., Tyson, R.V., Farrimond, P., 1998. High-resolution d13C stratigraphy through the early Aptian “Livello Selli” of the Alpine Tethys. Paleoceanography 13, 530–545. Moreno-Bedmar, J.A., Company, M., Bover-Arnal, T., Salas, R., Delanoy, G., Martínez, R., Grauges, A., 2009. Biostratigraphic characterization by means of ammonoids of the lower Aptian Oceanic Anoxic Event (OAE 1a) in the eastern Iberian Chain (Maestrat Basin, eastern Spain). Cretaceous Research 30, 864–872. Moullade, M., Kuhnt, W., Bergen, J.A., Masse, J.-P., Tronchetti, G., 1998. Correlation of biostratigraphic and stable isotope events in the Aptian historical stratotype of La Bédoule (southeast France). In: Comptes Rendus de l’Académie des Sciences, Paris, Sciences de la Terre et des Planètes, série IIa, 327. 693–698. Moullade, M., Tronchetti, G., Babinot, J.-F., 2009. Le Gargasien de Gargas (Vaucluse, SE de la France): synthèse des données de terrain et révision de la microfaune de foraminifères et d’ostracodes. Carnets de Géologie/Notebooks on Geology. Brest - Article 2009/10, 1–15. http://paleopolis.rediris.es/cg/CG2009_A10/ CG2009_A10.pdf (accessed 12.09.12.). Moullade, M., Kuhnt, W., Tronchetti, G., Ropolo, P., Granier, B., 2011a. OAE1a: late Early or early Late Bedoulian event? In: Grosheny, D., Granier, B., Sander, N. (Eds.), Platform to basin correlations in Cretaceous times Abstracts. Boletín del Instituto de Fisiografía y Geología 79–81, pp. 16–17. http://www.fceia.unr.edu.ar/fisiografia/ volumen79-81/BIFG_79-81_Grosheny.pdf (accessed 12.09.12.).
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Moullade, M., Granier, B., Tronchetti, G., 2011b. The Aptian Stage: back to fundamentals. Episodes 34, 148–156. Raspini, A., 2012. Shallow water carbonate platforms (Late Aptian–Early Albian, Southern Apennines) in the context of supraregional to global changes: reappraisal of palaeoecological events as reflectors of carbonate factory response. Solid Earth 3, 225–249. http://dx.doi.org/10.5194/se-3-225-2012. Reboulet, S., 22 others, 2011. Report on the 4th International Meeting of the IUGS Lower Cretaceous Ammonite Working Group, the “Kilian Group” (Dijon, France, 30th August 2010). Cretaceous Research 32, 786–793. Ropolo, P., Conte, G., Gonnet, R., Masse, J.-P., Moullade, M., 1998. Les faunes d’Ammonites du Barrémien supérieur/Aptien inférieur (Bédoulien) dans la région stratotypique de Cassis-La Bédoule (SE France): état des connaissances et propositions pour une zonation par Ammonites du Bédoulien-type. Géologie Méditerranéenne 3–4, 167–175. Ropolo, P., Moullade, M., Conte, G., Tronchetti, G., 2008. About the stratigraphic position of the Lower Aptian Roloboceras hambrovi (Ammonoidea) level. Carnets de Géologie/Notebooks on Geology, Brest, Letter 2008/03, 1–7. http:// paleopolis.rediris.es/cg/CG2008_L03/CG2008_L03.pdf (accessed 12.09.12.). Schlager, W., Philip, J., 1990. Cretaceous carbonate platforms. In: Ginsburg, R.N., Beaudoin, B. (Eds.), Cretaceous Resources, Events and Rhythms – Background and Plans for Research. NATO ASI Series, Series C: Mathematical and Physical Sciences 304. Kluwer Academic Publishers, Dordrecht, pp. 173–195. Skelton, P.W., Gili, E., 2012. Rudists and carbonate platforms in the Aptian: a case study on biotic interactions with ocean chemistry and climate. Sedimentology 59, 81–117. Swart, P.K., 2008. Global synchronous changes in the carbon isotopic composition of carbonate sediments unrelated to changes in the global carbon cycle. Proceedings of the National Academy of Sciences 105, 13741–13745. http://dx.doi.org/ 10.1073/pnas.0802841105. Tejada, M.L.G., Katsuhiko, S., Kuroda, J., Coccioni, R., Mahoney, J.J., Ohkouchi, N., Sakamoto, T., Tatsumi, Y., 2009. Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event. Geology 37, 855–858. Weissert, H., Lini, A., Föllmi, K.B., Kuhn, O., 1998. Correlation of Early Cretaceous isotope stratigraphy and platform drowning events: a possible link? Palaeogeography, Palaeoclimatology, Palaeoecology 137, 189–203.
Peter W. Skelton, Guest Editor* Department of Earth, Environment and Ecosystems, The Open University, Milton Keynes MK7 6AA, UK Bruno Granier, Guest Editor Département des Sciences de la Terre et de l’Univers, UFR des Sciences et Techniques, Université de Bretagne Occidentale (UBO), 6 avenue Le Gorgeu, CS 93837, F-29238 Brest Cedex 3, France E-mail address: [email protected] (B. Granier) Michel Moullade, Guest Editor Centre de Recherches Micropaléontologiques, Museum d’Histoire Naturelle, 60 Bd Risso, 06000 Nice, France Laboratoire de Géologie des Systèmes et des Réservoirs Carbonatés, Université de Provence (Aix-Marseille I), Campus St Charles, 3 Pl. Victor Hugo, 13331 Marseille Cedex 03, France E-mail address: [email protected] * Corresponding author. E-mail address: [email protected] (P.W. Skelton) Available online 16 October 2012
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Editorial
Introduction to thematic issue, “Spatial patterns of change in Aptian carbonate platforms and related events”
The Aptian Stage serves as a microcosm of the Cretaceous world, showing evidence for all the latter’s distinctive features, such as massive plume-related volcanism, strong climatic fluctuations within a broadly ‘greenhouse’ regime, and major perturbations of the global carbon cycle involving both oceanic anoxic events (most notably, OAE 1a) and crises of biocalcification, including the episodic demise of vast carbonate platforms (e.g., Ando et al., 2008; Blättler et al., 2011; Dumitrescu et al., 2006; Erba et al., 2010; Keller et al., 2011; Kuhnt et al., 2011; Méhay et al., 2009; Tejada et al., 2009; Skelton and Gili, 2012, from an extensive literature). Causal linkages between these various phenomena seem likely, but identifying exactly how they interacted requires (at least) precise documentation of the relative temporal and spatial patterns of the physical and biotic events involved. Pioneering studies in the last century initially held out the hope of a relatively simple scenario for the suppression of biocalcification based on excess nutrient flux (e.g., Hallock and Schlager, 1986; Schlager and Philip, 1990; Föllmi et al., 1994; Weissert et al., 1998). Subsequent detailed chemo-, and biostratigraphical analyses of Aptian successions over the last decade and a half, however, have confounded that earlier optimism, suggesting that the ‘Aptian story’ was more complicated than originally supposed. The following issues, over which consensus has proved elusive, especially merit further critical attention: (1) Correlation. Problems of biostratigraphical correlation between different basin and platform successions, reliant upon an assortment of different, non-ubiquitous taxa (e.g., planktonic foraminifers, calcareous nanofossils, ammonites, benthic foraminifers, especially orbitolinids, calcareous algae and rudist bivalves) have led to disagreements over the detailed chronostratigraphy of geochemical changes, particularly the carbon isotopic signature for OAE 1a (see Moreno-Bedmar et al., 2009, Fig. 1; Moullade et al., 2011a, Fig. 1). (2) Spatial variation of effects. Chemo-, and biostratigraphical studies around the northwestern (Franco-Swiss) Tethyan margin dominated the narrative in the earlier literature on the demise of carbonate platforms in the Early Aptian. But in order to assess any possible modulating effects of climate/ocean interactions, a more palaeogeographically inclusive survey of regional variations in the development of carbonate platform and pelagic facies, and associated biotic turnover, is necessary (Skelton and Gili, 2012). (3) Climatic and eustatic fluctuations. The amplitudes of climatic fluctuations assessed from different proxies, including postulated short-term ‘cold snaps’, are currently debated (see, for example, Jenkyns et al., 2012), while Masse and Fenerci-Masse (2011) have 0195-6671/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cretres.2012.09.001
proposed a stepped series of platform ‘drownings’ in the Early Aptian, rather than a single main event linked with OAE 1a. (4) Causal factors. Given the continuing lack of agreement on so many aspects of the Aptian record (as in 1–3, above), it is not surprising that a consensus is still lacking on the relative importance of various possible influences on the biota such as nutrient flux, seawater acidification and temperature in bringing about the changes seen in the marine geological record (cf., for example, Föllmi, 2012, and Skelton and Gili, 2012). These issues were variously addressed at the 4th French Congress on Stratigraphy held in Paris, France, in September, 2010 (‘STRATI2010’), especially in a thematic session that focused on the second issue mentioned above (Session 8: ‘Spatial patterns of changes in the Aptian’). Nine of the eleven papers in this volume are based on presentations that were given at various sessions in that conference and the remaining two (those by Graziano and by Masse and Fenerci-Masse) were subsequently submitted as additional contributions relevant to the theme. The papers herein are organized according to palaeogeography (Fig. 1), both in keeping with the main emphasis of Session 8 at STRATI2010 and also because several of them touch on more than one, if not all of the issues listed above, rendering an alternative, purely thematic ordering impractical. Our tour of the Aptian record thus begins with two studies from the traditional French heartland of Aptian stratigraphy, Provence (Moullade et al., 1998, 2009, 2011b). Lorenzen et al., (Fig. 1, #1) report on the geochemistry and foraminiferal biostratigraphy of three new drill cores from the historical Lower Aptian stratotype at Roquefort-La Bédoule. In agreement with previous outcrop studies in the area, they demonstrate an expanded record of the negative d13C excursion (C3 stage of Menegatti et al., 1998) that heralds OAE 1a extending up to the blowi/cabri foraminiferal zonal boundary situated within the Upper Bedoulian deshayesi ammonite zone, and calculate a duration for it of >200 kyr from estimated sedimentation rates, and >300 kyr for the subsequent main positive (C4) isotopic shift. A little further north, Masse and FenerciMasse (Fig. 1, #2) contrast the rudist-bearing limestones at Orgon – the type area for d’Orbigny’s ‘Urgonian Stage’, now defunct as a stratigraphical term, but still widely deployed in reference to such facies – with those in the Monts de Vaucluse–Apt region. Whereas the former belong to the uppermost Barremian, the latter are mostly confined to the Lower Aptian, though with an uppermost Barremian (‘U1’) equivalent of the Orgon limestones preserved in the western part of the Monts de Vaucluse, beneath a Palorbitolina lenticularis–Heteraster oblongus (‘Pa 1’) guide level.
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Fig. 1. Tethyan palaeogeography for the Early Aptian. Numbered stars refer to studies in this volume: 1, Lorenzen et al.; 2, Masse and Fenerci-Masse; 3, Clavel et al.; 4, Ivanov and Idakieva; 5, Cherchi and Schroeder; 6, Graziano; 7, Ruberti et al.; 8, Elkhazri et al.; 9, Peybernes et al.; 10, Granier and Busnardo; 11, Wilmsen et al. Map generated from ODSN Plate Tectonic Reconstruction Service (Hay et al., 1999).
The succeeding caprinid rudist-rich (‘U2’) Urgonian limestones are in turn terminated by a mid-Bedoulian discontinuity, overlain by a second (‘Pa 2’) Palorbitolina horizon, followed by bioclastic or coral (‘U3’) limestones, then marls with marly limestones. The demise of the rudist-rich U2 Urgonian limestones in this region thus pre-dated the d13C signature for OAE 1a as recognized at La Bédoule (see also Moullade et al., 1998; Masse and Fenerci-Masse, 2011). The third paper, by Clavel et al., extends the survey of platform stratigraphy northwards around the margins of the Vocontian Basin to the Swiss Jura (Fig. 1, #3). With a series of detailed palaeogeographical maps of successive late highstand facies distributions, dated by ammonites together with a wide range of other biota, these authors illustrate a step-wise centripetal progradation of carbonate platforms into the basin from the early Late Hauterivian. Platform development became most restricted in early Late Barremian times, coinciding with a marked turnover of orbitolinids. Subsequent transgressive re-establishment of the platform was then terminated in the Early Aptian, in late weissi Zone times, similarly to the U2 rudist limestones of the Monts de Vaucluse described in the previous paper – though in this case the discontinuity was directly overlain by hemipelagic facies. It is hypothesized that this striking change in sedimentation may reflect an increased connection with cooler northern waters accompanying the relative rise in sea-level. The next paper, by Ivanov and Idakieva takes us eastwards along the northern Tethyan margin to the uppermost Barremian to Lower Aptian succession in Bulgaria (Fig. 1, #4), focussing upon its ammonite biostratigraphy. Here they recognize, inter alia, a Roloboceras hambrovi Subzone in the upper part of the Lower Bedoulian Deshayesites forbesi Zone (replacing the former
Paradeshayesites weissi Zone; Reboulet et al., 2011), in agreement with Moreno-Bedmar et al. (2009) working in the Maestrat Basin of eastern Spain, though in contrast to its placement in the Upper Bedoulian Deshayesites deshayesi Zone at La Bédoule (Ropolo et al., 1998, 2008) (Fig. 2). The reliability of Roloboceras as a chronostratigraphical index thus remains open to question, as previously suggested by Ropolo et al. (2008). Interestingly, in this context, Ivanov and Idakieva also attribute an interval of thinly laminated, organic-rich clays in their (Lower Bedoulian) R. hambrovi Subzone to OAE 1a. The paper by Cherchi and Schroeder shifts attention to the central and southern Tethyan carbonate platforms (Fig. 1, #5), focussing upon orbitolinid biostratigraphy. Their recognition of a Praeorbitolina/Palorbitolinoides association characterizing the uppermost part of the Bedoulian in the Apulian, Adriatic and eastern Arabian platforms provides a valuable tool for refining the dating of such platforms. Next, Graziano synthesizes the Lower Aptian record of the Apulian carbonate platform margin from detailed studies of local exposures in the Gargano Promontory of SE Italy (Fig. 1, #6). Here, caprinid rudist-rich platform margin facies are shown to extend up into the lowermost Upper Bedoulian, where they are succeeded by a thin ‘crisis interval’, characterized by a distinctive brachiopod/microbial association attributed to a ‘hothouse’ regime and marking the inception of drowning of the platform margin. Physical correlation with isotopically calibrated pelagic successions of the Ionian Basin links this crisis and subsequent drowning interval with the negative d13C excursion heralding OAE 1a. The crisis also marks the eventual replacement here of an aragonite-dominated, ‘Urgonian-type’ platform association of caprinid rudists, corals and green algae by one dominated by
Editorial / Cretaceous Research 39 (2013) 1–5 Fig. 2. Two recent examples of divergent proposals for latest Barremian to earliest Albian chronostratigraphic subdivision and ammonite zonation: left, that from the ‘Kilian Group’ (Reboulet et al., 2011); right, that from Moullade et al. (2011b).
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more calcite-rich benthos such as chondrodontid bivalves, chaetetid sponges, bryozoans, echinoids and orbitolinids. A similar change in biota is recorded from the southern Apennine platform by Ruberti et al., (Fig. 1, #7), though at a higher stratigraphical level – around the Bedoulian/Gargasian boundary – and in a somewhat more internal setting affected by a phase of incipient tectonic restructuring. In this case the biotic changeover is mediated by an interval of thin paleosol-capped metric cycles containing laminites and oncoids with an impoverished biota that includes orbitolinids and small cerithiids, shortly followed by a horizon attributed to the Lower Gargasian Salpingoporella dinarica Abundance Zone. Again, the subsequent Upper Aptian to Albian platform benthos is dominated by calcite-rich rudists, including radiolitids, together with large chondrodontids. The next two papers concern North African successions. Elkhazri et al., report on a geochemical (including carbonisotopic) and microfaunal study of a section in northeastern Tunisia (Fig. 1, #8) in which they have identified the signature of OAE 1a in a 25 m interval of pelagic black limestones and marly limestones. They report the first appearance, followed by expansion to an acme of Schakoina cabri, accompanied by a radiolarian bloom, within the latter interval, which they ascribe to the C4–C7 stages of Menegatti et al. (1998), in broad agreement with the findings at La Bédoule reported in the first paper of the present volume. Further west, Peybernes et al., have studied a deepening-upward ramp succession of somewhat younger age (latest Early Aptian to Early Albian) in the Essaouira-Agadir Basin of southern Morocco (Fig. 1, #9). Here, a decrease both in calcium carbonate content and in Nannoconus abundance are noted at the Aptian–Albian transition, attributed either to climatic cooling or to an associated increased input of terrigenous sediments and nutrients. In one section (Tamzergout), an 8 m interval of dark marl in the Lower Albian that shows a marked decrease in calcium carbonate content and nanofossil productivity is correlated with OAE 1b. However, nanofossil fluxes here are considerably lower than those recorded in the Vocontian Basin of southeastern France, because, the authors suggest, of more arid conditions prevailing along the southern Tethyan margin. The last two papers focus on the Middle East. Combining bio-, litho-, and sequence stratigraphy with well log data and d13C-based chemostratigraphy, Granier and Busnardo review the Upper Barremian to Aptian stratigraphy of the eastern Arabian craton, using a reference core from offshore Abu Dhabi that covers the entire stratigraphical interval (Fig. 1, #10). They conclude that the Barremian/Aptian boundary falls within the third of four third-order transgressive–regressive cycles making up the Kharaib Formation. The succeeding Hawar Formation constitutes a single cycle, overlying a locally karstified surface regarded as time-equivalent to that terminating the Franco-Swiss Urgonian platform. OAE 1a most likely coincided with the forced regression that terminated the Hawar cycle, with a Bacinella proliferation occurring yet later, during the transgressive phase of the succeeding, single Shu’aiba cycle. Ammonites in condensed basinal facies at the top of the latter cycle show it to extend into the Deshayesites furcata Zone (¼ basal Gargasian instead of topmost Bedoulian in the convention followed by these authors; see Moullade et al., 2011b and Fig. 2 herein). The base of the succeeding Bab cycle is considered to correspond to that of the Kazhdumi intra-shelf deposits of the Zagros Mountains in western Iran. The final paper takes us to Central Iran (Fig. 1, #11), where Wilmsen et al., have also studied orbitolinid-, and rudistrich platform limestones of latest Barremian to Early Aptian age (Shah Kuh Formation), overlain by basinal deposits of the Bazyab Formation of (ammonite-dated) Late Aptian to Albian age. The Shah Kuh apparently developed from a narrow, high energy shelf to a broad, flat-topped rudist platform. Along with other such
regionally important ‘Orbitolina limestones’, it forms part of a widespread transgressive depositional megacycle. Exact dating of the demise of this Early Aptian platform remains to be established, though its rudist fauna bears some interesting resemblances to that of the Shu’aiba Formation in eastern Arabia, despite their wide palaeogeographical separation either side of the Neo-Tethys Ocean (Fig. 1, #10 and 11). So what conclusions can be drawn from this collection of papers in relation to the four interrelated issues cited at the start of this Introduction? With respect to correlation, divergences of opinion evidently persist over the exact dating of OAE 1a (cf., Lorenzen et al., and Ivanov and Idakieva, for example). But as Elkhazri et al., correctly observe, ‘In fact, for the moment no method demonstrates its isochroneity with certainty’. As a speculative alternative to this stratigraphical impasse, the (for some, heretical!) question could indeed be posed as to whether the iconic isotopic signals really were globally synchronous, as widely assumed; or could they have been regionally modulated by the isotopic feedback effects of independently occurring changes in platform carbonate production, along the lines proposed by Swart (2008)? Likewise, one might envisage distinct, asynchronous ‘Roloboceras bio-events’ possibly related to transient local OAE-related conditions. Hence neither carbon isotopic chemostratigraphy nor biostratigraphy alone can be considered infallible as chronostratigraphical tools without critical reference to each other as well as the implied context of relative sea-level change, as demonstrated in a recent critical study by Raspini (2012). Moreover, from both stratigraphical and palaeogeographical considerations, it is surely even clearer than it was before that the demise of Early Aptian Tethyan carbonate platforms cannot be interpreted as a single, OAE 1arelated event. While the termination of the northern, FrancoSwiss rudist-dominated platform facies can be dated to the midBedoulian (Masse and Fenerci-Masse; Clavel et al.), that of the Gargano margin seems to have followed somewhat later, in the early part of the Late Bedoulian (Graziano), while both the southern Apennine, and eastern Arabian (Shu’aiba) platforms persisted through the Late Bedoulian (Ruberti et al.; Cherchi and Schroeder; Granier and Busnardo). Nevertheless, the analysis of the Arabian depositional cycles by Granier and Busnardo suggests that the emersion of the earlier Kharaib platform was synchronous with that of the Franco-Swiss Urgonian platform, some time before the OAE 1a and microbial Bacinella proliferation events. The crucial difference is that the central and southern Tethyan platforms largely recovered in the Late Bedoulian, in contrast to the northern platforms (with the exception of those on the Iberian promontory), as previously stressed by Skelton and Gili (2012). It will be interesting to see how the Shah Kuh platform of Wilmsen et al., eventually compares with this pattern, given its palaeogeographical proximity to the northern margin, though in an even more southerly palaeolatitude than the Iberian promontory (Fig. 1). Unsurprisingly, the relative influences on such a complex history remain unresolved. Some authors emphasize variations in nutrient fluxes related to arid versus humid conditions to explain observed biotic contrasts (e.g., Peybernes et al.), while others additionally contemplate a role for CO2-driven changes in seawater chemistry and temperature, particularly noting apparent effects on the balance of aragonite and calcite in the platform biota (Graziano; Ruberti et al.). Also remarked upon is the influence of changing accommodation space on the shallow tops of platforms, whether in response to tectonic activity (Ruberti et al.) or to presumed eustatic fluctuations (Clavel et al.). Of course, these possible factors are not mutually exclusive, and they could, and probably did all have a part to play. So, despite some progress on revealing the stratigraphical complexities of the Aptian world, it seems that we still have some way to go to understand its workings.
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Peter W. Skelton, Guest Editor* Department of Earth, Environment and Ecosystems, The Open University, Milton Keynes MK7 6AA, UK Bruno Granier, Guest Editor Département des Sciences de la Terre et de l’Univers, UFR des Sciences et Techniques, Université de Bretagne Occidentale (UBO), 6 avenue Le Gorgeu, CS 93837, F-29238 Brest Cedex 3, France E-mail address: [email protected] (B. Granier) Michel Moullade, Guest Editor Centre de Recherches Micropaléontologiques, Museum d’Histoire Naturelle, 60 Bd Risso, 06000 Nice, France Laboratoire de Géologie des Systèmes et des Réservoirs Carbonatés, Université de Provence (Aix-Marseille I), Campus St Charles, 3 Pl. Victor Hugo, 13331 Marseille Cedex 03, France E-mail address: [email protected] * Corresponding author. E-mail address: [email protected] (P.W. Skelton) Available online 16 October 2012