Carbon-Isotope Anomalies and Demise of Carbonate Platforms in the Sinemurian (Early Jurassic) of the Tethyan Region: Evidence from the Southern Alps (Northern Italy)
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Geol. Mag. 154 (3), 2017, pp. 625–650. c Cambridge University Press 2016 625 doi:10.1017/S0016756816000273 Carbon-isotope anomalies and demise of carbonate platforms in the Sinemurian (Early Jurassic) of the Tethyan region: evidence from the Southern Alps (Northern Italy) ∗ ∗ DANIELE MASETTI †, BILLY FIGUS , HUGH C. JENKYNS‡, FILIPPO BARATTOLO§, EMANUELA MATTIOLI¶ & RENATO POSENATO∗ ∗ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Polo scientifico-tecnologico, Via Giuseppe Saragat 1, 44122 Ferrara, Italy ‡Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK §Dipartimento di Scienze della Terra, Università degli Studi di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy ¶UMR CNRS 5276 LGL-TPE, Université Claude Bernard Lyon 1, Ecole Normale Supérieure Lyon, Campus de la DOUA, Bâtiment Géode, 69622 Villeurbanne Cedex, France (Received 11 February 2015; accepted 22 March 2016; first published online 30 May 2016) Abstract – Despite its global impact on ecosystems, the Triassic/Jurassic boundary event had only a modest effect on the carbonate depositional systems of the Southern Alps, whereas a fundamental reorganization of the same palaeogeographic area took place during the Sinemurian Age. This paper investigates whether or not the well-documented demise of Sinemurian carbonate platforms in the Tethyan region was a response to a global event by examination of carbon-isotope anomalies in successions of different facies that record this interval of time. A chemostratigraphic transect from Lake Garda up to the eastern Italian border is illustrated by four stratigraphic sections; high-resolution (20 cm over key intervals) chemostratigraphic sampling allowed detection of a major negative δ13C anomaly of 1.5 ‰, preceded by a positive excursion, both in shallow- and deep-water successions, over the stratigraphical range of the ammonite genus Arnioceras. A comparison with sections from the UK suggests that the positive excursion belongs to the turneri Zone and the succeeding negative excursion falls within the obtusum Zone. In the deep-water Belluno Basin, the negative anomaly occurs in a biogenic chert-rich unit recording the onset of mesotrophic conditions in the basin. In the platform-carbonate successions, this major negative carbon-isotope excursion is developed within a calcarenitic unit corresponding to the lowest occurrence of the foraminifer Paleomayncina termieri. This evidence for deepening and transgression across the carbonate platform suggests pre-conditioning for drowning. Hence, rather than tectonic subsidence alone, environmental factors may have aided the demise of Tethyan carbonate platforms during the Early Jurassic Sinemurian Age. Keywords: carbon-isotope anomalies, carbonate platform demise, Tethyan continental margins, Jurassic, Southern Alps, Northern Italy. 1. Introduction: the demise of carbonate platforms in Winterer & Bosellini 1981; Masetti et al. 2012). Similar the Sinemurian Age of the Tethyan area shallow-water environments also existed elsewhere, for example in the Umbria–Marche Apennines of Cent- Global episodes of environmental change represent ma- ral Italy (Calcare Massiccio Formation), and in the jor turning points in the history of the Earth. Among Southern Limestone Apennines (Calcare a Paleodasy- these, the Triassic/Jurassic boundary (T/J) is character- cladus Formation; D’Argenio, Pescatore & Scandone, ized not only by a major extinction but also by disturb- 1973). ances in the carbon-isotope reservoir of the oceans and Although the T/J event had a negligible effect on atmosphere. Despite its global impact, the T/J event had the Jurassic depositional systems of the Southern Alps only a modest effect on the palaeogeographic configur- and Apennines, a fundamental reorganization of their ation of the Southern Alps as exposed in Northern Italy. carbonate systems took place during Sinemurian time The palaeogeography of this region at the beginning (Masetti et al. 2012), here summarized in Table 1, of the Jurassic Period was characterized by the wide- locally acting together with the onset of extensional tec- spread development of shallow-water carbonate plat- tonics. In Lombardy, starting from the interval between forms (Corna, Monte Zugna Formation), representing early and late Sinemurian time, the Corna Platform was the continuation of Late Triassic peritidal sedimenta- locally capped by open-marine crinoidal calcarenites tion, and stretching from Lombardy to Slovenia, only (Rezzato Encrinite; Schirolli, 1997; Meister, Schirolli interrupted by the deep Belluno Basin (Gaetani, 1975; & Dommergues, 2009); across much of the Trento Platform, the Hettangian–Sinemurian peritidal succes- †Author for correspondence: [email protected] sion of the Calcari Grigi Group (Monte Zugna Downloaded from https://www.cambridge.org/core. Open University Library, on 19 Jan 2020 at 08:40:25, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756816000273 626 D. MASETTI AND OTHERS Formation) is unconformably overlain by a Sinemurian–Pliensbachian condensed succession (Fanes Piccola Encrinite; Masetti & Bottoni 1975; Masetti et al. 2012); in the Northern Apennines, Marino & Santantonio (2010) described the early Sinemurian replacement of the peritidal Calcare peritidal platform, unconformably covered by spiculitic deposits the early/late Sinemurian boundary; condensation during late Sinemurian Massiccio platform by means of deep-water deposits Break-up of the of the Corniola Formation. In the structural highs of the same area this event is recorded later, at the base of the semicostatum Zone, by the superposition of a ‘drowning succession’ (‘Calcare Massiccio B’) over ; IFm – Inici Formation; MFm – Modica the underlying, peritidal, ‘Calcare Massicio A’. In the Ligurian Alps, the early Sinemurian carbonate platform ceased sediment production and was covered carbonate evolving in deeper setting’ ‘Dissected peritidal Generic Sinemurian Platform dissection at by deep-water deposits (Decarlis & Lualdi, 2010); in eastern Sicily, the progradational trend of the peritidal Inici Formation ceased at the early/late Sinemurian Zone boundary, just before the drowning of the carbonate platform that occurred in late Sinemurian time, or when the deep-water Modica Formation started to accumulate (Ronchi, Lottaroli & Ricchiuto, 2000). platform (IFm) overlain by deep-water deposits (MFm) semicostatum of the early Sinemurian Ammonites found close to the top of the Inici Forma- Demise of peritidal bucklandi tion in western Sicily also suggest that the carbonate Time’. More detail and references in the text platform, locally at least, ceased deposition at some point in the bucklandi or semicostatum Zone of early Sinemurian time or soon thereafter (Wendt, 1969; Arnioceras Jenkyns & Torrens, 1971). In the Betic Cordillera (Spain) Ruiz-Ortiz et al. (2004) described the Early Jurassic stratigraphic evolution of the rifted Iberian platform, overlain by deep-water pelagic deposits Sinemurian boundary Demise of peritidal Lower/upper margin indicating that widespread peritidal carbonates evolved with faulting to a more open and deep-marine setting. These authors refer the first dissection of the Zone platform by extensional faults to early Pliensbachian time, on the basis of a benthic foraminiferal associ- ation. However, previous interpretations have dated the faulting as intra-Sinemurian (Ruiz-Ortiz et al. platform (CMA) overlain by a drowning succession (CMB) referred to the semicostatum of the early Sinemurian 2004), so the evolution may be similar to the Italian Demise of peritidal Northern Apennines, Italy Ligurian Alps, Italy Sicily, Italy Betic Cordillera, Spain High Atlas, Morocco CMA/CMB boundary examples. In the High Atlas (Morocco) Merino-Tomé et al. (2012) described in detail the break-up of the peritidal Early Jurassic carbonate platform of Djebel Bou Dahar into smaller deeper water blocks as a result of tectonic processes at the boundary between the early and late Sinemurian. The same authors highlight a sudden contemporaneous decrease platform (CN) overlain by condensed succession (RE) Sinemurian boundary on the basis of ammonites in carbonate production leading to the generalized Demise of peritidal Western Southern Alps, Italy developing of hiatuses and the subsequent onset, in late Sinemurian–Pliensbachian time, of sub-photic siliceous sponge microbial facies. Despite the huge increase in area affected by the drowning of carbonate platforms in the Apennines and Southern Alps, the consensus to date is to view this event as regional and platform (ZG) overlain by condensed succession (ENF) Demise of peritidal Eastern Southern Alps, Italy Generic Sinemurian Lower/upper Ye sessentially Ye s due tothe Ye s increasing subsidence Ye s rate of Ye s Ye s Ye s fault-bounded blocks during extension of the Jurassic continental margin (Bernoulli & Jenkyns, 1974). Bearing in mind the widespread demise/drowning of carbonate platforms in the Tethyan area during a poorly dated interval or intervals in Sinemurian time, and that the Southern Alps contain outcrops of a former literature extensional tectonics passive Mesozoic continental margin affected by such Table 1. Summary of the variations in the carbonate sedimentationEffects affecting on some carbonate Tethyan areas occurringsedimentation as around ‘ described in the literature CN – Corna Formation; ZG – Zugna Formation; ENF – Fanes Piccola