Carbon-Isotope Anomalies and Demise of Carbonate Platforms in the Sinemurian (Early Jurassic) of the Tethyan Region: Evidence from the Southern Alps (Northern Italy)

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

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 association. 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 Encrinite; RE – Rezzato Encrinite; CMA and CMB – Calcare Massiccio Formation, A and B Type Formation. Age inferred in the Occurrence of phenomena, the main aims of this paper are:

Figure 1. (a) Mesozoic structural domain of the Southern Alps. The dotted line indicates the section in (b). (b) Section across the Southern Alps showing the extensional Mesozoic architecture of the Southern Alps at the end of Early Cretaceous time. After Masetti et al. (2012).

(1) to investigate whether or not the de- during Early Jurassic time. Extension shifted west- mise of Sinemurian carbonate platforms was the wards to the Ligurian–Piedmont area where oceanic result, wholly or partially, of a chemostrati- crust had formed by Middle Jurassic time (Bill et al. graphic/palaeoenvironmental event, by examining 2001). From this time until the Early Cretaceous, the carbon-isotope anomalies in successions across the Southern Alps underwent post-rift thermal subsidence whole area of the Eastern Southern Alps; (Bertotti et al. 1993; Fantoni & Scotti, 2003). (2) to provide high-resolution correlation between The Mesozoic extension resulted in the creation of shallow-water platform carbonates and deeper water N–S half-grabens, bounded by E- and W-dipping mas- pelagic ammonite-bearing successions across the East- ter normal faults (Fig. 1). In the Eastern Southern Alps ern Southern Alps by means of carbon isotopes in order three palaeogeographical-structural units are recogniz- to reﬁne, as much as possible, the exact position/timing able. These are, from west to east (Gaetani, 1975; of any palaeoenvironmental event. Winterer & Bosellini, 1981): a carbonate platform, (3) to ascertain the nature of carbonate-platform which drowned in Early Jurassic time, evolving into evolution coincident with this putative palaeoenviron- a pelagic plateau with condensed pelagic sedimenta- mental event in order to highlight the potential response tion during Late Jurassic time (Trento Platform) and of the sediment-producing carbonate factory. bordered to the west by the Lombardian Basin that ac- cumulated pelagic clays and carbonates; a basin, with similar pelagic facies, that developed in very Early Jur- 2. Geological setting: the Southern Alps assic time (Belluno Basin); and a carbonate platform Since the early 1970s, the Southern Alps have been that persisted from Jurassic until Cretaceous time (Fri- interpreted as a passive continental margin that, dur- uli Platform). ing Mesozoic time, experienced polyphase extensional tectonics that caused the fragmentation of the Adriatic 2.a. The Trento Platform Plate. The tectonic activity started during Late Triassic (Norian) time. After relative quiescence during Rhae- On the Trento Platform, the shallow-water sedimenta- tian (latest Triassic) time, renewed extension took place tion of the Early Jurassic is recorded in the thick pile

Figure 2. Chemostratigraphic transect across different types of Jurassic successions inside the main palaeogeographic units of the Eastern Southern Alps and location of the sections presented here. On the map, depicting the Pliensbachian stage, 1 and 2 represent, respectively, the areal distribution of central-western and the northeastern areas of the Trento Platform; 3 represents the Belluno Basin; 4 and 5 represent, respectively, the northern and southern areas of the Friuli Platform. ZG – Monte Zugna Formation; OL – Loppio Oolitic Limestone; RZ – Rotzo Formation; CG – undifferentiated Calcari Grigi; OM – Massone Oolite; ENF – Fanes Piccola Encrinite; OSV – San Vigilio Oolite + Tenno Formation; SOV – Soverzene Formation; IGNE – Igne Formation; ARV – Rosso Ammonitico Veronese; VJ – Vajont Limestone; CEL – Cellina Limestone. Modiﬁed from Masetti et al. (2012).

of the Calcari Grigi Group, and the overlying pelagic ation, a unit representing the Jurassic continuation of ‘condensed’ sedimentation recorded by the Rosso Am- the underlying, Upper Triassic, peritidal succession of monitico Veronese (Bajocian to Tithonian, Fig. 2). The the Dolomia Principale. The Loppio Oolitic Limestone Calcari Grigi Group is several hundred metres thick; and the Rotzo Formation represent, respectively, the its lower part corresponds to the Monte Zugna Form- middle and upper part of the Calcari Grigi Group; the

Figure 3. Lithostratigraphic relationships of the Jurassic units across the entire Trento Platform–Plateau from the Lombardian Basin in the west, to the Belluno Basin in the east. Red spots highlight the location of the negative CIE described in the text. Modiﬁed from Masetti et al. (2012).

most typical and renowned facies are those present in ness (Masetti et al. 1998), whereas the classic Calcari the Rotzo Formation, characterized by abundant plant Grigi succession with its well-known ‘Lithiotis’ beds remains and extensive banks of oyster-like ‘Lithiotis’, (Rotzo Formation) is present only in the central-western deposited in a dominantly subtidal environment (Ma- sector of the Trento Platform and passes westwards setti et al. 1998; Posenato & Masetti 2012; Franceschi to planar-bedded facies interpreted as marginal shoals et al. 2014). This subtidal environment, interpreted by (Massone Oolite; Beccarelli-Bauck, 1988). previous authors as lagoonal (the ‘Lithiotis Lagoon’ The northeastern sector of the Trento Platform of Bosellini & Broglio Loriga, 1971), passed later- (Figs 2, 3) is characterized by a widespread hiatus cor- ally to the western marginal oolitic complex (Massone responding to the Pliensbachian units, replaced by a Oolite, Figs 2, 3). The Monte Zugna Formation and the thin veneer of red cross-bedded crinoidal sand-bodies Loppio Oolitic Limestone, lacking faunas of proven corresponding to the Fanes Piccola Encrinite: where biostratigraphic value, have been referred to a generic this unit is missing, the Rosso Ammonitico rests dir- Hettangian–Sinemurian p.p. interval, whereas the age ectly on the Monte Zugna Formation (Masetti et al. of the Rotzo Formation is still debated and ascribed 2012). to a time interval spanning the late Sinemurian to late Pliensbachian, on the basis of foraminiferal biostratigraphy (Fugagnoli, 2004), or to early Pliensbachian to 2.b. The Belluno Basin late Pliensbachian, on the basis of ammonite biostrati- The inception of the Belluno Basin was linked to graphy (Sarti in Posenato & Masetti, 2012). Early Jurassic rifting that led to a roughly N–S- The unconformity surface capping the top of the oriented fault system (Masetti & Bianchin, 1987); shallow-water Calcari Grigi Group corresponds to a during Hettangian–Pliensbachian time, this basin was temporal hiatus that expands in duration eastwards ﬁlled by dark cherty, basinal micrites (Soverzene Form- (Masetti et al. 1998; Figs 2, 3). Based on the hiatus ation, Figs 2, 3). On the basis of data coming from at the top of the Calcari Grigi Group and the lateral the Verzegnis section, where sediments are free from variations displayed by the Pliensbachian units, Ma- heavy dolomitization, Masetti et al. (2012) suggested setti et al.(2012) proposed a further subdivision of that the birth of the Belluno Basin can be referred the Trento Platform into a central-western area (with to the Triassic–Jurassic boundary interval or even to the Pliensbachian Rotzo Formation) and a northeastern latest Triassic time. Above the Soverzene Formation, area (without the Rotzo Formation). The approximate the early Toarcian oceanic anoxic event (Jenkyns, 1988) spatial distribution of these areas is shown in Figure 2; is recorded by discontinuous levels of black shales Figure 3 illustrates the Hettangian–Sinemurian Monte and manganoan carbonates, contained within the Igne Zugna Formation crossing the entire Trento Platform Formation, which consists of decimetric rhythms of from west to east with little variation in facies and thick- grey marls and marly mudstones (Jenkyns et al. 1985;

Claps et al. 1995; Bellanca et al. 1999). This last been concentrated just below the unconformity surface unit is covered by the Vajont Limestone, composed at the top of the shallow-water succession where dis- of oolitic sands and biogenic skeletal debris redepos- tinctive carbon-isotope anomalies were predicted to be ited by means of gravity-flow processes that transferred present (Fig. 3). A summary of the principal character- oolitic sands from the western edge of the Friuli Plat- istics of the four sections is highlighted in Table 2. form into slope and basin environments (Bosellini & Masetti, 1972; Bosellini, Masetti & Sarti, 1981). The 3. Chemostratigraphic transect across the Eastern age of the Vajont Limestone, revised by Cobianchi Southern Alps (2002) on the basis of nannofossil biostratigraphy performed on several sections, can be ascribed to the late 3.a. Chemostratigraphic sampling and analyses Bajocian–Bathonian interval. The Fonzaso Formation 3.a.1. Analytical techniques (Callovian to lower Kimmeridgian) overlies the Vajont Limestone and consists of pelagic cherty mudstones Chemostratigraphic sampling was performed on the and skeletal-rich turbidites and debris-flow deposits. selected stratigraphic sections at a resolution of The Fonzaso Formation grades upwards into nodu- 20 cm, wherever allowed by the exposure conditions; lar, micritic red limestones very similar to the Rosso powdered samples were obtained directly by means of Ammonitico Veronese (Upper Member, upper Kim- a drill powered by a generator, with micritic matrix be- meridgian to lower Tithonian; Martire, 2007). ing preferentially sampled and skeletal fragments and veins being avoided, on the assumption that these com- ponents would have been more prone to vital effects 2.c. The Friuli Platform and diagenesis. The high-resolution sampling was pre- In a similar way to the Trento Platform, Masetti et al. ceded by a lower resolution sampling (a sample every (2012) proposed a further subdivision of the Friuli Plat- 2 m) in order to identify the main isotopic anomalies form into a northern and a southern area (Figs 2, 3). present in the section. The total thickness of the four The northern area is characterized by a stratigraphic sections presented here, on which this high-resolution evolution similar to that experienced by the eastern sampling has been performed, exceeds 570 m. For iso- and northern sector of the Trento Platform, in which topic analysis, the samples were analysed isotopically the shallow-water Rotzo Formation is missing and the for δ13C and δ18O using a VG Isogas Prism II mass spec- Monte Zugna Formation is overlain by the Fanes Pic- trometer with an online VG Isocarb common acid bath cola Encrinite. On top of this last unit lies a deep-water, preparation system. Samples were cleaned with hydro- Mid and Upper Jurassic succession, typical of the Bel- gen peroxide (H2O2) and acetone ((CH3)2CO) and dried luno Basin (VajontLimestone and Fonzaso Formation). at 60 °C for at least 30 minutes. In the instrument they To the south, the classic persistent carbonate platform is were reacted with purified phosphoric acid (H3PO4)at exemplified by the section cropping out along the Val- 90 °C. Calibration to PDB standard via NBS-19 was cellina Valley, located at the southern edge of the Friuli made daily using the Oxford in-house (NOCZ) Carrara Prealps, in which the exposed shallow-water succes- marble standard. Reproducibility of replicated stand- sion spans the interval from the Oxfordian through the ards was typically better than 0.1 ‰ for both δ13Cand whole of the Cretaceous (Cuvillier, Foury & Pignatti δ18O. Morano, 1968). For strontium-isotope analyses, c. 50 mg of carbonate per sample were dissolved in 6 ml of 2HNO3 for both 87Sr/86Sr and trace-metal analyses. A 1.5 ml ali- 2.d. Selected stratigraphic sections across the Southern quot of the dissolved solution was taken to perform Alps strontium purification for the 87Sr/86Sr measurements, Bearing in mind the above-mentioned stratigraphic set- and the remaining solution was diluted and meas- ting shown in Figure 2, four stratigraphic sections have ured for Sr, Mn and Fe concentrations. Strontium was been selected, each being representative of the different separated by a standard chromatography method us- sectors into which Masetti et al. (2012) subdivided the ing Eichrom Sr resin, and the purified solution was main palaeogeographic units of the Eastern Southern dried at 100 °C and re-dissolved in 2 % HNO3 prior Alps. Study of these sections allowed the generation to the isotopic analysis. The total blank was < 2ng of a chemostratigraphic transect from Lake Garda to Sr. The 87Sr/86Sr measurements were performed on a the eastern Italian border (Fig. 2). The stratigraphic Nu Plasma multi-collector inductively coupled plasma sections selected are, from east to west (Figs 2, 3): mass spectrometer (Plasma 1), and the trace-metal Monte Verzegnis, located in the Belluno Basin; Monte abundances were measured on a Thermo-Finnigan in- Cumieli, located in the northern sector of the Friuli Plat- ductively coupled plasma mass spectrometer (Element form; Foza, in which the Rotzo Formation is missing II) at the University of Oxford. Both instruments were and the Mid Jurassic Rosso Ammonitico rests directly coupled with a membrane desolvating system (Aridus, on top of the Monte Zugna Formation; and Chizzola, Cetac) to achieve high signal sensitivity and stabil- representative of the central-western area of the Trento ity. For 87Sr/86Sr measurements, all isotopes (88Sr, 87Sr, Platform (with the Pliensbachian Rotzo Formation). 86Sr, 85Rb, 84Sr and 83Kr) were measured in static In all these sections, chemostratigraphic sampling has mode. To achieve maximum precision and accuracy,

Table 2. Summary of the principal characteristics of the four sections selected for the generation of a chemostratigraphic transect across the Eastern Southern Alps.

Stratigraphic sections

Palaeogeographic unit Monte Verzegnis Monte Cumieli Foza Chizzola

Belluno Basin Friuli Platform, northern Trento Platform, Trento Platform, area northeastern area central-western area Depositional Deep basin Marginal, oolitic shoal Marginal, oolitic shoal Tidal ﬂats behind oolitic environment shoals Prevailing Cherty mudstones Oolitic grainstones Oolitic grainstones, lesser Oolitic grainstones and lithologies and wackestones wackestones stromatolitic boundstones Thickness of che- 261/94 m 137/47.5 m Only high-resolution, Only high-resolution, mostratigraphic 73 m 118 m section, low/high resolution No of samples 444 274 324 508 Palaeontological Ammonites and Foraminifers and dasyclad Foraminifers and dasyclad Foraminifers and dasyclad assemblage nannofossils algae algae algae

the 88Sr signal was kept between 13 and 16 V, and water diagenesis, the primary isotopic signal of carbon a minimum of 40 isotope ratios were collected with should not change substantially because the amount 20-second integration time per ratio. 83Kr was mon- of carbon in diagenetic fluids is low, unlike the case itored to correct for the interference of 86Kr on 86Sr, with oxygen, whose primary isotopic signal can also and likewise 85Rb was monitored for 87Rb correction be deeply modified during burial with recrystalliza- on 87Sr. 83Kr intensity was generally consistent and be- tion at relatively high temperatures (Scholle & Arthur, low 0.2 mV,and 85Rb varied between samples but was 1980). generally lower than 2 mV.Samples were measured us- Isotopic cross-plots of the data show some differ- ing a standard bracketing method with the NIST SRM ences (Fig. 4). As would be expected, the relatively 987 standard. The instrument mass fractionation was deep-water section of Monte Verzegnis shows the corrected internally using 86Sr/88Sr = 0.1194. The ex- most clustered set of values, with δ18O mostly fall- ternal reproducibility of 87Sr/86Sr in SRM 987 showed ing between −2 and 0 ‰ and more variability taken a value of 0.710258 ± 0.000049 (2 s.d.) from August to up with the range in δ13C that reflects the pronounced September 2013. negative excursion (linear regression coefficient, R = 0.12). The shallow-water sections, potentially prone to greater diagenetic effects, show considerably more 3.a.2. Diagenesis versus palaeoceanography scatter, particularly in the case of Chizzola (R = 0.10). The diagenetic behaviour of carbon and oxygen iso- Addition of diagenetic calcite may be implied by some topes in shallow-water carbonates is problematic be- degree of correlation between carbon and oxygen (neg- cause such materials are particularly susceptible to ative diagenetic trend) that is particularly apparent in meteoric-water diagenesis. Such facies accumulate the data from Foza (R = 0.37) and particularly Monte close to sea level, small changes in which can lead Cumieli (R = 0.58), although the latter shows most to periodic emergence. The resultant diagenesis would overlap with Monte Verzegnis by virtue of some rel- typically introduce fluids with relatively low δ18O and atively greater carbon-isotope values that exceed 3 ‰. δ13C values derived from rainwater after its interaction However, global warming forced by addition of iso- with atmospheric carbon dioxide and humus-rich soils topically light carbon dioxide and/or methane to the (Hudson, 1977; Marshall, 1992). Typically, horizons af- ocean–atmosphere system would potentially produce fected by such processes would have scattered and relat- negative excursions in skeletal and inorganic carbon- ively low carbon- and oxygen-isotope ratios compared ates for both carbon and oxygen isotopes: phenom- to normal marine values. Other possible causes for de- ena totally unrelated to diagenesis (Luciani, Cobianchi viation of isotopic values from primary signals include & Jenkyns, 2004). When chemostratigraphic analysis variable quantities of skeletal grains exhibiting non- has been simultaneously performed on the same strati- equilibrium fractionation, different quantities of arag- graphic interval in both shallow- and deep-water Juras- onite and calcite in the original sediment (Swart, 2008), sic successions, in conjunction with available biostrati- and the presence of void-filling secondary, low-Mg cal- graphy (calcareous algae and benthic foraminifera cite in cavities opened in the supratidal environment in platform carbonates and ammonites in deep- (e.g. Grötsch, Billing & Vahrenkamp, 1998;Davey marine carbonates), the presence of similar carbon- & Jenkyns, 1999). Despite the possible modification isotope anomalies in all sections proves the primary of shallow-water carbonates introduced by meteoric- nature of these major oceanographic and carbon-cycle

Figure 4. (Colour online) Isotopic cross-plot (δ13Cv.δ18O) of key Lower Jurassic sections in the Southern Alps. The deeper water section (Monte Verzegnis) displays the most tightly clustered data; shallower water sections are more variable in their isotopic composition. Most carbon-isotope values fall between 1 and 3 ‰ and likely record a primary signature, albeit with a diagenetic overprint.

perturbations (Woodﬁne et al. 2008; Trecalli et al. the palaeogeographic point of view, it belongs to the 2012; Sabatino et al. 2013). Bulk carbonate δ13C values northeastern sector of the Belluno Basin (Fig. 2). of between 0 and 3 ‰ in all four sections analysed here are certainly compatible with normal marine values for the time interval and, since distinct negative excursions 3.b.1. Lithostratigraphy are developed, rather than randomly scattered data, it is The Lower Jurassic ﬁll of the Belluno Basin is made of assumed that the platform carbonates contain a primary thin-bedded, cherty mudstones and wackestones with isotopic record, albeit with a diagenetic overprint. peloids, radiolarians and sponge spicules representing the Soverzene Formation (Fig. 5). This unit has been interpreted as peri-platform ooze (cf. Schlager & James, 3.b. The Monte Verzegnis section 1978) in which pelagic material falling through the wa- This section, 261 m thick (Fig. 5), has been measured ter column has mixed with the carbonate mud supplied and sampled in the homonymous mountain group loc- from adjacent carbonate platforms (Zanferrari et al. ated in the Carnian Alps, not far from the small town of 2013). The Soverzene Formation is 200 m thick Tolmezzo. The section contains a little dolomite. From and lies atop the peritidal deposits of the Dachstein

Figure 5. (Colour online) Monte Verzegnis and Monte Cumieli sections. In the upper part of the deep-water Monte Verzegnis section there is an abrupt positive carbon-isotope excursion followed by a broad negative excursion over the likely stratigraphical range of Arnioceras. In the shallow-water Monte Cumieli section, the same two negative excursions are tentatively recognizable, the ﬁrst just above the top of the peritidal unit of the Monte Zugna Formation and the second in the calcarenitic unit of the same formation, separated by a thin micritic intercalation corresponding to a positive CIE. LO Paleomayncina termieri indicates its lowest occurrence identiﬁed in the section.

Limestone containing Upper Triassic megalodontids by a hard-ground surface coated with Fe–Mn oxyhy- and the foraminifer Triasina hantkeni, thus allowing droxide crusts, on top of which lies the Monte Verzegnis the time of the initial development of the Belluno Basin Encrinite, a condensed unit, 20 m thick, character- to be fixed as close to the Triassic–Jurassic boundary. ized by the intercalation of cross-bedded, red crinoidal In its uppermost portion, corresponding to a thickness calcarenites and red nodular limestones in facies of of 16 m (Fig. 5), the Soverzene Formation is enriched the Lower Rosso Ammonitico, commonly showing pe- in white chert that forms thick bands (up to 40 cm) in- culiar stromatolitic/thrombolitic structures similar to terbedded with thinner limestones. The rise in the silica those described by Jenkyns (1971a) from western Sicily content corresponds to an increase in the proportion of and Massari (1981) from the Trento Plateau. Further sponge spicules in the rock, likely indicative of the on- upwards, this unit passes into the Vajont Limestone, set of mesotrophic conditions in the Belluno Basin (cf. largely constituted by redeposited oolitic grainstones, Föllmi et al. 1994). recording a return to basinal conditions on top of the The Soverzene Formation passes upwards, through underlying shallower water deposits. an unconformable boundary, to a pinkish bi-directional cross-bedded calcarenitic unit, 20 m thick, present at the top of this formation in the whole area of the 3.b.2. Biostratigraphy Carnian and Julian Prealps. This unit is composed of The Soverzene Formation is referred in the literature to light grey grainstones with small superficial ooids in the Hettangian–Pliensbachian (Zanferrari et al. 2013). which radiolarians and sponge spicules are mixed with During the measuring of the section, a specimen of benthic foraminifera and crinoidal fragments, record- an ammonite, identified by F. Venturi (Perugia Univer- ing an ephemeral shallowing-upwards evolution exper- sity) as pertaining to the genus Arnioceras, was dis- ienced during this time by the Carnian Prealps area of covered in a debris cone fed from a small cliff located the Belluno Basin (Zanferrari et al. 2013). This cal- 20 m below the top of the Soverzene Formation. carenitic unit of the Soverzene Formation is truncated Other specimens of Arnioceras have been described by

only revealed broken specimens of Schizosphaerella spp. Other species recorded are Parhabdolithus robustus, Parhabdolithus liasicus distinctus and Mitrolithus jansae (Table 3). Despite the very poor nannofossil record (Fig. 7), such an assemblage is consistent with the early Sinemurian age provided by the Arnioceras because Parhabdolithus liasicus distinctus and Mitro- lithus jansae are reported to first occur in the bucklandi Zone and P. robustus in the turneri Zone (see com- pilation in Bown, 1987). The recorded assemblage is compatible with the JL2 nannofossil zone, which spans the bucklandi to the base of oxynotum ammonite zones (Bown, 1987). The crinoid-rich calcarenitic unit at the top of the Soverzene Formation is devoid of ammonites and nannofossils (Erba, pers. comm. 2011); a 87Sr/86Sr ratio from a belemnite rostrum collected a few cen- timetres below its upper boundary gave a value of 0.707196 ± 0.000042 (normalized against a value of 0.710250 for the NBS-987 standard) suggesting an age interval either spanning the middle Pliensbachian to the early Toarcian or the early Bajocian (reference curve in Jones, Jenkyns & Hesselbo, 1994; Jenkyns et al. 2002). This evidence suggests that this unit could be considered the more open-marine counterpart of the mid-upper portion of the Rotzo Formation in the Vene- tian Prealps and could be ascribed to the Pliensba- chian p.p. (Masetti et al. 1998; Posenato & Masetti, 2012). In the Monte Verzegnis Encrinite, referred in the literature to the Toarcian p.p. – Bajocian p.p. (Pi- ano & Carulli, 2002), some ammonite specimens were collected in the lower part of the formation. Among these, G. Pavia (Turin University) identified Teloceras cf. triptolemus (Buckman) and Holophylloceras sp. ammonites, both referable to the early Bajocian. A 87Sr/86Sr determination for a belemnite collected in the lowermost part of the Verzegnis Encrinite gave a Figure 6. Ammonite stratigraphy of the Sinemurian Stage. Mod- ± ified from Dommergues, Ferretti & Meister (1994). value of 0.707062 0.000042 (normalized against a value of 0.710250 for the NBS-987 standard) indicating either the Pliensbachian–Toarcian boundary or a Piano & Carulli (2002), albeit from a poorly defined time interval spanning from the early Bajocian to the upper part of the formation. The Arnioceras genus has early Bathonian. Taken together, the data confirm an a distribution that embraces the semicostatum, turn- early Bajocian age for the lower portion of the unit. No eri, obtusum and, possibly, oxynotum ammonite zones ammonites have been found in the upper part of the (transition from the lower to the upper Sinemurian; Monte Verzegnis Encrinite and, by analogy with the Dommergues, Ferretti & Meister, 1994; Fig. 6), here similar Rosso Ammonitico cropping out in other areas informally called ‘Arnioceras Time’. The finding of of the Eastern Southern Alps (Martire, 2007), it likely the Arnioceras 20 m below the top of the unit indicates corresponds to the Bathonian–lower Callovian. deposition of the main part of the Soverzene Forma- tion, at least in this part of the Belluno Basin, during 3.b.3. Summary of original biostratigraphic data the Hettangian–Sinemurian interval. A section interval, 70 m thick (150–220 m, Fig. 5) and corresponding (1) In the Verzegnis section, the deep-water micritic to the upper portion of the unit from which the Arnio- unit of the Soverzene Formation, chert-rich unit in- ceras specimen comes, has been sampled for calcareous cluded, corresponds to peri-platform ooze delivered to nannofossil analysis. Smear slides (Bown & Young, the basin during Hettangian–Sinemurian p.p. time, up 1998) were thoroughly studied for 13 samples under to and including the so-called ‘Arnioceras Time’. New an optical microscope with polarized light at × 1000. calcareous nannofossil data are consistent with this age. Seven of the 13 samples bear very rare and poorly (2) 87Sr/86Sr data suggest that the calcarenitic unit preserved calcareous nannofossils; one other sample at the top of the Soverzene Formation belongs to the

Table 3. Distribution chart of calcareous nannofossils of the Monte Verzegnis section. For each slide, four transverses were studied (3, 2 cm each) under an optical microscope, polarized light at × 1000. Preservation was estimated according to Roth (1983).

Parhabdolithus Schizosphaerella Parhabdolithus liasicus Mitrolithus Sample Metre Preservation spp. robustus distinctus jansae

VC 8 216 B . . . . VC 5 212 O3 • .. . VC 3 211 O3 ••. • VC 1 210 B . . . . VD 16 203 B . . . . VD 14 201 B . . . . VD 11 199,5 B . . . . VD 5 187 B . . . . VD 3 183 B . . . . VB 49 179,5 O3 • . •• VB 47 178,5 O3 • .. . VB 46 178 O3 • . •• VB 36 150 F # . . .

• – present; # – fragment (F); B – barren sample; O3 – overgrown nannofossils.

Figure 7. Images of calcareous nannofossils recorded in the Monte Verzegnis section. White scale bar = 5 µm. In spite of poor preservation and important calcite overgrowth on coccoliths (three bottom pictures), diagnostic characters are still present.

mid–upper Pliensbachian, and it corresponds to the resolution interval (20 cm/sample) goes from 146 to mid-upper portion of the Rotzo Formation in the Vene- 240 m. The remaining under- and overlying segments tian Prealps, overlying the lower part of the same have been sampled with lower resolution (2 m/sample). formation with a hiatus corresponding to the upper Through the entire section, the carbon-isotope values Sinemurian – lower Pliensbachian; and the Monte mostly fluctuate between 1.5 ‰ and 3.5 ‰. In the Verzegnis Encrinite was deposited during early Ba- lowest 60 m, the curve mostly ranges between 2 ‰ jocian – early Callovian time and lies unconformably and 2.7 ‰: the relatively low resolution of the profile on top of the Soverzene Formation with a hiatus cor- prevents reliable identification of the negative peaks responding to the Toarcian–Aalenian interval. located at the T/J boundary. Stratigraphically higher, between 60 and 80 m, including an unexposed interval of 11 m, the curve shifts towards consistent values δ13 3.b.4. The Ccurve around 2.5 ‰. Up to about 160 m, the curve fluctu- A total of 444 samples have been analysed from a ates around a value of 2.5 ‰, with a pronounced pos- stratigraphic section 261 m thick (Fig. 5). The high- itive shift (to 3 ‰ around 114 m). A small (0.5 ‰)

negative followed by a small (0.5 ‰) positive excursion characterizes the 140–160 m interval. From here on up, the proﬁle describes a symmetrical oscillation that arrives at a minimum of 1.4 ‰ at 196 m then rises up to 3.6 ‰ at 236 m. This spectacular and symmetric oscillation of the carbon-isotope curve corresponds to about 60 m of section and is completely contained within the Soverzene Formation. The lowest value is located close to the cliff where the Arnioceras specimen was found, just at the base of the chert-rich unit located at the top of the typical micritic facies of the Soverzene Formation. At 237–238 m the carbon- isotope curve moves relatively abruptly to lower values ( 2.6 ‰) just below the unconformable boundary between the calcarenitic unit of the Soverzene Forma- tion and the Monte Verzegnis Encrinite before rising to > 3.5 ‰ at the base of this latter unit.

3.c. The Monte Cumieli section The Monte Cumieli section (Fig. 5) crops out in the Carnian Prealps, not far from the small town of Ge- mona, is 137 m thick, and exemplifies the Jurassic succession of the northern sector of the Friuli Platform, characterized by the early demise of the Early Juras- sic carbonate platform of the Monte Zugna Formation (Fig. 2) and by the lack of shallow-water carbonates younger than Sinemurian (Zanferrari et al. 2013). Figure 8. Microfacies from the Foza and Cumieli sections. (a) Micritic ooids showing thinly laminated outer tangential cortices, associated with Palaeodasycladus mediterraneus, visible 3.c.1. Lithostratigraphy in a transversal section in the lower part of the thin-section. The Monte Zugna Formation is further subdivided, as Lower part of the calcarenitic unit of the Monte Zugna Forma- tion, Foza section, scale bar = 1 mm; (b) radial-fibrous ooids in illustrated in Figure 5, into a lower peritidal and an which crystalline cortices are penetrated by dark microborings. upper calcarenitic unit. The lower unit is 87 m thick Everticyclammina praevirguliana at the nucleus of the largest and is composed of peritidal cycles representing the ooid. Upper part of the calcarenitic unit of the Monte Zugna Jurassic continuation of the depositional theme of the Formation, Cumieli section, scale as in (a). underlying Upper Triassic Dolomia Principale. The calcarenitic unit is 33 m thick and comprises metre-scale beds of oolitic grainstones that become progressively the oolitic cortex (Everticyclammina praevirguliana, richer upwards in echinoderm debris suggesting in- Fig. 8b). creasing open-marine influence (e.g. Jenkyns, 1971b). The Monte Zugna Formation is truncated by a dis- This calcarenitic body is interpreted as a subtidal shoal conformity surface on top of which lie the grey, loc- largely controlled by storm waves whose activity is re- ally reddish, cross-bedded crinoidal calcarenites of corded by plane-parallel lamination. Throughout the the Fanes Encrinite interpreted as sand-waves (Zan- entire unit, the ooids exhibit some degree of concentric ferrari et al. 2013). Resedimented deposits of the structure; however, in the lower part of the unit, the Vajont Limestone represent the youngest unit crop- cortex is dominantly micritic, in some cases with outer ping out in the Monte Cumieli section and record a thinly laminated tangential oriented crystals (Fig. 8a). deepening-upwards evolution of the northern sector of These micritic ooids are associated with oncoids, the Friuli Platform, which, during Middle Jurassic time, dasyclad algae (Palaeodasycladus mediterraneus, Pa- foundered to become effectively part of the Belluno laeodasycladus gracilis) and foraminifers (Aeolisaccus Basin where it received oolitic turbidites derived from dunningtoni, Siphovalvulina spp., Everticyclammina the southern portion of the same platform that per- praevirguliana). The micritic ooids are replaced up- sisted as a productive shallow-water carbonate source section by radial-fibrous ooids whose structure is inter- (Masetti et al. 2012; Zanferrari et al. 2013). rupted by dark microborings (Fig. 8b). The palaeontological assemblage associated with these radial-fibrous 3.c.2. Biostratigraphy ooids is characterized by foraminifera with a complex wall structure (Paleomayncina termieri, Tersella gen- The shallow-water assemblage recorded in the Monte otii, Rectocyclammina sp.), locally acting as nuclei for Cumieli section accords with the classic successions

of the Alpine–Mediterranean Tethys as documented in to the upper part of the LB1 and the lower part of the the Central Apennines and Southern Apennines (De LB2 assemblages sensu Barattolo & Romano (2005). Castro, 1991; Chiocchini et al. 1994, 2008), West- The biotic assemblage of the Monte Zugna Form- ern Croatia (Velic,´ 2007) and Morocco (Septfontaine, ation in the Monte Cumieli section is representative 1984, 1985). Barattolo & Romano (2005) recognized of typical shallow-water faunas in Tethyan successions the following four shallow-water carbonate-platform (Velic,´ 2007; Chiocchini et al. 2008), among which the assemblages as pertaining to the Upper Triassic – algae supply the most reliable indications of palaeoen- Lower Jurassic: vironment and water depth. In particular, the green al- (1) Algal and foraminiferal Triassic assemblage (TA gae Dasycladales are considered environment sensitive, assemblage, uppermost Rhaetian) characterized by the typically represented in the Lower Jurassic by taxa such occurrence of dasycladaleans, Griphoporella curvata as Palaeodasycladus mediterraneus, Palaeodasycladus (Gümbel) and Gyroporella vesiculifera Gümbel, in- gracilis and Tersella genotii. These green algae are volutinid foraminifera (essentially Aulotortus and Tri- thought to be characteristic of infralittoral, mostly trop- asina) and oncoids. The macrofauna is composed of ical to subtropical waters (Berger & Kaever, 1992). Ex- the large shells of megalodontid bivalves. Foraminifera tant species of Dasycladales, like Acetabularia, com- and megalodontids become more common up-section, monly grow in shallow protected lagoons and on roots whereas dasycladaleans become rare or are missing al- and shells in mangrove swamps; Dasycladales populate together. relatively quiet lagoonal floors, typically less than 10 m (2) Thaumatoporella and Aeolisaccus dunningtoni deep, and they commonly occur in tidal pools and pro- assemblage (LA assemblage, lowermost Hettangian – tected microenvironments within the barrier reef (e.g. upper Hettangian) characterized by the exclusive occur- Valet, 1979). With respect to the Lower Jurassic dasy- rence of Thaumatoporella and Aeolisaccus dunningtoni cladaleans, Tersella genotii is recorded from the Gran Elliott, mainly in the lower part of its range. Small si- Sasso area (Central Italy) in bioclastic sands interpreted phonous valvulinid foraminifera are rather rare, but up- as shoals located near to the platform margin (Barattolo section they become more common. Rare gastropods, & Bigozzi, 1996). In the Middle Atlas of Morocco, the bivalves and corals may also occur. Oncolitic coatings same species is found fossilized in situ in reef facies, on grains are common. whereas in the High Atlas it is found as mechanic- (3) Lower Jurassic dasycladalean assemblage (LB ally sorted debris in oolitic grainstones (Barattolo et al. assemblage, upper Hettangian – upper Sinemurian) 2008). In the Monte Cumieli and Foza sections, Tersella characterized by the occurrence of a variety of Liassic genotii occurs as fine debris in oolitic grainstones or species of dasycladaleans. The most representative as more complete specimens in packstones, thus sug- genera are Palaeodasycladus, Fanesella, Sestrosphera gesting a moderately turbulent lagoonal environment. and Tersella. Taxa of the previous LA assemblage con- In summary, the Palaeodasycladus–Thaumatoporella tinue into the base of the LB assemblage, which can algal assemblage is interpreted as characteristic of a be subdivided into lower (LB1) and upper (LB2) sub- subtidal setting with quiet to moderately agitated water, assemblages. In LB1, Liassic dasycladaleans are not presumably representing an open lagoon environment abundant and LA microfossils are still important. In (Barattolo & Bigozzi, 1996). LB2, dasycladaleans become dominant and large fo- The palaeontological assemblage of the overlying raminifera appear (e.g. Paleomayncina). Fanes Piccola Encrinite is characterized by the dom- (4) Large foraminifer assemblage (LC assemblage, inance of foraminifers (Involutina liassica, Agerella upper Sinemurian – upper Pliensbachian) marked by a martana, Lenticulina, Frondicularia, Ophtalmididae dominance of dasycladaleans, but with a relatively low and Nodosaridae) of little reliable stratigraphic signi- diversity. Larger foraminifera with a complex internal ficance. Zanferrari et al. (2013) interpreted this unit as skeleton are abundant, the most widespread genera be- the result of discrete, and virtually instantaneous, de- ing Orbitopsella, Lituosepta, Amijiella and Haurania. position of sand-waves bounded by long-lasting peri- The background fauna is always composed of an LA ods of non-deposition that overall occurred between assemblage. the age of the earliest ‘Arnioceras Time’ and the age The Monte Zugna Formation in the Monte Cumieli (Bajocian–Bathonian) of the base of the overlying Va- section is dominated by foraminifers (Aeolisac- jont Limestone. cus dunningtoni, Siphovalvulina spp., Meandrovoluta asiagoensis, Everticyclammina praevirguliana) and 3.c.3. The δ13Ccurve algae (Thaumatoporella parvovesiculifera, Palaeo- dasycladus mediterraneus, Palaeodasycladus gracilis, A total of 274 samples have been analysed from a 137 m Tersella genotii and Cayeuxia-like briopsidales); the thick stratigraphic section (Fig. 5). The first segment of calcarenitic unit records the first occurrence of Paleo- the section, 55 m thick, has been sampled at relatively mayncina termieri and an enrichment of foraminifera low resolution (2 m/sample), the second, 47.5 m thick, with a complex wall structure. According to Zanfer- extending up to the top, at a higher resolution (one rari et al. (2013), the Monte Zugna Formation has been sample every 20 cm). Within the section, the carbon- assigned a generic Hettangian–Sinemurian age; the mi- isotope values fluctuate between 2.9 ‰ (90 m) and cropalaeontological content of the section corresponds 0.4 ‰ (107 m). Up to 43 m from the base, within the

peritidal unit of the Monte Zugna Formation, the profile dasycladus sp.) and the foraminifer Everticyclammina evolves vertically with symmetric oscillations centred praevirguliana. In addition to this, the calcarenitic around a value of 2 ‰. Higher in the stratigraphy, ex- unit contains Paleomayncina termieri, Terquemella sp. tending up to 90 m, the curve moves to generally (dasycladalean calcified reproductive organs) and the more positive values (maximum value in the section: coprolite Favreina sp. The first occurrence of Paleo- 3 ‰) with a trend interrupted by a couple of negative mayncina termieri falls in the middle of the calcarenitic shifts (at 67 m and 84 m). This positive fluctuation of unit, 20 m below the unconformity. The Monte the curve corresponds to a thin intercalation of micritic Zugna Formation has been assigned to the Hettangian– deposits located in the upper part of the peritidal unit Sinemurian interval (Romano, Barattolo & Masetti, and to the first few metres of the calcarenitic unit of the 2005). The micropalaeontological assemblage of the Monte Zugna Formation (Fig. 5). The calcarenitic unit, Foza section, like that of the Monte Cumieli section, starting from the first occurrence of Paleomayncina corresponds to the upper part of the LB1 and the lower termieri, records a clear negative excursion in which part of the LB2 assemblages sensu Barattolo & Romano values shift from 2.9 ‰ (at 90 m) to 0.4 ‰ (at (2005). From a palaeoenvironmental point of view, the 107 m); higher in the section, the curve returns to- section is similar to that of Cumieli. In addition, the wards more positive values ( 1.7 ‰)upto 120 m, occurrence of Sestrosphaera liasina is recorded here. at the level of the unconformity between the Monte According to Romano & Barattolo (2009), this alga Zugna Formation and the Fanes Piccola Encrinite. most likely populated protected microhabitats at the boundary between the platform margin and the lagoon. The unconformity surface is covered by the Rosso Am- 3.d. The Foza section monitico Inferiore whose stratigraphical extent in the The Foza section (Fig. 9) is located in the eastern sec- Trento Plateau is referred to the upper Bajocian – lower tor of the Asiago Plateau (Venetian Prealps), is 73 m Callovian (Martire, 2007). thick, and has been sampled along the road connecting the small towns of Valstagna, located in Valsugana, to Foza, just below this last village. The Foza section 3.d.3. The δ13Ccurve exemplifies the Jurassic succession of the northeast- A total of 324 samples have been analysed from a 73 m ern area of the Trento Platform, characterized by the thick stratigraphic section (Fig. 9). The δ13C values, lack of Pliensbachian shallow-water carbonates (Rotzo which are highly scattered, mostly fall between −2 and Formation) and hence the early demise of the Early 2 ‰. Overall, the curve can be split into three minor Jurassic carbonate platform of the Monte Zugna Form- negative excursions separated by positive rebounds. ation (Figs 2, 3). The stratigraphically lowest negative excursion corresponds to the 0–16 m segment of the section, reaches a 3.d.1. Lithostratigraphy minimum value of −0.9 ‰ (at 10.6 m) and returns to a value of 2 ‰ (at 15.8 m); the second reaches The Foza section has been entirely sampled inside the −2.40 ‰ at 32 m from the base (not shown in fig- Monte Zugna Formation, up to its upper boundary with ure) then moves rather abruptly in a positive sense to- the Lower Rosso Ammonitico; in the shallow-water wards a value of 2.45 ‰ at the level of a bivalve bed carbonates of the Monte Zugna Formation the peritidal (Gervillia) located at about 40 m from the base of the features are less marked than elsewhere, but the depos- section, close to the lower boundary of the calcarenitic itional environment may be interpreted as the internal, unit of the Monte Zugna Formation. The third, relat- slack-water and muddy sector of the carbonate plat- ively broad negative excursion starts from a value of form (Romano, Barattolo & Masetti, 2005). As with 2.5 ‰ (highest value in the section: 42.5 m), then the Monte Cumieli section, a calcarenitic unit, 33 m falls abruptly, following the calcarenitic unit and the thick, is superimposed on the lower, mainly micritic stratigraphical distribution of Paleomayncina termieri, unit of the formation (Fig. 9); this granular unit is split to reach a minimum of −1.4 ‰ (56 m) and returns to into two parts by intercalated fine-grained deposits of 2.5 ‰ at the boundary with the Rosso Ammonitico. lagoonal character containing the bivalve Gervillia buchi (Fig. 9). Here the ooids also exhibit a stratigraphic evolution in which the micrite-coated grains with thinly 3.e. The Chizzola section laminated tangential cortices (Fig. 8a), present at the base, are replaced up-section by radial-fibrous ooids This section (Fig. 9) represents the central-western (Fig. 8b). areas of the Trento Platform (Fig. 2) in which the Pli- ensbachian shallow-water unit (Rotzo Formation) lies on top of the Monte Zugna Formation. Located in the 3.d.2. Biostratigraphy Adige Valley, the Chizzola section has been sampled The micropalaeontological content is similar to that along the road connecting the villages of Chizzola and present in the Monte Cumieli section. The micritic Mori. The top of the section, corresponding to the up- unit is characterized by dasycladaleans (Palaeodasy- per half of the Loppio Oolitic Limestone, has been cladus mediterraneus, Sestrosphaera liasina and Eo- measured near Nomi village.

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 CIE in the Sinemurian of the Southern Alps 639 Bed, but outcrop failure does not allow e readily discernible. The stratigraphically lowest Gervillia he Bed crops out, and the stratigraphically higher negative excursion in the Gervillia . Fig. 5 negative excursion is present just above the base ofgeneration the of data calcarenitic from unit stratigraphically of lower the strata. Legend Monte as Zugna in Formation where the Figure 9. (Colour online) Foza and Chizzola sections. In the Foza section the same two negative excursions described in the Monte Verzegnis section ar calcarenitic unit of the same formation. In the Chizzola section, the same isotopic pattern is repeated with the positive excursion coincident with t

3.e.1. Lithostratigraphy olution of 20 cm. The entire carbon-isotope curve fluc- tuates between values of −1.6 ‰ ( 4.5 m) and The Chizzola section exposes the upper portion of the 2.4 ‰ ( 25 m), with values being particularly Monte Zugna Formation (92 m) and the whole thick- scattered in the basal 20 m. The curve illustrates a pos- ness of the Loppio Oolitic Limestone (32 m). The first itive excursion ( 2.4 ‰) at 7.5 m, corresponding to unit is further subdivided into a peritidal calcarenitic the Gervillia Bed, declining abruptly to a relative min- unit (25 m) in which the peritidal cycles are made of imum at 13 m, before moving again to more positive cross-bedded, subtidal oolitic calcarenites (or, in one values that reach a maximum value of 2.4 ‰ at 25 m. case, micrites with Gervillia, Fig. 9) passing upwards in These positive–negative oscillations are entirely con- the cycle to inter–supratidal stromatolites, locally with tained within the calcarenitic unit of the Monte Zugna dinosaur tracks (Avanzini et al. 1997), and an upper, Formation, here characterized by peritidal features. On subtidal, nodular unit (67 m) cut by a neptunian dyke the basis of the chemostratigraphic correlation, it is filled with oolites derived from the overlying Loppio possible to correlate the Gervillia beds of the Chizzola Oolitic Limestone. The occurrence of peritidal facies in and Foza sections, located at the two opposite edges the calcarenitic unit of the Monte Zugna Formation is a of the Trento areas, across the entire platform, and peculiar feature of the Adige Valley, and the sediments even correlate the same horizons with the micritic unit, have been interpreted by Masetti et al. (1998) as depos- devoid of Gervillia, cropping out in the Cumieli section ited inside small tidal flats, developed behind marginal (Friuli Platform) corresponding to the same positive shoals, that were retrograding towards the inner part of excursion (Fig. 10). Above these excursions and ex- the Trento Platform during a transgressive phase. The tending for about 60 m up to 84 m, within the upper nodular subtidal units are interpreted as representing a portion of the Monte Zugna Formation in which per- deepening phase of the topmost portion of the Monte itidal structures are missing, the profile displays another Zugna Formation, which is missing in the other sec- broad negative excursion, centred around the neptunian tions; the Loppio Oolitic Limestone is an oolitic body dyke and reaching the lowest value of 0.1 ‰ at interposed between the Monte Zugna Formation and 53 m; positive indentations are present around 35 m the Rotzo Formation that spread across the main por- and 44 m. The top of the subtidal, mainly micritic unit tion of the underlying unit during a sea-level rise that of the Monte Zugna Formation, from 84 to 92 m ( pushed the marginal oolitic bars composing this unit 1.6 ‰), corresponds to a third negative excursion (min- from west to east across the Trento Platform (Fig. 3; imum value of 0.8 ‰ at 89 m) between two positive Masetti et al. 1998). excursions. The curve becomes more stable in the remaining part of the unit, fluctuating around an average ‰ 3.e.2. Biostratigraphy value of 1.3 , with a minor positive excursion around 114 m. The micropalaeontological content and the palaeoen- vironments of the Monte Zugna Formation are similar to those described in the two previous sections. As regards the Loppio Oolitic Limestone, the lack 3.f. Correlations of δ13C curves across the Eastern of fauna of significant value precludes reliable strati- Southern Alps and their comparison with coeval anomalies graphic attribution and, by analogy with other areas The proposed correlation of the above-described an- in the Eastern Southern Alps, it is generically re- omalies of the δ13C curves across the entire Eastern ferred to the Sinemurian Stage, although Franceschi Southern Alps, from Lake Garda to the eastern Italian et al. (2014) recognized the Sinemurian–Pliensbachian border, is illustrated by the grey band in Figure 10. Li- carbon-isotope boundary event in the uppermost por- thologies in which the carbon-isotope excursion (CIE) tion of the unit. The stratigraphic setting of the Rotzo has been recorded include the cherty micrites in the Formation is still a matter of debate: either from the basinal succession of the Monte Verzegnis section and upper Sinemurian to the upper Pliensbachian, if as- the oolitic grainstones locally interlayered with micrites signed on the basis of foraminiferal biostratigraphy of the shallow-water facies (Table 2). The excellent (Fugagnoli, 2004), or from the lower Pliensbachian matching between the single curves allows recognition, to the upper Pliensbachian, on the basis of ammon- both in the shallow- and deep-water units, of a dis- ite biostratigraphy (Posenato & Masetti, 2012). The tinct abrupt positive followed by broader negative CIE sharp bounding surface between the Loppio Oolitic located just below an unconformity surface (Fig. 3). Limestone and the Rotzo Formation, locally encrusted The primary origin of these excursions is supported with red ferruginous coatings, suggests that the contact by the following observations: all the coeval segments between these two units represents a regionally extens- sampled in different sections exhibit the same CIEs ive unconformity. with similar geometry and extending over similar stratigraphic thicknesses; all the curves are characterized by well-defined trends and not by single peaks that might 3.e.3. The δ13Ccurve represent diagenetic artefacts; the curves conform with A total of 508 samples have been analysed from a 118 m the stratigraphic occurrence of key faunal datum levels; thick stratigraphic section (Fig. 9) with a sampling res- the curves conform with similar facies developments.

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 CIE in the Sinemurian of the Southern Alps 641 tones and ws recognition, both in the shallow- Zone: this level records the first appearance of the oured grey in the figure, defines the major negative ates the correlation of the above-described anomalies turneri . Fig. 5 . Recognition and proposed zonal attribution of key carbon-isotope excursions in Monte Verzegnis derives from data from well-dated Sinemurian muds ). Legend as in Fig. 11 Paleomayncina termieri C curves across the entire Eastern Southern Alps, from Lake Garda to the eastern Italian border. The excellent matching between the single curves allo 13 δ and deep-water units, of theexcursion, abrupt and positive its followed lower by boundary broader negative (abrupt carbon-isotope positive excursion excursion)shales described from can in the be the UK text. correlated ( The with correlation the belt, peak col of the excursion to heavier values in the foraminifera of the Figure 10. (Colour online) Chemostratigraphic transect through the Eastern Southern Alps with pertinent biostratigraphic data. The figure illustr

In the basinal Monte Verzegnis section, the CIE in abundance of the thermophilic pollen Classopol- spans a stratigraphic thickness of 60–70 m (depend- lis classoides (Fig. 11), suggesting the occurrence of a ing on chosen baseline), referable, thanks to the finding warming event. of a specimen of Arnioceras, to a time interval ran- In Morocco, carbonate isotopic data from Lower Jur- ging from the base of the semicostatum, through the assic peritidal platform carbonates show a well-defined turneri to the top of the obtusum zones (Dommergues, negative excursion attributed to an early Sinemurian Ferretti & Meister, 1994; Fig. 6) and thus likely cor- age and a positive excursion in overlying open-marine responding to an interval of 2–3 Ma (Gradstein, Ogg sediments whose basal levels contain the ammonite & Schmitz, 2012). The same negative CIE is clearly Arnioceras and are placed in the upper Sinemurian recognizable, with a similar geometry, in the two pa- (Wilmsen & Neuweiler, 2008). In England and other laeogeographic domains situated either side of the Bel- parts of northern Europe, Arnioceras ranges from the luno Basin, namely in the Monte Cumieli section (Friuli bucklandi Zone of the basal Sinemurian into the ob- Platform) and the Foza section (northern sector of the tusum Zone (Page, 2010), in good correspondence with Trento Platform). Although values are scattered, this the distribution proposed by Dommergues, Ferretti & negative CIE signal, which is entirely contained within Meister (2004; Fig. 6). If high-resolution data from all the calcarenitic unit at the top of the Monte Zugna these sections are taken as a guide (Figs 5, 9, 10), it Formation (thickness 30 m), is not only preceded by seems that there are two possible negative excursions a positive excursion but also interrupted by positive in- over the likely stratigraphical range of Arnioceras. Con- dentations. Significantly, the positive to negative shift centrating on positive excursions, the most pronounced at the base of the major negative CIE correlates with the of which in the Dorset profile is in the upper part of first occurrence of the foraminifer Paleomayncina ter- the turneri Zone (Fig. 11), gives the suggested zonal mieri in both the Foza and Monte Cumieli sections. The equivalence in the Verzegnis profile as suggested in isotopic correlation with strata containing Arnioceras Figure 10. A negative trend covers the interval attrib- allows this foraminifer, once attributed a poorly defined uted to the semicostatum Zone and possibly some of the Sinemurian age, to be linked to the transition from the turneri Zone, whose defining feature, however, is a pos- lower to the upper part of the stage. In the Chizzola sec- itive excursion, albeit relatively small in the Verzegnis tion, located on the other side of the Trento Platform, profile. Following the turneri Zone positive excursion, this negative CIE is contained within the c. 25 m thick there is a well-defined negative excursion attributed to calcarenitic unit of the Monte Zugna Formation. the obtusum Zone, likely extending into the oxynotum Most Jurassic chemostratigraphical studies to date Zone, followed by near-symmetrical recovery to higher have been mainly focused on the Pliensbachian– values: a pattern matching that in the borehole ma- Toarcian interval; other stages are less well defined. terial in eastern England (Fig. 11). In the platform- In the Hettangian–Sinemurian interval, Jenkyns et al. carbonate sections, the abrupt positive excursions seen (2002) recorded carbon-isotope ratios from belem- in Chizzola, Foza and Monte Cumieli can thus be tent- nites and oyster shells from Portugal and Eng- atively referred to the turneri Zone and the ensuing land indicating a positive excursion in the lower negative excursion to the obtusum and oxynotum zones. Sinemurian followed by a negative excursion at the Sinemurian–Pliensbachian boundary. More detail is 4. Possible causes of the ‘Arnioceras Time’ negative given by Jenkyns & Weedon (2013), who illustrated δ13C excursion a high-resolution organic-carbon-isotope curve from Sinemurian black shales cropping out in the Wessex The ‘Arnioceras Time’ broad negative δ13C excursion Basin (Dorset, UK). These data show a negative excur- of 1.0 ‰ recorded in the Soverzene Formation of sion likely centred around the boundary of the semicost- the Monte Verzegnis section is well defined and ex- atum and turneri zones, a positive excursion extending tends over a thickness of 60 m. Unlike the negative through the upper turneri Zone before a fall at the close excursion that characterizes the lower Toarcian inter- of that zone into the lower part of the obtusum Zone, val, which is abrupt with a clear stepped profile in followed by a rise before the continuity of the section all sections (Jenkyns et al. 2002;Kempet al. 2005; is interrupted by a hiatus (Fig. 11). This positive turn- Hesselbo et al. 2007; Sabatino et al. 2009; Hermoso eri Zone excursion has been recorded also by Porter et al. 2012), the excursion at Monte Verzegnis appears et al. (2014) in marine sediments from North America more gradual with values dropping from 2.5 ‰ to (British Columbia, Canada) and manifestly represents 1.5 ‰. If values of 2.5 ‰, which characterize the a global marine carbon-isotope signature. Riding et al. lower parts of the section, are taken as background, (2012) studied a section from a borehole in eastern Eng- the negative shift may record introduction of isotop- land and documented a marked negative excursion in ically light carbon into the ocean–atmosphere system a δ13C curve from wood and palynomorphs centred in through oxidation of a formerly buried reservoir such the oxynotum Zone (lower part of the upper Sinemurian, as sub-seafloor clathrates or organic-rich sediments. Fig. 6), an interval that is missing in the Dorset profile. Alternatively, a reduction in the amount of global bio- This negative excursion, registered in both marine and mass or organic carbon buried in response to environ- terrestrial carbon (thereby indicating a response in both mental change could have caused movement to lower atmosphere and oceans) was coupled with an increase carbon-isotope values. Large Igneous Province (LIP)

Figure 11. Left-hand diagram illustrates the Copper Hill Borehole (eastern England) studied by Riding et al.(2012) and illustrating an increase of the dinoﬂagellate cyst Liasidium variabile and the thermophilic pollen grain Classopollis classoides in ammonite-bearing mudstones, suggesting the occurrence of a warming event coincident with a negative carbon-isotope excursion in marine and terrestrial organic matter largely corresponding to the obtusum and oxynotum zones of the Sinemurian Stage. The presence of the negative excursion in terrestrial pollen indicates that the atmosphere as well as the ocean was affected by the disturbance in the carbon cycle. Scale in metres is depth below surface. semicost.–semicostatum Zone; ob. – obtusum Zone; oxynot.–oxynotum Zone. Right-hand diagram illustrates high-resolution organic carbon-isotope stratigraphy of Sinemurian ammonite-bearing black shales from Dorset, southern England (Jenkyns & Weedon, 2013). A positive excursion is characteristic of the turneri Zone followed by a negative excursion in the lower part of the obtusum Zone; the upper part of the obtusum Zone and the whole of the oxynotum Zone are lost to a hiatus. Grey band illustrates the position of the principal negative carbon-isotope excursion, only partly developed in Dorset. Depth is given in metres below the top of the Black Ven Marls. semi. – semicostatum Zone; res. – resupinatum Subzone; obtu. – obtusum Subzone; rari. – raricostatoides Subzone.

volcanism cannot be invoked, because there are no isotopically light 12C into the atmosphere could have known provinces of this age (Courtillot & Renne, been played by syn-sedimentary tectonics that caused 2003): the main activity phase of the Central Atlantic fracturing and leakage of gas-hydrate reservoirs (cf. Magmatic Province (CAMP) occurred around 200 Ma Jenkyns, 2003). The negative excursion of ‘Arnioceras and terminated during Hettangian time (Marzoli et al. Time’ occurred coincidently with the reactivation of the 1999). rifting activity that affected large sectors of the Tethyan If isotopically light CO2 were to have been intro- areas from the Southern Alps, Apennines and Sicily to duced into the atmosphere, resultant global warm- the Betic Cordillera (Spain) and Moroccan High Atlas ing would have likely led in turn to acceleration of (Masetti et al. 1998; Ruiz-Ortiz et al. 2004; Marino the hydrological cycle and to the increase of global & Santantonio, 2010; Merino-Tomé et al. 2012). This weathering rates. Increased quantities of nutrients and coincidence in timing could explain why many authors fine-grained continental sediments delivered to oceans have ascribed local to regional drowning of carbonate would have favoured both increased organic productiv- platforms primarily to tectonic causes affecting subsid- ity and formation of salinity-stratified water bodies ence rate rather than climatic influence on the sediment leading to the onset of eutrophic conditions and de- factory itself. position of organic-rich clays and clay-rich limestones. No such markers of wet and humid conditions during δ13 the CIE have been so far identified in the Southern 5. Impact of the ‘Arnioceras time’ negative C Alps during ‘Arnioceras Time’, but a coeval warm- excursion on carbonate sedimentation ing event has been recognized in southern England The inferred position of the ‘Arnioceras Time’ negat- by Riding et al. (2012), based on palynological data ive δ13C excursion within the Lower Jurassic succession and extending over the obtusum and oxynotum zones along the transect crossing the Eastern Southern Alps (Fig. 11). A possible important role in the release of the is shown in Figures 3 and 10. In the Belluno Basin,

the negative carbon-isotope anomaly corresponds with which were located in many other areas around the the upper part of the lower micritic unit of the Sov- Mediterranean (e.g. Western Southern Alps, Ligurian erzene Formation, with the lowest values located just Alps, Northern Apennines, western and eastern Sicily, below the base of the chert-rich unit at the top of the and High Atlas). same formation (Figs 5, 9, 10). In the shallow-water Previous authors have postulated an interaction carbonates, both in the Trento and Friuli platforms, the between tectonic and eustatic processes as the most anomaly begins at the top of the lower, peritidal unit of viable mechanism able to explain the transgressive the Monte Zugna Formation, is interrupted by a short phase that occurred at the boundary between early positive δ13C pulse, and culminates in the calcarenitic and late Sinemurian time. Tectonic and/or eustatic pro- unit at the top of the same formation. cesses were probably important, given the significant The chert-rich unit of the Belluno Basin (Fig. 5, rise in the semicostatum Zone of the putative eustatic 16 m thick) correlates with an appreciable increase sea-level curve of Haq, Hardenbol & Vail (1988) and in sponge-spicule content, likely reflecting a more the north European relative sea-level curve of Hes- nutrient-rich mesotrophic environment. Such meso- selbo & Jenkyns (1998). However, taking in account trophic conditions could have been related to an ac- the coeval CIE described herein, this relative sea- celerated hydrological cycle linked to introduction of level rise in the Southern Alps and elsewhere in the isotopically light CO2 into the atmosphere and sub- Tethyan region could be interpreted not as purely eu- sequent global warming. The deleterious effects of static but as the consequence of a simultaneous de- nutrient excess on shallow-water carbonate produc- crease in carbonate production in shallow-water plat- tion by reducing water transparency, and encouraging forms caused by the introduction into the atmosphere– bioeroding organisms is well documented (Hallock & ocean system of isotopically light carbon that led to Schlager, 1986; Schlager, 2005). Consequently, the hi- increased introduction of terrestrially derived nutri- atus in the Verzegnis section between the chert-rich ents and ocean acidification, hence suppressing car- and calcarenitic units in the upper part of the Sov- bonate production and deposition (cf. Trecalli et al. erzene Formation, supposedly representing the upper 2012). Sinemurian – lower Pliensbachian interval, could in This climatic event, acting together with a react- part be related to the postulated coeval drop in the ivation of the syn-sedimentary extensional tectonics carbonate production on the neighbouring carbonate that reduced the productive areas of the carbonate plat- platforms. Since the lower portion of Soverzene Form- forms, would have ensured that the top of the carbonate ation represents peri-platform oozes in which pelagic platforms could no longer be readily maintained at sea material, falling through the water column, has been level. Such a proposed decrease in carbonate produc- mixed with carbonate mud supplied by the adjacent tion is also suggested by a fall in the sedimentation platforms (‘peri-platform ooze’ of Schlager & James, rate: the thickness of the calcarenitic units, in both the 1978), the drop in carbonate precipitation and secretion Friuli and Trento platforms is 30 m (Foza and Monte in the shallow-water feeder areas, acting together with Cumieli sections) and a little thinner ( 20 m) in the the Sinemurian/Pliensbachian carbon-cycle boundary Chizzola section, likely corresponding to the turneri, event recognized on the Trento Platform by Franceschi obtusum and oxynotum zones ( 10 m Ma−1 sediment- et al. (2014), could have produced the basinal starvation ation rate). Assuming a thickness of 100 m for the that caused this hiatus. lower peritidal unit of the Monte Zugna Formation (Ma- The onset of the calcarenitic unit at the top of the setti et al. 1998) and referring it to the remaining part Monte Zugna Formation in many localities represents of the lower Sinemurian (semicostatum and bucklandi a fundamental reorganization of the palaeogeography zones) and to the Hettangian, the corresponding time during the interval of the major negative CIE: tidal flat span could be estimated as about 4 Ma according to the areas across much of the Tethyan area were replaced time scale of Gradstein, Ogg & Schmitz (2012): that is, by subtidal, wave-controlled, oolitic shoals, in which a sedimentation rate ( 25 m Ma−1) more than twice peritidal facies are missing. The only exception in the that of the calcarenitic unit. Southern Alps is the Adige Valley (Chizzola section, Obvious evidence for the mesotrophic environments Fig. 9) where Masetti et al. (1998) interpreted these recorded by the chert-rich unit of the Verzegnis sec- remnants of the former peritidal system as small tidal tion in the Belluno Basin is apparently missing in the flats developed behind the western marginal shoals of palaeontological assemblage in the shallow-water cal- the Trento Platform. The calcarenitic unit at the top of carenitic units at the top of the Monte Zugna Form- the Monte Zugna Formation, like the overlying Lop- ation. On the contrary, the first appearance of Paleo- pio Oolitic Limestone, has been interpreted as due to mayncina termieri and, more generally, an enrichment the retrogradation of the marginal carbonate sand bars of foraminifera with a complex wall structure, seem to located at the western margin of the Trento Platform be related to an amelioration of the environment and to during a relative sea-level rise (Masetti et al. 1998). a shift towards an oligotrophic regime as proposed, for At its most extreme, this deepening and transgress- example, by Fugagnoli (2004) for the overlying Rotzo ive phase in the Eastern Southern Alps coincided with Formation. This anomaly could be explained assum- the definitive loss/drowning of early–late Sinemurian ing that trophic conditions are not the main parameter peritidal platforms, inherited from Late Triassic time, controlling the development of these foraminifera.

Analysing in detail the behaviour of the carbonate fact- pH may have directly controlled the whole production ory with respect to the isotopic excursions in all three of the carbonate platform and hence sedimentary accu- shallow-water sections suggests that the first negative mulation rate and relative sea level. CIE, centred around the boundary of the semicost- In mid-late Pliensbachian time, the carbonate factory atum and turneri zones (Fig. 10), coincides with the moved laterally from the top of the platform into the stratigraphic level recording the loss of peritidal sedi- neighbouring Belluno Basin, incorporating the portion ments; and their replacement by subtidal shoals coin- of the basin corresponding to the Monte Verzegnis area cides with the interval of transition between negative into a shallow-water domain where the cross-bedded and positive excursions of the isotopic curve. These calcarenitic unit at the top of the Soverzene Formation shoals are composed of ooids with a dense micritic ul- (interpreted as subtidal shoals) started to accumulate trastructure (Fig. 8a), typically associated with oncoids (Fig. 5; Zanferrari et al. 2013). During the same in- and dasycladaleans (Palaeodasycladus mediterraneus, terval, a lack of accommodation space is assumed to Palaeodasycladus gracilis). The recovery of the plat- have prevented deposition of shallow-water deposits form during the positive turneri Zone CIE caused its in the adjacent Friuli Platform (Monte Cumieli sec- progradation outwards and the deposition of the fine- tion; Zanferrari et al. 2013). The calcarenitic unit at grained lagoonal Gervillia buchi beds on top of the the top of the Soverzene Formation is in turn truncated marginal oolitic deposits of the Chizzola and Foza by a hard-ground surface intervening between its up- sections, respectively, at the western and eastern mar- per boundary and the Verzegnis Encrinites. Since this gins of the Trento Platform. In the Friuli Platform latter unit corresponds to the lower Bajocian to lower (Cumieli section) the Gervillia beds are not recorded Callovian interval, the hard-ground surface is equival- and are replaced by micritic deposits lacking obvious ent to a time span extending from the Toarcian to the macrofossils. Aalenian, a hiatus equivalent to that separating the top The negative obtusum–oxynotum Zone excursion, of the Rotzo Formation from the Lower Rosso Am- which may be interpreted as due to a further intro- monitico in the central-western sector of the Trento duction of isotopically light carbon into the ocean– Platform (see above and Masetti et al. 1998). atmosphere system, can similarly be linked to a reduc- After the postulated Sinemurian event, the Pliensba- tion in carbonate production and a new transgression, chian recolonization seems to have been largely con- recorded everywhere in the upper portion of the cal- trolled by an extensional tectonic phase that led to the carenitic unit of the Monte Zugna Formation. Being first differentiation of the central-western and northern- similar in bed organization and sedimentary structures eastern sector of the Trento Platform in which accom- to the lower part of calcarenitic unit, this upper portion modation space regulated the thickness of the shallow- shows a vertical evolution in which micritic ooids and water succession. In the more subsident central-western dasycladaleans are replaced upwards by radial-fibrous sector of the Trento Platform, the available accom- ooids and foraminifera with a calcitic wall structure modation space allowed the onset of a new type of (Fig. 8b). The degree of water turbulence and velo- carbonate factory characterized, at the beginning, by city, as denoted by sedimentary structures, apparently eutrophic deposits with abundant marls and lenses of changed little during deposition of the unit, ruling out black shales, followed by the development of the ‘Li- physical environmental factors as controlling ooidal thiotis’ beds (Rotzo Formation; Masetti et al. 1998; ultrastructure. Notably, the micritic ooids are associ- Bassi et al. 1999; Posenato & Masetti 2012). This ated with aragonitic dasycladaleans and correlate with early Pliensbachian environmental deterioration could transitions between positive and negative excursions of be also ascribed to the climatic events that occurred at the CIE, whereas the radial-fibrous ooids are associated the Sinemurian/Pliensbachian boundary (Korte & Hes- with foraminifera with calcitic walls during the acme selbo, 2011; Franceschi et al. 2014) or, more likely, to of the CIE. These associations point to seawater pH, the cumulative effects of both the preceding ‘Arnio- in turn forced by CO2 release, as a leading factor con- ceras Time’ and the Sinemurian/Pliensbachian bound- trolling both fossil occurrence and ooidal structure by ary events. According to Posenato & Masetti (2012), it promoting short-term changes in the dominant miner- is likely that the Rotzo Formation, overlying the Monte alogy (aragonite v.calcite) of the carbonate factory (e.g. Zugna Formation, is separated by a depositional hi- Sandberg, 1983; Wilkinson & Given, 1986; Zhuraviev atus referable to part of the lower Pliensbachian. Other & Wood, 2009). Following Strasser’s (1986) study of evidence of mesotrophic conditions during ‘Arnioceras Lower Cretaceous limestones in France and Switzer- Time’ comes from the Umbro–Marchean Apennines land, the micritic ooids are interpreted as originally where peritidal deposits of the Calcare Massiccio pass aragonitic, which, after inversion to calcite, preserved upwards into a deeper water succession characterized some degree of original concentric structure, whereas by the prevalence of sponges and other filter-feeding the radial-fibrous ooids, formed in waters of relatively organisms (‘Calcare Massiccio B’, from semicostatum low pH, preserved the original calcitic composition and (lower Sinemurian) to jamesoni zones (lower Pliensba- crystallographic orientation. Because a negative pulse chian) following Marino & Santantonio, 2010). of the carbon-isotope curve corresponds with a trans- In the northeastern portion of the Trento Platform gressive trend of the sedimentary succession, and the and on the northern edge of the Friuli Platform, where positive excursion records the contrary, the seawater no accommodation space was available, the top of the

Monte Zugna Formation is truncated by an uncon- Figs 5, 9, 10). The different ooidal structures and fossil formity surface corresponding to the gap embracing assemblages characterizing the two calcarenitic units the Pliensbachian, Toarcian and Aalenian stages (about (Fig. 8) could have been a result of changes in pH that, 20 Ma, Gradstein, Ogg & Schmitz, 2012), sporadically operating on a scale of a few thousands of years, fa- interrupted by the emplacement of the crinoidal sand- voured first ‘aragonitic’ and then ‘calcite seas’. The waves of the Fanes Piccola Encrinite (Masetti et al. virtual lack of fine-grained carbonate in the succession 1998; Zanferrari et al. 2013). The lithostratigraphic re- during the negative excursion may thus be explained if lationships depicted in Figure 3 suggest that this huge the switch to calcite seas also controlled the produc- hiatus could be related to the interplay between negli- tion of aragonitic mud, whether inorganically precipit- gible accommodation space, with the top of the Trento ated or derived from biological sources. These consid- Platform fixed at sea level for more than 20 Ma, and erations, coupled with a fall in sedimentation rate to the deleterious effects on the carbonate factory of at half the value of the preceding peritidal platforms, sug- least four climatic events: that of ‘Arnioceras Time’ gests that the transgression recorded by the calcarenitic described here, the Sinemurian/Pliensbachian bound- unit that came to overlie the lower peritidal unit of the ary event, the mid-Pliensbachian warming event and Monte Zugna Formation and well documented by sea- the Toarcian oceanic anoxic event. level curves reconstructed for the early/late Sinemurian interval, could be the consequence, not of a purely eustatic oscillation, but also a decrease in carbonate 6. Summary and conclusions production. The lateral spread of this calcarenitic unit Starting from the documented demise and/or drown- represents a potential first step towards the demise and ing of several Tethyan carbonate platforms at some ultimate drowning of the platforms. point during Sinemurian time, a chemostratigraphic The recolonization of the Trento Platform after the transect through this interval has been generated across ‘Arnioceras Time’ climatic event was controlled by the whole domain of the Eastern Southern Alps to in- extensional tectonics governing the availability of ac- vestigate the response of both shallow- and deep-water commodation space: where such space was negligible, domains during this time interval. Investigation has the top of the Monte Zugna Formation is truncated by concentrated on carbonate-platform successions, loc- a huge unconformity surface corresponding to a hiatus ally underlying open-marine and/or pelagic cover, and spanning the upper Sinemurian to the lower Bajocian one basinal peri-platform succession. An abrupt pos- (about 20 Ma); in the more subsident central-western itive followed by a broader negative CIE, typically in sector, the available accommodation space allowed the the range of 0.5–1.0 ‰, has been detected in all four onset of a new type of subtidal carbonate factory char- stratigraphic sections suggested to correspond with the acterized by the Pliensbachian ‘Lithiotis facies’. likely stratigraphical extent of the ammonite Arnio- The negative δ13C excursion may be attributed to ceras that is recorded from the adjacent deep-water a reduction of global biomass and/or burial of organic 13 basinal succession. The positive excursion is here at- matter and/or introduction of C-depleted CO2 into the tributed to the turneri Zone and the following neg- atmosphere–ocean system from a buried source of iso- ative excursion likely centred in the obtusum Zone topically light carbon. A coeval negative excursion in (Figs 6, 10). δ13C, described from southern England, is accompan- In the Belluno Basin, filled by peri-platform ied by palynological evidence suggesting the onset of a oozes (Monte Verzegnis section: deep-water Soverzene major warming event. The data presented here demon- Formation), the negative CIE spans a stratigraphic strate that the major negative CIE of ‘Arnioceras Time’ thickness of 60 m and its lowest value is located is represented across the entire Eastern Southern Alps, immediately below the base of a chert-rich unit cor- invariably associated with environmental reorganiza- responding to an increase in the sponge content of the tion of carbonate platforms and regional deepening. sediment. The hiatus between the chert-rich and cal- Taking in account also the widespread occurrence of the carenitic units in the upper part of the Soverzene Form- definitive demise and drowning of carbonate platforms ation is considered to embrace the upper Sinemurian during Sinemurian time in many parts of the Tethyan – lower Pliensbachian, an interval missing in both the area, the negative CIE likely represents a global cli- shallow-water Monte Cumieli and Foza sections, and matic event that pre-conditioned many carbonate plat- could be related to the coeval postulated drop in the forms for demise and ultimate drowning. sediment production of the feeder carbonate platforms. In the shallow-water carbonate succession of the Acknowledgements. We are grateful to: Federico Venturi Monte Zugna Formation (Monte Cumieli, Foza, (Perugia University), who identified the Arnioceras speci- Chizzola sections), the correlative major carbon- men in the field; Giulio Pavia (Turin University) helped us isotope negative excursion is mainly developed within in recognizing other ammonite specimens coming from con- the calcarenitic unit at its top, which is divided into densed units in the Verzegnis section; Elisabetta Erba ex- amined open-marine and basinal samples for nannoplank- two parts (semicostatum–turneri zones and obtusum ton; Marco Avanzini (MuSe, Trento) supplied the logistic Zone, respectively, for the lower and upper parts) by help that made possible the sampling of the Chizzola sec- the lagoonal Gervillia buchi beds, indicating an ap- tion; Guido Roghi, Jacopo del Corso (Padua University) and parent rapid recovery of the platform (turneri Zone, Marco Franceschi (MuSe, Trento) leant a hand in the field

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