Rhaetian Magneto-Biostratigraphy from the Southern Alps (Italy): Constraints on Triassic Chronology

Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 1–16 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Rhaetian magneto-biostratigraphy from the Southern Alps (Italy): Constraints on Triassic chronology Giovanni Muttoni a,b,⁎, Dennis V. Kent c,d, Flavio Jadoul a, Paul E. Olsen d, Manuel Rigo e, Maria Teresa Galli a, Alda Nicora a a Department of Earth Sciences, University of Milan, Via Mangiagalli 34, I-20133 Milan, Italy b ALP — Alpine Laboratory of Paleomagnetism, via Madonna dei Boschi 76, I-12016 Peveragno (CN), Italy c Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, U.S.A. d Lamont-Doherty Earth Observatory, Palisades, NY 10964, U.S.A. e Department of Geosciences, University of Padova, Via Giotto 1, 35137, Padova, Italy article info abstract Article history: New Late Triassic–earliest Jurassic magneto-biostratigraphic data have been obtained from three overlapping Received 6 October 2008 sections in the Southern Alps, Italy (Costa Imagna, Brumano, Italcementi Quarry), composed of ~520 m of Received in revised form 14 October 2009 shallow-marine carbonates outcropping in stratigraphic continuity. Characteristic magnetic components of Accepted 14 October 2009 presumed depositional age record a sequence of 9 normal and reverse polarity magnetozones (as defined by Available online 23 October 2009 at least three stratigraphically superposed samples) linked by conodont and palynofloral evidence from this study and the literature to Rhaetian to Triassic–Jurassic boundary age. This represents a significantly larger Keywords: Magnetostratigraphy number of polarity zones than previously recognized in more condensed Rhaetian sections from the Biostratigraphy literature, and by inference represents more time. These data are placed in a broader Late Triassic temporal Rhaetian framework by means of correlations to published magneto-biostratigraphic data from the Tethyan marine Triassic Pizzo Mondello section and the Newark astronomical polarity time scale (APTS). This framework is Time scale consistent with a position of the Norian–Rhaetian boundary (as defined at Brumano and Pizzo Mondello by the first appearance of Misikella posthernsteini) within Newark magnetozones E17r–E19r in the ~207– 210 Ma time interval, in basic agreement with the position originally estimated in the Newark using pollen biostratigraphy (E18 at 208–209 Ma). This framework is also consistent with the position of the Triassic– Jurassic boundary interval (placed at Italcementi Quarry at the acme of Kraeuselisporites reissingeri coincident with a negative carbon isotope excursion) correlative to just above Newark magnetozone E23r and just below the oldest CAMP lavas dated at ~202 Ma. Hence, we estimate the duration of the Rhaetian to be ~5.5–8.5 Myr (or even longer if the Triassic–Jurassic boundary is instead placed above the negative carbon isotope excursion as at Kuhjoch, which is the designated GSSP for the base of the Hettangian), and encompassing 9 magnetozones. This duration contrasts with a duration of ~2 Myr and only ~4 magnetozones in several alternative published magneto-biostratigraphic schemes. © 2009 Elsevier B.V. All rights reserved. 1. Introduction 2007). Uncertainty in the demarcation and correlation of stage boundaries is reflected in a debate on the calibration of the Triassic The Triassic is a ~50 Myr-long period of significant adaptive radi- time scale (Gradstein et al., 2004; Brack et al., 2005; Kent et al., 2006). ation of marine and continental life bracketed between two major For the Late Triassic, an astrochronology anchored to magnetostrati- extinction events, yet only three of the eight stage boundaries that graphy and radiometric dates is available from the continental sections define its chronostratigraphy have been thus far officially ratified by of the Newark Supergroup (Kent et al., 1995; Kent and Olsen, 1999; the International Commission on Stratigraphy (i.e., base Induan = Olsen and Kent, 1999), whereas magnetostratigraphy is available with base Triassic; Yin et al., 2001; base Ladinian = base Middle Triassic; conodont biostratigraphy and rare radiometric dates tied to the Brack et al., 2005; and base Carnian = base Late Triassic; Mietto et al., ammonoid zonation for the Tethyan realm. The Newark continental astronomical polarity time scale (APTS) and Tethyan marine biostratigraphy can therefore be correlated using the universal record of ⁎ Corresponding author. Department of Earth Sciences, University of Milan, Via polarity reversals for the construction of an integrated Late Triassic Mangiagalli 34, I-20133 Milan, Italy. Fax: +39 02 503 15494. time scale. While fundamental steps in this direction have already E-mail address: [email protected] (G. Muttoni). been taken, the issue is not yet settled, most glaringly for the Rhaetian. 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.10.014 2 G. Muttoni et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 1–16 Two options for the duration of the Rhaetian were recently proposed over, it is supposedly in part missing in the Newark APTS (Fig. 1,left using magnetostratigraphic correlation of conodont biostratigraphic panels ‘Tethyan composite I’ and ‘Oyuklu section’). In contrast, data from selected Tethyan sections to the Newark APTS. According to according to Channell et al. (2003) and Muttoni et al. (2004),the Gallet et al. (2007 and references therein), the Rhaetian has a short Rhaetian would represent a time of ~6–7 Myr from Newark magneto- duration (~2 Myr) straddling only about 3 polarity chrons, and more- zone E17 at ~209 Ma to the Triassic–Jurassic (T–J) boundary located just Fig. 1. Short duration vs. long duration options for the Rhaetian, the youngest stage of the Triassic. According to the compilation of magneto-biostratigraphic data from the Tethyan realm of Krystyn et al. (2002) and Gallet et al. (2003) (Tethyan composite I), and recent data from Oyuklu (Gallet et al., 2007), the Rhaetian is ~2 Myr in duration and in part missing in a supposed gap in the Newark APTS (Kent and Olsen, 1999). According to data from Pizzo Mondello (Muttoni et al., 2004), the Rhaetian should be ~6–7Myr long, virtually similar in duration to the original estimate of Kent and Olsen (1999). The Triassic–Jurassic boundary in Newark section based on palynofloral evidence occurs near the base of E24n. See text for discussion. G. Muttoni et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 285 (2010) 1–16 3 above magnetozone E23r at ~202 Ma, straddling 8 to 10 polarity chrons thick composite stratigraphic interval from the Zu1 member of the Zu and a duration similar to that of ~6 Myr originally proposed for the Limestone to the base of the Albenza Formation (Fig. 2A and B). The Newark section by Kent et al. (1995) and Olsen and Kent (1999) on the Costa Imagna section and the lower Brumano section straddle the Zu1 basis of pollen biostratigraphy (Olsen et al., 1996)(Fig. 1, right panel member up to the boundary with the overlying Zu2 member and have ‘Pizzo Mondello’; Muttoni et al., 2004). In this paper, we contribute to been correlated by tracing laterally lithostratigraphic marker beds the issue of the duration of the Rhaetian by presenting magnetostrati- (Fig. 3). In general, the Zu1 member consists of monotonous graphic data from a thick composite Tethyan marine section from the alternations of dark-grey marl and marly limestone horizons Southern Alps, Italy, and review the available magneto-biostratigraphic intercalated with predominately thin-bedded fine-grained lime- data from the literature that constrain the chronology of the Late Triassic stones. The lithofacies are arranged vertically into asymmetrical to for comparison with the recent Triassic time scale of Gradstein et al. nearly symmetrical, decameter-scale cycles. The upper Brumano (2004). section continues into the Zu2 member, composed of decameter- scale carbonate cycles with bioturbated or fossiliferous dark-grey 2. Geological setting and stratigraphy wackestones or coral patch reefs at the base, and intrabioclastic and/ or oolitic packstones–grainstones at the top (Fig. 3). The Zu3 member The uppermost Triassic succession of the Lombardian Basin in the encompasses three subunits (Zu3a–c), two of which are present in the Southern Alps north of Bergamo (Fig. 2A) consists of shales of the Riva upper Brumano section (Zu3a–b) (Fig. 3). The lower subunit (Zu3a) di Solto Shale (RSS) overlain by carbonates and shales of the Zu consists of 7.5 to 15 m thick marl and micritic limestone cycles Limestone; these are in turn overlain by Hettangian micritic lime- occasionally capped by thin iron-oxide crusts. The middle subunit stones of the Malanotte Formation and carbonate platform deposits of (Zu3b) is represented by 4 to 9 m thick marl and marly limestone the Albenza Formation (formerly Conchodon Dolomite) (Galli et al., cycles with frequent iron-oxide thin crusts at each cycle top. 2005; 2007; Jadoul and Galli, 2008). The Zu Limestone was The Italcementi Quarry section straddles the Zu3b subunit and traditionally subdivided into four members (Zu1–Zu4), but the continues into the overlying Zu3c subunit (Fig. 4), consisting of 40–50 m youngest member (Zu4) has been recently renamed the Malanotte thick bioturbated wackestones and packstones with benthic foramini- Formation (Galli et al., 2007)(Fig. 2B). These mainly shallow-water fers, bivalves,

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