Palaeogeography, Palaeoclimatology, Palaeoecology 517 (2019) 52–73 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo New magnetobiostratigraphic results from the Ladinian of the Dolomites T and implications for the Triassic geomagnetic polarity timescale ⁎ Matteo Marona, , Giovanni Muttonia, Manuel Rigob,c, Piero Gianollad, Dennis V. Kente,f a Department of Earth Sciences “Ardito Desio”, University of Milano, via Mangiagalli 34, 20133 Milano, Italy b Department of Geosciences, University of Padova, via Gradenigo 6, 35131 Padova, Italy c Institute of Geosciences and Earth Resources, Consiglio Nazionale delle Ricerche (CNR), via Gradenigo 6, 35131 Padova, Italy d Department of Physics and Earth Sciences, University of Ferrara, via Saragat 1, 44122 Ferrara, Italy e Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA f Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA ARTICLE INFO ABSTRACT Keywords: We investigated for magnetostratigraphy the Rio Nigra and Rio Frommer stratigraphic sections from Alpe di Magnetostratigraphy Siusi/Seiser Alm (Dolomites, northern Italy) in order to improve the calibration of the Triassic time scale. Both Geochronology sections are characterized by ammonoid and conodont associations typical of Longobardian (late Ladinian, Middle Triassic Middle Triassic) age. Moreover, the Rio Nigra section is constrained by a U-Pb zircon date of 237.77 ± 0.05 Ma. U-Pb dating Building on the recently verified Newark-Hartford astrochronological polarity timescale for the Late Carnian–Rhaetian (plus the Hettangian) and through magnetostratigraphic correlations of an updated inventory of Tethyan marine stratigraphic sections from the literature, some of which are provided with U-Pb zircon age constraints, we propose a revised Geomagnetic Polarity Time Scale for the entire Triassic. 1. Introduction Rio Nigra section, in conjunction with available geochronological data from the late Ladinian–Carnian and estimates of regional sediment The continuous addition of relevant magnetostratigraphic, radio- accumulation rates, to derive an age of ~237 Ma for the Carnian base, metric, and astrochronologic age data warrant an update of the Triassic older than in previous timescales (e.g., Hounslow and Muttoni, 2010). geomagnetic polarity timescale (GPTS). An astrochronological polarity The aim of this study was to improve the chronology of the Middle timescale (APTS) for the Early–Middle Triassic has been recently ob- Triassic by conducting a magnetostratigraphic study of the U-Pb-cali- tained through astronomically-tuned magnetostratigraphic sections brated Rio Nigra section as well as of the largely coeval Rio Frommer from South China (Li et al., 2016, 2018). The magnetostratigraphy of section from the Dolomites. These new data are used in conjunction the Late Triassic has been improved with studies at Pignola-2 (Carnian; with data from a selection of 33 Tethyan marine sections (Fig. 1A, B) Maron et al., 2017), Wayao (Carnian; Zhang et al., 2015), and Pignola- from the literature (10 of them from the Southern Alps; Fig. 2A), con- Abriola (Norian–Rhaetian; Maron et al., 2015; Rigo et al., 2016), and strained by an updated inventory of radiometric age data and key their correlations to the reference Newark-Hartford APTS (Carnian–- biostratigraphic events useful to define stage boundaries, to construct Hettangian; e.g., Kent et al., 2017), which has been recently confirmed an updated GPTS spanning from the recently recalibrated age of the by new U-Pb zircon dates from the Petrified Forest drill core (Kent Permian/Triassic boundary (Burgess et al., 2014) to the Carnian (Late et al., 2018). The central thread of the Middle Triassic GPTS derives Triassic). This GPTS is then appended to the Late Triassic Newark APTS from radiometrically-calibrated magnetostratigraphic sections in the (Kent et al., 2017 and references therein; Fig. 1A, B) where stage Dolomites of northern Italy (e.g., Muttoni et al., 2004a), where addi- boundaries are defined by correlations to Tethyan marine sections some tional U-Pb zircon dates have recently become available from tuff layers of which of recent publication. Our Triassic GPTS is then discussed in at Seceda (239.04 ± 0.10 Ma, 240.28 ± 0.09 Ma, 240.58 ± 0.13 Ma; comparison with previous timescales (e.g., Szurlies, 2007; Hounslow Wotzlaw et al., 2018) and Rio Nigra (237.77 ± 0.05 Ma; Mietto et al., and Muttoni, 2010; Li et al., 2018). 2012). In particular, Mietto et al. (2012) used the U-Pb date from the ⁎ Corresponding author. E-mail addresses: [email protected] (M. Maron), [email protected] (G. Muttoni), [email protected] (M. Rigo), [email protected] (P. Gianolla), [email protected] (D.V. Kent). https://doi.org/10.1016/j.palaeo.2018.11.024 Received 11 July 2018; Received in revised form 2 November 2018; Accepted 21 November 2018 Available online 23 November 2018 0031-0182/ © 2018 Elsevier B.V. All rights reserved. M. Maron et al. Palaeogeography, Palaeoclimatology, Palaeoecology 517 (2019) 52–73 Fig. 1. Panel A: Global map with the location of the stratigraphic sections discussed in the text. Panel B: Paleogeographic reconstruction of Pangea and the Tethys Ocean in the earliest Late Triassic at ∼225 Ma (from Muttoni et al., 2015). Due to general north- ward motion of Pangea during the Triassic, strati- graphic sections older than 225 Ma, such as the Early Triassic sections from South China, were located closer to the paleoequator. Magnetostratigraphic sections discussed in the text are indicated by num- bers (legend in figure). 8°E 12°E 3 4 5 B Creek N N 0 1 2 Km 0 20 40 Km 6 2 Siusi Pathway Bolzano 1 N Compaccio (Bozen) Dolomites 7 Road ITALY N 46°32’ Rio Frommer 44°N 46°N Town Belluno Sciliar-Catinaccio Rio Freddo section Trento 0 250 Km Nature Park Mountain 8 ALPE 10 Lombardy Peak Alps Rio Nigra DI 46°31’ N 46°31’ section Stratigraphic 46°N 9 SIUSI Mt. Sciliar Section Bergamo Garda Lake 11°33’ E 11°35’ E 11°37’ E Venezia C Volcanites Brescia (Fernazza Fm.) Verona Padova Adriatic Frommer mb. Sea (Fernazza Fm.) A Po Plain Sciliar Fm. (Sciliar III) 10°E 11°E 12°E RIO FROMMER (~38 m) Marmolada Cgm. River Sea/Lake City Stratigraphic Section RIO NIGRA (Wengen Fm.) (~38 m) 1 - Rio Nigra/Rio Frommer; 2 - Frotschbach; 3 - Bulla/Siusi; 4 - Seceda; Wengen Fm. 5 - Pedraces; 6 - Prati di Stuores; 7 - Belvedere; 8 - Margon; 9 - Italcementi Quarry; 10 - Brumano Erosive Surface Fig. 2. Panel A: Map of north-eastern Italy, with position of the main stratigraphic sections from the Southern Alps. Panel B: Map of the Alpe di Siusi/Seiser Alm area, Dolomites, Italy. The Rio Frommer stratigraphic section is located near the village of Compaccio/Compatsch and the Rio Nigra section closer to the Sciliar/Schlern massif, within the Sciliar-Catinaccio/Schlern-Rosengarten Nature Park. Panel C: Stratigtraphic framework of Alpe di Siusi in which are represented the boundaries between the Fernazza Fm. (Volcanites and Frommer member), the Sciliar Fm., and the Wengen Fm. (including the Marmolada Conglomerate). 2. Stratigraphy of Rio Nigra and Rio Frommer sections Schlern along the Rio Nigra Creek (Fig. 2B). The section is ~38 m-thick and straddles the Frommer member of the Fernazza Formation The Rio Nigra section (coordinates: 46° 30′ 56.1″ N; 11° 35′ 43″ E) is (Gianolla et al., 1998; Stefani et al., 2010; Mietto et al., 2012; Bernardi located in the Alpe di Siusi on the north-eastern flank of Mount Sciliar/ et al., 2018)(Fig. 3A). It starts with pillow lavas at the base overlain by 53 M. Maron et al. Palaeogeography, Palaeoclimatology, Palaeoecology 517 (2019) 52–73 Rio Nigra section D. Magnetostratigraphy A. Lithostratigraphy B. Biostratigraphy C. Thickness IRM0.3/SIRM (m) Declination Inclination VGP Latitude Scilliar Fm 38 CNI 1 36 RNM 31.53 RNM 31.00 34 Frankites regoledanus RNM 30.00 CNI 2 RNM 28.69 32 RNM 27.74 RN2n CNI 3 30 CNI 4 Zestoceras cf. nitidum RNM 24.60 28 CNI 5 26 RNM 22.86 237.77 ± 0.05 Ma RNM 21.60 24 RN1r.1n RNM 20.80 RN1r 22 20 CNI 6 Paragondolella foliata RNM 16.61 RNM 15.89 18 Gladigondolella tethydis RNM 15.28 Pseudofurnishius murcianus murcianus RN1n.2r RNM 14.47 16 RNM 13.68 Frommer mbr RNM 12.99 RN1n.1r FernazzaFm Budurovignathus diebeli RNM 12.38 14 Paragondolella inclinata RNM 11.45 RNM 10.01 12 RNM 9.22 RNM 7.92 10 RNM 7.34 Budurovignathus sp. RN1n RNM 7.00 8 RNM 5.66 Budurovignathusmostleri RNM 5.31 6 4 Budurovignathus mungoensis RNM 2.80 Anolcites ? neumayri RNM 1.50 2 RNM 1.10 Gladigondolella malayensis malayensis 0 Pseudofurnishius murcianus praecursor 10.90.8 270 0 18090 270 09- -30-60 0 3060 90 54-09- 0 9045 Ammonoids Conodonts )E°( )°( (°N) Fig. 3. The Rio Nigra stratigraphic section. From left to right: A) lithostratigraphic log, where on the left are the samples for magnetostratigraphy (red lines) and for conodonts (blue lines), while on the right are the positions of the ash-beds (black triangles), including the one at ∼28 m dated with U-Pb at 237.77 ± 0.05 Ma (Mietto et al., 2012); B) biostratigraphy, represented mainly by conodonts and ammonoids attributed to the neumayri and regoledanus subzones interval; C) IRM0.3T/ SIRM ratio, showing a general increase of high-coercivity minerals in the upper-part, suggesting a decrease of magnetite relative to hematite; D) magnetostratigraphy (ChRM declination, ChRM inclination, Virtual Geomagnetic Pole latitude, magnetozones), revealing dominant normal polarity along the entire section (magneto- zones RN1n, RN2n), punctuated by a reverse magnetozone (RN1r) and three single-sample reverse intervals (RN1n.1r, RN1n.2r, RN1r.1n). volcaniclastic sandstones and marls. The basalt-sediment contact is 3.