Rhaetian magneto-biostratigraphy from the Southern Alps (Italy): Constraints on Triassic chronology |
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| Authors: | Giovanni Muttoni Dennis V Kent Flavio Jadoul Paul E Olsen Manuel Rigo Maria Teresa Galli Alda Nicora |
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| Institution: | 1. Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark;2. University of Innsbruck, Institute of Geology, Faculty of Geo- and Atmospheric Sciences, Innrain 52, Innsbruck 6020, Austria;3. Biogeosciences, UMR CNRS/uB 6282, Univ. Bourgogne Franche-Comté, 6 Boulevard Gabriel, 21000 Dijon, France;4. Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark;5. University of Exeter, Camborne School of Mines & Environment and Sustainability Institute, Penryn Campus, Penryn, Cornwall TR10 9FE, UK;1. Department of Earth and Environmental Sciences, Kumamoto University, Kumamoto 860-8555, Japan;2. Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan;3. Department of Earth and Sea Sciences, University of Palermo, Via Archirafi 22, 90123 Palermo, Italy;4. Department of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padova, Italy;5. IGG-CNR, Via Gradenigo 6, 35131 Padova, Italy;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;2. Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA;3. Geological Survey of Canada, 1500 - 625 Robson St., Vancouver, B.C. V6B 5J3, Canada;4. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;5. Institute of Geology and Paleontology, Tohoku University, Aoba-ku, Sendai 980-8578, Japan;1. Department of Petroleum Geosciences, Khalifa University of Science and Technology, PO Box 12778, Abu Dhabi, United Arab Emirates;2. Dept. of Geological Sciences, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA;1. Dipartimento di Scienze della Terra “Ardito Desio”, Universita'' di Milano, via Mangiagalli 34, 20133 Milan, Italy;2. Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA;3. Department of Earth and Planetary Sciences, Rutgers University Piscataway, NJ 08854, USA;4. Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA |
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| Abstract: | New Late Triassic–earliest Jurassic magneto-biostratigraphic data have been obtained from three overlapping sections in the Southern Alps, Italy (Costa Imagna, Brumano, Italcementi Quarry), composed of ~ 520 m of shallow-marine carbonates outcropping in stratigraphic continuity. Characteristic magnetic components of presumed depositional age record a sequence of 9 normal and reverse polarity magnetozones (as defined by 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 number of polarity zones than previously recognized in more condensed Rhaetian sections from the literature, and by inference represents more time. These data are placed in a broader Late Triassic temporal framework by means of correlations to published magneto-biostratigraphic data from the Tethyan marine Pizzo Mondello section and the Newark astronomical polarity time scale (APTS). This framework is 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. |
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