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Towards Accurate Numerical Calibration of the Late Triassic: High- Precision U-Pb Geochronology Constraints on the Duration of the Rhaetian

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Towards Accurate Numerical Calibration of the Late Triassic: High- Precision U-Pb Geochronology Constraints on the Duration of the Rhaetian Towards accurate numerical calibration of the Late Triassic: High- precision U-Pb geochronology constraints on the duration of the Rhaetian Jörn-Frederik Wotzlaw1*, Jean Guex2, Annachiara Bartolini3, Yves Gallet4, Leopold Krystyn5, Christopher A. McRoberts6, David Taylor7, Blair Schoene8, and Urs Schaltegger1 1Section of Earth and Environmental Sciences, University of Geneva, 1201 Geneva, Switzerland 2Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland 3Muséum National d’Histoire Naturelle, 75005 Paris, France 4Equipe de Paléomagnétisme, Institut de Physique du Globe de Paris, 75238 Paris, France 5Institute of Paleontology, University of Vienna, 1090 Vienna, Austria 6Geology Department, State University of New York Cortland, Cortland, New York 13045, USA 7Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA 8Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA ABSTRACT an is characterized by a worldwide regression, which induced important Numerical calibration of the Late Triassic stages is arguably the changes in marine lithofacies between the Norian and Rhaetian Stages most controversial issue in Mesozoic stratigraphy, despite its impor- (Golebiowski, 1990). tance for assessing mechanisms of environmental perturbations and This prelude to the end-Triassic crisis was marked by several extinc- associated biologic consequences preceding the end-Triassic mass tion episodes and/or by a stepwise reduction in biodiversity. To better un- extinction. Here we report new chemical abrasion–isotope dilution– derstand this complex biodiversity pattern and its relationship with abiotic thermal ionization mass spectrometry zircon U-Pb dates for volcanic mechanisms, temporal constraints are of critical importance. Despite this ash beds within the Aramachay Formation of the Pucara Group in importance, the duration of the Rhaetian stage is strongly debated, leading northern Peru that place precise constraints on the age of the Norian- to the concept of a short (~4 m.y.) versus long Rhaetian (up to ~10 m.y.; Rhaetian boundary (NRB) and the duration of the Rhaetian. The for a synthesis, see Ogg, 2012). These different estimates are primarily sampled ash bed–bearing interval is located just above the last occur- based on magnetostratigraphic correlations of biostratigraphically con- rence of the bivalve Monotis subcircularis, placing this stratigraphic strained marine sections and the continental Newark Basin sequence (east- sequence in the uppermost Norian, perhaps ranging into the earliest ern North America). In this study, we present new high-precision zircon Rhaetian. Zircon U-Pb dates of ash beds constrain the deposition age U-Pb dates obtained from volcanic ash beds intercalated with uppermost of this interval to be between 205.70 ± 0.15 Ma and 205.30 ± 0.14 Ma, Norian to Rhaetian marine sediments in northern Peru. To our knowledge, providing precise constraints on the age of the NRB. Combined with this is the only locality worldwide where biostratigraphically defi ned up- previously published zircon U-Pb dates for ash beds bracketing the per Norian-Rhaetian strata are associated with zircon-bearing volcanic ash Triassic-Jurassic boundary, we estimate a duration of 4.14 ± 0.39 m.y. beds. These data allow us to independently constrain the duration of the for the Rhaetian. This ends a prolonged controversy about the dura- Rhaetian Stage, which has important implications for the construction of a tion of this stage and has fundamental implications for the rates of reliable Late Triassic geological time scale. paleoenvironmental deterioration that culminated in the end-Triassic mass extinction. BIOSTRATIGRAPHIC DEFINITION OF THE NORIAN- RHAETIAN BOUNDARY AND THE SIGNIFICANCE OF INTRODUCTION BIVALVE BIOCHRONOLOGY IN THE PUCARA BASIN The uppermost Triassic stage, the Rhaetian, is framed by two biotic The biostratigraphic defi nition of the NRB has been the subject of a and environmental crises. At its top, the end-Triassic crisis is marked by long debate. After a thorough discussion through the Subcommission on one of the most severe mass extinctions of the Phanerozoic, followed by Triassic Stratigraphy (Krystyn, 2010), the base of the Rhaetian is taken as a recovery phase associated with important perturbations in the carbon the fi rst occurrence (FO) of the conodont Misikella posthernsteini. This cycle and strong climatic changes during the earliest Jurassic (e.g., Olsen option appears as the most satisfactory because the NRB is then located et al., 2002; Guex et al., 2004; McElwain et al., 2009; Ruhl and Kürschner, just above the great Norian crisis marked by the disappearance of almost 2011). This crisis was likely driven by several pulses in volcanic activ- all monotid bivalves and a signifi cant turnover in ammonoid and conodont ity during the emplacement of the Central Atlantic Magmatic Province faunas (e.g., McRoberts et al., 2008; Krystyn and Kürschner, 2005). (CAMP; Marzoli et al., 2004; Schoene et al., 2010; Blackburn et al., We recently carried out detailed stratigraphic research in the Utcu- 2013). At its base (Norian-Rhaetian boundary, NRB), the Norian crisis is bamba Valley, northern Peru, where the Aramachay Formation (Pucara much less pronounced than the end-Triassic extinction, but it nevertheless Group) contains a complete upper Norian to lower Sinemurian marine affected a large proportion of ammonoids and saw the near total disap- sedimentary sequence (Fig. 1; Schaltegger et al., 2008). The thickest pearance of the bivalve Monotis, which was dominant in the late Norian sequence is located near Levanto (Fig. 1B) and consists of soft brown seas almost globally (e.g., McRoberts et al., 2008). Low-paleolatitude siltstones alternating with slightly more calcareous beds. The Rhaetian conodont faunas of the Tethys Ocean show a dramatic turnover from ro- of that sequence is poor in ammonoids; the few collected specimens al- bust Epigondolella-dominated to fragile and tiny Misikella associations low, however, a reliable correlation with the standard ammonoid zonations at the NRB (Krystyn and Kürschner, 2005; Krystyn, 2008). The abiotic used in the upper Rhaetian and lower Jurassic. The base of the Jurassic causes of the NRB crisis are not well known, but the very end of the Nori- (Triassic-Jurassic boundary, TJB) is marked by the FO of ammonite Psilo- ceras spelae (Fig. 2). The highest Rhaetian ammonoid-bearing beds con- *E-mail: [email protected]. GEOLOGY, July 2014; v. 42; no. 7; p. 571–574; Data Repository item 2014201 | doi:10.1130/G35612.1 | Published online 16 May 2014 GEOLOGY© 2014 Geological | July Society2014 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 571 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/42/7/571/3544892/571.pdf by Princeton University user on 19 October 2020 a 200 201 202 203 204 205 206 207 Ma 80°W 78°W 76°W Chachapoyas Chachapoyas U Levanto N t ash beds c Fig. 1B V Fig. 1C 5 mm 70 u Hettangian LM4-100/101: 201.32 ± 0.13 Cajamarca PUCARA a GROUP b FO P. spelae l Maino l e a LO C. LM4-90: 201.39 ± 0.14 y m 8°S 6°S b Rio crickmayi a LM4-86: 201.51 ± 0.15 PACIFIC N Maino 5 mm OCEAN 04 km b A B 50 LM4-76/77: 201.87 ± 0.17 to Chachapoyas 1b 3' 1d 40 LEVANTO 3a LM4-58/59: 202.16 ± 0.27 c 30 LM4-50/51: 202.38 ± 0.16 0 500 m C to Maino 10 mm 20 V. saximon- tanus Figure 1. Geographical framework and geological setting 10 of study area. A: Digital elevation model of northern and d LM4-23B: 203.71 ± 0.11 central Peru showing location of study area and extent 1 mm of Pucara Group. B: Google Earth™ image with details of Rhaetian 0 TRIASSIC JURASSIC study area. C: Google Earth™ image of study area with location of the studied sequence with sampled fossilif- erous horizons and volcanic ash beds indicated by red 1 mm -10 labels (see the Data Repository [see footnote 1]). 10 mm -20 tain Choristoceras crickmayi and C. marshi, and the lowest contain a rich 10 mm -30 LP2010-1d: 205.30 ± 0.14 middle Rhaetian fauna with Vandaites saximontanus. Otapiria aff norica LP2010-1b: 205.40 ± 0.09 Most signifi cant for the defi nition of the NRB in the Levanto sec- Oxytoma cf NRB inequivalis -40 LP2010-3a: 205.70 ± 0.15 tion is the last occurrence of large monotid bivalves, identifi ed as Monotis (m) LO Monotis Nor. subcircularis (Fig. 2; Fig. DR2 in the GSA Data Repository1), which oc- subcircularis cur in hard gray and massive silty limestones at the base of the section. Figure 2. Summary of biostratigraphy and U-Pb geochronology for M. subcircularis and other large monotid bivalves underwent nearly com- Late Triassic of the Pucara Group, northern Peru. Stratigraphic col- plete extinction at the top of the Norian. Only two dwarf species occur in umn shows occurrences (FO—fi rst occurrence; LO—last occurrence) the lowest Rhaetian of the western Tethys, one crossing the NRB and the of characteristic ammonites and bivalves and locations of ash beds (colored dots). Photographs to the left of stratigraphic column show other fi rst appearing in the basal Rhaetian (McRoberts, 2010). The extinc- representative specimen of collected fossils: (a) Psiloceras spelae; tion of large Monotis at the top of the Norian is correlative with the top (b) Choristoceras crickmayi; (c) Vandaites saximontanus; (d) from of the Sagenites quinquepunctatus ammonoid zone in the Tethys and the top to bottom, Oxytoma cf. O. inaequivalvis, Otapiria aff. O. norica, top of the Gnomohalorites cordilleranus ammonoid zone in the Americas. and two specimens of Monotis subcircularis. Recalculated previously At the proposed global boundary stratotype section and point (GSSP) at published (Schoene et al., 2010; Guex et al., 2012; see text) and new U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) Steinbergkogel (Austria), Monotis beds are rare and disappear ~14 m be- dates from single zircons are color-coded to the corresponding ash low the NRB (Krystyn, 2008).
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