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Triassic Eustatic Variations Reexamined






          Bilal U. Haq, Smithsonian Institution, Washington, D.C., USA, and Sorbonne University, Paris, France
          ABSTRACT                           INTRODUCTION                       documentation includes sections from the
            Documentation of eustatic variations for   The Triassic Period encompasses 50.5   Sverdrup Basin, Svalbard, and the Barents
          the Triassic is limited by the paucity of the   m.y., spanning an interval from 251.9 to   Sea. This paper serves to complete a review
          preserved marine stratigraphic record,   201.4 Ma (Ogg et al., 2016). By this time,   of the entire Mesozoic as both Cretaceous
          which is confined mostly to the low and   the megacontinent of Pangaea had already   and Jurassic sea-level variations have
                                                                                already been reappraised (Haq, 2014, 2017).
          middle paleolatitudes of the Tethys Ocean.   assembled, surrounded by the Panthalassa   For a background of the paleoenvironmental
          A revised sea-level curve based on reevalu-  Ocean that covered >70% of Earth’s
          ation of global stratigraphic data shows a   surface, and by the mid-Triassic the   conditions (oceans and climates) in the
                                                                                Triassic see the GSA data repository (see
          clear trend of low seastands for an extended   Pangaean landmass was almost evenly    footnote 1).
          period that spans almost 80 m.y., from the   distributed in the two hemispheres around
          latest Permian to the earliest Jurassic. In the   the paleo-equator (see Fig. S1 in the GSA    TRIASSIC TIME SCALE UPDATES
                                                         1
          Early and Middle Triassic, the long-term   Data Repository ). The interval from latest
          sea levels were similar to or 10–20 m   Permian through the earliest Jurassic, a   A succinct discussion of the method-
          higher than the present-day mean sea level   time span of nearly 80 m.y., represents the   ological advancements and modifications
          (pdmsl). This trend was reversed in the late   longest spell of low seastands of the   to the Triassic time scale can be found in
                                                                                Preto et al. (2010), and a detailed discussion
          Ladinian, marked by a steady rise and cul-  Phanerozoic. The Triassic is also bracketed
          minating in peak sea levels of the Triassic   by two major biotic extinctions near the   of Triassic stratigraphy has been presented
                                                                                by Ogg et al. (2014). Since the last update
          (~50 m above pdmsl) in the late Carnian.   Permian-Triassic (P-T) and Triassic-Jurassic   of the Triassic third-order sea-level varia-
          The trend reverses again with a decline in   boundaries, the one at P-T boundary being   tions (Haq and Al-Qahtani, 2005) that was
          the late Norian and the base level remain-  the most severe biotic turnover of the   calibrated to an earlier version of the time
          ing close to the pdmsl, and then    Phanerozoic (Raup and Sepkowski, 1982;   scale, there have been several refinements
          dipping further in the mid-Rhaetian to    Hallam and Wignall, 1997; McElwain et   to the Triassic chronostratigraphy. The
                                             al., 1999). The Late Triassic experienced
          ~50 m below pdmsl into the latest Triassic   the beginning of the lithospheric swell,   latest version of the time scale (Ogg et al.,
          and earliest Jurassic. Superimposed upon   ushering the breakup of Pangaea and its   2016) modifies the boundaries of Triassic
          this long-term trend is the record of 22   eventual split into discrete continents in    standard stages (ages) by anywhere
          widespread third-order sequence boundaries   the later Mesozoic (see Fig. S1 [see footnote   between <1 m.y. to almost 6 m.y. Like
          that have been identified, indicating sea-  1]). The definite signs of the beginning    earlier versions, the new time scale is
          level falls of mostly minor (<25 m) to   of Pangaean fragmentation were clearly   mainly based on biostratigraphy, anchored
          medium (25–75 m) amplitude. Only six of   manifest by the end of the Triassic with the   by selected radiometric dates, with some
          these falls are considered major, exceeding   basaltic outpouring of the massive Central   intervals refined by astronomical and
          the amplitude of 75 m. The long interval    Atlantic magmatic province (see, e.g.,   cyclostratigraphical fine-tuning, and others
          of Triassic oceanic withdrawal is likely to   Marzoli et al., 1999, 2004; Davies et al.,   aided by magnetostratigraphy. Conodonts
          have led to general scarcity of preserved   2017).                    and ammonoids constitute the mainstay
          marine record and large stratigraphic    In the past two decades substantial new   of the Triassic biostratigraphic correlations.
          lacunae. Lacking evidence of continental   stratigraphic data from Triassic sections   Special problems concerning wider correla-
          ice sheets in the Triassic, glacio-eustasy as   has come to light, and there have been sig-  tions using these fossil groups in the
          the driving mechanism for the third-order   nificant refinements in time scales, making   Triassic include taxonomic standardization,
          cyclicity can be ruled out. And even though   a review and revision of the Triassic sea-  rarity of markers, potential diachroniety
          transfer of water to and from land aquifers   level variations timely. The documentation   in conodontsʼ first and last appearance,
          to the ocean as a potential cause is plausible   for the revised Triassic sea-level curve,   and provinciality among ammonoids.
          for minor (a few tens of meters) sea-level   though still largely from northwestern    Since much of the Pangaean landscape was
          falls, the process seems counter-intuitive   and central Europe (western Tethys), now   dominated by terrestrial sediments, regional
          for third-order events for much of the   also includes sections further east from   correlations often rely on palynology,
          Triassic. Triassic paleoenvironmental    other parts of the Tethys, such as the   ostracods and tetrapods that do not lend
          scenarios demonstrate a close link between   Arabian Platform, Pakistan, India, China,   themselves to wider correlations with
          eustasy, climates, and biodiversity.  and Australia. From the boreal latitudes   marine records. Ogg et al. (2016) ascribe a

          GSA Today, v.28, https://doi.org/10.1130/GSATG381A.1. Copyright 2018, The Geological Society of America. CC-BY-NC.

          1 Data Repository item 2018390, background and documentation of depositional sequences for the new Triassic sea-level curve, is online at
          www.geosociety.org/datarepository/2018/.

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