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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

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/.

4 GSA Today | December 2018

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