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Ogg and Hinnov (2012) and Ogg et al.           foraminifera, nannoplankton, and calpi-         of the seafloor of Jurassic age has since
(2016). The last version of the Jurassic time  onellids). In that Meso-Cenozoic synthesis      been subducted.
scale is partially based on constraints from   (Haq et al., 1988; Hardenbol et al., 1998), a
best fits of numerical radiometric ages,       special attempt was made to study all             The documentation of the shorter-term
partially on cyclostratigraphy in strata of    available stage stratotype (or neo-strato-      sea-level changes (third-order events) are,
various stages and oxygen and other isoto-     type) sections (including those from the        of course, based on sequence-strati-
pic data. Magnetostratigraphy was helpful      Jurassic) that form the basis (or a global      graphic information from some relatively
only in the Bajocian through Tithonian         standard) for biochronostratigraphy. For        longer duration sections, but in most loca-
interval (with a hiatus at Callovian-          the Mesozoic, most of these sections hap-       tions this information is pieced together
Oxfordian transition) where the low-ampli-     pen to have been chosen in NW Europe.           from several sections within the Jurassic.
tude seafloor magnetic anomalies (from         Another reason for the Eurocentricity of        Data from these studies were evaluated
Ocean Drilling Program site 801 on the         the Jurassic sea-level curve was the limita-    (and sequence-stratigraphically reinter-
older part of eastern Pacific Plate) could be  tions posed by the provinciality of the         preted, as needed) before inclusion in the
tied to magnetostratigraphy. The attempts      ammonite zones that do not permit precise       current synthesis. The Jurassic paleonto-
to astronomically fine-tune discrete inter-    correlations for a truly globally based         logical cross-correlations (i.e., zonal
vals of the Jurassic (see, e.g., Strasser,     chronology of eustatic events. These cor-       schemes based on different fossil groups
2007, and a summary by Huang in Ogg            relations become somewhat easier in the         and in different regions; Hardenbol et al.,
and Hinnov, 2012) may help with duration       latest Jurassic (Tithonian) where one can       1998) proved to be invaluable in aiding
of some zonal intervals, but such piece-       draw on multiple correlative tools, but for     correlations in some cases. The sequence-
meal efforts do not alleviate the precision    much of the Jurassic the correlation limita-    stratigraphic interpretation criteria are
issues of all of the stage boundaries that     tions persist. In the current synthesis, all    well established and do not need repeti-
are exacerbated by the lack of reproducible    available additional studies in Jurassic        tion; however, in addition to these, other
radiometric control for much of the Middle     stratigraphic sections (from 1988 through       lithological and paleontological criteria
and Late Jurassic. This implies that, in       2017) with good biostratigraphic data           (originally listed in Haq and Schutter,
general, the time scale of the Jurassic and    were reevaluated. As a result, the correla-     2008; Haq, 2014) can also aid in the iden-
precision of the ages of many biostrati-       tion net has now been widened somewhat          tification of system tracts, depositional
graphic zonal boundaries still remain less     to include other areas to the east in the       surfaces, and sequence boundaries in out-
than well constrained. As Ogg and Hinnov       Tethyan realm and to the Southern               crop and well-log sections. These include
(2012) state, the Jurassic scale “should be    Hemisphere; i.e., Argentina’s Neuquén           forced regressive facies, condensed sec-
considered a work in progress” and             Basin, where a nearly complete Jurassic         tion deposits, transgressive coals, evapo-
although new constraints have refined the      record is preserved (e.g., Legarreta and        rites, carbonate megabreccias, exposure-
overall numerical chronology, “several         Uliana, 1996). The heavy dependence on          related deposits (i.e., incised valley fills,
intervals lack adequate constraints.” Any      ammonite zones for correlation means that       autochthonous coals, eolian sandstones,
future modifications of the time scale will    there is a built-in uncertainty in the ages of  and karst in carbonates), as well as later-
obviously necessitate the recalibration of     the sequence boundaries. While the              ite/bauxite deposits. General trends in
the sea-level chronology.                      sequence boundaries are placed according        oxygen-isotopic data, in as much as they
                                               to their relative stratigraphic position        reflect broad climatic trends, can also
REVISION OF THE JURASSIC                       within an ammonite zone (e.g., at the base,     lend greater confidence to the longer-term
SEA-LEVEL CURVE                                middle, top, or at the zonal boundary),         eustatic trends, and when the shorter-term
                                               theoretically the error bar could extend to     isotopic excursions are distinctive, they
  The main correlative tool in the Jurassic    the entire duration of the zone or subzone      can aid in the positioning of the timing of
marine strata is ammonite biostratigraphy,     in question.                                    the sequence boundaries within a long-
occasionally assisted by other fossil                                                          duration biostratigraphic zone (see Haq,
groups, such as dinoflagellates, radiolaria,     The long-term sea-level trends are simi-      2014, for further discussion). In this syn-
calcareous nannofossils, and calpionellids     lar to those shown in Haq et al. (1987, 1988)   thesis, d18O isotopic data from Jurassic
(the last only in the Late Jurassic). In the   and Hardenbol et al. (1998). The original       belemnites collected from the European
earlier Meso-Cenozoic sequence chrono­         long-term curve for the Jurassic was based      sections (from the Sinemurian through
stratigraphy of third-order sea-level          on continental flooding data, but unlike        Tithonian interval; see Martinez and
changes (Haq et al., 1988, and later by        the Cretaceous (see Haq, 2014), knowledge       Dera, 2015) were plotted against the
Hardenbol et al., 1998), the Jurassic          of the oceanic crustal production rates for     sea-level curve (and smoothed by Robust
sequence chronostratigraphy was based on       the Jurassic (i.e., variations in the mean      Lowess Regression) for comparison
sections in northern and central Europe        age of the oceanic lithosphere, variations      (see GSA Data Repository Fig. S11). The
(northern and southern coasts of England,      in the production rates at mid-ocean            general trends in these data (which repre-
west-central France, southern Germany,         ridges, duration of the emplacement of          sent broad climatic variations) show an
and Switzerland) and their ammonite and        seamounts, and large igneous provinces on       apparent similarity to the long-term sea-
microfossil content (mostly dinoflagellates,   the seafloor) is fragmentary because most       level curve, even though the ice-volume

1GSA Data Repository Item 2017387, documentation of depositional sequences comprising the new Jurassic sea-level curve, is online at www.geosociety.org/pubs/
ft2017.htm.

                                               www.geosociety.org/gsatoday                                                                                     5
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