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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
