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component in the oxygen-isotopic signal
is considered negligible in the Jurassic.
An examination of the available
sequence-stratigraphic reports of the
Jurassic (up to 2017) revealed that many
sections around the world cannot be cor-
related with precision with the European
stage stratotypes due to the provincial
nature of ammonites, though other fossil
groups can be helpful for cross-correla-
tions. The earlier syntheses presented by
Haq et al. (1988) and Hardenbol et al.
(1998) still form the basis of the current
synthesis. Additional information on third-
order sequences that form a part of this
reappraisal comes from Britain and France
(Wignall, 1991, from Kimmeridgian of
Dorset and France; Partington et al., 1993,
Kimmeridgian to Ryazanian of North Sea;
Herbin et al., 1995, Kimmeridgian and
Tithonian of Dorset and Yorkshire in the
UK and Boulonnais Basin in France;
Taylor et al., 2001, Late Jurassic of Wessex- Jurassic is presented as two figures (Fig. 1
Weald Basin; Williams et al., 2001,
Kimmeridgian and Tithonian of Wessex
Basin; Hesselbo, 2008, from the Jurassic
onshore sections of Britain); Poland
(Pienkowski, 2004, Early Jurassic of Polish
Basins); Greenland (Surlyk, 1990, Jurassic
of East Greenland); Russia (Sahagian et
al., 1996, mid- to Late Jurassic of Russian
Platform; Pinous et al., 1999, Callovian to
Oxfordian of western Siberia); Portugal
(Leinfelder, 1993, Kimmeridgian of
Lusitanian Basin); Denmark (Johannessen
et al., 1996, and Johannessen, 2003, Late
Jurassic of North Sea and Danish Central
Graben); and northern Switzerland (Gygi
et al., 1998, Oxfordian-Kimmeridgian;
Colombié and Ramell, 2007,
Kimmeridgian). Other areas of the Tethys
include the Arabian Platform (Sharland et
al., 2001, 2004; Haq and Al-Qahtani, 2005,
mid- to Late Jurassic; Al-Husseini and
Matthews, 2006, Oxfordian–early
Kimmeridgian), and India (Krishna, 2005,
mid- to Late Jurassic of Kutch Basin). For
the depositional cycles identified in Tibet,
where a nearly complete Jurassic record
exists (Li and Grant-Mackie, 1993), direct
correlation with the sub-boreal third-order
cycles of Europe and those from the west-
ern Tethys could not be established due to
differences in ammonite assemblages, but
the authors show similarity in trends, and
even tie some of the major sequence
boundaries with those in Europe. From the
Southern Hemisphere the only data that
could be considered for this synthesis
6 GSA Today | January 2018
come from Argentina (Mitchum and
Uliana, 1985; Legarreta and Uliana, 1996,
Jurassic of the Neuquén Basin). A number
of other studies of the Jurassic that were
undertaken at the broader (second-order)
scales were not considered relevant for a
third-order scale synthesis, but they do
sometimes provide additional constraints
for the long-term trends. As our ability to
more precisely correlate sequences
improves in the future (through ancillary
fossil biozones and other multiple, overlap-
ping, correlative criteria, such as che-
mostratigraphic methods), these deposi-
tional cycles may be extended to other
parts of the globe where the marine
Jurassic record is well preserved, such as
New Zealand.
RESULTS
The cycle chart resulting from the reap-
praisal of global stratigraphic data of the
for the Early Jurassic and Fig. 2 for the
Middle and Late Jurassic). The figures
represent the established biochronostratig-
raphy of the Jurassic, plotted against the
results of the current synthesis of the
sequence cycles, their revised ages (and an
updated numbering system partly adapted
from Hardenbol et al., 1998). A sea-level
curve based on the onlap record is the final
product of the synthesis. The biochro-
nostratigraphic parts of the figures show
the latest (GTS 2016 of Ogg et al., 2016)
age model from the Rhaetian (latest
Triassic stage) through Berriasian (the
early Cretaceous stage). This is tied to a
composite paleomagnetic reversal scale
that remains tentative below the Bajocian.
The seafloor magnetic anomaly record is
fragmentary below this level because the
older Jurassic oceanic lithosphere has been
largely subducted. Even for the Bajocian
through Tithonian interval, it is dependent
on a single site from the eastern Pacific
(ODP site 801). The polarity scale from the
Oxfordian to Tithonian is, nevertheless,
fairly well verified in multiple sections and the latest Toarcian, the sea levels fell again
basins. The stages, Hettangian to
Tithonian, currently considered standard
subdivisions for the Jurassic, are tied to
ammonite zones that, much like in the
Cretaceous, are the most common fossil
group for correlation in the Jurassic. The
cross-correlation between zones from the
relatively warm-water Tethyan regions and
cooler-water boreal/sub-boreal regions
(though still a part of the western Tethys)
follow those suggested by Hardenbol et al.
(1998) and later by Ogg and Hinnov (2012).
Calcareous nannofossil zones of the
Jurassic, also included, are mostly of long
duration and of limited correlative utility
in this period. However, sometimes they
do provide additional criteria for
correlations.
The two columns on the right in Figures
1 and 2 show sea-level events (mostly
third-order and some consistent fourth-
order sequence boundaries) and sea-level
curves (long-term and short-term) for the
Jurassic. When sequence boundaries are
correlatable in several basins they are con-
sidered widespread (though global validity
cannot be verified due to the Eurocentric
nature of most of the data). The criteria for
the long-term curves (shown in the last
column on the right) have been discussed
earlier in this paper, and the shorter-term
sea-level curve that is derived from the
sequence-stratigraphic data to its left. The
amplitudes of third-order sea-level changes
(rise and falls in meters) shown here are
averaged from stratigraphic estimates in
several basins and should be considered
approximate (see discussion in Haq, 2014).
They are subdivided into three magnitude
categories of sea-level falls: major (>75 m),
medium (25–75 m), and minor (<25 m).
Most sea-level events fall within the
medium category.
The long-term sea-level envelope (indi-
cating the maximum flooding of continen-
tal margins and interior basins) shows that
sea level remained close to or below pres-
ent-day mean sea level (pdmsl) from the
latest Triassic through the Hettangian and
early Sinemurian, rising only a few tens of
meters above pdmsl in the late
Sinemurian-Pliensbachian, and by the late
Pliensbachian it reverted back to levels
comparable to pdmsl. In the Toarcian,
there is an apparent long-term rise that
may have peaked at ~75 m above pdmsl. In
to a few tens of meters above pdmsl, a
trend that continued into the early
Aalenian. From the late Aalenian onward,
there is a gradual sea-level rise trend, with
a few tens of meters of dip in the Bajocian
and another in the latest Callovian–earliest
Oxfordian that culminated in the peak high
of the Jurassic in the late Kimmeridgian–
early Tithonian. Near the Kimmeridgian-
Tithonian boundary, the sea level may
have been as high as ~140 m above pdmsl.