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Jurassic Sea-Level Variations: A Reappraisal
Bilal U. Haq, Smithsonian Institution, Washington, D.C., 20013; and Institut des Sciences de la Terre, Sorbonne Universités, UPMC,
Paris 75252, France, bhaq@inbox.com
ABSTRACT
An accurate chronostratigraphy of the
timing and magnitude of global sea-level
trends and their short-term variations is an
indispensable tool in high-resolution cor-
relations, exploration, and paleoenviron-
mental and geodynamic models. This
paper is a reappraisal of the Jurassic sea-
level history in view of recent updates in
time scales and a large body of new chro-
nostratigraphic data accrued since 1998,
when the last such synthesis was pre-
sented. A review of the Jurassic sea-level
history has also been keenly awaited by
explorationists given that the Jurassic con-
tinues to be a major exploration target for
the industry. As in previous eustatic mod-
els of this period, the updated Jurassic sea- highstands. The broad trends in Jurassic
level curve remains largely Eurocentric
due to the limitations imposed by biostrati-
graphic correlation criteria (provinciality
of ammonite and microfossil zones),
though it can now be extended to some
parts of the Tethys toward the east. The
updated long-term curve indicates that
there was a general rise of sea level
through the Jurassic that began close to a
level similar to or below the present-day
mean sea level (pdmsl) in the early
Jurassic, culminating in the peak high in
the late Kimmeridgian–early Tithonian
interval, before stabilizing in the earliest
Cretaceous at ~110 m above pdmsl. Within
this long-term trend are relative second-
order highs in the Toarcian and Aalenian,
and at Bathonian-Callovian and
Kimmeridgian-Oxfordian boundaries.
Superimposed are 64 third- and fourth-
order fluctuations of which 15 are consid-
ered major with base-level falls of more
than 75 m, although precise amplitudes of
drawdowns are often difficult to establish.
Higher resolution fourth-order cyclicity
(~410 k.y.) is also observable in many
Jurassic sections whenever sedimentation
rates were high. Causes for the third-order
GSA Today, v. 28, no. 1, doi: 10.1130/GSATG359A.1. Copyright 2017, The Geological Society of America. CC-BY-NC.
4 GSA Today | January 2018
cyclicity, in the absence of major ice sheets
in the Jurassic, remains enigmatic.
INTRODUCTION
A record of sea-level variations of the
past inferred from the stratigraphy of con-
tinental margins and interior basins (where
the movements of the shoreline can be best
documented) is a key predictive tool in
hydrocarbon exploration. These data can
provide insights into several pre-drill
assessment criteria, including the migra-
tion of reservoir facies in response to rises
and falls of sea level, the frequency and
duration of subaerial exposure during low-
stands, and the generation and preservation
of source rocks during transgressions and
sea-level variations have been known for
some time (Vail et al., 1977; Hallam, 1978,
2001; Haq et al., 1987, 1988; Hardenbol et
al., 1998; Haq and Al-Qahtani, 2005), but
recent updates of time scales and the
accrual of new stratigraphic data from the
period dictate a reappraisal of Jurassic
eustatic history, especially at the third-order 2008). Although actual global tempera-
(shorter-term) time scales. A reappraisal
of the long- and short-term trends of
the base level would also be useful for
academic research because such informa-
tion can be the basis of stratigraphic,
paleo­ environmental, and geodynamic
models. In this communication, a brief
summary of the updated version of the
Jurassic sea-level history is presented so
that it can be expediently made available to
the research community.
The Jurassic period is currently esti-
mated to have lasted some 55.6 m.y.
(201.3–145.7 Ma) (Ogg et al., 2016). The
period saw relatively low sea levels in the
Early Jurassic, with the exception of the
early Toarcian, which witnessed a relative
high, a variable overall lowstand in the
Middle Jurassic, and a gradual rise there­
after that lasted through much of the Late
Jurassic. Climates also paralleled these
trends. Faunal and isotopic data imply rela-
tively warm climates for most of the
Jurassic, with some exceptions, lacking
credible evidence for widespread glacia-
tions in much of this period. However, the
relative warmth of the Hettangian through
Toarcian interval seems to have been inter-
rupted by a cooler late Pliensbachian
through early Toarcian (Hinnov and Park,
1999; Dera et al., 2009; Suan et al., 2010;
Korte and Hesselbo, 2011; Korte et al.,
2015). Korte and Hesselbo (2011) believe
that the Early Jurassic may have fluctuated
between greenhouse and icehouse condi-
tions. There may also have been some
cooler intervals in the Aalenian, Bajocian,
Bathonian, and early Callovian (Rogov
and Zakharov, 2010), as well as a cold spell
near the Middle–Late Jurassic transition
(in the late Callovian) (Dromart et al.,
2003). Most of the Late Jurassic is inter-
preted to have been relatively warmer and
equable, experiencing peak warmth in the
Kimmeridgian (Frakes et al., 1992;
Zakharov et al., 2006; Brigaud et al.,
tures and atmospheric or oceanic latitudi-
nal thermal gradients of the Jurassic are
only conjectured, modeling indicates that
pCO2 levels may have been a minimum of
four times the present-day levels (see, e.g.,
Sellwood and Valdes, 2008). The long-
term sea level and climatic trends also
show an apparent correspondence.
JURASSIC TIME SCALE
Jurassic time scales have been in a sig-
nificant state of flux since the last third-
order sea-level curve for this period was
published by Haq et al. (1988) or the later
update by Hardenbol et al. (1998).
Considerable advancements have been
made to better delimit the stage boundaries
of the Jurassic, and the most recent effort
to update this time scale was presented by