Jurassic Sea-Level Variations: A Reappraisal

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 correlations, exploration, and paleoenvironmental and geodynamic models. This paper is a reappraisal of the Jurassic sealevel history in view of recent updates in time scales and a large body of new chronostratigraphic data accrued since 1998, when the last such synthesis was presented. A review of the Jurassic sea-level history has also been keenly awaited by explorationists given that the Jurassic continues to be a major exploration target for the industry. As in previous eustatic models of this period, the updated Jurassic sealevel curve remains largely Eurocentric due to the limitations imposed by biostratigraphic 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 secondorder highs in the Toarcian and Aalenian, and at Bathonian-Callovian and Kimmeridgian-Oxfordian boundaries. Superimposed are 64 third-and fourthorder fluctuations of which 15 are considered 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 Bilal U. Haq,

A new Pliensbachian elasmobranch (Vertebrata, Chondrichthyes) assemblage from Europe, and its contribution to the understanding of late Early Jurassic elasmobranch diversity and distributional patterns

“…Significantly, the Pliensbachian-Toarcian transition and the immediately following Toarcian-Oceanic Anoxic Event (T-OAE) mark two global episodes of palaeoenvironmental perturbations that were accompanied by well-known secondorder mass extinctions of marine invertebrate biota (e.g., Little and Benton 1995;Cecca and Macchioni 2004;Wignall et al 2005;Caswell et al 2009;Dera et al 2010;Caruthers et al 2014;Rita et al 2016), a pronounced eustatic sea-level rise (e.g., Hallam 2001;Haq 2017), and a marked increase in seawater temperature associated with global warming (e.g., Hesselbo and Jenkyns 1998;Bailey et al 2003;Rosales et al 2004;Gómez et al 2008;Suan et al 2010;Korte and Hesselbo 2011). The upper Pliensbachian of Grimmen therefore documents a marginal marine ecosystem immediately before the two global palaeoenvironmental perturbation episodes that occurred at and immediately following the Pliensbachian-Toarcian transition.…”

Triassic Eustatic Variations Reexamined

“…The documentation for the revised Triassic sea-level curve still comes largely from low to temperate paleolatitudes of the Tethys, but also includes its boreal counterpart sections from the Sverdrup Basin, Svalbard, and the Barents Sea. As indicated previously for the Jurassic (Haq, 2017), the reliance mainly on ammonoids and conodont biostratigraphies for correlations means that the built-in uncertainty in the proposed ages of sequence boundaries is equal to the duration of the biozone (or subzone) that is used to date the boundary. This means that error-bars are relatively small in the Induan through Anisian interval (average zonal duration = 0.34 m.y.…”

“…What solid-Earth tectonic controls could have potentially influenced the sea-level changes in the Triassic when the planet was characterized by a large supercontinent with no apparent major ice accumulations on land? Most of the recent insights into the understanding of tectonic influences on sea level (as measured locally) either try to explain it on very short time scales (isostatic response to elastic and viscous loading and unloading) or on the very long time scales of multiple millions of years (see discussions in Haq, 2014, 2017, and Cloetingh and Haq, 2015. These influences cannot account for the third-order cyclicity in the Early and Middle Triassic (average duration of ~1 m.y./cycle in the Early and ~1.4 m.y./ cycle in the Middle Triassic).…”

“…From the boreal latitudes documentation includes sections from the Sverdrup Basin, Svalbard, and the Barents Sea. This paper serves to complete a review of the entire Mesozoic as both Cretaceous and Jurassic sea-level variations have already been reappraised (Haq, 2014(Haq, , 2017. For a background of the paleoenvironmental conditions (oceans and climates) in the Triassic see the GSA data repository (see footnote 1).…”

Aquifer-eustasy as the main driver of short-term sea-level fluctuations during Cretaceous hothouse climate phases

“…the constant ups and downs at several hierarchical temporal scales that are beautifully displayed by sequence stratigraphy for both icehouse and greenhouse phases of the Earth system (e.g. Haq et al 1987;Haq 2014Haq , 2017aHaq , b, 2018. This intrinsically gives an argument by itself for a cyclic process.…”