Jurassic Sea-Level Variations: A Reappraisal
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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 sea-level 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 sea-level 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 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 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 cyclicity, in the absence of major ice sheets in the Jurassic, remains enigmatic.
Manuscript received 5 Sept. 2017. Revised manuscript received 26 Sept. 2017. Manuscript accepted 26 Sept. 2017. Posted online 24 Oct. 2017.
© The Geological Society of America, 2017. CC-BY-NC.