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, [email protected] ABSTRACT cyclicity, in the absence of major ice sheets Jurassic. Climates also paralleled these An accurate chronostratigraphy of the in the Jurassic, remains enigmatic. trends. Faunal and isotopic data imply rela- timing and magnitude of global sea-level tively warm climates for most of the trends and their short-term variations is an INTRODUCTION Jurassic, with some exceptions, lacking indispensable tool in high-resolution cor- A record of sea-level variations of the credible evidence for widespread glacia- relations, exploration, and paleoenviron- past inferred from the stratigraphy of con- tions in much of this period. However, the mental and geodynamic models. This tinental margins and interior basins (where relative warmth of the Hettangian through paper is a reappraisal of the Jurassic sea- the movements of the shoreline can be best Toarcian interval seems to have been inter- level history in view of recent updates in documented) is a key predictive tool in rupted by a cooler late Pliensbachian time scales and a large body of new chro- hydrocarbon exploration. These data can through early Toarcian (Hinnov and Park, nostratigraphic data accrued since 1998, provide insights into several pre-drill 1999; Dera et al., 2009; Suan et al., 2010; when the last such synthesis was pre- assessment criteria, including the migra- Korte and Hesselbo, 2011; Korte et al., sented. A review of the Jurassic sea-level tion of reservoir facies in response to rises 2015). Korte and Hesselbo (2011) believe history has also been keenly awaited by and falls of sea level, the frequency and that the Early Jurassic may have fluctuated explorationists given that the Jurassic con- duration of subaerial exposure during low- between greenhouse and icehouse condi- tinues to be a major exploration target for stands, and the generation and preservation tions. There may also have been some the industry. As in previous eustatic mod- of source rocks during transgressions and cooler intervals in the Aalenian, Bajocian, els of this period, the updated Jurassic sea- highstands. The broad trends in Jurassic Bathonian, and early Callovian (Rogov level curve remains largely Eurocentric sea-level variations have been known for and Zakharov, 2010), as well as a cold spell due to the limitations imposed by biostrati- some time (Vail et al., 1977; Hallam, 1978, near the Middle–Late Jurassic transition graphic correlation criteria (provinciality 2001; Haq et al., 1987, 1988; Hardenbol et (in the late Callovian) (Dromart et al., of ammonite and microfossil zones), al., 1998; Haq and Al-Qahtani, 2005), but 2003). Most of the Late Jurassic is inter- though it can now be extended to some recent updates of time scales and the preted to have been relatively warmer and parts of the Tethys toward the east. The accrual of new stratigraphic data from the equable, experiencing peak warmth in the updated long-term curve indicates that period dictate a reappraisal of Jurassic Kimmeridgian (Frakes et al., 1992; there was a general rise of sea level eustatic history, especially at the third-order Zakharov et al., 2006; Brigaud et al., through the Jurassic that began close to a (shorter-term) time scales. A reappraisal 2008). Although actual global tempera- level similar to or below the present-day of the long- and short-term trends of tures and atmospheric or oceanic latitudi- mean sea level (pdmsl) in the early the base level would also be useful for nal thermal gradients of the Jurassic are Jurassic, culminating in the peak high in academic research because such informa- only conjectured, modeling indicates that the late Kimmeridgian–early Tithonian tion can be the basis of stratigraphic, pCO2 levels may have been a minimum of interval, before stabilizing in the earliest pale oenvironmental, and geodynamic four times the present-day levels (see, e.g., Cretaceous at ~110 m above pdmsl. Within models. In this communication, a brief Sellwood and Valdes, 2008). The long- this long-term trend are relative second- summary of the updated version of the term sea level and climatic trends also order highs in the Toarcian and Aalenian, Jurassic sea-level history is presented so show an apparent correspondence. and at Bathonian-Callovian and that it can be expediently made available to Kimmeridgian-Oxfordian boundaries. the research community. JURASSIC TIME SCALE Superimposed are 64 third- and fourth- The Jurassic period is currently esti- Jurassic time scales have been in a sig- order fluctuations of which 15 are consid- mated to have lasted some 55.6 m.y. nificant state of flux since the last third- ered major with base-level falls of more (201.3–145.7 Ma) (Ogg et al., 2016). The order sea-level curve for this period was than 75 m, although precise amplitudes of period saw relatively low sea levels in the published by Haq et al. (1988) or the later drawdowns are often difficult to establish. Early Jurassic, with the exception of the update by Hardenbol et al. (1998). Higher resolution fourth-order cyclicity early Toarcian, which witnessed a relative Considerable advancements have been (~410 k.y.) is also observable in many high, a variable overall lowstand in the made to better delimit the stage boundaries Jurassic sections whenever sedimentation Middle Jurassic, and a gradual rise there- of the Jurassic, and the most recent effort rates were high. Causes for the third-order after that lasted through much of the Late to update this time scale was presented by GSA Today, no. 1, doi: 10.1130/GSATG359A.1. Copyright 2017, The Geological Society of America. CC-BY-NC. 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 The documentation of the shorter-term best fits of numerical radiometric ages, special attempt was made to study all sea-level changes (third-order events) are, partially on cyclostratigraphy in strata of available stage stratotype (or neo-strato- of course, based on sequence-strati- various stages and oxygen and other isoto- type) sections (including those from the graphic information from some relatively pic data. Magnetostratigraphy was helpful Jurassic) that form the basis (or a global longer duration sections, but in most loca- only in the Bajocian through Tithonian standard) for biochronostratigraphy. For tions this information is pieced together interval (with a hiatus at Callovian- the Mesozoic, most of these sections hap- from several sections within the Jurassic. Oxfordian transition) where the low-ampli- pen to have been chosen in NW Europe. Data from these studies were evaluated tude seafloor magnetic anomalies (from Another reason for the Eurocentricity of (and sequence-stratigraphically reinter- Ocean Drilling Program site 801 on the the Jurassic sea-level curve was the limita- preted, as needed) before inclusion in the older part of eastern Pacific Plate) could be tions posed by the provinciality of the current synthesis. The Jurassic paleonto- tied to magnetostratigraphy. The attempts ammonite zones that do not permit precise logical cross-correlations (i.e., zonal to astronomically fine-tune discrete inter- correlations for a truly globally based schemes based on different fossil groups vals of the Jurassic (see, e.g., Strasser, chronology of eustatic events. These cor- and in different regions; Hardenbol et al., 2007, and a summary by Huang in Ogg relations become somewhat easier in the 1998) proved to be invaluable in aiding and Hinnov, 2012) may help with duration latest Jurassic (Tithonian) where one can correlations in some cases. The sequence- of some zonal intervals, but such piece- draw on multiple correlative tools, but for stratigraphic interpretation criteria are meal efforts do not alleviate the precision much of the Jurassic the correlation limita- well established and do not need repeti- issues of all of the stage boundaries that tions persist. In the current synthesis, all tion; however, in addition to these, other are exacerbated by the lack of reproducible available additional studies in Jurassic lithological and paleontological criteria radiometric control for much of the Middle stratigraphic sections (from 1988 through (originally listed in Haq and Schutter, and Late Jurassic. This implies that, in 2017) with good biostratigraphic data 2008; Haq, 2014) can also aid in the iden- general, the time scale of the Jurassic and were reevaluated. As a result, the correla- tification of system tracts, depositional precision of the ages of many biostrati- tion net has now been widened somewhat surfaces, and sequence boundaries in out- graphic zonal boundaries still remain less to include other areas to the east in the crop and well-log sections. These include than well constrained. As Ogg and Hinnov Tethyan realm and to the Southern forced regressive facies, condensed sec- (2012) state, the Jurassic scale “should be Hemisphere; i.e., Argentina’s Neuquén tion deposits, transgressive coals, evapo- considered a work in progress” and Basin, where a nearly complete Jurassic rites, carbonate megabreccias, exposure- although new constraints have refined the record is preserved (e.g., Legarreta and related deposits (i.e., incised valley fills, overall numerical chronology, “several Uliana, 1996). The heavy dependence on autochthonous coals, eolian sandstones, intervals lack adequate constraints.” Any ammonite zones for correlation means that and karst in carbonates), as well as later- future modifications of the time scale will there is a built-in uncertainty in the ages of ite/bauxite deposits. General trends in obviously necessitate the recalibration of the sequence boundaries.