Triassic Eustatic Variations Reexamined
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Triassic Eustatic Variations Reexamined Bilal U. Haq, Smithsonian Institution, Washington, D.C., USA, and Sorbonne University, Paris, France ABSTRACT INTRODUCTION documentation includes sections from the Documentation of eustatic variations for The Triassic Period encompasses 50.5 Sverdrup Basin, Svalbard, and the Barents Sea. This paper serves to complete a review the Triassic is limited by the paucity of the m.y., spanning an interval from 251.9 to of the entire Mesozoic as both Cretaceous preserved marine stratigraphic record, 201.4 Ma (Ogg et al., 2016). By this time, and Jurassic sea-level variations have which is confined mostly to the low and the megacontinent of Pangaea had already already been reappraised (Haq, 2014, 2017). middle paleolatitudes of the Tethys Ocean. assembled, surrounded by the Panthalassa For a background of the paleoenvironmental A revised sea-level curve based on reevalu- Ocean that covered >70% of Earth’s conditions (oceans and climates) in the ation of global stratigraphic data shows a surface, and by the mid-Triassic the Triassic see the GSA data repository (see clear trend of low seastands for an extended Pangaean landmass was almost evenly footnote 1). period that spans almost 80 m.y., from the distributed in the two hemispheres around latest Permian to the earliest Jurassic. In the the paleo-equator (see Fig. S1 in the GSA 1 TRIASSIC TIME SCALE UPDATES Early and Middle Triassic, the long-term Data Repository ). The interval from latest A succinct discussion of the method- sea levels were similar to or 10–20 m Permian through the earliest Jurassic, a ological advancements and modifications higher than the present-day mean sea level time span of nearly 80 m.y., represents the to the Triassic time scale can be found in (pdmsl). This trend was reversed in the late longest spell of low seastands of the Preto et al. (2010), and a detailed discussion Ladinian, marked by a steady rise and cul- Phanerozoic. The Triassic is also bracketed of Triassic stratigraphy has been presented minating in peak sea levels of the Triassic by two major biotic extinctions near the by Ogg et al. (2014). Since the last update (~50 m above pdmsl) in the late Carnian. Permian-Triassic (P-T) and Triassic-Jurassic of the Triassic third-order sea-level varia- The trend reverses again with a decline in boundaries, the one at P-T boundary being the most severe biotic turnover of the tions (Haq and Al-Qahtani, 2005) that was the late Norian and the base level remain- Phanerozoic (Raup and Sepkowski, 1982; calibrated to an earlier version of the time ing close to the pdmsl, and then Hallam and Wignall, 1997; McElwain et scale, there have been several refinements dipping further in the mid-Rhaetian to al., 1999). The Late Triassic experienced to the Triassic chronostratigraphy. The ~50 m below pdmsl into the latest Triassic the beginning of the lithospheric swell, latest version of the time scale (Ogg et al., and earliest Jurassic. Superimposed upon ushering the breakup of Pangaea and its 2016) modifies the boundaries of Triassic this long-term trend is the record of 22 eventual split into discrete continents in standard stages (ages) by anywhere widespread third-order sequence boundaries the later Mesozoic (see Fig. S1 [see footnote between <1 m.y. to almost 6 m.y. Like that have been identified, indicating sea- 1]). The definite signs of the beginning earlier versions, the new time scale is level falls of mostly minor (<25 m) to of Pangaean fragmentation were clearly mainly based on biostratigraphy, anchored medium (25–75 m) amplitude. Only six of manifest by the end of the Triassic with the by selected radiometric dates, with some these falls are considered major, exceeding basaltic outpouring of the massive Central intervals refined by astronomical and the amplitude of 75 m. The long interval Atlantic magmatic province (see, e.g., cyclostratigraphical fine-tuning, and others of Triassic oceanic withdrawal is likely to Marzoli et al., 1999, 2004; Davies et al., aided by magnetostratigraphy. Conodonts have led to general scarcity of preserved 2017). and ammonoids constitute the mainstay marine record and large stratigraphic In the past two decades substantial new of the Triassic biostratigraphic correlations. lacunae. Lacking evidence of continental stratigraphic data from Triassic sections Special problems concerning wider correla- ice sheets in the Triassic, glacio-eustasy as has come to light, and there have been sig- tions using these fossil groups in the the driving mechanism for the third-order nificant refinements in time scales, making Triassic include taxonomic standardization, cyclicity can be ruled out. And even though a review and revision of the Triassic sea- rarity of markers, potential diachroniety transfer of water to and from land aquifers level variations timely. The documentation in conodontsʼ first and last appearance, to the ocean as a potential cause is plausible for the revised Triassic sea-level curve, and provinciality among ammonoids. for minor (a few tens of meters) sea-level though still largely from northwestern Since much of the Pangaean landscape was falls, the process seems counter-intuitive and central Europe (western Tethys), now dominated by terrestrial sediments, regional for third-order events for much of the also includes sections further east from correlations often rely on palynology, Triassic. Triassic paleoenvironmental other parts of the Tethys, such as the ostracods and tetrapods that do not lend scenarios demonstrate a close link between Arabian Platform, Pakistan, India, China, themselves to wider correlations with eustasy, climates, and biodiversity. and Australia. From the boreal latitudes marine records. Ogg et al. (2016) ascribe a GSA Today, v.28, https://doi.org/10.1130/GSATG381A.1. Copyright 2018, The Geological Society of America. CC-BY-NC. 1Data Repository item 2018390, background and documentation of depositional sequences for the new Triassic sea-level curve, is online at www.geosociety.org/datarepository/2018/. composite error of between 0.2 and 0.59 are similar to the sea-level curve, recording duration = 0.77 m.y.), and expand for the m.y. for the stage boundaries in the Triassic only a long-duration signal in the Late remainder of the Carnian through Rhaetian depending on the type of data (see also the Triassic (Trotter et al., 2015). This singular interval (average ammonoid zonal duration GSA data repository for further discussion attribute of the Triassic stratigraphy (i.e., = 2.43 m.y.). Using multiple overlapping of time scales [see footnote 1]). the potential of missing marine strati- criteria (i.e., several fossil groups), these graphic record and large time gaps that uncertainties can sometimes be narrowed. LARGE TIME GAPS IN THE shows up in sequence-stratigraphic signal) The long-term sea-level envelope for the RECORD OF THE MIDDLE requires further thought and inquiry. Triassic is similar to those shown in Haq et AND LATE TRIASSIC? al. (1987, 1988) and Hardenbol et al. (1998). Even a cursory look at the most recent REEVALUATION OF THE The original long-term curve for the update of the Triassic time scale (Ogg et al., TRIASSIC SEA-LEVEL CURVE Triassic was based on continental flooding 2016) reveals its extreme lopsidedness: As stated above, the main correlative data and this is still the case, because other while the Early Triassic spans only 5.1 m.y. criteria for the Triassic marine strata are constraints for this envelope, such as mean and the Middle Triassic increases to 9.1 ammonoid and conodont biostratigraphies. age of oceanic crust, are not available since m.y., the Late Triassic jumps to a substan- The distribution of Triassic ammonoids almost all of the Triassic age oceanic crust tial span of 35.6 m.y. Some unevenness is to taxa in the boreal latitudes (e.g., British has since been subducted, with the excep- be expected, but this extreme asymmetry is Columbia, Siberia), however, was not the tion of a limited area of the seafloor on also witnessed in the time spans of the same as those in the Tethyan realm, and Exmouth Plateau west of Australia (von stages (ages) and biostratigraphic zones this provinciality poses limitations for direct Rad et al., 1989). Recently van der Meer within the stages, as well the lengths of the correlations. The detailed cross-correlation et al. (2017) have produced independent sequence cycles and corresponding sea- schemes provided by Hardenbol et al. (1998) estimates of the long-term sea level based level events that all increase in duration in that have attempted to tie marine and on Sr-isotope data, which show close the Middle to Late Triassic. terrestrial biostratigraphies from the similarities to the continental flooding If the above apparent chronostratigraphic Tethyan and high latitudes are invaluable curves and to the long-term Triassic curve asymmetry is real, then the large differ- for the longer distance correlations. The presented here, although the interpreted ences in the duration of fossil zones imply correlation chart of these authors also pro- amplitudes differ. The documentation for that evolutionary rates (as measured by vides links to the stratigraphic distribution the short-term (third-order) sea-level events appearance of new species/m.y.) were of other Tethyan fossil groups, such as is based on sequence-stratigraphic informa- relatively rapid in the Early Triassic (thus calcareous nannofossils, dinoflagellates, tion pieced together from several available the availability of a high-resolution biozonal larger foraminifera, ostracods, radiolarians, longer duration sections and augmented by subdivision), declining somewhat in the and spore and pollen, which can be invalu- some shorter-duration records. In addition Middle Triassic, and slowing down to an able in constraining some of the long- to sequence-stratigraphic interpretive extreme thereafter (characterized by a few distance correlations (Hardenbol et al., criteria that are now well established and long-duration biozones), especially in the 1998, chart number 8).