Page 5 - i1052-5173-28-12_GSAT-web-compressed
P. 5

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).   do not require repetition, other features that
          later Late Triassic. However, the temporal   In this reappraisal of the Triassic sea-  were particularly helpful in stratigraphic
          lengths of sequence cycles (based on sedi-  level variations, which uses all available   interpretations (originally listed in Haq
          mentary facies shifts) do not have to follow   sequence-stratigraphic data published    and Schutter, 2008) in the Triassic include
          the biotic evolutionary trends, and yet they   since the last such compilation (Haq and   forced-regressive facies, organic-rich facies
          do. Their long time spans (average of ~5   Al-Qahtani, 2005) as well as older studies,   of the condensed sections, transgressive
          m.y./cycle in the Middle and Late Triassic)   was reevaluated before inclusion in the    coals, evaporites, exposure-related deposits
          would imply a built-in bias in the record   current synthesis. The documentation for   (including incised valley fills, autochtho-
          expressed as a lack of preserved marine   the revised Triassic sea-level curve still   nous coals, eolian sandstones, and karst),
          stratigraphic record. This seems plausible   comes largely from low to temperate paleo-  and laterite/bauxite deposits. These fea-
          in a scenario where the long-term trend of   latitudes of the Tethys, but also includes    tures can often aid in the identification
          low seastands for the period means fewer   its boreal counterpart sections from the   of depositional surfaces and system tract
          marine records in favor of more terrestrial   Sverdrup Basin, Svalbard, and the Barents   boundaries on outcrops and in well logs.
          sedimentary records. This is exacerbated   Sea. As indicated previously for the   The earlier syntheses for the Triassic
          by mostly type-1 sequence boundaries   Jurassic (Haq, 2017), the reliance mainly on   period (Haq et al., 1987, 1988; Hardenbol
          (when the base line withdraws beyond the   ammonoids and conodont biostratigraphies   et al., 1998; Haq and Al-Qahtani, 2005)
          shelf edges) that may incorporate large    for correlations means that the built-in   continue to be the basis for the new
          erosional time gaps. Large temporal    uncertainty in the proposed ages of   revision presented here. The Triassic cycles
          lacunae in the stratigraphic record could   sequence boundaries is equal to the dura-  presented in Haq et al. (1987, 1988) were
          explain the potentially specious signal that   tion of the biozone (or subzone) that is used   based on sections from NW Europe (Italy,
          comes across as slowdown in the biotic   to date the boundary. This means that   Austria), the Arctic (Svalbard, Bjørnøya
          evolutionary rates, as well as the dearth of   error-bars are relatively small in the Induan   in the Barents Sea), and Pakistan (the Salt
          sequence cycles for the interval in question   through Anisian interval (average zonal   Range). Hardenbol et al. (1998) consider-
          (i.e., Middle and Late Triassic). An oxygen-  duration = 0.34 m.y.), increase to medium   ably augmented this Triassic data from
          isotopic record of the Triassic derived from   levels for the Ladinian through earliest   the European basins, putting it on better-
          Tethyan conodont apatite shows trends that   Tuvalian interval (average ammonoid zonal   defined biostratigraphic footing and


                                                 www.geosociety.org/gsatoday                                       5
   1   2   3   4   5   6   7   8   9   10