In particular, the new schemes have a more logical nomenclature in which most of the revised zones begin with the first occurrence of the index species. In particular, the IUGS Lower Cretaceous Working Group (Kilian Group) for the Lower Cretaceous has been striving to develop a systematic zonation for the Tethyan region that can be applied over large regions ( Hoedemaeker et al., 1993 Reboulet et al., 2006, 2009, 2011, 2018). For example, many of the high-resolution “zones” of the western interior of North America would be classified as “horizons” in the broader zonal schemes used in Europe. The relative grouping into zones also varies among regions and stages. Only a few of the Cretaceous ammonoid zones compiled by Hancock (1991) are currently used by the various Cretaceous working groups (e.g., Rawson et al., 1996b Reboulet et al., 2011, 2018). To partially rectify this situation, the grouping of ammonoid datums into zones and subzones has undergone significant revisions since 1990. comm., 2019).Įxtreme faunal provincialism necessitated the establishment of different regional scales throughout most of the Cretaceous, and these regional scales were commonly nonstandardized among publications. Early Cretaceous calcareous nannofossil zones are Boreal (BC Bown et al., 1998) and Tethyan [CC with selected zone/subzone markers with calibrations compiled from Bergen, 1994 Bralower et al., 1995 and other sources, including Watkins, pers. comm., 2019) and Early Cretaceous calpionellid zones ( Remane, 1998). Selected microfossil zones are planktonic foraminifers (composite from Coccioni and Premoli-Silva, 2015 Petrizzo et al., 2014 and other sources, including Huber and Petrizzo, pers. Marine macrofossil zones for Late Cretaceous are ammonoids of the Western Interior of United States (a full list of zone names is in Table 27.3) and inoceramids of North America and Europe ( Cobban et al., 2006 with partial modification by Walasczcyk, 2019). Selected marine macrofossil biostratigraphy columns for Early Cretaceous are ammonoid zones for the Tethyan Realm (Sub-Mediterranean province Reboulet et al., 2018) and Subboreal Realm ( Mutterlose et al., 2014). (“Age” is the term for the time equivalent of the rock-record “stage.”). Summary of numerical ages of epoch/series and age/stage boundaries of the Cretaceous with selected marine biostratigraphic zonations and principal trends in sea level. Pollen-spore zonations and clam-shrimp conchostracans are useful in some terrestrial and marginal-marine settings.įigure 27.9. During the Tithonian, single-celled calcareous calpionellids appeared and are an important biostratigraphic tool in moderate-depth settings until their extinction in the mid-Valanginian of the early Cretaceous.ĭinosaurs are the famous Jurassic Park vertebrates, but the schematic Pangea-wide zonation of characteristic assemblages is only loosely calibrated to marine stages (e.g., Lucas, 2009, Fig. Siliceous radiolarians, which had originated in the early Paleozoic, are a major component of deep-sea sediments, including “ribbon radiolarites” of the Tethyan basins. These single-celled pelagic phytoplankton originated in the middle of the Triassic. The other major marine biostratigraphic zonations utilize calcareous nannofossils and organic-walled cysts of dinoflagellates. However, to show the taxonomic relationships of these ammonite zones, the full genus–species-name taxa are used in the stratigraphic charts in this book. A synthesis of major evolutionary trends, regional paleogeographic distribution, and ammonite zones is compiled by Schweigert (2015). Gradstein, in A Concise Geologic Time Scale, 2016 (1) Biostratigraphy (marine terrestrial)Īmmonite workers in the Jurassic, in contrast to formalized practices in other periods, often use a “standard zone” that is only indirectly associated with the biotic range of the ammonite taxon that lends its name, and these are usually indicated by a nonitalicized name (e.g., as indicated above, the basal Oxfordian “Mariae Zone” does not begin with the lowest occurrence of Quenstedtoceras mariae).
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