Worldwide ammonite correlation at the Pliensbachian Stage and Substage Boundaries (Lower Jurassic) Christian Meister Natural History Museum of Geneva, Department of Geology and Paleontology, 1 Rte de Malagnou, CP 6434, CH-1211 Geneva 6, Switzerland email: christian.meister®ville-ge.ch ABSTRACT: The present paper is an inventory of the biostratigraphical ammonite data at the boundaries of the Sinemurian and Pliensbachian Stages and of the Lower-Upper Pliensbachian Substages. Sinemurian and Pliensbachian Stages belong to the Early Jurassic (Lias) and the age of their boundary is 190 m.y. following the last version of the Geologic Time scale com­ piled by Walker and Geissman (2009). Since 2006, the Global boundary Stratotype Section and Point (GSSP) for the base of the Pliensbachian Stage is formally defined at Wine Haven in Yorkshire Coast (UK). This level coincides with the ammonite association Bifericeras donovani Dommergues and Meister and Apoderoceras sp. which define the base of the Taylori Sub- chronozone of the Jamesoni Chronozone. For the Lower-Upper Pliensbachian boundary, estimated at 186.5 m.y. there is no formal definition with a GSSP until now. Several options remain open as well in the Euroboreal Domain (Hebrides, Yorshire, Dorset in UK, Causses Basin in France, Cordillera Iberica in Spain, Lusitanian Basin in Portugal) as in theTethyan Domain (Subbeticas in Spain, Apennines in Italy, Bakony in Hungary). Worldwide correlations (Euroboreal, Tethyan and East Pacific Domains) at these boundaries are proposed based on ammonites after critical review of their taxonomy and biostratigraphy. Indeed for Lower Jurassic, ammonites represent the best fossil group for precise biostratigraphy and correlation and that is why the standard chronostratigraphic framework (at zonal level) has been based on them since Oppel (1856-58). INTRODUCTION is the smallest subdivision of a chronostratigraphical scale that should be defined at its base like a stage (see below). A potential Since d'Orbigny introduced the notion of Stage and Oppel the zonule is still not defined. concept of Zones near the middle of the 19th century (the sub- zone concept emerged more gradually during the 20th century), The basic idea is to present guide ammonites or ammonite as­ work has continued to refine and to define the succession of chro­ nostratigraphic units so as to arrive at a reliable continous time semblages which characterize biohorizons in different localities scale. of the Euroboreal, Tethyan and Pacific Domains and to correlate them. With this aim, the International Commission of Stratigraphy Historically, the fossiliferous sequences of NW Europe are taken (ICS) indicates that boundaries must be formally defined by as reference to establish standard zonations, especially for the a stratotype; a standard Global Stratotype Section and Point Jurassic, and the most effective standard zonalion for this pe­ (GSSP). riod is based on ammonites. This is the case for the Sinemurian- Pliensbachian and Lower-Upper Pliensbachian Boundaries. Despite radiometric dating and possibilies of correlations with magnetostratigraphy, stable isotopes and Milankovich cycles, This chronostratigraphical interval is a period of strong am­ biochronology remains of key importance due to its high preci­ monite provincialism and the correlations between different pa- sion and its resolution even if paleobiogeography and paleoecol- leobiogeographical domains (Euroboreal, Tethyan, Pacific Do­ ogy sometimes restrict its potential. mains) remain difficult. The purpose of this paper is not to discuss again all the concepts of chronostratigraphy and biostratigraphy. For this, we refer to Two possible responses Dean et al. (1961), Salvador (1994), Callomon (1995) and Page 1) The use of several regional zonaUotvs.lf vUsaxvahs.ol.vite neces­ (2003). Herein we just present a table to summarise my opinions sity first to describe and documents the succession in any Teg\OTi concerning the smallest biostratigraphical and chronostratigraph- and to propose a biohorizon succession, it better, for us, at this ical units: biohorizons and zonules (text-fig. I). Hedberg (1976 p. stage of resolution to directly correlate with standard zonation 49, 67) defined the concept of chronohorizons which is, for us, without to establish regional biozones or subbiozones (see an ex­ quite similar and can be integrated into the more precise concept ample for ammonite succession in Mexico, Meister et a). 2009). of zonules well discussed by Phelps (1985) and Page (1995). The Regional zonations raise more proMems than they solve ana"ia- concept of horizons is discussed in detail by Callomon (1995) and Blau & Meister (2000). The definitions of biohorizon and crease the difficulties of correlations, axAeasXfoc Mwcwatvvtes (see zonules, as used herein, are briefly presented. A biohorizon is Meister et al. 2006). For hierarchical consistency it is necessary the smallest biostratigraphical unit that corresponds to a bed or to select one scale as primary standard and to refer directly to it a serie of beds, characterized by a specified taxon or assemblage even it remains some uncertainties. of time-diagnostic guide-fossils, within which no further strati- graphical differentiation of the fauna can be made. A biohorizon 2) The second solution is to use the best zonation with the most is locally recognizable and a standard horizon is reproductible at accurate resolution - in this case the NW European zonation [re­ a large scale (ideally paleogeographic domain scale). A zonule inforced by the choice of the Pliensbachian GSSP in the NW European Domain (Meister et al. 2006)] - as a reference and to Christian Meister: Worldwide ammonite correlation at the Pliensbachian Stage BIOSTRATIGRAPHY (used herein) biohorizon = bioevent = niveau a => at a local, regional, lithologic profile scale e = local unit bas r i faunal horizon = characteristic faunal horizon the y = unitary association => already synthetic results at supra-regional, basin scale b d ne i def standard horizon = reproductible units => at province or domain scale " t (Unitary Association assemblage) no biozone/biosubzone including e.g. assemblage zone, range => at province or domain scale zone, acme-zone, interval-zone, partial or total range zone 2) [ e CHRONOSTRATIGRAPHY bas (= BIOCHRONOLOGY) r (used herein) thei y Zonule = chronohorizon (sensu Hedberg, 1976) b d e n Subchronozone = Standard subzone defi y ; Chronozone = Oppel zone = Standard zone formall TEXT-FIGURE 1 Summary of biostratigraphical and chronostratigraphical classifications used in this work with proposed synonymy. 1) biozone, biosubzones reveal only regional information. 2) standard horizon is a potential zonule but not officially defined at its base. correlate with it. During recent years, several studies presenting For the Lower-Upper Pliensbachian Boundary several possible very precise biostratigraphic frameworks have been published GSSP candidates can be selected to define the base of the Upper allowing the proposal of a more or less precise correlation be­ Pliensbachian. For this interval the proposed correlations remain tween paleogeographical domains. informal but are quite precise because of the association of Eu­ roboreal and Tethyan ammonite in some key regions such as the All the basic stratigraphical data have been published by the re­ Austrian and Hungarian upper Austroalpine units. spective authors quoted in the figures and they have been criti­ cally reviewed for taxonomy and/or stratigraphy. The informa­ Remark: In the following text the Sinemurian - Pliensbachian tion came from range charts or from faunal lists based on bed by Boundary is abbreviated SPB and the Lower - Upper Pliensba­ bed collections of ammonites. The references are given in each chian Boundary to LUPB. figure. These represent the successions of biohorizons across both boundaries throughout the world. Some significant ammo­ DISCUSSION nites are illustrated. The Pliensbachian Stage To identify precisely and to correlate the Sinemurian-Pliensba- The Pliensbachian is the third Stage of the Jurassic System. Its chian boundary is often very difficult because of sedimentologi- name derives from Pliensbach, a small village in SW Germa­ cal and tectonic problems which affect the fossil record. More­ ny not far from Holzmaden and the original definition of this over, the Lower-Middle Lias is a period of strong provincialism stage dates back to Oppel (1858, p. 248-249, 256), previously and/or endemism, and resulting in addition difficulties for am­ d'Orbigny's Liasien (1849-1852). monite correlations. Consequently, only one outcrop was deemed suitable to be selected for the GSSP of the Sinemurian-Pliensba- The base of the Pliensbachian is located at a level of important chian boundary (see Meister et al. 2003) and formally defined in faunal changes corresponding roughly to the disappearance of 2006 (Meister et al.) at Wine Haven (Yorkshire, UK). the Echioceratidae (Psiloceratoidea) and the subsequent full de- 84 Stratigraphy, vol. 7, no. /, 2010 Age Walker & Estimated £ c Estimated Geissman Estimated duration Estimated 15 Eg duration • 200" age Chronozones of one Zonule Chronozones age c o< one Zonule 1 •183 m.y. Z> P. -186.5 m.y. C3 Davoei Davoei O Ibex 25 -140.000 y. Ibex -I R -188.9 m.y. Jamesoni Jamesoni 157.000 y. 190 m.y. Raricostatum Raricostatum -58.000 y. -191.1 m.y. Oxynotum 40 -87.500 y. Oxynotum —> c Obtusum Obtusum -193.5 m.y. si CO 197 m.y. TEXT-FIGURE 2 Ages and estimated duration of zonules for the Upper