The PaleocenelEocene boundary Global Standard Stratotype-section and Point (GSSP): Criteria for Characterisation and Correlation.
MARIE-PIERRE AUBRY and the WORKING GROUP ON THEPALEOCENE/EOCENE BOUNDARY *
Abstract:. The choice ofa Paleocene/Eoeene (PIE) Global Standard Stratotype-section and Poínt (GSSP) is complicated by the fact tllat there exísts confusíon on the exact denotation ofthe Paleocene and Eocene Series and theír constituent lower rank (stage) units. Whíle we can now resolve tllis problem by recourse to rigorous hístodeal analysis, actual placement of ¡he GSSP ls further exacerbatect by an embarrassment of dches (in regards to 7 criteria suitable for characterising and correlatíng a PIE GSSP but whích span a temporal interval of>2 my). Followíng the precept that the boundaries between higher level chronostratigraphic units are to be fbunded upon the boundaries oftheir lowest constítuent stages in a nested híerarchy. we note that one of tlle critería providing global correlat1on potential (a stable !Solope excursíon in marine and lcrrestríal stratígraphles) Hes at a stratígraphic leve! more than Imy older than the base ofthe slratotypic Ypresían Stage to whieh lhe base of the Eocene Series has been subordillated untíl now. Lowering a ehronostratigmphíc unn by this extent risks a sígnificam modifieation to tlle original geohistorieal denotatíon ofilie Ypresian Stage and the Eocene Series. We discuss bere four options that are open lo Votíng Members ofthe Paleogene Subcommission. One solution eonsísls in adjusting slíghtly tlle base ofthe Ypresian Stage (and, tbtL". the Eocene Series) so as lo be correlatable on the basis oftllc lowest occurrencelFirst Appearance Datum (LOIFAD) oftbe ealeareous nannofossíl species Tríbrachíatus.dígítalis. Another solution would be to decouple series and stages so that the Ypresian Stage remains essentially unaltered but the base ofthe Eocene 18 relocated so as to be corre1ated on tlle basis ofthe Carbon lsolope Excursíon (elE). Two (compromise) solutions eonsist in erecting a new stage for the upper!termínal Paleocene (between the Thanetian [sensu Dollfus] and Ypresian characterised at lts base by the global 8table isotope excursion. The PIE GSSP may then be placed at the base of the stratotypíc Stage (thus preserving historical continuity and conceptual denotation and stabilíty) or at the base 01' the newly erected stage (faeílítating correJadon of the base 01' tlle Eoc\"'l1e series, at least in principie). Both GSSPs sbould be placed in suitable maríne ,tratigraphic seetions yet to be detennincd but upon which there ís considerable eurtent investigative activity.
M.-P. AUBRY Instítut des Scíences de r Evolution, Université Montpellier 1I, 9 Place Bataillon, 34095 Montpellier Cedex 05. Current address: Department ofGeological Scíences, Rutgers University, Piscataway, NI 088854, USA * See end for Iist and addresses ofthc Working Group participants. Accepted: 24th April2000
"The ofthe earth, with aH its varied events, proposed definition of chronostratigraphic is written foc us only in the sequence ofrock strata boundaries associated with the designation of a section to making up the earth's erust. These strata carry the serve as reíerence for the boundary detlnition (Cowie, 1986; story, such as we ean know ít, like pages in a Cowrie et al., 1986; Remane et al., 1996). Since then, book. This book is already printed - without our various subcommissions 00 Stratigrdphy have been active in help and without our advice. \Ve can still divide it describing Global Standard Stratotype - sections and Points into chapters to suit ourselves, if we wish, but we (GSSPs). Thc Paleoccne/Eocene (PíE) boundary ls the last can do this ooly by dividing it iuto grOllps of high-rank boundary still W1der consideration by the pages. There may be endless arguments among us Subcommission on Paleogene Stratigraphy, the proposals tor to what events in the story should be the bases for the CretaceouslPaleogene, Eocene/Oligoceoe and Paleogene! the chapters, depending on individual interests and Neogene boundaries having been already ratified by the individual vicwpoints, but the pages will remain Intemational Uníon ofGeologist Scientists (IUGS). the same regardless of how we group them. And, Afier 10 years of active research and discussions, we are like the pages ofthe book, so the strata ofthe earth now in a position to select the best suited to are our only fixed basis ofreference for chapters in characterise the PíE boundary. The purpose ofthis paper the history ofthe earth --tor the definition of our IS to describe seven events that occurred in Chron C24r and chronostratigraphic scale." mal' serve to characterise the PIE bnundary, and to evaluate (Hedberg, 1961: 509-510) their stratigraphic reliability and usefulness in order to provide the scientific community interestcd in this prnblem INTRODUCTION in Paleogene sttatigraphy with the crítical elements needed Stability in stratigraphic nomenclature and classification has to make an infomled choice of the boundary criterion/ia. become a necessity, bnth for the student of stratigraphy Prior to this description, the shifi in chronostratigraphic whose efforts in correlating distant sections is made difficult philosophy that has occurred since 1986 i8 briefly di8cussed by the use of various stratigraphic concepts and lack nf so that the reader understands the issues to consider in precise definitinn of chrnnostratigraphic units. and ti)! the scIecting the criterionla that will serve to characterise the PI non-speciali8t who mal' be confused by the multitude of E boundary. Further díscussíon on these can be found concepts hidden under a single chronostratigraphic term aodl elsewhere (Aubry el al., 1999; Aubry, 2000). or by the use 01' the same coocept The Intemational Commission nn Stratigraphy (ICS) has thus 58 AUBRY ETAL
ACTIVITIES OF THE WORKING GROUP ON THE correlations (see Aubry, 1995, 1997, 1998b). Using the PIE BOUNDARY AND IGCP 308 method of temporal interpretation of stratigraphic sections Since its inception in 1989 at the 28th International we have shown that most upper Paleocene-Iower Eocene Geological Congress in Washington, the Working Group on deep sea and land sections (among which those considered the PIE boundary has been active under the auspices of as potentíal GSSPs) constitute a discontínuous record of UNESCO in the fonu ofIGCP Project 308, and has devoted Chron C24r (Aubry et al, 1996,2000; Aubry, 1998a, b). Of rnuch of its efforts to describing and correlating marine the two dozen deep sea and marine land sections that have (including deep sea) and terrestrial upper Paleocene-Iower bcen examined so far, only two can be confidently said to bc Eocene sequences ín key areas of thc world. Much effort essentially contínuous across the carbon isotope excursion has been placed on delineatíng the events that occurred (CIE), one ofthe candidate criteria for characterising the PI during Magnetic Chron C24r, in a ~1.5my interval that E boundary (see below). However, the same powerful tools encompasses various ínterpretatíons of the PIE boundary in have allowed us to construct a firm relative chronology of marine and terrestrial stratígraphy (see Berggren & Aubry, events from disjunct andlor discontinuous stratigraphic 1996, 1998; Aubry, 2000). records. The consequence is that whereas there ís a choice of criteria to characterise the PIE boundary and corrclate it The achievements of IGCP Project 308 were reviewedl around the world, we have no suitable GSSP to propose. discussed at thrce successive mcetings, a Penrose Because much effort has been placed in soundly correlatíng Conference in Albuquerque, New Mexico (Berggren et al., the numerous sections studied, we believe that decísion on 1997), a Société géologique de France Séance Spécialisée in the criteria(ion) which will characterise the boundary prior Paris, France (Thiry et al.. 2001) and an intemational to íts definition (Le., the formal desígnatíon of a meeting in GOteborg (Sehmitz et al., 2000). The magnitude lithostratigraphic horizon in a chosen section) is tenable. and abruptness of changes that the earth underwent during Magnetic Chron C24r were never as well appreeiated than PHlLOSOPHICAL APPROACH TO CHRONO as a result ofthese conferences. It has become clear that the STRATIGRAPHY: TWO OPPOSITE PRECEPTS world as we know it today largely stems trom the major Stríct applicatíon of the principies of chronostratigraphy changes that took place during that time, a significant dictates that the base ofa series be defined by the base ofits tuming episode in the hístory ofour planet. oldest constituent stage (see Hedberg, OO., 1976; see also Boundary Workíng Group activíties have focused on two Remane et al., 1996, aIthough formulation of this principie major issues. One is the characterisation of a boundary and is unclear and commonly ignored by the ICS when GSSPs íts correlation, the other is the prospect for "boundary are ratified: see Aubry et al., 1999). In thís perspective, stratotype sections" in order to pinpoint the most suitable working groups involved with the detlnition of section to serve as the GSSP. In this context, among the chronostratigraphic units should be concemOO with the greatest achievements oflGCP Project 308 are: definition ofthe lowest stage ofa series, Le., in our case, the formal and ICS-ratified definition of the base of the l. A composite chronologic succession of events Ypresian Stage (votedlapproved as the lowest constítuent constructed from fine scale analyses of disjunct stage of the Eocene Series by the International stratigraphies in oceanic, shallow maríne and terrestrial Subcommissíon on Paleogene Stratigraphy, 28th realms (Aubry et al., 1996; Berggren & Aubry, 1996, 1998). lntemational Geological Congress, Washington D.C., 1989). Hedberg (ed., 1976: 85) recognised the need for mutual 2. The reappraisal of stratigraphic sections in key boundary stratotypes "to serve both as the top of one stage epicontinental areas such as northwestem Europe (e.g., and the bottom of the next younger stage", and added (op. Laga, ed. 1994; Knox et al., eds., 1996), the Gulf Coast (e. cit., p. 86) that "the boundary stratotypes between two g., Mancini & Tew, 1995) and the New Jersey Coastal Plain stages could be selected so that certain ones could serve also (e.g., Gibson et al., 1993, 1994). as the boundary-stratotype between larger units (series, 3, The detailed description of sectíons regarded as potential systems, etc.)", remarking that "The procedure thus lends GSSPs. itself readily to a complete hierarchícal scheme of chronostratigraphic division with no gaps and no overlaps". Among these latter are outcrops in the Apennines (Corfield Because aH Paleogene stages are based on unconformable et al., 1991), the central Negev (Benjamini, 1992), the stratigraphic units (Hardenbol & Berggren, 1978; Aubry, eastern pyrenees (Molina et al., 1992), the Betic Cordillera 1985), mutual boundary stratotypes can only be defined (Molina et al., 1994; Canudo et al., 1995; Lu et al., 1996), outside of the type areas. Thus, foHowing a strict the Basque Country (Canudo et al., 1995; Orue-Extebarria, Hedbergian approach to chronostratigraphy, the Working 1996), the Nile Valley (Schmitz et al., 1996; Aubry et al., Group on the PIE boundary should be concerned with the 1999) and the Tyrolian Alps (Egger, 1997). dcsignation of a ThanetianlYpresian boundary stratotype in which the age of the boundary horízon is coeval with the Remane et al. (1996: 78) stress that "Corretation precedes base of the Ypresian Stage in its type area. We recogníse definition" ín the selection of a GSSP, adding however that here tbat there may be a correlation problem, as did "it would be unrealistic to demand that a given boundary Hedberg (ed., 1976), but definition is unique and stable. be recognisabte all over the wortd before it can beformally This would be in harmony with the philosophy followed in defined" . We should Iike to poínt here that the two the establishment of Cenozoic chronostratigraphic schemes issues ~orrelation and definition- are quite distinct, at (e.g., Berggren et al., 1985; 1995; Haq el al., 1987) in which least with regard to the Cenozoic stratigraphic record. the bases of the standard stages essentially correspond to the Powerful correlation tools used in connection with time bases ofthe unit stratotypes in their type area. scales such as the Integrated Magneto-Biochronologic Scale The guidelines to chronostratigraphic procedure enunciated (IMBS; Berggren el al., 1995) provide a means towards by Cowie et al. (1986) and codífied by Remane et al. (1996) rigorously estimating the completeness of stratigrapbic have introduced a fundamental change to the Hedbergian sectíons, and, consequently, for establishíng true temporal chronostratigraphic procedure. Stratigraphic units are also PIE BOUNDARY 59
) defined by their base (Cowie et al., 1986: 8, stated: 1999). The base of the standard Ypresian "Although there is no scientific prínciple ínvolved in chronostratigraphic unít corresponds to this trangressive considering the base of a unit any more important than the surface, with an estimated age of 54.37 Ma (see below). top of a stratigraphic unit, lCS bodies (e.g., Ihis particular surface has thus a double significance: one Subcommissions) are responsible by convention for the is chronostratígraphic (i. e., temporal), the other is base of their units"), but the role of the stage so geohistoríc because the widespread Ypresian transgression fundamental in Hedberg's vísíon, ís now subordínated to the was associated with the end of íntensive compressional expediency of correlation (see Aubry et al., 1999). The tectonism in the North Atlantic (Knox, 1996). It can be salíent difference for our purpose is however of a different seen that redefining the Ypresian stage on the basis of a nature: where Hedberg recognised the importance of much older stratigraphic level (likely to refiect another definition, the current ICS emphasises the importance of event in historical geology) would create a confusíng correlation. Whereas Hedberg's boundary stratotype situation. preserves hístorical precedent (í.e., the definítíon ofthe base ofa stage in itSlype area), the GSSP ignores it (Cowie, 186; THE PIE BOUNDARY INTERVAL AND THE PIE Cowie et al., 1986; Remane et aL, 1996; see Aubry et al., BOUNDARY EVENTS 1999, for quotes and discussion). The GSSP is chosen on Aside from the potentíal problems alluded to above, the the basis of its correlation potential, independently of prior, Working Group on the PIE boundary has been concemed albeít informal, chronostratigraphic definítion: "Even that placement of the base of the redefined Ypresian Stage though a chronostratigraphic boundary is defined by a point does not violate the historícal definition of the successive in the rock, its formal definition should be preceded by a regional/standard stages, Ypresían and Thanetian. The thorough test of correlation potential of the envísaged phílosophy and methodology followed by the Working boundary level" (Remane, 1997; 3; bold in the orígínal Group have been clearly expressed by Knox (1994) and text). In summary, under the ICS rules, the mandate ofthe Berggren and Aubry (1996, 1998). In short, the Working Working Group on the PIE boundary is to define a Group has delineated a stratigraphic interval, ofien referred Iithostratigraphic horizon that is essentially correlatable to as the Bounda¡:v lnterval, bracketed by the top of the globally, and which will constitute the base ofthe redefined Thanet Sand and the base of the leper Clay Formations Ypresian Stage. Because this horizon may be of quite (Text-figure 1) in order to determine which new locatíon of different age than the base ofthe stratotypic Ypresían Stage the base ofa redefined Ypresian Stage would not víolate the of the Belgíum Basin (an age that has been accepted by original concepts of Thanetian and Ypresian Stages. The most stratígraphers) ambiguity and confusíon may quíckly "Boundary Interval" represents a temporal duration of arise. If the base ofthe redefined stage is placed at a level ~2.3my. Because the Eocene stratigraphic record in the eíther much younger/older than the regional/standard stage, London-Hampshire Basin(s) is more amenable to precise the same term (stage name) will serve to characteríse two correlation with the deep sea record by means of extremely dífferent eoncepts. To summaríse, the sítuation biostratigraphy, magnetostratigraphy and tephrastratigraphy is as follows; than that of Belgium (where the Ypresian stratotype is 1. Following Hedberg's guidelines, the base of the global located (Dumont, 1849», we have substituted the base of Ypresian Stage essentially corresponds to the base of the the London Clay Formation (= base ofthe Walton Member; regional/standard Ypresian Stage. Its definition is Ellison et al., 1994) to the base ofthe leper Clay (as defined independent ofany means ofcorrelatíon. by the base of the Mont Héríbu Member, de Coninck et al., 1983) as the informal definition of the base of the Eocene 2. Followíng the guidelines expressed in Cowíe et al. and Series as followed by most stratigraphers. Through Remane et al., the base ofthe global Ypresian Stage would sequence stratígraphy it has been shown that the base ofthe possibly not correspond to the base ofthe regional/standard Walton Member (= Unit A2; King, 1981) correlates with Ypresian Stage. Redefinition ofthe base will be contingent the base ofthe Mont Héribu Member; e.g., Steurbaut, 1998 upon the selectíon of a criterion for global correlation. and references therein). However, wíth regard to this situation, one may ask how far above or below the base of the regional/standard Ypresian Direct correlations indieate that the top ofthe Thanet Sand Stage can the base ofthe global (Le., ICS-ratified) Ypresian Formation lies within Magnetozone C25r and Biozone NP8 Stage be located? There is no provision for this question in (AH & Jolley, 1996), and has an estimated age of-56.6 Ma the current ICS guidelines (Remane et al., 1996). (see Berggren & Aubry, 1996). The base of the London Clay Formatíon (= base ofthe Walton Member) lies -60% It is beyond the scope of this paper to present the above the base of Magnetozone C24r, very c10se to the unfortunate consequences of this situation thoroughly NPI0alb bio(chrono)zonal boundary and has an age díscussed in Aubry et al. (1999), Aubry & Berggren estimate of 54.37 Ma (see Berggren & Aubry, 1996, and (2000a, b) and Aubry (2000). However, in response to Aubry, 1996; these age estimates are based on the possible concem over seemingly undue emphasisl Geomagnetic Polarity Time Scale [GPTS] of Cande and distinction placed here upon the regional versus the Kent, 1995 [see below]). As it happens, major redefined Ypresian Stage, one should recall that the paleontologic events occurred during the ~2.3my interval stratigraphic units that constitute Paleogene regíonal/ bounded by these two stratigraphic levels. There were, standard stages (and many other stages) are natural entitíes among others, a major turnover among terrestrial mammals that reflect the geodynamic evolutíon of the basin where (Wood et al., 1941; RusselI, 1968) that vertebrate they were deposited. The Ypresian rock unit constitutes a stratigraphers have used to characterise their placement of broad synthem unconformable with both the Thanetían and the PIE boundary (e.g., Russell et al., 1982; Gingerich, the Lutetían synthems, and its base (Mont Héríbu Member 1989, following Pomerol, 1977), the largest extinction of of the leper Clay, Walton Member of the London Clay the Late Cretaceous Period and Cenozoic Era among the Formatíon) corresponds to a major transgressíve surface deep water benthic foraminifem (see Thomas, 1992, 1998), that can be followed throughout northwestem Europe (see a distinct turnover among the calcareous nannoplankton for instance Knox, 1996; Steurbaut, 1998; Aubry et al., (Aubry, 1998c; Aubry et al., 2000) and the marine and 60 AUBRY ETAL
...... ' a:z 00 LL-~5 a:w v wa: UJ ca: Ü a:o C\l 00 U ... ~ -----f--- S/SUfiJOOS8/fM "VV U Sl/riJ¡lfJ¡P 1 -l UJ Ü * V 111 3d 11I-13d -~ 10~ nev SdN lVGQ SIlL _ __1 NOll'tV'lI:lO:l NOIIVVIII::JO:I AVl:::>NOaNOl H:::>IMl:lVH NOIIVlI\Il:::lO.:l dnOl:::l~ H138VNl aNV'S dnol:::l8 S3l1\1VHl 13N\fHl PIE BOUNDARY 61 (Dockery, 1998; Hartman & Roth, 1998), and the global SEVEN CRITERIA TO CHARACTERISE THE PIE dispersal of the holozoic dinoflagel1ate Apectodinium spp. BOUNDARY (Bujak & Brinkhius, 1998; Crouch et al., 2000). In the Seven potential criteria have been retained by the PIE WG meantime, there was also a reduction in the intensity of as valuable for long distooce correlations (Text-figure 1). atmospheric circulation (Rea et al., 1990), the warming of They are introduced from younger to older in reference to the high latitudes ood of the deep )ea, an event so the fact that, until now, the base of the regional/standard distinctive, sharp ood short-termed that it has been dubbed Ypresian Stage determines the base of the Eocene series. the Late Paleocene Thermal Maximum (LPTM, Zachos et The criteria are of different nature: biostratigraphic, al., 1993), explosive volcanism in the North Atlootic paleomagnetic and isotopic. Biostratigraphic events have (Morton and Parson, 1988; Knox, 1996) and in the been general1y preferred to any other for the Caribbean (Bralower et al., 1997). Perhaps this time is best characterisation of chronostratigraphic boundaries, but Imown, however, for the 3-4 %0 negative excursion (ClE) in magnetostratigraphic events have been recentIy proposed the carbon isotopic composition ofthe oceoo first identified and accepted (e.g., Steininger et al., 1997). The criteria are in Ocean Drilling Project (ODP) Hole 690B on Maud Rise, 13 described, their correlation potential is given and their pros Antarctica, by Kennett & Stott (1991). This ;S C anomaly, and cons are listed. which is superimposed on a long-term Chron C26r to C24n decrease of the mean d 13 C of the ocean (Shackleton et al., The estimated ages assigned to the different events are 1984; Shackleton, 1986; Zachos et al., 1993) has now been those derived from the GPTS (Cande & Kent, 1992, 1995) identified in numerous marine (but see Aubry, 1998a, b) as based on a composite reference stratigraphic section (Aubry wel1 as terrestrial sections (Koch et al., 1992; Stott et al., et al., 1996; Berggren & Aubry, 1998). While we believe 1996). the relative chronology of events in Chron C24r to be firm, we recognise that the numerical chronology wil1 require Whereas the chooges in the marine ood terrestrial biotas are revision (although it is unclear how much). The location of now well documented (see papers in Aubry et al., I998a, the 55 Ma-calibration point in the chron used by Cande & Schmitz et a!., 2000, Thiry et al., 2001), there remain large Kent (1995) in their GPTS (1992, 1995) is unlmown uncertainties on the oceanographic significance of the (Aubry, 1998b). In addition, a magnetostratigraphic isotopic records. For instance sorne records suggest a reinvestigation concluded that Chron C25n is not temporary shift from high latitude temperature-driven to represented in ÜDP Hole 690B (AIi et a!., 2000). A low latitude salinity-driven ocean circulation (Pak and stratigraphic level in that hole thought to correspond to the Miller, 1992) that other records do not confirm (Schmitz et Chron C25n1C24r reversal boundary served as a tie point in al., 1996). As a whole, the cause(s) of the profound the construction of the composite reference section. changes that occurred during Chron C24r remain(s) e1usive, Younger ages (~55 Ma) have now been proposed for the and model1ing reveals how complex the situation is (e.g., ClE based on independent methodologies (Norris and Rahl, 81000 and Thomas, 1998; Bice, 2000; Bice & Marotzke, 1999; Wing et a!., 1999). However, no revised numerical 2000). It has been proposed (Bralower et al., 1997) that chronology for the whole ofChron C24r is available as yet. intensive volcanism triggered dissociation of gas hydrates and subsequent release of methooe into the atmosphere, causing a sharp global warming (Dickens et al., 1995). CRITERION 1 (C1) Whereas active tectonism, including widespread vo1cooism The First Appearance Datum (FAD) oC Tribrachialus in the NE AtIantic linked to rifting and spreading between digilalis or the NP10a/b subchronal boundary; estimated Greenland and Eurasia (see Morton et al., 1988; Knox, age 54.37 Ma (Aubry el al., 1996; Berggren & Aubry, 1998), ood in the Caribbean region (Bralower et al., 1997), 1996) and massive metamorphism and erosion linked to the India Asia collision (Kerrick & Caldeira, 1994; Beck et a!., 1995) Tribrachiatus digitalis Aubry 1996 is a recentIy introduced, are well documented, evidence is slowly building that characteristic ca1careous noonofossil species with a wide supports the dissociation of gas hydrates (Katz et a!., 1999; geographic distribution. Its stratigraphic range defines Dickens, 2000), viewed by many as the most plausible Subzone NPlOb. Its FAD immediately fol1owed the end of mechooism to explain the CIE. eruptive vo1canism (Ash Series 2.3, Knox, 1996) in the North Atlantic. Through indirect correlation between ash layers and biostratigraphy in DSDP Site 550 (Porcupine Abyssal Plain, at the seaward edge of the Goban Spur), it has been shown (Aubry, 1996) that the FAD of T. digitalis can be used to estimate the age of the base of the London Text-ftgure 1. Loeation of the seven potential eriteria to eharaeterise the PIE boundary with respeet to the U.K. Iithostratigraphy. Lithostratigraphic framework from King (1981), Ellison el al. (1994) and Jolley (1996); Magnetostratigraphy (1) fmm Ali el al. (1993), Ali and Jolley (1996) and ElIison el al. (1996); ealcareous nannofossil stratigraphy (2) from Aubry (1985, 1996), ElIison el al. (1996), and Aubry & Curry (unpublished data); Dinoflagellate stratigraphy (3) fmm Powell el al. (1996); Charophyte stratigraphy (4) fmm Riveline (1986); Vertebrate stratigraphy (5) from Hooker (1996); Isotope stratigraphy (6) from Sinha (1997) and Thiry et al. (1998); (7) Tephrastratigraphy from Knox (1990). . The drawing ofthe lithostratigraphic units and the gaps that separate them are not to seale. It is diffieult to infer the 10eation ofthe NP9/NPI0 bio(ehrono)zonal boundary in this lithostratigraphie sueeession. It is loeated here in the stratigraphie gap that separates the Lambeth and Thames Groups, but it may be as 10w as in the uppennost part oflhe Woolwieh-Reading Fonnation. BY: base of the standard Ypresian Stage stratotype, = base of the Mont Héribu Member. Tht-3: upper surfaee of Thanetian sequenee 3 of Powell el al. (1996) = surfaee Tht-4 of Hardenbol (1994). Cl to C7: eriteria for eorrelation as diseussed in the tex!. Ahy: Apectodinium hyperacanthum Zone; Aau: Apectodinium augustum Zone; Gor: Glaphyrocysta ordinata Zone; Was: Wetzeliella aslra Zone, Wme: Clay Fonnation (ElIison et al., 1994; = Walton Member of Cons: King, 1981)). Tribrachiatus digitalis has now been 1: This criterion is not applicable in shallow identified at the base ofthe Thames Group (Aubry & Curry, (epicontinental) settings unpublished data). In DSDP Site 550, the LO ofthe species 2: There is, as yet, no indirect means ofcorrelation with tbe t~ is immediately abovc ash series. Jolley (1996) terrestria1 record dístinguished two unconfonnable units in the Wrabness 3: Because ofpossible reworking, un HOlLAD may not be Member. The lower Unit A is a tuffaceous silstone whereas as suitable as a lowest occurrencelFirst Appearance Datum Unit B is a sund. Thus, tentatively, the LO of T. digifalis is (LOIFAD) to characterise a chronostratigraphic boundary. located bctwcen Units A and B. The tenns Lowest OccurrencclFirst Appearance Datum and CRITERION 3 (C3) Highest Oceurrence/Last Appearance Datum (LOIFAD and Tbe FAD of Tribrachiatus bramlettei; estimated age: 55 HOlLAD, respectively) are taken as defined by Aubry Ma (Swisher and Knox.1991) (1995), with LO and HO having stratigraphic and FADI LAD temporal significanee. LOIFAD and HOlLAD are The NP9INP 1° zonal boundary has become loaded with used in conjunction here to indicate that the LO is meant to ambiguity, first for correlation reasons, seeond for represent a FAD, and HO a LAD taxonomie reasons. The pros and cons ofthis eriterion are as follows: The NP9INP10 zonal boundary has ofien been used to approximate the PIE boundary. The base of thc standard! Pro: regional Ypresian is younger (-0.63my) than the 1: Use of this criterion would cnsure continuity in NP9/NPI0 bio(chrono)zonal boundary (Aubry et al., 1996 stratigraphic nomenclature. The base of a formally-ICS and Aubry & Curry, unpublished data). Reference to tbe PI ratificd Ypresian Stage would essentially correspond to the E boundary without indication that its reeognition was base ofthe regional/standard Ypresian Stage. based on the NP9/NPlO zonal boundary has thus caused eonfusion over the years. Further confusion has arisen from 2: This entedon is applicablc in almost all marine settings, the use ofseeondary markers (thc Ha ofFasciculithus spp., from the deep sea to epicontinental arcas. the LO ofD. diastypus) to detennine the zonal boundary. 3: This species has a very short range. Its biochron is estimated of ~0.2my (Aubry et al., 1996) A Iineage from Rhomboaster to Tribrachiatus involving a change in symmetry from rhombohedron-shaped fonns (= 4: The nannoliths of digitalis are diagcnetically resistant T. genus Rhomboaster) to fonns with a radial, six or three·fold and easily identified. symmetry (= genus Tribrachiatus) has been well 5: The FAD of T. digitalis appears to be correlative with the established by Romein (1979; see also Aubry et al., 2000). acme ofDeflandrea oebisfeldensis In this Iineage, Tribrachiatus bramleuei is the fifSt speeies with a radial symmetry, and its LO defines the base ofZone Con: NP10 (Martini, 1971). The CIE oCCUfS at -mid·level in 1: There is, as yet, no possible correlation with the Zone NP9 and very c10se to the LO of Rhomboaster spp. tcrrestrial, particularly with thc mammalian, record. The introduction of a synonymy between T bramleuei and Rhomboaster spp. (BybeH & Self-Trail, 1995) has fueled an amicable eontroversy, and resulted in an unfortunate CRITERION 2 (C2) confusion because ifthe two taxa are regarded synonymous, The Last Appearance Datum (LAD) of Morozovella the CIE is shown to occur at the NP9INP1 Ozonal boundary, velascoensis or the P51P6 biochronaI boundary; not in mid-Zone NP9. Failure to recognise the estimated age: 54.48 Ma (Berggren & Aubry, 1998) Rhomboaster-Tribrachiatus Iineage results in the loss of significant stratigraphic and evolutionary infonnation. The Planktonic foraminiferal stratigraphy has played an use of the HO's of Fasciculifhus alanU and F. important role in chronostratigraphy and several planktonie tympaniformis can help in locating the two main foraminiferal datums serve to delineate series boundaries (e. evolutionary steps in the Rhomboaster-Tribrachiatus g., the top ofthe Eocene is eharaetcrised by the HOlLAD of Iineage. The LO/FAD of Rhomboaster spp. (and the ClE) Hantkenina spp.; Premoli Silva & Jenkins, 1988). The is essentially synchronous with the HOlLAD of highest oeeurrenee (HO) ofM. velascoensis defines the top Fasciculithus alonU whereas the HOlLAD of F. of Zone P5 (Berggren & Miller, 1988). The LAD of this tympanijormis appears to be very c10se to the LO/FAD of T. species is slightly older (~O.3my) than the base of the bramleuei. stratotypic Ypresian Stage. Cross correlation with DSDP Site 550 indicates that it would líe within the Harwich The NP9/NPlO zonal boundary (= LO of T. bramleue¡) Fonnation (as defined by ElIison et al., 1994), within the with an age of 55 Ma serves as a calibration tie-point in the interval comprised between the upper part of the Orwell GPTS ofCande and Kent (1992,1995). In this GPTS, tbe Member and Unit A of the Wrabness Member (as defined PIE boundary was taken at the NP9INPlO zonal boundary, by Jolley, 1996). und thus bears an age of 55 Ma. Indirect correlations indicate that the NP9/NP10 zonal Pro: boundary probably falls within the stratigraphic gap 1: Morozovella velascoensis is a distinctive fonn. between the Lambeth and the Thames Group. 2: An easy datum to delineate in oceanic settings 3: It is marginally older than the base of the regional! Pro: standard Ypresian Stage. 1: Use ofthe NP9INPIO zonal boundary to eharacterise tbe PIE boundary would confonn to the current GPTS 2: This criterion is applicable to praeticaHy aH marine PIE BOUNDARY 63 environments in the deep sea benthíc realm (the BFE), the other in the 3: It has ofien been used to characterise the PIE boundary North American continental realm (the MDE) 4: It is further characterísed by a rapid diversificatíon in the Cons: marine calcareous plankton, with the occurrences of two 1: There are currently no known means of correlation wíth compressed acaríniníd species and one morozovellíd, of the terrestrial record several short-range calcareous nannofossil taxa and the 2: The taxonomíc controversy regarding the Rhomboaster acme of Apeclodinum spp. (Zone Aau of Powell et al., Tribrachiatus lineage has not been resolved as yet, although 1996) the recent recognitíon of bramlettei as a discrete taxon 5: It occurs ín mid Zone P5 and at the NP9a/b subzonal ("Rhomboaster bramlettei bramlettei"; Von Salís in Von boundary. Salís et al., 2000)-with the corollary that the ClE occurs in Zone NP9 not at the NP9INPlO zonal boundary Cons: cDnstitutes a step towards settlement 1: It ís >I my older than the base ofthe Ypresían Stage as 3: T. bramlettei may be scarce in some settíngs stratotypífied in Northwest Europe 2: 1t faHs within a líthostratigraphíc unit (the Reading CRITERION 4 (C4) Woolwich Formation ofthe Lambeth Group (ElIison et al., 1994) that is currently (and has been tradítíonally) assígned The 013C excursion (CLE); estimated age: 55.52 Ma to the Thanetian Stage (Berggren & Aubry, 1996) 3: Biostratigraphic control ís required for its firm The ClE was inítially ídentified at Site 690 on Maud Rise, identification Antarctíc Ocean (Stott et al., 1990). Sínce then, ít has been reportcd from many deep sea sites (e.g., Pak & MilIer, CRITERION 5 (C5) 1992; Bralower et al., 1995; Norrís & Rohl., 1999) and land se<.tions from bathyal (e.g., Lu et al., 1996; Schmitz et al., The LAD of the Slensionia beccariiformis assemblage; 1996; et al., 2000) and epicontínental (e.g., Thomas estimated age: 55.52 Ma (Berggren & Aubry, 1996) et al., 1997; Cramer et al., 1999) settings. It has been A sígnificant change at bathyal and abyssal depths in shown, however, that many such exeursíons are benthic foraminiferal assemblages around the PIE boundary pseudoevents that result from anomalous juxtaposition of was first documented by Tjalsma & Lolunann (1983). ísotopíc records caused by unconformities (Aubry, I998a, However, the significance of this event was not fully b; Aubry et al., 2000). Correct ídentificatíon of the apprecíated untíl the discovery of its synchrony with the exeursion requires an ísotopíe deerease of 3 to 4 %0, and ClE as recognísed at Maud Ríse Síte 690 (I'homas, 1990). correlation with míd-Zone NP9 (Aubry et al., 1996; = Sínce then the benthic foramíniferal extinction and the ClE NP9a/b subzonal boundary, Aubry, 1998a, Aubry el al., have been found in association in many deep sea (bathyal 2000) and mid-Zone P5. In deep sea sectíons, the ClE is and abyssal) sectíons (see Thomas, 1998 fbr a review). The synchronous wíth the benthic foramíniferal extinetion location of the benthic foraminiferal event wíth rcspect to (BFE). In Tethyan, Atlantie and tropical Pacific sections, calcareous mícrofossil stratígraphy is, as the ClE, ín míd• the CIE is associated with a rapíd diversification in the Zone NP9 (at the NP9a/b subzonal boundary). A turnover calcareous plankton, íncluding the oecurrenee of two ín shallow water benthíc foraminífera, but of lesser compressed acaríníníd (A. africana and A. sibayensis) and a amplítude than ín the deep sea faunas, has also been morozovellíd (/11. allisonensis) species (e.g., Kelly et al., documented (Speíjer, 1994; Thomas el al., 1997; Cramer et 1996; Lu el al., 1996; Norrís and Rohl, 1999) and of al., 1999). awk'Ward, ofien asymmetrical calcareous nannofossíls among whích Rhomboasler calcitrapa and Discoaster Through associatíon ofthís event with the CIE, the level of Graneus (Aubry, 1999; Aubry et al., 2000). The CIE ís also the BFE correlates ín the shallow maríne record of NW associated wíth the apparently global acme of the Europe with a level withín the Sparnacían and withín the dínoflagellate Apectodinium (Bujak & Brínkhuís, 1998; Readíng-Woolwich Formation. Crouch el al., 2000). The ClE marks the onset of the LPTM (Zachos el al., 1993). As for the ClE, unconformities may truncate the record of the benthíc foraminiferal extinctíon as díscussed by Aubry The ClE has also been identified in the North American (1998a, b). terrestríal sections (Koch el al., 1992, 1995) where it was shown to occur at the base ofWasatchían Zone WaO and to Pro: be assocíated with the mammal dispersal event (MDE; see 1: thís constítutes a majar (stratígraphic) event ín the Berggren el al., 1997). In Europe, the ClE has been bathyal and abyssal realms ídentífied in the lower part of the Sparnacian Argiles 2: this deep water event ís correlatable to the nerítíc realm Plastiques bariolées (Paris Basin), well below the Meudon 3: it is assocíated with the ClE mammalían fauna, and in the lower part of the Reading 4: ít provides índirect correlation wíth the terrestríal record Formation (London Basin; Stott el al. 1996, Sínha et al., via the ClE. 1996; Sinha, 1997; Thíry et al., ín review). Cons: Pro: 1: 1t is >1my older than the base ofthe Ypresian Stage as 1: lt allows dírect correlation between marine and stratotypified ín Northwest Europe terrestríal records 2: Correlatíon with the terrestríal record is only índírect 2: This ís a major event ín ítself; the ClE is well chardcterísed and oflarge amplítude 3: lt Is assocíated with two signíficant bíotíc turnovers, one 64 AUBRY ETAL CRITERION 6 (CÓ) 1. Chronostratigraphic boundaries may be simply arbitrary, Chron C2Sn/C24r magnetic reversal; estimated age: in which case a criterion may be chosen for pragrnatic 55.964Ma (Cande & Kent, 1995) reasons. Magnetic reversals serve this purpose well. The Chron C25n/C24r magnetic reversal would ensure global Chron C25n is represented in many of the sections correlation ofa "PIE" boundary. investigated by IOCP Project 308. The Chron C25n/C24r reversal boundary has been identified at the base of the 2. Chronostratigraphic subdivisions may be drawn in such Upnor Formation in the Thanetian-Ypresian type area (AH a way that they constitute natural divisions of the el al., 1996; Ellison el al., 1996) where it constitutes a stratigraphic record. Both stages and series as originally valuable anchoring point for global correlations. conceived (Lyell, 1833; d'Orbigny, 1852) were natural subdivisions of the stratigraphic record, but based on different criteria. However, it is essential to recognise that Pro: chronostratigraphy must remaín an independent means of 1: A magnetic reversal is a nearly synchronous global subdividing the stratigraphic record, independent of any event aspect of earth history, either paleobiologic, tectonic or 2: Use of this criterion would permit correlation between climatic (Hedberg, 1976; see discussions in Aubry el al., the marine and terrestrial records. (' 1998b, 1999; Aubry, 2000; Van Couvering, 2000). 3: In the marine record Chron C25n is easily identified through biostratigraphy (the LO/FAD ofD. mulliradialUS is Two among the aboye criteria hold privileged places. They associated with ~mid-Chron C25n and the HOlLAD of are the FAD of T. digitalis and the CIE. Choice of the Globanomalina pseudomenardii is associated with Chron FAD of T. digitalis would immediately guarantee continuity C25n(y) ; see Berggren el al., 1995). in stratigraphic nomenclature. lt would favour traditíonal approaches to chronostratigraphy in which stages are the basic ehronostratigraphie unit (as speeies are the basic Con: element in taxonomic nomenclature). It would allow global 1: It is >1.5my older than the base ofthe Ypresian Stage as marine eorrelation of the base of the standard/regional stratotypified in northwest Europe Ypresian Stage, and consequently ofthe base ofthe Eocene 2: The maln events that make early Paleogene evolution so Series. We have currently no means of identifying this distinctive occurred about OAmy after this magnetic level in the terrestrial realm, but this does not imply that reversal further research would not provide such means (see Van 3: lts identification requires biostratigraphic control. Couvering, 2000). We have established a posteriori that this level of chronostratigraphic significance dates a major CRITERION 7 (C7) geological moment in the history ofnorthwestem Europe, i. Top oC the Thanet Sand Formation; estimated age: 56.6 e., a change in sedimentary regime following widespread Ma (Berggren & Aubry, 1996). transgression as the result of thermal subsidence of the North Atlantic region. The top of the Thanet Sand Formation is an erosional surface which líes in Chron C25r (AH & Jolley, 1996), Choosing the ClE would ensure truly global correlation calcareous nannofossil Biochon NP8 (Aubry, 1985) and from the deep sea to the terrestrial record, and this criterion dinoflagellate Biochron Ahy (Apectodinium h)peraCanlhum is supplemented by a host ofadditional means ofcorrelation interval Zone; Powell et al., 1996) 1t is the upper bounding ofthe boundary level. We have established a priori that the surface of Sequence Th-4 of Hardenbol (1994) or Sequence ClE points to a decisive moment in the evolution of the Tht-3 ofPowell el al. (1996). There is general agreement planet, well documented, albeit unexplained. lt would mark that the stratigraphic gap between the Reculver Silts and the the time of fundamental changes in faunas and floras, much Upnor Formation (ex Woolwich Bottom Bed) at the base of in agreement with Lyell's concept (1833) of epoch, the Upnor Formation is the largest in the Thanetian although, as discussed in Aubry (2000), changes did not Ypresian succession in the London.Hampshire Basin (and occur simu1taneously at the time of the CIE, and they probably of all NW Europe; e.g., Aubry el al., 1986; AH & occurred at different rates in different paleontologic groups Jol!ey, 1996). over a spread of ~ 1.5my or more. However, a drawback of concem is that the CIE is appreciably older (> 1my) than the base ofthe standard/regional Ypresian Stage. Pro: 1: This level constitutes thc top ofthe Thanetian Stage s.s1. The following examples may help clariJ}: concerns that (= sensu Dollfus, 1880, not sensu Renevier, 1873, 1874; have been enunciated aboye. An excellent means ofglobal and not as currently understood) in its typc area. correlation of the MiocenelPliocene (Messinian/Zanclean) boundary was available to the working group on that boundary. This was the base ofthe Oilbert Chron (= Chron Con: C3r/C3An.1n reversal boundary). This event is only 0.6my 1: This level is poorly dated and cannot be correlated with older than the unconfom1able base of the standard/regional any degree ofconfidence outside NW Europe. Zanclean Stage. However, it was not retained by the Subcommission on Neogene Stratigraphy (SNS; see van DISCUSSION Couvering et al., 1998) because it would have resulted in It is clear from the list abo ve that sorne criteria are better the marine evaporites traditionally assigned to the suited than others to characterise a chronostratigraphic Messinian Stage and Miocene Series being transferre Aubry et al. (1999) have discussed the pitfalls ofredefining ACKNOWLEGMENTS. stages on the basis of chronostratigraphic units of higher ranks (systems or series), and proposed that the decoupling We take this opportunity to thank UNESCO and IUGS for of stages and series (for the Cenozoíc Erathem) was a way their support in the form of IGCP Project 308. We also lo preserve the meaning of stages at the same time as thank aH our colleagues interested in Paleogene history, in complying with the ICS's requirement for globally particular aH the participants of the Penrose Conference, correlative boundaries. They thus suggested that the PIE Albuquerque (I997), who have shared data and ideas with boWldary could be characterised and correlated on the basis uso ofthe CIE, but that the base of the Ypresian Stage remaín unchanged (Option 2; Text-íigure 1, columns 1 and 2 combined). 66 AUBRY ETAL REFERENCES Tarnished Golden Spikes. In Gradstein, F. M., and van der Zwaan, B., (eds.), Earth Science Reviews, Elsevier., 46: 99-148. ALI, J. R., HAILWOOD, E. A., & KING, C., 1996. The "Oldhaven magnetozone" in East Anglia: A revised interpretation. AUBRY, M.-P.,BERGGREN, W. A., CRAMER, B., DUPUIS, In Knox, R. W. O'B., Corfield, R. M. and Dunay, R. E., c., KENT, D. V., OUDA, K., SCHMITZ, B., STEURBAUT, E., Correlation of (he Early Pa/eogene in Northwest Europe. 1999. Paleocene!Eocene Boundary Sectíons in Egypt. In Duda, Geological Society Special Publication No 101: 195-203. 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High-resolution holostratigraphy of during the Paleogene: A marine perspective. The Journa/ of middle Paleocene to early Eocene strata in Belgium and adjacent Geology, 101: 191-213. areas. Pa/aeontographica Ab!. A, 247: 91-156. STOTT, L. D., KENNETT, J. P., SHACKLETON, N. J., & CORFIELD, R c., 1990. The evolution of Antarctic surface waters during the Paleogene: Inferences from the stable isotopic composition of planktonic foraminifers, ODP Leg 113. In Barker, P. F., Kennett, J. P., et a/., Proceedings of the Oeean Drilling Program, Scientifie Results, 113: 849-863. 70 AUBRY ETAL The Working Group was constituted at the onset ofthe first IGCP Project 308 meeting held on the 1st and 2nd June 1991 . the Natural History Museum, London and organized by R. O'B. Knox and J. Hardenbol. New members were welcomed in tt course oflast year to replace retired or deceased members (J. de Coninck, L. Stover and G. Jenkins, respectively). As ofApril 2000 the Working Group ineludes: DrJason AH Dr Jan Hardenbol Dr Petcr Laga Earth Scíences 826 Plainwood Drive Service géologique de Belgique University ofHong Kong Houston D13, me Jenner Pokfulam Road Texas 1000 Bruxelles Hong Kong 77079 Belgium USA Dr Marie-Pierre Aubry Dr Eustoquio Molina Institut des Sciences de l'Evolution Dr Claus Heilman-Clausen Departamento de Geologia Université Montpellier JI Geologisk Institut Facultad de Ciencias Place Eugene Bataillon Aarhus Universitet Universidad de Zaragosa 43095 Montpellier cedex 05 DK-8000 50009 Zaragoza France Aarhus Spain Denmark Dr WiIlillm A. Berggren Dr Birger Schmitz Departemnt of Geology and Dr Jeremy Hookcr Earth Sciences Center Geophysics Department ofPalaeontology Goteborg University Woods Hole Oceanographic lnst. The Natural History Museum 413 8 1 GDteborg Woods Hole CromweIl Road Sweden Ma 02543 London USA SW75BD Dr Etienne Stcurbaut United Kingdom Department 01' Marine Geology Dr Henk Brinkhuis Institut Royal des Sciences Laboratory of Palaeobotany and Dr Dennis Kent NatureIles de Belgiquc Palynology Department ofGeological Scíences 29 rue Vautier University ofUtrecht Rutgers University B-I000 Bruxellcs Budapestlaan, 4 Piscataway, NJ 08854 Belgium CD Utrecht USA The Netherlands NL-3584 David Ward Dr Chris King Crofton Court Dr Christian Dupuis l6A Park Road 81, Crofton Lane Faculté polytechnique Bridport Orpington 9, Rue de Houdain Dorset DT6 5DA KentBR5 !lID B-7 000 Mons United Kingdom United Kingdom Belgium Dr Robert Knox Dr Philip Gingerich British Geological Survey Museum of Paleontology (1514 Keyworth Museums Bldg.) Nottingham University 01' Michigan NGl2 5GG Ann Arbor, MI 48109-1079 United Kingdom USA As this paper goes to press, the WG has voted on these issues (December 1999). A majority 01' 86% of the WG mernbers voted in favor ofthe introduction 01' a new stage while 58.8% ofthem voted for the lowering ofthe PIE at the level of the excursion. As a majority of 60% is an ICS requirement, the working group has submitted a proposal to the ISPS for introducíng a new stage. lt wíll rc-vote rcgarding the location ofthe PIE.