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Chronostratigraphy and : A proposed realignment

Jan Zalasiewicz, Dept. of , University of Leicester, hierarchies would remain available for use, as recommended by a University Road, Leicester LE1 7RH, UK; Maria Bianca Cita, Dept. formal vote of the International Commission on in of , University of Milano, via Mangiagalli 34, 20133 2010. Geological context helps determine the appropriate usage of Milano, Italy; Frits Hilgen, Stratigraphy/Palaeontology, Dept. of the component units. Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, the Netherlands; Brian R. Pratt, Dept. of Geological Sciences, INTRODUCTION University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Geology is the natural in which plays a central role. Canada; André Strasser, Département de Géosciences, Université The passage of that time and its events (small and large) and de Fribourg, Chemin du Musée 6, CH-1700 Fribourg, Switzerland; intervals (short and long) are recorded in Earth’s rocks, Jacques Thierry, Université de Bourgogne et UMR CNRS 5661 particularly in stratigraphic successions and by the various Biogéosciences Dijon, 6 Bd Gabriel, 21000 Dijon, France; and lithologic, paleontologic, magnetic, and chemical signals within Helmut Weissert, ETH Zürich, Geologisches Institut, Sonneggstrasse them. Study of these rocks has yielded the 4.6-billion- 5, 8092 Zürich, Switzerland of Earth—study that is ongoing and is now being extended to other planets. Stratigraphy is the means of analyzing and ordering these phenomena, with and geochronology ABSTRACT dealing explicitly with the relations of and time. We propose a realignment of the terms geochronology and The primary means by which geological time information is chronostratigraphy that brings them broadly into line with current conveyed is by the use of the Geological Time Scale (GTS = use, while simultaneously resolving the debate over whether the International Chronostratigraphic Chart [ICC] of the International Geological Time Scale should have a “single” or “dual” hierarchy Commission on Stratigraphy [ICS]) and its units. The most of units: Both parallel sets of units are retained, although there familiar of these units are the geological periods of geochronology, remains the option to adopt either a single (i.e., geochronological) sensu stricto, or, more simply, of time (e.g., , ) or a dual hierarchy in particular studies, as considered appropri- and the corresponding systems of chronostratigraphy, sensu ate. Thus, geochronology expresses the timing or of events stricto, or time-rock on which they are based. Historically, the (depositional, diagenetic, biotic, climatic, tectonic, magmatic) in systems were built from, or subdivided into, and stages; the Earth’s history (e.g., glaciation, - periods, epochs, and ages were then used to refer to the intervals mass ). Geochronology can also qualify rock bodies, of time in which the strata encompassed were deposited. Thus, stratified or unstratified, with respect to the time interval(s) in conceptually, there has been a “dual and parallel hierarchy” of which they formed (e.g., Early Ibex ). In chronostratigraphic (time-rock) units used to designate rock addition, geochronology refers to all methods of numerical bodies that formed contemporaneously and geochronologic (or 1 http://doc.rero.ch dating. Chronostratigraphy would include all methods (e.g., time) units used to designate intervals in which they formed or , , , during which other events occurred (e.g., , extinction, cyclostrati graphy, ) for (1) establishing the deformation, transgression). Many of these units were originally relative time relationships of stratigraphic successions regionally set up as (and remain fundamentally) relative time-rock units. and worldwide; and (2) formally naming bodies of stratified rock These are typically of the last half billion (the that were deposited contemporaneously with units formally Eon), where there are good assemblages (i.e., biostratig- defined at their base, ideally by a GSSP (Global Boundary raphy) that remain key to their definition, recognition, and Section and Point = “golden spike”) that represents a correlation. Wherever feasible, additional tools, such as specific point in time. Geochronologic units may be defined and magnetostratigraphy, chemostratigraphy, sequence stratigraphy, applied generally by either GSSPs or—as currently in most of the cyclostratigraphy, and are employed (e.g., —by Global Standard Stratigraphic Ages (GSSAs). Strasser et al., 2006; Weissert et al., 2008; Langereis et al., 2010; Geochronologic units would continue as the time units eons// Catuneanu et al., 2011; Gradstein et al., 2012). Most of the periods/epochs/ages, and chronostratigraphic units as the time- Precambrian units of the GTS, which largely lack useful fossil rock units //systems/series/stages. Both assemblages, remain defined by Global Standard Stratigraphic

1 By “formed” we mean when the main fabric of the rock was constructed; in sedimentary rocks, this is taken as when the sedimentary particles were deposited; in igne- ous rocks, this typically means intrusion or crystallization (although these processes may not be precisely synchronous). The “timing of formation” of any individual is often more problematic, because such a rock commonly includes components that crystallized at different along a pressure-temperature- time path.

 http://doc.rero.ch 2004) have argued for unification of the geochronologic and and geochronologic ofthe forunification argued have 2004) Jensen, 2007; Carter, 2004; al., et Odin 2004; al., et Gong 2007; 2004b, 2004a, al., et Zalasiewicz (e.g., Some more generally). used word “age” the with confused be “age” may geochronologic the that means also (which ofrock unit aphysical other the and oftime unit “intangible” one an meanings, two having “Jurassic” as such toterms leads This names. formal same the with scale time parallel and adual ofretaining necessity over the debate been has there of time, spans are units geochronologic whereas 2012). 2004, al., et Gradstein (see established been have ones toapply, regional difficult are units global which in Forcircumstances geology. in way same the in used widely are Grenvillian, and Caledonian as such terms, informal Even banking). literature, architecture, art, (e.g., products human its and Renaissance) (e.g., interval time foradistinctive used are terms when history, human with as just represent, they intervals time and events tothe ofreference method practical and venient acon- provide They ). Hirnantian the (e.g., environments global time-constrained record) distinctive, (and encompass often unit atime during formed rocks however, the More importantly, phenomena. ofrock ages numerical the toestablish than correlations relative toestablish moreuseful and easier usually itis because partly and atleast) (to units ofthe convenience and familiarity ofthe because partly is This of years. bynumbers than rather units ofGTS terms in information time geological convey to morecommon itremains Nevertheless, ages. numerical with GTS the tocalibrate expended is Today,1902). effort considerable Jukes-Brown, (e.g., dating radiometric before days the in effectively, instances. such in chronostratigraphy of application the complicates this and 2007), al., et Zalasiewicz (cf. ofbioturbation effects blurring bythe example, for compromised, be commonly may resolution time high such at analyzed deposits in ,(year, , ).Superposition time human using occur they as dated and recorded, observed, are events Heregeologic . tothe leads turn in this and 2009), al., et (Walker core ice aGreenland in GSSP a with redefined been has numerically, only defined recently until , The tuning. astronomical through resolution athigh dated precisely and integrated are that ages numerical and encompassed time forthe signals magnetostratigraphic and biostratigraphic significant all them within have section same the in byGSSPs defined lower boundaries and upper with ) and ( stages Neogene 2006). al., et (Hilgen concept unit-stratotype ofthe revival tothe led has signatures Milankovitch complete effectively with successions long oceanic used. presently that from different very be will likely that scale time aPrecambrian in result will projects These ages. numerical chosen byarbitrarily than rather 2004) Bleeker, (e.g., record rock bytheir and the forsubdividing GSSPs todefine efforts initiating is Stratigraphy onPrecambrian Subcommission ICS the Furthermore, byGSSPs. ofthe subdivision and forthe aGSSP considering are subcommissions Cryogenian and Ediacaran the and 2006), al., et (Knoll 2004 in byaGSSP defined was /Period However, Ediacaran Ma. the at2500 set is forinstance, boundary, Archean–Proterozoic the (GSSAs); Ages While traditionally chronostratigraphic units consist of rocks, ofrocks, consist units chronostratigraphic traditionally While functioned and created, were GTS ofthe versions Early of recognition the scale, time geologic ofthe end other At the  to reexamine these terms and their conceptual value. conceptual their and terms these to reexamine timely itseems Thus, . astratigraphic within levels at all both provide simultaneously which astrochronology, as such by methods blurred now considerably is methods dating absolute and relative evidence. that from derived history temporal on the ofgeochronology definition the in material—and or evidence, physical ontangible example, bodies—for rock the on ofchronostratigraphy definition this in afocus also is There “geochronologists”). tobe themselves consider dating radiometric in specialists most indeed (and “absolute” ages todetermine dating numerical suggests moreambiguously rather geochronology while rocks, stratified typically ofrock, ofbodies relations time relative Earth.” ofthe history the in ofevents sequence time bodies.” ofrock ages and relations time relative the with PREVIOUS DEFINITIONS PREVIOUS GEOCHRONOLOGY: AND CHRONOSTRATIGRAPHY PROPOSED REALIGNMENT usage, and provide examples and explanations of good practice. ofgood explanations and examples provide and usage, proper their discuss units, oftheir and terms ofthese application and definition the weconsider Next, meanings. ofwider variety a acquired have “chronostratigraphy” and “geochronology” terms the Furthermore, usage. dual the tomaintain no, 0abstain) 2 (15 yes, overwhelmingly recommended members voting ICS the workshop, the following ballot 2010. June in aformal In Republic, Czech Prague, in on Stratigraphy Commission International ofthe ataworkshop and 2006 June in Austria, Graz, in held GSSP” the Beyond “Chronostratigraphy: Conference Penrose GSA atthe extensively discussed was issue The record. stratigraphic hierarchy. dual long-established ofthe forretention argued have 2006) al., et Hilgen Aubry, 2007; 2004; Walsh, 2004; Narkiewicz, 2004; al., et Bassett 2004; Lucas, and Heckert (e.g., others while hierarchies, chronostratigraphic these two terms are defined as defined are terms two these 1999), Salvador, and Murphy 1994; (Salvador, Guide Stratigraphic with numerical dating, whereas, before the wide application of application wide the before whereas, dating, numerical with determined is rocks metamorphic and ofplutonic of formation time the Furthermore, present. are relationships superpositional where strata, sedimentary in possible only are astrochronology and biostratigraphy Forexample, classification. and study oftheir means ofthe hence and rocks, non-stratified and ofstratified properties geological fundamental the between differences considerable remain also Yet, there sciences. Earth ofthe nature disciplinary inter- increasing ofthe because notleast relations, time related rock- toall refers that aterm in value is there Nowadays, 1994]). [Salvador, ordiscussion explanation little with introduced was philosophy in change the which in edition, the in (as rocks 1976]) tocover [Hedberg, Guide extended orbe Stratigraphic International ofthe edition first the in (as rocks to stratified Today, certainly, the clear separation that used to exist between between toexist used that separation clear the Today, certainly, with explicitly deals chronostratigraphy approach, this In Geochronology This debate represents subtle but distinct perspectives on the on the perspectives but distinct subtle represents debate This Chronostratigraphy International ofthe versions latest tothe According One might consider here whether stratigraphy should be restricted restricted be should stratigraphy whether here consider might One numerical and relative dates, once calibrated by biostratigraphy, bybiostratigraphy, calibrated once dates, relative and numerical —“The science of dating and determining the the determining and ofdating science —“The —“The element of stratigraphy that deals deals that ofstratigraphy element —“The all

GSATODAY|MARCH2013 radiometric dating, it was determined by cross-cutting relations with embarked on a program of defining new chronostratigraphic stratified rocks. This suggests a means of sharpening the distinction units and corresponding geochronologic units in the Precambrian between the two terms, as follows: stratigraphic record, to be defined by GSSPs for which numerical Chronostratigraphy, consistent with its general use today, is the ages will then be calculated. establishing of time relations in stratified rocks. The term is gener- Accordingly, a formal chronostratigraphic unit is the material ally restricted to deposition-related processes in which the super- stratal (time-rock) body interpreted to have been deposited positional properties are present, and hence the detailed historical contemporaneously and with lower and upper boundaries defined record is accessible. Chronostratigraphy is the application of disci- by GSSPs that afford the most reliable stratigraphic signals for plines such as biostratigraphy, magnetostratigraphy, chemostratigra- their temporal correlation. A formal geochronologic unit is the phy, cyclostratigraphy, sequence stratigraphy, and numerical continuous time interval between the deposition of the lowest and dating to stratigraphic successions in order to interpret temporal highest strata within the unit. In the case of non-stratified rocks, correlations. Furthermore, it involves the development of formally the rock body is referenced in terms of the time it formed (e.g., named and defined chronostratigraphic units and hierarchies, Early El Capitan Granite). This does not mean that the which comprise the ICS as well as regional chronostratigraphic rock is part of a time unit, for rock and time are separate and classifications. On Earth, chronostratigraphy effectively starts in distinct phenomena: It simply conveys that a dominant in the Archean, ca. 3.8 Ma, when a stratal record begins. the granite’s formation (crystallization of the component Geochronology denotes time relations in all rocks, specifically ) took place during a particular time unit, as deduced, for when they formed, whether stratified or non-stratified. It also instance, from radiometric ages. The boundaries of the time unit denotes the time of processes in which rocks not only formed but in this example, the Epoch (and simultaneously also were eroded (unconformities) and deformed (structural and of the equivalent Lower Cretaceous Series), are established using cross-cutting relationships). It is used to denote the timing of chronostratigraphic methods at GSSP sections and numerically events throughout all of Earth’s history that are interpreted from calibrated, for example, by radiometric dating of volcanic ash the rock record (e.g., climatic, biotic, tectonic, and oceanographic). layers within fossiliferous, correlatable successions. The geochronologic units for much of the Ediacaran to Thus, detailed analysis and correlation of the stratal record are the intervals in time during which corresponding chrono- establishes both the chronostratigraphic framework and the stratigraphic bodies of strata were deposited. Thus, the boundaries equivalent and parallel geochronologic units, while, as noted of chronostratigraphic units defined by GSSPs, chosen for their earlier, for much of the Precambrian, geochronologic units are potential for precise global correlation, mark the beginnings and currently defined by GSSAs. ends of the respective geochronologic units. Furthermore, geochron- Chronostratigraphy (time-rock) Geochronology (time) ology is commonly used to denote the practice of radiometric (e.g., Phanerozoic) Eon (e.g., Phanerozoic) dating (the term “” is available to separately denote (e.g., ) (e.g., Mesozoic) the process of numerical dating, though it has not been widely System (e.g., Cretaceous) Period (e.g., Cretaceous) adopted). Thus, geochronology can be expressed in numerical Series (e.g., Upper Cretaceous) Epoch (e.g., ) ages and durations, though the dating of geologic events Stage (e.g., ) Age (e.g., Cenomanian) and intervals is most often expressed in terms of the geo- Series for several systems have been formally named with the chronologic units. adjectives Lower, Middle, and Upper added to the system name; The succession of global geochronologic units, equivalent to the the respective epochs have been formally named with the units of the ICC, comprise the GTS, and these are calibrated by adjectives Early, Middle (Mid in the UK), and Late added to the http://doc.rero.ch numerical ages. In some instances, ash layers associated with period name. For some systems/periods (e.g., Cambrian, , GSSP sections have provided high-precision ages for boundary , , Neogene, and Quaternary), the series/epochs levels (e.g., Brack et al., 2005, for the Stage of the are given formal names without adjectives added to the system/ ). Astronomical tuning of complete, continuous Neogene period name. If used informally for any chronostratigraphic or and Quaternary sections that include GSSPs provides very precise geochronologic unit, the adjectives (lower, middle, upper, early ages for boundaries as well as for enclosed stratal packets within middle, late) are not capitalized. We omit, for the time being, the sections, but these may be subject to revision with alternative formal subdivisions of stages/ages (i.e., /chrons). tunings and/or new astronomical solutions. Most GSSPs lack such This is a complex question beyond the scope of this paper. Such ash layers and need be calibrated with numerical ages (themselves small-scale units now dominate the chronostratigraphy of subject to revision and refinement) from elsewhere. For these younger strata (e.g., the numbered stages of the reasons, boundaries of the chronostratigraphic units are not Quaternary calibrated by astrochronology—but see Cita and defined by numerical ages; instead, they are defined by GSSPs Pillans, 2010), but the necessity or means of formally defining chosen within intervals with stratigraphic signals that offer the them as higher-order chronostratigraphic units remains unresolved. most reliable and most widespread time correlation. The age of a That aside, the schema outlined here reflects the current GSSP is estimated using mainly a radioisotopic age determination standard meaning and use in practice of these units, both chrono- in its stratigraphic vicinity. In contrast, the Archean and Proterozoic stratigraphic and geochronological, although the emphasis has were first defined as, and subdivided into, geochronologic units been modified to be more clearly expressed in terms of the funda- defined by numerical ages chosen as large round numbers (3600 mental stratified/non-stratified divide. Strata and the stratigraphic Ma, 2500 Ma, 1200 Ma) rather than to reflect accurately the signals they contain can be assigned to chronostratigraphic units, Precambrian rock record and the global events it records. Now, which can be mapped, studied, and sampled. However, they (or though, the ICS Subcommission on Precambrian Stratigraphy has more precisely the events that shaped them) can also be referred

 http://doc.rero.ch THE DISTINCTION BETWEEN TIME AND TIME-ROCK UNITS TIME-ROCK AND TIME BETWEEN DISTINCTION THE Epoch). Ordovician Early the during occurring oran evolving (e.g., hierarchy geochronologic ofthe terms in made be would them, between relationships the and ornon-stratified, stratified whether rocks, ofall study the from interpreted ofevents history the Then, form they that weemphasize, not, and Epoch, Ordovician Early the during deposited were strata these that means Group” Ibex Ordovician “Early phrase (the Epoch Ordovician Early tothe referred be also can and Series Ordovician Lower ofthe part form Group can Ibex the Thus, formed. they which during time bythe them identify that units to geochronologic omnipresent in the recording of Cenomanian time by deposited bydeposited time ofCenomanian recording the in omnipresent incompleteness an is there Thus, years. ofamillion fraction ofasubstantial uncertainty involves but itusually trajectory, because fossil evolution essentially possesses a unidirectional means, bypaleontologic normally is today, correlation practice In correlation. in uncertainty onthe depends Stage Cenomanian the of recognition the GSSPs, separated geographically typically these from away far worldand the in elsewhere addition, In unit. of the succession stratigraphic global ofthe part small avery representing sections stratigraphic short within are GSSPs many Furthermore, succession. the ortopwithin base atthe unconformities or non-sequences moresignificant be may there and present, be will orscale level atsome hiatuses because incomplete invariably almost is Age Cenomanian ofthe record sedimentary the one place, Atany base/beginning. ofthe that than continent on adifferent often removed, far location, atanother is usually which Age/Stage, overlying ofthe GSSP bythe fixed are end/top the while section, type atthe GSSP bythe fixed are units ofthese respectively, base, and beginning the (Fig. 1). Thus, Cretaceous Late/Upper the of Stage Cenomanian the and Age Cenomanian the between say units, (time-rock) chronostratigraphic and (time) geochronologic between obviously toit—most assigned successions stratal the A distinction may be made between a geological time unit and and unit time ageological between made be may A distinction part of that epoch).  SUGGESTIONS FOR BEST USAGE BEST FOR SUGGESTIONS the Early Cretaceous”; “the isotope excursions in the early early the in excursions isotope “the Cretaceous”; Early the in evolved species Taconic“this orogeny”; Ordovician to Late English] UK in [orMid Middle “the (e.g., processes and events of totime referring when used are late and early and Late and Early Stage). Hirnantian ofthe orlower part series Ordovician Upper ofthe lower part the in occurs reversal magnetic-polarity the ofScandinavia; Ordovician Lower the from samples in recorded is excursion isotope the strata; ofcarbonate consists primarily America ofNorth Ordovician Lower the (e.g., successions stratigraphic within positions and torocks referring when used be should lower Silurian) lower , (e.g., upper and lower and Cretaceous) Upper Ordovician, Lower (e.g., Upper and Lower informally. used orwhen name ofaformal part as modifiers ortime superpositional with those are units some More trouble- Hirnantian). ofthe base the from upward ranges Hirnantian, the in occurred (e.g., used is rank but notits name proper the which in asentence orin section) Creek Vinini the in used as adjectives (e.g., Hirnantian glaciation, are units most when Hirnantian noproblem presents strata This rocks. stratified non- with associated tophenomena and totime referring when unit ageochronologic and rocks tostratified referring when unit achronostratigraphic touse is method Asimple writing. logical geo- in units geochronologic and ofchronostratigraphic usage correlation. chrono- through possible only generally is record stratigraphic Cenomanian the in gaps oftemporal recognition Regardless, strata). Cenomanian orsimply Cenomanian, the during deposited (strata ageo oras hierarchy) dual unit the chronologic employing Stage, Cenomanian (i.e., terms chronostratigraphic in strata the classifies one whether true holds distinction This bydefinition. on Earth, every where identical remains Age Cenomanian bythe encompassed time ofgeological However, span the rock. in events Cenomanian identifying in imprecision an and place atany We offer the following suggestions for consistent and effective effective and forconsistent suggestions following the We offer added to the analytical uncertainty. analytical tothe added is GSSPs tothe correlation/interpolation in uncertainty then elsewhere, from taken if obtained; precision ofanalytical levels the to essentially relates uncertainty the then GSSPs, respective their to adjacent immediately from taken If define. they that interval time ofthe hence and GSSPs two forthe estimates age numerical provide dates Radiometric 2. 1and byGSSPs defined age) the best placed within the stage (and thus probably depositeddensed deposit within that is poorly stratigraphically con- constrained, ahighly Cshows although Locality section. type respective its from is uncertainty butthere (shown section, type atits located as precisely is GSSPs ofthe theEach gray shading) in locatingthat includes each the GSSP away of the gaps succeeding sporadic with time asuccession Bshows unit near Locality the base. top. the near aGSSP includes that succession anear-continuous Ashows ity Local- rock. and oftime relation the illustrating 1. Diagram Figure

GSATODAY|MARCH2013 Neogene”). However, rocks can also be referred to by the time at Gong, Y.-M., Yin, H.-F., Zhang, K.-X., Du, Y.-S., Feng, Q.-L., Lai, X.-L., Xie, S.-C., which they formed (e.g., the Early Ordovician Windfall Tong, J.-N., Gu, S.-Z., Shi, X.-Y., Weldon, E.A., Shi, G.R., and Ma, X.-P., Formation), but superposition and time terms should not be 2004, Simplifying the stratigraphy of time: Comments and Reply: COMMENT: Geology, p. e59, doi: 10.1130/0091-7613-32.1.e59. mixed in the same modifier group, sentence, or paragraph (thus Gradstein, F.M., Ogg, J., and Smith, A., 2004, A Geological Time Scale 2004: “early Stage,” “lower Epoch,” and “early Upper Cambridge, Cambridge University Press, 610 p. Ordovician strata” are incorrect). If one is concerned about the Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M., 2012, The Geologic proper unit rank term to use or if one is not comfortable using Time Scale 2012: Oxford, Elsevier, 1144 p. age as the equivalent geochronologic unit for stage, then one can Heckert, A.B., and Lucas, S.G., 2004, Simplifying the stratigraphy of time: either not use the rank term or use “rock” or “time” instead Comments and Reply: COMMENT: Geology, v. 32, no. 1, p. e58, doi: (e.g., the species evolved in the Hirnantian or during Hirnantian 10.1130/0091-7613-32.1.e58. Hedberg, H.D., 1976, International Stratigraphic Guide: New York, John time; the fossil occurrences in the Hirnantian or low in the Wiley, 200 p. Hirnantian succession). Although meaning is often clear from Hilgen, F., Brinkhuis, H., and Zachariasse, W.-J., 2006, Unit stratotypes for global context, appropriate usage of chronostratigraphic and geochron- stages: The Neogene perspective: Earth-Science Reviews, v. 74, p. 113–125. ologic units can help, for instance, express succinctly the dis- Jensen, S., 2004, Into focus: Paleontologica Electronica, v. 7, no. 1, 3 p. tinction between data and observations and interpretations Jukes-Brown, A.J., 1902, The Student’s Handbook of Stratigraphical Geology: (e.g., “the successive, closely spaced lowest occurrences of species London, E. Stanford, 589 p. of Normalograptus in the lowest Hirnantian Stage worldwide Knoll, A.H., Walter, M.R., Narbonne, G.M., and Christie-Blick, N., 2006, The Ediacaran Period: A new addition to the : Lethaia, succeeding the abundant and diverse assemblages of diplograptid- v. 39, p. 13–30. dicellograptid-orthograptid species in the upper Stage Langereis, C.G., Krijgsman, W., Muttoni, G., and Menning, M., 2010, reflects a major faunal turnover associated with the Hirnantian Magnetostratigraphy—Concepts, definitions and applications: glaciation that began in the latest Katian”). Newsletters on Stratigraphy, v. 43, p. 207–233. Murphy, M.A., and Salvador, A., 1999, International stratigraphic guide—An CONCLUSIONS abridged edition: Episodes, v. 22, p. 255–271. Narkiewicz, M., 2004, Time—Stolen or regained?: Palaeontological Association, The scheme outlined in this paper seems a reasonable way to The Palaeontology Newsletter, no. 56, p. 75–77. retain the two widely used terms chronostratigraphy and Odin, G.S., Gardin, S., Robaszynski, F., and Thierry, J., 2004, Stage boundaries, geochronology in both an informal and a formal (classificatory) global stratigraphy and the time scale: Towards a simplification: Brest, sense, establishing a clear and practical difference between them France, Carnets de Géologie/Notebooks on Geology, Brest, Article more or less in line with current practice and also in line with 2004/02, p. 1–12. Salvador, A., 1994, International Stratigraphic Guide: A Guide to Stratigraphic their etymology. Both parallel sets of units are retained, though Classification, Terminology, and Procedure, Second Edition: Trondheim, there remains the option to adopt either a single (i.e., geochron- Norway, International Subcommission on Stratigraphic Classification of ologic) or a dual hierarchy in particular studies, as considered IUGS, International Commission on Stratigraphy, 214 p. appropriate. Within the framework proposed here, this question Strasser, A., Hilgen, F.J., and Heckel, P.H., 2006, Cyclostratigraphy—Concepts, may be allowed to be effectively determined, ultimately, by definitions, and applications: Newsletters on Stratigraphy, v. 42, p. 75–114. majority usage. Nevertheless, clarity and precision of stratigraphic Walker, M., Johnsen, S., Rasmussen, S.O., Popp, T., Steffensen, J.-P., Gibbard, P., expression seem currently achievable, within the guidelines Hoek, W., Lowe, J., Andrews, J., Björck, S., Cwynar, L.C., Hughen, K., Kershaw, P., Kromer, B., Litt, T., Lowe, D.J., Nakagawa, T., Newnham, R., we suggest. and Schwander, J., 2009, Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using ACKNOWLEDGMENTS http://doc.rero.ch the Greenland NGRIP , and selected auxiliary records: Journal of We thank Stan Finney for his thoughtful and constructive input toward , v. 24, p. 3–17. the construction of this paper, although he does not necessarily agree with all Walsh, S.L., 2004, Time and time-rock again: An essay on the (over)simpli- of our proposals. 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